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Geological Society of Australia

Joint Specialists Groups Meetings, Canberra, February 1972

ABSTRACTS

GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED JOINT SPECIALIST GROUPS MEETING CANBERRA, FEBRUARY

13-18, 1972

ORGANIZING COMMITTEE CHAIRMAN:

Dr. I.B. Lambert

COMMITTEE:

Dr. P.J. Cook Dr. K.A.W. Crook Mr. H.F. Doutch Dr. R.A. Eggleton Dr. M.J. Rickard Dr. G.F. Taylor Dr. P.H. Walker

SECRETARY:

Mrs. D. Moss

SOCIAL ORGANIZER:

Mr. K.A. Townley

ACKNOWLEDGEMENTS Professor D.-A. Brown, Head of the Geology Department, Australian National University, and Dr. N.H. Fisher, Director of the Bureau of Mineral Resources, are thanked for kindly lending personnel, duplicating facilities and projectors, and for their assistance in a variety of situations which came up during the organization of the Meeting. The efforts of the convenors in organizing interesting programmes of speakers for each of the ten symposia are gratefully acknowledged.

CONTENTS

PROGRAMME

1 - 8

II. ABSTRACTS Symposium A: Rock-Forming Silicates ••••••••.••••«A1 - 20 Symposium B:

Chemistry and Mineralogy of Gossans ••••••••••••••••••••••••••••#B1 - 17

Symposium C:

Sedimentation and Ore Formation' in the Near-Shore Environment •#••##•01 - 25

Symposium D:

Chemical Differentiation in Weathering and Sedimentation ••••••••SI - 46

Symposium E: Tectonic Maps of Australia Symposium F: Tectonics of the Tasman Geosyncline

••••E1 - 13 ##F1

- 16

Symposium G: General Papers, Sedimentology and Surficial Geology ••••••• Symposium H:

G1 - 17

General Papers, Ore Genesis •••••••• #H1 - 18

Symposium I: Structural General Papers, Tectonic and Geology #••••••••#

##H

- 29

Symposium la: Tectonics and Structure of the Axunta Region ••••••••••••••#Ia1 - 16 Symposium J: !!!• AUTHOR

INDEX

General Papers, Geochemistry and Mineralogy

••#J1 - 28

2 CHEMISTRY LECTURE THEATRE

PHYSICS LECTURE THEATRE

3.00 F.C. Loughnan and R. Goldbery Problems with the genesis of dawsonite in the Sydney Basin ££9

Map of N.S.W.

2.30 R,G, Warren Metallogenesis in the mainland section of the Lachlan Belt E4* 3.00 T.G. Vallance The fetamo rphic Map of Australia £6

3.30 Afternoon tea

Chairman: Professor F.C. Loughnan

Chairman: Professor E.S,Hills 4.00 M.J. Rickard Introductory remarks - the Tectonic Map of Australia and New Guinea, 1971 (no abstract)

4.00 B.R. Senior Morphology and geochemistry of chemically altered rocks in southwest Queensland

D43

4.30 G.Scholz^ R.J. Gilkes and G.M. Dimmock Lateritic deep weathering of granite

4.30 H.F. Doutch The concept of Transitional Tectonism in Australia EIO

- 0 0 P»L.C.Grubb Evolutionary trends in residual bauxite deposits

5.00 K. Plumb The tectonic evolution of Australia El3 D35

Note It is proposed that further discussion of the days contributions will take place on a generally acceptable free evening

TUESDAY, FEBRUARY 15 SYMPOSIUM D (continued) Chairman; Dr. J.C. van Moort 9

* 0 0 C*R.M. Butt The influence of chemical differentiation on geochemical surveys D^

9.30 M.J. Mulcahy, E.Bettenay and P.J. Hingston The distribution of superficial deposits, soils and weathered mantles in southwestern Australia in relation to minerals exploration

10.00 Morning tea

SYMPOSIUM I: GENERAL PAPERS / TECTONIC AND STRUCTURAL GEOLOGY. (Convenor: H.J. Harrington) Chairman: Professor S.W.Carey 9,00 R.Green and P.H.Sydenham Measurement of tidal strains in the crust II 9 30 ' H.L. Davies Underthrusting to explain metamorphic zonation in the Goropu Mountains, Papua 12. 10.00 H.L.Davies, M.Norvick R.J.Ryburn, D.S.Hutchison. I.P.Sweet Tectonics of the Western Central Ranges of Papua New Guinea I 5

10.30 Morning tea

JOINT SPECIALIST GROUPS MEETING PROGRAMME MONDAY»

FEBRUARY 14,

CHEMISTRY LECTURE THEATRE

PHYSICS LECTURE THEATRE

SYMPOSIUM D: CHEMICAL DIFFERENTIATION IN WEATHERING AND

SYMPOSIUM F: TECTONICS OF TASMAN GEOSYNCLINE.

SEDIMENTATION. (Convenor:

J.C. van Moort)

Chairman:

Professor H. Bloom Page

9.00 D.J. Swalne Factors governing the rapid weathering of a siltstone 9

* 3 0 J«D« Saxby Metal organic compounds in sediments

10s0

° W * E * B aker Humic acids and their role in weathering

1972

Dl 05

(Convenor:

E. Scheibner)

Chairman:

G.H. Packham

Rage 9.00 K.A.W. Crook Some observations on the development of the Tasman Geosyncline p| 9.30 B.S. Oversby Plate tectonics and the evolution of the Tasman Geosyncline p^. 10.00 M. Solomon and J.R. Griffiths Tectonic evolution of the Tasman orogenic zone p£>

10.30 Morning Tea

Chairman: Dr. D.J. Swaine

11.00 E. Heidecker Evolution of the Ravenswood-Lolworth Block; influence upon Devonian tectonism in north-eastern Queensland F7

11.00 H. Bloom Chemical enrichment of trace metals in the oceans 11.30 G.B. Allison and J.W.Holmes Natural tritium as a tool in groundwater hydrology

DIZ

[)I5

11.30 E. Scheibner A model of the Palaeozoic tectonic history of N.S.W. f\l

12.00 R.J. Gilkes and R.Young Biotite weathering rate and the oxidation state of octahedral iron D3Z

12.00 R.J. Gunthorpe The Nowendoc Ultrabasic Belt and associated metamorphic rocks p|2

12.30 G.H. Packham Preliminary report on Deep Sea Drilling Leg. 21, Suva to Darwin(no abstr*

12.30 - 2.00 Lunch

1.00 - 2.00 Lunch

Chairman: Dr. M.J. Mulcahy

SYMPOSIUM E: ASPECTS OF TECTONIC MAP COMPILATION

2,00 J.C. van Moort The K ? 0 , CaO > MgO and C0 2 content of shales DI9 2.30 P.J. Davies and B. Radke The mobility of certain trace elements during diagenesis

(Convenor: H.F. Doutch) Chairman:

D23

H.F. Doutch

2.00 E. Scheibner Theoretical principles of the Tectonic

CHEMISTRY LECTURE THEATRE SYMPOSIUM H: ORE GENESIS (Convenor:

PHYSICS LECTURE THEATRE

GENERAL PAPERS

Chairman: K.L. Burns Page 11.00 J.R. Griffiths The evolution of the Tasman Sea 14

Pay

Dr. G.F. Taylor)

Chairman: B.W. Hawkins iX 10.30 W.C. Lacey Design of exploration programmes 11.00 A. C. Purvis Nickel mineralization in the Carr Boyd Complex, W.A.

l±r30 R.W. Nesbittj^ A. C.Purvis ^ ' O O J.Barry and N.G.McKay. The Archaean greenstone peridotites of the Eastern Goldfields • an example of the peridotitenickel sulphide association H3 12.00 J.W. Smith Sulphur isotopes as a guide to sulphate reduction in sediments

11.30 G. Derrick Carbonate breccia in the Precambrian Corella Formation of northwest Queensland - tectonic or sedimentary? IS 12.00 E.S.T. 0fDriscoll The contact or interface as a locus of maximum deformationj9 12.30 C.McA. Powell Tectonic dewatering - a geological process producing slatey cleavage

12.30 J.R. Richards Ore formation hypotheses and lead isotopes HIO

1.00 - 2.00 Chairman:

Lunch

Dr. G.H. Taylor

2.00 D.L. Forsyth Application of the synthetic Au-Ag-Te system at Vatukoula, Fiji HI O 2.30 A.M. Giblin and D.J. Swaine Brine-rock interactions H M

1.00 - 2.00 Lunch Chairman:

Dr. C.McA.Powell

2.00 M.S. Paterson The brittleductile transition in rocks 2.30 J.N. Boland Electron microscope studies of deformation structures in olivine 119

3.00 D.E. Ayres and P.J.Eadington Trace element distribution in wall rocks as indicators of ore genesis , Rock hole Mine, N.T. HI 2.

3.00 M.A. Etheridge Physical and chemical controls on recrystallization of mica

3.30 Afternoon tea

Chairman: Professor W.C. Lacey

Chairman: E.S.T. O'Driscoll

4.00 B. Lambert Replacement textures in sedimentary ore deposits Hjtf A.30 I.H.Wilson, G.M.Derrick and R.M.Hill Copper mineralization (excluding Mt.Isa) in the

00 K.L. Burns An algebra of . geological events 12.9 4.30 G.S.Gibbons Contouring of structural (Schmidt) diagrams J

5.00 General discussion

A CHEMISTRY LECTURE THEATRE

PHYSICS LECTURE THEATRE

Precambrian Cioncurry Complex of northwest Queensland, Australia 5*00 General discussion WEDNESDAY, FEBRUARY 16 SYMPOSIUM C: SEDIMENTATION ASD ORE FORMATION IN NEAR-SHORE ENVIRONMENTS (Convenors: P.J.Cook/S,M.Richards) Chairman:

A* J« Gaskin

9.00 Rc L <»> Stanton A general view of ore types in the near shore environment (no abstract) 9.45 Speaker to be announced: Sedimentation in the nearshore environment (no abstract)

SYMPOSIUM la: TECTONICS AND STRUCTURE, ARUNTA REGION (Convenors: MoJ.Rickard and Professor H.J.Harrington) Chairman: Dr. R.G. Dodson 9.00 R. Shaw Introduction. Tectonics and structure of the Arunta region ]a ! 9.30 A.J.Stewart and M.Kahn T'he mechanics of emplacement of the Arltunga Nappe Complex 1& 10.00 J.Funk and R.Marjoribanks Interaction of basement and cover in the Arltunga Nappe Complex

10,30 Morning tea

10.30 Morning tea

11.00 M«C« Brown Stratigraphic and palaeogeographic setting of the HYC Pb-Zn deposit, McArthur River, N.T.

11.00 M.J. Rickard and R. Shaw Preliminary interpretation of the basement structures north of the Arltunga Nappe Com-

11.30 0»H a Taylor The euxinic environment and carbonaceous sediments CI,

plex

la 6

12.00 He van den Heuvel Sedimentation and diagenesis of the upper formations of the Mount Isa Group, Queensland C6 12.30 - 2.00 Lunch 2

K.A.W. Crook Volcanic ^rrigenous sedimentation vno abstract) 2.30 I«B. Lambert Metal enrichments in volcano-sedimentary-exhalative areas, PNG 3,00 M. Solomon Ore deposits associated with geosynclinal vulcanism in Tasmania and a genetic model for the Mt. Lyell pyrite-chalcopyrite ores C

12.30 - 2.00 Lunch SYMPOSIUM J: GENERAL PAPERS GEOCHEMISTRY AND MINERALOGY (Convenor: Dr. R.A.Eggleton) Chairman: Dr. R.A. Eggleton 2

* G>J>H. McCall Geochemical characteristics of differentiated dykes and greenstone suites of the Yilgarn Block, W.A. j,

PHYSICS LECTURE THEATRE

CHEMISTRY LECTURE THEATRE ^ ^

2.30 A.Y. Gllkson Primary^* geochemical patterns of Archaean igneous suites and a model of early crustal evolution ^ 3.00 D.C. Gellatly The petrology of the Strangways Range Carbonatite J10

3.30 Afternoon tea

4.00 B. Daily Aspects of carbonate sedimentation in the Cambrian of South Australia

CIO

4.00 K.G. Bowen Arsenic as a guide to gold mineralization at the Wattle Gully and Sambas Mines ^^

4.30 P.J. Davies Diagenesis in carbonates

CI A

5.00 K. Johns Occurrence of Pb-Zn deposits in Cambrian carbonates of South Australia

4.30 K.G. Henley and P.K. Schultz The Bi-Pb-S-Se mineral in the Juno orebody J145.00 J.D. Cocker Mineral layers in north-east Tasmanian granites JI4 5.15 G.S.Lister Coincidence in grain boundaries J(5

THURSDAY FEBRUARY 17 SYMPOSIUM A: ROCK-FORMING SILICATES

SYMPOSIUM C: (continued) Chairman: Professor P.F.Howard

(Convenor: Dr. R.H.Vernon)

9.00 C.E.B.Conybeare Ore genesis in the delta environment

CI5

9.30 S.M. Richards Proterozoic uraniferous conglomerates» Pilbara, W.A.

cia

10.00 R.G.Dodson Sandstone type uranium deposits

CIS

Chairman: Professor J.F,G. Wilkinson

9-00 R.H.Flood and S.E.Shaw Co-existing biotite and hbrnblende from granitic intrusions, New England Batholith, N.S.W. ai 9.30 T.H.Green and A.E. Ringwood Experimental crystallization of garnet in a natural rhyodacite composition AT

10.00 R.L. Oliver Metamorphic olivine in ultramafic rocks, with special reference to rocks from Pioneer, near Kalgoorlie, W.A. A& 10.30 Morning tea

10.30 Morning tea

6 PHYSICS LECTURE THEATRE

CHEMISTRY LECTURE THEATRE

Racjs 11.00 K.R.Walker, N.G.Ware and J.F. Lovering Compositional variations in the pyroxenes of the differentiated Palisades Sill, New Jersey A7

11.00 P»J»Cook The estuarine environment and phosphate deposition 11^30 P,S.Roy Recent estuarine sediments and mineralization on the central N.S.W. coast

r 2|

12*00 M.S.Shepherd Mineral sands on the east coast of Australia

C-25

12.30 - 1,30 Lunch

12.00 D.E. Ayres Silicate mineral relationships in the Brockman Iron Formation AI2. 12.30 - 2.00 Lunch

SYMPOSIUM G: GENERAL PAPERS S EDIMENTOLOGY/SURFICIAL GEOLOGY

SYMPOSIUM A: (continued) 2.00 A.C.McLaren Transmission electron microscopy of feldspars A12.

(Convenor: Dr. G.E.Williams) Chairman: Dr. V.A. Gostin 1*30 A.J.Moss Breakage of granitic quartz in rivers

11.30 S.Y. OyReilly The origin and nature of hourglass zoning in certain clinopyroxenes H'J

2*45 J« Boland Electron microscopy of defects in kyanite i

2,00 P,H. Walker and J.Hutka Experimental fragmentation of granitic detritus transported in water ^with special reference to size of fragmentation products 2.30 D.L. Gibson and K.A.W. Crook Weathering and sediment formation in the upper Genoa River drainage basin 3.00 G.E. Reinson Hydrology of the Genoa River estuary, Mallacoota , Victoria 3.30 Afternoon tea 4.00 G. Taylor Quartz gravels northern N.S.W* < 3 0 G.F. Taylor Formation occurrence and diagenesis of monohydrocalcite 5.00 W, Mayer Diamictite sedimentation in the NowendocMt.George area, northeastern N.S.We

3.15 Afternoon tea

(as Yh

G 14

3.45 R.A.Eggleton Stilpnomelane - structure, composition and properties hiri 4.15 P.B. Moore General crystal-chemistry of the calcium orthosilicate polymorphs, merwinite,and the alkali sulphate structure types A (9 5.00 General discussion

7 FRIDAY, CHEMISTRY LECTURE THEATRE SYMPOSIUM B:

FEBRUARY, 18

Page

GEOCHEMISTRY AND

MINERALOGY OF GOSSANS (Convenor: Dr. P.R. Donovan) Chairman: 9s0

Dr. P.R. Donovan

^ D ra<je SYMPOSIUM J: (continued) GENERAL PAPERS GEOCHEMISTRY AND MINERALOGY (Convenor; R.A.Eggleton) Chairman: Dr.C.E.B. Conybeare

Donovan - Introductory remarks (no abstract)

9.15 A.W.G.Whittle Gossan interpretation as a method in exploration

PHYSICS LECTURE THEATRE

9.00 P.M. McKirdy Eometamorphism of organic matter in Precambrian and Cambrian sediments JZO

10.00 G»M«Derrick Geochemistry of gossans from the Mount Isa-Cloncurry region, north-west Queensland B7

9.45 T.G.Powell and P.M. McKirdy The relation of Australias petroleum compositions to source, maturation and migration JfJ

10.30 Morning tea

11.00 D.C.Green, P.Gregory and W e Cooper Redrock and gossan geochemistry, Duchess-Malbon River area, north-west Queensland B5

11.00 K.A.W. Crook and G.R. Kelly Burial metamorphic hydrogarnet JZS

11.30 W . L Baker An early Tertiary (?) fossil gossan, at Oliver Hill, north-west Tasmania &Q 12.00 K.Rs Yates Gossan studies and their importance in exploration programmes IM5 12.30 General discussion 1.00 - 2.00 Lunch SYMPOSIUM B: (continued) Chairman: Dr. A.W.G.Whittle 2.00 W. Hancock and J.Wilmshurst Geochemical observation on some Ni-Cu gossans BIO 2.30 I.Re Pontifex The mineralogy and geochemistry of some Western Australian gossans and pseudo-gossans 3.00

J.M. Clema and N.P. Stevens-Hoare A method of geochemical classification for gossans, laterites and sediments from the Yilgarn and Pilbara Shields of W . A . B l Z

3.30 Afternoon tea

11.30 G.M. Derrick Pseudomorphous replacement of tourmaline by prehnite, and kyanite andalusite by topaz J23 Vacant

8 CHEMISTRY LECTURE THEATRE

<pa^e

4 9 00 R. Dalgarno Geochemistry of the Redross Nickel Prospect, Widgiemooltha area, W.A.

g|X

4*30 J.R. Beevers Some observations on the analysis of gossanous and weathered material from the Western Australian Shield B17 5.00 R.R. Keays Platinoids as discriminators of gossans derived from nickeliferous sulphides 5.30 - 5.45 General discussion

EXCURSIONS: A:

Sunday, February 13, 1972.

Leave Burton Hall, A.N.U. 9.00 a.m.

Stratigraphic Revisions, Canberra-Yass district (Leader Dr. K.A.W. Crook)

B/D : Structure of the Queanbeyan - Tarago district/ recent developments in knowledge of the geology of the Captains Flat district

(Leaders:

Dr. M.J. Rickard/ E.Z. Geologists)

C : Metamorphic and igneous rocks of the Cooma district (Leader: BARBECUE:

Dr. B.W. Chappell).

Sunday February 13, 1972.

The post-excursion barbecue will be held at the A.N.U. Staff Centre, Acton.

The bar will be open from about 5.30 p.m. and

cooking will commence between 6.30 and 7.00 p.m.

Only those

who have enrolled for the barbecue may attend. DINNER:

Wednesday, February 16, 1972. The Meeting Dinner will be held at Bruce Hall at 7.30 p.m. for 8.00 p.m.

Admission by ticket only.

Dress informal.

A1 CO-EXISTING BIOTITE AND HORNBLENDE FROM GRANITIC INTRUSIONS, NEW ENGLAND BATHYLITH, NEW SOUTH WALES R.H. Flood and S.E. Shaw, School of Earth Sciences, Macquarie University Chemical analyses and optical properties of co-existing hornblende and biotite from a number of calc-alkali plutons in the New England area of New South Wales suggest that these minerals may be subdivided into three groups:(1) tho se characterised by red-brown biotite and pale green—brown 34~ 34" 24(Z) hornblende of low Fe

/Fe

4-Fe

. Granodiorites, tonalites and

diorites containing these minerals have low Fe3+/Fe3+4-Fe24* and have minor ilmenite as an opaque oxide mineral phase. (2) those characterised by green-brown biotite and blue-green (Z) hornblende and occurring in rocks and adamellite 3+ 3~of t" granodiorite 2.1 composition with a higher Fe

/Fe

4-Fe

than group (1).

Magnetite

rather than ilmenite co-exists as an opaque oxide mineral phase. The differentiation index (D.I.) of rocks in groups (1) and (2) is less than 80. (3) those characterised by dark-brown biotite and dark green (z) hornblende and generally occurring in leucocratic adamellites with higher F e O ^ / (FeO^+MgO) than either groups (1) or (2),

The D.I.

of rocks in this group is greater than 85 and the opaque oxide mineral phase, where present, is magnetite.

Group 3 mineral assemblage can

not be distinguished from groups (1) and (2) on the basis of Fe 3+ /Fe 3+ + Fe 2+ . Hornblende compositions fall mainly within the magnesio- to ferro-hornblende field, with members of group (1) extending into the actinolite field and members of group (3) into the ferrotschermakitic field.

Tetrahedral A1 shows an increase from group (1)

to group (3), while octahedral A1 remains low throughout.

Biotite

displays little variation in Al, the main compositional change being in the mg number between groups.

Thus groups (1) and (2) contain

biotites with a variable but higher mg number than (3). For groups.. (1) and (2) the. co-existing hornblende and biotite 34. 34* 24have approximately, the. same Fe

/Fe

4-Fe

but differing mg numbers,

and in. each case hornblende has a higher mg number than biotite. Group (3) hornblende-biotite pairs by a hornblende 34. are 34- characterised 24with a significantly higher Fe /Fe 4-Fe than the co-existing

A2 biotite, while the mg number of each hornblende and biotite of a pair is similar. 3-1Based on the similar Fe

3-|-

/Fe

2-f

+Fe

of the hornblende-biotite

pairs in groups (1) and (2), it is suggested that crystallisation of hornblende and biotite took place under buffered conditions of oxygen fugacity.

The biotite solid solution curves for the buffers

Fe^O^-Fe^O^ and Ni-NiO (Wones and Eugster, 1965) would suggest that the biotites of group (2) crystallised under conditions of oxygen fugacity between these two buffer systems.

Those of group (1) indicate

oxygen fugacities either on or lower than the Ni-NiQ buffer i.e. under less oxidising conditions than group (2). 3+ Possible explanations of the variable Fe

/Fe

34. +Fe

2+ and constant

mg numbers between the co-existing hornblende and biotite of group (3) are briefly discussed. METAMORPHIC OLIVINE IN ULTRAMAFIC ROCKS FROM WESTERN AUSTRALIA WITH SPECIAL REFERENCE TO THOSE AT PIONEER, NEAR KALGOORLIE 1 1 2 R.L. Oliver , R.W. Nesbitt , D.M. Hausen

3 and N. Franzen

1 Department of Geology and Mineralogy, University of Adelaide 2 Newmont Exploration Limited 3 Department of Geology, University of Western Australia Olivine is a common constituent of altered ultramafic rocks in the Greenstone Belts of Western Australia (e.g. Nesbitt 1971) and elsewhere (e.g. Peters 1968).

It is generally assumed (e.g. Woodall &

Travis 1969) that such olivine is a primary phase, wholly or partially preserved as relict grains in a rock otherwise composed of secondary minerals. At Pioneer, 65 miles south of Kalgoorlie, Western Australia, olivine where present, commonly occurs as megacrysts in a matrix of tremolite, chlorite, serpentine and opaques, anthophyllite, talc, carbonate, phlogopite and clinopyroxene.

The olivine megacrysts average 1-2 cms.

diameter and give to the hand specimens a 'spotted* appearance., The possibility that these megacrystic olivines are not relict pheno crysts but are in fact pojrphy rob lasts of metamorphic origin was alluded to by,Oliver & Ward (1971) and is indicated by certain commonly observed textural relations, viz: (1) Olivine grains include abundant tremolite prisms sipiilar in

A3 size, shape and orientation to those constituting the matrix.

These

olivines appear to have 'engulfed1 the matrix tremolite prisms or to have crystallised synchronously with them (Fig.l). A metamorphic origin for the tremolite is probable, and this demands a similar origin for the olivine 'poikiloblasts'. (2) Lines of magnetite 'granules' extending through the secondary mineral matrix are continuous through the olivine porphyroblasts; the magnetite lines are considered to be pre-olivine.

Such a relationship

is difficult1 to account for other than by ascribing a metamorphic origin to the olivine. Similar megactysts are found at Coolgardie (Fig.2). Perhaps the most convincing secondary olivine porphyroblasts are those occurring at Spargoville in spinifex-textured rocks.

In these,

olivine 'patches', uniformly extinguishing under crossed niqols appear to be selectively replacing, parallel alternating chlorite-rich and tremolite rich zones representing, fomer spinifex-textured igneous olivine and intervening matrix (Fig.3).

These were originally

interpreted by Nesbitt (1970) as indicating a metamorphic origin for some spinifex-textured rocks. The metamorphic range of the secondary mineral phases which accompany the porphyroblastic olivine has been estimated at Pioneer (Oliver & Ward 1971) as extending from 450° to 680° (Greenschist to Upper Amphibolite facies).

This range includes the stability field

of olivine in the presence of water and at the pressures expected (Fig.4), so that a metamorphic crystallisation of olivine (forsterite) in these rocks is at least theoretically possible. Whole-rock analyses of olivine-bearing rocks at Pioneer indicate a

peridotitic composition (Oliver & Ward 1972) with up to 7.2% CaO

containedr presumably, in tremolite derived from clinopyroxene of the original peridotite.. Very few of the Pioneer rocks contain relict clinopyroxene; it is much less abundant than olivine.

However at low

temperatures, in the presence of water, olivine is less stable than is clinopyroxene and hydration of these minerals should preferentially remove the olivine.

That this is not the case is evidence

in support of the olivine having a metamorphic origin. The megacrystic olivines in the Pioneer rocks range from 13% to 25% fayalite.

This must be taken into account in a consideration of

the univariant curves in Fig.4.

The temperature range for the

A4 stability of olivine is likely to be somewhat lower than is shown as a consequence of the above mentioned fayalite contact. The relatively high fayalite content of the olivines under considerati on is not in support of their metamorphic origin according to Gabrielse (1963) who refers to the extremely low iron content of metamorphic olivine.

Peters (1968) on the other hand, describes olivines

grown during the Alpine metamorphism in Europe which are somewhat richer in Fe than the olivines in the primary lherzolite, the difference, however, being considered too small to be of any use in the distinction of primary and secondary olivines.

The significance of the fayalite

content of the olivine With regard to its metamorphic or igneous origin warrants further investigation. The regeneration of olivine as suggested above implies considerable mobility of components and fluctuation of metamorphic conditions. Apparent growth of olivine as in Fig. 3, at the expense of chlorite and tremolite involves the removal of calcium, aluminium and water. Incorporation of the lines of magnetite granules in olivine porphyroblasts as an inheritance from a pre-existing texture (see Fig.2) implies that olivine regeneration in this case at least, was an open system constant volume process. At Pioneer a tentative metamorphic sequence is as follows. Initial metamorphism of peridotite (primary olivine and pyroxene) resulted in the development of chlorite and tremolite (+ serpentine?) at temperatures of 400-500°C.

Progressive temperature increase

was responsible for the growth of olivine porphyroblasts, poikiloblastically including tremolite, followed, at higher temperatures, up to 680°C, by the crystallisation of anthophyllite.

That clino-pyroxene

did not crystallise significantly at this stage is perhaps due to moderate partial pressures of CC^ stabilising tremolite preferentially (see Winkler 1967, Fig,5) and also the olivine in preference to serpentine (see Greenwood, 1967, Fig.7).

Subsequent lowering of

temperatures to below 500°C resulted in the development of mesh lizardite or chrysotile and of antigorite coronas round some olivine porphyroblasts.

Yet another temperature rise was marked by partial

steatisation of serpentine, tremolite and anthophyllite. The extent that activities of H^O, CO 2 &nd other components have affected these postulated temperature fluctuations is unknown, but it could well be considerable.

A5 REFERENCES GABRIELSE, H., 1963: McDame map area, Cassiar District, British Columbia. Canada Geol. Survey, Mem.319 GREENWOOD, H.J., 1967rMineral equilibria in the system tyg0-Si02-H20-CQ2. In Researches in Geochemistry (Ableson, P.Hf, Ed.). J. Wiley & Son, New York, pp.542-567 NESBITT, R.W., 1970: Comments on the term 'Spittifex Texture' as applied to the serpentinites of the Eastern Goldfields.

Abstracts

of Geol. Soc. Australia Symposium, Perth/Kalgoorlie 1970 NESBITT, R.W,, 1971: Skeletal crystal forms in the ultramafic rocks of the Yilgarn Block, Western Australia; evidence for an Archaean ultramafic liquid.

Jour. Geol. Soc. Australia, Spec.Publ. No.3

OLIVER, R.L> & WARD, M., 1971;

A penological study of serpentinous

rocks associated with nickel sulphide mineralisation at Pioneer, Western Australia. Jour. Geol. Soc. Australia, Spec* Publ. No. 3 PETERS, Tj.,1968; Distribution of Mg,.Fe, Al, Ca and Na in coexisting olivine, orthopyroxene a$d,clinopyroxene in the Totalp serpentinite (Davos, Switzerland) and in the Alpine metamorphosed Malenco serpentinite (N, Italy) Contr. Min. Pet., 18, pp.65-75 WINKLER, H.G.F., 19^7:

Petrogenesis of metamorphic rocks.

Springer-

Verlag, New York WOODALL, R. & TRAVIS, G,A., 1969:

The Kambalda nickel deposits,

Western Australia. Proc. 9th Commonwealth Mining & Metallurgical Congress 1969, Vol. 2, pp .517-533. FIGURES Fig. 1

Diagrammatic sketch of olivine porphyroblast poikiloblastically including matrix tremolite prisms.

Pioneer.

Width of field

approximately 1 cm. Fig. 2

Semi-diagrammatic sketch of olivine porphyroblast enclosing strings of magnetite granules released by alteration of earlier igneous spinifex texture.

Fig. 3

Coolgardie. Width of field 8 mm.

Semi-diagrammatic sketch of olivine porphyroblast growing at expense of tremolite and chlorite.

Note the preferential

replacement by olivine of the chlorite.

The chlorite represents

original spiriifex;textured olivine and the tremolite has replaced the inter-olivine matrix (glass + pyroxene). Width of field $ p ,

Spargoville.

A6 Fig. 4

Univariant curves in the system MgO - Si©2 - H 2 G, after various workers, showing the stability range of forsterite in relation to that of associated secondary minerals S = serpentine; Q • quartz; T = talc; B = brucite; F = forsterite; A « anthophyllite; E = enstatite

A 6(a)

Trcmolite

Chlorite

Olivine

Magnetite

p0| 3

Chlorite

Tremolite Olivine

•pressure

9 Johannes (1968)

Yoder (1952)

7 6 Greenwood (1963)

5 U 3 2 1

300

400

500

600

700

800 °C temperature

A7 EXPERIMENTAL CRYSTALLIZATION OF GARNET IN A NATURAL RHYODACITE.COMPOSITION Trevor H. Green1 and A,E, Ringwood 2 1 School of Earth Sciences, Macquarie University 2 Department of Geophysics and Geochemistry, Australian National University A high-pressure, high-temperature investigation of a glass prepared from a natural garnet-bearing rhyodacite from Victoria has been conducted under anhydrous and hydrous conditions (2, 5 and 10% by weight of water added).

Garnet and quartz are near-liquidus

phases from 13.5 to 27 kb with 2% water present, but with 5-10% water present garnet is the liquidus phase from 18-27 kb and the appearance of quartz is depressed significantly below the liquidus. All garnet crystals analyzed over the pressure range 11 to 27 kb have > 12 mol.% grossular, higher than that found in the natural garnets.

It was not ppssible to obtain garnets with composition

corresponding in grossular content and Fe/Mg ratio to the natural garnets for the experimental conditions of this study.

COMPOSITIONAL VARIATIONS IN THE PYROXENES OF THE DIFFERENTIATED PALISADES SILL, NEW JERSEY 1 2 3 K.R. Walker , N.G. Ware , and J.F. Lovering 1 Bureau of Mineral Resources 2 Department of Geochemistry, Australian National University 3 Department of Geology, University of Melbourne The Palisades Sill of the eastern United States was emplaced in the Stockton Formation of the Newark Group during the Upper Triassic. A complete section of the sill is exposed opposite New York City where it is about 1000 feet thick, and where the renowned Mg-olivine (hyalosiderite dolerite) layer may be seen about 70 feet above the base. The sill is a multiple, intrusion of oversaturated tholeiitic magma which, .apajrt from crystal, settling, that formed the Mg-olivine layer at the internal contact between the two main magma phases comprising...the_sill,^differentiated larg§ly, as a result of .fractional crystallization. This differentiation followed a normal course of strong iron- and slight alkali-enrichment to a stage where fayalite granophyre formed, and then the trend changed to one of alkali- and

A8 silica-enrichment in the very late fractionation stages. The ferromagnesian mineral series of this fractionation sequence are augite/mauve-brown ferroaugite (WoQQ. Enc1 Fs-- to Wo 00 En 1n Fs„), JO JI 11 Jo LU JZ pale green ferroaugite ( W o ^4 E n 2 7 F s 29 t 0 W °44 E n io F s 46^ Pig e o ni te (Wo g En 54 Fs 38 to Wo 12 En 34 Fs 54 ), bironzite/hypersthene (Wo^ 5 En 7 5 Fs 2 Q 5 to Wo 3 En 5g Fs 39 ), and ferrohypersthene (Wo 3 En 34 Fs 63 to Wo 4 En 26 Fs 7Q )• Associated with these pyroxenes are olivine (Fo-,-, „ to Cr and F o13 // to Fo55 Fo 4 . 5 ^ biotite whose Fe/Hg.gpe^. from 1 to 23, arid hornblende whose Fe/Mg is between 5 and 10. The compositional trends of the pyroxenes in the pyroxene quadrilateral

re fairly normal for the augite/mauve-brown ferroaugite, the

bronzlte/hyperstheue,, an£ the pigepnite series, except that there is a regular but only slight; (WQ33 to Wo3g). variation of Ca in the augite/ ferroaugite series with fractionation, and that the inversion of pigeonite to hypersthene occurs commonly near E n ^ rather than the En^Q found in many other tholeiitic intrusions. The other distinctive features of pyroxene behaviour occur from the late middle fractionation stages on, where alteration of the mauvebrown ferroaugite in residual stages of crystallization led to the formation of a separate ferroaugite series, a more Ca-rich pale green one whose trend parallels that of the normal ferroaugite trend but is nearer the Di/Hd join. As cooling proceeded into the subsolidus, other accompanying reactions took place; these involved the Ca-*poor pyroxene series.

the early late stages ferriferous pigeonite inverted to ferrohypersthene, and for a short period ferriferous orthopyroxene crystallized directly from the magma.

This then became unstable and the reaction clinopyroxene

+ orthopyroxene+liquid clinopyrpxene+olivine+quartz-l-liquid took place. With further iron-enrichment in the magma after this, ferrohortonplite changed to fayalite, and the,coexisting ferroaugites moved toward f§rrohedenbergitic compositions.

In the final stages where silica- and alkali-

enrichment became more pronounced laprnblende largely replaced the ferroaugites as in the granophyric dolerite.

A9 THE NATURE AND ORIGIN OF HOUR-GLASS ZONING IN CERTAIN CLINOPYROXENES Suzanne Y. O'Reilly, School of Earth Sciences, Macquarie University Abundant euhedral clinopyroxene phenocrysts

(up to 6 mm long)

showing excellent development of hour-glass zoning, occur in a latestage differentiate in an undersaturated alkali basaltic sequence from the Southern Highlands, N.S.W.

The degree of perfection of1 the

zoning provides an ideal opportunity to determine the morphological, optical, and chemical properties of different zones.

Detailed

investigation of such clinopyroxenes within one rock type eliminates ambiguity in the interpretations of chemical variations which could otherwise be due to compositional diversities of host rocks. Typical hour-glass form is best developed in the _a/£ plane, i.e. perpendicular to ID, and is evident by colour variation in the different sectors.

Considering the hour-glass zones on (010) as

four interlocking triangles (see fig. 1), those triangles whose bases are parallel to a_ are pale, those whose bases are parallel to c. are deeply coloured and more intensely pleochroic.

Either both

pale or both deeply coloured sectors may be contiguous in different crystals, thus no consistent chronological order of formation exists. (Pale sectors are contiguous in fig. 1) exhibit patchy and irregular zoning.

(100) and,(001) sections

The suggestion of hour-glass

shape may be evident on (100) sections, but is absent on (001) sections where zoning is similar to the complex zoning described by Wilkinson (1957).

Thus the type and degree of morphological

perfection of the zoning is dependent on crystallographic

orientation.

The mutual boundaries of the pale and deeply coloured sectors are i

•

X" •

crystallographically related to the inclination of faces in the (010) zone.

A model wherein the deeply coloured sector has the

form of two triangular prisms, the bases of which are parallel to (100) faces, is consistent with observed data. As well as the hour-glass sectorial zoning, narrow bands, concentric with the euhedral, crystal outline (see fig. 1), occur continuously around all sectors of the hour-glass

zones.

Morphologically,. this zouingîs ..eqîv^le^t to normal and oscillatory zoning Ixi pâgigclases,..

Thejre. are no discontinuities at sector

boundaries, and these zones appear to represent time boundaries within a given crystal.

They are distinguishable by different colour

A10 intensities and slightly different extinction angles. Many of the hour-glass zoned crystals are completely rimmed by a zone of dark purplish-red clinopyroxene chemically and optically equivalent to quench groundmass clinopyroxene.

This outer zone is

continuous, non-sectorial, and probably due to late-stage precipitation of a quench clinopyroxene rim. Measurements of optical orientations of sectors in over 50 grains on a universal stage established that there is no variation in crystallographic orientation of sectors within individual crystals other than slight-differences in extinction angle which are attributable to minor chemical differences. Electron microprobe analyses were carried out on various sectors of the hour-glass zones on five different grains.

A clear corres-

pondence between colour (i.e. particular zone sector) and chemistry is established.

Ti02, A1 2 0 3 , and Si02 variations are responsible

for the zoning.

In deeply-rcoloured sectors, Ti02 ranges from 5.8

to 5.2%, Al^O^ from 10.9 to 10.1%, while Si02 shows antipathetic variation, ranging from 41.5 to 42.6%.

These minor fluctuations

are due to the concentric normal and oscillatory zoning.

In pale

sectors, Ti02, A l ^ , and Si02 show ranges of 2.5 to 2.9%, 5.8 to 6.8%, and 50.4 to 47.9%.

The continuous outer rim of deep pink

clinopyroxene, compositionally identical on both pale and deeply coloured sectors, is chemically distinct from the hour-glass sectors in its relative enrichment in iron at the expense of magnesium. Volcanological interpretation of the associated lava sequence and the topographic form, indicate that.the host rock was sluggishly erupted close to a small (and probably final) vent, this representing the most differentiated effusive rock of this volcanic pile.

reasonably long period of shallow iritr^telluric crystallisation can be inferred.

The chemistry and mode of the host rock shows that

clinopyroxene was the major precipitating phase containing MgO (modal olivine <2%).

The pyroxene is envisaged as crystallising

under an environment of relatively slow ionic diffusion (due to increasing magma viscosity) and of relatively slow crystal growth compared to quenching conditions; The hour-glass zoning, produced mainly, by. variation of H Q g and A1

2°3 a n t i P a t h e t i c a l l y

Si02 is considered to derive from inter-

action between growth rate of the crystal and diffusion rate of

Figl

All ionic species from the liquid to the growing crystal surface.

The

obvious crystallographic control of the orientation of these sectors suggests selective substitution of A1 for Si in tetrahedral sites parallel to the (101) direction, charge imbalance being overcome by 3+ concomitant increase of in octahedral M^ and M2 sites. It is suggested that this hour-glass sectorial zoning depends on . the rate of nucleation of clinopyroxene from a magma saturated in potential pyroxene,

Using the diopside structure of Zussman (1968),

it is evident that tetrahedral sites would be simultaneously exposed on (010) and (001) layers, thus creating an ideal environment for complete substitution of A1 and Ti.

Two specific growth mechanisms

may account for the hour-glass zoning. One is the complementary 2j ^j coupled substitution of A1 (for Si in tetrahedral sites) and 34Ti

(in M^ sites) within the same growth layer (i.e. (010) or

(001)).

The other is an "oscillatory11 effect wherein charge

imbalance in one layer of a growing crystal (e.g. parallel to (110) or (101)) is satisfied by completion of the coupled Al-Ti substitution in the next layer. REFERENCES WILKINSON, J.F.G., 1967:

The clinopyroxenes of a differentiated

teschenite sill near Gunnedah, New South Wales.

Geol. Mag.,

94, 123-134 ZUSSMAN, J., 1968: 1. Pyroxenes.

Fig. 1

The crystal chemistry of pyroxenes and amphiboles,

Earth Sci. Rev., 4, 39-67 FIGURES

(010) section of a clinopyroxene showing hour-glass zoning delineated by solid lines.

Normal and oscillatory zoning

is shown by dashed lines.

Note indentations at hour-glass

sector boundaries coinciding with normal and oscillatory zonal changes * Hour-glass sectors with triangular bases parallel to (100) and normal and oscillatory zones parallel to c are deeply coloured.

Sectors with triangular bases parallel to

(001) are pale and, in this example, contiguous .

A12 SILICATE MINERAL RELATIONSHIPS IN THE BROCKMAN IRON FORMATION D.E. Ayres, C.S.I.R.O., Division of Mineralogy Chert, iron oxide and quartz-iron oxide mesobands comprise a major portion of the mesoband types within the banded iron formation macrobands from the Dales Gorge Member of the Hamersley Iron Formation.

Chert,

magnetite and hematite are the major mineral constituents and predominate over the various iron silicate minerals.

Because of this the textural

relationships between the iron oxides and the silicates can be readily observed. In the absence of a precursor, stilpnomelane is considered to be a primary constituent.

Greenalite and an iron-rich chlorite are ^lso

thought to be primary constituents from which minnesotaite and biotite formed during later metamorphism.

Riebeckite has apparently replaced the

primary minerals in mainly quartz-iron oxide mesobands to form massive riebeckite mesobands and has used magnetite and other minerals as sites for nucleation and growth.

The sodium necessary for the forma-

tion of riebeckite may have been present locally although since both biotite and riebeckite have developed later than magnetite euhedra it is possible that they both formed during metamorphism. It is considered that pressures of 4 to 6 kilobars produced by the load of overlying strata and temperatures of 200°C to 400°C, due to a geothermal gradient of 15°C/km, were sufficient to initiate metamorphism which resulted in the development of minnesotaite and biotite in mesobands of the appropriate composition.

TRANSMISSION ELECTRON MICROSCOPY OF THE FELDSPARS A.C. McLaren, Department of Physics, Monash University The development of ion-bombardment thinning has now made it possible to use the full potentialities of transmission electron microscopy (and t\xe associated technique of selected area electron diffraction) for the study of submicroscppic features and crystal defects in minerals and rocks. At Monash we are using these techniques to obtain direct information about the nature, distribution and interaction of crystal defects and impurities in important,rock-forming minerals (and related synthetic crystals) and to assess the role of such defects in geological processes, in particular metamorphoric and tectonic processes.

A13 Currently, a major part of our effort is concerned with the feldspars and this paper will be essentially a review of this work. However, particular attention will/be given to the problem of determining the basic structure of the intermediate plagipclase feldspars. On the basis of the observed X-ray diffraction patterns, the ordered ("low-temperature") plagioclase feldspars are divided into four main structural divisions: (i)

Low-albite structure

- AnQ to An 2

(ii)

Peristerite structure

- An 2 to An 25

(iii)

Intermediate structure

- An2^ to A n ^

(iv)

Anorthorite structure

- A n ^ to A^QQ

The primitive anorthorite structure is characterised by four types of reflection: Type (a): h + k « 2n,

I - 2n

Type (b):h + k = s 2 n + l, I = 2n + 1 Type (c) : h + k - 2n,

£ - 2n + 1

Type (d): h + k - 2n + 1, I « 2n where n is an integer.

The a-reflections are characteristic of all

feldspars. In the intermediate range there are no reflections of types (b), (c) and (d).

However, there are now pairs of e-reflections equidistant

from the positions of the b-reflections in the anorthorite structure. The position in reciprocal space of an e-reflection may be specified by the vector

drawn from the nearest "b-position".

In addition, the

intermediate structure exhibits pairs of f-reflections equidistant from the a-reflections,. The position of an f-reflection may be specified by a vector reflection.

in reciprocal space drawn from the associated a-

Bown and Gay (1958) found that S_T = 2S^ and varies in

magnitude and direction with anorthorite content.

Smith and Ribbe

(1969) suggested that the e- and f-reflections imply the presence of a superlattice with spacing given by 1/ | s j . McConnell has implied that two periodicities are involved.

Since

= 2_S, he concluded that

the superlattice spacing associated with the f-reflections must have half the spacing of the superlattice associated with the e-reflections, and that the "two effects are strongly correlated statistically". Figure 1 is an idealized drawing of a typical electron diffraction pattern of an intermediate plagioclase showing clearly the pairs of e- and f-reflections.

This type of pattern may be considered as the

A14 superposition of two nets in the corresponding reciprocal lattice plane. The larger net corresponds to a sub-cell, the smaller to a long-period superlattice based on the sub-cell and arising from the ordering of . some kind of point, line or plane defect.

The observed reflections

are centred approximately on the sub-cell nodes,

It is clear that

the superlattice period D is given by 1/ |jSf.| - 1/2 | sj . If this interpretation of the diffraction pattern is correct then it should be possible to observe the superlattice directly in the electron microscope by forming an image of the crystal by combining (i) the transmitted beam 000 with its two f^-satellites or (ii) any areflection with ;its f-satellites or (iii) any pair of e-reflections. Simple considerations show that the superlattice image will have the same period D - 1/ |S_f| as the superlattice for all three imaging conditions. Direct resolution of the superlattice provides localised information about the arrangement of the superlattice planes.

This is in

contrast to that obtained from the diffraction pattern alone which provides information averaged over a comparatively large volume. example, for a sample of

For

(the highest An content of the inter-

mediate structure) the superlattice was not uniformly developed throughout but was confined to domains.

At this composition D is

about 8QA and jSr is near normal to (141), The nature of the superlattice', and,, its variation in magnitude and orientation with An content are veyy incompletely understood, but will be discussed along the lines given by Megaw (1960). Superimposed on the superlattice there is often a much coarser quasi-periodic lamellar-substructure which can give rise to optical schiller when the period is of the order of the wavelength of light. The nature of these substructures will be discussed. Examples of submicroscopic substructures in peristerites and alkali feldspars will also be shown and discussed.

A 13 (a)

Figure 1: Idealized drawing of the electron diffraction pattern of an intermediate plagioclase feldspar (An^). The electron beam is near normal to (001).

A15 ELECTRON MICROSCOPY OF DEFECTS IN KYANITE J.N. Boland, Department of Geophysics, Australian National University The defect structures in kyanite were investigated by transmission electron microscopy.

The predominant defect structure observed was

microtwinning on (100).

The twin width varied from about 50 A° to a

maximum of 0.8 micron (8000A°).

The twin laws were determined by analys-

ing the electron diffraction contrast at the twin boundaries.

In all

the samples examined, only two of the possible three twin laws were observed (see Deer, Howie and Zussman, 1966) . These laws were the (100) lamellae twins with twin axes (a) 'perpendicular to (100) and (b) parallel to z.

The absence of the third (100) twin law with twin

axis parallel to y was considered to be related , to the shear directions in the naturally occurring kyanite crystals (i.e. if the first two twin laws are observed to operative, the resolved shear stresses on the third system must be very small).

This assumes that the origin of

these microtwins is a defprmational process. Previous research on the deformation of kyanite found that the only slip system operative was (100) [001].

The fine striae on (100)

reported by Mugge (1883) were shown not to be twins because of their optical properties.

These results were confirmed by Raleigh (1965).

However, a more thorough examination by optical microscopy in conjunction with electron microscopy revealed that deformation twinning can be an important mode of deformation along with slip, polygonization and kinking.

A simple dislocation mechanism using emissary

dislocations was used to account for the formation of the twins. Other types of lattice defects were observed in kyanite and a detailed examination of the atomic structures of these defects was made using the fringe analysis method previously given by Remant et al (1965).

This analysis revealed antiphase domain boundaries in the

(110) of the kyanite lattice but this is directly related to the closepacked (111) planes of oxygen that can be considered as making up the structure.

This planar defect was not produced by deformation and

consequently appears to be a growth phenomenon of kyanite, A second type of antiphase domain boundary was observed on (100).

This planar fault was always terminated by dislocations and

the fringe analysis of the fault revealed that faulting vector, R, was 1/2 [001].

Since the slip direction in kyanite is [001] it is

A16 energetically more favorable for the dislocations with Burgers vector, b, of [001] to dissociate into partial dislocations by reaction [001]*l/2 [001]1 + [001]2 This results in a stacking disorder between the two 1/2 [001] dislocations; the separation of the partial dislocations will depend on the energy created by introducing this disorder which is effectively an antiphase domain boundary.

Such dissociations were frequently observed

in both naturally and experimentally deformed kyanite. The possibility of polymorphic transformations was considered. However, there was no evidence of the kyanite transforming to either andalusite or sillimanite.

This negative result supports the theory

that the A^SiO^. polymorphic transformations are reconstructive and are not accomplished by simple shear processes. REFERENCES DEER, HOWIE and ZUSSMAN:

An Introduction to Rock-Forming Minerals,

Longmans, Green and Co.Ltd., London, 1966 MUGGE, 0.,

NEUES, JAHR:

RALEIGH, C.B.:

6 Geol. Palentol. Abhandl. 1, 71, (1883)

Glide Mechanisms in Experimentally Deformed Minerals,

Science, 150, 739-741, (1965) REMAUT, G., GEVERS, R., LAGASSE, A., AMELINCKX, S.:

Dynamical

Theory of the Images of Microtwins as observed in the Electron Microscope,

phys. stat. sol*, 10, 121-139, (1965)

STILPNOMELANE - STRUCTURE, COMPOSITION, M P PROPERTIESR.A. Eggleton, Department of Geology, Australian National University Stilpnomelane is a layer silicate* similar to biotite in chemistry and appearance, though inore brittle.

It occurs as a major rock forming

mineral in the Brisbane, New Zealand, Japanese and Californian metamorphics, and in the Hamersley, Lake Superior and South African Precambrian Iron Formations.

It is found as a minor mineral in such varied

parageneses as the Great Cobar and Mt Isa ore horizons, in vughs in the Skaergaerd intrusion, at Franklin Furnace and at Broken Hill. Stilpnomelane contains about 20% Fe, and is unique among silicates in permitting large scale variation in ferrous/ferric ratio with no change in the amounts of other cations. 4K(Fe Mg)12(Si Al)^g[0,(OH)]^.nH^O.

A simplified formula is

Other elements reported in

A17 stilpnomelane are Na, Ca, Mn, Zn, Ti, as well as traces of NH3« The extent of major element variation is shown in Fig.l. In contrast with the continuous single tetrahedral sheet of biotite and other micas, the tetrahedral sheet of stilpnomelane is broken up into groups of 24 tetrahedra connected by paired rings of 6 tetrahedra.

The tetrahedral sheet is thus an open four-layer

structure, having some resemblance to zeolites.

The silicon to

oxygen ratio of this sheet (6:14) is between that of micas (6:15) and framework silicates (6:12).

The lower half of the tetrahedral sheet

is shown in Fig.2. The silicon sheets are connected by Fe and Mg in octahedral sites, as in biotites.

However, not all metal cations are near a

silicon, and those more distant can oxidize readily.

Of the 48

octahedral sites in the unit cell, 4 very easily accommodate Fe

oI ,

8 fairly easily, 18 only under oxidizing conditions, and 18 can not. 2+ Oxidation of Fe is accomplished by oxygen permeating the zeolite-like silica sheet and then combining with hydrogen present as hydroxyl in the octahedral sheet. the silica sheet.

The water so formed tends to remain within

Ferrostilpnomelanes are those in which only the

easily oxidized 12 cations (or fewer) are ferric iron; ferristilpnomelanes have all but the last 18 cations oxidized. Few samples fall between these extremes, as oxidation is rapid once started.

For this reason samples taken from outcrop may not represent

the composition of the mineral in its formation environment. Although most stilpnomelanes from regional metamorphic rocks appear to have crystallized as ferrostilpnomelane, some vein samples crystallized as ferristilpnomelane.

Experimental work may prove the

usefulness of the mineral as an indicator of oxidation-reduction conditions. The oxidation of stilpnomelane is a 3-step process; within each step stilpnomelane acts as an oxygen buffer.

Designating the number

of ferric ions by a subscript, and as yet unknown threshold oxygen fugacities by f^, we can write: (Stilp) + °2 r 0

(stiiP)4

0 < f(o2) = f x

4 +

(Stilp)12

12 +

(Stilp)3Q

2< f(o2)

f(o2) = f 2

A18 Oxidation of ferrostilpnomelane causes a reduction in a and an increase in

these changes can be used to estimate the oxidation

state of the mineral (Fig.3).

Host samples of stilpnomelane have a

range of compositions, resulting in diffuse X-ray powder diffraction maxima, so that diffractometry rarely yields precise data.

By con-

trast, powder photography from a single grain can give precise cell dimensions, but these may not represent the whole sample.

Published

graphs relating refractive index to ferrous ferric ratio take nq account of other compositiqnal variations and so can only give very approximate compositions.

A plot of y refractive index versus a 2-f 34° , Fe , and Mg content.

can be contoured to give approximate Fe

Mg Fe

K HaO 0

STILPNOMELANE CELL

F a 2*

CONTENTS

A 13(b)

12-4

12-3

doo,

1 2 "2

• 8O

Ferri

121 -

12-0

21-7

Ferro

21-8

21-9 a

22 0

22 1

A19 GENERAL CRYSTAL CHEMISTRY OF THE CALCIUM ORTHOSILICATE POLYMORPHS, MERWINITE, AND THE ALKALI SULFATE STRUCTURE TYPES P.B. Moore, Department of the Geophysical Sciences, The University of Chicago The calcium orthosilicate, Ca2[SiO^], polymorphs and merwinite, Ca^Mg[SiO^]£ are of great importance to the cement and blast furnace industries and the literature on the subject is enormous.

The alkali

sulfate structure types such as glaserite, K N a ^ S O ^ and g-K^SO^] are the basis for no less than 100 .'compounds. I wish to demonstrate that the geometrical and topological principles behind all these compounds are closely connected and that their structural relationships can be placed within a rational framework which is both simple and efficacious. The glaserite structure type is the best introduction to the problem. One large alkali cation is ideally 12-coordinated and resides in a polyhedron of oxygen atoms of which six define the vertices of an elongate trigonal antiprism ("octahedron") and six are situated in an hexagonal ring in the plane of the large alkali.

Such an

arrangement is obtained by a "pinwheel" pf tetrahedra each of which shares a corner with the trigonal antiprism and whose vertices define the hexagonal ring of coordinating oxygens. 46+ of K

to S

The crystal radius ratio

is favorable since the height of the antiprism down tfre

3 axis is about double that of the tetrahedron down the 3-fold axis. The pinwheels polymerize by corner sharing to form an infinite flat sheet in a plane which, in many respects, is related to close-packed geometry.

The pinwheel (the trigonal antiprism and its associated

six tetrahedra) has point symmetry 3 in the glaserite structure. Once- its arrangement is specified, the oxygen coordination about the other non-equivalent alkali cations is automatically defined.

For

glaserite, the second alkali is coordinated by six oxygens defining a trigonal antiprism which is situated above the pinwheel, and a third circumjacent alkali coordinated to ten oxygens witl> point symmetry 3m. The glaserite pinwheel is mathematically treated as a bracelet. A bracelet is a loop with n nodes and involves m symbols, where m<n. For the pinwheel, n=6 (a hexagonal ring) and m=2 (tetrahedra point either up (u) or down (d)). distinct bracelets is 12.

Combinatorially, the total number of

Each bracelet may have.a distinct

A20 complementary bracelet, obtained by switching the u and d symbols. Some bracelets are their own self-complements•

Once the bracelet is

known, the coordination number of the oxygen anions about the cations is automatically defined, at least for the ideal structure (i.e., no distortions).

The space group and cell shape of the ideal model also

is retrieved.

In real crystals, this corresponds to the maximum

coordination number the central cation can have, The bracelet complementary to glaserite (3u+3d) is found in merwinite.

The bracelet for g-K^tSO^] is (4u+2d) and is its own

self-complement.

Larnite, af-Ca2[SiO^],

this arrangement.

Starting with a given bracelet, it is evident that only certain combinations of bracelets are permissible.

Thus, hypothetical

arrangements have been retrieved and their symmetries and cells derived.

Several of these may exist in complex crystals.

One

example offered is a possible arrangement for bredigite, ca. Ca^Mg[SiO^]^, also known as the "T" phase in the system Ca0-Mg0-Si02. The desirable qualities of this approach include systematic retrieval of all arrangements both real and hypothetical and the maximum ("ideal") coordination numbers of the large cations. Degrees of solid solution and unusual compositions (as for the ,! fl

phase) can be explained on this basis.

B1 GOSSAN, INTERPRETATION AS A METHOD IN EXPLORATION A.W.G. Whittle & Associates, Adelaide There was widespread interest in this subject in the 1920/1940 period when geologists such as Locke, Boswell and Blanchard were carrying out pioneer work on the interpretation of leached outcrops. The earliest work was on the outcrops of copper orebodies, but interest soon spread to the outcrops of other base metal orebodies. The development of this technique diminished in the ensuing period because of the rapid growth of geoehemical and geophysical methods of exploration.

However, during the last five years there

has been a renewed interest in this subject, and many companies now carefully sample conspicuous ironstone outcrops to have these examined, and categorised as false or true gossans. The examination of leached outcrops as an exploration method can be more usefully applied now, than 30 years ago.

This improved

application arises from the facility to combine these observations with other readily available and well-developed techniques.

In the

field for instance, the ironstone outcrop can be studied in the context of.its surface geochemical characteristics and its subsurface geophysical responses.

In the laboratory, the ironstone is no longer examined only

by stereobinocular microscope, but polished-thin sections are made, and very detailed work is carried out with the polarising microscope.

The

uncertainties encountered in the microscopic examination may then be further resolved by the electron probe analyser. The typical gossan outcrop is characterised by the accumulation of yellow, brown, red or black limonite with other oxides, metal oxysalts, quartz, silica and silicates in country rock which has a bleached appearance.

The gossan commonly stands out in topographic

relief, and while broadly massive in its structure it is usually cellular in its detailed texture. Apart from entrained country rock, vein quartz and secondary chalcedonic or opaline silica, the main component of a gossan is the so-called limonite.

This generally consists mainly of goethite, but

it may also contain lepidocrocite, hematite and maghemite.

According

to the nature of the original vein formation the limonite is completed with diverse varieties of both the jarosite minerals, and the manganese oxide minerals.

In addition there may be metal oxysalts, oxides and

native metals as well as gypsum, clay minerals, calcite and silica.

B2 It is seldom that the limonite component of the gossan is entirely indigenous in its origin.

While the key boxworks and replicas are

built by strictly indigenous goethite with or without silica, there is always more or less exotic goethite which, as microcrystalline or colloform banded aggregates, filled leached cavities, There are three major features of the gossan which are important in exploratory work: (1) its physical relationship to the host rock in terms of structural attitude, and its involvement within zones of rock alterations; (2) its geochemical characteristics; (3) its intrinsic ^ross structure; its finer textural characteristics; and its mineralqgical constitution. These three features, when defined by the joint effort of the field geologist, the geochemist and the mineralogist, should in most instances facilitate the interpretation of the gossan.

From this may

be postulated the classification of the original vein formation, and its original metal bearing characteristics at better than a 75% confidence level. The structure, texture and mineralogy of gossans may be determined simultaneously by the use of thin and polished sections in conjunction with the observations made with the stereobinocular microscope.

many cases surface staining by specific phemical reagents, or contact and electrographic printings with,gelatine coated papers, are utilised to locate the secondary metal oxysalts.

Where unusual difficulties are

encountered, X-ray and electron probe analyses are also employed. The structure of the gossan must be distinguished from the finer textural features which are mainly determined by the highly important boxworks and replicas of the oxidised and leached sulphides.

The gross

structure of the gossan is usually that of the original vein formation, and it is manifest in the shapes and alignments of entrained wall rock, quartz and other persistent gangue components, as well as in the distribution of the indigenous limonite within the gossan.

Shears,

fractures and brecciation are readily recognised both in the entrained rock and in the persistent; gangue components.

Such structures may

have been further accentuated by the preferential distribution in them, of exotic silica and exotic limonite. By virtue of these observations and those of the field geologist, it is possible to classify the gossan as having developed from:

B3 a mineralised shear; an open space filling; a pyrometasomatic skarn deposit; a stratabound syngenetic deposit, etc. It is most important to determine the mineralogy of the persistent gangue components, and of the entrained wall rock, as well as that of the protected-unoxidised primary ore mineral inclusions in the gangue components.

The results are important in classifying the original vein

formation, and in confirming the conclusion arrived at by the consideration of the structure. The optical determination of the components is without difficulty apart from the clay mineralsthe jarosites, and the microcrystalline oxysalts and oxides.

If these are too fine for determination in thin

section, or by refractive index media, X-ray or probe analysis may be required, but in order to keep costs at a minimum, optical methods, aided by microchemical and staining techniques are employed where practicable. The diagnostic texture of the gossan is commonly determined mainly by the boxworks and replica forms of the oxidised and leached primary sulphides, oxides, silicates and carbonates.

Replica forms in goethite

or silica are mainly a simple expression of external crystal morphology, whereas boxworks are more complex.

The external shape of the entire

boxwork, and the configuration of internal and external cell walls, reflect both the external morphology, and the internal crystal structure of the former mineral, i.e. cleavages and partings are important elements of the boxwork. The mode of development of boxworks and replicas during the oxidation of ore, depends in detail upon climate, host rock environment, the composition of the gangue, topography, groundwater conditions, and so on. Apart from the obvious influence of oxygenated waters, it is considered that the initial oxidation of sulphides may also proceed by their reaction with either CC^ or K^C^ in groundwaters. Most important however, is the direct oxidation which is initiated in orebodies by the relatively high susceptibilities of pyrite and pyrrhotite.

The indigenous goethite,_which is required for the form-

ation of both boxworks and replicas, thus develops directly or indirectly, according to the Eh/pH condition of the environment. trends are as follows:

The fundamental

B4 (1) Direct oxidation to goethite at moderate Eh and moderate pH. (2) The formation of an interim ferrous salt at moderately high Eh and moderately low pH, 3-5. (3) The formation of an interim ferric salt at high Eh and low pH, 3. The subsequent reactions involving the interim products are very important and, depending upon the Eh/pH conditions: indigenous goethite is progressively formed: ferrous salt is preferentially produced and remains stable: or there is a tendency for excessive production of ferric salt. In the environment where ferrous salt is the major and continuing product, the circumstance is optimum for the formation of false gossans. In the environment where ferric salt is the major and continuing product there is rapid progress to cyclic reactions which increase the rate of dissociation of the sulphides.

Of the excessive amounts

of ferric sulphate continually regenerated by these cyclic reactions, a proportion is continually being hydrolysed in place to build up boxworks and replicas. It should be emphasised, that the trend in the progress of oxidation according to the broad terms suggested, depends largely upon factors such as: the proportions of pyrite and pyrrhotite in the ore assemblage: the presence or absence of reactive gangue minerals and wall rocks: and the degree of air access as determined by lode structure. The usual megascopic configuration of the boxworks from the common sulphides is well-presented in Blanchard's "Interpretation of leached outcrops'1.

These are also the forms which are seen in polished sections,

often at much finer sizes.

The variations in the boxwork forms of some

of the sulphides, and of a number of silicates, carbonates, sulphates and oxides have been worked out.

A selection of these is illustrated.

B5 RED ROCK AND GOSSAN GEOCHEMISTRY, DUCHESS-MALBON RIVER AREA, N.W. QUEENSLAND 1 2 2 D.C. Green , P. Gregory and W. Cooper 1 Department of Geology and Mineralogy, University of Queensland 2 Geophoto Resources, Brisbane Introduction and definition of "Red rock": The term "red-rock" was first used by Edwards and Baker (1954) to describe the rock formed by hydrothermal alteration of paraamphibolites, ortho-amphibolites,.ortho-amphibolites and hornblende schists in the Duchess, Trekelano and Dugald River areas of N-W Queensland.

They showed that the formation of "red-rock" involved

leaching of most of the MgO and much of the CaO and FeO from these rocks and redistribution of the remaining constituents with little change in volume.

The main product of the hydrothermal process is plagioclase

impregnated with finely divided haematite dust, which imparts the brick red colour to the rocks.

Accessory minerals produced are

actinolite, epidote, quartz, calcite, scapolite, magnetite, sphene and sericite with minor amounts of apatite, tourmaline and fluorite. Gossaneous blebs of chrysocolla, chalcopyrite and malachite are associated with concentrations of actinolite. Field characteristics: This study includes observations made in the Duchess, Trekelano, Mt. Hope and Pelican-Malbon River areas. Intense hydrothermal activity is confined to zones of brecciation and shearing within metabasalts, metadolerites and para-amphibolites of the Lower Proterozoic Corella Formation. In these areas the "red-rock" takes several forms.

In general,

it has a pervasive nature but is concentrated along conduits such as joints, fractures and bedding planes.

The process may cause

either bleaching of the country rocks outwards from these passages> or is confined to discrete veinings of red or pink plagioclase. In the larger conduits which haVe suffered more intense action, the leached material is transported and precipitated as dykes 10 cms. to 2 metres wide with inclusions of brecciated country rock and well formed prisms of actinolite. Further intense action produces broad zones consisting

B6 entirely of red rock with 110 relict features of the host rocks. Actinolite occurs as foliated segregations, as pods or discrete prisms.

In brecciated areas, finely crystalline pink plagioclase

encrustations occur on fragments of the country rock.

Voids within

the breccia remain open or are filled with sheaves of columnar epidote. "Red rock'1 associated with brecciated and open fissured areas contains crystals of brick red plagioclase which are pseudomorphs after actinolite, epidote or scapolite. Petrography: Host rocks invaded by the hydrothermal solutions are bleached to a light pink or altered to a brick red colour.

Ferroaugite and

ferrohastingsite of the banded para-amphibolites are reconstituted to epidote, actinolite, and actinolite with remnant hornblende centres. Laths of hastingsite in metadolerites and metabasalts are converted to actinolite. Original plagioclase is partially or completely replaced by fibrous scapolite, or is recrystallised to haematite impregnated, granular, secondary plagioclase.

The plagioclase produced by the

"red rock" process is distinguished by feathery and columnar structures with shadowy extinction, characteristic of hydrothermal precipitation. Twinning is largely observed by the intense haematite colouration. The plagioclase consists of both albite and oligoclase-andesine (An^^ and k n ^ ^ ^ but is not distinguishable as separate grains of a unique composition. Gossans: Small patches of haematite gossan, frequently with a good boxwork after chalcopyrite and minor pyrite are common in the intensely altered "red rock" areas.

They are associated with shears

and fractures on a micro- to meso- scale and also with faults (near Duchess).

Most gossans have dimensions of 3 metres x 1 metre or less,

although a few larger gossan patches attain lengths of over 70 metres with a width of less than 2 metres.

Malachite and chrysocolla are

frequently associated with the gossans as are calcite lenses which occupy near vertical NNE trending .fractures.

Disseminated

chalcopyrite within the calcite is locally replaced by secondary chalcocite, covellite and m<alachite-chrysocolla. found.

Pyrite is rarely

Silicification and jasperization of these zones is common.

B7 At Duchess, N-S trending calcite fracture fillings are partially or completely replaced by iron rich solutions leaving a gossan and/or jasper with secondary associated chalcedony.

Repeated

movement along these fault zones has produced jasper breccias. Replacement of the calcite occurs from the margins inwards.

The

effects of copper-^bearing solutions have extended up to 1 metre into the country rock (hornblende-biotite amphibolite and schists)< Perfect rhombohedral boxworks after calcite are often preserved. Geochemistry of the gossans: The range of base metal contents of 23 gossan samples is given in Table 1. Table 1 range

average

800 ppm -

41.25 ppm

40 PPP "

765 ppm

6.18% 192 ppm

45 ppm - 1500 ppm

211 ppm

30 ppm - 1530 ppm

167 ppm

10 ppm - 1950 ppm

111 ppm

1 ppm -

6 ppm

2.5 pi

Origin of the copper mineralization: The combined action of shearing and of hydrothermal solutions (possibly associated with adjacent granite intrusions) on metabasalts and metadolerites, which have background Cu values in excess of 200 ppm, served to release Cu from pyroxenes and/or from accessory fine grained chalcopyrite.

The copper (together with other base

metals) was taken into solution,.and transported to shear, fracture and fault zones where it was precipitated as chalcopyrite which is preserved within calcite bands.

Gossan boxworks after chalcopyrite,

and containing malachite, occur in areas now often devoid of calcite.

GEOCHEMISTRY OF GOSSANS FROM THE MOUNT ISA - CLONCURRY REGION, NORTHWEST QUEENSLAND G. Derrick, Bureau of Mineral Resources About 50 samples, of gpssan have been analysed by atomic absorption for the following elements: Cd, Mn, Cr, Ag, Mo and As.

Cu, Pb, Zn,.Ni^ Fe,.Co,

The samples come from Proterozoic rocks

between Cloncurry and Mount Isa, and include gossans from

B8 unminer&lized and mineralized zones and mines in the Argylla Formation (acid volcanics), Marraba Volcanics and Soldiers Cap Formation (metabasalts) Marimo Slate, Corella Formation (limestone), Mount Isa Group (shales), and various granitic intrusives.

Most of

these gossans are derived from copper-bearing lodes^although some are from lead-zinc and pyrrhotitic strata-bound deposits. The gossans show various forms of boxwork, abundant massive, earthy limonite and hematite, some quartz, chalcedony and calcite gangue and in some samples traces of both chalcopyrite and secondary copper minerals. Analytical results are not yet available.

However, it. is hoped

that the following points could be evaluated: 1. Comparison of gossans from copper deposits in e.g. black slate with those from deposits in basalt, dolerite or granite could indicate significant differences between gossans from stratabound-type deposits and epigenetic fissure-type lodes respectively. 2. Comparison of gossan and unaltered sulphide equivalent (e.g. Mount Colin, Big Boot Mines) should indicate the relative movement of elements during alteration of the ore vein.' Similarly, gossans from slate could be compared with unaltered slate. 3. In the absence of good boxworks, determination of critical indicator elements in known mineralized gossans could allow rapid evaluation of gossans developed over veins of unknown economic value. 4. Comparison of calcite, quartz and gypsum-bearing gossans could indicate the nature of the contribution of the gangue minerals to ore formation.

AN EARLY TERTIARY (?) FOSSIL GOSSAN AT OLIVER HILL, NW TASMANIA W.E. Baker, Dept. of Mines, Tasmania Oliver Hill is situated about 12 miles south-west of Sheffield in north-western Tasmania.

It lies amongst the western foothills

of a range of mountains composed.of Precambrian and Palaeozoic rocks which swing .around in an arc from the west coast of the State. The predominant outcrops on Oliver Hill are of Ordovician sandstone. Cambrian quartz-porphyry and Tertiary basalt also occur.

Minor

Ironstone

IRON

MANGANESE

LEAD

ZINC

oo cG

Figure 1:

Iron, Manganese, Lead, Zinc,Copper Distribution in Ironstone at Oliver Hill.

COPPER

B9 outcrop and float occurs sporadically over a distance of about a mile along the north-eastern flank of Oliver Hill.

No base

metal deposits are known from the area although the Round Hill mine lies 2 miles to the north.

Near the southern extremity of

the ironstone outcrop a small amount of alluvial gold was won from the Devonian mine workings whilst minor tin was taken from the Tin Spur section at the northern extremity of the exposure.

The

ironstone first attracted the attention of the Department of Mines in 1958 when part of it was mapped and sampled by R.G. Robinson. This work was compiled by I.B. Jennings (1963).

Analysis of 35

samples for gold, silver, tin, tungsten and lead showed these elements to be absent or occur only in traces in a large proportion of them.

A summary of results for positive samples is given in

Table 1. Table 1

Analytical Results from 1958 survey of Oliver Hill Ironstone

Element

No. of -fve Samples

Maximum value

Gold

7 dwts/ton Au

Silver

2 oz/ton Ag

Tin

5.6% Sn02

Tungsten

1.6% W0 3

Lead

8.8% Pb

Despite the above data no further work was undertaken since the interpretation then favoured was that the ironstone was an iron rich sediment. The current Project was commenced in 1969 when 200 samples were taken over the whole area of outcrop and float. for iron, manganese, lead, zinc and copper.

These were analysed

The analytical data are

presented as frequency and cumulative per cent curves in Figure 1. With the exception of iron all elements show strongly positively skewed distributions.

Comment on the bimodal character of the iron

distribution will be deferred until later.

Of the base metals,

only lead shows values of interest with a maximum of 6,84% and a quarter of the samples exceeding 1.5%. ,The median values for lead, zinc: and copper, arg, 0,67$,. 0.11% and. 0.01% respectively and even allowing for possible mobility differences for these metals during development of the ironstone these results suggest th^t

BIO the parent mineralisation was dominated by lead.

Because of the

wide departure from the normal distribution, tests of correlation between the metals are of limited use.

Tests based on logarithmically

transformed data reveal a strong correlation (0.666) between lead and manganese whilst all other metal pairs show far weaker or no correlation. Detailed mineralogical studies of the ironstone have not yet been undertaken.

The degree of correlation between manganese

and lead together with the fact that no secondary lead minerals are visible in samples carrying up to 6% lead suggests the possible presence of a complex oxide such as coronadite. The age of the ironstone is uncertain.

It crops out in the

vicinity of the remnants of basalt flows and is everywhere at a lower level than these.

At one point towards the northern end of

the exposure it can be traced to the lower edge of a thin flow. This suggests that it is of pre-basalt age, at least early Tertiary. Evidence of the recycling of iron is apparent in the bimodal character of the iron distribution curve.

This has been checked in

the field where it was found that small areas of iron cemented Quartern?ary grits occur in association with the ironstone at the northern extremity of the exposures. Reference JENNINGS, I,B. 1963. Middlesex,.

One mile geological map series. K/55-6-45. Explan. Rep . geol. Surv.Tasm.

GEOCHEMICAL OBSERVATION ON SOEffiiMiCu GOSSANS W. Hancock 1 and J. Wilmshurst 2 1 Australian Selection 2 G.S.I.R.O., Sydney The examination and interpretation of leached and/or oxidized outcrop has a recognized place in mineral exploration.

However

detailed information on 'gossans' from the West Australian 'Nickel Belt' is not yet generally available. For the present study we have been examining^ such material by chemical and petrologiaal methods with a view to sharpening the decidedly hazy line between the (low in Ni and Cu) gossan and the non-gossan.

In this context the term 'gossan' covers both the

Bll massive sulphide equivalent and that derived from rock in which sulphide was more highly dispersed.

Thus we are dealing here with

material which ranges from more or less siliceous limonite - goethite hematite rock to oxidized and leached ultramafic. At this time we will discuss the chemical data which has been obtained for outcrop specimens inclusive of the above types.

The

samples have been analysed by means of emission spectrometry, XRF and AAS - the data thus includes information on the conveniently determined elements, including the precious metals Pd, Pt and Au.

PLATINOIDS AS DISCRIMINATORS OF GOSSANS DERIVED FROM NICKELIFEROUS SULPHIDES. R.R. Keays, Department of Geology, University of Melbourne A neutron activation analysis scheme is being used to assess the potential of the platinum metals in the search for nickeliferous sulphides.

Two factors which affect the .potential of these metals as

indicators of nickel sulphide mineralization are: (1) they must be concentrated in primary nickel sulphides relative to their abundance in other crustal material, and (2) they must be fixed in the gossan produced by the oxidation and weathering of rocks carrying such mineralization.

Ir, Pd, Au,

Cu, Co and Cr have been determined in copper-nickel sulphides from eight different localities along with ultramafic host rocks, nickeliferous and non-nickeliferous gossans, laterites and iron-caps, all from Western Australia. Nickeliferous sulphides are significantly enriched in the

platinoids and gold relative to normal crustal rocks as is readily -9 apparent from the following data, in p.p.b. (10 gm/gm): Au

Ultrabasic rocks

6.2

•

Basic rocks

4.5

0.2

Granite

7.1

0.04

1.6

Nickel sulphides, Western Australia

20-600

100-200

500-2500

Gossans derived from nickeliferous sulphides, are similarly enriched iri platinoids, relative to the normal crustal rock, non-nickeliferous gossans, iron-caps and laterites.

Ir behaves in

an inert manner during gossan formation and is concentrated in the

B12 gossan,

Pd is depleted relative to Ir but is still present in

sufficient quantities to characterize the gossan as nickeliferous. Au displays an erratic pattern, evidently being easily mobilised by the processes which affect the formation of gossans.

Preliminary

data indicate that Pd and Ir occur dispersed in gossans.

A METHOD OF GEOCHEMICAL CLASSIFICATION FOR GOSSANS, LATERITES AND SEDIMENTS FROM THE YILGARN AND PILBARA SHIELDS OF WESTERN: AUSTRALIA 2 J.M. Clema and N.P. Stevens-Hoare 1. Exploration Manager, Kia Ora Gold Corporation No Liability, Adelaide 2. Geologist, Consolidated Goldfields Australia Limited, Kalgoorlle ABSTRACT A study of trace elements of ironstones from the Yilgarn and Pilbara Shields of Western Australia has shown that AAS geochemical data may be used to classify ironstones statistically into populations of gossans or laterites and sediments.

These

groupings, are supported by geological observations and drilling information.

The method is quick, cheap, easy to use and

samples need not be collected in any complicated or systematic manner.

Nickel gossans can be identified with certainty,

however, at this stage the distinction between base metal gossans and pyrite-pyrrhotite gossans is not particularly marked.

GEOCHEMISTRY OF THE REPROSS NICKEL PROSPECT WIDGIEMOOLTHA AREA WESTERN AUSTRALIA R. Dalgarno - Project Geologist,Anaconda Aust.Inc. The Redross nickel gossan was located in November 1967 during geological mapping of the southern part of the Widgiemooltha Dome. The locality is two miles south-east of the granite which intrudes the core of the dome and which is dated at 2600 m.y. The succession flanking the granite comprises pillowedmetabasalt and dplerite with unit$ of tremolite-chlorite rack (? after pipr.itic basalt), and metadunite with silt atone and. lojcal conglomerate, black shale and iron formation.

Laminated siltstpnes

with tuffaceous units and interbedded acid and basic volcanics

B13 occur higher in the succession. The dome is a relatively open structure, the core of which is 12 miles long by 4 1/2 miles in width.

The limbs generally dip between

45 and 65 degrees but are locally steeper and certain belts are isoclinally folded and overturned.

Regional metamorphism is green

schist facies with only very restricted contact effects at the granite margin.

Granite and porphyries were intruded at a late

stage and the youngest event in the area was the intrusion of E-W trending basic dykes (Widgiemooltha dyke suite). Nickel mineralization occurs in serpentine and talc altered dunite and peridotite sills which intrude the basalt pile, commonly along shears and seams or cleavages resulting from interleaved cherty and iron sulfide rich sediment bands.

Nickel sulfides occur

both as disseminations and massive remobilized ore Associated with footwall structures.

Embayments and structural irregularities in

the footwall contact are important ore locations. The ore minerals comprise pyrrhotite and pentlandite with accessory chalcopyrite.

A supergene sulfide zone comptising vio-

larite and pyrite occurs below the oxidized zone and grades down with decreasing violarite alteration to primary sulfides at approximately 500-600 feet vertical depth. The Redross gossan outcrops sporadically as ironstone rubble within fine grained metabasalt over a strike length of 400 feet. It varies from massive siliceous ironstone with traces of relict pyrite to earthy and leached cellular limonite with rare malachite flecks.

Results from a series of samples collected along the gossan

outcrop averaged 2200 ppm nickel and 1100 ppm copper. At the discovery site there is a topographic relief of the order of 80 feet with scattered talc-carbonate float at creek level on unexposed ultrabasic rocks, grading upslope to rubbly and then outcropping basalt along ridges east and west of the Redross ultrabasic unit.

The talc-carbonate unit is 250 feet in thickness and

the footwall basalt exceeds 1000 feet.

The eastern basalt is 450

feet thick and is followed above by a second ultrabasic similar to the Redross body.

The sequence strikes N25°E and dips 45 degrees

easterly. The Redross Vein has an average dip of 55 degrees and in plan

B14 and section is slightly oblique to the footwall contact.

The ore-

body plunges at approximately 30 degrees to the south and will be intersected at depth by the Redross Shaft which is collared in the eastern basalt unit. Exploration was concentrated on the gossan and its extensions by means of soil geochemistry and bulldozed costeans.

The first

drill hole intersected the distal part of the shoot which appears as a fractured quartz vein invaded by chalcophyl sulfides of the massive ores in the Redross structure.

Further costeans revealed

the geometry of the ore structure and the second diamond drill hole successfully intersected mineralization in the Redross vein.

Limited

auger testing and shallow percussion drilling were the methods applied to extend information south into the area of overburden. The discovery section was drilled for geochemical data and illustrations are presented here of the distribution of nickel values within the oxidized zone and supergene sulfide zone.

The variation

of Ni Cu and Co values in the leached profile is illustrated by slides. Figure 1

Sketch Plan Redross Nickel Prospect, Widgiemooltha Western Australia

Figure 2

Geological Cross Section Redross, discovery drill section (RD2)

Figure 3

Interpolated Contours for Nickel within the weathered zone of the Redross Ultrabasic (composite section)

(GOSSAN

100

103

100

Talc carbonate rock

REDROSS NICKEL PROSPECT^ WIDGIEMOOLTHA WA SCALE

GOSSAN

101

TrtmohH chtorif /rock —

LEACHED ZONE

200 feet

Talc carbonate rock

b 14^

GEOCHEM. SECTION Limit of oxidation

PARTLY OXIDIZED ORE

Metabasalt

SUPERGENE SULFIDE ORE

Metabasalt

REDROSS NICKEL PROSPECT WIDGIEMOOLTHA W.A. SCALE

Fig I

Fig 2

40 feet

REDROSS NICKEL PROSPECT WID6IEM00LTHA W.A.

SCALE

INTERPOLATED CONTOURS FOR NICKEL WITHIN THE WEATHERED ZONE OF THE REDROSS ULTRABASIC VERTICAL EXAGGERATION x2 AMPHfBOUTE

ULTRABASIC

LEGEND ppm Ni < 200 200 - 400 4 0 0 - 600 6 0 0 - 800 800 - 1000 1000 - 1500

AMPHiBOUTE

1500 - 3500 3500 - 4000 > 4000 UMfT

OF OXIDATION

Fig 3

B15 GOSSAN STUDIES AMD THEIR IMPORTANCE IN EXPLORATION PROGRAMMES K.R. Yates, McPhar Geophysics The importance of gossan studies in any exploration programme in Australia is emphasised.

Prior to the current

spate of active exploration in Australia, gossan studies were largely limited to the work of Locke (1926) and Blanchard (1925-1968) in his numerous papers and excellent hook. The latter work was largely based on Australian experience. Many parts of the world's major mining countries exhibit only a very limited development of leached outcrops, because of glaciation or relatively recent close attention and documentation on the subject is not very extensive. Most work has been confined to the study of the leached outcrops of porphyry copper deposits in south-western U.S.A. and Blanchard*s astute observations on base metal deposits in northwest Queensland. In Australia, where much of the stable land surface has been subjected to prolonged periods of exhaustive weathering, deep leaching of sulphide bodies is prevalent. An understanding of the textures, mineralogy and geochemistry of gossans is thus a prime consideration for exploration in this type of environment. Although textural studies have been -underway for over 40 years, trace element geochemical studies have only come to the forefront in the past decade with the development of instruments such as the atomic absorption spectrophotometer.

The geochemical

approach is likely to yield rewarding results with continued research. The ultimate answer should be in a combination of geochemical studies with megascopic and microscopic observations on textures, mineralogy and colour of iron oxide cappings.

The results of

such research can be applied to mineral exploration of a number of underdeveloped countries with geological and climatic environments similar to Australia.

B16 Examples are given of important Australian ore deposits which have "been proved, as a result of gossan discoveries in the past decade; thus illustrating that outcropping orebodies are still to be found by well-directed scientific gossan search. This is followed by a brief discussion of gossan search methods and interpretation of ironstone analyses, together with suggestions for further research. THE MINERALOGY AND GEOCHEMISTRY OE SCME WESTERN AUSTRALIAN GOSSANS AMD PSEUDOGOSSANS I.E. Pontifex, McPhar Geophysics During the past 3 to 5 years the examination and classification of gossans and related leached outcrops has been widely accepted as forming a significant part of mineral exploration. This work is a revival of the importance attached to the recognition of such materials by exploration geologists in about 19J0, largely due to the successful application of up-dated techniques, notably in the fields of chemical and mineralogical analysis, which allow a more precise classification of leached outcrop. The correct assessment of the importance of a gossan lies in the three following areas 1.

Geological mode of occurrence

Geochemical analysis

3. Mineralogical analysis The author!s work has involved examination of gossans in thin and polished section, interpretation of their genesis, and correlation of the mineralogy with geochemical analyses. This paper presents examples of the mineralogical assessment of gossans.

It does not offer profound, all-embracing conclusions

on gossan formation etc., but rather examples, in the form of projected photomicrographs, of textures, structures and related mineralogical features which can be determined microscopically and used as a practical, and what commonly proves to be an essential, part of gossan classification.

B17 It will be demonstrated that the examination of gossans in thin and/or polished sections, and the co-ordination of this work with chemistry, can be used as a routine method to identifytrue and false gossans of nickel and other base metal sulphides, ferruginised rock, laterite, and various other leached outcrop. The mineralogical features will be presented together with the corresponding chemistry of some different types.

The field

occurrence of some samples will also be discussed. SOME OBSERVATIONS OH THE ANALYSIS OF GOSSANOUS AND WEATHERED MATERIAL FROM THE WESTERN AUSTRALIAN SHIELD J.R. Beevers, McPhar Geophysics A number of gossans have been subjected to AAS analysis following a variety of acid leaches.

These values show

considerable variation. The results are compared to total analyses by XSF and an * attempt made to explain the anomalies.

C1 A GENERAL VIEW OF ORE TYPES IN THE NEAR SHORE ENVIRONMENT R.L. Stanton, Department of Geology, University of New England, Armidale.

SEDIMENTATION IN THE NEAR SHORE ENVIRONMENT Speaker to be announced.

STRATIGRAPHIC M P PALAEOGEOGRAPHIC SETTING OF THE HYC ZINC-LEAD DEPOSIT, McARTHUR RIVER, N.T. M,C. Brown, Canberra Collejp of Advanced Education A brief review of the hydrology and sedimentology of restricted marine basins will be followed by an account of the stratigraphic aiid jalaecgeographic setting of the Proterozoic HYC zinc-lead sulphide deposit. The sulphides were deposited at the bottom of a local depression on the floor of an otherwise extensive shallow sea. This downwarp developed following a long history of deposition of intertidal and shallow marine carbonates over the region. Conditions appear to have been ideal for deposition of sulphides in the downwarp for the following reasons: (a) strongly reducing conditions would be expected in a local closed depression on the sea floor; (b) except for some nearby low islands, which shed some terrigenous sediment into the depression, there were no sources of terrigenous sediment to dilute any chemical sediments in the depression. Any other land areas would have been a long way from'thfe-depression and would have been composed largely of carbonate rocks. (c) Sabkha-type evaporites are present in the sequence about 20001 below the HYC sulphides.

The structure of the area

after downwarping in the HYC area would have allowed brines derived from the evaporites to form a local pool on the sea floor.

C2 THE EUXINIC ENVIRONMENT AND CARBONACEOUS SEDIMENTS GoH. Taylor, CSIRO, Division of Mineralogy The objective of the symposium is to explore the common ground between sedimentology and ore genesis.

My task is 'to present the

results of current CSIRO work on carbonaceous sediments and ore deposits'.

I have therefore used this opportunity to sketch out a

framework and to indicate by means of (*) where recent or current CSIRO work is relevant.

These areas of work will be discussed during

the presentation. 1°

The environment of deposition The conditions under which shale-like rocks accumulate can be

partially described in terms of pH and Eh.

These in turn depend at

any one time on the type and amount of material brought in to the area, the depth of water, the temperature, the availability of oxygen, and other factors.

These properties are, of course, inter-related and

will in general vary in all three dimensions of the body of water through which sedimentation is occurring.

Thus it is possible to have

a body of water in contact with atmospheric oxygen at its upper surface which contains virtually no oxygen at its interface with the newly deposited sediments.

Such a spatial variation in properties

may persist for only-quite short periods (e.g. where tidal or seasonal effects are important) or be maintained for very long periods (as in the deposition of such rocks as the Chattanooga Shale)*. Into this environment is discharged the sedimentary material a process implying a degree of turbulence and (during post-Archaean times at least) the introduction of oxygen.

Where the sedimentary

matter includes more than enough reductants to react with the oxygen and turbulence is not too great, it will still be possible to maintain highly reducing conditions near the sea floor. The sedimentary matter can thus be thought of as comprising (a) inorganic matter, which in the case of shales is extremely fine in size, down to colloidal dimensions and especially rich in layer silicates;

and (b) organic matter.

not at, distinct as may be supposed.

However these two categories are Thus virtually every flake of clay

entering a basin of sedimentation already has a thin surface film of organic matter*•

This film is very thin, but nevertheless has a marked

effect on properties such as adsorption.

Such organic matter as well

C3 as that in free grains may bring with it metals adsorbed or chemically incorporated*• Some of the high-carbon material entering the basin may be graphite or other carbonaceous matter weathered from earlier-formed rocks, but in the general case, the great majority will have been derived from organisms of various kinds®

Most Precambrian matter of biological

origin is suspected to have been algal and possibly bacterial in nature; Phanerozoic carbonaceous matter included the remains of the whole range of organisms.

Despite chemical changes which must have occurred, there

may have been little physical change in the organic remains if these quickly arrived in highly reducing conditions*» But the great majority of carbonaceous matter in rocks (and especially in shales) does not retain morphological features by which it may be related to the preexisting organisms, which could have been brought in by streams, have existed as surface 'blooms0, or have grown and died within the water column itself*.

This lack of morphology could be understood if the

organisms had been extremely small and very numerous, like bacteria, or if the organisms were so easily degraded that no morphology survived diagenesiso

In fact most carbonaceous matter appears to have been

chemically degraded and reprecipitated as very fine-grained, amorphous particles at the time of deposition« Much of the carbonaceous matter in shales, then, represents not' so much the actual organisms as derivatives from them.

The release of

fine particulate material probably involves a number of processes of which oxidation may be one*. Partial oxidation in turn suggests a degree of selectivity in that some chemical bonds will have been oxidized preferentially.

In the process, the particulate residue, despite

considerable variability in the original organisms, seems to have been of fairly constant elemental composition at least since the Proterozoic*. Additional to such 'humic* material are the waxy remains such as the exines of pollen grains9 which are remarkably resistant to oxidation under some conditions*o Mention should be made of the occurrence of porphyrins - large stable organic molecules - in some carbonaceous shales.

These have a

high affinity for metals such as vanadium, and concentrations such as those at Julia Creek in Queensland may be built up over long periods in this way*.

C4 The proportion of these varieties of carbonaceous matter differs, and the total amount of such material in different rocks varies from mere traces, to 10 per cent and more by weight.

The reasons for this

variation clearly lie in the proportion of organic to inorganic matter introduced and in the degree of imbalance between available oxygen and organic matter at the time of deposition.

It seems likely that even

for a highly carbonaceous shale, the great majority of organic matter was destroyed before it could be incorporated in the sediment.

The

achievement and maintenance of conditions sufficiently reducing for organic matter to be preserved requires that a considerable amount be consumed.

Not a great deal seems to be known about the process, but

it is likely that carboxyl groups and eventually carbon dioxide account for most of the oxygen*. 2«

The process of sedimentation What happens as this mixture of organic and inorganic matter

reach the sea floor?

As we have seen, there may well have been little

dissolved oxygen in the bottom waters, although at the very slow rates of deposition of some black shales, even low levels of oxygen would have been important.

The absolute level of oxygen and the Eh would to

some extent be related to the configuration of the sea floor. The most^highly reducing conditions occur ih depressions or 'pools1.

particularly interesting situation arises where such pools (as in the Red Sea) are occupied by brines dense enough for the organic matter to float at the brine/sea-water interface*.

Such a layer may act as a

well-defined barrier to oxidation although probably the density stratification alone would make it difficult for oxygen to penetrate far into the brine layer. Both mineral particles and organic matter are likely to contain much water as they reach the sea floor. For the organic matter, this means that the particles contain about 90 per cent water.

A small part

* of this water is very loosely held and if permeability allows, will escape under a load of a few feet or tens of feet of sediment.

But some

of the water is held much more strongly, so that 20 per cent or more may still be present in carbonaceous matter under some thousands of feet of younger sediments*. This f rain1 of fine particulate organic matter during sedimentation will thus form coatings over all or much of the inorganic sedimenting

C5 material - coatings which, by later loss of water and other diagenetic changes, will be extremely thin, although remarkably continuous*® However, the euxinic environment may also permit the activity of anaerobic bacteria.

These consume more organic matter than they

contribute and in the process may affect its chemical constitution. The most obvious effects of anaerobic bacteria result from their ability to reduce sulphate to sulphide under such conditions.

One consequence

of this reduction is that much sulphur can be incorporated in the carbonaceous matter itself, to remain fixed until later stages of diagenesis and metamorphism, when this same sulphur may be made available, possibly to form sulphides*.

Recent work has shown that such addition

of sulphur to carbonaceous matter may take place many feet or even tens of feet below the sediment-water interface, presumably through the medium of the formation water*.

The presence of H^S, of course, also results

in the precipitation of metal sulphides, most of which are extremely insoluble.

Pyrite is the ubiquitous sulphide formed, frequently appear*

m g as framboids if certain conditions are fulfilled*.

Other sulphide-

forming metals will similarly precipitate (although not usually as framboids) when the solubility product is exceeded. Other enrichments can occur during deposition.

Thus uranium

and some other trace elements tend to occur in greater concentrations in carbonaceous shales than in nearly all other rocks and this almost certainly represents extraction from normal sea-water.

Consideration

of the enrichment factors, however, shows that base metal deposits of ore grade cannot have formed in this way from normal sea-water. 3*

Processes occurring after sediment deposition As the newly deposited sediments become progressively more deeply

buried, water is released as the sediment compacts.

However, the

carbonaceous films, in general, remain intact and tend to serve as barriers isolating inorganic grains from one another, thereby inhibiting grain growth and recrystallization and probably also reducing the permeability of the rock, especially across the bedding*. The deposition of sulphides continues during this early diagenetic stage as evidenced by overgrowths on framboids*.

While the likeli-

hood appears to be that these metals entered the sediments from above (as is thought to have occurred in the Red Sea deeps) the possibility of upward movement of metals through the sediment pile has also been considered. this way*.

It has been shown that sulphide banding can be formed in

c6 Carbonaceous shales continue to have a role in ore gensis long after burial prevents exchange, between them and the overlying water® As indicated above, carbonaceous matter soon after deposition may contain small amounts of combined or adsorbed metals and up to several per cent of organic sulphur®

In the course of diagenesis, the carbonaceous

matter undergoes progressive changes in composition and there comes a stage at which most of the metals and sulphur will be lost from the carbonaceous matter to become available as new phases or for transport and deposition elBewhere* <> This release of elements occurs at a)Dout the stage when the formation water is likely to be highly saline and capable of extracting metals righte

®e® from inorganics as well) in its own

Such a means of generating an ore-forming solution seems entirely

consistent with the known behaviour of the materials under conditions of aiagenesis*® Even later in the geochemical cycle the importance of carbonaceous matter in ore genesis remains, for example in the localization of uranium mineralization*, and in other ways which may be beyond the scope of the present symposiurru

SEDIMENTATION AND PI AGENESIS OF THE UPPER FORMATIONS OF THE MOUNT ISA GROUP HoBo van den Heuvel, Mineral.Resources Appraisal Pty®Ltd® A general study has been made into the environmental conditions that existed prior to, during and after the deposition of the Precambrian syngenetic silver-lead-zinc and copper mineralization of Mount Isa® The mineralization is contained within an at least 5000 feet thick sequence of well laminated siltstones, the latter characterized by the presence of a persistent primary dolomite and locally very high concentration of authigenic albite or microcline®

The close similarity with North

American examples suggests that the Upper Mount Isa sediments were laid down in an intermontane basin, in which at times volcanic tuff beds were deposited®

A higher than normal salinity was able to build up and persist

in the basin, providing an environment for the deposition of primary dolomite, and the conversion of tuffaceous material and other alurninosilicates into authigenic feldspar®

The high salinity also greatly

affected the subsequent diagenetic changes in the sediment, producing abundant authigenic crystal growth®

C7 The salinity of the basin water influenced the deposition of the micaceous clay fraction. due to flocculation.

This settled out nearer the basin margins

The deeper water zone, containing the copper

mineralization within the ore-bearing Urquhart Shale Member, received' little clastic sediments, and is principally composed of chemically deposited dolomite and chert.

The persistent vertical, opposed to the

lateral facies similarities, would indicate a depositional basin fixed within rather rigid limits.

The. fixed depth control in the basin gave

a stable location to the density stratification in the saline water, which controlled the physico-chemical conditions that effected sediment and chemical deposition. The bottom water, isolated for considerable periods from the oxygenated surface water, was able to dissolve Fe and other base metals from the marginal basin areas and precipitate them in the deepest part of the basin.

The energy required for these chemical reactions was

provided by the decomposition of abundant organic matter, and some of the latter decomposition products were utilized for renewed diagenetic and primary precipitation of dolomite. The dolomite in the sediment is characterized by a moderate to high iron and manganese substitution, the two elements showing a direct linear relation in this substitution.

The Fe/Mn replacement in the

dolomite varies within the. basin sediment.

In the Urquhart Shale Member

it is highest near the interphase of the Ag-Pb-Zn and Cu mineralization. The Urquhart Shale sediments with a high Fb, Zn, Cu or total iron content have a wide compositional range, but a common low Ti/Zr ratio, suggesting their common linkage. Quite independent of the mineralogical composition of the Upper Mount Isa sediments is the high La and Y content.

The high CI content

of the sediments supports the concept of a saline sedimentary and diagenetic environment.

VOLCANIC TERRIGENOUS SEDIMENTATION K.A.W. Crook, Department of Geology, Australian National University.

METAL ENRICHMENTS IN VOLCANO-EXHALATIVESEDIMENTARY ENVIRONMENTS, NEW BRITAIN, PNG. I.B. Lambert, CSIRO, Division of Mineralogy

c8 Certain stratiform base metal deposits occur within sedimentary rocks that contain significant amounts of fragrnehtal volcanic materials. This has led a number of geologists to postulate that these could be syngenetic ores which formed in volcano-exhalative-sedimentary environments, with volcanic exhalations providing the necessary metals, and perhaps the sulphur.

In an attempt to test this suggestion, the Baas

Becking Laboratory has investigated the metal abundances of thermal springs, fumaroles, harbour sediments and volcanic rocks from the present day volcano-exhalative-sedimentary areas at Matupi Harbour and Talasea, both on the island of New Britain, PNG. Matupi Harbour near Rabaul, is a sheltered basin with acid thermal waters entering at its eastern and northern shores.

These appear to

be sea water which has circulated deeply in the volcanic pile, and been rendered acid by addition of magmatic gases and near surface oxidation of H^S,

They have leached considerable amounts of metals from the rocks

through which they pass;

they carry up to 100 ppm iron, 100 ppm

manganese, 2.5 ppm zinc, and slightly less than 0.1 ppm copper and lead. In comparison with the hot brines under the Red Sea, the Matupi Springs have generally higher iron and manganese contents, comparable zinc contents, and roughly one tenth of the copper and lead contents. The sediments in Matupi Harbour are being, significantly enriched in iron manganese and zinc by these thermal waters, but not to near the extent of the Atlantis II sediments.

Reddish-brown coloured silt and

sand layers in the harbour contain up to about 17% iron, 1.2% manganese and

zinc but do not have high contents of copper or lead.

Their

red-brown colouration is due to amorphous iron oxide - hydroxides which precipitate from the springs as soon as they mix with the harbour waters.

The lower metal concentrations in the Matupi sediments may be

largely a function of the very high rate of influx of detrital materials there - an average of about 1.2 cm per year over the last 35 years. There is very little iron sulphide forming in the Matupi area. Test tube studies indicated that sulphate-reducing bacteria are not noticeably active in the Matupi Harbour sediments unless organic matter is added.

The scarcity of organic matter on the floor of the harbour

was quite unexpectedo

C9 The Talasea geothermal area is some 270 km southwest of Matupi Harbour*

Thermal activity occurs along parts .of the shoreline

around Talasea Harbour and at several inland localities. The inland thermal waters are strongly acid and have low salinities. They appear to represent shallow-circulating, fresh waters that have been acidified by volcanic gases and near surface oxidation of H^S. 'They often have high Fe and Mn contents, and a few have moderate Cu and Zn contents.

The waters from the abundant small intertidal thermal

springs have salinities up to # that of seawater, but relatively low metal contents.

They appear to be a mixture

of the inland acid-

sulphate waters with interstitial waters from the coastal sediments. There are only localized minor enrichments of metals in the Talasea Harbour sediments.

This reflects the lower metal contents of

the spring waters and their relatively low rates of discharge into the harbour. However, iron sulphide minerals (pyrite, marcasite and nhydrotroilite,!) are quite abundant in the immediate vicinities of a number of the thermal fields.

Bacteriological studies have established the presence

of sulphate-reducing bacteria in the thermal fields, but sulphur isotopic and other data suggest that most of the sulphide ions which are precipitating the Fe are of volcanic origih. It appears fea.sible that stratiform base metal deposits could form, under certain conditions, in volcano-exhalative-sedimentary environments.

These conditions include a low influx of detritus, cir-

cumstances suitable for separation of the metals before, during or after their precipitation from the exhalations, ahd an inorganic or bacterial sulphide ion source (for sulphide deposits). For ore grade deposits to be generated in this fashion it is probable that there would have to be rather greater than normal amounts of metals in the rodks being leached by the circulating fluids and/or the fluids would have to leach a large volume of rocks over a long period of time.

ORE DEPOSITS ASSOCIATED WITH GEOSYNCLINAL VULCANISM IN TASMANIA AMD A GENETIC MODEL FOR THE MT LYELL PYRITECHALCOPYRITE ORES. M. Solomon, Department of Geology, University of Tasmania.

C10 The three major ore deposits of volcanic association in Tasmania are representatives of three important ore-types - the iron oxide ores (Savage River, Czechoslovakia, Germany),

the-

pyrite-sphalerite-galena-chalcopyrite ores (Rosebery, Buchans, Brunswick) and the pyrite-chalcopyrite cores (Mt Lyell, Rio Tinto, Cyprus)* The Savage River magnetite lenses are interbedded with basaltic volcanics in unstable shelf sediments, and the orebodies lie on an important fault zone that acted as a hinge line during sedimentation. The Rosebery and Mt Lyell deposits occur in calc-alkaline, acidintermediate volcanics of the Mt Read Volcanic Arc, which in the Middle Cambrian consisted of extensive volcanic islands flanked by lagoons and sedimentary basins. The bulk of the Mt Lyell suphides can be shown to be of replacement origin and are believed to have precipitated from heated seawater rising up conduits developed at fault intersections. Solutions reaching the surface in local basins precipitated pyrite-sphalerite-galena deposits, or massive sulphur bodies that later converted to pyrite. High metal:

S minerals were precipitated near

the surface in sinter-like chert pipes. Hydrothermal activity and ore deposition followed a pause in volcanic activity and was succeeded by deposition of several thousand feet of sandy limestones, shales and volcanic rocks.

ASPECTS OF CARBONATE SEDIMENTATION IN THE CAMBRIAN OF SOUTH AUSTRALIA B. Daily, Department of Geology and Mineralogy, University of Adelaide An early Cambriah shallow epeiric sea progressively inundated the erosional surface cut across Older Precambrvian crystalline rocks (Yorke Peninsula) and across flat to gently folded and faulted Late Precambrian sediments of the Adelaide 1Geosyncline1.

Low-lying islands, involving

sands of the Pound Quartzite and sediments of older formations as well as the cores of diapiric structures, broke the surface of this sea during the depositi on of the initial transgressive sands and associated finegrained clastics.

Interbeds of thin lenticular carbonates, where present,

consist of either sandy dolomitised ooid grainstones, often glauconitic,

C11 or fine grained dolomites, many of which, because of frequent desiccation and reworking of these tidal flat deposits, gave rise to intraformational dolostone breccias and conglomerates.

Often the sandstones are bio-

turbated and the trace fossils Piplocraterion Torell and Plagiogmus Roedel are widespread.

Shelly faunas are rare and confined to thin

intervals of glauconitic or calcareous quartz-rich sandstones. Depths of water and rates of subsidence played major roles in determining the facies and thicknesses of the subsequently deposited Lower Cambrian limestones.

In general, thin pale coloured carbonate formations

(normally less than 600 metres thick) like the Kulpara, Wangkonda and Wilkawillina Limestones were deposited on the relatively stable shelves, whereas in the more negative basinal areas, or across hinge lines, much thicker darker coloured carbonates (up to 3*000 metres thick) were laid down.

Examples are the Parara and Sellick Hill Limestones.

Lateral

and vertical facies changes between these two contrasting carbonate types are often rapid and are readily demonstrable.

Water depths for

both types varied from sealevel to probably much less than 50 metres for the pale coloured carbonates and from sealevel to possibly as much as 100 metres for dark limestones in the basinal areas where subsidence was most rapid. The stable shelf carbonates can be subdivided into several distinctive types, most of which can be readily interpreted in the light of present day models of carbonate deposition, for example Shark B a y , Western Australia.

The fallowing examples represent deposition of

carbonate sediments under progressively less restrictive conditions and in general reflect a gradual deepening within the basin of deposition. 1.

Limestones and dolomites with laninoid - fenestral or 'bird's-

eye 1 fabrics and indicative of the supra-tidal environment.

Un-

fossiliferous 1 bird's-eye 1 limestones with the vughs filled with spar are common-in the Kulpara, Wangkonda and Andamooka Limestones. However, less general restrictive conditions probably pertained in the upper parts of the Hawker Group in the Mt Frome area where scantily fossiliferous 'bird's-eye' limestones intertongue over shore distance with red-beds on the one hand and ooid grainstones and archaeocyathid limestones on the other. 2.

Flat algal-laminates or planar stromatolites are phytogenically

induced fabrics characteristic of sheltered intertidal zones in

C12 modern hypersaline environments such as Shark Bay, Western Australia. Similar structured limestones, frequently brecciated due to desiccation and commonly dolomitised, are very widespread in the upper levels of the Kulpara Limestone.

They are generally devoid of associated faunas and

are regarded as ancient examples of planar stromatolites formed on tidal flats under restricted conditions.

Similar planar stromatolites

without accompanying faunas and generally dolomitic and occurring repetitively with interbeds of 'bird's-eye

limestones, ooid grain-

stones and intraclastic limestones are common in the Andamooka Limestone. Traced laterally many of these flat bedded crypt-algal carbonates develop into domed stromatolites with up to kO cms of relief.

Other examples

occur in the Wangkonda and Ajax Limestones and the Hawker Group. 3. Digitate, columnar and domed stromatolites as well as oncolites. and thrombolites, all of which are products of algal activites, are closely associated with fine crystalline dolomites, planar stromatolites and ooid grainstones.

They are indicative of hypersaline intertidal

to very shallow infratidal zones as are their modern analogues. These structures are widespread, occur as thin beds and mounds and are associated with transgressive and regressive phases of carbonate formations.

Rarely do these limestones contain a fauna because of

the restrictive nature of the environment.

However, columnar stromat-

olites binding ArchaeocyathS are known from near the top of the Hawker Group. 4.

Ooid grainstones, commonly quartz-rich and cross-bedded, together

with associated intraclastic carbonates constitute a significant proportion of the stable shelf carbonates.

The general absence of

faunal elements and the facies of the accompanying carbonates, including fine grained dolomites (primary or early diagenetic), together attest to the warm highly saline shallow seas in which the ooliths' normally formed.

Accompanying dolomitisation and silicification features are

almost ubiquitous.

Under less restricted conditions ooid grainstones

intertongued seawards with fossiliferous limestones carrying rich faunas. 5.

Limestones with shelly faunas probably constitute well "under 50%

of the Lower Cambrian Shelf carbonates.

Generally, they are.non-

argillaceous and pale to pink coloured.

They include archaeocyathid,

trilobite, brachiopod and hyolithid rich limestones, fine to coarse grained, recrystallised and patchily dolomiti§ed. .. Archaeocyathid biostromal limestones predominate but small mounds or bioherms occur

C13 and inter-tongue with other carbonate beds. Associated calcareous algae are abundant and indicate deposition well within the photic zone and much less than 50 metres deep, and in warm clear waters of normal salinities. In the shelf carbonate sequences evidence of unconformities is widespread.

They are recognised by red weathering zones often associated

with calcreted surfaces, calcrete pisolites and solution cavities with associated travertine.

Sedimentary dykes wedging down from the

weathered zones are common. The darker foetid carbonates such as the Parara Limestone vary in colour from medium grey to dark grey to almost black. ?erroancalcite, ferroan-dolomite and fine disseminated pyrite and carbon together with clays are common constituents and act as pigments.

Very

strong reducing conditions below the sediment-water interface are implied.

Prolonged sub-aerial exposure, which would have oxidised and

lightened the colour of the sediment, must be precluded.

Relatively

rapid burial of sediment was probably the case. Several distinctive types of dark coloured limestones can be recognised and as for the pale coloured carbonates are discussed in terms of increasing water depth. A.

Dark grey planar stromatolitic limestones are relatively uncommon and no associated 'bird's-eye1 limestones are known. '

Dark grey poorly argillaceous limestones including stromatolitic, oncolitic, thrombolitic and oolitic types are common associates in the oldest Parara type limestones of the Hawker Group.

Laterally

they may intertongue with pale coloured carbonates of similar types or basinwards with dark grey mottled limestones. C.

Dark grey poorly argillaceous mottled limestones of diverse types and uncertain genesis are the most distinctive and puzzling of any of the Cambrian carbonates. Mottled types described under this category (C) are fine grained and range from banded to pseudobrecciated types.

In outcrop, a typical mottled rock consists of

dark coloured fine grained calcite or ferroan calcite bands which are interbedded or intricately interwoven with paler (generally yellowish when weathered) and more argillaceous or silty coarser grained dolomitic rock (ferroan-dolomite).

The mottles may be

C14

primary or very early diagenetic as a mottled structure was present prior to lithification as seen from intraformational slumps and draping effects around bioherms. origin.

Possibly the mottles are of algal

These limestones contain relatively sparse faunas (phosphatic

brachiopods, Archaeocyatha, sponges and hyolithids) and intertongue with small dark grey archaeocyathid bioherms which are not mottled. Some oolitic interbeds are frequently present in the mottled limestones. Basinwards mottled limestones may intertongue with flaggy argillaceous limestones. D . Fine grained, medium to dark grey bedded, mottled and nodular argillaceous limestones are characteristic of the type Parara Limestone. Where good circulation prevailed the limestones are richly fossiliferous and contain abundant trilobites and other shelly fossils. Archaeocyatha are rare except in areas flanking biostromal banks. The argillaceous content is moderate to high and in the thicker sections dark to black shales are interbedded with the flaggy limestones. In regions of rapid subsidence and where restrictions to water circulation existed bottom conditions were stagnant and faunas were generally sparse.

The darkest limestones were deposited in these

situations.

DIAGENESIS IN CARBONATES P.J. Davies, Department of Geology, University of Queensland. Carboniferous, Devonian and Jurassic reefal and subreefal carbonates exhibit regional distribution patterns of diagenesis.

Similar

carbonate facies pass through predictable cycles of diagenesis which are linked to their depositional history.

A "normal*1 path of diagenesis for

such rocks may therefore be delineated.

A regional effect may be superim-

posed on this "normal" path as a primary or secondary dolomitization, or as an early or late silicification.

However all the limestones studied

showed that certain diagenetic processes occur in a predictable sequence. This does not mean however that the boundaries between particular diagenetic events are time zones.

Only the onset of a first phase of cementation may

be considered a time zone.

This is often early i.e. either syndepositional

or immediately post depositional.

C15 OCCURRENCE OF Pb-Zn DEPOSITS IN CAMBRIAN CARBONATES OF SOUTH AUSTRALIA R.K. Johns, Geological Survey of South Australia Significant concentrations of lead and zinc with minor copper are widely distributed within lower Cambrian sediments of the Adelaide Geosyncline, the adjoining Torrens Sunkland and the Stuart Shelf.

The

basal Parachilna Formation is locally host to copper mineralization, while stratiform lead deposits (with associated zinc, and copper) occur in the overlying carbonates that comprise the Wilkawillina Limestone. Galena was mined (1870-1900) from shallow pits and underground galleries along the western margin of the Ediacara Basin and recent drilling has delineated reserves of 30 million tons averaging 1% Pb in laminated algal dolomites and sandy dolomites.

Small production was

achieved from similar developments on the eastern flanks of the Flinders Ranges, near Wirrealpa.

Weakly disseminated lead and zinc sulphides

are present elsewhere but production has been unimportant. The setting of the various deposits which have been investigated are described.

They may be classified as Mississippi Valley Type.

Oxidized lead and zinc minerals, derived from sulphides in this environment and transported by groundwater, have been concentrated to form mineable ore bodies by partial replacement and cavity filling in carbonate hosts.

At Puttapa, reserves of 1 million tons of principally

willemite containing 35$ Zn and a lead-rich zone (10% Pb) have been outlined.

Structural control of these deposits is apparent.

ORE GENESIS IN THE DELTA ENVIRONMENT C.E.B. Conybeare, Department of Geology, Australian National University Deltas, viewed in three dimensions,' are complex sedimentation provinces including sheets of pro-delta or shelf, muds and silts;

shore-

line sand bodies such as bars, barrier islands, and associated beaches; river distributaries forming anastomosing, linear bodies termed 'shoestring sands1;

intercalating layers of organic-rich muds, silts"and

carbonized peat formed in bays, lagoons, and" fringing marsh;

carbon-

aceous backswamp muds grading laterally into river flood plain silts and fine sands;

river point bar deposits of sand, grit, and gravel;

C16 and soil.

Deltas range in width from a few to several hundred miles;

they form in all climates, from the Artie to the Tropics, and in both salt and fresh-water environments.

Sediments in sections through the

lower reaches of larger deltas, being farther from their provenance are, in general, finer grained than those cored in the

upper reaches which

merge with the lower extremities of alluvial valleys where point bar deposits of sand form a larger percentage of the sediments. The composition and grade of these sediments will vary, depending on the lithology and distance of their provenance, rate of erosion of the provenance which influences the supply of sediment and consequently the rate of sedimentation, and on the climate. throughout the history of the delta.

These factors can vary

During a period of time some

part of the delta may, for seasons of sedimentation environment, nature of sediment supply, or hydrodynamic conditions, be favourable for a particular type of ore genesis such as the formation of deposits of mineral sands, including cassiterite, and of diamonds, gold, and uraninite. Triassic and Jurassic formation of the Colorado Plateau, deposited under fluviatile and lacustrine conditions in a delta environment, contain uranium and vanadium deposits in river channel sediments, the mineralization being commonly associated with carbonized vegetation in point bar deposits.

The genesis of the ore may have involved several

cycles of solution and deposition.

Originally, the uranium may have »

existed as uraninite in primary deposits or disseminated in rocks. Subsequently, the uraninite was oxidized within the zone of weathering to form complex associations of copper, lead barium, and other cations with hydrous uranium-vanadium oxides, carbonates, and phosphates such as carnotite, autunite, dewindtite, torbernite, and others.

These

oxidation products are slightly soluble in groundwater and consequently can be carried in solution through sandstone where hydrodynamic conditions exist.

Moving formation water will seek the most permeable

zones which are, in the Colorado Plateau section, the coarser sandstones and gritstones filling ancient river channels.

The association of

uranium-vanadium minerals with carbon and hydrocarbons is well known in other parts of the world, the carbon-hydrocarbon material apparently acting as a catalyst to the precipitation of uranium-vanadium compounds in solution.

C17 The upper part of the Witwatersrand

System of South Africa

comprises several thousand feet of quartzose conglomerates and argillaceous quartzites.

Gold values occur in particular lenses of

conglomerate, commonly concentrated near the base*

Although some .

solution and redistribution of gold has occurred, it is generally thought that the first concentration was of placer gold.

The gold-

bearing conglomerates are crossbedded and of fluviatile origin, forming lenses that thicken and thin from several feet to a few inches.

The

thicker portions fill linear scours in the underlying quartzite. Of particular interest is a thin layer, up to an inch thick but commonly much less, of uranium-bearing thucholite (a carbon-hydrocarbon association of uncertain composition and origin) which usually occurs at the base of particular conglomerate lenseso

Uranium values, present

as disseminated uraninite, are always associated with the thucholite. Higher gold values are also found within the few inches of conglomerate adjacent to the thucholite layer, but the association of gold with o thucholite is probably not genetic. The uranium-thucholite association, on the other hand, may be genetic, although there are proponents for the idea that the uraninite was primarily of placer origin.

Until the

origin of the thucholite can satisfactorily be explained, its paragenetic relationships with uraninite remain unsolved,

i f , as some geologists

believe, the thucholite is of organic origin, possibly representing an algal mat covering sand that was subsequently buried and locally scoured by a sheet of gravel, then the relationship may be similar to that of the carbon and uranium-vanadium minerals in the Colorado Plateau. Although many of the extensive geological investigations of the Witwatersrand System have been directed to mining problems, sedimentological studies undertaken s6 far have indicated a northwest provenance for the rudites' and,£reiiites, and have suggested an origin in which detritus from alluvial fans was swept into a large body of water to form a delta.

Gold values were concentrated in the coarse fluviatile

facies, and to a Very minor extent in sands which included those of both river and beach origin.

Muds were carried farther out to sea and

are not present in the area mined. With fluctuations in sea or lake level the quartzite layers may represent conditions of deeper water on which algal ma.ts may have grown.

C18 Periodic shallowing of the sea, or local progradation of river distributaries, could have locally scoured and buried the sand layer and its algal mat.

Subsequently, if ground water containing uranium in solution

migrated into buried, permeable beds of gravel, there may have been precipitation of uranium oxides where the waterfcamein contact with carbonized remains of the algal mat* Exploration for economic minerals and metals in the delta environment must consider the sedimentological and petrophysical factors that control or influence the genesis of ore as syngenetic or epigenetic deposits®

In this context, mineral exploration in the delta environment

comes well within the domain of stratigraphy and sedimentology®

PRQTEROZOIC URANIFEROUS CONGLOMERATES, PILBARA, W.A. SoMo Richards, Cominco Exploration, Adelaide Uraniferous conglomerates occur in Lower Proterozoic rocks in the south eastern sector of the Pilbara Block, WoA*

They occur in a

narrow, elongate basin, trending NNE, informally named the Nullagine basin, and adjoining the major Hamersley Basin.

The trend of the

basin is paralleled by Archaean basement linears. The stratigraphic section represents the lower part of the Lower Proterozoic Fortescue Group*

It is a sequence transgressive onto the

Archaean basement and is dominated by sandstones and basalts, with minor shales and siltstones.

The maximum thickness of the section to

the base of the Kylena Basalt Formation is approximately 1000 feet. The sandstones, frequently conglomeratic quartzites and feldspathic quartzites, may be massive or crossbedded trough crossbedding in

with large scale, unimodal,

to 6tf sets commonplace.

Scouring is common,

and cosets (averaging 2 or 3 r in thickness) tend to be broadly lenticular.

Interbedded, thin, finer grained sandstones lend a cyclicity

to the sequence, which together with the sedimentary structures, stamps the rocks as alluvial. variations.

Associated conglomerates show rapid packing

Intercalated shales and siltstones contain algal biscuits,

mudcracks, ripple marks and bimodal crossbedding, indicating a shallow marine, possibly estuarine environment.

Current directions in the

alluvial sections appear to be from the west and south. Volcanics in the sequence are essentially basaltic flows, although rhyolitic flows

C19 immediately underlie the main alluvial interval. Mineralisation occurs in quartzites and conglomerates in the main alluvial interval in the western half of the basin.

The conglom-

erates are typically pyritic, and comprise largely siliceous pebbles in a green arkosic matrix.

Conglomerates -containing the better miner-

alisation are well packed and sorted, and up to 30 feet thick. The heavy mineral assemblage comprises pyrite, anatase, monazite and ziron. Uranium is associated with carbonaceous pellets, which occur in narrow layer concentrations in the conglomerates, and dispersed in the quartzites. Comments will be made on the world wide distribution of this kind of mineralisation, and particular attention will be given to the comparison with the Blind River, Canada and Witwatersrand, South Africa, occurrences*

SANDSTONE TYPE URANIUM DEPOSITS R.G, Dodson, Bureau of Mineral Resources Approximately ninety perccent of the- Western World's known economically mineable uranium deposits occur in sediments, either as conglomerate-type deposits, or as sandstone,-type deposits.

In the

sandstone environment uranium deposits occur in a variety of forms, the most common being "rolls11 and "blanket11 deposits.

The host rocks

containing such deposits extend over a wide age range.

The deposits

do however share certain common characteristics which provide guides for exploration. Although the uraniferous deposits of Wyoming, Colorado, and New Mexico, U.S.A., are the largest and best known sandstone type deposits, similar deposits are known in many parts of the world, including Australia. A brief description is given of the characteristic features of "roll11 and "blanket" type deposits, and an outline given of the geological setting of the sedimentary environment in which they occur.

C20 THE ESTUARINE ENVIRONMENT AND PHOSPHATE DEPOSITION P.J. Cook, Bureau of Mineral Resources Estuaries are an important feature of most coastlines (both modern and ancient).

They may also be economically important as sites

of syngenetic ore deposits such as uranium and phosphate, and placer deposits such as mineral sands, gold and tin.

An understanding of the

geochemical and sedimentary processes in the estuarine environment is therefore vital to the elucidation of the genesis of a number of mineral deposits.

It has for instance been suggested that the phosphate deposits

of northwest Queensland are in part estuarine. In an effort to better understand the estuarine environment in general and the distribution of phosphorus in particular, the Bureau , of Mineral Resources has initiated a study of the two East Australian estuaries, Broad Sound and Mallacoota Inlet (the latter being a joint ANU-BMR project).

Broad Sound is a large ephemeral tropical estuarine

complex on the Queensland coast (in contrast to Mallacoota Inlet which is a small perennial temperate estuary on the Victorian coast), which in common with most other estuaries shows a marked intermingling of various types of depositional environments.

These environments include

supratidal mud flats, mangrove, mangrove inlet, intertidal muds and sands, and shallow marine muds, sands and gravels;

each may be

characterized by textural and/or geochemical criteria.

Eh and pH were

measured in all the accessible environments both in situ and on the extracted interstitial waters.

Eh values ranged from slightly negative

in some of the intertidal muds to strongly positive in most other environments.

Ph was rather variable;

strongly acid conditions occurr-

ing in old mangrove deposits (3-3 measured in situ;

5.3 on the

extracted pore fluids) and alkaline conditions in most other environments, maximum values occurring on the supratidal mud flats in areas of algal growth.

As phosphate solubility is pH dependant, this variability of

pH has a very marked effect on the phosphate content of the'water. In mangrove inlets with acid water, phosphate concentrations of up to several hundred times that for normal seawater were noted•

Overall

however, the phosphate content of the water in Broad Sound tended to be fairly low.

Similarly the estuarine sediments tend to range in P 0 2 5 content from average to a little below average. Relative concentrations

do however occur in the carbonate gravels at the northern end of Broad Sound away from the diluting effects of terrigenous sedimentation,

C21 and in the older (minimum age 2000 BP) mangrove deposits where contents of up to 20,000 ppm have been recorded.

This is marked contrast

to the present day mangrove deposits which have ^2^5 content of only 500 to 1000 ppm. Excess phosphate in modern estuaries derived from fertilizers and. the comparatively recent development of Rhizophora mean that caution must be exercised when attempting to apply conclusions reached from presentcday estuaries to for instance Cambrian phosphorites of possible estuarine origin. The Queensland phosphate deposits fairly certainly formed under nearshore conditions.

Some phosphorites

may be of primary origin, but others possibly formed postdeposit ionally below the sediment-water interface, by the leaching out of phosphate by acid interstitial waters, subsequently followed by the phosphatization of calcareous or siliceous grains, or the direct precipitation of interstitial apatite as conditions became more alkaline.

RECENT ESTUARINE SEDIMENTS AND MINERALIZATION ON THE CENTRAL N.SoW. COAST PoS. Roy, Geological Survey of N.S.W., Dept. of Mines The unconsolidated sediments underlying four estuaries on the central N.S.W. coast have been studied by seismic and. drilling techniques: Lake.

Tuggerah Lake, Fullerton Cove, Port Stephens and Wallis

Their thickness ranges from 170 feet in Tuggerah Lake to 250

feet in Fullerton Cove. Fluvial gravels, sand and clays, estuarine clays and sandy clays, quartzose sands (shelly and non-shelly), humate, and soft black muds have been encountered.

These sediment type have

been grouped into two sediment associations. Association^) is found in Tuggerah Lake and Western Port Stephens. Fluvial sand and gravels infill bedrock channels and grade upwards into sandy clays and clays of both fluvial and estuarine origin. These" are overlain in turn by soft black estuarine muds which cover most of the bottom of these estuaries. Association (2) is found in eastern Port Stephens and Wallis Lake. Dark grey estuarine clays generally overlie basement except in some deep bedrock channels where coarse grained fluvial sediments probably occur.

The estuarine clays are overlain by thick sequences of

C22 of quartzose sands, in part shelly and in part non-shelly, the latter with hurnate beds. . Black muds and sandy muds overlie these sands in some areag,notably in Fullerton Cove.

The sediments

beneath Fullerton Cove are similar to those of association (2), however the past influence of the Hunter River has resulted in more extensive development of fluvial gravels than in either eastern Port Stephens or Wallis Lake. Associations (1) and (2) are basically facies equivalents.

The

fluvial influence is dominant in association (1) while in association (2) the marine, or estuarine, influence is dominant. The seismic has delineated a number of erosion surfaces in the sequence of unconsolidated sediments.

The main disconformity is

accompanied by a marked change in sediment type and is believed to have originally formed as a result of river erosion during a late Pleistocene regression, and was later partly re-excavated during the late Wisconsin low sea level stand.

The black surface muds of

association (1) and both the shelly and non-shelly quartzose sands of association (2) overlie this erosion surface. In the context of mineralization the quartzose sands and . estuarine muds are of most interest.

The former as a potential host

for heavy minerals (rutile and zircon etc), and the latter as an environment in which authigenic minerals form. •

(A) The quartose sandsS') have been divided into non-shelly and

shelly units which belong to two depositional phases. (i) The non-shelly sand unit often contains sequences of humate (up to 55 feet thick and extending to 80 feet B.SoLo in Wallis Lake).

Dates from humate in similar

sequences elsewhere give an age of about 30,000 years B.P. which suggests that these non-shelly sands probably formed as dunes during the Wisconsin interstadial*

The absence of shells in these sands can

be attributed to post depositional leaching during the Holocene low sea level stand. (ii) Since their deposition the shelly sands have not been exposed to sub-areal weathering, and are believed to have formed during the active part of the last (Holocene) transgression in open estuarine or beach

C23 %

environments. These quartz sands were largely derived through re-working, by tidal currents and waves, of the non-shelly sands. It seems probable that the development of the Pleistocene non-shelly sands and the Holocene shelly sands are related to the developement of the inner and outer coastal barrier systems respectively.

However

in the Port Stephens area there is some stratigraphic evidence for a 3rd and earlier stage of coastal barrier development.

The !linner,f

barrier along the southern side of western Port Stephens pisedates the erosion of a channel system cut into the sediments beneath the estuary. (This is the disconformity mentioned above.) Elsewhere both shelly and non-shelly sands have been deposited in channels of this age and are therefore younger than the so called "inner" barrier on the southern side of western Port Stephens.

This barrier presumably formed during

a high sea level stand that preceeded the Wisconsin interstadial. Sands adjacent to the estuaries contain economic grades of rutile and zircon.

The possibility that similar concentrations occur in the

sediments beneath the adjacent estuaries has long been recognised. At a very rough estimate sea level changes occurred at the rate of about 1 foot per 30 or

years between the Wisconsin interstadial (about

30,000 years ago) and 6,000 years ago when the sea reached its present level.

It is generally accepted that concentrations of heavy minerals-

initially result from waves re-working beach sands during periods of relative sea level stability.

It is unlikely that these conditions

would have been achieved during the rapid changes in sea level accompanying the Holocene transgression.

The shelly sands deposited

at this time would therefore be unpromising hosts for heavy mineral concentrations.

The potential for heavy mineral concentrations in the

Pleistocene non-shelly sands depends upon the neight sea level reached during the Wisconsin interstadial and the length of time sea level remained stable at this level.

There is some geomorphic evidence along

the east coast to suggest t&at the sea at this time approached present day sea level in which case much of the non-shelly sand would have presumably been deposited during the active stage of the Wisconsin transgression (they occur between 01 and 110! below S.L.).

As with

the Holocene shelly sands conditions would have been fairly unfavourable for heavy ^mineral concentrations.

C2k

A cursory look at the heavy minerals in drill hole samples of these sands shows very low concentrations of heavy minerals ( 9 o f the samples contain less than 0.k% total heavy minerals). (B) The soft black surface muds are of interest in view of their authigenic mineralization.

They reach thicknesses of up to 75 feet

in Port Stephens and Fullerton Cove and extend-to a maximum depth of 105 feet B.S.L.

The mud accumulates in the low energy estuarine

environment formed during the last transgression.

Fine fluvial

material is distributed by tidal currents and is deposited from suspension.

Hud accumulation commenced when sea lev-el rose during the

last transgression and drowned those parts of the estuaries which were remote from either river deltaic, or higher energy coastal environments of deposition.

Althoughi the muds are extensively bioturbated

at their surface, their high organic content produces a reducing environment from near the surface downwards.

Pyrite, gypsum, glauconite,

benthonic foraminifera and sponge specules are relatively common.

Tests

have been carried out to determine the presence of concentrations of vanadium, uranium and phosphate in these muds. The continuous and thick sequence of black mud provides a record of the estuarine micro fauna possibly covering the last 10,000 years.

The foraminiferal

assemblages are being categorised in drill hole samples in Port Stephens and Fullerton Cove. The dark grey estuarine clays that occur beneath the quartzose sand units in eastern Port Stephens and Wallis Lake and also occur in the fluvial clay sequences in western Port Stephens and Tuggerah Lake are believed to be Pleistocene equivalents of the 55 feet black surface muds. However the micro fauna and authigenic mineral in these clays are generally less abundant than in the surface muds.

C25 MINERAL SANDS ON THE EAST COAST OF AUSTRALIA M.S. Shepherd, Associated Minerals, Southport, Queensland Heavy mineral i^and deposits are described as found on the east coast of Australia.

Origin, methods of formation and age of deposits

are briefly discussed. In some older deposits, post depositional changes have led to the use of dry excavators such as bucket wheels, bulldozers and loaders, instead of the suction cutter dredge which is the traditional method of excavation in the mineral sands industry.

Examples of these are

described. After nearly twenty years of intensive mining of high grade deposits, the trend to larger mining plants working in low grade deposits is considered.

f M

FACTORS GOVERNING THE RAPID WEATHERING OF A SILTSTONE D.J. Swaine, C.S.I.R.O. Division of Mineralogy, Sydney The rapid weathering of a siltstone near Liddell, New South Wales, was investigated*

The main aim was to understand the rapid

breakdown of the siltstone on exposure to atmospheric conditions* The fresh siltstone and the weathered siltstone were carefully sampled in an excavated area, and a piece of core was used as representative of the sandstone.

The chemical and mineralogical compositions of the samples were determined by various methods, the salient results being summarised ' as follows:(i) The chemical compositions are given in Table 1. Calcium, carbonate CO2, organic carbon, total sulphur and sulphate are markedly lower in the weathered siltstone compared with the siltstone.. A special determination of pyrite showed 1 - 1 . 5 $ FeSg in the siltstone, 0 - 0.051o in the weathered siltstone and 0,05$ in the sandstone. No unusually high concentrations of trace elements were found in any of the samples, indicating that trace elements are not a major factor in the weathering of these rocks• (ii) The mineralogical examination of the samples showed that two of the samples of siltstone should be classified as subfeldspathic lithic wacke and one as subfeldspathic lithic arenite. The weathered material is weathered greywacke and the basement rock is lithic arenite. X-ray diffraction identified quartz and feldspar as major components, with lesser amounts of muscovite and kaolinite.

Dolomite was detected in

the siltstone and sandstone samples, but not in the weathered material, in keeping with the results for carbonate.

Crystalline material found

in the weathering zone, especially in horizontal layers, was gypsum. A thin white crust on some of the weathered rocks contained epsomite; ^^these rocks were in a wet area where changes in water level may have been occurring.

The microscopical examination showed that pyrite,

mainly framboidal, occurs ubiquitously in the matrix of the siltstone. Although there is no consistent association of pyrite with organic matter, some organic matter may be found with the pyrite. Pyrite was not detected in the weathered siltstone, but traces of detrital pyrite were seen in the sandstone sample.

D2 Although dolomite is the main carbonate mineral, lesser amounts of siderite and calcite are probably present in the siltstone and sandstone samples,

(iii)Examination of the siltstone samples by a differential thermal analysis technique showed that the organic carbon is similar to bituminous coal, surely not a surprising result for sediments adjacent to coal seams.

Discussion of results

The above results are explained by oxidation of pyrite, during weathering, to give some free sulphuric acid, which has then reacted with carbonate minerals forming sulphates, which have been largely washed out of the weathered material.

Crystalline gypsum, found

mainly in horizontal layers, is then a primary product of the oxidation of the pyrite.

It will be appreciated that an investigation of

chemical weathering of this type can neither predict the state of a rock at a particular time during weathering nor the rates of relevant reactions.

However, in view of the presence of framboidal pyrite, that

is a very finely divided material with a large surface area, it seems reasonable to suggest that oxidation would be rapid in the presence of air and water.

The additional effects of bacteria and some increase

in temperature would enhance oxidation.

At the same time, the coaly

organic matter would also be oxidized.

Perhaps the principal unanswered

question arising from this investigation is the relative roles of organic matter and pyrite.

In other words, does the presence of

organic matter retard the oxidation of pyrite by providing reducing conditions locally or does it enhance the oxidation of pyrite by attracting oxygen or even by forming a peroxide?

However, even with-

out knowing the mechanisms of these reactions, it seems that the rapid weathering of the siltstone is caused by the oxidation of pyrite and coaly matter leading to the breakdown of the carbonate minerals, all of these minerals being disseminated throughout the rock.

The above discussion is related to atmospheric oxidation. status of the siltstone in situ is probably different.

The

Bacteria and

elevated temperature are improbable at depths of some hundred metres. Lateral movement of water into the siltstone is likely to be slow and most of it will be from the bottom and sides of a permanent body of water.

The material sedimenting from this water contains some organic

matter and possibly some gypsum, thereby providing a medium for

D3 bacterial oxidation-reduction reactions conducive to the removal of oxygen from the bottom waters.

This in turn means that the water

seeping into the siltstone should be very low in oxygen and hence potentially unable to oxidise pyrite appreciably.

Under the con-

ditions suggested, the percolating water may contain carbon dioxide from the oxidation of organic matter, but this should not favour the oxidation of pyrite.

The mechanism proposed for the rapid weathering of the Liddell siltstone exposed to atmospheric conditions, may well apply to similar rocks containing framboidal pyrite, organic matter and carbonate minerals finely disseminated through the rock.

In general,

the status of the rock in situ depends on the availability of oxygen and water, in other words on the stability of the pyrite and organic matter.

It is a pleasure to thank the Electricity Commission of New South Wales for their support, and various colleagues who assisted me, especially Mr G. Leblang.

TABLE 1 Composition of Samples (as per cent) SILTSTONE

WEATHERED SILTSTONE

SANDSTONE

Si02

68.7

68.3

65.5

66.1

69.1

76.2

A1 2 0 5

12.5

12.1

13.8

14.7

13.0

10.4

Pe203 (i)

3.7

3.6

4.3

5.6

5.2

2.4

1.2

0.8

1.0

1.3 1.0

1.0

Sad

1.3 1.1

0.08

0.07

1.1

Na„0

1.4

1.2

1.8

1.4

1.6

1.8

K2O

2.8

2.7

2.8

2.5

2.4

2.7

MgO

H 2 O~

2.0

2.1

2.7

5.4

2.3

0.9

Ti02

0.7

0.2

0.7 0.2

0.7

P2O5

0.7 0.2

0.3 0.2

0.1

CO^

2.4

C (organic)

0.9

S_(ii) S04 (as S)

1.0

1.1

1.2

0.2

0.3 0.2

0.2 —

0.2

2 trace elements (iii)

0.2

1.7 1.0

0.2

1.4

0.1

3.0

1.7

0.2

0.3

0.06

0.2

0.06

0.05

0.1

0.2

D4 Notes:

(i)

Total iron

(i i) To tal sulphur (iii) Sum of trace elements (determined spectrographically)

THE INFLUENCE OF CHEMICAL DIFFERENTIATION ON GEOCHMICAL SURVEYS C.R.M. Butt, C.S.I.R.O* Division of Mineralogy, Perth Geochemical surveys utilizing stream sediments are widely used as a reconnaissance or follow-up tool for mineral exploration and, to a lesser extent, for agricultural and other purposes.

The success of

the technique depends upon the representivity of the samples, so that patterns shown by sediment metal contents reflect similar variations in bedrock geochemistry.

In some situations, however, spurious patterns

occur, resulting from the processes of weathering and erosion.

Such

patterns represent regional or local variations in the secondary (weathering) environment and are unrelated to bedrock geochemistry* -The most important of these is produced by the chemical differentiation of iron, manganese and some associated metals*

This can lead

to a secondary enrichment in the stream channel and on occasions, give rise to substantial false anomalies* The mobility of metallic species released during weathering is largely controlled by Eh and pH.

Chemical differentiation occurs as

a result of differing responses of these species to the varying conditions prevailing throughout the soil profile or sequence*

The

differentiation is essentially vertical if solution and reprecipitation are completed within the profile but becomes lateral where mobile species enter the groundwater and reprecipitate elsewhere in the sequence*

A number of elements, for example Fe, Mn, Co, Cu, Ni, and

Zn, are selectively mobilized where acid reducing conditions prevail in soils*

Subsequently iron and manganese are reprecipitated as

hydrous oxides in an environment of rising Eh and/or pH and scavenge a number of the other metals, p^^t^c^l^^l^jc^

-t, nickel and zinc.

In areas of acid podzolic and gleyedwsoils, for example, considerable vertical and lateral differentiation can occur in the soil sequence by this mechanism.

In temperate regions of high rainfall, such soils

can be widespread so that lateral migration and precipitation may d&ly reach completion in the oxidized waters of the stream channel. The consequent enrichment of the stream sediment gives rise to misleading results which can hinder the interpretation of drainage surveys if not recognized.

D5 The processes involved are discussed in terms of some chemical and micromorphological examinations of soils and sediments made in connection with a regional geochemical survey carried out in Northern Ireland.

The same phenomena, however, are to be expected to occur

wherever environmental conditions favour active leaching of soils. Some parallels can also be drawn with differentiation that has occurred in lateritic soils.

METAL ORGANIC COMPOUNDS IN SEDIMENTS J.D. Saxby, C.S.I.R.O. Division of Mineralogy, Sydney Most sedimentary material is inorganic in origin with organic matter being only a minor component (<1

However the percentage

of carbonaceous material increases to—1 fo in many black shales, to —10% in oil shales and to —100$ in coals. Metals are associated with organic matter in various ways and metal-organic compounds in a sediment are an interesting intermediate between completely inorganic and organic components.

In certain cases metal-organic complexes

may act as a source of metals during diagenesis or metamorphism. Although geochemists have suggested that metal-organic compounds are present in certain geological environments, in most cases the exact structure of the molelcules is unknown and the available chemical data has rarely been applied to specific situations.

The distribution

of compounds of this type in carbonaceous sediments is governed largely by their thermal stability which restricts their possible occurrence to species involved in sedimentation and to rocks which have not undergone extensive metamorphism.

It is possible that the presence

(or absence) of particular metal-organic complexes in a sediment will prove to be an indicator of the extent of diagenesis.

The term metal-organic can be used to describe molecules in which the metal is bonded to organic matter via (a) carbon atoms giving rise to organometallic compounds, (b) carboxyl groups leading to salts of organic acids, =(c) electron-donating atoms (0, N, S, P, As, etc.) forming co-ordination complexes or (d)ttelectron-donating systems (olefinic bonds, aromatic rings, etc.).

Bonding types (b) and (c)

are likely to be much more common in sedimentary environments than types (a) and (d). An apparent association between carbonaceous matter and metals in a sediment does not necessarily indicate that any

D6 of these bonding types is present*

Many weak interactions "between

metals and organic material may be attributed to Van der Waal!s or physical adsorption*

Metal-organic compounds are characterized by

true chemical bonds, although the distinction between physical and chemical adsorption may not always be clearcut*

One can envisage two interrelated ways in which metal-organic compounds can participate in sedimentary processes and be incorporated in sediments? (a) Biological Metal-Organic Molecules*

Living organisms are able to

concentrate certain metals and the resulting compounds may be introduced directly (or in a slightly changed form) into a sedimentary situation*

However to be important geologically, such molecules must

be sufficiently stable to survive the partial decay of the organism and the oxidizing or reducing conditions in the basin.

The following

list shows the organic groups which bind metals most commonly in living organisms®

The metals are arranged in approximately decreasing

order of occurrence0 Fe

: porphyrin, imidazole, -KH^, =NH, S^S, -S , -COO ,

: -COO ,

s ~C00~9

: -BH2?

porphyrin, imidazole porphyrin, imidazole RgS, -S~

: -HH2, = m , R2S,

: -C00~, =P0~, imidazole

: -NHg, =KH, corrin, benzimidazole, carbanion of a sugar

: -MH2, =NH, R2S, ~S~

« R2S,

: -COO"", =P0~

•Ni

: uncertain (porphyrin?)

t uncertain (not porphyrin)

Of these classes of metal-organic compounds, metal porphyrin complexes are the most stable and have been detected in a wide range of recent and ancient carbonaceous sediments*

In most cases the

reactions during sedimentation and diagenesis have involved removal of magnesium from chlorophyll or iron from related tetrapyrrole systems, modification of the ring and its substituent groups and complex formation with nickel or vanadium*

H7.CW3

CM^ t k I OCM CO

chlorophyll - a

nickel complex of deoxophylloerythroetioporphyrin

Porphyrin and other metal complexes may in turn react to give species of higher molecular weight which are incorporated into crosslinked, polymeric kerogen molecules. (b) Chemical Reaction in the Sedimentaiy Basin. A wide variety of organic molecules may be carried into marine or freshwater basins either in solution or as particles or colloids.

In the sedimentary

environment reaction may occur with metal ions and, if the solubility product of the resulting complex is exceeded, precipitation may occur directly. Alternatively, a soluble metal-organic species may adsorb physically onto other particles and be incorporated in the sediment. Thus, organic acids and a range of organic molecules possessing donor properties towards metals due to lone electron pairs on nitrogen, oxygen and sulphur atoms may bind or even concentrate metals in a sediment.

Carboxyl groups will tend to react with metals such as

Ag, Al, Ba, Ga, K, Mg, Na and Pb, while co-ordinate bonds will usually be favoured for Co, Cu, Fe, Ni, V and Zn, and in each case the resulting compound can become a minor (or rarely major) component of the sediment.

The formation of complexes from humic acids generally

involves both electrovalent bonds from carboxyl groups to metals and co-ordinate covalent bonds from phenolic oxygen atoms to metals.

Among the most interesting potential complexing molecules to be found in natural waters are amino acids, which may act as monodentate, bidentate or terdentate ligands (through 0, N or S atoms).

Three of

the species which may be formed from the sulphur-containing amino acids cysteine and methionine are:

CH -NHj. - M *

;

MHz

H i r e H ,

a Mx

Simple compounds such as these may in turn form part of complex humic polymers,,

In all cases however, the identification of metal-organic

compounds at any stage of the sedimentary process without removal of the bonded metal is a difficult practical problem*

Fate of Metal-Organic Compounds during Diagenesis.

Many factors

determine the subsequent history in a sediment of a metal complexed to a porphyrin, fatty acid, amino acid, or any other organic molecule * Each type of compound needs to be considered individually but in general a sterile, slightly reducing environment will favour preservation through the early stages of diagenesis.

The long-term

fate of a metal complex in a carbonaceous shale, oil shale or even coal will be determined by the temperature to which the sediment is heated.

Most compounds of this type decompose at less than 250°C,

although porphyrins can be distilled at — 500°C.

When a metal-organic compound contains sulphur, the formation of metal sulphides during diagenesis is possible.

Experiments with

complexes and salts of various metals with the amino acid cystine have shown that at low temperatures organic sulphur becomes available for metal sulphide formation.

Under reducing conditions at 200°G

gaseous, oily and kerogen fractions have been formed as well as metal sulphides.

Although the deposition of sediments is the major area in the geochemical cycle where metal-organic compounds are likely to be encountered, the solubilization of minerals by organic m;olecules during weathering may also involve complex formation.

Naturally

occurring reagents such as amino acids, salicylic acid, 2-ketogluconic acid and adenosinetriphosphate are capable of dissolving oxide, sulphide or carbonate minerals by chelation of the metal to the organic molecule through nitrogen or oxygen atoms.

Bacterial

leaching almost certainly involves similar fundamental reactions. The resulting metal complexes could in favourable circumstances be reincorporated into sediments as the geochemical cycle continues*

D,9 HPMIC ACIDS AND THEIR ROLE IN WEATHERING W.E# Baker, Department of Mines, Tasmania Humic acids appear to have been named by a German scientist, Sprengel in 1826, although the term humus was used earlier by Wallerius in his 'Agricultural Fundamenta Chemica1 published in Sweden in 1761. Throughout the 19th century many conflicting ideas on the origin, character and properties of humic acids were presented• Numerous substances were extracted from a variety of sources. Each was held to be a specific compound and the literature became cluttered with analyses, empirical formulae and names. By the close of the century the accumulated data were so confusing that a number of investigators doubted the very existence of these complex substances. The confusion was largely set aside by the work of the Russian, Shmuk between 1914 and 1930. Shmuk proposed that humic acids were a group of structurally similar compounds which he found to be of aromatic nature. He also demonstrated the presence of carboxyl and phenolic hydroxyl groups. Prom this work has developed the present day concept of humic acids which are regarded as complex heteropoly condensates of aromatic character. Properties of humic acids which are relevant to the study of some geological processes ares (1) Possession of numerous functional groups which result in high exchange capacities. (2) Very high molecular weights and consequently size which places them in the colloid range. (3)

Ability to complex metals. %

The exchange reactions of humic substances have been applied to the enrichment of metals in bioliths by Szalay in Hungary (1952-1967) and more recently in Australia by Beevers (1966). The reaction between metals and humic acids has been considered as salt formation, colloidal absorption and complex formation. Most soil scientists would now accept 'that complexation is the main factor in humic acid ~ metal interaction although a few still consider colloidal properties are involved.

D10 The possible influence of humic substances on geological processes has been discussed for a'very long time*

Sprengel (1826)

believed that humic acids displaced silicic acid from silicates and similar views, very much expanded, were expressed by Julien (1879)• Other studies of more recent times present evidence conforming the attack of silicates by humic acids (e*g# Ponomareva and Ragim-Zade, 1969)#

Recent text books by Krauskopf (1967) and Loughnan (1969)

question the efficiency of humic acids in the weathering of silicates* The role of humic acids in the degradation of the minerals of economic deposits has not received the attention given to the silicates.

Gruner

(1922) demonstrated that peat waters attacked silicates, iron oxides and siderite, but had little effect on pyrite*

Freise (1931) claimed

that humic acids dissolved gold but Fetzer (1934) f G-arces (1942) and Ong and Swanson (1969) contested this claim*

Fetzer (1946) investi-

gated the possible solubilization of a number of economic minerals and concluded that humic acids were ineffective solvents* The writer!s work at the Department of Mines has been largely centred around a re-appraisal of the effectiveness of humic acids as solvents*

Whereas earlier studies have made use of fine grained

(-2004}=) mineral powders and humic acid strengths in the range 1-5$ (w/v) the present studies have been carried out with -50 +50# powders and O.V/o solutions of humic acids*

Most experiments were

carried out in a 'perfusion type apparatus and comparisons were made between solubilities in water equilibrated with atmospheric carbon dioxide and in humic acid*

For silicates the results agree with

previous studies that have indicated attack on these minerals* There is a far greater release of metal ions than of silicon and this supports the view that metals are removed by complexation* Ultimately this would result in collapse of the silicate lattice but it is not known if the silica becomes bonded to the humic acid or is removed as an independent colloid*

For minerals other than

silicates the effect of humic acids is devastating*

Sulphides

release metal at a rate ranging from 270p g Cu/day (chalcopyrite) to 43,200 |j g Ni/day (Heazlewoodite) compared with <10 and 1080 respectively from the action of water*

Calcite releases 92,800 |jg

Ca/day to humic acid compared with 1180 to water*

Contrary to the

findings of Fetzer (1945) the present studies indicate a slight solubility for gold and a very much greater solubility for the less noble metals©

D11

A percolation unit was designed, to enable experiments to be carried out under controlled atmospheres. A few minerals were subjected to the action of water in the presence of an atmosphere consisting initially of 2 % (v/v) carbon dioxide. Under these conditions calcite released 2700 Jj g Ca/day. Experiments under nitrogen showed little variation in the activity decreased suggesting that the presence of oxygen accelerates humic acid attack on these. REFERENCES

BEEVERS, J.R. (1966) A Chemical Investigation into the Role of Sorption Processes in Ore Genesis. B.M.R. Report No. 106. FETZER, W.G. (1934)

Transportation of gold by organic solutions.

Econ. Geol. 29, 599. PETZER, W.G. (1946) Humic acids and true organic acids as solvents of minerals.

Econ. Geol. 41$ 47*

PREISE, P.W. (1931)

The transportation of gold by organic underground

solutions. Econ. Geol. 26, 421. GARCES, H . (1942)9 .Solubility of gold in humic acids. Anales.

cong. panamericano ing. minas. y . geol. 3* 1135* JULIEN, A.A. (1879)

On the geological action of the humus acids.

Proc. Am. Assoc. Adv. Sci. 28, 311. KRAUSKOPP, K.B. (1967)

Introduction to Geochemistry.

McGraw-Hill.

LOUGHNAN, P.C. (1969)

Chemical Weathering of the Silicate Minerals.

Elsevier. ONG, H.L. and SWANSON, V.E. (1969)

Natural organic acids in the

transportation, deposition and concentration of gold.

Q. Col. Sch.

Mines 64, 395• P0N0MAREVA, V.V. and RAGIM-ZADE, A.I. (1969)

Comparative study of

fulvic and humic. acids as the agents of silicate mineral decomposition. Pochvovendenie, 3* 26.

D12 SPRENGEL, C. (1826) Tiber Pflanzenhumus, Humussaure und humussaure Salze. Kastriers Arch. Ges.. Naturlehre.

8, 145•

SZALAY, A* (1967) *The association of vanadium with humic acids. Geochim. Cosmochim. Acta 31, 1. *The earlier work of Szalay largely appears in Hungarian journals, CHEMICAL ENRICHMENT OF TRACE METALS IN THE OCEANS H. Bloom, Chemistry Department, University of Tasmania Natural elution into the oceans and the concurrent processes of removal by such means as sedimentation, produce fairly steady concentrations of trace metals in sea water for which average values can be tabulated.

Biosphere - hydrosphere interactions /O (< There exist also, methods by which concentrations of trace metals can be altered substantially, either in localized areas or over the oceans as a whole.

For example, concentration of trace

elements in the marine biosphere relative to the hydrosphere occurs with remarkable specificity of certain organisms for the uptake of particular minor elements. Other interactions Considerable variations of trace element concentrations occur according to location, depth of sampling, effects of ocekn currents etc., e.g. for mercury, investigators have found variations from 0.00001 to 0.0004 p.p.m. depending on the above factors.- In regions of mercury mineralization values of 0*1 p.p.m. have been reported. The variation in concentration of this element in estuarine bottom muds is also considerable.

There are reported instances of much

higher concentrations of this element in localized areas on land. Pollution processes by man have resulted in marked influence on the concentrations of at least several of the trace elements. For example the rate of introduction of lead into the oceans-has

D13 been dramatically altered by the combustion of lead tetra-alkyls .in motor car fuels•

In the past three decades, at least part of the

enormous quantities of lead compounds produced in the world from this source, have been washed into the oceans with the result that the present rate of introduction of lead is estimated to be 27 times greater than in the Pleistocene Period wit.h 3 x 1011 g of lead now being washed out of the atmosphere into the oceans annually.

In sea waters off the coast of southern California the

surface concentrations of dissolved lead vary between 0.08 and 0.4 pg/l with an average value of 0.2pg/l (i.e. 0.0002 p.p.m.), whereas in the deep waters the lead concentrations are almost uniform at 0.03 pg/1.

The higher surface concentrations are attributable

to pollution.

Pollution can cause changes in trace metal concentrations by means other than direct introduction of the element. Various metals, e.g. manganese, have solubilities which are greatly affected by oxygen content of the water (see under Hydrosphere - Lithosphere interactions), hence their solubilities can be greatly changed in anoxic waters produced by the introduction of organic matter, e.g. by sewage.

Hydrosphere - Lithosphere interactions Many variables affect the local concentrations of trace elements in the oceans, e.g. pH, oxygen concentration and mineralization. Elements with vastly different solubilities in different available oxidation states are most affected, e.g. the transition metals. Enrichment by marine organisms Many minor constituents of sea water, including beryllium, mercury, cadmium, lead, and arsenic, are known to be enriched by marine organisms such as molluscs and edible fish.

The enrichment

factors are in many instances large, depending on the animal concerned, hence there is often danger of poisoning to other living beings using the marine animals as food.

The following table gives "average" values for the composition of sea water with respect to certain minor elements and enrichment factors for a number of marine organisms.

M ^ n )

D14

Average abundances of certain trace elements in the earth. s crustt sea waterf and concentration factors in selected sea organisms (All values are p.p.m.) Element

Earth's Crust

Ocean Water

Oceanic residence time (years)

2.8

0.000001

150

0.07 0.2 100

0.0001

2 x 10 6

0.00005 0.0006 0.003 0.002 0.01 0.002

5 x 10 5 550 5 x 10 4 1400

Cd Cr Cu Mn Mo Hi Pb V Zn Hg

55 950 1.5 75 12.5 155 70 0.08

0.00003 0.002 0.005 0.00005

5 x 10 5 4 1.8 x 10 2 x 10 5 1 x 10 4 1.8 x 10 5 4 4.2 x 10 \

Scallop -

2,300 2,300,000 200,000 3,000 55,500 90 12,000 5,300 4,500 28,000

Enrichment Eactors Organisms Mussel Oyster 1,000,000 18,700 320,000 60,000 14,000 4,000 30 4,000 3,300 1,500 110,000 100,000

Other Pish 300,000 4,000,000

330 100,000 320,000 3,000 13,500 60 14,000 4,000 2,500 9,000 100,000

10,000 to 100,000 depending on size and species

For marine organisms, the selective and general accumulation of trace metals is probably initiated by complexing reactions including chelation at the exposed external surfaces of the organisms. This would account for the considerable enrichment factors for the transition metals* Conversion of trace elements, from less to more toxic chemical compounds The mechanism of conversion of certain metals and their inorganic compounds into more toxic organic compounds, e.g. mercury dialkyls, has undergone considerable investigation. It appears that natural pathways exist, e.g< by the direct conversion of inorganic mercury into dimethyl mercury by reaction with Vitamin B12. The concentrations of mercury and other metals in various'organs of the animal also vary considerably.

The results of an investigation of mercury concentrations in Tasmanian oysters (both the "mud" oyster and the cultivated Japanese species), will be presented*

Mechanisms will be proposed to explain

the wide range of concentrations observed and the probable main chemical species involved in the dissolution of mercury and other trace elements, in sea water, will be discussed*

NATURAL TRITIUM AS A TOOL IN GROUNDWATER HYDROLOGY G.B. Allison and J.V. Holmes,C.S.I#R,0. Division of Soils, Adelaide; Discipline of Earth Sciences, Flinders University Tritium has been released to the stratosphere by thermonuclear explosions.

These have occurred principally in the northern hemisphere

and fallout from these has completedly masked that from natural production processes.

The tritium, which eventually reaches the earth in precipitation, had a maximum in concentration in the southern hemisphere in 1965-66.

1 2

At this time, annual mean concentrations of 40-50 T.U.* 9 observed in mid latitudes.

were

The tritium concentration of rainfall is

now declining and will presumably approach the prebomb concentration of about 6 T.U. Considerable enthusiasm was engendered by the early claims of 3 4 Libby,

Brown

and others in the early 1960!s that determination of

tritium concentrations in underground water samples could be interpreted to provide useful estimates of groundwater resources.

However,

for any such interpretation to be made, a suitable model for flow behaviour must be devised. Many attempts have been made to formulate models.

One of the most commonly used for confined aquifers is the 5 piston flow model with dispersion first outlined by Nir . However,

water table aquifers (which are common in humid regions) in contrast to confined ones, receive recharge via infiltration over their whole surface.

In this case, provided the correct sampling technique is

used, the aquifer can be treated as well mixed and it becomes quite easy to calculate the ratio of recharge to total volume of the aquifer. To enable interpretation of the natural tritium concentration of groundwater to be made, it is essential to know all tritium inputs to the aquifer.

For this reason we have reconstructed a history of

tritium fallout for southern Australia . -J Q * 1 Tritium Unit (T,U„) = 1 tritium atom in 10 atoms of other hydrogen species.

D16 The tritium concentration, 1 , of a well mixed aquifer (or of

water taken from a stratified aquifer using a technique which gives a well mixed sample) is given by oo T =1 a Va

n n=o

n n d _i ) e- * n ^ V ; a

where v is the annual recharge, V is the total storage volume of the a aquifer and T n is the tritium concentration of water being recharged to the system n years before sampling. An example of the use of this model is given by calculations based on results obtained from samples taken from the Polda Basin, on Efyre Peninsula, South Australia.

The results of tritium

analyses of samples taken from the aquifer together with the estimate of recharge for each site are given in Table 1. Assuming a porosity of 3 0 t h e mean recharge is calculated at about 30 mm per year.

The mean annual rainfall for the area is about 400 mm, 50%

of which falls between May and August when potential evaporation is at its minimum.

Soil moisture deficits are small for the skeletal

soils in the area.

Tritium analyses can be a powerful qualitative tool in hydrology.

This is demonstrated by some measurements from the North

Adelaide Plains, near Adelaide.

In this case the sampling prograjnme

was not suitable for a quantitative description of the hydrology of the area to be made.

The area is of economic importance for agriculture, and in response to heavy pumping, a large drop in potentiome'tric surface has occurred.

An extensive geophysical survey has been carried

out in the area^.

During the latter part of this investigation,

samples of underground water were taken for tritium analysis in the hope that these might clarify sources of recharge.

The tritium concentrations for all samples were quite low, indicating a low ratio of recharge to total volume.

It is unlikely

that recharge could occur via surface infiltration because of the low rainfall and high soil moisture deficits throughout the area. Likely sources of recharge are two ephemeral streams which cross the plain.

D17 No tritium was found at four sites which were> 0.5km from the rivers. This indicates no component of recent recharge at these sites. However, all sites on or near the rivers showed measurable tritium - indicating that recharge does occur through the river beds. This conclusion is supported by the observation that an area of lower salinity groundwater forms an envelope about the rivers. The results can be used to show that much more water moves laterally from the river in the unconfined aquifer than moves vertically into the deeper confined aquifers. At one site water moves 600m laterally in about 11 yrs, but the time taken to move 100m vertically downwards is 30-60 yrs. An approximate calculation of recharge to the confined aquifers using the results of tritium 6

analysis, yields a value of 0.2-0.5x10 m /yr. Using classical hydrogeologic. techniques, the throughflow within the confined 6 3 aquifer has been estimated at 10 m /yr. Our results therefore suggest, in addition to some recharge from the river beds, that significant recharge to be confined aquifers occurs via the alluvial fans at the base of the scarp which forms the eastern boundary of the plain. Measurement of natural tritium in an aquifer allows the age structure of the aquifer to be determined, and in this way it may be possible to make estimates of aquifer parameters. It is well known that the flow of groundwater in most situations is quite slow and may be delineated by streamlines. The Dupuit-Forchheimer approximation for a water table aquifer which is uniformly recharged by surface infiltration and which drains to a line sink parallel to the divide, allows the time of travel for a parcel of water in a stream tube to be computed. In this way, the age structure of an aquifer can be built up. If the effluent stream at outflow can be regarded as being well mixed, the tritium concentration at outflow, T, is given by

1000

1 T = ttt^t: 1000

where T

— n=o

n 2 v < 1 + 2i "I^A. E Tn exp< * SxTO^K i=o /JlL* (5999-2i) 4 x 10 6 K

(1+21)4.7?) 0

is as defined earlier, L the length of aquifer from divide

to outflow, K the hydraulic conductivity, q the annual recharge rate and ZQ is the height of water in the stream channel above the "base of the aquifer.

E'18 A portion of the Gambier Plain in South Australia approximately corresponds to the geometry required for the above model. A tritium concentration of 0*6 T.U. is obtained for samples of water taken from springs which represent the outflow from the aquifer (this value shows no annual fluctuation). Substitution of K, calculated from 7 8 pump test data and q, obtained from a lysimeter experiment , show that the equation gives a reasonable approximation to field behaviour. However, the low tritium concentration at outflow makes a check of the model difficult because of the large relative errors involved in analysis at this concentration level. A search is currently being undertaken to find an aquifer of suitable geometry and with a relatively short turnover time for water in order to better test the model. The advantage of the use of this type of model is that the values obtained for q or K are regional estimates, rather than for a particular site as more traditional methods give. Although accurate modelling of aquifer behaviour is difficult, qualitative evaluation of the characteristics of groundwater bodies can be undertaken at much lower cost and effort than by classical hydrogeologic techniques. REFERENCES 1.

Taylor, C.B« Tellus 20, (1968) 559*

Allison, G.B., Holmes, J.W. & Hughes, M.W. J. Hydrol* 1j[ (1971),19.

Begemann, P. & Libby, W.F. Geochinu et Cosmochim. Acta T2 (1957) 277.

4«

Brown, R.M. Geochim. et Cosmochim. Acta 2^ (1961) 199.

Nir, A.J. Geophys. Res. 69, (1964) 2589.

Dept. of Mines, South Australia, Report 1964/67, (1968).

Bleys, C. & Warner, K.R. Geol. Surv. of S.A. Report Book No. 56/91 (1965).

Holmes, J.W. & Colville, J.S. J# Hydrol. 10, (1970) 38.

1519' TABLE 1 TRITIUM CONCENTRATIONS OBTAINED IN SAMPLES FROM THE POLDA BASIN Bore No. Thickness of aquifer penetrated by the bore (m)

Tritium concn« (ToUe) (r .3)

PT8

24.4

1.0

*PT10

J0.5

0.3 3.0

PT11

7.6

PT13 PT14

7.0 6.5

4.9 2.2

PT15

6.7

3.0

Ratio of Estimated recharge recharge to total assuming a porosity of volume (- .002) 50fo (cm) 4±2

• 005 .012

3*1

.020

4-«5 2±1

.009 .012

2±1

*Confined THE K 2 0 , CaOt MgO and C0 2 CONTENT OF SHALES J.C. van Moort, Geology Department, University of Tasmania Introduction The carbonate and non-carbonate fraction of one hundred and forty shales from the Australian platform were analysed for their calcite/dolomite/siderite content, clay mineralogy and CaO, MgO, K 2 0 and C0 2 content*

Pre-Ordovician shales prove to be dolomitic

and sideritic, contain essentially 2M illite and chlorite and have a high MgO and K 2 0 content and a low CaO/MgO ratio*

Mesozoic and

Cainozoic shales are essentially calcitic, have a variable clay mineralogy with illite

illite polymorph ratio varying widely),

montmorillonite, kaolinite, randomly interstratified montmorillonite/ illite, have a low MgO and K 2 0 content and a high CaO/MgO ratio # Shales of intermediate age have an intermediate composition*

The

composition of young and old sedimentary shales is compared with data from about six hundred young and old metamorphic slates, phyllites, schists and pelitic hornfelses* Parallel trends in the chemical and mineralogical composition of shales as a function of geological age were observed in North America and Eastern Europe (cf0 Ronov and Migdisov, 19711 Weaver, 1967)o

The older shales are richer in potassium and exhibit, like

D20 the older carbonate rocks, a lower CaO/MgO ratio*

Conflicting

evidence exists on the trend of magnesium content from older to younger shales*

In the present study the global chemical evolution

of shales from the Proterozoic onwards is traced.

Current hypothesis concerning the differences between younger and older shales are reviewed*

Geochemical models involving storage

of calcium in the Precambrian ocean prove to involve an unrealistically high CO2 pressure of the atmosphere.

The high magnesium and

potassium contents of pre-Ordovician shales are explained by introduction of these elements from altering sandstones, and volcanics within the sedimentary pile. Constancy or evolution of the chemistry of shales after deposition The differences in chemical and -mineralogical composition of the older shales could result either from different patterns of sedimentation in the past, or from post-depositional changes in the mineralogy and chemistry of the rocks. Studies on trends in the major element composition with time of sediments other than shales are rare and inconclusive. According to Ronov, Mikhailovskaya and Solodkova (1963)* sandstones from the Russian platform also exhibit a decrease of KpO from ancient to young sands and an increase from g g . No trend can be observed in the Palaeozoic MgO and Mesozoic averages, but the sandstones in the Russian Precambrian CaO exhibit high KgO and low values due to the appearance of highly micaceous greywackes and arkoses. According to the data provided by the same authors, the chemical and mineralogical composition of the Russian platform is also a function of their depth below the presentday land surface, the deeper levels being reduced in K20, MgO and COg* This observation weakens the importance of the previous observations because

the

Precambrian sands analysed were taken from shallow

depths. Pettijohn (1941) showed that older sandstones have a very restricted mineralogy, their unstable minerals having been dissolved slowly. Very little is known about the chemistry of the important group of volcanogenic sediments. Constancy of the chemistry of the silicate fraction of shales is generally not doubted and mineralogical differences between older and younger shales are usually explained as primary.

On the other hand

D21 it has been claimed by most workers on older carbonate rocks, that much dolomite is of secondary origin and that magnesium was introduced into the sediment after deposition*

These two views contra-

dict and one is tempted to explore the consequences of admitting chemical changes of both carbonate and silicate fractions in shales, or no post-depositional chemical change at all.

Geochemical consequences of Ronov*s geochemical model

The Russian school of thought adheres a geochemical model with constancy of post-depositional composition (e.g. Ronov and Migdisov, 1971)#

According to Ronov the steady increase of the carbonate rocks

since the Proterozoic is out of proportion with the possible supply from the continents, particularly with respect to calcium.

Consequently

he assumed an eight-to-nine fold concentration of calcium in the ocean at the end of the Archaean.

The relative and absolute amounts

of dolomite and calcite precipitated subsequently are, according to this hypothesis, largely dependent on the atmospheric CC>2 pressure, which is supposed to have increased in the Precambrian and to have dropped in the Palaeozoic.

Thus the high dolomite content of the

older shales and other sediments is thought to be of primary origin. The quality and quantity of the Proterozoic river discharge is claimed by Ronov to have compensated the dolomite formation.

The magnitude of the amounts of calcium to be stored can also be estimated as follows:

in the post-Cambrian weathering products

of the crust (with a probable

ratio of 1#5-1«7)> up to 1.7 times

more CaO than MgO will have been accumulated in the sediments. (1.7 x 83 x 10 1 5 = 141 x 10 1 5 ton CaO; see table 1).

Actually the

younger sediments, more calcium-rich than the source rocks would 15 indicate, contain 173 x 10 ton CaO. The difference, when stored in a pre-Ordovician ocean of similar volume as today would cause an approximately sixty-fold concentration of calcium in the old ocean. The calcium concentration, according to this estimate and also Ronov *s lesser estimate, would require completely unrealistic CO2 pressures and in this respect the storage aspect of Ronov1s hypothesis has to be rejected. Postulated post-depositional change in the chemistry of sediments Any such a hypothesis must account for a possible source for potassium and magnesium (to be introduced into the older shales and carbonate rocks) and also a source for the increased calcium content of the younger sediments.

Hypothetically such a source can be found

D22

in buried sandstones. According to Ronov, Mikhailovskaya and Solodkova (1963) the deep seated sandstones are depleted of several minerals (e.g. glauconites, amphiboles, pyroxenes, carbonates), which can be a source of potassium, magnesium and calcium, and these deficiencies are of secondary origin.

In spite of the weak chemical

expression of the trends the mineralogical changes are spectacular enough to accept the following as a working hypothesis:

Potassium

and magnesium are transferred to the shales and carbonate rocks, where they are fixed, whilst calcium migrates to higher levels during the continuing consolidation process of the sediments.

However the envisaged component transfer from the sandstones to the shales and carbonate rocks is insufficient in magnitude, because of the small proportion of sandstones in the stratigraphic record* To obtain satisfactory masses it is necessary to take also the volcanogenic sediments into account.

It is known from microscopic

evidence that the volcanogenic sediments are leached considerably underground, but chemical information is largely leaching. The suggested hypothesis of chemical readjustment within the sedimentary pile explains several problems of the sedimentary petrology in a much simpler way than Ronov1s hypothesis, whilst still leaving the way open to primary differences in sedimentation. Table 1

Relative absolute differences in K20, CaO, MgO and C02 content between old and young non-volcanogenic sediments (van Moort, 1972). B

Upper Proterozoic and Cambrian

Post-Cambrian non-volcanogenic

non-volcanogenic sediments

sediments

940-500 million years B.P.

500-0 million years B,P, 15 Total mass 1,632 x 10 J ton

Total mass

424 X 10 15 ton

k2O

2.9%

12 x 10 15 ton

CaO

4*2$

24 x 10 15 ton

MgO

3.6$

<cL 5*7^ CaO/MgO = 1 . 2

CO,

K20

1.7$

28 x 10 15 ton

16 x 10 15 ton

CaO MgO

10.6% 5.11o

173 x 10 15 ton

29 x 10 15 ton

C02

12.6% ,

CaO/MgO

2.1

83 x 10 15 ton 206 x 10 15 ton

3*2-3 References Pettijohn, F.J. (1941)• Persistence of heavy minerals in geologic age, J. Geol. 49:

610-625.

Ronov, A*B. and Migdisov, A.A. (1971)•

Geochemical history of the

crystalline basement and the sedimentary cover of the Russian and North American Platforms,

Sedimentology 16:

137-185,

Ronov, A.B., Mikhailovskaya, M.S. and Solodkova, I.I. (1963).

"Evolution

of the chemical and mineralogical composition of sandy deposits.t! In A.P. Vinogradov (ed.) Chemistry of the Earth's Crust, 1. Akad. Nauk. S.S.S.R.

p. 201-212 (in Russian).

Van Moort, J.C. (1972). The magnesium and calcium content of sediments in general and of pelites in particular as a function of geological age and degree of metamorphism.

Chem. Geol. (in press).

Weaver, C.E. (19^7)• Potassium, illite and the ocean.

Geochim. Cosmo-

chim. Acta J1: 2182-2196. THE MOBILITY OF CERTAIN TRACE ELEMENTS DURING- DIAGENESIS P.J. Davies and B. Radke Department of Geology, University of Queensland The concentration, present site position and mobility of strontium, iron and manganese in limestones from three Jurassic sequences in Europe and two Carboniferous sequences in Central Queensland, have been studied. The sediments are predominantly reefal and subreefal shoal facies. Elemental analysis was conducted by means of optical spectrograph, x-ray fluorescence, atomic absorption, and electron probe micro-analysis. Mean results of the analysis are shown in Table 1. The paths of diagenesis shown by the * rocks are shown in Pig. 1.

STRONTIUM a.

Concentration In the Upper Jurassic (Corallian) rocks of Europe, the highest

levels of strontium occur in the Oxford region (up to 3000 p*p*m.),

D24 intermediate levels occur in the Yorkshire region (up to 2000 p.p.m.), and very low levels occur in the rocks from Northern Switzerland (<300 p.p.m.).

Thus, marked regional variations are evident®

the Oxford and Yorkshire sequences, strontium increases upwards from the subreef into the reef horizons. Highest strontium levels are found in the Solenopora (algal) limestones which form the top of reef. At Oxford the reef:subreef ratio: is almost 2:1.

Yery little

variation occurs between reef and subreef in Northern Switzerland. At Cannindah in Central Queensland, strontium levels in the Carboniferous rocks are low (250 p.p.m.).

Very little variation occurs.

At Ubobo, in Central Queensland, strontium levels are lower (mean 123 p«p.m«) than at Cannindah.

Little variation is again seen.

b. Mobility Recent carbonate sediments are composed of high strontium aragonite. Assuming the uniformitarian principle, then it may be concluded that the ancient rocks studied exhibit a loss of strontium which has occurred during diagenesis.

Further, it is obvious that

large inter- and intra-regional variations of strontium occur, even within rocks of the same age.

Therefore- it is incorrect to suggest

that certain periods of geological time are characterized by rocks with certain levels of strontium.

It is however likely that Cambrian

rocks will contain less strontium than Jurassic rocks because of the greater length of time during which diagenesis has been operative®

Assuming that the majority of strontium is originally located in aragonite, then the nettloss of strontium from rocks must involve two processes, with resultant problems:

(i) loss of strontium from

aragonite i.e. the mechanism of the aragonite to calcite transformation, and, (ii) movement of strontium out of the rocks to produce a nett loss i.e. the nature of the diagenetic environment under which strontium mobility occurs.

The evidence from each locality is examined in the

light of these two problems.

Europe A comparison of the geochemistry and the diagenetic history of the rocks allows the following speculations

D25 1. Many samples show abundant finely preserved ooliths in rocks containing in excess of 1000 p,p,m, strontium* Further, this level of strontium is high, even though the percentage of ooliths is relatively low compared with modern oolitic sands. 2, High strontium levels correlate with high solution and low recrystallization,

Low strontium correlates with

high recrystallization and low solution, 3*

The present strontium levels are not likely the result of precipitation from solutions of the type described by Kinsman (19^9)> -2 of 0,9 x 10

solutions with a (Sr/Ca)T ratio

, Many reasons suggest this®

Ficst, most

of the Jurassic carbonates studied would require the strontium to have been precipitated from solution with a (Sr/Ca)JTJ ratio far in excess of this value*

Secondly,

the strontium concentrations are much higher in the intraclastic sediments than in the oolitic sediments, in spite of the fact that the oolites probably had a higher original strontium concentration®

Thirdly, the strontium

levels in many of the Jurassic carbonates are very high for pre Pleistocene sediments, even though the percentage ooliths is not high compared with present day oolitic sands.

Fourthly, the Oxford and Yorkshire rocks exhibit

evidence of a major solution phase after the precipitation of the first formed cement.

Depletion of some strontium

must have occurred at this time.

Therefore, the present

high values are a reflection of what must have been even higher levels in the period prior to the precipitation of the second phase cement. The timing of the mobilization and removal of strontium is different in the Oxford/Yorkshire and Swiss sediments. In all three areas, two possible flushing periods for strontium have occurred i,e, prior to Cement 1 (early), and prior to Cement 2 (late).

At Oxford and Yorkshire, the present high strontium values indicate that much strontium was not lost, probably because it was not available, i,e, the aragonite

calcite transformation had not

occurred, and aragonite existed after the time of precipitation of phase 2 cement.

D26 In the rocks from Northern Switzerland, strontium is at present low*

The major loss of strontium is likely to have occurred prior

to cement phase 2, i*e* at a time of lowered pH, maximum flow transfer and maximum solution*

The rocks from Northern Switzerland are also

characterized by low strontium and high recrystallization*

It is

therefore reasonable to assume that the marked loss of strontium is associated with the recrystallization process* Cannindah - Central Queensland The following conclusions may be drawn from a statistical comparison of the diagenetic and geochemical data (Table 2):

1 * Strontium is low, and is at present situated largely within calcite cements although its original site position was probably within oolitic aragonite*

It is interesting

to note that the oolitic content of these rocks is generally much higher than, the Jurassic rocks of Britain, but is comparable to the Swiss rocks* 2*

An early stage of silicification is evident*

Higher

levels of strontium are seen in the rocks which have undergone this early silicification*

Therefore the

process of silicification has "frozen" strontium within the oolitic structure* 3*

The levels of strontium in the silicified rocks show that almost QOfo of original strontium had been lost from the aragonite structure prior to silicification*

At least some portion of the remaining 20% must have been lost during the later stages of diagenesis, suggesting that an open chemical system was operating*

Ubobo - Central Queensland The results of a diagenetic and geochemical comparison are shown in Fig* 2*

The following conclusions may be drawn.

1 * Strontium was originally housed at two locations within the sediments a* oolitic aragonite, and b* micritic matrix*

Strontium is at present located within calcite

cements and within recrystallized micrite*

D27 2. Higher than average strontium levels are the result of two factors*

a. The presence of depositional aragonitic

micrite within the sediments.

This restricts the

permeability, and acts as a break on the loss of strontium from the sediment,

b. Early silicification "freezes"

strontium within oolitic lattices.

Strontium not lost

in the aragonite inversion prior to silicification is trapped within grains.

IRON AMD MANGANESE Concentration

IRON

Oxford

8280 P<•P»m. mean (large variations).

Yorkshire

1940 P<•P•m. mean (large variations)•

Switzerland Cannindah

158 P<•P•m. mean (small variations). 1000 P<• P.m. mean (small variations).

Ubobo

2410 P •P•m. mean (large variations).

Oxford

CO —J

Yorkshire Switzerland

P • P•m. mean (large variations). 45 P • P•m. mean (small variations)•

Cannindah

14 P •P•m. mean (small variations). 1000 P •P•m. mean (large variations).

Ubobo

1620 P • P.m. mean (large variations).

b. Yalence State Both iron and manganese occur in the divalent state. c. Present Position The present position of iron is within the carbonate lattice of the second stage cement (Fig. 1). Mossbauer spectrographic studies, and staining methods (Davies and Till 1968, and Davies 1971) shows that iron substitutes for calcium in the octahedral site of the calcite lattice. Manganese probably occupies similar sites to iron.

Statistical Relations The statistical relationships are shown in Pigs 2 and 3 and

Table 2.

In all the areas studied a good iron/manganese relationship

may be seen.

In two cases (Cannindah and Ubobo) the precision of

the analytical method is such that it may be concluded from the regression line that iron is present as an original constituent

D28 of the rock and as a fraction introduced during diagenesis.

the three other areas studied the same relationship was suspected, but the precision of the analytical method was such that it did not allow this interpretation.

J. Source

Oxford

External

Yorkshire

External

Switzerland

Indeterminable

Cannindah

Internal

Ubobo

External

Divalent iron substituting for calcium serves as an important diagenetic indicator.

It is likely that divalent iron was introduced

into the pore solutions after a period of solution, during burial of the sediment.

Sediments which form under a regressive depositional

environment often show a diagenetic history involving two phases of cementation, the second of which is composed of ferroan calcite. Sediments which have had a transgressive depositional history are often characterized by cements composed almost entirely of ferroan calcite.

Thus the diagenetic history is to some degree dependent on

the depositional history.

f,i i DEPOSITION

PZ 8(b)

Fi| I

POSmVE

CORRELATION

NEGATIVE CORRELATION Correlation*

are

obovo th«

9 5 % »lflfilflcance lovol.

D28(c)

12-

Fe (ppm) 7 x 10*

100

300 JFt (ppm)

• ft

o o

200

100

Mn(ftpm)-

# ^ U L

L X 8 9 10 • Mn (ppm) X10*

X 13

J 15

1 16

I 17

I 18

I 19

I 20

Sorting

Roundness

Iron

Manganese

Lead

Copper

Lead Zinc Insolubles

x + +

Intraclasts j Ooliths

o o o

Crinoids

Compaction

Grain Size Silicification

+ x~ o

+ TABLE Jt

Correlation half matrix*

99.9/o

level

itZTJr

Silicification

rstrontiim

Grain Size

Micrit©

Sorting

"Soundness

Compaction

Bioclasts

Micrite

Crinoids

x~ o~ o"

Zinc

Bioclasts

Iron

Ooliths

Intraclasts

Copper

Insolubles

Manganese

DZ $ W

1529

PROBLEMS-WITH THE GENESIS OF DAWSONITE IN THE SYDNEY BASIN, N.SJ, F#C» Loughnan and R. Goldbery, School of Applied Geology, University of New South Wales

Dawsonite (NaAlCOjCOH^) is a rare mineral which to date, has been found as a constituent of sedimentary rocks in only three localities, viz, in Tanzania, in the Wyoming-Utah-Colorado area of UeS*A0 and in the Sydney Basin of N*S*W«

Near Olduvai Gorge, Tanzania, dawsonite is prevalent in arid, sodarich and highly alkaline soils of Pleistocene age where it is associated with zeolites, principally phillipsite and natrolite, and is believed to have formed through reaction of nepheline and trachytic glass with sodium carbonate solutions•

According to Hay (1963)1 the soils have pH values

ranging from 9*5 - 10»6.

In the Wyoming-Utah-Colorado area, dawsonite is widely disseminated in the extensive varved dolomites and oil shales that comprise much of the Eocene Green River Formation (Smith and Milton, 1966).

Analcite, other

zeolites, various carbonates of sodium, magnesium and calcium and an array of rare authigenic silicates accompany the dawsonite, and again there is every indication that the assemblages represent the products of reaction of soda-rich waters with volcanic glass at high pH values.

The presence of

dawsonite in the Green River Formation has provoked much interest for although the mineral is rarely detectable in hand specimen, it is nevertheless, sufficiently abundant to warrant consideration as a potential ore or aluminium.

In fact, according,to Hite and Dyni (1967), one square

mile of these deposits may yield more aluminium than that obtainable from the total reserves of bauxite in the United States

Dawsonite is particularly prevalent in the Sydney Basin despite the fact that there is a little beyond the presence of the mineral to indicate that unusual soda concentrations or high pH values prevailed during deposition of the sediments,,

The mineral is present in both marine and

coal measure rocks of the Permian and extends into the overlying nonmarine Triassic and, although it appears restricted to the northern two-thirds of the basin, it is nevertheless probably more abundant than in the Green River Formation*

The Sydney Basin appears to be the

only known area of occurrence of dawsonite in marine beds0

It commonly

occurs as a cement in sandstones but in the marine Berry Formation in the

D30 western parts of the basin* it forms pseudomorphs after coarse crystals of glauberite (Na2S0£fCaS0if) and is also present as spherulitic masses in lenticular beds of dolomite® Pseudomorphs of single and twinned crystals of glauberite, known generally as "glendonites", are relatively abundant in parts of the marine formations of the Sydney Basin*

They range up to a foot or more

in length and mostly are composed of calcite although dolomite, gypsum — - and dawsonite-bearing glendonites have been recorded®

They have no

specific orientation within the sediment which would suggest that their development took place below the surface of the mud in a tidal or supratidal environment*

It is not known whether the dawsonite replaced

the glauberite prior to consolidation of the host sediment nor is the mechanism of the replacement understood*

In fact, it is difficult to

envisage a chemical environment that on one hand promotes solution of calcium and sulphate ions and on the other, causes precipitation of sodium® The spherulitic masses in the dolomite range up to 2 cm in diameter and appear disconnected*

They are aligned with the general direction

of bedding which tends to suggest that their development was essentially contemporaneous with that .of the dolomite.

In places the dawsonite

masses have been replaced by kaolinite and calcite while nordstrandite is commonly associated with the dawsonite and tends to occur as subhedral grains around the margins of the spherulites*

Nordstrandite is

a polymorph of gibbsite and was originally synthesised by Nordstrand et al* in 1956*

Subsequently, it has been found in two natural

occurrences, both in limestones, from Guan (Hathaway and Schlanger, 1962) and Sarawak (Wall et al*, 1962)*

Although the origin of the

Guan and Sarawak.occurrences is not known, there seems little doubt that the nordstrandite in the Sydney Basin has formed from dawsonite through the loss of soda*

Dawsoni t e NaAlC05(0H)2 + H2 p

Nordstrandi t e ^

Al(OH)3 + NaHCC>3

A recent petrographic and X-ray examination of bore core samples obtained from the lower 1700 feet or so of the Singleton Goal Measures in the Hunter Valley has shown that dawsonite is common throughout much of the sequence®

Moreover, the mineral is not restricted to any

D51

particular rock type but occurs alike in deltaic mudstones, siltstones and coarser sediments as well as in fluvial sandstones and conglomerates o Furthermore, the dawsonite does not appear to be associated with a particular clay mineral.

Near the base of the measures

illite accompanied by mixed layered clay minerals, predominates to the virtual exclusion of kaolinite whereas, near the top of the sequence, the relative abundance of the two clay minerals is reversed and kaolinite forms the bulk of the matrix.

The presence of kaolinite,

a mineral generally believed to form under acid conditions, in association* with dawsonite is of interest.

Feldspars are a common constituent of these coal measure sediments and in the coarser grained rocks evidence of alteration of the feldspars to dawsonite can be seen under the microscope.

Nevertheless, it would

be difficult to attribute all the dawsonite in these sediments to this origin,

Dawsonite is also present in the Singleton Coal Measures as joint and fissure infilling and, with other carbonates, it is prevalent as cleat-facings in the coal.

The occurrence of the mineral in this form

indicates deposition from circulating groundwaters, presumably of high pH values, and it would appear that at least some of the dawsonite is of epigenetic origin.

Dawsonite was initially synthesised by Bader (1938), who passed C0 2 through a solution of sodium hydroxide and sodium aluminate, and later Chesworth (1971) who immersed aluminium amalgam in a saturated solution of sodium carbonate at 25°C.

According to Chesworth, a pH

value approaching 12 is necessary for the formation of dawsonite. this is correct, it is not surprising that the mineral is rare.

What

is difficult to comprehend however, is the presence of the mineral in sediments of the Sydney Basin where evidence of unusual pH conditions is lacking and minerals such as trona Na^CO^NaHCO-^HgO and nahcolite NaHCO^, which elsewhere are intimately "associated with dawsonite, are absent.

Since silica is readily mobilised at pH values above 9-5

(Krauskopf, 1959), silicates such as analcite, rather than the carbonate or sodium and aluminium would be expected in the silicarich sandstones.

However, to date analcite and other zeolites have

not been found with dawsonite in the Sydney Basin.

D32 Recently, Grubb (1970) has recorded the presence of a small amount of dawsonite in detrital sediments derived from the Gove bauxite deposit.

The dawsonite occurs with gibbsite, kaolinite, hematite,

goethite, boehmite, quartz and phillipsite and overlies carbonate deposits in the intertidal zone. Transported bauxites are known to occur in parts of the Permian-Triassic succession of the Sydney Basin but as yet, dawsonite has not been found in these deposits. REFERENCES

Bader, E. (1938) - Neues Jahrb. Mineral. Teil,

kk9-k65»

Chesworth, W. (1971) - Nature Physical Sci., 231 Goldbery, R and Loughnan, F.C. (1970) - Amer. Mineral., 55, ^77-^90 Grubb, P.L. C. (1970) - Mineral Deposita, 5, 2^8-272 Hathaway, J.G. and Schlanger, S.O. (1962) - Nature, 196, 265-266 Hay, R.L. (1963) - Geol. Soc. Amer. Bull., 7^, 1281-1286' Hite, R.J. and Dyni, J.R. (1967) - Q. Journ. Colorado School of Mines. 62, 25-38 Krauskopf, K.B. (1959) - Soc. Econ. Palaeont. & Mineral Symp., 7, Nicholas, E. and Ozimic, S. (1970) - Bur. Min. Res. Record 1970/7 Nordstrand, R. A. van, Hettinger, W.P. & Keith, C.D. (1956) - Nature, 177, 713-714 Smith, J. W. and Milton, C. (1966) - Econ. Geol., 61, 1029-10^2 Wall, J.R.D., Wolfenden, E.B., Beard, E.H., and Dean, T. (1962) - Nature 196, 26^-265. BIOTITE WEATHERING RATE DEPENDS ON THE OXIDATION STATE OF OCTAHEDRAL IRON R.J. Gilkes and R. Young* Department of Soil Science and Plant Nutrition, University of Western Australia, Perth Much of the agricultural land in Western Australia has a soil cover developed from granite or related rock types.

Large areas of this

region have soils that are deficient in nutrient trace elements leading to the widespread application of Cu, Zn, Mo and other fertilisers.

general terms these de'ficiences may be ascribed to the low content of some nutrient ions in acid rocks and to the maturity of the soils of this region.

These soils include laterite and bauxite deep weathering

profiles of depths in excess of 50m as well as more recent soils

D33 developed on various parts of truncated laterites and bauxites.

The detailed mechanisms by which the processes of weathering (leaching, oxidation, etc.) remove trace elements from weathered rocks or render them unavailable to plants are poorly understood.

In this

investigation the influence of oxidation state of octahedral iron in biotite (on the release of structural ions during weathering) has been determined.

An understanding of this process is of considerable agricultural significance since biotite is an important source of K, Mg, Mn and Zn in some Western Australian soils. A biotite specimen from London, Ontario supplied by Wards was used for the laboratory studies described below.

The structural

formula of the mineral is:

(Si

2! 9 1 Al l!o5 Ti o!o4 ) I I -1.05 C+.O] \

I . [2.83]

—J

+1«02

+0.03 '

[1.01]

-1.02

A series of partially oxidised specimens was obtained by shaking the mineral in saturated bromine water for periods up to six weeks. X-ray diffraction, electron-optical and chemical analyses of the products revealed that apart from minor dissolution and/or ejection of structural cations to preserve electrical neutrality the oxidised minerals were otherwise identical to the original biotite.

The oxidised biotites of different ferrous iron contents (but constant total iron) were artificially weathered in frequently changed . NaCl and HC1 solutions.

Some results are shown in figures 1 s^nd 2 which

demonstrate the marked increase in stability acquired by oxidised biotites.

The highly oxidised biotites act like dioctahedral micas

with respect to their low levels of potassium release in NaCl solutions. This analogy is consistent with measurements made on infra-red absorption spectra of these specimens.

The vacancy peak at 3520rrr>' which is

a measure of the dioctahedral nature of the oxidised biotite (figure 3)

064 increases in intensity with increasing ferric iron content.

Tfye con-

centration of vacancy sites is linearly related to ferric iron content

and inversely to the potassium released by ten NaCl extractions ( figure k). These results and others for structural ions in biotite indicate that the oxidation/reduction environment during biotite weathering will greatly influence the stability of this mineral and hence the distribution of some micronutrient elements in soils. •/•Fe3*

230 26

K 2-2

5 CD

4-46

6-54

O 14 >

730

9-71

: 06

11.31

02.

•/• Mg REMOVED FROM BIOTITE

0 1 2 3 4 5 6 7 8 9 10 EXTRACTION NUMBER Fig 1 Removal of K by 1M NaCl extraction from <lOp oxidised biotite.

44 6 654 7.80 260 971

100

80 60

1131

40 20

34 5 6 7 8 9 1 0 EXTRACTION NUMBER

Fig 2 Dissolution of Mg from oxidised biotite by 0-1 M HO.

WAVE HUMBERfcrrT') Fig 3 Infra red absorption ipectra f o r oxidised <lOp' biotites in KBr {}isc;s heatecf at 3501:.

;'4

a 5 FfiJ

Fig 4 The variation of I.R. absorption a t s a c i a t e * with

octahedral vacancies in <iQi bkjtite as a

function ot

octahedral f e r r i c iron cjontent.

EVOLUTIONARY TRENDS IN RESIDUAL BAUXITE DEPOSITS P.L.C. Grubb, Institute of Mining Research, University, Salisbury

Evolutionary trends in residual bauxite deposits Most bauxite being products of intense tropical leaching are residual in origin, yet extensive horizons of detrital bauxite can be easily demonstrated and there is also evidence to suggest that some bauxites (or part thereof) have been chemically precipitated.

assume however, that all residual bauxites are fundamentally alike is untrue and instead it is becoming increasingly clear that these may be subdivided into two main groups:

those which are associated with

peneplain land surfaces and those located in more rugged hilly terrain. This somewhat arbitrary distinction goes a lot further than that as is reflected by the relative situations of these bauxite types, their profile characteristics (including thickness and textural properties), and finally, their composition«

To begin with, peneplain bauxites, which are the predominant deposit type in Australia, are characteristically confined to coastal regions«

Here their formation seems to have been closely defined by

tectonic warping (or faulting) with corresponding slight changes in sea levels, the end result being that the bauxite deposits themselves tend to be located along the hinge zones of successive (land) erosion cycles.

Consequently, apart from buried horizons, these deposits are

probably the best preserved residual bauxites in geological history, being least affected by subsequent physical planation.

Added to this,

the bauxite horizons themselves are fairly constant in thickness seldom exceeding fifteen feet in depth, while their basal contacts show comparatively little vertical variation.

Significantly,

peneplain bauxites tend to derive mainly from sedimentary type source rocks and frequently possess fairly permeable seat rocks. These factors tend to facilitate a fairly advanced form of textural evolution culminating in a fine pisolitic and boehmite-rich 'A* horizon*

Con-

sequently in Australian deposits a well defined A, B, C and D zonation can be readily established in purely residual profiles, (the validity of such zonal distinction being amply demonstrated by sizing and mineralogical analyses).

From this it is apparent that depending on

the relative thicknesses of the above zones, an approximate gauge of

D36 profile maturity exists.

For instance in the Weipa profiles the fAf

zone is predominant while the fBf and' fCf horizons are vestigial,

may be less than fortuitous therefore that of all the Australian bauxite deposits, it is the upper levels of the Weipa profiles which are so enriched in boehmite,

Quite a different situation is encountered in volcanic upland type bauxites*

These, as the name implies, derive mainly from volcanic

or igneous source rocks and are characteristically located in more rugged rejuvenated terraines - often at considerable elevations above sea level and commonly situated some distance from the coast,

In the field

these bauxites are characterised by a remarkable depth of weathering some

bauxite horizons being in excess of forty feet in depth.

Basal

contacts with the underlying mottled clay horizon are highly irregular making accurate ore reserve calculations a somewhat hazardous task® This feature is particularly evident in the more mountaineous bauxites of Mt Mian je (Malawi), Penhalonga (Rhodesia) and Mocambique, but less conspicuous in the more coastal bauxites of Johore, Malaya and Sarawak, Textural differentiation within individual profiles is characteristically rudimentary and for the most part shows abundant evidence of direct bauxitisation of the igneous source rock in the form of extensive pseudomorphic textures.

Thus only in pre-Quaternary deposits,

where conditions have also changed as apparent in Bihar, India, is there some tendency towards texturalevolution similar to that encountered in peneplain-type deposits.

As a result of their great depth of weather-

ing, immature textural relations and topographic situation, volcanictype bauxite deposits are remarkably susceptible to subsequent erosion, and it is probably for this reason that their pre-Quaternary members are preserved only by burial (as in Gippsland) or as rubble horizons (as on Mitchell Plateau, Western Australia),

Assuming that such a broad distinction is acceptable, this behoves some more fundamental explanation.

To achieve this, the following

discussion is limited to bauxite deposits in Australia, Malaya, Borneo, Malawi, Mocambique and Rhodesia,

Of all these territories, only in

Australia are true peneplain-type bauxites to be found, where they are essentially pre-Quaternary in age. The"remaining bauxites are all characteristically of the volcanic type and Recent,

D37 Imperative to this investigation is a knowledge of the amount of material which was leached from a source rock to produce a bauxite horizon of known composition, and a comparison of results between peneplain and volcanic type bauxites.

Three figures can be derived

by one or two methods:

(1)

Selection of a truly in situ residual bauxite profile, and estimation of bulk and average composition of bauxite in that profile of known dimensions.

Finally calculations to determine

how much material would need to be leached from the underlying parent rock to produce that quantity and grade of bauxite. (2)

The insoluble heavy mineral residues from bauxites collected as successive .5 - 1 meter channel samples throughout the profile are plotted and corrections applied for the determined specific gravity or porosity variation.

From this it is

possible to estimate the actual degree of concentration or leaching which has taken place.

Yet here again although

very high recoveries of insoluble heavy minerals may be achieved by sodium hydroxide leaching followed by a prolonged sequence of dispersion, decantation, etc., the figures are underestimates. The reason for this is that the experimental recovery (intact) of the same insoluble heavy mineral species from fresh parent rock has proved to be impracticable.

Consequently in this

work the concentration of insolubles in bauxites is related only to that in the intermediate method and pallid clay horizons, which are themselves weathering products. The results so far from this second method are nevertheless quite significant.

In broad outline they suggest that the overall

degree of leaching in volcanic-type bauxites was considerably more intense than in the peneplain-type bauxites - even when comparing equivalent profile thicknesses.

Thus for the volcanic-type bauxite

horizons there is a concentration factor in excess of 2.2 over the intermediate clay zones, whereas in peneplain-type bauxite this figure is within the region of only 1.5. To explain this divergence it is necessary to point to the respective water table fluctuations in these two contrasting bauxite types.

D38

During the active formation of peneplain-type bauxites for instance, the water table has only a limited degree of fluctuation and is never far removed from the base of the bauxite*

On the other

hand in the typically upland volcanic-type bauxites the fluctuation levels of the water table are extremely wide, yet as these deposits were generally formed in localities where the annual rainfall exceeded 2500mm, downward leaching is at a premium.

Only in the now more coastal

bauxites therefore as in Malaysia and Gippsland, is there any evidence of upward movement of the water table with resultant alumina reprecipitation under the influence of a more seasonal climatic regime. The probable nature of this secondary enrichment is best demonstrated by the Gippsland bauxite deposits for in the basal horizons of some of these profiles almost equal amounts of gibbsite and nordstrandite may be detected together with a little scarbroite.

Apparently then a

rise in sea level relative to these residual bauxite deposits resultedin the initial precipitation of aluminium as scarbroite followed by its transformation under the influence of leaching to nordstrandite and eventually gibbsite.

Probably then this mechanism is in large

measure responsible for the high gibbsitic content, the well defined zonation, relatively constant bedrock elevation and the lower leaching figures encountered in peneplain-type bauxite.

In summary then we may distinguish two main residual bauxite types representing differing modes of formation.

On the one hand there

are the volcanic type bauxites, which are characteristic of fairly hilly country and although confined to between only 75 and 125 ft. elevations in Malaysia, are in Southern Africa located on rejuvenated plateaux remnants of more than 6000 ft. above sea level.

Profile thicknesses

in this bauxite group are extremely irregular and may involve either intermediate kaolinisation of parent rock or direct bauxitisation depending on the irregular contouring of the bedrock-weathered mantle interface.

Bauxite horizons here are commonly very thick but con-

spicuously lack any significant textural development.

As a result these

deposits are readily susceptible to rekaolinisation and physical erosion as a result of even marginally changing conditions.

This is

most strikingly apparent in the eastern districts of Rhodesia where the average annual precipitation is now between 800 and I600mrr compared with 1600 - 3200 mir. in the Mt Mlanje area of Malawi.

Rainfall

is also rather more seasonal in distribution than in the latter etrea.

D39 Consequently only in the Penhalonga district of Rhodesia and bordering Mocambique where precipitation is in the order of 2500 mnu are bauxite profiles similar to those on Mt Mlanje observedo

North and south of

this area, however, extensive rekaolinisation and strip erosion of former bauxite horizons are the general rule®

Quite different conditions are apparent in peneplain-type bauxite for as a result of their unique setting and hydrological conditions, these have envolved a mature profile textural zonation culminating in a pisolitic fA' horizon and accompanied by far higher tolerance levels to rekaolinisation or strip erosion following changing external conditions,

As a direct result of this, peneplain-type bauxite deposits

are still abundantly preserved in Australia - despite the highly seasonal climatic conditions prevailing there (especially in the Darling Ranges area of Western Australia)® A final but nevertheless significant feature of these two bauxite types is that in volcanic upland type deposits bauxitisation is highly selective in nature favouring more aluminous source rocks, such as syenit es or albitites as in Malawi, Rhodesia and Mocambique or acidic to intermediate volcanics as in Malaysia.

Peneplain-type bauxites, on

the other hand, show a wide variety of source rocks including shales, arkoses, sandstones or even basaltic detritus, but are for the most part clastic in origin®

THE DISTRIBUTION OF SUPERFICIAL DEPOSITS, SOILS AND WEATHERED MANTLES IN SOUTH-WESTERN - AUSTRALIA IN RELATION TO MINERALS EXPLORATION MeJ® Mulcahy, E® Bettenay and F®J® Hingston*C®S®IeR*0® Division of Soils, Perth The Western Australian Pre-Cambrian Shield (see Locality Plan), an area of major mineral interest, is one of low relief in which laterization, including the development of surface crusts of iron and aluminium oxides, and deep weathering resulting in the formation of kaolinite clays containing only the resistant quartzose skeleton grains, has been extensive® to any depth®

Only at its margins are streams and rivers incised'

There are only limited areas where shallow in situ soils

or outcrops occur, and those of economic significance have largely been investigated, long since®

Elsewhere streams are not competent to

remove the products of arid erosion so that there are developed

D40

extensive mantles of sands on divides, and of shallow colluvial, alluvial and aeolian sediments on valley sides and floors.

In Australia these extensive superficial deposits have until recently attracted little attention from geologists and geomorphologists despite the fact that they obscure and hide the fresher bedrock where ore bodies are to be found*

However, since Soil Scientists have

traditionally been concerned with the nature of soil parent materials they have frequently investigated their patterns of distribution and the subsequent development of soil profiles within them*

The degree

of profile differentiation, which may be expressed as depth functions of clay, carbonate, soluble salts, etc. is related to the time for which the soil parent materials have been emplaced, and therefore more strongly differentiated profiles may reflect secondary haloes due to deep seated ore bodies, while youthful profiles will probably not* Through stratigraphic and soil profile evidence a relative age sequence between deposits can be established, and this may serve as a framework in which background levels and patterns of variation for elements in each unit may be determined by appropriate sampling and analysis.

In Western Australia Soil Scientists have investigated the occurrence of weathered mantles and superficial sedimentary deposits, and subsequent soil formation, over a wide range of climatic and landscape conditions*

These studies have largely been in areas of agricultural

importance, but have also extended into the more remote pastoral districts in the north of the State*

Mapping for the Atlas of

Australian Soils (Northcote 1960-1968) has enabled experience gained during these more detailed studies to be supplemented by observations throughout the whole of the State, including areas of major minerals interest*

Thus a basis already exists for the development of soil

studies in relation to minerals exploration and this may be illustrated by reference to a number of examples largely drawn from published work of the CSIRO Division of Soils0 Examples range from relatively intact laterites and sandplain, through a range of surfaces cut by erosion and deflation into lateritic and subsequent soil profiles, to depositional sites in which a variety of colluvial, alluvial and aeolian sedimentary bodies may be found* Three examples are included*

* see Section 1

The first* of these, after Mulcahy and

D42

valley floors, and mask deep zones of lateritic weathering beneath. Outwash plains are common in the Murchison but also extend southeastwards towards the Nullarbor Plain.

Here "hardpan" occurs

extensively as a cemented layer in thickness of up to 100 feet, Lateritic profiles occur as minor areas on divides, and in a truncated form below the hardpan layer• Lakes and parnas occur most extensively in the south-east of the area mapped, but are also present as smaller areas elsewhere.

Calcar-

eous soils on lake parna are morphologically similar to soils developed in situ over basic rocks in the eastern goldfields, and here they may be so general as to mask the occurrence of rocks of economic importance. Alluvial valleys are of limited extent on the Pre-Cambrian Shield. To the north the upper reaches of the rivers are marked by calcretes which have moderate supplies of alkaline groundwaters, while further downstream the alluvial deposits are more frequently associated with saline and acid groundwaters.

Saline valley floors or salt lake chains

drain the arid interior of south-western Australia. REFERENCES

BETTENAY, E. and HINGSTON, F.J. (196*0.- Development and distribution of soils in the Merredin area, Western Australia.

Aust. J.

Soil,Res. 2: 173-86. BETTENAY, E., KEAY, J. and CHURCHWARD, H.M. (1971).- Soils adjoining the Gascoyne River near Carnarvon, Western Australia. CSIRO Aust. Div. Soils, Soils and Land Use Ser. No.51®

MULCAHY, M.J., and HINGSTON, F.J. (1961).- The development and distribution of the soils of the York-Quairading area, Western Australia, in relation to landscape evolution. CSIRO Aust. Soil Publ. No,17. NORTHCOTE, K.H. et al. (1967-8).- Atlas of Australian Soils, Sheets 1-10. CSIRO Aust., and Melbourne University Press.

D43

MORPHOLOGY AND GEOCHEMISTRY OF CHEMICALLY ALTERED ROCKS IN SOUTHWEST QUEENSLAND B.R. Senior, Bureau of Mineral Resources, Canberra Chemically altered sediments were studied in an area of 210,000 2 km in southwest Queensland. Within this area approximately half of the exposed Mesozoic sediments and a large proportion of the early Tertiary sediments, have been altered by chemical differentiation. In addition, large areas of altered rocks are downfolded below quite thick Cainozoic sediments or are mantled with superficial Quaternary cover. If the concealed altered rocks are included it can be inferred that preserved chemically differentiated sediments occupy about 75% of the area. Two major weathering phases are postulated, one in the lateCretaceous and a second in early to middle-Tertiary time, which produced three groups of chemically differentiated rocks. The first and most widespread event was the late-Cretaceous deep chemical alteration of Mesozoic sediments. The parent sediments altered during this phase are labile sandstone, siltstone, mudstone and minor limestone of the Lower to Upper Cretaceous Rolling Downs Group«

These dominantly feldspathic sediments are particularly suscep-

tible to alteration, and were kaolinized, partly silicified and ferruginized to a depth of approximately 130m. The altered profile does not have a crust and differs from a laterite in that the chemical differentiation is not arranged in regular decomposition zones. Instead the white or varicoloured kaolinitic beds are randomly interbedded with "ironstone", mottled beds and siliceous argillaceous rocks (porcellanite). Continuous core drilling into the late-Cretaceous chemically altered sequence recovered 90m of altered rock overlying a gradational interface above unaltered labile sediments. Analysis of clay minerals at approximately 3® intervals from this hole (BMR Canterbury 1) shows a sharp decrease upwards in the proportion of smectite (Montmorillbnite group) and a more gradual increase in kaolin. Chemical analysis, by contrast, show more variation from bed to bed than from bottom to the top of the altered sequence. Outcrop samples of black heavy

D44 "ironstone11 from concretions and lenses which are numerous throughout the sequence, contain between kk% and 85$ Fe^Oj, and 11% to 32$ SiC>2. Minor erosion and slight folding of the late-Cretaceous chemical weathering profile occurred prior to deposition of widespread, fluviatilef early Tertiary sediments of the Glendower Formation. The Glendower Formation is dominantly a quartzose arenaceous sequence and contrasts markedly with underlying, quartz-deficient labile sediments of the altered Cretaceous sequence® Following deposition of the Glendower Formation a second chemical differentiation phase occurred in early to middle Tertiary time, which resulted in widespread silicification. During this episode, beds of silcrete were formed in the Glendower Formation, but in areas where this unit was thin or not deposited the upper part of the late Cretaceous chemical weathering profile was re-weathered, and a hard crust or "weathered mantle11 formed. Silcretes in southwest Queensland are extensive and individual beds may be up to 5m thick. They formed by surface and sub-surface silicification of quartzose sediments. In places where the Tertiary deposits were thin (< 5m) the entire sequence may be altered to silcrete. In thicker deposits (5 to 60m) several beds of silcrete may occur separated by unaltered quartzose sandstone. Sedimentary bedding features, such as channel sands and lenticular beds, strongly controlled the precipitation of silica from groundwater, and such features are often selectively silicified. Multiple silcretes probably formed contemporaneously, with precipitation of silica controlled by vertical differences in permeability. Argillaceous beds or lenticular sand bodies bounded by less permeable sediment formed barriers to migrating siliceous solutions. At the present time silcrete beds form prominent residual cappings to many landforms and as a result they have been regarded as a remnant landsurface or "duricrust". The evidence suggests that they are not the product of surface silicification and are not necessarily duricrusts which implies silicification of a former landscape.

D431N

Prior

£ 8

2 c; C

£ 5

stream

| 6

Gascoyne

flood-plain e

ridge

Prior

•

River

Levee

Recent

Mood-plain

stream Flood-plain

ridge

Distributary P521

P519

channel j8 | P517 P 5 i 6 t e r r a c e C 1 G 2 GG

Gascovne 1

i l l

layer ' Properties Doorawarrah

of G a s c o y n e

layer

Properties

E3 £

textured <

of D o o r a w a r r a h 1

D e c r e a s i n g thickness ot layer

D e c r e a s i n g s a n d size

D e c r e a s i n g s a r d size

I n c r e a s i n g clay + silt

layer

I n c r e a s i n g clay + silt

I n c r e a s i n g salt

saline layer I n c r e a s i n g salt 0

(»

20 40

SO 100 120 140 160

chain.

A p p r o x . h o r i z o n t a l scale A p p r o x . vertical scale

80 feet

Section 3 Generalized section of the Gascoyne delta After Bettenay, Keay and Churchward (1971)

OLD PLATEAU

SPILLWAY DEPOSITS

Qe K Qd M Bg B Mg Y

Quailing Erosional Kauring Quailing Depositional Monkopen Belmunging Balkuling Malebeiling York <1 Ironstone Pallid Zone : .

Transitional Zone ,

Country Rock Soils and land surfaces at Kauring.

Section I (After Mulcahy and Hingston 1961)

Section 2 Modification of the laterized Tertiary landscape After Bettenay and Hingston

(196k)

T)k5 Thin sections of silcrete were examined and were found to consist of angular quartz clasts and rare anatase cemented within a matrix of amorphous and cryptocrystalline silica0

Detailed examination of the

quartz grains show pitting and embayment of the peripheries®

In some

grains a gradual transformation of quartz into chalcedony occurs from the core to the edge of the grain*

These features indicate that the

matrix was enriched partly at the expense of the quartzose host sedimentso

Fourteen samples of silcrete were analysed by X-ray diffraction and XRF techniques®

Only quartz and traces of anatase were identified

by X-ray diffraction0

Major oxide analyses showed the composition of

the silcretes to be remarkably similar, and they average 96% Si02 and consistently have greater than

TiO^®

Silcretes formed in the

Witteberg Series (South Africa), and on gneissic granite of the Arunta Complex (Northern Territory), were compared and were found to have very similar compositions*. Simultaneously with the formation of silcrete a weathered mantle was developing in areas which were free of Glendower Formation sediments*

In these areas the exposed chemically altered Cretaceous

sediments were again weathered and partly reworked by pedogenic processes to a depth of about 15m forming a hard crust or weathered mantle®

The weathered mantle forms prominent hill cappings with vertical escarpments*

Parent texture of the host rock is completely destroyed,

and volume changes as a result of alteration give the unit a well developed columnar structure*

In detail the rock is fine grained,

rather massive and structureless, although it is cut with a mosaic of fine fractures*

The original kaolinitic sediments are cemented and

moderately indurated with iron and silica®

The upper 2 or 3m of the

weathered mantle may contain indurated quartzose material with included silcrete cobbles and boulders®

Pisolitic rubble infilling pipe-like

structures are numerous and extend vertically into the weathered mantle*

During this second and final major weathering phase some of the opaline silica was probably re-mobilised within the late Cretaceous

m chemically altered sequence and precious opal was precipitated into a capillary and joint system inherited from the first late Cretaceous phase of chemical weathering*

Queensland precious opal deposits are

restricted to the late Cretaceous chemically altered sequence and are closely related to areas where these rocks are modified to a weathered mantle in their upper part0

Conversely precious opal does not seem to

occur where Glendower Formation sediments with silcrete overlie* LATERITIC DEEP WEATHERING OF GRANITE G. Scholz1, R.J. Gilkes2 and G.M. Dimmock^ 1 , 2 Department of Soil Science and Plant Nutrition, University of Western Australia, Perth; 3C.S.I.R.O; Division of Soils, Perth

Lateritic soils are very extensive in Western Australia covering ¥+,000 Km2 in the South-western part of the State alone. They are associated mostly with the gently undulating surface of the Great Plateau of Western Australia (Jutson, 191*0 and generally comprise profiles with large amounts of ferruginous gravel and laterite boulders in the surface overlying mottled and pallid zone clays. Several workers have described various mineralogical and chemical features of laterites in Western Australia (Turton et alf 1962; Grubb, 1966) but an intensive study of complete profiles relating changes in mineralogy during soil formation to associated changes in chemistry has not previously been undertaken. The two profiles described in this investigation were chosen as exhibiting common sequences of horizons (figure 1) and were in fact used as demostration sites during the Ninth International Congress of Soil Science in 1968. The mineralogical composition has been plotted for both profiles in the form of depth functions to provide a simple pictorial representation of laterite composition and to indicate weathering trends (figure 2). The mineral species present in these soils and the transformations whereby the parent granite alters to a surface soil rich in aluminium and iron oxides are similar to those described by other workers for laterites and bauxites from many localities. The essential difference between these laterites and local low grade bauxite deposits (Grubb, 1966) is due to the leaching of silicon from both quartz and kaolin during bauxite formation. This only occurs on a minor scale during laterite formation (figure 3)«

Changes in the abundance of sixteen elements during; laterite formation have also been determined and may be explained in terms of the relative stabilities of the mineral species present and the isomcjrphous substitutions commonly occurringin these minerals. REFERENCES Grubb, P.L.C. (1966)•

Some aspects of laterisation in Western

Australia. Roy* Soc. West. Aust. J. Jutson, J.T. (191*0.

117-12^

Bull. Geol. Surv. W. Ayst., 61.

Turton, A.G., Marsh, N.L., McKenzie, R.M. and Mulcahy, M.J, (1962). The chemistry and mineralogy of lateritic soils in the south* *

west of Western Australia.

Soil Publication No. 20,

C.S.I.R.O., Australia.

fiju*t f;

KAWUH6

prof ile

(PfSZA)

Ka UHIN6

prpf I{*.

^ARTlCLf*

1'Z£

OmuUiM

(*SSL4)

Loose irrty. Ft ymvt/ in light gr*4f land if matrix. PI STY

FTRRUJINAUN

duricruft Msft, tft/icw'th rtdt Itgrtif yittu e,l»ifi ibund irrty. ft yavtJ

Whit* jr-ifttf tilt Ifam ( tp*r*4ie

l*ry* 4»rk

rul prwy.

stfrtfthcM.

granitic- fabric evident.

I'prtcut 'u >n«tt whdt tftl/o nv rtrena br*ytn jritfi/ sift lOMrrj (ateennp. aranfc)

YaLANBEC.

cemmgrj

bictifa

flakes

be/ewZSnp.

profifi.(°553A)

iocit pitelicit, fwnq. jrsrcJ in ife//ew'th irewn tandu matrix

Meftvt'/ewish-brewn » rid tan fits riv •'{»*»*> Strnmon /*}$. fc'yra\JL

' fj * jf. " IJ GtNER.ALfZ.tO

mOKPHOLJOtY

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ptjTA.'tvT/OV,

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Figure 1: Morphology and particle size distrifcutipn for Kauring and Yalanbee profiles.

t)L7 (a) W

t.l.nt... pr.f.U.. S.m- 1U.nl,t.t„. nirHr.l.fy

Dtp+hXcm)

(A) Kaolins profile

Ocp'fh (m)

9 - Qujrtz. F - Ftldtpar K - Kaolin . 6 - Oiibsift

Oo - Gotlhiit. H - Htm*1,U

(b) YA LAN BEE Profile

> Z *r\rr> Fraction

Whet*. Soil

< I rryrrt Fraction

Figure 2: Kauring and Yalanbee profiles• Semi-quantative mineralogy expressed as functions of depth.

kaolin

Altera fion

Depth increasing

inert a s <ng

QUARTZ.

GIBBSITE

QOEHMlTE GOETHITE H.EMAT/TE

G/86SITE

SECONDARY SILICA

K.AOLIN M/XED LAYER. minerals

QUA R rz

FELDSPAR.

RIOTITE

G0 9.TH/TE

HORNBLENDE,

*tc.

Figure 3 Mineral transformations during I'&ter)tic. weathering of granite.. Full lints represent major pathways ; broken lines represent minor pathways.

Figure 3: Mineral transformations during lateritic weathering of granite. Full lines represent major pathways} r * broken lines represent minor pathways#

E1 THEORETICAL PRINCIPLES Off THE TECTONIC MAP OF NEW SOUTH WALES AT SCALE 1:1,000,000 E. Scheibner, Geological Survey of New South Wales

1. The Tectonic Map of New South Wales was compiled on the basis of plate tectonics theory using actualistic models. The rules of plate tectonics (McKenzie, 19&9) c a n

quantitatively applied only to plates

which may be observed and to most oceanic plates. However, enough is known about the interaction of plates, that, on the basis of analyses of products of plate interactions, a qualitative reconstruction of past plates can be "undertaken (Hamilton, 1969a,b;

1970; Dewey and Bird, 1970* and

others). 2. In the author!s opinion structural geology involves the study and description of primary structures and products of structural deformations in the earth1s crust. On the other hand, tectonics (tectonic geology) is a study of a succession of tectonic units in the evolution of the earth!s crust by reconstruction based on actualistic (but non-uniformitarian) concepts.

Thus structural geology is descriptive, while tectonic geology

is historical, and describes the evolution and history of structures. In the relativistic sense a tectonic unit can be visualized as a structural unit before its deformation. A tectonic map should illustrate the tectonic history, the process of crustal evolution in time, and the origin of the architecture of the earth's crust in time. A Tectonic map, besides showing the boundaries of structural units (as they are at present), should illustrate the boundaries of rock bodies belonging to various past tectonic units. The relationship of tectonic units can be illustrated in the palinspastic reconstructions - diagrammatic sections, tables and tectonograms. Names of past tectonic units (like troughs, rises, mobile zones or "geosynclines", etc.) should not appear on the face of any tectonic map compiled for present time. These past tectonic units were transformed into structural units (like fold-belts composed of synclinoria and anticlinoria, nappes, blocks, etc.). The names of structural units should be on the map, and names of tectonic units on diagrammatic palinspastic sections, tables and tectonograms.

E2 %

During the tectonic mapping it is necessary to define and to

map boundaries of structures and structural zones (anticlinoria, synclinoria, syneclises, margins of basins, nappes, thrusts, blocks with different type or intensity of deformation), faults, and any major structural discontinuities, and all important structural information related to folding, faulting, metamorphism and igneous activity. Geophysical information related to structures, depth of basement in basins, etc. should be evaluated.

It is necessary to analyse the

character of rock complexes or rock bodies and, as much as possible, to make an objective interpretation of these. Based on facies analyses, and on the character of volcanism, plutonism and metamorphism, the past ephemeral tectonic units are reconstructed and the boundaries of rock bodies belonging to different units are mapped*

The basic tectonic units

can be grouped into higher rank tectonic units by relation to the process of cratonization, i.e. as stages in the process of cratonization. These may be shown by colours, which may be chosen from the standard colour in the stratigraphical column. The basic tectonic units can be shown by a pattern in the colour of the stage. Sometimes the boundaries of tectonic units may cut the boundary of lithostratigraphic units. Tectonic mapping brings to light any inconsistencies in rock and lithostratigraphies. 4.

The basic tectpnic units are determined by tectonic realm.

The tectonic realm can be oceanic, continental, and transitional. In the transitional realm, either the crustal/mantle properties are -unevenly distributed as in the small ocean basins, or the crustal thickness is transitional between oceanic and continental, representing a certain' transitional stage in the crustal evolution. The transitional realm characterizes zones where interaction of major plates is taking place, i.e. marginal mobile zones (they are marginal to plates). Tectonic province is an area characterized by its tectonic realm and bounded by terminal deformation (orogeny), which exchanges the existing tectonic realm for another. This change is reflected in sedimentary facies, volcanism, plutonism, metamorphism, metallogeny, etc. Four progressive types of tectonic provinces can be recognized: oceanic, pre~cratonic, transitional, and epi-cratonic.

Due to

rernobilization this evolutionary sequence of tectonic provinces can be offset©

E3 Tectonic stage is a rock body comprising the tectonic province and characterizing a certain stage of development.

Usually several

tectonic stages can be recognized within a tectonic province, and these are stages of progressive cratonisation.

Basic tectonic unit is a rock body which is part of a tectonic stage originating in a certain specific, well defined area of a tectonic stage originating in a certain specific, well defined area of a tectonic province«

Because of the evolutionary character of processes of

sedimentation, volcanism, plutonism, metamorphism, and metallogeny, tectonic units develop and change during their existence-

When due to

structural changes, orogeny, crustal accretion, or simply to deposition, the changes reach such a magnitude, that a new morphotectonic form can be distinguished, this is considered as a new tectonic unit. REFERENCES DEWEY, J.F., and BIRD, J.M., 1970. Mountain belts and the new global tectonics.

J. geophys. Res., 75 04)> 2625-47•

HAMILTON, V., 1969 a. Mesozoic California and the underflow of Pacific mantle.

Geol. Soc. Amer. Bull. 80, 2409-30. 1969 b.

The volcanic central Andes - a modern model for

the Cretaceous batholiths and tectonics of western North America in Proceedings of the Andesite Conference (A.R. McBirney, ed.). Bull. Dep. Geol. Miner. Ind. St. Oregon - 65, 175-84. 1970.

Tectonic Map of Indonesia - a Progress Report.

U.S. Geol. Surv. Open File Report, 29 pp. (unpubl.). McKENZIE, D.P., 19^9• plate motions.

Speculations on the consequence and causes of

Geophys. J.R. astr. Soc., 18, 1-32.

METALLOGENESIS IN THE MAINLAND SECTION OF THE MCHLAN BELT R.G. Warren, Bureau of Mineral Resources, Canberra*

In the light of new interpretations of the Lachlan Belt in mainland Australia (cf. Scheibner, this symposium and symposium P), it is now possible also to re-appraise the distribution of mineralization within it.

The distribution of mineralization in general with respect

to a mobile region can be considered as being space-dependent in so far as there is a recognized pattern progressive from stable craton through its more mobile, "unstable zone.

The distribution of mineralization with

respect to time also has a recognized pattern evolving during the stages of deposition, !orogenesis1, and transitional tectonism. There appear to be five significant time intercepts in the development of the Lachlan Belt and hence in its metallogenic evolution. These are the early Ordovician, circa 487 m.years, in the late Ordovician, circa 415 m.years, in the late Silurian, in the mid-Devonian and in the Carboniferoust In the early Ordovician the Kanmantoo region was folded at about 487 m. years. Rocks deformed in this event crop out in the Kanmantoo region proper, in the Padthaway Ridge in South Australia, and in the Tibooburra Block of north west New South Wales.

This deformation event

introduced a spectrum of gold, copper, barite, and base metal mineralization into both the mobile belt and the mobile craton edge.

The craton from

then on must have extended as far east as White Cliffs.

The rocks of the

Parkes Platform may have also been deformed at this time, and therefore the copper deposits of Tottenham and Hermidale districts could be synorogenic copper deposits formed in the mobile zone.

(Equally, since

the deformation may have been late Ordovician, these deposits may be synorogenic but late Ordovician in age.) The rocks deformed in the late Ordovician form the largest area within the Lachlan Belt.

They may be considered in four "units - the

Ballarat Trough with the Heathcote line as its eastern margin, the Wagga-Omeo Metamorphic Belt, the Cooma-Molong Arch and the South Coast Belt. *

The pre-orogenic phase seems metallogenically "unimportant,

Presented with the permission of the Director of the Bureau.

TABLE

GENERAL

TECTONIC

UNITS

FOR T E C T O N I C

MAPS

OfFP

DMP

BASINS TRfNCHFS

(1965)

AUB01NE 0 T E M GEOSYNCLINES

MIOGE OS Y NC UN AI F U R R O W S & RIOGFS , 0 R F

(1951)

(1965)

BADGLEY

KRUMBElN 8. SLOSS

( ON I E MPOR A R Y r™trnonn*.v *°"* ° F P IEUCEOS Y N C M N F < N T R A CftA T 0 N IC P O S I T I V E CONTINENTAL t P 11 U G F OS Y NC L IN E RIFT V A L L E Y SHE IF RISF I SLOPE * Y G E 0 S Y N C I I M E S^ SENSU S IftIC T 0 B A C K OFF P iN TftAGftA T ONMC F U R R O W S G E O S Y N C l INE

(1950)

SINITZYN & P E Y V E

RESIDUAL G E O S Y N C l INE

S Y~ S ~1 F M S P OS T - 0 R OG F N IC BASIN CRATON IF T V A L L F Y S T A B l E SHELF

(1947, 1951)

KAY G

SECONDARY GE OSYNC L INE

S O F G A R A P

F EPIFUGFOSYNCLINE IN I R ACftA T ON IC G E O S Y N C l INf E It OGE OS Y NC I INE A U T O G E O S Y N C l INF T A P H R O G E O S Y N C t INE Z F U G E O S Y N C l INE AUTOGF OSYNCLINF T A P H R O G E O S Y N C L INE {'RATON M l r H T S 0

MIOGF OS Y NC t INE S

(1900)

STIILF

E \ N C I I

HAUG

0 ^

D*frct-«cn. ol M.^, Dot! ?l» |J J(j

f U GEOSYNCl INAL F U R R O W S & RIOGES

I 1

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CLASSIFICATIONS OF THE TECTONIC ELEMENTS OF DIFFERENT AUTHORS TECTONIC REALM (OB DOMAIN DF T E RM^NED B' THt UPE Of THE CRUST MAMUf L'THOS^MERE:

E5 but the synorogenic mineralization may include some of the gold deposits of western Victoria, the gold in the Parkes-Forbes line, in the Cmeo-Cargelligo zone, and in the Snowy Mountains, and minor copper in the Cooma-Molong Arch*

The intracratonic Silurian depositional troughs, the Melbourne and Gobar, contain mineralization, chiefly copper, that may be considered as being closely allied to the depositional environment.

The late

Silurian granites that cut the Wagga-Omeo Belt are in effect granites cutting the mobile craton edge, or alternatively late granites which introduced a typical molybdenum - wolfram - tin suite of mineralization in the zone from Mount Wills to Tallebung. This suite may also include fluorite and some minor copper.

The Silurian to mid-Devonian Hill End Trough, which may have been intracratonic or have involved fractures reaching to the mantle, contains mineral deposits whose metal concentration originated in the suitable sedimentary environment along the Captains Plat - Tarago

- Tuena line.

The Carboniferous late phase granites introduced gold across the southern portion of the Lachlan Belt and in the Hill End Trough - South Coast Belt.

In the east the gold is associated with pyrite but in

Victoria in the Melbourne Trough and Ballarat Trough the gold occurs with antimony, an association more typical of mobile craton edge or near craton edge of a mobile zone than the virtually stabilized craton that appears to have existed there at that time.

E6 THE METAMORPHIC M P OF AUSTRALIA: A PROGRESS REPORT T.G. Vallance, University of Sydney A metamorphic map of Australia at a scale of 1:5,000,000 is being compiled as part of a project for a World Metamorphic Map organized through the Commission for the Geological Map of the World.

It is hoped

that the metamorphic maps will form valuable complements to the tectonic map series. The major aim is to record distribution of the various metamorphic facies and facies series recognized in regionally metamorphosed terrains. For mapping metamorphic belts at the scale adopted a simplified scheme of facies has been devised (Zwart et al#, 1967)# A three-fold division into low-, intermediate-, and high-pressure facies series is used, corresponding in broad terms to Miyashiro!s (19&1) andalusite-sillimanite plus low-pressure intermediate, kyanitesillimanite, and high-pressure intermediate plus jadeite-glaucophane types, though provision is made for less-sophisticated representation of metamorphic characters in terrains at present imperfectly known. Progress in compiling the Australian map has been severely limited by the all-too-common absence of critical information regarding metamorphic mineral assemblages in published works relating to our metamorphic terrains. The problem is acute especially with regard to the older Precambrian regions. Apart from a few notable cases such as parts of the Arunta Complex in Central Australia, the Lamboo Complex in northern Western Australia and the Willyama Complex in New South Wales, Australian Precambrian terrains have not been subjected to detailed metamorphic mapping of isograds or facies.

There is an urgent need for

more careful petrological detail in works descriptive of metamorphosed areas. The pattern emerging from the compilation to date suggests a general absence of typical high-pressure and a relative abundance of low-pressure metamorphic belts in Australia.

Glaucophane is known only in association

with epidote and albite in parts of the Brisbane Metamorphics of southeastern Queensland and with lawsonite and .jadeite (but not jadeite + quartz) in tectonic blocks at Port Macquarie, New South Wales.

E7 The Palaeozoic metamorphic "belts of southern and eastern Australia are typically of low-pressure character. Metamorphic zonal features in the Kanmantoo Group of South Australia such as the appearance of andalusite + staurolite before sillimanite in prograde sequences of pelitic schists indicate a facies series approaching the transition to intermediate-pressure type and akin to that recognized by H.H. Read in the Buchan region of the northeastern Grampian Highlands, Scotland.

In eastern Australia the Palaeozoic metamorphic belts are younger and, generally, of lower-pressure character.

Thus in the Cooma district of

New South Wales and in the Wagga Metamorphic Belt of New South Wales and northeastern Victoria, the typical prograde succession involves andalusite+ K-feldspar followed by sillimanite + K-feldspar suggesting breakdown of muscovite + quartz within the aadalusit* stability field m & pressures lower than those prevailing during metamorphism in the Abukuma region of Japan.

Only rarely are phases such as almandine and staurolite reported from

these Australian occurrences but cordierite is common in both intermediate and high grade rocks. Metamorphic assemblages in rocks of the Wongwibinda Complex of the New England region, New South Wales, and in those of the Cairns - Tully district of northern Queensland, indicate higher pressures perhaps nearer those that prevailed at Abukuma than at Cooma but still within the field of the low-pressure facies series.

True regional granulite facies are restricted to the Precambrian regions of Australia where representatives of both low- and intermediatepressure two-pyroxene facies are reported, usually though by no means exclusively in association with appropriate amphibolite facies. Extremely low-pressure granulites are probably uncommon but welldocumented examples occur in the Lamboo Complex,

There two-pyroxene basic

granulites are associated with, .inter alia, calcareous rocks bearing wollastonite.

Such observations are contrasted with those cases of

Ca-rich members of the granulite facies in which calcite + quartz and dolomite in silica-poor environments remain stable (e.g. in parts of southern Eyre Peninsula and in the far north-west': of South Australia) presumably in response to greater pressures.

Information is needed

concerning the mineralogy of rocks of contrasted compositions, in particular of pelitic and Ca-rich types, coexisting with many basic granulites in Australia before their facies series affiliations can be established.

E8 In the Willyama.Complex of western New South Wales and eastern South Australia, prograde metamorphic successions transitional from amphibolite to granulite facies are well known.

The rocks have been attributed by

R.H. Vernon to Miyashirofs low-pressure intermediate group of facies series which, for the purposes of the metamorphic map, is classed with the low-pressure series.

Kyanite and staurolite, recognized in several parts of

the Broken Hill field, are regarded as products of retrograde action.

The role of retrogression is doubtless relevant to many other Precambrian metamorphic associations in Australia but at present data that would serve to resolve the problem of whether particular lower grade rocks associated with higher grade materials have prograde or retrograde status are practically non-existent.

Considerable areas of Western Australia are occupied by rocks of greenschist facies. Among these, metabasalts or greenstones commonly retain relict gross fabric characters of igneous origin.

In such cases

there is no reason to doubt that they are prograde products.

However, the

facies series to which these rocks should be assigned is far from clear at least to this author.

Perhaps low- and intermediate-pressure series

will be recognized in the greenschist terrains, as appears to be the case with amphibolite and granulite (two-pyroxene) facies areas, but greenschist facies mineral assemblages are not notably pressure-diagnostic and isolated greenschist terrains may long offer problems. The draft map is offered here for criticism and comment.

It is

clearly an imperfect document, based in part on guess-work or pure ignorance.

Corrections, additions and, indeed, any assistance to make the

map a more useful contribution will be most welcome. Finally, I should like to repeat my plea for more detailed and meaningful information in publications on metamorphic areas.

The mere use of terms like slate,

phyllite, schist, mica schist, calc-silicate rock, greenstone, amphibolite and, even, granulite is not particularly informative. REFERENCES MIYASHIRO, A., 1961. Evolution of metamorphic belts. J. Petrol., 2, 277-311.

E9 ZWART, H.J., J. CORVALAN, H.L. JAMES, A. MIYASHIRO, E.P. SAGGERSOIT, Y.S. SOBOLEV, A.P." STORAMANIAM, T.G. VALLANCE, 196?i A scheme of metamorphic facies for the cartographic representation of regional metamorphic belts. IUGS Geol. Newsletter (for 1967)» no. 2, 57-72.

E10 THE CONCEPT OF TRANSITIONAL TECTONISM IN AUSTRALIA H.F. Doutch, Bureau of Mineral Resources, Canberra*

The concept of Transitional Tectonism establishes itself when analysing the regional geology of Australia for natural tectonic domains. Many domains are immediately obvious on a geological map of the continent; they fall into two main categories, sedimentary basins and their basements, which are respectively domains of the Platform Covers and Orogenic Provinces of the new Geological Society of Australia Tectonic Map of Australia and New Guinea, 1971* Inevitably there are some natural domains which do not fit easily into any of these classifications.

In particular, this paper is concerned

with domains in eastern Australia which are made up of the Upper Devonian Lambie Group and Lower Carboniferous Drummond Group and their equivalents• Their tectonic status becomes clearer when both time and space relationships between Platforms Covers and Orogenic Provinces are investigated.

Similar time and space relationships between domains such as the Hamersley area and its Pilbara Block basement, the McArthur Basin and the Pine Creek Geosyncline, and the Amadeus Basin and the Musgrave Block suggest that the continent grew by way of a series of tectonic sequences, each consisting of Platform Cover events following on from Orogenic Province events.

One set of sequences, based on the West, North, Central

and Trans-Australian Platform Covers is obvious on the new map, and others can be derived from it. «

In eastern Australia the Permo-Triassic Galilee, Cooper, Bowen and Sydney Basins Platform Cover domains have for basement in many places the Lower Palaeozoic Orogenic Province domains of the Anakie Inlier and the Lachlan

Geosyncline.

This is a somewhat misleading statement as it

glosses over the problem of whether Upper Devonian and Lower Carboniferous events of the Tabberabberan-Kanimblan interval in this tectonic sequence are best regarded as part of Platform Cover or Orogenic Province histories, or as constituting a third type of tectonic phenomenon.

Presented with the permission of the Director of the Bureau.

E11 Within the Tabberabberan-Kanimblan interval the Upper Devonian Lambie, Catombal and Hervey Groups are commonly considered to be part of the Lachlan Geosyncline.

The equivalent Cocoparra, Mulga Downs and

Amphitheatre Groups further west can be treated similarly, but they have also been grouped together as the Darling Basin in a petroleum exploration context.

Also within the Tabberabberan-Kanimblan interval, the Lower

Carboniferous Drummond Group of the Drummond Basin is lithologically and structurally similar to the Lambie Group and its equivalents;

although the

basin has been called a miogeosyncline it is more usually thought of as a sedimentary basin in part underlying the Galilee Basin and with the Anakie Inlier as its basement.

The tectonic status of the adjacent, mostly

Upper Devonian, Adavale Basin, which lies sandwiched between parts of the Galilee and Cooper Basins and their basements, will be uncertain if analogous reasoning is used.

The ambivalence here is the result of various independent tectonic syntheses from local data, usually made within a framework of classical tectonic theory of European or North American origin.

On this basis the

tectonic status of these domains of the Tabberabberan-Kanimblan interval seems to be neither uniquely basinal nor uniquely geosynclinal, but perhaps something between the two.

The complementary approach of objectively analysing the regional geology of Australia results in recognition of families of domains with contrasting properties®

Thus, quoting from the new tectonic map reference, Platform

Cover domains are broadly characterized by 'widespread generally thin sequences of shallow water and continental sediments; basaltic sheets;

rare small plutons,

mild deformation - broad warping and block faulting1.

These characteristics in sum - as a gestalt, perhaps - contrast strongly with those of Orogenic Province domains: extensive linear troughs;

thick flysch-like. sequences in

abundant and varied volcanics and plutonics;

intense deformation and widespread metamorphism of fill*.

This is almost

exactly the contrast observed between the Galilee-Cooper-^owen-Sydney Basins and the Anakie Inlier-Lachlan Geosyncline, a contrast which is enhanced by the different sorts of tectonic histories deduced for them.

It is this complementary analytic approach which clarifies the tectonic status of domains of the Tabberabberan-Kanimblan interval. domains of the Lambie Group and Drummond Group and their equivalents

The

E12 (including the Adavale Basin) contain moderately thick fluvial and terrestrial sandstone, shale and conglomerate, redbeds "being common; rapid changes of thickness and lithology occur, and in many places deposition is molasse-like;

acid volcanics and marine rocks are part of

successions in domains in the east.

Granites intruding eastern domains

are logically a characteristic of the interval, being noneonformably below Platform Cover (e.g. the Kanimblan granites underlying the Sydney Basin).

Large broad open folds are typical, with the most intense

deformation confined to areas of faulting (cf. J. Geol. Soc. Aust* 16(1))•

In sum these characteristics are transitional between those of Platform Covers and Orogenic Provinces, as are the events proposed by local syntheses*

The total impression is that of tectonism milder than that of

the preceding Orogenic Province but more vigorous than that of the succeeding Platform Cover, and constitutes the basis of the concept of Transitional Tectonism*

The concept extends naturally to the Tabberabberan-Kanimblan domains of the Avon River and Grampian Basins of Victoria, adding cauldron phenomena to the list of characteristics, and the Bundock, Clarke River and Burdekin Basins of Queensland.

It could be applied to molasse-like deposits of

this age in Platform Cover domains in the Warburton, Amadeus and Georgina Basins (it is not so applied on the new map), but not to contemporaneous Orogenic Province deposits in the Hew England Geosyncline.

However some

domains of this age in the Yarrol Basin have similar properties to those of Transitional domains, and this may indicate that parts of the New EnglandYarrol Geosyncline developed on a reactivated Lachlan Geosyncline (equivalent) basement*

The concept has been extended to cover time-rock associations such as the 'post-tectonic1 granites and acid volcanics which immediately precede the Platform Cover deposits of the Carpentarian and Adelaidean Systems*

E13 TEE TECTONIC EVOLUTION OF AUSTRALIA K.A. Plumb, Bureau of Mineral Resources, Canberra*

The tectonic evolution of Australia will be illustrated by extracting a series of palaeotectonic maps from the new Tectonic Map of Australia and New Guinea. Emphasis is given to the factual history deducible from the geologic record rather than hypothesis of origin.

The data presented is directly available from the Tectonic Map so that some of the concepts and applications of the map will be illustrated.

Some interesting tectonic problems which still require solution and which became apparent during compilation of the map will be mentioned.

* Presented with the permission of the Director of the Bureau.

FX SOME OBSERVATIONS ON THE DEVELOPMENT OF THE TASMAN GEOSYNCLINE K.A.W. Crook, Geology Dept., A.N.U., Canberra The contrasts already noted between geosynclines of Atlantic and Pacific type (3) are probably fundamental, reflecting their different origins as ocean basins associated with spreading zones, and marginal seas associated with island arcs, respectively. The evolution of both types of geosynclines produces new continental crust, by a process which may be termed 1kratonizatidn'. This involves first the production of new lithosphere by contributions (chiefly volcanic) from simatic sources, and by contributions (chiefly sedimentary) from sialic sources.

Next this new lithosphere becomes

progressively more continental in character, as a result of tectonic, metamorphic and magmatic processes which, among other things, produce granite plutons.

Finally the new lithosphere becomes completely

stabilized, and of typical continental character, and is added to an adjoining kraton as a neo-kratonic tract (5). The volumes and proportions of simatic and sialic contributions to the new lithosphere vary as between the two types of geosynclines. The areas of new continental crust vary consequentially.

Atlantic

geosynclines typically result in narrow linear neo-kratonic tracts with a minor simatic component.

Neo-kratonic tracts formed by Pacific

geosynclines are typically more extensive lensoid areas with a major simatic component.

Pacific geosynclines are the more important

transformers of oceanic into continental crust. Suggestions have been made that sea-floor spreading models can explain the pre-kra.tonic history of Precambrian shields (9).

The

greater importance of Pacific geosynclines as transformers of oceanic into continental crust suggests that geosynclines of this type may have played

the major role.

Interpretation of structurally complex and poorly fossiliferous geosynclines tracts of all ages would be simplified by the recognition of various indicators of stages in geosynclinal evolution.

These may

be termed 'indicators of kratonization1. A simple indicator of kratonization, at least for Pacific geosynclines, is the mode of occurrence and origin of quartz in the geosyncline at various stages of its development.

The stages,

illustrated diagrammatically for the Lachlan Geosyncline, are as

F2 follows: 1. Micro-quartz stage:

This is the initial stage of accumulation in

an off-shore geosynclinal trough.

The only quartz entering the trough

from the land is a small amount of silt-sized quartz transported by wind and quartz in the clay fraction of pelagic sediments.

However in

the biogenic rocks abundant in this stage of the geosyncline microquartz (chert) is common, and may predominate.

In, this stage

essentially unmodified oceanic crust is present. 2. Autochthonous vein-quartz stage:

Deep burial of the rocks in the

trough leads to the formation of quartz veins in the chert sequence. Furthermore burial metamprphism of the volcanic components and their sedimentary derivatives may yield vein-quartz (1).

This vein-quartz

may remain in situ or, where earth movements bring it to the surface, it may be incorporated in sediments of the next stage. 3.

Stage of allogenic terrigenous quaytz:

The filling of .the trough

adjacent to the continent enables quartz sand of ultimate continental derivation to enter the off-shore trough.

It is there found in

greywackes and the interbedded lutites.

In the greywackes of this

stage

quartz makes up in excess of 15% of the framework.

In some

geosynclinal troughs it may exceed 65% of the framework (4). 4.

Stage of autochthonous plutonic quartz:

The generation of granitic

magma deep in the geosynclinal pile and its intrusion as regional aureole granites provides a new mode of occurrence for quartz, namely quartz of autochthonous plutonic origin.

A slightly later substage

of this stage is the appearance of plutonic quartz at higher levels in the pile in contact aureole granite.

This stage corresponds to

the end of flysch sedimentation in a trough and probably marks the appearance of a crust intermediate between oceanic and continental. 5.

Stage of volcanic quaytz:

With the breakthrough of granitic magma

to the highest level of the ;crust, volcanics containing appreciable quantities of volcanic quartz are developed. the appearance

This stage is marked by

of such quart2 either in volcanic in the sequence or

in their sedimentary derivatives.

The appearance of volcanic quartz

in sediments in the geosyncline will be approximately coeval with its first appearance in volcanic rocks. Stage of reworked autochthonous plutonic quartz:

This stage is

marked by the appearance in sediments of appreciable quantities of quartz derived from granites that intrude the geosynclinal pile. It corresponds to the transitional tectonic regime which follows the final disruption of a geosynclinal tract.

F3 Stage of detrltal quartz in platform-sandstones;

In this stage

quartz is very abundant, frequently forming more than 90% of the framework of sandstones.

It is of diverse provenance although much

of it is derived from the everted intruded and metamorphosed geosynclinal pile.

Its mode of occurrence as extensive platform sand sheets is

quite distinctive, and evidences completion of kratonization. Paleogeotectonicss

I have shown (4) that three major and distinct

types of greywackes can be recognised, and have suggested that these are of geotectonic significance, indicating different types of crustal discontinuities, as follows: 1. 2.

Quartz-poor greywackes (<15% quartz) indicative of island arcs. Intermediate-quartz greywackes (15-65% quartz) indicative of tectonically active continental margins.

Quartz-rich greywackes (>65% quartz) indicative of tectonically quiescent continental margins. All three types are represented in the Tasman Geosyncline, at

certain times simultaneously, and at certain places successively. Maps showing their distribution, and the inferred character and position of source areas,, have been prepared.

They are necessarily

incomplete, but suggest a complex tectonic history, for the Lachlan Geosyncline at least. Extension into Antarctica;

Close inspection of currently accepted

Gondwanaland reconstructions (2, 8) reveals certain anomalies in the fit of Australia and Antarctica.

These include (a) disjunction

between the platform,cover of the Adelaide "Geosyncline" and that of the "Ross Orogen"; (b) disjunction between the flysch sequences of the Kanmantoo Trough and the "Borchgrevink Orogen"; (c) implied abrupt termination of the Ballarat Trough between Australia and the north coast of Victoria Land; Australia

(d) paleomagnetic pole positions for

through the Paleozoic and Mesozpic that are disparate

with those of other Gondwana continents (7). A new reconstruction is proposed in which north Victoria Land adjoins the shelf south of Kangaroo Island and Tasmania adjoins the Ford Ranges of Marie Byrd Land.

This implies an extension of the

Tasman Geosyncline through the Ford Ranges (6), Horlick, Whitmore and Ellsworth Mountains and the Antarctic Peninsula to link eventually With South America.

F4 References 1.

Coombs, D.S., et al., 1959.

Craddock, C., 1970.

Geochim.Cosmochim.Acta 17, 53-107.

Geological Map of Antarctica, Plate XXIII,

Amer. Geogr.Soc., New York. 3.

Crook, K.A.W., 1969.

Crook, K.A.W., in press.

Earth Plan.Sci.Letts, 5, 429-438.

Folk, R.L., 1968.

Graywackes:

Encyclopaedia Brittanica.

Petrology of Sedimentary Rocks;

Austin, Tx,

Hemphills. 6.

Halpern, M., 1968.

McElhinny, M.W., Pers. comm.

Earth Plan.Sci.Letts 5, 159-167.

Smith, A.G. & Hallam, A. 1970.

Nature, 225 (5228), 139-144.

White, A.J.R. et al., in press.

Geol.Soc.Aust.Spec.Pubn, 3.

PLATE TECTONICS AND THE EVOLUTION OF THE TASMAN GEOSYNCLINE B.S. Oversby, Bureau of Mineral Resources, Canberra The evolution of the Palaeozoic Tasman Geosyncline in eastern Australia is interpreted in terms of plate tectonics.

A number of

related geotectonic elements can be demonstrated or inferred to have been present within the geosy.nclinal belt at different times. The areal distribution of these elements is consistent with the belt having evolved under the influence of a western continental lithosphere plate, and an eastern oceanic plate. During Cambrian time an island arc was present in Tasmania and Victoria;

northwards its course is obscured.

To the west of

the arc lay the Dundas Trough and equivalents, and the craton.

The

Dundas Trough was probably a marginal sea, the presence of ultrabasic intrusives and basic volcanics suggesting that rifting and spreading took place.

Modern island arcs are associated with subducting

oceanic plates, the surficial expression of subduction being the development of trench complexes.

The site of the Cambrian trench

in the Tasman Geosyncline is not exposed, although it might have been where the. Hill End Trough later developed. By Ordovician time volcanism in the geosyncline had shifted east, and the Canberra-Molong and Anakie highs, as well as the Hill End Trough, lay along the island arc.

Presumably the eastward shift

followed migration of the consuming margin of the oceanic plate and its trench.

Deformed abyssal plain sediments and alpinotype

serpentinites characterise trenches;

such rocks are present in the

F5 Woolomin Beds, Brisbane Metamorphics, and other units of the eastern part of the geosyncline.

In the south a marginal sea to the west of

the island arc might be represented by the Melbourne and Wagga troughs; serpentinites in these troughs suggest that some rifting and spreading took place. During Silurian time all of the Ordovician tectonic elements persisted in more or less modified form.

The greatest modifications

took place in, and adjacent to, the island arc as a result of intermittent tectonism.

Although volcanic activity continued in the

arc during early Devonian time plate subduction along the associated trench had evidently ceased following another eastward shift of the consuming plate margin.

A new volcanic arc was established on the

inactive trench during late Silurian to early Devonian time, as indicated by rocks in the Tamworth and Yarrol troughs, and in the Bowen Basin.

The medial Devonian Tabberabberan Orogeny essentially

stabilised the old arc, as well as associated elements to the west of it.

Orogeny was most intense in the southern part of the geosyncline,

possibly because the sediments in the inferred marginal sea there were deformed around rifted basement blocks. The Chillagoe Shelf and Hodgkinson Trough, in the extreme northern part of the geosyncline, are interpreted as the shelf and rise of an Atlantic-type continental margin until medial or late Devonian time. Major tectonism, and a marked increase in volcanic activity, took place during late Devonian and early Carboniferous time,- when the continental and oceanic lithosphere plates probably broke apart.

The oceanic plate began to be consumed

along a trench somewhere to the east of the present coastline, and both the Chillagoe,Shelf and Hodgkinson Basin, as well as succeeding elements, became part of an island arc.

The structurally distinctive

Broken River Embayment probably represents the continentward extension of a transform fault zone which separated a static oceanic plate to the north from a moving one in the south until medial or late Devonian time. By Carboniferous time a continuous island arc extended from the New England region, throughvthe Bowen Basin and Broken River Embayment,_ into the Hodgkinson Trough. is no longer present in Australia.

The associated trench

The evolution of the arc and other

geotectonic elements was similar to that of the preceding ones further west;, stabilisation followed the Permian Hunter-Bowen Orogeny.

F6 The relatively mild intermittent tectonism characteristic of the Tasman Geosyncline throughout much of its history is reflected in the predominance of steep structures.

It is suggested that the

inferred easterly migration of the consuming oceanic plate margin, combined with slow westward movement of the plate, weakened the effects of continental-oceanic plate interaction.

As a consequence

the Tasman Geosyncline is structurally quite different from many other Cordilleran-type belts which have been described.

: TECTONIC EVOLUTION OF THE TASMAN OROGENIC ZONE M. Solomon and J.R. Griffiths>University of Tasmania

The Paleozoic history of the Lachlan and New England orogenic provinces is reviewed in terms of plate tectonics.

Early Mesozoic

reconstructions indicate that the Paleozoic rocks of eastern Australia are part of a much larger tectonic belt which extends south into Antarctica and includes parts of New Zealand, the Campbell Plateau and Lord Howe Rise.

This complex belt formed between the Precambrian

shield of Australia and Antarctica, and the Paleozoic "Pacific Ocean11. The large area involved and the comparative lack of geological data make it difficult to render a complete synthesis.

Generalised

sections through Tasmania, western and central New South Wales and New England are used to illustrate the pattern of the tectonic evolution of the belt. It is possible to distinguish two phases of tectonic

activity,

which are geologically similar and presumably reflect similar lithosphere-plate interactions.

The first (Cambrian) is seen in

Antarctica and Tasmania and the second (Ordovician-Permian) in New South Wales and Queensland. In Antarctica and Tasmania late Proterozoic-Cambrian continental shelf and slope sediments accumulated from the west.

During the middle

Cambrian an island arc developed above a west dipping subduction zone in western Tasmania.

An extensive north-south trending continental

block was carried westwards and collided with the island arc complex in the early Ordovician.

Succeeding stable and unstable shelf

sedimentation was continuous with that in New South Wales and represents part of the second phase of tectonic activity.

F7 In western and central New South Wales, an eastward progression of foreland, shelf, continental slope and volcanic arc is recognised in the Ordovician-Devonian.

Following formation of a trench and

west dipping subductIon zone,

orthotectonic deformation occurred

along this continental margin, and syn- and post-orogenic sediments were deposited further to the east. Ordovician-Silurian sediments in New England appear to be unrelated to this activity and may have been derived from an older continental block lying well to the east and separated from mainland Australia by an ocean basin at this time.

This ocean basin became

progressively smaller and eventually filled with sediments as the block was carried westwards.

In the Devonian the block collided with

the orogenic belt complex and the basin sediments were paratectonically deformed.

Continued westward movement of the block was accompanied

by east-dipping thrusts along which serpentinite material of mantle origin was dragged up.

By the Triassic, movements in the Tasman

Orogenic Zone had virtually ceased, and a new continental margin was developing well to the east along the Norfolk Ridge and through New Zealand.

. EVOLUTION OF THE- RAVENSWOOD-LOLWORTH BLOCK; "INFLUENCE UPON*DEVONIAN TECTONISM IN NORTH-EASTERN QUEENSLAND E. Heidecker, University of Queensland The Block bears spatial relationships of genetic significance to: (i) the principal structural elements of north-eastern Queensland represented in figures 1 and 2, particularly the west-trending group; (ii)

the Burdekin, Drummond and Bowen Basins shown in Fig 2

terminating against the Block or following structures defining its margins; (iii)

igneous activity in the Palaeozoic, Cretaceous and Late

Tertiary to Recent; (iv)

the Ravenswood - Charters Towers - Cape River Metallogenetic

Province. Analysis of the evolution of the Block in the context of the following tectonic phases depends on, and at the same time helps to develop these relationships:

F8 Phase 1 - Marginal * Basin 'Expansion (Cambrian to Ordovician) A monotonous sequence of flyschoid sediments (Cape River Beds, Charters Towers Metamorphics) accumulated in a basin which developed along the craton's margin,south of a hinge in north-eastern Queensland. The marginal basin was transitional between types represented by the Sunda Shelf of Indonesia and the Sea of Japan in that it was underlain by continental crust, albeit disrupted and thinned.

Further east lay

a volcanogeuic-carbonate facies, only remnants of which survive (the Argentine Metamorphics). Westerly transcurrent faults absorbed sinistral shear so that the craton-ocean boundary remained intact north of the hinge, whilst south of the hinge oceanic crust underthrust the continent. Irregularities in the strikes of essentially parallel transcurrent faults were probably accommodated in the asthenosphere.

In the less

mobile lithosphere, troughs with ultrabasic intrusions developed in areas of divergence, whereas the crust thickened in compressed convergence zones.

The earliest expression of the Ravenswood -

Lolworth Block was such a,zone of crustal thickening.

It resembled

transverse belts directed towards the craton from hinges between the festoons of Island Arcs bordering Asia.

Evidence of flattening is

provided by fabrics and indicated by convergence of the Inkerman Alex Hills Shear Zone in Figure 1 and other westerly zones further south. The depressed lower margin of the thickened crust melted in a region of rising geothermic flux.

The acid Windsor Ranges Volcanics

shown in Figure 1 accumulated along the belt of transcurrent faults. In a similar fashion the Trans-Mexico Neovolcanic Plateau follows onshore continuations of the Clarion Fracture Zone, a prominent feature of the East Pacific sea floor. Phase 2 - Plutonism (Middle Ordovician to Late Silurian) The pile of sediments was thick enough for granite formation in its lower portions once the usually hijgh geothermic gradients of marginal basins were established.

Synorogenic granites rose to

shallower levels below a broad terrain undergoing high temperature low pressure metamorphism similar to that of the Abukuma Highlands.

F9 A mobile welt suffering sinistral shear on westerly zones yielded by folding with NNW axial-plane foliation.

Older strongly

overprinted trends in the cratonic infrastructure also rotated towards the WNW. Tectonic activity and heat flow waned. Mobility contracted to the westerly transcurrent fault zones, which suffered cataclasis and hydrothermal activity.

Transcurrent movements then ceased, and

postorogenic plutons intruded the westerly fault zones in the Late Silurian. Phase 3 - Cratoni^satlon v ~ (Late Silurian to Middle Devonian) Greywacke stripped off the deformed welt developed in Phase 2 accumulated in the Kangaroo Hills Deep.

This basin developed across

what had been the northern limit of the mobile zone bordering a marine shelf further north.

The shallow granite core of the welt was

extensively exposed in the Lower Devonian. Erosion and the final collapse of thermal disturbances in the mantle in the Middle Devonian reduced the region to a monotonous peneplain fringed by broad shelves. . Deep oxidation of an arid land surface, on which primitive plants failed to gain a foothold, produced a red felspathic mantle (the Big Bend Arkose).

extraordinarily prolific coralline fauna (Burdekin Formation and limestones of the Broken River Embayment) flourished on the broad shelves, stimulated by severe wave action. As the arid peneplain provided very little mud., this wave action did not stir up pollutants which would have inhibited the benthonic fauna. Phase 4 - Taphrogenesis (Upper Devonian to Lower Carboniferous) The stable carbonate shelf north of the Ravenswood - Lolworth Block contrasted with areas to the south where tectonic instability increased during the Middle Devonian.

Andesite flows and tuffs

are interbedded with limestone beds (Douglas Creek Limestone Theresa Creek Volcanics, Dunstable Volcanics, Capella Creek Beds). This instability was associated with a new cycle of fragmentation, particularly in the Yarrol Basin, As heat flow increased, granites were emplaced at the Middle-Upper Devonian boundary (Retreat Granite,

F10 Mount Morgan Tonalite in Figure 2).

Like the Miocene granites of

Asia's Pacific margin, these granites, though "Post-orogenic" in appearance, actually ushered in a cycle of marginal basin development. During this stage the Anakie High rose between NNW faults (trend group (iii) Figure 1). A marine incursion deep onto the craton was cut off by the High and evaporites (Etonvale Formation) accumulated in the Adavale Basin. The Ravenswood - Lolworth Block resisted northward propagation of the Anakie High, the Drummond Basin, and probably the Yarrol Basin.

It acted as a thick, obdurate, transverse crustal segment.

Although elements to the south could not spread north, they continued to spread eastwards by sinistral shear on reactivated westerly faults following the Ravenswood - Lolworth Block.

Once

disengaged, along these faults, thickened sections of the Block rose isostatically.

The deep mantle of oxidized debris was rapidly

stripped of these areas, choking the Burdekin Basin with red-beds of the Dotswood Formation which accumulated in much the same setting as the Buckabie Formation of the Adavale Basin. Sinistral movements on the westerly zones produced NE trending tensional zones, (group (iv) Figure 1).

Swarms of porphyrite dykes

as at Charters Towers followed such structures which developed where younger intrusives lay astride the westerly fault zones.

Larger NE

trending tensional structures, the Broken River Embayment and the Oweenee Zone, were intruded by upwelling mantle material, and preserved down-faulted portions of formerly extensive shelf sediments. Eventually fractures marginal to the Anakie High propagated across the Ravenswood - Lolworth Block.

A line of porphyrite dykes

and lode systems followed the western structure through Charters Towers, north into, the Dalrymple Giraben (Figure 1).

The fractures

on the eastern margin of the Anakie High extended north as the Ravenswood Fracture Zone (Figure 2).

Considerable adjustment between

blocks ensued. Uplifted segments of the Fanning Group were stripped to give the Deadman Gully Conglomerate.

Folding in a Germanotype Tectonic.

Setting affected recently-intruded porphyrite dykes and flows.

These

movements which post-date the Middle Devonian, but had less effect on

Fll

Upper Devonian -acid dykes (discussed below), were synchronous with the Tabberabberan Orogeny of south-eastern Australia. High geothermic gradients were established in a taphrogeniQ setting similar to that of the Basin and Range province of the United States, With melting of lower crust, extensive acid volcanicity extended northwards across the Ravenswood - Lolworth Block. Large quantities of coarse reworked acid volcanics and tuffs were added to the Dotswood Formation.

Hydrothermal solutions

deposited gold and base metals in numerous low-wangle gravity faults which earlier had shed slabs off the Ravenswood - Lolworth High during its uplift and penetration by NNW fracture zones. These lodes were cut by west-trending porphyrite dykes in the Lower Carboniferous as activity along the western section of the Ravenswood - Lolworth Block waned.

However further east activity

intensified climaxing in a Permian plutonic phase.

F12 THE NOWENDOC ULTRABASIC BELT AND ASSOCIATED METAMORPHIC ROCKS R.J. Gunthorpe, University of North Queensland A major structural discontinuity trending NW - S E through the Nowendoc district in the southern part of the New England.Tableland separates rocks of widely differing structural style and metamorphic grade.

This discontinuity has been the site of emplacement of a belt

of ultramafic intrusions.

These consist mainly of schistose and

"brecciated" serpentinite, but a gradation into foliated, unserpentinised harzburgite occurs in one of the bodies.

Smaller pods of albitised

dolerite, rodingite and albitite are also present. Low grade metamorphic rocks on the northeastern side of this discontinuity consist of irlterlaminated cherts and,jaspers, layered micaceous schists, and metabasalts. history is apparent.

A complex metamorphic and structural

Early episodes of metamorphism show a regional

zonation with respect to a gneissic granodiorite complex approximately 20 miles to the north.

An inferred later episode of metamorphism is

characterised by the phases crossite, pumpellyite, and stilpnomelane in the low grade metabasalts adjacent to the belt of serpentinite intrusions. The rocks on the southwestern side of this discontinuity are folded but unmetamorphosed Upper Palaeozoic sediments in which bedding and other sedimentary structures are undisturbed.

A MODEL OF THE PALAEOZOIC TECTONIC HISTORY OF N.S.W. E. Scheibner, Geological Survey of N.S.W. The theoretical principles and basis (modified plate tectonics) on which the Palaeozoic tectonic history of N.S*W, was analysed are dealt with elsewhere

(Scheibner, in press), but is necessary to mention here

at least a few modifications of plate tectonics. To explain the episodic extensions (tensile stresses) at the margin of the Australian plate during Palaeozoic time, a new model for the origin of marginal seas is suggested, based on Elsasser's (1971) model of plate motion and retrograde motion of the sinking lithospheric slab at the Benioff Zone.

Tensile stresses in the lithosphere at leading

plate margin are cheated by.retrograde motion, of the Benioff Zone. Transmitted tensile stresses at the leading.,pJLgJ:e^jnajgin will t£igg§r off the mechanism of crustal separation and the coupled process of ctustal generation, i.e., either axial or non-axial sea-floor spreading

CORAL

TOWNSVIL: TOOMBA

BASALT ( R E C E N T )

GRANITE .

DRUMMOND INTRUSIVE

GROUP

Argentine

HORNTON GAP

RHYOLITE.

A C I D V O L C A N I C S . C O N G L O M E R A T E . RED SILTSTONE.AR MARINE

SEDIMENTS.

SANDStoH

LSMESTONE , ARKOSE LOfVVCWTH IGNEOUS C O M P L E X RAVENSWOOO GRANODfORITE COMPLEX CHARTERS MOUNT

TOWERS

WINDSOR

STRUCTURAL FRAMEWORK OF N.E. QUEENSLAND

M E T A M O R P H 5 C 3 C A P E RIVER BE VOLCANO

"^V.

Q U A R T Z ITE. ARGENTINE

METAMORPHJCS

Faults Joints Lodes Dykes

8chistosifcy Volcanic ^teat

* Woodstock

. HEIDEC KER

5ft. St* Michael

Hillgrove

MARBLE. A M P H I B O L I T E SCHIST GNEISS

Inferred positions of lineaments between points defined by the above structures. Numerical code indicating which of the trend groups 1 to 4 , listed below, a lineament or structural zone belongs to.

H a u g h t o n Valley

' T h e Mingete

y Dawn-V illiant \ de^System)

Blu«

Charters Towers Goldfield RAVENSWC i f f

'MMM

% C a p e River "Goldfield ^

l & J W A R G ]

STRUCTURAL TREND

PENTLAND

GROUPS

WESTERLY. F O L I A T E D , I N T E N S E L Y M Y L O N I T I S E D ZONES OF O R D O V I C I A N A G E OR OLDER, CUT BY G A R N E T A N D CARBONATE V E I N S , REACTIVATED L A T E R IN THE PALAEOZOIC W I T H AURIFEROUS VEINS A N D DYKES. C O N T R O L L E D J O I N T GORGES IN MESOZOIC SANDSTONE.' A N D Q U A T E R N A R Y V O L C A N I C VENTS. 2. W E S T N O R T H W E S T TO N.W. GMEISSJC ZONES W I T H 1. DVKES. 3. N.N.W. F A U L T S PROPAGATED PARALLEL TO THE A N A K I E HIGH IN T H E UPPER D E V O N I A N , D Y K E S , VEINS. G R A B E N S , DYKES, ELONGATE

INTRUSIVES.

jgggh&i kilometres

fil(k)

FX3 will start in fields of tensile stresses.

This is an alternative

hypothesis for the origin of marginal seas, differing from the diapiric pull^apart hypothesis of Karig (1971). On the leading edge of the advancing plate in the trench (upper and lower slope), oceanic sediments are scraped off the sinking plate, and continental rise pr frontal-arc-basin sediments accumulate to form a flysch wedgew

The flysch wedge is characterized by strong repeated

deformation and the presence of basic volcanics and ultrabasics< The oceanward rotation of axcontinental plate accounts for most structural defprmatioris in the marginal mobile zone. The model of orogenie granite derivation as proposed by Hamilton (1969) accounts for the orogenic granites; however, one. subduction zone cannot account for all the Palaeozoic granites in NVSivti

Some granites

and acid volcanics have been derived during subduction of oceanic/ crust of marginal seas, quite far from the major trench situated at the !

major plate margin.

Serpentinite belts in N.S.W. mostly represent obduction zones, rather than subduction zones.. The obduction zones were formed during quick advancement of leading edges of a continental plate, the neighbouring oceanic crust and upper mantle being encountered and up»thrust. In early Palaeozoic time the studied area was the continental marginal of the Proterozpic Australian-Antarctic Continent^haviiig been in contact with the hypothetical Palaeo-Pacific Ocean.

In the early Cambrian,

decoupling of the Palaeo-Pacific oceanic crust from the neighbouring continental crust occurred.

A system of marginal seas, island arcs and

trenches originated on the present e ate tern margin of the Australian plate. In early Cambrian in N.S;W., at the margin of the Australian plate the following tectonic units can be recognized;

Kanmantoo Marginal Sea

and its continuation the Bancannia Trough, the Mount Wright Volcanic Arc, the Gnalta Shelf separated by^ th§ Kopnenberry Fracture Zone from the White Cliffs Deeper, Terrace/and both underlain by ...the. Proterozoic Wonominta Complex

and th$. Girilambone Flysch Wedge (trench).

These tectonic units

were parts of the Kanmantoo-Tyennan Pre-Cratonic Province which was sufficiently cratonized during the Delamerian Orogeny for the Gnalta Transitional Province to be formed above it. The Kanmantoo-Tyennan Pre-Cratonic Province is represented in Victoria by the Heathcote Zone and in Tasmania by the Mount Read Volcanic

F14 Arc and Dundas Trough, and a similar situation can be observed on New Zealand (Cambrian volcanic arc), helping to position the old core of New Zealand south of Tasmania for the Cambrian.

A similar situation

possibly also existed in Queensland (comp. Heidecker, this Symposium) . During the Lower-Middle Ordovician a new strong extension occurred at the margin of the Australian plate.

A substantial part of the

Girilambone Flysch Wedge was separated from the main plate and the Wagga (-Ballast Chert) Marginal Sea was formed.

On the leading edge of the

separated Girilambone Complex and on the oceanic basement, the Molong Volcanic Rise (volcanic arc) developed. accumulated (Monaro Basin).

Eastward the new flysch wedge

Oceanward rotation of the Australian plate

caused the Benambran Orogeny.

After this orogeny new location was

established in lowermost Silurian.

This again caused a strong extension

during which three marginal basins were created, from west to east, the Cobar Trough, the Cowra Trough, and the Hill End Trough and its southern continuation the Captains Flat Trough.

The central and eastern troughs

originated by splitting of the Molong Volcanic Rise into three segments. The western segment was left attached to the Parkes Shelf, the central segment formed the Molong-Canberra Shelf, and the eastern segment contributed to the formation of the frontal arc area. During a later part of the Lower Silurian, the Quidong Orqgefly occurred.

During this orogeny high tectonic,granites were emplaced,

and the Capertee Rise formed.

Also, acid volcanism started, and orogenic

plutonism lasted throughout the Silurian and Lower-Middle Devonian.

Sedi^-

mentation in the three marginal basins continued. During the Middle and Upper Silurian a new location of the trench was established.

A new inter-arc basin developed, the Murruin Basin

east of the Capertee Rise.

A frontal volcanic arc, which developed at

the end of the Silurian, is concealed at present, but it supplied volcanic detritus into the Tamwprth Frontal Arc area.

Within this area a deeper

frontal arc trough developed, the Tamworth Trough, characterized by deepwater sedimentation and spilite volcanism.

On the upper and lower slope

the Woolomin Beds accumulated to form the new flysch wedge. During the Upper Silurian and Lower Devonian, the Australian plate rotated oceanward, Cowra Trough.

Firstly the Cobar Trough was closed and then the

The oceanic crust was subducted, but in the terminal phases

also obduction zones developed, the Coolac and Tumut Pond Serpentinite Belts.

Shelf sedimentation spread over the area of the deformed troughs,

F15 while the Hill End Trough was further filled by flysch of terrestrial •and volcanic provenance.

The volcanic island arc east of the Murruin

Basin shed quantities of volcanic debris into the Tamworth Unstable Shelf (frontal arc basin) area.

At the leading plate margin the

Woolomin Flysch Wedge was further accumulated.

The main subduction

zone accounted for the volcanic frontal arc but the acid and intermediate volcanics and extensive plutonics were derived by subduction of the oceanic crust of marginal seas and by partial fusion of accreted crust. During the Middle Devonian Tabberabberan Orogeny extensive areas of former marginal seas and rises, forming the Lachlan Pre-Cratonic Province, were structurally deformed, metamorphosed, intruded by plutons, and so cratonized.

The Tamworth Frontal arc area and the Woolomin•Flysch

Wedge were also cratonized, however to a lesser extent. Lachlan Pre-Cratonic Province the Lambie established.

Above the

Transitional Province was

The area which continued in its pre-cratonic development

might be called New England Pre-Cratonic Province, and here two tectonic units can be recognized: the Mandowa Shelf,and the Texas Unstable Shelf (frontal arc area).

The Lambie Transitional Province was composed of

the Ravendale and Hervey Terrestrial Basins on the west, and the Lambie Intramontane Depression on the east, with the shortlived Lambie Shelf during Frasnian time. During the Kanimblan Orogeny (Upper Visean) the Lachlan Fold Belt (craton) was formed.

Quick movement of the Australian plate caused

the oceanic crust and mantle of the Woolomin-Texas complex to be encountered and an obduction zone (Peel Thrust) formed.

In the marginal part

of the Lachlan Fold Belt the discordant Bathurst granites were emplaced during the Namurian.

Subsequently, subduction at the Woolomin-Texas

trench terminated.

The cratonie Woolomin-Texas complex emerged to form

a landmass (New England Arch), while to the west and south the Kullatine Shelf was formed, and further west the terrestrial-paralic Ayr Basin. Retrograde motion of the subduction zone started to cause extension in the future Sydney-Bowen Basin in the Upper Carboniferous.

While an

island arc probably developed in Queensland, a volcanic rift existed in N.S.W. In Lower Permian time new extension was expressed in formation of the Sydney-Bowen Basin, and inland epircratonic basins.

Crustal separa-

tion had cut across the older structures and the Nambucca Basin was in contact with two zones:

the Drake Shelf having the Woolomin-Texas

F16 complex, and the Yessabah Shelf having the Tamworth Group, as a basement.

It is indicated that the Nambucca Beds with some

ophiolites were laid down on oceanic crust possibly in a marginal sea and repeatedly deformed. During the Hunter Orogeny (mainly Upper Artinskian) the Carboniferous Peel Thrust (obduction tine) was strike-slip displaced in a clockwise sense.

The strike-slip movement amounted to about 150 km.

The northern section of the Woolomin-Texas Block was displaced along a series of meridionally trending strike-slip faults.

Along one of

these faults, the Demon Fault, the Demon Block moved in a clockwise sense southward squashing the Nambucca Beds against another rigid block, the Kempsey Block. zone was formed,

South of the Demon Block a minor obduction

An enormous amount of granites was derived from

the subduction zone and by fusion of crustal material.

A few granites

were emplaced during the structural deformation, while most of New England granites are post-kinematic and post-strike-slip movement. The point of "collision" migrated northward, the orogenic granite^ being derived in Permian time in N.S^W. but in Triassic time and later in Queensland. with time.

Similarly, structural deformation migrated northward :

' ?

A transitional province was established in the Upper Permian in N.S.W. and definitive cratonization occurred during the Bowen Orogeny.

G1 BREAKAGE OF GRANITIC QUARTZ IN RIVERS A, J, Moss, CSIRO, Division of Soils,

The downstream fining of detritus in rivers has been attributed to both hydraulic selectivity and physical particle size reduction The former has been widely studied and its importance is now beyond doubt;

the importance of the latter, because of difficulties

involved in its study, has never been properly assessed,,

Arbitrarily, physical removal of only relatively minute fragments from a particle's surface may be called 1 attrition1, whereas the fragmentation of a particle by more deeply penetrating fractures into relatively large products may be called 1 breakage \ The action of attrition is readily discernible in nature because of its rounding effects.

Breakage of unrounded particles is almost

impossible to detect in streams by mere observation, a hydraulic explanation of downstream fining being always available.

The

purpose of the present project was to find and apply a technique that would assess such breakage.

The breakage of detritus represents a large number of discrete events,. i,e* the fragmentation of individuals. particles

undergo varying stress events the weaker must, in

general, be broken first, leaving a residue of the stronger. Although the breakage products of partidee fragmented by a process usually cannot be found, strong particles that have survived the process usually can.®

The difference in strength

between a suite of particles that have survived a fragmentation process and the strength of an otherwise identical suite that has not been involved in the process should give some measure to the effectiveness of the process* A general measure of the resistance of a suite of particles to fragmenting processes can. be made by loading them individually between parallel plates and recording the load on them when they fragment - the'fragmentation load1®

This has been done with a

slightly modified unconfined compression test machine*

50, 100

or more particles from a single sieve fraction are thus fragmented6 The resulting data are usually plotted as cumulative curves*

G2 Initial breakage of coarse granitic quartz in rivers was first investigated because of the general importance of this material as a source of sand and silt.

Samples from weathered Shannons Flat

Granodiorite, from a headwater stream

(Gibraltar Creek) draining

it, from a larger stream flowing over it (Paddys River), and from a 50 kilometre length of the Murrumbidgee River were studied* Results closely followed prediction.

In Gibraltar Creek major breakage of coarse granitic quartz more than compensated lateral addition of new material with a resulting strength increase downstream.

In Paddys River breakage

and new feed approximately balanced each other.

The Murrumbidgee

River was followed out of granite country through reaches where breakdown could not be compensated by new feed.

Under these

circumstances kmm quartz was almost eliminated in 11km and 2-^-mrn quartz greatly reduced in kjkm*

Finer grades of granitic quartz

are not only fed to the streams but produced in them in large quantities by breakage of coarse grains.

As a result, the common

plutonic quartz mode of bed sediments fined from about 2mm in the headwaters of Gibraltar Creek to about O.^mm in the lowest investigated reaches of the Murrumbidgee River,

Quartz-bearing detritus at any point can be characterised, in terms of physical maturity, by plotting mean fragmentation load against grain size.

Breakage of granitic quartz in this river system is clearly a major phenomenon.

There is no reason to suppose that the area

is unusual for a mountainous source region.

By comparison with

breakage, attrition can only be a very minor mechanism of size reduction.

In this fluviatile environment fine grains are able to outlast coarser, stronger ones.

All the streams studied have

pebble-rich beds, and movement as bed-load, in intimate association with colliding pebbles, is thought to cause most of the breakage of granitic quartz.

Particles of diameter 2-5mm are thought to

be highly vulnerable to this form of breakage for hydraulic reasons. Their rapid destruction in young streams accounts for the general scarcity of material of this size in natural deposits*

G3 Downstream fining in young streams is clearly due to both hydraulic processes and breakage.

However, the effect of breakage

probably wanes downstream as fluviatile environments become gentler and pebbles become rarer,

EXPERIMENTAL FRAGMENTATION OF GRANITIC DETRITUS TRANSPORTED IN WATER, WITH SPECIAL REFERENCE TO SIZE OF FRAGMENTATION PRODUCTS P»He Walker, J• HutkafCSIRO, Division of Soils0 Fluviatile transport causes a degree of grain fragmentation and results in a separation of fragmentation products through bed and suspended load movemento

Qualitative and quantitative

aspects of fluviatile fragmentation in nature are difficult to evaluate because, at a given point in a stream, grains produced by fluviatile fragmentation are rapidly disseminated and not readily distinguishable from those produced by weathering breakdowno

The experiments described here on grain frag-

mentation exploited a situation where the initial material and all the fragmentation products were available for analysis$

The main series of experiments was carried out in an unlined steel drum 0,31m diameter and 0«38m long, rolling axially at 1*05 rev/sec to give a nominal distance of travel of H.^km in 4 hours*

The material used was coarse, quartz-rich bed load

sediment from the predominantly granitic catchment of Gibraltar, Creek, A^C^T*

A bulk sample of this sediment was first washed

and hand-sieved through the range 0*5 - ^o76rnme

Individual drum

experiments were carried out with 800g subsample from one 0/b sieve size range in 6 litres glass-distilled water*

The granular

composition ranged from k2% quarts (>90% quartz in each grain) at the coarse end to 7 q u a r t z at the fine end of the range* The products of fragmentation were analysed by sieve (>0e063mm)? by the Coulter Counter resistive pulse method based on particle volume (0e02-0e063mm), by pipette (2ji-0®02mm), and by centrifuge (0.06-2JJCC All grain size distributions of fragmentation products for the thirteen 0/k fractions of the granitic sediment showed two distinct components.

A coarse component of much greater quantit-

ative importance was produced close in size to the original

material of each run.

A fine component, of upper size limit

between 15 and 20jUL, extended through the remainder of the fine range to 0.06JJU

Replicated runs using the same granitic sediment

(1.68-2.00mm sieve size) with water in an end-over-end shaker, and with pipette analysis of the fragmentation products up to 31*3}Ai gave the same two components in the particle size distribution.

similar result was obtained by using only quartz grains selected from the granitic sediment (3»36-^+•00mm) and fragmented with water in the end-over-end shaker.

Since the same.kind of coarse and fine

size components were obtained despite variations in experimental methods and composition, it appears that these components are a general feature resulting from contact between grains of similar / size in water.

Drum experiments were continued using vein quartz pebbles of different size with a range of size grades of the granitic sediment.

Grain fragmentation in the presence of moving pebbles

was clearly quantitatively significant.

Fragmentation was greater

as pebbles were larger relative to the size of granitic sediment used*

The particle size distribution of fragmentation products

when small pebbles were used with the granitic sediment still showed the same fine component (<20^lO as in the drum runs without pebbles.

With large pebbles, the coarse component of "the particle

size distribution of fragmentation products tended to merge with the fine component, yielding an almost linear plot of log diameter/ log cumulative wt

The runs with large pebbles also gave the

clear indication of an upper size limit to fragmentation products of 0.25mm irrespective of the size (>0.5mm) of granitic alluvium used.

The coarse fragmentation component (or breakage) close in size to the initial material was most readily explained in terms of the observable fractures initially within the grains.

This was

especially so for the granitic quartz which showed a marked degree of incipient fracturing in thin section.

Fine fragmentation

products could have had several origins, the most likely of which were fine chipping (or attrition) of external grain surfaces, release of fine grains packed in internal fractures, and breakdown of partially weathered grains of feldspar, mica etc.

The presence

G5 of quartz, identified by x-ray diffraction, as a major component in the < Zjx fraction of drum runs without pebbles suggested that attrition of quartz had contributed significantly to the fine fragmentation products.

Where large pebbles were used in the

drum, however, the distinction between coarse and fine fragmentation components was obscured.

This suggests that the distinction

between breakage and attrition may be arbitrary under certain conditions.

The relevance of the present experimental work to natural fluviatile transport has been shown by the grain strength data for granitic quartz discussed in the preceding paper of this Symposium by Dr A.J. Moss.

Fragmentation of granitic quartz occurred in

the drum experiments to a degree commensurate with that occurring in streams.

The present investigation shows, therefore, that

fragmentation in fluviatile transport is a means of producing significant quantities of fine quartz.

Similarly it is concluded

that breakage is a quantitatively important mechanism for grain size reduction during stream transport.

The present observations,

together with those recently published by Blatt (Geol. Soc. Amer. Bull. 81:

255-262, 1970), suggest also that a major proportion

of the clastic quartz in sediments has its origin in breakage during fluviatile transport.

WEATHERING AND SEDIMENT FORMATION IN THE UPPER GENOA RIVER DRAINAGE BASIN 4 D.L. Gibson and K.A.W. Crook, Department of Geology, Australian National University A study of the mineralogy and geochemistry of residual soils and contemporary deposits in a large (about 830 sq. km) homogeneous humid climato-topographic drainage basin with 71% granitic bedrock has shown that pedogenic and weathering processes account for major differences between granitic bedrock and comtemporary sediment* Comparison of the composition of contemporary sediment in the lower parts of the drainage basin and arenites shows that quartz-rich arenites may result from detritus that has travelled onl;* a relatively short distance from a predominantly granitic source.

G6 HYDROLOGY OF THE GENOA RIVER ESTUARY, MALLACOOTA, VICTORIA G.E. Reinson, Department of Geology, Australian National University Estuaries are partially restricted bodies of water unlike open oceanic areas, and therefore display definite hydrological conditions which are governed not only by tidal forces and river discharge, but by their physical dimensions as well®

The Genoa

River estuary, at Mallacoota, Victoria, just south of the New South Wales - Victorian border, shows a definite hydrological pattern which is controlled mainly by magnitude of river discharge and physical dimensions, in particular bottom topography, of the estuarine basin.

The estuary is situated between two protuding

headlands, Cape Howe and Bastion Point, and a landward prograding barrier-bar has formed between them.

The estuary thus can be con-

sidered as a composite system, consisting of an outer embayment partially enclosed by the barrier-bar, and a major drowned river valley in its upper reaches.

The inlet connecting the estuary

with the ocean is relatively small compared to the dimensions of the sound within the barrier, thus tidal action is considerably reduced within the estuary (Fig. 1). The bathymetry of Mallacoota estuary is quite complex, and water depth, instead of increasing seaward as in the classical open-ended estuary, increases in a landward direction®

Marine

detritus is prograding landward in the lower part of the estuary, while terrestrial material is accumulating in a seaward direction in the upper reaches.

This trend is reflected in the bathymetric

profile along the main axis of water and sediment movement, which reveals four major environments with respect to water depth (Fig. 1): 1•

Flood-tidal delta and lower shallow basinal areas

lower main river channel

upper shallow basin upper main river channel.

The complexity of bottom topography controls in large part the water circulation pattern within the estuary.

As the sea water

intrudes over the flood-tidal delta it sinks down into the deeper areas in the lower part of the estuary, where it remains.

During

Figure 1:

Genoa River Estuary

66(b)

Figure 2:

Generalized salinity profiles

G7 the successive flood-tidal stage the same procedure occurs again*, Salt water is entrained in the seaward moving upper layer and this results in a compensating landward movement of intruding sea water. The sea water moves through the upper basin and sinks into the deeper-zones of the main river channel.

The upper fresh-water

layer flows seaward, leaving the major part of the water column undisturbed.

Sea water probably remains within the deeper parts

of the estuary for long periods of time before being exchanged* Only abnormally high river discharge completely removes it from the upper river channel, but never completely from the deeper parts of the lower river channel and basins.

Thus the nature of

the bottom profile prevents the complete flushing of sea water from the estuary.

Figure 2 illustrates generalized salinity profiles in the estuary at three different stages of river discharge.

During

normal stages of river discharge (approx0 113 cusecs), the vertical stratification takes the form as in Fig. 2a, and remains the same, except near the entrance, throughout the tidal cycle. This is essentially a two—layer flow system with entrainment of salt water into the upper layer of fresher water as it flows seaward. During abnormally low river discharge, the salinity stratification is more diffuse, and vertical mixing occurs throughout the water column (Fig 2b).

During catastrophic stages of river discharge,

(approximately 22,000 cusecs at half-flood stage), as occurred in February 1971, the estuary is still not completely flushed of sea water (Fig0 2c).

Gross entrainment of salt water into the column

of seaward-moving freshwater occurs, but because of the nature of the bottom topography, a small lens of salt water still remains in the lower channel and basins of the estuary.

QUARTZ GRAVELS IN NORTHERN NEW SOUTH WALES G. Taylor, Geology Department, Australian National University Introduction Stretching across the north of New South Wales from the Darling River at Collarenabri west are a series of low ridges of Cretaceous outcrop locally known as Murillo. is discussed here;

One of the easternmost of these

those further west have been partially described

(Dury and Langford-Smith, 1965)0

G8 The area examined is in the Cumborah region 4.5km north of Walgett.

Here the ridges reach a maximum altitude of 160 m, but

average 20 m above the black-soil plains of the Darling River Valley. The ridges are undulating, with a series of small gullies draining Several 'peaks1

into palaeolakes around the margins of the ridges.

in the area reach 158 m or above and these are residuals capped by a silcrete.

The quartz, gravel deposits in the area all occur above 152 m and have been previously noted by Abbott (1881), Whiting and Relph (1958), Pogson and Smart (1967)1 and Offenberg

(1968).

Stratigraphy The stratigraphy of the surficial deposits on the Murillo is complex and nowhere is a complete section exposedo

Fig, 1 shows a

section together with the present topographic relationships of these units.

The Cretaceous Rolling Downs Group forms the ridges over which four surficial units are deposited (Fig. 1).

It consists of sand-

stones (with claystone pebbles in places), siltstones, and claystones which are in part silicified and contain the opal mined in this area. sand.

It contains no quartz pebbles and very little quartz

Unconformably overlying the Rolling Downs Group are quartz

gravels ("Cumborah Gravel11)©

T*hey occur topographically between

158 and 152 m and in close proximity to a silcrete.

They reach

a maximum thickness of 3 m at Cumborah®

Unconformably overlying the "Cumborah Gravel" is a thin (<0.5 m) deposit containing clasts of "Llanillo Silcrete11 and Rolling Downs Group sediments.

These gravels are easily distin-

guished from the "Cumborah Gravel" as they have a heavy coating of sesquioxides.

In topographic lows (<152 m) the iron-stained gravels

are commonly mixed with reworked "Cumborah Gravel",

In areas of low

relief and in the flat central regions of the ridges these units are covered by sand which reach maximum thicknesses in the peripheral gullies.

In most localities augered pebbles from the silcrete gravel

are about 0.7 m below the, surface or less.

At least two pedologic units are identified; unrecognised.

others may remain

The first is a massive silcrete, the "Llanillo

Silcrete", developed on the Rolling Downs Group.

It is best exposed

at Lianillo homestead where it is about 0.5 m thick.

.CH-wm-. Qhnt c

The second,

Oumborah Gravels11.

G9 This unit is of unknown thickness.

It outcrops

at only two localities, Mt Charlotte and Pine Opal Lodge.

It consists

of well cemented "Cumborah Gravel."

Nature of the "Cumborah Gravel" A stratigraphic section measured at the Cumborah gravel pit reads: 0 - 0.4 m

Silt, sand and gravel (clasts are silcrete and Rolling Downs Group sediments) - gravel highly iron stained.

0.4 m - 2.0 m Quartz gravel with sand and silt matrix. Pebbles of chert, jasper, agate, silificied wood and quartz. Some rude cross-stratification dipping northwesterly and a washout trending N-S.

Occasional small boulders

of Rolling Downs Group. 2.0m - 3*0m As above but with zones of quartz-cemented gravels, No sections at other localities are available for detailed examination.

The gravels are only rudely bedded and show few other

structures except those noted above, in places a rough pebble imbrication, the pebbles dipping in a northerly direction, and occasional poorly sorted and sand laminae in the lower parts of the section. The "Cumborah Gravel" contains about 50% gravel and 50% sand and silt matrix.

The gravel fraction is 90% quartz and 10% chert,

jasper, silicified wood and agate.

The quartz is 85% sound crystal

quartz, the remainder is vein or milky quartz with no typical igneous or metamorphic quartz.

The majority of the pebbles are well rounded

and often the jasper, chert etc. are highly polished;

the quartz is

more commonly frosted and often exhibits crystal faces.

The sand

fraction is composed almost entirely of crystal quartz.

The shape

and surface textures of the sand are more complex.

Pig. 2 shows

the roundness/size relationships in the sand range.

The grains

fall into three shape ranges, equant (spherical, irregular and euhedral), platy (usually angular and parallel sided), and shard like (bounded by smooth but stepped conchoidal fractures).

The surface textures

also fall into three categories, frosted, smooth-stepped and crystal surfaces.

These are due to chemical etching, breakage,

and crystal growth respectively.

Of the grains examined, very few

examples of mechanical percussion pits are evident®

This is due to

G10 the sustained silica mobility in the section and the resulting modification of the grains by solution and precipitation of quartz. The quartz bipyramids examined range from perfect forms; or etched, through to perfect spheres.

undamaged

Intermediate cases examined

show no abrasion, pitting etc. on the crystal edges but may show solution features® Age of the "Curnborah Gravel" Since no fossils or minerals suitable for dating occur in or near the gravels the best estimates for datimg them are the silcrete formation. The HCumborah Gravel11 unconformable overlies Cretaceous rocks and includes reworked blocks of Rolling Downs sediments and nLlanillo Silcreten0

At least one phase of silicification post-dates the

"Curnborah Gravel11 deposition, the "Mt Charlotte Silcrete11.

Stephens

(1971) suggests most silcretes in Australia are a post-Miocene development.

He also suggests that the majority of Australian silcrete

and laterite occurrences are co-genetic.

Dury et al. (1969) propose

a minimum age for a pre-basalt laterite in the Warrumbungle Ranges of

i 0.3 m.y. BP, Exon et al» (1970) have dated a deep weather-

ing profile in S.E. Queensland as Early Miocene or older.

Similarly

Wyatt & Webb (1970) have dated laterites in eastern Queensland as Early Miocene.

It is thus reasonable to assume that the silcretes

on the Murillo could be older than Late Miocene regardless of Stephens1 (1971) findings.

In summary, the "Curnborah Gravel" is

most probably Miocene but it possibly may be older. Origin of the "Curnborah Gravelft The gravels at Curnborah are fluvial because they are imbricate, cross-stratified, channelled, and contain interbedded poorly sorted sand laminae and lenses.

The only data available show that any

depositing currents came from the southeast quadrant. The gravel source is probably in the Mesozoic sequence of the Great Artesian Basin, this being the obvious source for the fossil wood, agate and porcellanite found in the gravels.

It does not

however explain the abundance of crystal quartz and the quartz bipyramids, the source of which is most likely to be from the Palaeozoic volcanics and quartz veins in the Eastern Highlands*

G11 Plots of Tertiary deep-leads in New England show a westerly trend, converging on Bellata south of Moree in line with an outcrop of "Cumborah Gravel1* near Collarenabri and the gravels discussed herein.

The New England deep-leads are of Eocene and Oligocene age

(Wellman, 1971); or Oligocene.

similarly those at Bellata are probable Eocene

This would point to the gravels being distributed

by westerly flowing streams during the Eocene or Oligocene. Wellman (1971), in a study of the East Australian basalts, shows that the eastern margin of Australia rose some 800 m during the period from Early Palaeocene to Mid Mjtocene. This would provide a sufficient relief to derive such gravels from the area.

This explains the majority of features observed in the gravels, except the origin of the perfect euhedral quartz bipyramids©

References ABBOTT, J®R., 1881 - Jour. Proc. Roy. Soco N.S.W, 15, 41-70* DURY, G.H., LANGFOR&-SMITH, T., & Mc DOUG ALL, I., 196$; - Aust. Jour, Sci.» /51a 362-363o EXON, N*F., LANGPOREUSMITH, T., & McDOUGALL, I 0 , 1970 - Jour6 Geol, Soc. Aust., 17, 21-30. LANGFORD~SMITH, T., & DURY, G*Hc, 1965 - Am. Journ, Sci,,

170-

190. OFFENBERG, A.C., 1967 - Geological notes to accompany the 1:250,000 Angledool Sheet.

N.S.W. Geological Survey, 33 P*

POGSON, D,J., & SMART, J.V., 1967 - Geol0 Survey N.S.W,« File GS67/075

(unpublo).

STEPHENS, C.G., 1970 - Geoderma, 5, 5-52o WATKINS, J*Ro, 1967 - Jour. Geol. Soc. Aust.t

153-168*

WHITING, J.W., & RELPH, R.E., 1961 - Techo Rept* Dept. Mines, N.S.W, 6, 7o WYATT, D.H., & WEBB, A.We, 1970 - Jour. Geol. Soc. Aust., 17, 39-51• WELLMAN, P.., 1971 - The age and Palaeomagnetism of the Australiant Cenozoic Volcanic Rockst

Ph.D* Thesis, A.N.U. (unpubl.).

G12 FORMATION, OCCURRENCE AND DIAGENESIS OF MONOHYDROCALCITE G.F. Taylor, CSIRO, Division of Mineralogy

Recent discoveries of monohydrocalcite in the Southeast of South Australia may provide further information in the understanding of processes of dolomitization.

The prograding coast of this area of South Australia has given rise to a series of parallel dunes and* large interdunal lagoons and swamps®

Pleistocene to Recent carbonate sediments

of the region overlie Tertiary sediments and Palaeozoic granites. At the present time, only the Coorong lagoon and a number of large salt lakes south of Robe contain water throughout the year*

In recent years there have been extensive studies of the sediments associated with the Coorong lagoon and a large number of "ephemeral" lakes stranded by the rapidly silting Coorongp ALDERMAN and co-workers have shown that the fine-grained carbonate sediments consist of aragonite, calcite, magnesian-calcites, protodolomite, dolomite, magnesite and hydromagnesite and have proposed a diagenetic sequence for their existence® Three small lakes occurring in depressions in the dunes around Robe are quite different:

Lake Fellmongery with per-

manent brackish water, Lake Butler now opened to the sea as an artifical harbour, and Chara Swamp. Beach rocks surrounding Lake Fellmongery were first described by MAWSON (1935) who noted their algal origin.

Recent investigations of these grape-stone textured beachrocks have shown that they consist entirely of monohydrocalcite, a mineral only once before having been reported occurring in a geological environment (SAPOZHNIKOV and TSVETKGV, 1959)0

Chemical

analysis of the material indicates a molecular formula of CaCO^. 0.9^ H 2 0 (cf. CaCO^.H^O for synthetic and CaCO^.O.6^ H 2 0 for Russian material).

X-ray diffraction patterns, IR spectra and DTA/DTG

traces are similar to those previously reported for natural and synthetic material®

Red Sand Iron stained gravels with reworked "Cumborah Gravel1' "Mt Charlotte s i U r e t c " "Cumborah Gravel11 "Llanillo silcrete11 Rolling Downs Group

Stratigraphic section

152m.-a.s.l.

Hypothetical cross section to show the topographical relationships

1 v. coarse sand 2 med. sand 3 fine - v. fine sand

30 20 10

Roundness S (P>

Figure 2 Shows the relationship between size and roundness for the sand fraction of the "Cumborah Gravel"

G13 Those areas of the beach-rock which are submerged are covered with red and blue/green algae.

Initial characterization has re-

created Rivulariaceae, Shizothrix calcicola and Chlorella,

Micro-

scopic and isotopic means are being used to determine whether the monohydrocalcite is being secreted by the algae, or whether it is formed by fortuitous combinations of Ehf pH, temperature and salinity as described in the literature for laboratory syntheses*

The friable tuff-like rock is readily broken by wave-action so that at the edge of the beach-platform, sediments consist entirely of gravel®

almost

There is a gradation of particle size in the

deeper portions of the lake from a coarse sand to extremely finegrained muds.

Cores taken from these muds have been examined.

The

more recent top sediments are monohydrocalcite, but with increasing depth there is a diagenetic succession from calcite through a range of magnesian-calcites to dolomite,

As the monohydrocalcite is

stable in the surface waters, the interstitial waters have been examined for differences in Mg/Ca ratio9

salinity, Eh and pH to

account for these diagenetic changes.

Throughout the sediment pile there is a characteristic red/ brown coloration due to algal remains.

The presence of large

quantities of E^S indicate reducing conditions in these muds, Chara Swamp which is adjacent to Lake Fellmongery has similar sapropelic sediments, and although not surrounded by monohydrocalcite beachrrocks was once part of Lake Fellmongery9

Lake Butler also has a wide beach-platform composed entirely of grape-stone textured monohydrocalcite, and its sediments are ELS-rich sapropelic oozes.

Although these three lakes are now

unique, the recent discovery of traces of monohydrocalcite in "ephemeral11 lakes near Salt Creek and the widespread occurrence of sapropelic sediments may indicate that these conditions were once more widespread.

Investigations of dolomitization in Lake

Fellmongery may add further to our understanding of such processes throughout the Southeast

of South Australia,

Mk DIAMICTITE SEDIMENTATION IN THE NOWENDOC-MT GEORGE AREA, NORTHEASTERN NEW SOUTH WALES W. Mayer, Riverina College of Advanced Education, Wagga Wagga.

A thick sequence of Permian marine sedimentary rocks, containing at least 5,000 feet of diamictites, occurs in the Nowendoc-Mt George area.

The geological setting and the sedimen-

tation of these rocks are outlined here.

Figure 1 shows the area

under dicussion.

The dominant tectonic features of this area are two major fault zones striking E-W to NW-SE, along which large masses of serpentinite, dolerite, and other basic and ultrabasic rocks have been intruded,

The southernmost of these fracture zones represent©

an extension into this area of the Great Serpentine Belt or Peel Fault System, which forms a tectonic boundary between what has been termed the New England Complex of highly deformed rocks to the east and northeast, and the more gently folded strata of the Western Belt of Folds and Thrusts to the west and southwest.

The Nowendoc

Fault System forms the northern zone of dislocation. The Permian sediments crop out in the area bounded by the two fault zones.

They generally occupy the downthrown areas

between horsts of elongate fault-bounded blocks of chert.

two areas, however, they appear to rest conformably on the highly deformed cherts.

Fossils at a number of localities in this area

indicate a Lower Permian age for these rocks. The Permian rocks reach a maximum thickness of over 15*000 feet, and may be subdivided into six litholqgical units of formation status. Formation 6.

Dark grey to black mudstone or siltstone, apparently massive, with thin interbeds of sandstone. 1,200 ft

Formation 5»

Thick beds of conglomerate, sandstones with largescale cross-bedding, occasional thin beds of diamictite and limestone. 2,500 ft

•

G15 Formation k.

A thick succession of laminated siltstones and mudstones, rare beds of turbidite sandstone, some horizons of diamictite, and thin lenses of conglomerate• 1,000 ft

Formation 3*

Apparently massive beds of diamictite up to 150 feet thick, some turbidite sandstones, siltstones and mudstones, and thick conglomerate lenses, 6,000 ft

Formation 2C

Turbidite sandstones and siltstones with interbedded layers of diamictite. 3,000 ft

Formation 1.

Basal conglomerate and breccia in massive beds up to 60 feet thick, with some beds of sandstone* 2,000 ft

The diamictites are distributed through the greater part of the Permian sequence in this area, and form the dominant lithology in Formation 3*

The range of particle sizes in these rocks forms

a complete spectrum from mudstones or siltstones with rare pebblesize clasts, to pebbly sandstones, to conglomerates with a dispersed framework.

Individual clasts average 1-3 ins but occasionally

reach a maximum diameter of more than 1 foot. to be independent of bed thickness.

Clast size appears

The clasts are generally

rounded to subrounded and only rarely angular.

The diamictites occur in beds ranging from

ins to 150 feet

thick, which reach their greatest thickness in Formation 3* Pebbly mudstones or pebbly siltstones form the thickest layers; pebbly sandstone may be up to 25 feet thick.

In many diamictites

the clasts are more or less evenly distributed throughout, in some they tend to be concentrated in the basal part of a bed, whereas in two instances a vertical size grading was observed. A number of diamictites show a patchy irregular distribution of coarser and finer material suggestive of incomplete mixing of the various size grades.

Rafts of conglomerates and contorted shale

fragments occur in some beds.

G16

As with many diamictite deposits in various parts of the world, the question as to whether they represent marine glacial deposits or are the products of mass-movement processes is the central issue in this area.

Voisey (1939) interpreted diamictites in the eastern

part of this area as marine tillites and assigned them to the Upper Carboniferous Kullatime Series.

These rocks are here

regarded as belonging to Formation 3 of the Permian sequence.

The present work suggests that the emplacement of the diamictites in this area as submarine mudflows is more in keeping with the sedimentary features displayed by these rocks.

Submarine

gravity failure of large volumes of sediment of differing lithologies, and the subsequent mixing and churning of gravel, sand and mud into a slurry, as the material flowed to deeper parts of the basin, is the process here suggested for the formation of the diamictites. In many cases the mixing of the various, size grades remained incomplete, suggesting perhaps a highly viscous flow or a short slope which caused the deposit to come to rest before the mixing process could be completed.

In more diluted flows the clasts would tend to

sink through the slurry and to be concentrated nearer the base of the deposit.

The few graded diamictites observed are probably due

to a flow developing high enough velocities to take clasts into suspension and to become transformed into a turbidity current. Some of the mudflows had sufficient energy to erode the sediment over which they flowed, as shown by the inclusion within the diamictites of twisted chips and contorted rafts of mudstone. One of the most commonly accepted criteria for the recognition of marine glacial deposits is the presence of megaclasts in laminated sediments which, because of their penetration and distortion of a number of laminae, are assumed to have dropped into the sediment. Although many hundreds of feet of finely laminated sediment are interbedded with diamictites in this sequence, only in two instances were clasts which may be regarded as dropstones found associated with these.

Compelling evidence exists elsewhere to suggest that parts of New South Wales were glaciated during Upper Carboniferous and Lower

G17 Permian times.

Although marine ice may have made a small

direct contribution to sedimentation in this area, the bulk of the sediments are believed to have been deposited by a process of mass-movement.

VOISET, A # H. (1939), Proc. Linn- Soc. N.S.W.. 38, ^90*

LEGEND

GEOLOGICAL MRP OF THE NOWENDOC-NTGEORGE flRER

vvvvvvv vvTETCHM^ Basalt

V V V V V

+- ± + + +

Gt<wi Lte Dolerlte a* A GakUro £ UJ

DiamLcUH , Coâlomeraie, SauAstouc , SitUûe DEVONIAN j CatBONlFE^RQUS Con-^tomevflite, Sa*dt- SilHtone a 5 Cêvts, J assets, States, L 3 Altered âsic Lavas M A / v a / W ac Lovz-GraAettetaw.otj>tyi.cKocVs H /K

SC R L E 1 -.500,000 MikEl 10 *0 FIGURE 1

=S3O ¥0

tcru

H1 DESIGN OF EXPLORATION PROGRAMS W.C* Lacyf Professor of Geology, James Cook University of North Queensland Using the Southwestern United States as an example, the three stages of an exploration program — sance, and evaluation —

target generation, reconnais-

are examined* v Discovery with minimum risk

and expenditure and maximum economic reward is the objective of the program•

Target generation is guided by the selection of metallogenic provinces, limitations imposed by specific regional and local geological control, past production records, and value judgements as to risks.

Discounted cash flow models can then be generated to

indicate probable returns on "most likely11 discoveries.

Reconnaissance and evaluation stages should be outlined in advance by critical path planning of the various techniques to be used and budget requirements (including money, land and personnel) with carefully selected decision points and risk limits.

NICKEL MINERALISATION IN THE CARR BOYD COMPLEX, WESTERN AUSTRALIA A.C. Purvis, Great Boulder, Western Australia, R.W. Nesbitt, Department of Geology, University of Adelaidej and

Hallberg, CSIRO Division of

Mineralogy• 2 The Carr Boyd Rocks Complex is a,, small (75km ) layered intrusion of Archaean age 100 km NNE of Kalgoorlie W.A. It intrudes basic volcanics and interbedded sediments assigned to the Mulgabbie Formation by Williams (1970)f and is intruded by Archaean granite, and by doleritic dykes of the Widgiemooltha suite (Sofoulis, 1966, Turek, 1966), The bulk of the complex is variously uralitised, serpentinised or otherwise altered, but studies in the western part have revealed sufficient unaltered rock to give a reasonable reconstruction of that part of the complex*

In this area the following elements make

up the layered complex:- i)

a series of layered ultramafic and

mafic- rocks, showing broad-scale cyclic layering of three types, representing three different orders of crystallisation, and three

H2 different, but related, magnas;

ii)

a group of bronzitite bodies,

from a few cms to 250 metres across, including three large bronzitesulphide bodies composing the ore-shoots;

iii)

a series of minor

intrusions, ranging from olivine-augite micronorite to biotite microgranodiorite and iv) marginal sheets of coarse-grained norite with minor Ni-Cu-Fe sulphides•

Detailed chemical and mineralogical studies (Purvis, Nesbitt and Hallberg, in preparation) indicate that the layered complex formed at a depth of some 25km, at temperatures between 1,300 and 1,200°C, from sulphur-rich tholeiitic magmas with progressively increasing normative plagioclase, and progressively increasing normative orthopyroxene, and that the ore-shoots axe related to the third of the series of magmas.

The ore-shoots consist essen-

tially of bronzite, pyrrhotite, pentlandite, chalcopyrite and pyrite, with variable amounts of olivine, chrome-titanomagnetite, augite, plagioclase and green hornblende all of which, except the pyrite, have been analysed.

The indicated temperature of form-

ation (1200°C) is lower than that for the layered series rocks, and the mineralogy is similar to that seen forming post cumulus phases in the immediate host-rocks.

The ore-shoots are essentially pipe-like breccia bodies, with large numbers of xenoliths of the layered series derived from the immediately surrounding country rock.

The host to this breccia

varies from massive sulphides with minor bronzite to sulphidepoor bronzitite. Co.

The massive sulphides average 6°/q Ni, 27° Cu and 0.2fo

Similar sulphides are found throughout the layered series, with

variations represented by very rare occurences of cubanite or valeriite.

In addition supergene phases (violarite, pyrite, marcas-

ite, greigite, bravoite) occur in the deep weathering profile, or in low temperature veins. References; Purvis^ A.J., Nesbitt, R.W., and Hallberg, J., in preparation; The geology of a part of the Carr Boyd Rocks Complex, W.A. Sofoulis, J., 1966: Widgiemooltha,

West Australia Geol.

S. Survey 1:250,000 Geol. Explan. Notes. Turek, A., 1966:

Rubidium-strontium isotopic studies in the

Kalgoorlie-Norseman Area, Western Australia.

Australian

Nat. Univ. Ph.D. Thesis (unpubl.), Williams, I.A., 1970:

Kurnalpi, W.A., West Australia Geol.

Survey 1:250,000 Geol. Series Explan. Notes.

H3 THE ARCHAEAN GREENSTONE PERIDOTITES OF THE EASTERN GOLDFIELDS: AN EXAMPLE OF THE PERIDOTITE - NICKEL SULPHIDE ASSOCIATION ReWe Nesbitt, A.C. Purvis, J. Barry and K.G. McKay, Geology Department, University of Adelaide A study of several mineralised ultramafic belts in the Eastern Goldfields of Western Australia has been carried out with particular emphasis on the Scotia ultramafic.

The iron—nickel—sulphide

deposit at Scotia is perhaps a model example of this type of mineralisation, since it develops the major characteristics seen in the Australian and Canadian occurrences.

These are:-

(a) an association with a particular type of peridotite designated a fkomatiitic peridotite1 by Viljoen and viljoen (1970) and as an 'Archaean greenstone peridotite1 by Nesbitt (1971); (b) the sulphide is concentrated on a marked flexure at the basal contact of the ultramafic host and one other rock type (which is a sediment in the case of Scotia); (c)

the sulphide is almost free of silicate matrix at its base (so called massive sulphide zone) and is overlain by a zone of mixed sulphide-silicate (matrix ore or disseminated zone),

In this paper, the discussion will centre around the peridotite sulphide association.

Two points will be emphasised, viz. the unique

nature of the Archaean greenstone peridotites and the association of massive nickel sulphide with these rock types. (a) Archaean Greenstone Peridotites In the last few years there has been several papers dealing with the tectonic and mineralogical characteristics of various ultramafic types.

In two recent articles, Wyllie (19699 1970) discusses various

classifications and modes of origin of these rock types.

It is

perhaps not surprising that Wyllie ignores those peridotites occurring in the Archaean shield areas since they are commonly grouped in the ophiolite suite. However, Hess (1955) recognised the fundamental difference between the serpentinised peridotites of Rhodesia and the Canadian shields and those of the younger ^lpinotype1 belts. This view was forcibly re-emphasised by the Viljoen brothers (1970) in a major contribution to the study of Archaean ultramafic belts.

There, are several major differences between the two ultramafic assemblages. Perhaps the most obvious is the presence of massive

H4 nickel sulphides in the Archaean and chromitites in the younger serpentinites.

Another difference, recognised by Hess (1955) is

that the older serpentinites are uniformly spread throughout the greenstone portions of the shield areas whereas the alpinotype occur in restricted "belts. Thayer, in several papers describing the characteristics of alpine serpentinites, has stressed the dominance of flow layering, a feature he attributes to emplacement in a partially crystalline state.

Such features are totally absent from the

Archaean greenstone peridotites.

Perhaps the most striking difference between the two peridotite types is the development in the Archaean rocks of a texture, colloquially known as 'spinifex1 texture (Nesbitt 197"!) • The textural type is common in the Australia, Canadian and South African greenstone belts and the term appears to have a general acceptance. The interpretation of the texture is that it results from the fast crystallisation of a supercooled liquid and analyses of the rock indicate that the liquid was ultrabasic in composition. When this interpretation is used on the Scotia ultramafic belt, the body appears to be made up of several units, separated by the spinifex cooling surfaces.

The units vary in thickness and

lateral extent and are believed to represent a pile of volcanics and interflow sediments. A typical unit' (Fig. 1) contains the following features;

a brecciated upper surface, passing pro-

gressively down through a fine grained random spinifex to a coarse plate spinifex textured peridotite. preserved in these rock

Coirmpnly olivine is

types. The junction between the chilled

top and the main body below (interpreted as a crystal mush) is sharp and commonly marked by a preferred alignment of olivine crystals, parallel to the contact.

We interpret the breccia zones as flow top breccia, perhaps hyaloclastites (Hittman 1962) and these, together with the identification of vesicles, point to a shallow depth of emplacement, if not an extrusive origin. The vesicles we have identified are rather rare and commonly''are ..widely spaced.

They are similar to those

reported by the Western Australian Geological Survey from Edjudina

H5 (I. Williams, persecomm.) and Baxter (1971) from Ravensthorpe. The implication of the spinifex units to sulphide ore genesis is quite clear*

If a magmatic origin is proposed for the sulphide

(a view supported by the writers) then the sulphide could not have settled through the total thickness of the ultramafic body (the presence of sediment bands also would prevent this). Nesbitt (1970, 1971) has argued that the volume of sulphide which accompanies the ultramafic in most cases could not have been completely miscible within it. As a consequence, it is suggested that the bulk of the sulphide segregated elsewhere and was emplaced as a slug of sulphide liquid. Indeed, some of the Western Australian deposits)e.g. Carnilya Hill, where only massive SLilphides are present) may well represent the injection of a sulphide magma into a random position within the ultramafic.

Mantle Evolution, Greenstone Peridotites and Nickel Sulphides The geochemistry of the Archaean greenstone peridotites is now reasonably well documented (Naldrett & Mason 1968; Tiljoen 9170; Nesbitt 1971)o

Yiljoen &

A particularly important feature

is the high level of sulphur in some of the rock types, which in places reaches sufficient concentrations to form ore bodies. This feature, together with the development of significant quantities of ultrabasic liquids suggests that the mantle had reached a particular stage of development through which it has since passed. The evidence of ultrabasic liquids reaching the surface suggests that either the heat flow during the Archaean !

was unusually high or the liquids were pushed rapidly through a very thin crustal layer (or both). It is also suggested that the presence of large concentrations of magmatic sulphides in the Archaean compared to younger ultramafic belts, points to a significant depletion of sulphur from the mantle since the Archaean. Ringwood (1966) has pointed to the discrepancy between sulphur abundance in carbonaceous chondrites compared to pyrolite and concludes that this is due to loss during the primary differentiation of the earth. However, the average S content of pyrolite quoted by Ringwood is 300 p.p.m. which is not significantly different to that found in the greenstone peridotites away from the mineralised zones.

H6 This apparent contradiction

can be explained if the sulphur

was released and segregated away from the parental material prior to the production of the peridotitic liquids. It is envisaged that sulphur would largely be held in the form of an immiscible sulphide liquid.

Continued heating of mantle material would produce liquids,

ultrabasic in composition with a maximum sulphur carrying capacity of some 500 p.p.m.

(MacLean 1969)•

References BAXTER, D.R., Honours Thesis, University of Adelaide, 1971. HESS, H.H., Geol. Soc. Amer. Spec. Paper 62, p.391* MacLEAN, W.H., 1969:

Econ. Geol., 6± p.865.

NALDRETT, A.J., & MASON, G.D., 1968: NESBITT, R.W., 1970: _ _

1 1971:

RINGW00D, A.E., 1966:

Can. J. Eaxth Sci.,

p#111.

Geol. Soc. Aust. Abst. Archaean Conf# Geol. Soc. Aust. Spec. Publ. 3. Advances in Earth Sciences, p.287.

Ed.

P.M. Hurley, M.I.T. Press. RITTMAU, A., 1962; Yolcanoes and their Activity. Vincent.

Trans. E#A*

J. Wiley & Sons. Publ#

VILJOEN, M.J., & VILJOEN, R.P.:

Geol. Soc. South Africa Spec.

Pub « 2. WYLLIE, P.J., 1969s , 1970s

Tectonophysics,

p.437

Min. Soc. Amer. Spec. Pap. 3* P* 3*

Base of overlying peridotite flow

Breccia Zone at flow top. Breccia fragments may have vesicles, and/or spinifex texture, Thickness of the Zone varies from 1 cm. to greater than 10 metres.

Random Spinifex Zone, progressively coarser to base. CO § LO CN

Coarse plate Spinifex Zone.

a* <ex

Base of chill zone marked by alignment of elongate skeletal shaped olivine grains.

Main body of peridotite.

Fig. Is

Diagrammatic section through a Spinifex Zone at the top of a flow. This represents a generalised case since there are many variations. Commonly a devitrified glass is present instead of the breccia. In other examples, the base of the overlying flow is marked by a chill (= Spinifex) unit also.

H7 SULPHUR ISOTOPES AS A GUIDE TO SULPHATE REDUCTION IN SEDIMENTS J.W. Smith, CSIRCj, Division of Mineralogy In attempts to explain the mode of genesis of sulphide ore bodies, sea water sulphate is frequently cited as the source of the sulphur*

If this is so, an essential reaction prior to the pre-

cipitation of the metal sulphides is the reduction of the sulphate to hydrogen sulphide.

The known methods for this reduction are

very limited*

In aqueous systems at normal pressures sulphate reducing bacteria perform this function readily at the expense of oxidisable organic compounds;

whereas in the absence of biological agencies,

recourse must be made to extremely powerful reducing agents e.g. phosphorus and hydriodic acid, to effect reduction.

At temper-

atures of 300°C in the presence of elemental sulphur, or more particularly the disulphide ion, the direct reduction of sulphate with a wide variety of organic compounds has been accomplished. At extreme temperatures carbon and solid sulphate react directly.

Prom the variety of measurements (fluid inclusion filling temperatures, stable light isotope ratios etc) it has been concluded that the temperature of formation of many ore bodies has not been in excess of 200°C.

If this is so, and those power-

ful reducing agents which are only available in the laboratory are excluded, our present knowledge suggests that reduction of sulphate must have resulted from bacterial activity or from direct reaction with readily oxidisable organic compounds, possibly in the presence of elemental sulphur.

Thus as has been previously suggest-

ed, in order that massive deposits of metal sulphides shall be produced at acceptable temperatures, a very considerable reservoir of oxidisable organic materials must be available.

The close association frequently found between finely dispersed carbonaceous material and metal sulphides provides further support for this argument, although the age of such deposits and the extent of metamorphic and other changes undergone have tended to obscure successfully the exact mechanisms by which such deposits originated.

Whilst the reduction of sea-water sulphate with the subsequent formation of pyrite is easily recognisable in the bottom muds from most reducing marine environments, such systems do not in general produce massive, highly concentrated zones of metallic sulphides®

H8 In fact it would appear that to date no well documented system has been examined in^hich passage of sea water through a bed of organic matter Mas lead unequivocally to the formation of massive sulphides*

This is indeed surprising since the situation

appears not to be uncommon and is apparently easily recognisable. Not infrequently well defined coal seams are overlain, sometimes very closely, by extensive marine measures.

In such

circumstances the downward percolation of sea water sulphate might be expected and, provided the coal was immature and reactive at this stage, reduction of sulphate and the precipitation of metal sulphides principally towards the upper margin of the seam could occur®

In fact, massive accumulation of pyrite under

these conditions are frequently reported. Two such seams in which these characteristics are well marked are the Garrick, and Greta Pel ton seams both of which are overlain by marine measures.

In order to determine the source of

the pyrite sulphur in these and the mechanisms possibly responsible for its formation, subsections of the seam were selected and the distribution and isotopic composition of the various forms of sulphur present measured.

These results are shown diagrammatically

in Fig. 1.

Prom this Figure it is at once apparent that the general distribution of the forms of sulphur in both seams is very similar, with the pyritic, sulphate and elemental sulphur contents decreasing very rapidly with increase in distance from the seam roof, whilst the organic sulphur contents remain relatively unchanged. Marked divergences in the isotopic composition of the various forms of sulphur in the two coals are however very obvious. In general the S34 contents of all forms of sulphur in the Greta Pelton coal are at a maximum in the roof and towards the 34 top of the seam (dS values for inorganic forms of sulphur ranging from +20 to +27 per miL ) whereas in the Garrick those samples taken at the top of the seam are isotopically much "lighter" and a very marked maximtim in S 5 4 content occurs some 4ft below the seam roof.

In both coals the isotopic composition

of the organic sulphur shows least variation and in both9 the> 34 S contents of all forms of sulphur are least at the seam floor#

GRETA

PELTON

SEAM

ROOF

SULPHUR,

<5S

GARRICK *>

cn 6 0

\ \

\ X \

NOT \ \ \

X X

EXAMINED \ \ \ \ \ \ \ >_

t r * — X - ^ "

1 4-0

10 0

SULPHUR,

6-0

120

• X

so4& 2-0

TROILITE

EXAMINED

NOT

W W W \ w w w w w \ \ \ \\ \ \ \

M ETEORITIC

SEAM

ROOF

20-! : / | J AO

%o,

FLOOR

-10

"V X " ' x \ \ \ \ \ \ \ \

o'" +10

6 S 3 * °/oo, M E T E O R I T I C

+20

+30

TROILITE

DISTRIBUTION AND ISOTOPIC COMPOSITION OF SULPHUR IN COALS a

•

PYRITIC

ORGANIC

x——x

SULPHATE ELEMENTAL

H9 The main geological difference between these coals is in the distance from the roof to the overlying marine measures* In the Greta sequence this is virtually nil since marine fossils have been reported within 1 metre of the upper coal surface®

In this case encroachment by the sea occurred pre-

sumably very soon after, or even terminated, the initial coal forming stage•

Freshly buried plant residues are extremely

reactive and thus any sulphate coming into contact with these might be expected to be rapidly and completely reduced, either by bacterial action or possibly by direct reaction with organic residues•

If this occurred no fractionation of the sulphur

isotopes could be expected during the conversion of sulphate sulphur to pyrite*

On the other hand, the Garrick coal seam is separated from the marine measures by some 100ft of sandstone, siltstone and shale.

This separation suggests a considerable time lapse

between the end of the coal-forming stage and invasion by the sea*

During this period, the reactivity of the buried organic

residues would decrease owing to elimination of the more reactive groupings and, when soluble sulphate reached this material, the rate of reduction could be much slower and less complete*

This

type of situation favours the development of a kinetic effect in which the isotopes of lower mass and activation to react preferentially•

onergy tend

If sulphate is being reduced in such

a situation this effect will be manifested by the formation of isotopically "lighter11 E^S + pyrite (relative to the sulphate) at the surface of the reducing material, and by the production of progressively "heavier" BUS 4- pyrite as the sulphate, depleted

in the more reactive S

, is reduced at deeper levels within

the bed of organic material* The isotopic data is thus in accord with a rapid and complete reduction of sulphate in the uppermost

level of the

Greta coal and with the operation of a kinetic effect in the Garrick seam leading to the formation of the "heaviest" pyrite deep within the seam as the last traces of sulphate are reduced*

H10

These results are of particular interest as they demonstrate 1.

In well defined systems the extent of penetration by sulphate 34 32 may often be determined from S yS ratio measurement* 2,

Different mechanisms for sulphate reduction may be recognised. In many cases a sharp difference in isotopic composition

between sulphur of land plant and sea water origins may be observed, 4,

Whilst the determinations of

ratios may be quite

informative if a complete system is examined in detail, single isolated values are unlikely to be meaningful and could well be misleading. 5.

Although very substantial quantities of sulphate are reduced

this is not accompanied by any increase in the organic sulphur contents of the coals. ORE FORMATION HYPOTHESES AND LEAD ISOTOPES J.R. Richards, Department of Geophysics and Geochemistry, The Australian National University A review of some of the more recent work on lead isotopes in mineralized regions shows that a fresh approach to interpretation is needed. It would now seem that lead isotope ratios have nothing to contribute to arguments about the mode of deposition of lead ores;

except in the very broadest terms they cannot be used by

themselves to "date" an ore sample. Because of their great sensitivity to past presence of U and Th, lead isotope patterns offer however a powerful means of correlation with the lead found in other rocks. Thus wellplanned studies should be able to give a great deal of information about the source rocks from which the ore lead could have been concentrated, and perhaps in some cases also about the route followed between source and site of deposition.

APPLICATION

OF THE SYNTHETIC Au-Ag-Te SYSTEM AT VATUKOULA T

FIJI

D#L. Forsythe, Billiton Exploratie Maatzchappy Australia en an experimental system has been applied with some success to natural ore mineral assemblages, the presence of "forbidden" or seemingly incompatible assemblages in an ore suggests the influence of a number of factors, one of which is multistage deposition accompanied by partial replacement of older by younger minerals in the system.

H11 Recent studies of the artifieal Au-Ag-Te system, notably those by Markham and Cabri, have considerably advanced the understanding of the phase relations in the system, although the workers differ considerably in detail and many questions remain unanswered. Attempts to apply the artifieal system to naturally occurring gold-silver telluride assemblages by the above and several subsequent workers have been considered fairly successful, notably when the tellurides had been deposited simultaneously®

In the Emperor gold mine at Vatukoula, the tellurides were deposited in-more than one stage, and a number of forbidden" assemblages and even ore body zoning resulted : the synthetic system could not be readily applied, although within the separated stages the telluride assemblages are compatible with the system.

BRIKE-ROCK INTERACTIONS A*M. Giblin and D.J. Swaine, CSIRO, Division of Mineralogy After the discovery of metal-rich brines in the U.S.A. (Salton Sea), the U.S.S.R. (Cheleken Peninsula) and the Red Sea, there was renewed interest in brines as ore-forming solutions. A better understanding of the role of brines in ore genesis requires laboratory experimental work to ascertain, for example, the extent of interactions between brines and rocks, the effects of the passage of brines through rocks, the formation of brines, the nature and conditions of stability of metal complexes in brines.

The results, of such investigations should improve the interpretation of certain field observations relevant to ore-prospecting. This paper gives the results of some work on brine-rock interactions•

Amongst the factors considered are temperature

and composition of brine, pH, Eh, rock type, time. The following results will be discussed:(i) The amounts of Cu, Pb, Zn, Fe and Mn extracted from an unmineralized shale by calcium chloride, by potassium chloride and by sodium chloride solutions at 60°C« (ii) The amounts of the above metals extracted by a brine of similar composition to the Salton Sea brine«

112 (iii)Differences for basalt, granite, an unmineralized shale and other rocks• Brief details will be given of the apparatus used, of the means of controlling the pertinent factors, and of the methods of analysisf These experiments show that brines can extract Cu, Pb, Znf Fe and Mn from various rocks at 60°C to give a solution which is potentially ore-forming, TRACE ELEMENT DISTRIBUTION IN WALL ROCKS AS INDICATORS OF ORE GENESIS ROCKHOLE MINE N»T/ D.E. Ayres and P.J. Eadington, CSIRO, Division of Mineralogy The Rockhole Mine is one of the deposits of the South Alligator Uranium Field which contains six major deposits and a number of prospects and minor deposits. These deposits are located near an unconformity between carbonaceous shales and siltstones of the Lower Proterozoic Koolpin Formation and sandstones and rhyolites of the Middle Proterozoic Edith River 7olcan±cs« The ore deposits can be divided into (a) those containing pitchblende and (b) those consisting entirely of secondary uranium minerals (e*g* torbenite, saleeite). This paper is concerned only with the genesis of the pitchblende deposits* The mineralisation at Rockhole is predominately coarse- to fine-grained

pitchblends with sparse fine grained arsenides,

sulphides and sele^ides* Native gold is consistently present as inclusions and along shrinkage cradkjs in pitchblende* In general pitchblende was found as pods and lenses at the unconformity between sandstones and carbonaceous shales and in faults within the carbonaceous shales• Trace element distributions have been determined in wallrocks for the Rockhole Mine* lig# 1 shows the distribution of U, Y and C for a traverse from the lode out into the wallrocks# It can be seen that U and V decrease systematically out into the carbonaceous shales and are independent of the C content* However, in the footwall volcanic rocks (No 27881) U and V are low indicating the importance of the carbonaceous shales in

H13 controlling precipitation of uranium. In addition U and V are higher in shales from No 3 Adit (av* II 290 ppm, av# V 94O ppm) than in carbonaceous shales from the lower No 1 Adit (av. U 37 ppmf av. V 266 ppm)*

This decrease with depth is attributed

to precipitation from the mineralising solutions at higher levels and their consequent depletion as they move downwards along the faults*

Groundwater is thought to be the transport-

ing agent for the uranium since the suite of elements associated with uranium in the wallrocks (V, Ni, B and Na) are all mobile in slightly acid groundwater® It is suggested that the uranium in the groundwater was derived by leaching from the Edith River Volcanics*

These

rocks have higher than average uranium and thorium contents, 30 - 56 ppm Th, 1 0 - 3 0 ppm U compared with 28 ppm Th and 8*5 ppm U world average*RoshoIt and Noble (1969) have shown that crystallised silicic volcanics have most likely lost 40 - ^Ofo of their original uranium content by leaching and if this is also the case for the Edith River Volcanics, then they have lost approximately 20 ppm uranium to the groundwater*

The

uranium would have been in solution as a complex involving the uranyl ion, e*g* UO^(CO^)^ ^ * In this form the uranium would be stable under oxidising conditions but under reducing conditions would precipitate as UOg (pitchblende)• It is concluded that the South Alligator uranium deposits were formed by leaching of uranium from the Edith River Volcanics by meteoric water which then moved as groundwater through the underlying sandstones*

Deposition of pitchblende occurred when

the groundwater containing ppm levels of uranium moved down or along faults into or close to carbonaceous shales of the Koolpin Formation* Interaction with the carbonaceous shales resulted in reducing conditions and precipitation of uranium* Reference R0SH0LT, J*N*, and NOBLE, D.C*, (1969) Loss of uranium from crystallized silicic volcanic rocks* Science Letters, 6, 268-270*

Earth and Planetary

114 REPLACEMENT TEXTURES IN SEDIMENTARY ORE DEPOSITS I.B. Lambert, CSIRO Division of Mineralogy, Baas Becking Laboratory Stratiform ores of the type exemplified by the Pb-Zn deposits at Mount Isa and McArthur River, the Cu-Pb-Zn deposits of the German Kupferschiefer, and the Cu deposits of the African Copperbelt are characterized by large numbers of thin, closely-spaced metal sulphide bands which are conformable within the enclosing sedimentary strata.

The sedimentary structures and considerable

lateral extent of many of these metal sulphide bands point to their formation before consolidation of the host sediments.

Nevertheless, a very significant proportion of geologists consider that the replacement textures exhibited by some of the sulphides in such deposits provide unequivocal evidence that they were formed by intricate hydrothermal replacement of the sedimentary strata. The replacement textures include: (i)

veins of sulphides cutting across conformable sulphide bands;

(ii)

rims of one sulphide mineral around another;

(iii) inclusions of one sulphide mineral in anothers (iv)

embayments of 6ne sulphide mineral filled by another.

In this paper it is argued that such textures could develop as a result of diagenetic and metamorphic modifications of syngenetically precipitated metal sulphide bands, and therefore they do not necessarily indicate that the ores were formed wholly, or largely, by hydrothermal replacement.

There are a number of possible ways in which replacement relationships could be generated in sedimentary sulphide deposits. Two that I have investigated experimentally are: (i) The diagenetic breakdown of sulphur bearing organic matter. This could lead to later generations of metal sulphides at temperatures above those at which bacterial reduction of sulphate can occur. Experiments have demonstrated that cystine, a sulphur bearing amino acid found in natural organic matter, will react with a wide variety of metals (in solution or solid forms) under diagenetic temperature and pressure conditions, to form metal sulphides.

In runs of

H »3Ca)

Fig. 1 -

Geochemical Rockhole

Profile

I Adit

Along

Part

( 820' Level)

C %> u ( x 10 ppm ) v ( x I 0 2 ppm )

A <\ ^

*V * v

V ^ v

A * a^ " v v

^V v

27881 N Lab. No.

Rock

type

27881

Hematised volcanic

27860

Carbonaceous shale

27861 27862 27863 2786£

27866 Portal 820 Level Adit

27865

Siliceous carb. shale

27866

Ferruginous carb. shale

Eadington

H15 several weeks duration, reaction could be observed at temperatures as low as 70°C#

At temperatures of

around 150°C, reactions generally were completed within a day or so®

The presence of sulphur in a variety of organic compounds in natural sedimentary environments should lead to the release of sulphur over quite a wide range of post-depositional conditions. Metal sulphides could also form from diagenet.^.c breakdown of metal-organic-sulphur complexes formed during sedimentation*

) The low grade metamorphic breakdown of pyrite in the presence of organic matter to form pyrrhotite•

Exper-

iments have shown that the addition of lignite or bituminous coal can lower the breakdown temperature of pyrite by several hundred degrees•

In the presence of

about 15-2C$ weight organic matter, the pyrite is progressively converted to pyrrhotite over a temperature range of some 150°C.

The lowest temperature at which

pyrrhotite can be detected in runs of 1 weekfs duration was approximately 350°C.

Increasing pressure only

slightly increases the temperature for the appearance of pyrrhotite,

The breakdown of pyrite liberates

sulphur species which could generate later metal sulphides in sedimentary ore deposits•

Other causes of replacement textures are possible: Metal sulphides precipitated during sedimentation could be replaced by, or act as nuclei for, growth of other metal sulphides.

This could occur as the compositions

of the metal-bearing solutions are modified by precipitation, exchange with sediments, mixing with waters expelled from deeper sediments, changing conditions at their source, etc. Furthermore, with increasing temperatures, the pore solutions of the sediments would partially dissolve some of the early formed sulphide and gangue minerals, and subsequently reprecipitate them with replacement relationships9

H16 (ii) Metals could be released during diagenetic dolomitization; precursor minerals could contain trace metals in greater amounts than can be incorporated in the dolomite structure. The"excess" metals could be precipitated as metal sulphides with replacement relationships. (iii)Some metal sulphides precipitated under certain conditions may have a high degree of non-stoichiometry, and could subsequently develop exsolution textures (rims, blebs or lamellae). (iv) Zones of relatively low pressure (fold crests, cracks, etc.) which develop after sedimentation may be filled by metal sulphides which could precipitate from metalliferous solutions, or, in the cases of more easily deformed minerals, migrate there in the solid state. It would appear that all of the processes noted above may act in varying degrees on sedimentary metal sulphide deposits and that they can adequately explain the replacement textures that occur in such ores.

COPPER MII-IERALIZATION (EXCLUDING MOUNT ISA) IN THE FRECAMBRIAN CLONCURRY COMPLEX OF NORTHWEST QUEENSLAND, AUSTRALIA I.H. Wilson, Geological Survey of Queensland, Brisbane; G.M. Derrick and R.M. Hill, Bureau of Mineral Resources, Canberra. p The Precambrian Cloncurry Complex, covering 64,000 km of north-west Queensland, consists of igneous, volcanic, and sedimentary rocks metamorphosed in the greenschist and amphibolite facies. It is structurally complex, and is intruded by polyphase acidic and basic rocks. The region has produced one million tons of copper from about 1,000 mines. Mount Isa Mines have produced 90 percent of this. Semi detailed regional mapping of a 50 km wide east-west strip across the region has shown that most mines are located in the upper Argylla Formation (acid volcanics), the Marraba Volcanics (meter-basalt), and the middle Corella Formation (scapolitic quartz of eldspathiccalcgranof els, para-amphibolite, and slate). See Table 1 and Fig. 1).

H17 Most copper deposits axe epigenetic, occurring v/ith quartz and/or calcite gangue in discordant fractures« Supergene enrichment has been a major concentrating process. Dolerite shows a close spatial relationship to most of the deposits;

granite is associated with some deposits near

Mary Kathleen.

In the Corella Formation large remobilized

calcite "bodies with chalcopyrite axe partly fault-controlled; vein copper deposits in black slate have no obvious igneous source, and axe subeconomic below the water table.

One stratibound

occurrence has been noted, consisting of mainly chalcopyrite blebs in calc-silicate rock. Most of the copper appears to be genetically related to basic magma;

hydrothermal saline fluids derived from meta-

morphosed, locally evaporitic sediments axe possibly orecarrying associates of the basic rock,

Geochemical work has

shown that copper mineralization is most likely to occur in rocks which originally had high trace copper contents.

Table 1«, Stratigraphic distribution of copper occurrences Formation

Copper Occurrences

Possible Equivalents Detailed Ma PP in g

Younger Granites Lawn Hill Fmn Surprise Ck Beds (b) Gunpowder Ck Fmn (b) Deighton Quartzite (b) Magazine Shale (c) Kennedy Siltstone (c) Spear Siltstone (c) TJrquhart Shale (c) Upper Corella Fmn (d) Middle Corella Fmn (d) Lower Corella Fmn (d) Mitakoodi Quartzite

Ploughed Mt Beds, Paradise Ck Fmn Mingera Beds Roxmere Quartzite, Knapdale Quartzite

;i,o,o) ,0,2)

(0,1,0)

Kuridala Fmn (?) Staverley Fmn, Native Bee Siltstone (c) Marimo Slate, Answer Slate, Breakway Shale (c) Moondarra Siltstone (c), Overhang Jaspilite(d) Toole Ck Meta-volcanics (e), Ballara Quartzite, Judenan Beds, Myally Beds 24 (5,14,4,0,1) Marraba Volcanics' Eastern Ck Volcanics, Weatherly Ck Quartzite (e) 126 (98,20,8) Mt Guide Quartzite Leander Quartzite, May Downs Gneiss (f) 1 (1,0) Snake Cr Meta-turbidite(e) 5 Older Granites Kalkadoon Granite, Wonga Granite 12 (12,0) Argylla Fmn Cliffdale Volcanics (a) 96 (0) Magna Lynn Basalt (g) 10 Leichhardt Met amorphics Yaringa Metamorphics 57 (57,0) Notes

(a) (b) (c) (d) (e) (f) (g) (h)

Relative age unknown; possible time equivalent of Argylla Fmn For relationships, see Pig. 2 Subdivisions of Mount Isa Shale (Bennett, 1965) Informal name (Derrick et al. .Bur, Miner. Resour. Aust. Rec. 1971/56 n 11 (Glikson and Derrick. Birr. Miner. Resour. Aust. Rec. 1970/24 11 11 (Derrick et al. Bur. Miner. Resour. Aust. Rec. 1972/b H 11 (Derrick et al. Bur. Miner. Resour. Aust. Rec. 1972/a Subtotals for each formation and possible equivalents, respectively.

Total Outcrop 31 1 , 151 (50^36,65) 55 (53,2)

1 (0,1,0)

0 0 63 (5,58) 32 (7,20,5) 327 (245,52,28,2) 122 (104,15,3) 113 (11,32,4,14,52) 299 (181,90,28) 20 ( 1 7 , 3 )

58 (58,0) 209 (5)

10 126 (126,0)

F I G l : RELATIONSHIPS OF LOWER PROTEROZOIC LAWN HILL

GUNPOWDER

MAY DOWNS

MT ISA

STRATIGRAPHIC

UNITS

MARY KATHLEEN CLONCURRY INTRUSION

GRANITE-

KURIDALA

SOLDIERS CAP

Lawn Hill Fmn

Deighton

Surprise /Ploughed

\Mountain \ Bed 8

Minoera Bd

Judenan Beds

Myally

Eastern <CLeonder Qte

Beds

Creek I

May

Kuridala Fmn

Yaringo

Mmlcs

Corella

Fmn

Shale Ballara Qte

Beds Volcanics

-INTRUSIONS

Mt Guide ftH

Downs Gneiss ^

Qte

Mount Ito

Gunpowder Ck Fmn

Roxmere, Knapdale

Qte

Paradise Ck Fmn

Marlmo Staveley Fmn Slate Answer Slate Overhang Jaspilite Mitakoodi

Qte

Marraba

Volcanics

Snake Ck Meta-Turbidltes

Argylla Fmn Magna

Lynne

Toole Ck Mela-Vbl Weatherly Ck Qte

Basalt

Leichhardt Metamorphics

Transitional or arbitrary boundary between contemporaneous units Modified from

Carter et al. (1961)

II MEASUREMENT OF TIDAL STRAINS IN THE CRUST Ronald Green and Peter H. Sydenham, Department of Geophysics, University of New England, Armidale In the past twenty years there has been a revolution in the orthodox view of the development of the Earth's crust.

"Crustal

Stability11 was the orthodox view and continental drift was a barely tenable hypothesis. The early palaeomagnetic work when conceded the "dipole hypothesis11, was at first only accepted as indicating polar wandering.

Subsequently,

with the accumulation of polar wandering curves for differing continents and the acceptance of the validity of "magnetic field reversals11 and the statistical alignment of the magnetic and rotational poles the validity of "continental displacement11 became a distinct possibility.

It was

not until the palaeomagnetic data was wedded to age determination work and the magnetic field over oceanic ridges was mapped, that the reality of the displacement of large rafts of crust was accepted.

With the

bequest of the title of "plate tectonics n o n the kinematics of the relative motion of the plates the formal acceptance of the hypothesis was assumed, but nevertheless, there still remains a large amount of room for discussion about the question of where and how the plates are consumed.

In addition, there is the question of how or why the

plates are moved.

It is known from seismic and gravity information

that the region from 70 Km to 250 Km is of low rigidity, lower density and low "figure of merit" (Q) and that tidal deformations lead to a 19 release of 2.75 10 ergs/sec. This figure is accurately known. The uncertainties arise as to where precisely the energy is released. The atmosphere and the oceans are numerically insignificant, and the core as an energy sink is untenable because it leads to restrictive core-mantle coupling conditions and in all cases is inadequate.

This

raises the possibility of re-examining the mantle and specifically the low-velocity layer as a £idal energy sink - i.e. tidal energy of 19 2 x 10 ergs/sec. would be liberated in the depth range of 70 Km 250 Km. At the University of New England, a well instrumented observatory (Cooney International Observatory) has been set up. the University and occupies an old mine.

It is 25 Km from

Electric power, telephone

and two-way radio have been connected and a vehicular access provided. The instrumentation includes a 10m Quartz-rod strainmeter, tensionedwire strainmeters, He-Ne laser strainmeter and two tiltmeters plus

12 seismological arid supporting instrumentation.

The instrumentation

will provide accurate amplitude and phase information for tides.) With the results from the Cooney Observatory as a reference,

a mobile tidal

recording unit has been constructed to measure tidal strains at other sites in the Commonwealth and to examine the phase changes throughout the country.

The instrumentation will be discussed.

The two questions to be asked and can be answered are:1.

Is the phase change across the country constant?

Is the phase change related to the surface geology or tectonics? Based on this type of basic measurement, information about the

viscosity at tidal and long-time frequencies and ease of movement of plates, will be forthcoming. UNDERTHRUSTING TO EXPLAIN MEIAMORPHIC ZQNATION IN THE GOROPU MOUNTAINS H.L. Davies, Bureau of Mineral Resources, Canberra The Goropu Mountains in southeastern Papua occupy an area of 1600 sq km (9°32! - 9 0 58 ! S, l A S ^ ' - l A ^ r E ) and have watershed elevations between 1800 and 3676 m.

The western part of the range is

higher and more deeply dissected (maximum elevation Mount Suckling, 3676 m).

The central and eastern parts are lower (maximum elevation

Mount Dayman, 2850 m) and have the form of an elongate half dome (Dayman,dome) with only shallow stream dissection. The Goropu Mountains are made up of about 4000 m of mafic sqhist, (Gwariu Metabasalt) overlain by 500 - 1000 m of calcareous schist (Bonenao Schist), which two rock units together form the Goropu Group.

The Goropu Group metamorphics grade,southeastward into un-

metamorphosed Upper Cretaceous submarine basalt and minor limestone (Kutu Volcanics; Fig. 1).

To west and southwest rocks of the Goropu

Group are in fault contact with peridotite and gabbro of the Papuan Ultramafic Belt (Musa valley area);

to the east they are in contact

with ultramafic fault slices and ultramafic cataclasite, and to the north they are bounded by yemnger sediments and volcanics.,, The metamorphics are broadly folded into an east-west anticline, 60 Km long and 25 Km wide.

The structure is defined by probable bedding

in the Bonenao Schist and by possible bedding in the Gwariu Metabasalt; in both rock units bedding appears to be parallel to metamorphic foliation. The anticline is fault-bounded, and some of the faults are low angle, i.e. 0 - 30° dip.

13 Metamorphic grade in the Goropu Mountains is greenschist facies, and pumpellyite-bearing (i.e. lower grade) greenschist facies.

Charac-

teristic minerals in metabasalt are actinolite, epidote, albite, chlorite, sphene, and, less commonly, pumpellyite, stilpnomelane, glaucophane, calcite, quartz and muscovite. the lower grade schists.

Relict clinopyroxene is preserved in

Higher grade (but still greenschist facies)

completely reconstituted schists typically occur in a narrow zone close to the bounding faluts. Characteristic minerals of the calcareous schists are calcite, quartz, muscovite, chlorite, albite, with less commonly, graphite and, in some instances, accessory pyrite.

In some specimens from the

eastern Goropu Mountains metamorphic aragonite substitutes for calcite, either partly or wholly, and in some others the carbonate minerals are dolomite and magnesite.

Lawsonite is present in some metabasalt and

calcareous schist near Mount Suckling. Southeast of the Goropu Mountains, between 9°54fS - 9°58!S there is an area of weakly metamorphosed basalt and limestone which forms a transition zone from Goropu Group metamorphics in the north to the unmetamorphosed Kutu Volcanics in the south.

Basalt in the transition

zone is massive or only weakly sheared, with prehnite and/or pumpellyite in veins or replacing primary minerals.

The associated limestone is

schistose but only very finely recrystallized (.01 mm). Any interpretation of the geological evolution of the Mount Suckling - Mount Dayman area must take into account the following points: a)

The generally low angles of dip of metamorphic foliation and of

The apparent parallelism of metamorphic foliation and former bedding.

The dynamic nature of the metamorphism as demonstrated in the

The occurrence of minerals which indicate high P/T conditions,

some faults.

cataclastic texture of most of the schists.

namely aragonite and lawsonite. e)

The metamorphic zonation of Cretaceous rocks from greenschist to pumpellyite-bearing greenschist to pumpellyite-prehnite and unmetamorphosed rocks.

The presence of large areas of peridotite and gabbro of the Papuan Ultramafic Belt in the west, and of ultramafic fault slices elsewhere. The following sequence of events is proposed (Fig. 2):

14 1)

Cretaceous:

The extrusion of about 4000 m of basalt at

rift zones or a spreading centre in the Pacific Ocean produced the, basalt lavas and accompanying minor intrusions which now make up the Gwariu Metabasalt and Kutu Volcanics.

During periods of volcanic

quiescence it was possible for pelagic limestone* to accumulate on the sea floor in sequences as thick as 1000 m (Bonenao Schist). 2)

Palaeocene or Eocene:

In response to norths-south or northeast-

southwest compression, the basalt and limestone of the Cretaceous oceanic crust was thrust over itself on a low-angle fault (probably a subduction zone).

The lower plate was metamorphosed to greenschist

facias or pumpellyite^bearing greenschist facies, with local development of minerals which indicate high P/T, namely lawsonite and aragonite8

Grade of metamorphism in the lower plate decreased

downward away from th§ thrust plane, and decreased southward and eastward where burial by underthrusting was not as deep (Fig. 4). 3)

Palaeocene or Eocene:

Movement on the*subduction zone ceased

when Australian continental crust entered the zone as part of the lower plate, further to the west. 4)

Oligocene:

No record.

5) Middle Miocene (lower Tf) to Hplocene:

This was a period of

rapid vertical adjustments by block faulting and anticlinal folding, in response to isostatic or compressive shear forces. The metamorphosed basalt and limestone (Goropu Group) was arched upwards into a broad anticline, and sediments derived from the uplifted basement were subsequently themselves elevated and eroded. The occurrence of lawsonite and aragonite in the underthrust plate indicate metamorphic confining pressures of 4 - 6 kb, which is equivalent to burial at 15 - 25 km below surface.

This approximate

depth of burial is in fair agreement with the estimated thickness of the overthrust plate (Papuan Ultramafic Belt) which is about 18 km. Erosion alone would be hardly adequate to explain the subsequent removal of the overthrust plate.

Presumably it was removed by gravity sliding

and/or reversal of movement on the thrust plane. The occurrence of a narrow zone of higher grade greenschist facie$ metamorphics close to the thrust plane may be due to increased cataclasis near the thrust plane and to higher temperatures occasioned by (a) friction on the thrust plane and (b) proximity to high temperature mantle rocks of the upper plate.

13 I So'

METAMORPHIC

ZONES

REFERENCE unmefamorpfios* d

g/aucophstne

S Pliocene, C Quaternary seo/fments

prehn/fej

Idwsonife

m mon^onite

srjtgonite

gr qranite

pumpeJtyite

pumpef/ifit'e - bearing greenscA/sf ' • horn ft /s

jneeascA/sf

Eocene submarine basa/t

u uHramaftc rocks

Metamorphic jon^tion c*n be explained

by burial caused by

underthrusfinq

10 KM

Contours miner*J

(dbove) show maximum depth of

assembles

Possible

sfeges

E OCENE

——

and

orera//

degree

de ve/opmenf;

underthrusfinq

buri*!

of rock

now

df surface.

Depth

is inferred

recrysfaf/i&tion.

B„

upper piste

unmet+nvorpho secf pnehnilt y pumpeJ/yite - be*ann q qrctnGchttf

uplift

MIOCENE

upper p/dte slides off

3. PRESENT

PAY

from me/amorphic

TECTONICS OF THE WESTERN CENTRAL RANGES OF PAPUA NEW GUINEA H.L. Davies, M. Norvick, R.J. Ryburn, D.S. Hutchison & I.P.Sweet Bureau of Mineral Resources, Canberra In 1971 the Bureau of Mineral Resources mapped the geology of the western central ranges of Papua New Guinea between Porgera and the West Irian border, and from the Sepik plains in the north to the Fly-Strickland plains in the south.

The area overlaps previous surveys by BMR in the

northeast and east (South Sepik, Wabag), and by oil search companies in the southeast and south.

The area is of economic interest for its

petroleum, copper and gold potential, and of structural interest because it straddles the border zone^between relatively stable Australian continental crust and the mobile Pacific margin. The area may be described in three provinces:' a northern province, which extends from the Sepik plains to the line of the main divide; a central province, which includes the Om and Lagaip drainage basins and the Sepik headwaters; and a southern province, which coincides with the drainage of the Fly River and the Strickland River below the OmLagaip junction.

The northern province is dominated by igneous,

metamorphic and volcanic rocks, the central province by slightly metamorphosed shallow and deep water marine Mesozoic sediments, and the southern province by unmetamorphosed Mesozoic and Cainozoic sediments. The main rock units of the northern province are (a) metamorphics (generally sialic), (b) mafic and ultramafic rocks, and (c) Younger (Middle Miocene) volcanics, volcanic sediments, and intrusives.

The

mafic and ultramafic rocks, with associated limestone and high P/T metamorphics, may represent oceanic crust and mantle in a former subduction zone (active in Cretaceous-Eocene or mid-Miocene ?)•

has been suggested that such'a subduction zone was made up of Pacific oceanic crust to the north and Australian continental crust to the south, but this simple model does not explain the present day occurrence of sialic metamorphics north of the supposed fossil subduction zone (Fig. 1).

North-south structural trends in the western part of the

northern province suggest the possibility of an original north-south trend to the subduction zone, which trend could have been masked by later east-west left-lateral strike-slip faulting (Frieda-Lagaip fault zone). The central province consists of Jurassic, Cretaceous and

16 possibly Eocene marine clastic sediments (mostly siltstone and shale) which are folded, faulted and slightly metamorphosed.

Metamorphic

grade increases from the south (unmetamorphosed) to the north (greenschist facies).

Total thickness of sediment is guessed at 5000 - 10,000 m,

and the nature of basement is not known.

The great lateral extent of

some,of the rock units and the strong rectilinear nature of the faults (strike around 290°) are probably evidence of strike-slip faulting. The southern province consists of broadly folded unmetamorphosed Mesozoic and Cainozoic sediments, which, in one area, are seen to overlie a Permo-Triassic granite basement.

About 2000m of Jurassic and

Cretaceous marine clastic sediments are overlain conformably by Oligocene to Middle Miocene limestone, which is in turn overlain by Upper Miocene and Pliocene marine clastic sediments, and some Plio-Pleistocene volcanics. Plio-Pleistocene stocks and dykes intrude throughout the province.

Some

time after upper Miocene or lower Pliocene, the Mesozoic and Cainozoic strata were arched into broad major folds (e.g. the Mueller anticline which is 40 km across) and minor folds, and disrupted by high-angle normal faults and low-angle reverse faults.

Sense of movement in the

low-angle reverse faults is upper plate southwards, in all cases.

This,

and the configuration of some of the structures (as recorded in airphotographs and radar imagery), suggest origin by gravity sliding rather than north-south compression.

Some normal faulting postdates the low-angle

reverse faulting, and has tilted and folded the thrust sheets.

THE EVOLUTION OF THE TASMAN SEA J.R. Griffiths, Department of Geology, University of Tasmania Australia, Antarctica, New Zealand and surrounding submarine plateaux and ridges were all part of a single plate during the early Mesozoic* A thick pile of sediments accumulated along the Mesozoic Pacific continental margin* and this was deformed as a result of lithosphere plate consumption during the late Jurassic - early Cretaceous.

Subsequently

sea-floor spreading in the area has shifted fragments of the original, continental block,to their present positions.

Sufficient marine magnetic

profiling data is available to allow construction of a model which shows progressive stages in the evolution of the region. The first stage of sea-floor spreading began about 80 m.yrs. ago, when the Campbell Plateau and- Antarctica separated and a triangular ocean basin opened northwards into the future Tasman Sea.

Increasing strain

I4!°E

I42°E

Fig. I

I43°E

STRUCTURE AND SIMPLIFIED GEOLOGY WEST CENTRAL RANGES, P.N.G. KuE

PROVINCE

Alluvium Volcanic sediments,volcanics

PQi

Intrusives

U.Mio - Plio.

MuP

Sediments

M.Mio.

Volcanic sediments, volcanics

Intrusives

Limestone

U.Cret-Eoc.

KuE

Interpreted oceanic crust and mantle: mafic and ultramafic rocks and some sediments

Cret.

Clastic sediments

Clastic sediments, mostly black slate and schist; metamorphic grode increases northwards

Jur-Cret?

Metomorphic rocks,generally sialic

Permo- THossic

PTR

Granite basement

re M

Jur.-Cret. Jur.

PQs a Ningerum

Oa PQs

lOOOOm

Fault

B 9000m

SOOOm lOOOOm-

Geological boundary

lOOOOm

5000m

• Om

- 5000m lOOOOm

4 0 Km

PQs

KuE

PROVINCE

Qa Plio.- Pleist.

KuE

Thrust fault Anticline Province boundary

17 within the Campbell Plateau-Lord Howe Rise block was relieved by formation of small sphenochasms - the Bellona Gap, Emerald Basin and Bounty Trough - and by internal rifting within the block forming the New Caledonia Basin (Fig. 1A) . Around 50 m.yrs. ago sea-floor spreading began between Australia and Antarctica.

Marine magnetic profiling data shows that from 50 to

10 m.yrs. ago the rate of northward movement of Australia was about twice that of the Campbell Plateau, relative to Antarctica.

The

resultant differential movement between Australia and the Campbell Plateau is accommodated in the model by further spreading in the Tasman Sea and dextral transcurrent offset of the Lord Howe RiseNorfolk Ridge relative to the Campbell Plateau (Fig. IB). Bending and stretching along a zone through New Zealand initially accounted for this offset, but later rupture of the New Zealand geosynclinal axis occurred along the Alpine Fault.

The total amount of

offset is about 1200 km, of which 480 km has occurred on the Alpine Fault itself.

As the Lord Howe Rise moved north-easterly relative to

Australia, further spreading occurred in the Tasman Sea along roughly north trending axes, but there is insufficient data to closely define this spreading pattern at present.

The Alpine Fault remained linked

to the spreading mid-ocean rise to the south by the ;Macquarie Lineament, which thus originated as a major tr^nscurrent fault on the ocean floor (Fig. IB). At about 10 m.yrs. ago the spreading rates of Australia and the Campbell Plateau from Antarctica became equal and spreading ceased in the Tasman Sea.

The Australian-Pacific plate boundary along the

Alpine Fault-Macquarie Lineament assumed a different character after this time, as relative rotation between these plates became centred on the boundary to the south of New Zealand (Fig. 1C).

Relative

movements across this boundary since 10 m.yrs. ago have been small but complex, resulting in the Kaikoura Movements recorded in New Zealand and the uplift of blocks of oceanic floor to form the present Macquarie Ridge.

18 CARBONATE BRECCIA IN THE PRE CAMBRIAN CORELLA FORMATION OF NORTHWEST QUEENSLAND - TECTONIC OR SEDIMENTARY? G. Derrick, Bureau of Mineral Resources, Canberra The Corella Formation is a widespread unit covering nearly 10,000 2 km

in the Carpentaria^ Cloncurry Complex of northwest Queensland.

consists of calc-silicate granofels, limestone, dolomite and schist, and abundant calc-silicate breccia.

Honman (1939) regarded these breccias

as agglomerate, and Carter et al. (1961) considered that the majority of them were of sedimentary origin, though some could be fault breccias or a result of intense folding. The breccia forms black weathering knobbly hills and ridges, and consists of clasts of laminated marl, calcareous quartzite and calcsilicate rock,3 to 20 cm diameter, though some larger blocks are present. The matrix is an aggregate of fine fragments, quartz, feldspar, calcite and hematite.

It is associated mainly with dolerite and the more

calcareous units of the Corella Formation in linear synclinal fold belts and fault zones, and predates the regional metamorphism(s) which affected the area. Three types are distinguished:

(a) intraformational breccia;

(b) transported-injected breccia and (c)

fault breccia.

Intraformat-

ional breccia forms in zones parallel to bedding, and clasts in the breccia represent slightly disrupted blocks of more competent strata. Transported-injected breccia is chaotic, and displays sharp contacts with regularly bedded Corella Formation;

breccia pipes and irregular,

steep-sided masses and veins are common.

Fault breccia contains

elements of both the preceding breccia types, and is separated from them only because of the association wit;h major faults. Origin of the breccias: Two alternative modes of origin of the breccia are considered: (a)

The breccia represents reef detritus formed by "slumping and density current action at or near the foot of a slope, below wave base".

(b)

(Carter et al., 1961)

The breccia is of tectonic origin.

Considering the first possibility, detailed mapping in the area has shown that there was an overall lack of submarine relief, including reefs, during deposition of the Corella Formation.

Since little or no

slumping has been observed, and since no relation appears to exist between breccia and probable shore-line areas, a syndepositional origin

iSld)

Fig.l

Develop merit of breccia in the CorelU Formation, N.W. Queensland.

ib)

(tide

formed

minted

finfest

one

- a r e n i t e

F r o c t a ring

Competent

beds

during

f o l d i n g

Bfock

r o t a t i o n

a d r a » c e

Pfate/et

- type"

apart

s t a g e

fens

Ce)

ioh

pic

J>cd

Fray

W//i»

u n d e r

Corc//ct

< F<*e yra' co/car40u{ matrix

Whf'te ymmff* •frmfmG*

structure

c o n t / o e f e t f

the

+4<?of hm/mfe cf La/C - A///<*Tc #oc *

Sracc.

i a

17(3)

KEY Hh

M I D - O C E A N RIDGE A N D T R A N S F O R M FAULT (ARROWS INDICATE SPREADING V E C T O R S )

D I A G R A M A T I C A S Y M M E T R I C SPREADING A X I S IN TASMAN SEA

MINOR S P H E N O C H A S M S A N D S P R E A D I N G A X E S

M O T I O N OF BLOCK W I T H R E S P E C T TO ANTARC-

A X I S O F N E W ZFALAND GEOSYNC-L 1NE

ABBREVIATIONS B . G . - Bf LLONA G A P ; B . I . - BALLENY ISLANDS; B . T . - BOUNTY TROUGHf,.». - L M E R A L D B A S I N ; M . I . - M A C O U A R I E ISLAND; N . C . - NFW C A L E D O N I A

FIGURE 1

T h r e e stages in the evolution of the Tasman

Sea.

(A)

P a T e o c e n e (60 in.yrs.ago). The Campbell Plateau and New Zealand have rifted away from A n t a r c t i c a .

(B)

O l i g o c e n e (30 m . y r s . a g o ) . Australia and A n t a r c t i c a have s e p a r a t e d . T h e Campbell Plateau is lagging behind Australia in its northward m o v e m e n t , with resultant t r a n s c u r r e n t m o v e m e n t on the A l p i n e Fault and further spreading in the Tasman S e a .

(C)

P l i o c e n e (5 m . y r s . a g o ) . Australia and the Campbell Plateau are m o v i n g northwards at the same r a t e . Spread inn has ceased in the Tasman Sea and only small relative m o v e m e n t s are occurring on the A l p i n e F a u l t - M a c q u a r i e L i n e a m e n t .

19 for the breccia is unlikely. A tectonic origin for the breccias is strongly favoured because of the regional association of breccias with fold and fault zones.

The

mechanism by which breccia is developed from well-bedded limestones and calc-silicate rocks is shown in Fig. 1.

Brecciation is initiated by

folding of the calc-silicate strata, during which the more competent beds are fractured by incipient cleavage development.

Increasing deforma-

tion causes block rotation and the development of platelet structures in the competent strata.

The resultant breccia is the intraformational

type. As the intensity of folding increases the competent blocks are further rotated and separated in a carbonate matrix which increasingly undergoes plastic flowage and recrystallization, and this results in chaotic breccia typical of the transported-injected type and fault type. The former type results when extreme deformation causes piercement or injection of relatively unbrecciated strata by chaotic breccia;

the

latter results when fold stresses are eventually relieved by faulting. The association of breccia with dolerite suggests the latter was intruded along weakened strata, and may have further contributed to brecciation.

Silicification and the development of large bodies of

secondary "remobilized" limestone commonly accompany the formation of breccia throughout the Corella Formation. References CARTER, E.K., BROOKS, J.H., and WALKER, K.R., 1961 - The Precambrian Mineral Belt of North-Western Queensland.

Bur. M&ir; Resour.

Aus t. Bull., 51. HONMAN, C.S., 1939 - The Soldiers Cap area, Cloncurry district. A£r. Surv.

N. Aust., Qld Rep. 18.

THE CONTACT OR INTERFACE AS A LOCUS OF MAXIMUM DEFORMATION E.S.T. O'Driscoll, Western Mining Corporation An interface or contact in its simplest form may be regarded as a boundary surface between two domains which may be isolated by virtue of differing physical or other qualities.

The description implies that

the interface constitutes a surface or zone of discontinuity, either sharp or gradual.

110 In geological contexts, common types of interface include the surfaces of (a) lithplogic layering (sedimentary or igneous); fault or fracture planes;

(b)

(d) intrusive contacts (conformable or transgressive).

These types

are not mutually exclusive, but may occur in combination.

They may

be gradational from one to another, and may be considered at various scales ranging from that of continental boundaries to that of microscopic intergranular boundaries.

It is well known that particular

types of interface are commonly characterised by supervenient anomalies, including accumulations of hydrocarbons and concentrations of metalliferous minerals. Where an interface is involved in tectonic movements, it plays a conspicuous role as a locus of maximum deformation.

This

"anomalous11 behaviour is to be clearly seen in flexur.al-slip folding of sandstone-shale sequences where the resistance of the arenaceous layers to deformation is compensated by a corresponding greater deformation in the argillaceous layers.

This is expressed in the

shale as sigmoidal flow cleavage which reaches a clearly defined shear maximum at the actual sandstone-shale interface, but decreases outward into the shale.

The overall deformation in this context is represented

by the summation of each local surplus or deficiency of deformation taken from layer to layer. It is suggested that the simple example of shear maximisation at the interface of layers in flexural-slip folding is merely a particular case of a general condition of tectonism - one which tends to generate a peripheral "shell" or envelope of maximum deformation around a relatively rigid body where it is enclosed in a matrix that is deforming by plastic failure.

The deformation along the interface

between the body and its matrix is maximised either in dilatation or contraction, or in a combination of either of them with differential shear.

The proportions vary according to the directional relationship

between the interfacial plane and the axes of the deformation ellipsoid of the adjoining plastic matrix. The most familiar example of this principle of boundary dilatation may be seen on a small scale in the development of pressure fringes at opposite ends of a rigid phenocryst in a schist.

The fringes normally

accentuate the direction of extension as a rather uniform increment polarised in a linear manner.

A model analogue suggests, however, that

pressure fringes represent areas of plastic deformation which is at a

Ill sharp dilatational maximum at the boundary of the rigid body, but which decreases progressively outward to assume the average deformation of the overall matrix*

The indication is also that this inter-

facial dilatational maximum is only one phase of a continuous peripheral maximum which may be traced around the body from one pressure fringe to the other, passing in the process from maximum dilatation to maximum differential, to maximum contraction, to maximum differential, to maximum dilatation.

The model incidentally

suggests that the length of pressure fringes at the ends of a resistant phenocryst may be a function not only of intensity of strain, but significantly, also of the original length of the phenocryst parallel to the stretch direction (A-axis of the deformation ellipsoid). This would appear to explain why a long phenocryst of given width may have terminal pressure shadows that are longer and sharper-tipped than those of an adjacent shorter phenocryst of the same width.

The

difference is due not so much to a difference in individual strain as to the relative dimensional ratios, of the phenocrysts. Interfacial deformation maxima may have special applications to layered rocks at a regional scale in domains of en enchelon "wrinkle" folds that are generated along regional trends at approximately 45 degrees to the direction of transcurrent differential movements of an underlying basement.

Here the regional deformation ellipsoid

(relative to the basement) lies essentially with its long A-axis and its short C-axis both horizontal, and its intermediate B-axis vertical. The overlying folds, with their horizontal axes parallel to the long axis of this ellipsoid, may thus be termed A-folds in terms of regional basement deformation.

Essentially, these folds may be re-

garded as being generated from layers originally lying horizontally in the AC plane of the regional ellipsoid and in the ac kinematic plane of the postulated basement movement where basement shear zones are dipping steeply. The model analogue suggests that the folds develop with a significant degree of flexural dip-slip within the layers where the local deformation ellipsoids have their long A-axes essentially vertical, and their intermediate B-axes horizontal and parallel to the major fold axes.

The folds and subsidiary drags on them, may therefore be

alternatively regarded as B-folds in terms of their supporting array of internal local deformation ellipsoids.

These ellipsoids in turn

have an opposite kind of symmetry on opposite flanks of a fold.

112 These divergent ellipsoids of regional and local deformation respectively are regarded as developing syntectonically, and interacting to product "resultant" deformation ellipsoids which are aligned edge-up, with AB planes steeply dipping, and their long A-axes tilted and plunging into the flanks of anticlines in opposite directions from opposite sides of the structure.

In this form, the flattened ellipsoids may be regarded

as representing elliptical planes (or lenses) related to areas of maximum deformation where they intersect particular interfaces.

Such intersec-

tions then represent a lineation corresponding to bedding-cleavage lineation of the kind which is commonly found to coincide with anomalous material concentrations (e.g. ore shoots).

A cleavage plane in this

context must therefore be regarded as a non-isotropic non-equimetric quantity, i.e. an elliptic rather than a circular plane, giving it a connotation of plunge as well as strike and dip.

Its interactive effect

will vary according to whether it intersects an interface end-on or with its side-edge (i.e. with its A-axis or its B-axis). At the interface between rigid and plastic material the maximum interfacial disturbance appears to occur where the A-axis of the deformation ellipsoid is at right angles to the interface - i.e., where it penetrates the interface end-on.

The point of penetration then marks

an area of maximum contraction of the interfacial surface (folds and reverse faults) as well as a maximum dilatation perpendicular to the interface system, expressed as a separation of interfaces by an increase in their spacing in a manner analogous to the development of pressure fringes. The foregoing argument presents a context in which fold systems are characterised by local strain phenomena which are typically expressed as linear features with a preferred orientation which crosses the fold trends at an acute angle. These linear features mark the trace of strain maxima in lithological interfaces and contacts, and they change their directions of plunge as they slant across anticlinal ridges or elongated domes from one limb to the other.

The asymmetry of the resulting pattern

provides a clue to the type and direction of the initiating tectonic movements.

In this context, model analogues have been devised to simulate

the different kinds of local deformation maxima that may be expected in common structures.

The models are in terms of a selected interface

which marks the junction of a rigid layer with a plastic layer, which are deforming together. The models show that in folded sequences undergoing slow deforma-

113 tion, the interfaces or contacts that are parallel to the BC plane of the deformation ellipsoid are the ones along which maximum dilatation is developed.

For thick relatively rigid and resistant layers,

the maximum dilatation is located immediately over the overwalls of the rigid anticlines and under the underwalls of the rigid synclines. This is due to different relative movement in the direction of the A-axis of the deformation ellipsoid, as well as to a protective umbrella of rigid curvature. Where reverse faulting occurs on the limb of a rigid anticlinal layer,

the formation of small graben structures is accompanied by a

sharp interfacial deformation maximum immediately over the upper surface of the subsiding rigid block.

This maximum is confined between the

reverse faults that bound the structure and is projected diminishingly for a short distance out into the plastic overlayer.

A converse

behaviour is found in thin resistant layers which lie in the acute sector between the kinematic ac plane (average) and the BC plane of the deformation ellipsoid.

Here the formation of sharp buckles results in

a local severe contact dilatation maximum either under the anticlines or over the synclines, i.e., on the concave side of the layer.

This is

due to the buckle deformation moving,faster than the average rate of extension of the A-axis of the deformation ellipsoid. In summary, the application of conventional kinematic principles to the study of fold structures can be clarified by comparisons with kinematic deformation models.

These models suggest that contacts and

interfaces function as a locus of maximum deformation when involved in tectonic movements.

Comparisons may be drawn between deducible tec-

tonic structural features and the dispbsition of ore-shoots in known metalliferous deposits.

If comparisons are valid over a large range

of scales, it may mean that pressure fringes, sigmoidal asymmetry and typical fracture patterns are equally common to a small phenocryst in a tectonite, and to a large scale hard-core domal structure in the field, and that anomalous material dispositions around each are the result of the same fundamental principle. The writer is indebted to the Australian Mineral Industries Research Association for permission to present these ideas which were developed at the University of Adelaide under an A.M.I.R.A. Research Fellowship during 1963-65.

114 TECTONIC DEWATERING - A GEOLOGICAL PROCESS PRODUCING SLATY CLEAVAGE C. McA. Powell, School of, Earth Sciences, Maequarie University Cleavage in slates has been studied for more than 130 years, and the wealth of debate about its origin attests to the elusive nature of a completely satisfactory theory. account for

Two main processes are usually invoked to

slaty cleavage, viz

(a) mechanical rotation of existing particles (b)

crystallization of new minerals.

Studies of deformed objects generally show that slaty cleavage lies perpendicular to the direction of maximum finite shortening in a rock, and this association has led many geologists to propose that strain produces the cleavage.

Thus, a commonly-held view is that cleavage in

slates is a product of deformation or recrystallization, or of both in varying proportions depending on the geological environment.

Cleavage

in the interbedded sandy rocks is regarded as a set of fractures produced by one of a variety of mechanisms, but with no close genetic relationship to the flow cleavage in slates. In 1962, Maxwell proposed that slaty cleavage forms by dewatering of argillites under high pore-water pressures prior to lithification. Maxwell reasoned that in rapidly buried pelitic sequences where pore water can be trapped, deformation could cause the pore pressure to reach, and even exceed, the lithostatic pressure.

The resulting instability

promotes rapid dewatering as the loosely packed, water-bearing pelite reorients its components, and compacts during the escape of the highpressure water.

Platy clay minerals become oriented parallel to the

channels of water movement, and thereby initiate a slaty cleavage.

Sub-

sequent pressure solution, crystallization and flattening in the cleavage plane enhance the preferred orientation. The main evidence supporting this process, tectonic dewatering, is the presence of intrusive clastic dykes parallel to slaty cleavage. The sandstone dykes appear to have intruded as disaggregated particles in a fluid medium, implying that pore-fluid pressures were sufficient to separate the grains.

Maxwell suggested that the sandy beds were

completely unconsolidated, but further work by Powell (1969) shqwed that only certain beds were susceptible to liquefaction.

I have found

intrusive clastic dykes parallel to slaty cleavage in a number of areas including Northern Michigan (Fig. 1), Vermont, New York, Tennessee and the

115 Lake District of England (Fig. 2). The cleavage in both the slate and the interbedded sandstone is produced by thin films of pelite, fractions of a millimetre thick, which separate zones an order of size wider in which there is no surface of easy parting.

These thin films of pelite, pelitic foliae, can

commonly be traced across several thin silty laminae.

The main differ-

ence between the structure of the cleavage in the sandstone and that in the slate is one of size.

The cleavage foliae in the slate are much

thinner and more closely spaced than those in the sandstone.

In many

cases recrystallization in the slate has been more extensive than in the coarser clastic layers, and has masked the cleavage folia. Maxwell (1962) invoked tectdnic dewatering to account for cleavage in the argillites, but ascribed the "fracture" cleavage in the interbedded sandstones to interlayer shearing. out (1968, 1969, 1972

I have pointed

a,b) that cleavage in both the psammites and

the pelites was formed during tectonic dewatering as a quasi-planar structure sub-parallel to the fold axial surface. Continued deformation after tectonic dewatering causes rotation of the cleavage in the competent layers producing a characteristic fan convergent towards the fold core (Fig. 6).

Plastic flow in the

pelite adjusting to fill the spates between the more competent layers produces characteristic cleavage antifans slightly divergent towards the fold core.

The initial orientation of platy particles by tectonic

dewatering is strictly a mechanical rotation, but subsequent movements and recrystallization during folding enhance the preferred orientation. Tectonic dewatering producing slaty cleavage can only occur in certain geological environments, viz. thick deposits of rapidly "pouredin" sediments.

Many of the details of the process are yet to be

determined, but some indications of the nature of the sediments at the time of dewatering can be determined from microstructures in thin sections.

Only a few of the sandy layers appear to have been

liquefied, indicating that many of them were quite cohesive during tectonic dewatering.

Interparticle forces may be electrostatic, as

between clays or very small grains, or chemical in the form of cement, matrix, or intergrowths and overgrowths.

When the pore pressure

reaches, or even exceeds, the lithostatic pressure, only those beds having interparticle cohesive forces less than the excess pore pressure

116

will be subject to liquefaction.

In many of the sandy layers the

tensile strength of the layer as a whole may be lower than individual interparticle forces, and thus there will be hydraulic fracturing of the layering with apparent internal cohesion of the -fragments.

Platy

particles transported along'the flow channels thus established, may be deposited to remain as pelitic foliae. References: MAXWELL, J.C., 1962,

Origin of slaty knd fracture cleavage in

the Delaware Water Gap area; New Jersey and Pennsylvania, p. 281-311, in Engel, A.E.J., James, H.L., and Leonard, B.F., Editors, Petrologic Studies - A Volume in Honor of A.F, Buddington:

Geol. Soc. America, 660 p .

POWELL, C. McA., 1968, Structural analysis of the Siamo Slate, Marquette Trough, Michigan.

Northwestern Univ, Rept. No.

18, NASA Research Grant NGR 14-007-027, Star X69-10963, p. 15-45. 1969, Intrusive sandstone dykes in the Siamo Slate near Negaunee, Michigan:

Geol. Soc. America Bull.» v . 80,

p. 2585-2594, 1972a (in press) Tectonic Dewatering and Strain in the Michigamme Slate, Michigan:

Geol. Soc. America Bull,

v . 83. 1972b (in press) Tectonically Dewatered Slates in the Ludlovian of the Lake District, England:

Geol. Jour.,

v. 8, pt. 1.

THE BRITTLE-DUCTILE TRANSITION IN ROCKS M.S. Paterson, Department of Geophysics, Australian National University This paper aims to give a current picture of the brittle-ductile transition in rocks, taking into account recent observations that have given new insight into behaviour in the brittle and ductile fields at relatively low temperatures, especially the following: (1)

In the brittle field, inelastic dilatation occurs at stresses

appreciably below the maximum s t r e s s ^ . This can be attributed to micro(2) seismic measurements . Also, microcracks can be observed microscopically in specimens in tests interrupted before the load maximum is

1 ItHeSJi Fig. 1o A group of intrusive siltstone dykes parallel to slaty cleavage in Precambrian Michigamme Slate. In adjacent outcrops whole zones of sediment have been liquified and partially homogenized into a gray, massive rock. Some of tnese mobilized zones are more than 10 metres wide, and in places banding produced by multiple siltstone dykes parallel to cleavage, obliterates the bedding.

Fig. 2 (a)

Coupled pelitic and siltstone dykes in Ludlovian rocks of the Lake District. Siltstone dykes intruding down are intimately coupled with pelitic dykelets, and in places the entire depositional fabric of the rock has been disrupted.

(b)

Foliated siltstone adjacent to outcrop in Fig. 2a. Banding is produced by intrusion of pelitic and siltstone dykelets along the escape qhan.nels of the highpressure pore fluid during tectonic c^ewatering.

FigureS. Characteristic eleavage structure in sandstone, traced frojp thin sections. (Clear, quartz; light shading, muscovite; dark shading, chlorite; black, iron oxide; irregular stippling, indeterminate crystallization.) a. (left) Cleavage foliae extend a considerable distance in the rock, and individual foliae may be traced throughout a thin section. Original, detrital (?) rounded quartz grains arc outlined by iron-oxide dust rings, small fluid inclusions, and a few sericite flakes. b . (right) Augen structures are produced where cleavage foliae diverge aroujid rounded quartz grains. T h e quartz overgrowth, optically continuous with the rounded corp, is intergrown with small flake*, of muscovite. In the intergrowth area, which is elongate parallel to the cleavage, the newly crystallized flakes of muscovite do not penetrate the old quartz-grain boundary.

0.1 mm Figure 4 Characteristic cleavage in slate, traced from thin sections. (Clear, quartz; light shading, muscovite; dark shading, chlorite; black, iron oxide; irregular stippling, indeterminate crystallization.) a. (left) Cleavage foliae in originally silt-sized material. The phyllosiiicates in the dark, iron-stained foliae are strongly oriented parallel to the cleavage direction, whereas in the interfolial zones the platy particles are more randomly oriented. Recrystallization has destroyed most of the original grain shapes, and only the larger quartz grains can be recognized. b. (right) Cleavage foliae in very fine-grained slate. The foliae arc not as prominently marked as in coarsergrained rocks, and recrystallization has completely obliterated old grain shapes. Nevertheless, the phyllosiiicates (mainly chlorite) in the foliae are strongly oriented parallel to the cleavage direction and contrast with the more random orientations in the interfolial zones.

SANDSTONE

SLATE

SANDSTONE ^ H ! DYKES PLANAR

Fig# 5. Planar cleavage initiated bytectonic dewatering part-way through deformation.

Fig. 6. Characteristic fan (competent bed) and antifan (incompetent bed) developed by continued deformation after initiation of a planar cleavage.

117 (3) reached; these microcracks are mostly oriented parallel or at relatively low angles to the maximum principal compressive stress. In some cases it has been shown that the maximum in the load corresponds to a weakening of the specimen due to proliferation of microcracking and not to the onset of a macroscopic fracture which only appears at a later stage. (2)

Inelastic dilatation, as well as a relatively high pressure

sensitivity of the flow stress, can accompany deformation at con(4) fining pressures well into the ductile field . This indicates that even in rocks such as marble there may be a significant component of cataclastic deformation in the ductile field at low temperatures. (3) There have been many observations in materials such as metals and magnesium oxide, and even possibly in rocks, of microstrain occurring within some grains prior to general plastic yielding so called ,!preyield microstrain11. Understanding of the brittle to ductile transition calls for an explanation of how the tendency to macroscopic fracture is suppressed with increasing confining pressure and/or temperature.

The simplest

view is that stress for propagation of a shear fracture increases with pressure because of friction and that transition to ductile behaviour occurs when the yield stress is reached, the yield stress being relatively insensitive to pressure.

However, while this may be a

valid picture for simple cases such as homogeneous single crystals, it is an oversimplified view for real rocks, especially in view of the several possible mechanisms of deformation in the ductile field, namely:

(1) cataclastic deformation, involving the proliferation of

microcracking and relative movement on the crack surfaces (analogous to deformation of sand);

(2) crystal plasticity, involving slip and

twinning within the grains;

(3) diffusional deformation, involving

change of shape by mass transport of material by diffusion processes (this is a relatively high temperature phenomenon and I shall not consider it further in this paper). An essential factor in understanding the brittle-ductile transition in most rocks is the micro-heterogeneity of the material in its response to stress.

Thus with increasing stress the following sequences

of behaviour may occur: (1) microcracking

»cataclastic flow:

at stress levels well

below the stress for macroscopic fracture or flow, micro-cracking occurs

118 in some of the g r a i n s ; with increasing stress the microcracking becomes more profuse and interconnects until eventually the material can undergo a bulk deformation by relative movement of the particles into which the microcracking has granulated it.

Increasing, the,

pressure increases the difficulty of each of these processes but possibly has less effect on the early stages of propagation of microcracks

^ .

Macroscopic fracture results from an instability in this development which may involve the catastrophic propagation of a single microcrack, the linking up of a number of microcracks, or the development of a zone of more intensive microcracking which later becomes the locus of a macroscopic fracture; some recent observations favour the last of (3)

these

. The increase in pressure then tends tp suppress the develop-

ment of macroscopic fracture by stabilizing the microcracking but an essential aspect of this is the heterogeneity of the material in limiting the distance to which an individual microcrack tends to run. (2)

microyielding

^general plastic flow:

At stress levels

below the macroscopic yield stress, slip or twinning may occur within individual weaker or more favourably oriented grains.

However, higher

stresses are needed to produce yielding in the majority of the grains. Also multiple slip is required within the grains in order to keep grain boundaries in contact during general deformation and this may require even higher stresses to bring the harder slip systems into activity and so achieve macroscopic deformation. In a given rock, both sequences can be expected simultaneously with increase in stress and which predominates will depend on the material. In most rocks at relatively low temperatures, the microcracking to cataclastic flow sequence takes precedence over the yielding sequence and so the suppression of macroscopic fracture and hence the transition froth b rittle to ductile behaviour is due to the stabilizing of the microcracking sequence.

Only at very high pressures do the stresses

for cataclastic flow come to exceed those for general crystal-plastic flow, although this condition will be attained more readily at higher temperatures on account of the greater influence of temperature on crystal plasticity. References (1)

BRACE, PAULDING and SCHOLZ, J.G.R.

(2)

MOGI, Bull. Earthqu. Res. Inst, Tokyo Univ. 1962, 40, 125;

1966, 71, 3939.

SCHOLZ, J.G.R. 1968, 23, 1417. (3) WAWERSIK and FAIRHURST, I.J.

Rock Mech. Min. Sci. 1970, 7, 561;

119 WAWERSIK and BRACE, Rock Mech. 1971, 3, 61. (4)

EDMOND and PATERSON, I.J. Rock Mech. Min, Sci. in press.

(5)

FRANCOIS and WILSHAW J. appl. Phys. 1968, 39, 4170.

ELECTRON MICROSCOPE STUDIES OF DEFORMATION STRUCTURES IN OLIVINE J.N. Boland, Department of Geophysics, Australian National University Electron microscopy has been used to determine the nature of the deformation features observed in naturally deformed olivine.

The

commonly reported features are kink-bands, extinction bands, polygonisation and less frequently deformation lamellae - Raleigh (1968). However, it has become important to determine the basic defect structure of these features in order to interpret their possible modes of formation.

Previous interpretations have relied upon the similarity

between the optical characteristics of these deformation features and those reported and interpreted by Christie et al (1964) for quartz. Electron microscopy of deformation lamellae in quartz - McLaren et al (1970) - showed that the interpretation by Christie et al (1964) was not correct.

In quartz, the deformation lamellae were found to be

composed of complex tangles of dislocations.

A similar type of investi-

gation was undertaken on olivine and the initial results presented by Boland et al (1971) showed that the situation for olivine was more complex than originally proposed and the simple analogical arguments of Raleigh (1968) were incorrect.

It was shown that there was a

need to make definite correlations between structures observed in the optical microscope and the dislocation substructures observed in electron microscopy. An investigation of the optical characteristics of deformation features in a dunite-mylonite from Anita Bay, New Zealand showed that the optical microscope was not able to distinguish adequately between the kink-band boundaries, extinction bands and polygonal walls. result led

This

to a closer study of the theory of image formation in the

optical microscope- McLaren et al, 1970.

It was shown that the image

produced by deformation lamellae and certain kink-band boundaries was a function of the numerical aperture of the lens and it was not possible to determine the type of defect structure producing this image. detail was not resolvable in the optical microscope.

Such

120 The large grains in the dunite-mylonite contained two essentially distinct structural characteristics.

One type of grain (type A) consisted

of lamellae structures typical of kink-band boundaries while the second type (type B) exhibited a cellular pattern of boundaries.

The kink-

band boundaries of type A were shown to have optical characteristics typical of those reported for deformation lamellae - Raleigh (19,68). They were confined to the (100) or (110).

The cell walls of type B

were confined to the following planes: (100), (001), (010), (110). However, the cell walls associated with these latter planes did not produce images typical of deformation lamellae. Samples of olivine from ultrabasic nodules from Mt. Dun, N.Z*, were observed to contain similar features except the range of planes containing the cell walls of type B was more varied. The increased resolution and the higher magnifications obtainable in electron microscopy made this technique suitable for deciphering the, structural details of these deformation features. The (100) kink-band boundaries were shown to consist of arrays of edge dislocations lying in the (100).

The (110) boundaries were

found to be either edge or screw dislocation arrays.

These linear

arrays of dislocations were found to interact with each other to form Y, L and H type junctions.

The cell-walls of grain-type B were shown

to consist of similar uniform arrays.. It became apparent that the dislocation substructures found in these naturally deformed olivines had a remarkable similarity with the substructures observed in metals deformed under creep conditions. In order to test the validity of this comparison, samples of olivine showing very little deformation were cored and deformed experimentally under appropriate creep conditions in the Paterson gas-apparatus (Paterson, 1970).

In order to get significant amounts of deformation

into the samples, the temperatures required were between 730°C and 900°C.

The confining pressure was 3 Kbars and the creep stress was

chosen within the range of 3 to 5 Kbars.

The amount of creep strain

introduced by the deformation was never greater than 1.5 per cent. However, there was severe deformation introduced into the samples during the loading sequence, in one specimen 7.6 per cent was introduced before the creep stress of 5 Kbars was reached.

Because of the

scatter of the strain versus time results, no constitutive equation of flow was determined but samples were prepared for both optical and

121 electron microscopy. The optical microscopy revealed that the deformation lamellae were generally in the (100) and (110). However, lamellae features were also observed in the following planes: (001), (010), (101), (111) and (okl).

Some grains showed two intersecting lamellae systems or two

different systems operating in different areas of the one grain.

The

optical characteristics of these lamellae were investigated in both crossed-nicols and bright-field plane-polarised illumination.

While

images were easily recognised in crossed-nicols, only 50 per cent of the lamellae were found to give rise to bright-field.images.

The

apparent width of the "object" giving rise to these images was found to be related directly to the numerical aperture of the objective lens by the relationship established previously for quartz lamellae McLaren et al (1970).

It was concluded that the optical images of

these lamellae features did not reveal sufficient detail to allow an interpretation of their possible.defect structure. Electron microscopy of the deformed samples revealed in detail the dislocation substructures and possible mechanism involved in the deformation of olivine.

The very early stages of deformation involve

the motion of screw dislocations with Burgers vector b^ • [001], there was,a tendency of the dislocations to form dipole, triple and multipole clusters.

Besides these clusters, pile-up arrays of screw

dislocations in (100) and (110) were observed (Stage 1 deformation). With increasing deformation, the dislocations from secondary slip systems were activated and interacted with the primary slip dislocations ,to form tangles (Stage II deformation).

These tangles were

generally on (100) and (110), although tangles on (okl) planes were observed.

For much higher strains (7.6%), there were instances of

grains containing polygonal walls parallel to (100) but this was rare. The deformation structures so far described resulted from the loading sequence and it was equivalent to constant strain-rate deformation (for a strain rate of 10

sec

on these substructures.

. The creep deformation was superimposed

In general, the creep deformation did not

substantially alter the dislocation substructures described previously even for creep times of 1200 minutes.

This failure to generate creep

structures typical of metal creep structures was thought to be due to the small amounts of creep strain introduced.

For metal samples,

creep strains of up to 40% may be introduced since the creep rate for

122 these materials is substantially higher than for olivine.

The occurr-

ance of polygonal structures in the most severely deformed sample of olivine is evidence of this strain dependency but this polygonal structure was more a dynamical recovery effect than an inherent creep mechanism. These results indicate that for constant strain rate deformation, the dislocation substructures and mechanisms are similar to those observed in metals.

Experimentally crept olivine may produce structures

equivalent to those observed in naturally deformed olivine but verification of this point is dependent on being able to introduce sufficient creep strain into the specimens.

The maximum attainable creep strain

was only 1.5 per cent which Was not sufficient to produce the dynamic equilibrium dislocation arrays commonly observed in crept metals. References BOLAND, J.N., McLAREN, A.C., HOBBS, B.E., (1971):

Dislocations

Associated with Optical Features in Naturally-Deformed Olivine. Contr. Mineral, and Petrol., 30, 53-63. CHRISTIE, J.M., GRIGGS, D.T., CARTER, N.L;, (1964):

Experimental

evidence of basal slip in quartz J. Geol. 72., 734-^756. McLAREN, A.C., TURNER, R.G., BQLAND, J.N., HOBBS, B.E., (1970): Dislocation structure of the deformation lamellae in synthetic quartz; A study by electron and optical microscopy. Contr. Mineral, and Petrol. 29, 104-115. PATERSON, M.S., (1970):

A high-pressure, high temperature apparatus

for rock,deformation. Int. J. Rock Mech. Min. Sci. RALEIGH, C.B., (1968):

517-526.

Mechanism of plastic deformation of olivine.

J. Geophys. Res. 73 (14) ,5391-5406.

PHYSICAL AND CHEMICAL CONTROLS ON RECRYSTALLISATION IN MICA Michael A. Etheridge, Geology Department, University of Adelaide INTRODUCTION When a mineral recrystallises during a regional metamorphic event, the metamorphic petrologist determines its chemistry and ascribes the process to changes in mineral stability fields.

A structural geologist

studying the same rock,would tend to measure the relevant crystallographies orientations, and discuss the results in terms of the effects of stress and/or strain on particularly oriented grains.

This paper,

describes three examples of deformed and recrystallised mica (one

123 experimental, and two natural), and attempts to show that nucleation and growth of the recrystallised phase is driven by both chemical and deformational forces. The experimentally recrystallised mica specimens were taken from large phlogopite single crystals from the Strangways Range, central Australia.

The natural examples were a green-brown biotite single

crystal of unknown origin,and brown biotite phenocrysts in a dacite from near Tumut, N.S.W. (cf. Ashley et al., 1971).

The experiments

were carried out on 1/4" diameter cores approximately 3/4,! in length, in a Griggs-type solid pressure medium apparatus at pressures of 10 kbars and temperatures up to 1150°C.

Most specimens were deformed at

300°C and a strain rate of about

sec"1, followed by hydrostatic

annealing at 900°C to 1150°C.

Due to a defect in the apparatus, the

annealing time was limited, especially at the higher temperatures to less than 1/2 hour.

This reduced the range of possible experiments,

and the study was thus confined to the microstructural and chemical effects of recrystallisation rather than the full range of P-T conditions under which it took place.

The history of the green-brown biotite is

unknown, but it is extensively kinked and locally recrystallised. It is considered that the brown biotite-bearing dacite has been annealed and recrystallised in a granite aureole after pre-intrusive deformation caused kinking of the biotite (Ashley et al., 1971). RESULTS Data from the experiments include description of the microstruetures observed in thin section, some grain orientation data and mi croprobe chemical analyses.

The microstructural observations include migration

of kink boundaries, nucleation and growth of new grains and other effects associated with these. New grain orientations were measured with respect to the deformed host, and angular misorientations across mobile and sessile kink boundaries were compared•

Chemical analyses

of the host grain and recrystallised grains were taken from various localities within one specimen. The naturally deformed single crystal of biotite was found to have recrystallised almost exclusively along kink boundaries.

The

orientation of these grains with respect to the host grain and kink boundary, and the relative chemical compositions were measured. evidence for mobility of kink boundaries was also noted.

Some

The biotite

phenocrysts in the deformed dacite can be traced from normal kinked grains outside the granite aureole to completely recrystallised patches

124

close to the intrusion.

Once again, measurements were made of host

grain - new grain relative orientation, as well as the orientation of kink boundaries relative to both.

Unfortunately, the new grains associa-

ted with relict host grain were too small to be analysed with any precision, so no chemical data are available for this example. The details of the above data will be included in the lecture, but the main points may be summarised thus: 1.

Small but significant changes in major element composition take place from host to new grains during recrystallis.ation;

2. Deformation features and crystal lattice orientation control the nucleation sites and growth orientation of the new grains. DISCUSSION The results are discussed in,terms of the established nucleation, grain

growth and phase change theories of the simpler metallurgical

and chemical systems.

It is found that none of the main deformation-

induced nucleation theories, viz: 1. Homogeneous or classical nucleation (Becker, 1938); 2. Bulge nucleation (Bailey and Hirsch, 1962); 3.

Subgrain growth or coalescence (Hu, 1962);

satisfactorily explains the orientation relationships observed.

However,

it is evident that the deformation features do control the final position and orientation of the new grain, and may also influence nucleation. In view of the changes in chemical composition involved, the first of the above nucleation mechanisms is invoked, with the addition of a chemical potential term to the standard free Energy equation, viz: AG = (gg - gi + Ag ) 4- oA - ooV s where, A » overall free energy change from phase 2 to phase 1; gi and gz are the chemical potentials of the two phases; Ag is the increase in elastic strain energy involved in the phase change; a and 63 are the surface and stored plastic strain energies respectively, and A and V are the surface area and volume , of the nucleus of phase 1.

Jf AG is

negative, the phase change is accompanied by a lowering of the overall free energy of the system, and is thus enhanced (Christian 1965, p.418). A combined increase in the terms gi and a) is expected to occur outside the host phase stability range and in the vicinity of kink boundaries respectively.

It is suggested that this combination leads

to preferential nucleation along such boundaries.

After nucleation,

there are two possible factors which could control growth of new grains:

125 1.

Anisotropic grains tend to grow with their elongation (fastest

Growth is controlled by some lattice coincidence between new

growth direction) along a zone of high energy (kink boundary);

grain and host* as has been shown for metals (Aust 1969;

Gordon

and Vandemeer 1966). The present evidence does not distinguish absolutely between these two, but suggests that both may be important. References ASHLEY, P.M., B.E. CHENHALL, P.L. CREMER & A.J. IRVING (1971): The geology of the Coolac Serpentinite and adjacent rocks east of Tumut, N.S.W,

J. Proc. Roy. Soc. N.S.W., 103, in press.

AUST, K.T. (1969):

Interface migration.

In: Interfaces, ed.

R.C. Gifkins, Aust. Inst. Metals Conf., Butterworths, Australia, pp. 307-334. BAILEY, J.E. & P.B. HIRSCH (1962): some polycrystalline metals.

The recrystallisation process in

Proc.Roy.Soc. (London) Ser.A.,

267, pp. 11-30. BECKER, R. (1938):

The formation of nuclei by precipitation in metallic

solid solutions.

Ann. Physik, 3_2, pp. 128-140.

CHRISTIAN, J.W. (1965): alloys.

The theory of transformations in metals and

Pergamon Press, London, 973 pp.

GORDON, P. & R.A. VANDEMEER (1966):

Grain boundary migration.

In:

Recrystallisation, grain growth and textures, ed. H. Margolin. Am.Soc.Metals, pp. 205-266. HU, H. (1962):

Annealing of silicon-iron single crystals.

and recrystallisation of metals, ed. L. Himmel.

In:

Recovery

Interscience Publ.,

New York, pp. 311-362.

AN ALGEBRA OF GEOLOGICAL EVENTS K.L. Burns, Mineral Research Laboratories, CSIRO Geological events, such as "emplacement of granite", "growth of slaty cleavage",, may be ordered into a sequence by two methods.

One

is to assign each event to a place in a scale of radiometric ages, which amounts to putting the events into one-to-one correspondence with the set of real numbers. numbers applies.

In ordering such a list, the algebra of real

A second method, which is more widely applicable, is

to determine the time relations of events in pairs, such as "a fold is of type (SI, S2)" or "granite intrudes conglomerate".

These binary

relations between events may be used to order events into a sequence

126 using the transitive properties of the relation older than* It is shown, however, that the binary relations between events do not follow the familiar rules for the algebra of real or integral numbers and it is necessary to erect a new system of relations called the algebra of events. The fundamental relation is older than or equivalent to and this may be used to define the relations older than, younger than, equivalent to, incomparable to, and is covered by.

The essential

difference from the algebra of integers is that the reflexive relation (equal to) is replaced by two such relations (equivalent to and incomparable to) in the algebra of events. A number of binary relations between events may be assembled into an event matrix which is basically a truth table for the relation older than. This may be "ordered" and "stacked11 by operations termed ORDER and STACK.

The relations of each event to every other may be determined by

simple inspection of an ordered, stacked matrix and from this a geological history may be assembled.

If there are contradictions in the field data,

ordering into a proper sequence is impossible and may be detected.

there are ambiguities in the field data, there are several different orders that are still proper sequences so that the event matrix may be ordered, however, the ambiguities appear as voids in the stacked matrix. Formal methods of ordering events have become essential in the structural analysis of large regions, such as the Willyama Complex at Broken Hill, Australia, using numeric methods of data recording and machine systems of analysis.

CONTOURING OF STRUCTURAL (SCHMIDT) DIAGRAMS G.S. Gibbons, The New South Wales Institute of Technology Contours When point plots of structural data have been prepared on a Schmidt projection, it is customary to "contour" them before interpretation, mainly because the process clarifies the diagram, tending to eliminate errors of perception caused by groups of points which chance to lie in some particular configuration (e.g. a straight line) on the projection. These errors are especially significant in judging symmetry, since the configurations of points exert a stronger influence on "the mind's eye" than do the actual point-frequencies in different parts of the diagram. However, in terms of sample representation,

the contour-diagram

contains less information than does the point-plot; it is in fact a smeared-out representation of the points plotted.

127 If the contour-diagram is a means of extracting from the pointplot the significant aspect (point frequency) while suppressing irrelevant aspects (e.g. distracting point-configurations), then interpretation of the contour-diagram is in fact an interpretation of the finite number of grains actually studied; with such an approach the contouring process has no direct relationship to the parent population, but only to the observed data*

However, if two

sets of data are contoured and then compared for reproducibility, each set is clearly being considered as a sample of a larger population. In the literature, there is a general recognition that the contour-diagram has direct relevance to the finite set of datapoints treated; but there is also a general tendency to use the contoured-diagram as having direct relevance to the parent population.

In using the terms "fabric diagram"fand "point density";

in contouring by percentage instead of by number; and in testing validity by comparisons of reproducibility, this inference can be seen.

It is seen also in one of the arguments used to justify

contouring:

that it enables direct comparison of samples containing

different numbers of points. Analysis of the Contouring Procedure In contouring, a small area of the point plot is examined and the number of points within that small area, or "test cell", is noted. If the total number of points plotted is N, the number within the test * n cell is n* and the test cell area is A, then the parameter ^ is the mean frequency (per unit area) of points in the test cell; and if one assumes that the points represent a random sample of an infinite population, then this mean frequency is an estimate of the mean point density within the test cell. It is general practice to plot the observed frequency as "point density" at the centre of the test area, and to contour the values obtained in this way. Unfortunately there is no direct relationship between the true point density at the test area centre, and the mean frequency observed. Because of this, direct statistical evaluation of diagrams has not previously been possible.

However, providing one assumes a continuous

distribution of point densities throughout the test cell area, there is a simple alternative which allows for a rigorous statistical

128 approach.

For a continuous distribution, the true point density

at some point or points within the test cell is equal to the mean true density over the cell area; and the observed frequency is, in fact, a valid estimator of this mean density. Where a frequency is used as a basis for contouring, therefore, there are two uncertainties between the frequency observed and the corresponding point-density of the parent population; (a) There is the numerical uncertainty associated with as an estimate of the mean density. (b) There is the angular uncertainty associated with the centre of the test cell as an estimate of the point(s) for which true density = mean density. IT IS PROPOSED THAT THIS IS THE ONLY STATISTICALLY VALID BASIS FOR CONSIDERING POINT-FREQUENCY OBSERVATIONS AS AN INDICATION OF POINT-DENSITIES IN THE PARENT POPULATION. Numerical Uncertainty Interpretation of a contour diagram is initially in terms of a null hypothesis, H q , for which the parent population is uniform, and the expected frequency E^ in any test cell equals NA.

At points of the

diagram with high or low observed frequencies n, one is concerned with alternative hypotheses H^, expectation E^ ^ Eq.

Ordinary statistical

tests may be used to relate n to E q , E^ in any given case. Angular Uncertainty Essentially, the angular uncertainty is the angular radius of the test cell, and represents the greatest possible angular divergence between the centre of the test cell an<| the direction for which, in the parent population, the true probability density equals the true mean density over the cell. ual test cells:

The concept is obvious enough for individ-

in contouring, its effects are less obvious, and

will therefore now be specified. Essentially, there are three effects.

The first is obviously the

position of the contours:

in extreme cases this may be inaccurate to

almost a full cell radius.

Secondly there is *the matter of discrimina-

tion:

if a cell is centred on a small high-probability area surrounded

by a large area of below-average value, and if the cell covers a large part of this outer zone, then the small high will not normally be detected.

(Features having diameter less than the radius of the test

cell will not generally be delineated unless probability contrast is

129 very high indeed).

The third effect arises from the first two, arid

has a numerical aspect.

If true probability densities fall away from

a point maximum then it is clear that the mean density over a cell must be less than the maximum point.

Thus in the highest- and lowest-

density regions of any contoured diagram certain contours may actually be missing as a result of the angular uncertainty effect. Recommended Procedure THE PRESENT STANDARD PROCEDURE USING A CIRCULAR TEST CELL, OF AREA 1% OF THE FULL PROJECTION CIRCLE, IS A SUITABLE BASIS FOR THE APPROACH PROPOSED. 11°.

ANGULAR UNCERTAINTY IS THUS CONSTANT AT APPROXIMATELY

E q (FOR CELLS WITHIN THE PROJECTION AREA) IS QUOTED FOR EACH

DIAGRAM, AND STATISTICAL EVALUATION (EITHER FOR DIRECT INTERPRETATION, OR FOR COMPARISON) IS CARRIED OUT IN TERMS OF THIS QUOTED VALUE.

FOR

THIS REASON, CONTOURS ARE DRAWN WITH VALUES WHICH ARE FRACTIONS OR MULTIPLES OF E . o Statistical analysis of such diagrams must be essentially in terms of the binomial distribution.

If N is sample size, the

relevant parameters are N, Eo/N for H o , and N,Er/N for other models H^.

Where N is above 50, it is convenient to use the Poisson approxi-

mation, with parameter E q (or E^). Examples of graphs, useful for evaluation as proposed, are attached.

1290)

Curve poire showing observed frequencies

confidence interval n . oC» Q* O S

for

various

Xal TECTONICS AND STRUCTURE OF THE ARUNTA REGION, CENTRAL AUSTRALIA *

R.D. Shaw, Bureau of Mineral Resources, Canberra

The Arunta Complex is a multiply-deformed sequence of gneiss and schist that was overlain by 10,000 metres of shelf sediments between the Adelaidean and the Carboniferous.

The basement was remobilized and

the cover deformed along a number of zones, that now form.the structural margins of younger sedimentary basins.

The remobilization perhaps began

in the Devonian and probably reached a maximum in the early Carboniferous.

This paper reviews present knowledge of the tectonic features of

the region; particular emphasis being given to the remobilization zone at the northern margin of the Amadeus Basin.

The papers that follow

will review two nappe complexes within the Amadeus deformed zone, and the structure of the region of adjoining basement rocks that escaped the Carboniferous deformation. The Arunta Complex consists of metamorphosed sediments, possibly marine shelf sediments (mainly quartzo-feldspathic and pelitic gneisses, quartz-rich and calcareous meta-sediments) as well as orthoamphibolites. The first major metamorphism to affect the Complex is possibly Archaean, since the Lower Proterozoic Warramunga Group is underlain by gneisses and schist south-east of Tennant Creek.

(This group -underwent lower"

greenschist metamorphism and deformation probably about 2000-1950 m.y. ago).

The Archaean metamorphism was possibly accompanied by multiple

deformation, although the folding could equally well have occurred during the major event dated at 1700-1800 m.y. ago.

In the Chewings

Range, early folds with north-south axes have been refolded about steeply-plunging E-W axes and an early mineral lineation has been refolded.

South of the Harts Range early folds with N-trending axial-

planes and steep northerly plunges have been refolded about axes o r similar plunge to produce folds with E-W-trending axial planes. Not a great deal is known of the folding history elsewhere in the Arunta Complex.

The multiple folding was accompanied by variable and even

more widespread metamorphism to the amphibolite and lower granulite fades. The complex underwent a major episode of granite intrusion and probable metamorphism about 1700-1800 m.y. ago.

Granites of this age

occur along the southern margin of the Georgina Basin and possibly in the Harts Range. *

Presented with the permission of The Director, Bureau of Mineral Resources

Ia2 Low-grade metamorphic rocks of pre-Adelaidean age occur withi,n the Arunta region in the Reynolds Range area, in the Mount Leisler area Western Australia and in the Jervois area, where they form part of a prograde metamorphic sequence (R.H. Vernon, pers. comm.).. In the northern part of the Alcoota Sheet area, similar low-grade rocks unconformably overlie higher-grade metamorphic rocks. The Arunta Complex was overlain by a succession of shallow-water shelf sediments now preserved in the Amadeus, Ngalia and Georgina Basins. The beginning of sedimentation closely followed^ and perhaps preceded, an igneous episode in the Musgrave Block at about 1000 m.y.

Sedimenta-

tion ended with deposition of the molasse-like sediments in the late Devonian and early Carboniferous. The Arunta basement was reactivated along a number of clearly defined zones at the margins of the sedimentary basins.

K/Ar dates by

Stewart (1971a) in the Arltunga part of the deformed zone at the margin of the Amadeus Basin show that metamorphism of both basement and cover rocks reached a maximum in the early Carboniferous.

This age can

probably be roughly applied to all three reactivated zones. The zones at the northern margins of the Amadeus and Ngalia Basins are characterized by thrusting, some overfolding, and retrograde metamorphism, and are flanked on one side by folded and thrust sedimentary rocks and on the other side by lower granulite and amphibolite facies rocks.

Deformation at the southern margin of the Georgina Basin is

much less intense, and is characterized by high-angle basement faults and, within the basin, assymetrical synclines without compleRfcntary anticlines. On the regional Bouguer anomaly map, the deformed zones at the northern margin of the Amadeus and Ngalia basins coincide with major gravity gradients.

Positive anomalies occur over the areas of high-grade

metamorphic rocks, and iiegative anomalies occur over the sedimentary basins and retrograde rocks.

In other words, the gravity gradient

occurs well to north of the basin margin. The Bouguer anomaly gradient over the deformed zone at the northern margin of the Amadeus Basin is of such a magnitude (160 mgals) that it has been suggested that the lower crust has been deformed into a warp (Marshall and Norain, 1954;

Forman in Wells et al, 1971) or deformed

by upthrusting of crust and mantle above a north-dipping deformed zone (Forman in Wells et al, 1971) or deformed by upthrusting of crust and

Ia3 mantle above a north-dipping deformed zone (Forman in Wells et al, 1971; Forman and Shaw, 1971).

Basement-cored nappes (such as occur

in the deformed zone at Arltunga, Ormiston Gorge, and Alice Springs) are typical of Cordilleran-type belts where continental and oceanic plates interact (Dewey and Bird, 1970),

They are not generally known

from areas well within old cratonic shields, as is this case in central Australia.

The Amadeus Basin, unlike the orthogeosynclines

of many orogenic belts where nappes typically occur, includes negligible volcanic material and consists of marine shelf sediments that totalled a moderate 6000 metres before orogenesis began. The reactivated zone at the northern margin of the Amadeus Basin consists of a number of narrow, highly-deformed zones in which the older basement structures have been obliterated and a new schistosity formed. Patchy regions of pseudomorphous retrograde metamorphism define the limits of the broader reactivated zone which is up to 25 kilometers wide and extends over 500 kilometers (300 miles) latitudionally• The Bureau of Mineral Resources and the Australian National University are now mapping in detail the southern flank of the Harts Range, the Reynolds Range and adjoining areas, and the Ormiston and Arltunga Nappe Complexes.

The Bureau of Mineral Resources has also

carried out regional mapping on the Alcoota Sheet area.

Up until now,

most work has been done in the Arltunga Nappe Complex.

Here the

simple stratigraphic succession formed by the Heavitree Quartzite and Bitter Springs Formation in the deformed zone has enabled a detailed re cons t ructi on. The nappes were first described at Ormiston and Arltunga by Forman (Forman et al., 1967) and at Alice Springs by Stewart (1967). In outline, the nappe complex at Arltunga consists of a single large nappe, the White Range Nappe, underlain by two smaller, related thrust nappes - the Ruby Gap Nappe in the east and the Trephina Gorge Nappe in the west.

In the lower thrust nappes, the basement rocks are

comparatively undeformed except near the main thrusts or slides.

The

White Range Nappe developed by a combination of thrusting and overfolding but the amount of overfolding involved is problematical.

The

presence of inverted basal conglomerate in the Arltunga area indicates some overfolding has occurred.

Although the possibility of major

overthrusting was recognized after the regional mapping (Forman et al, 1967) it was not until the detailed mapping in 1969 that £he presence

Ia4 of major overthrusting was proven., Ductile deformation in the rear or northern part of the White Range Nappe resulted in reclined folding, strong E-W boudinage, and sub-vertical flattening as well as thrusting, whereas in the front of the nappe,: em^acement was principally by thrusting. The deformation sequence suggests that formation of the Ruby Gap and Trephina Gorge Nappes may have closely followed the development of the larger, overlying White Range Nappe. : Thrusting began early in the formation of the White Range Nappe, and was accompanied in its later stages by the formation of E-W upright folds.

The main

schistosity of the most deformed and retrogressed zone developed as a second stage, and is axial-plane to

N-S-plunging reclined folds.

In the Ruby Gap Nappe both E-W folds in the autochthon and N-S recumbent folds in the allochthon probably formed at the same time as the main thrusting episode.

The formation of minor E-W upright

folds and kink folds followed the development of the schistosity ir* parts of both nappes. Major thrust surfaces within the nappe complex probably originated near the basement-cover contact and migrated upwards to join a major decollement in the Bitter Springs Formation. Subsidiary thrust surfaces or slides formed underneath the main fault nea.r the Heavitree-Bitter Springs contact.

The thrusts have subsequently been rotated about E-W

axes, particularly in the front of th§ complex. References DEWEY, J.F., and BIRD, J.M., 1970 - Mountain belts and the new global tectonics.

Jour, geophys. Res., 75, 2625-2647.

FORMAN, D.J., MILLIGAN, E.N. AND MCCARTHY, W.R. , 1967 - Regional geology and structure of the northeastern margin of the Amadeus Basin, Northern Territory.

Bur. Miner. Resour. Aust. Rep., 103.

FORMAN, D.J., 1971 - The Arltunga Nappe Complex, MacDonnell RangesK Northern Territory, Australia.

J. geol. Soc. Aust., 18, 173-183.

FORMAN, D.J., and SHAW, R.D., 1971 - Relationship between regional metamorphism, structure and Bouguer gravity anomalies in central Australia.

Aus t. N.Z. As soc. Adv. S ci.. 43rd Cpngr.

Brisbane, Abstr. 3, 36. MARSHALL, C.E., and NARAIN, H., 1954 - Regional gravity investigations in the eastern and central Commonwealth. Univ. Sydney. 1954/2.

Mem. Dep. Geol. Geophys.

1*5 STEWART, A.T., 1967 - An'interpretation of the structure of the Blatherskite Nappe, Alice Springs, Northern Territory. J. geol. Soc. Aust., 14(2) 175-84. STEWART, A.T., 1971a - Potassium-argon dates from the Arltunga Nappe Complex, Northern Territory.

J. geol. Soc. Aust,,

17, 205-211. STEWART, A.T., 1971b - Structural evolution of the White Range Nappe, central Australia. . Ph.D. thesis, Yale University. Microfilms;

University

Ann Arbor, Michigan*

WELLS, A.T., FORMAN, D.J., RANF&RD, L.C., and COOK, P.J., 1970 Geology of the Amadeus Basin, central Australia.

Bur. Miner.

Resour. Aust. Bull. 100.

MECHANISMS OF EMPLACEMENT IN THE CENTRAL PART OF THE ARLTUNGA NAPPE

COMPLEX

1 A.J. Stewart

2 and M. Yar Khan

1 Bureau of Mineral Resources, Canberra 2 Australian National University Emplacement mechanisms in the central part of the Arltunga Nappe Complex are of two fundamentally different kinds, viz., those associated with thrust-faulting and sliding along thin zones of dislocation, and those involved in penetrative deformation of larger volumes of rock where faults are absent.

Thrust-faulting is particularly important in

the eastern and western parts of the nappe complex, and gave rise to stacks of thrust sheets under the Ruby Gap Nappe, in the Atnarpa Range between the Ruby Gap and White Range Nappes, and in the Mt Laughlen area of the White Range Nappe (Fig. 1).

In contrast, penetrative

deformation is more important than faulting in the central and northern parts of the White Range Nappe.

An increase in the amount of pene-

trative deformation is also evident from south to north across the nappes, and is believed to have been caused by a rise in the geothermal gradient in the centre of the northern part of the nappe complex (Fig. 1). 1.

Cover Rocks (Heavitree Quartzite) In the southern part of the nappe complex, the Heavitree Quartzite

is unmetamorphosed and almost undeformed;

sedimentary and diagenetic

structures (bedding and cross-bedding, ripple marks, and silica overgrowths and concretions) are undamaged except where the quartzite is

Ia6 brecciated near faults.

Folds are rare, and are large, upright to

south-verging, flexural-slip folds with subhorizontal axes trending approximately east-west,

Undulatory extinction and sub-basal deforma-

tion lamellae are abundant, but the principal stress directions defined by the lamellae differ from locality to locality, and cannot with any certainty be related to the emplacement of the nappe complex.

Quartz

c-axes are in general randomly oriented, but a very weak preferred orientation is evident in brecciated specimens from the northern.part of the Atnarpa Range. In the central and northern parts of the White Range Nappe, the quartzite becomes more and more deformed and metamorphosed northwards. Recumbent to reclined folds are common, and their axes plunge gently north.

The quartz grains become increasingly flattened vertically and

elongated north-south and to a lesser extent east-west;

the resulting

foliation and lineation parallel the axial planes and axes (respectively) of the north-plunging folds.

Undulatory extinction becomes more marked

northwards, and grades intq sharply defined serrate kink bands that tend tp lie in the foliation plane.

Minute xenoblastic quartz grains

form at the edges of and along kink band boundaries in the large flattened grains, and increase in size and amount and become more polygonal with distance northwards;

in the northern-most parts of the nappe

the small grains form by polygonizattion of the large strained grains.. Certain preferentially oriented grains (with east-west c-axes) have enlarged in the north-south and east-west directions, and form unstrained paddle-shaped crystals up to 2 cm long.

The c-axes of the flattened

grains form weak crossed girdles which intersect in the foliation plane at right angles to the lineation, together with two maxima situated between the girdles and close to their intersection;

the girdles

become clearer and the maxima stronger with distance northwards, and thus correlate with the northwards increase in strain in the quartzite. The c-axes of the small polygonal grains form very weak but similarly oriented cleft girdles with associated double maxima in the foliation plane at right angles to the lineation.

The symmetry of both fabrics

is approximately orthorhombic, and the symmetry axes have a constant orientation throughout the area. In the Atnarpa Range, a similar sequence of changes is observed, except that thrust faults and associated brecciation are much more important here, and most of the quartzite in the area is virtually undeformed.

The brecciated quartzite consists of large, subrounded to

Xa7 angular grains in a very fine-grained groundmass of irregular, angular quartz fragments.

There is some preferred dimensional orientation of

the large grains, and the c-axes of these grains form a very weak girdle oriented

at a high angle to the bedding.

Deformed quartzite is restric-

ted to the northwestern flank of the range, and even here the deformation in the southwestern area is restricted to narrow zones.

The quartz c-axes

in this area form a single girdle oriented at right angles to the foliation, together with two strong maxima situated symmetrically within the girdle. In the northeastern part of the range, deformation is quite penetrative, and the textures and cross-girdle microfabrics produced are similar to those in the northern part of the White Range Nappe, However, in quartzite taken from very close to thrust faults in this area, one of the crossed girdles is lacking, and the symmetry of the fabric is approximately monoclinic. 2.

Basement Rocks (Arunta Complex) The basement rocks of the White Range Nappe display structures of

two different ages, namely, an early set that formed in Middle Proterozoic time, and a later set that formed simultaneously with those in the cover rocks in Middle Palaeozoic time.

The early set is present in the southern

part of the nappe, and the various structural elements (foliation pi-axes, linear dimensional grain orientations, and fold axes) have a northwest trend; the folds in this area are upright.

Structures in the northern

part of the nappe plunge gently north and have the same symmetry as the corresponding structures in the cover rocks; basement rocks are recumbent to reclined.

folds in the northern

Microscopically, the rocks

in the southern area show mild deformation (kinking of mica, bending and minor cataclasis of feldspar, deformation lamellae in quartz) and patchy retrogression which is characterized by chlorite and sericite. In the northern area, cataclasis of feldspar is ubiquitous, and quartz has re-crystallized to aggregates of small polygonal grains; kinking of mica is of minor importance.

Retrogression is widespread in the

northern area, and is characterized by the alteration of plagioclase + microcline to albite + epidote + white mica; biotite is chloritized in some areas, but elsewhere it appears unaltered." There thus appears to be a slight northwards increase, from the lowest to„ the1 middle part of the greenschist facies, in the grade of the retrogressive assemblages. As in the cover rocks, quartz c-axes are randomly oriented in the southern basement rocks, but form crossed girdles in the northern area. The symmetry of the girdle fabric is approximately orthorhombic, and

Ia8 similar to that in the cover rocks, indicating that basement and cover deformed together as a single tectonic unit throughout the area. The preferred orientation of the quartz c-axes in the White Range Nappe appears to have arisen during syntectonic recrystallization, as the observed intragranular flow is not enough to achieve the marked flattening of the grains, and furthermore, sub-basal slip moves the c-^axis away from the foliation plane, not towards it as found in this area.

The

steps in the orienting process appear to have been the following: 1. Mild deformation of the original round grains of quartz takes place by intragranular flow. 2. Diffusion (i.e. dislocation motion) begins, as evidenced by the serrate nature of the grain boundaries and kink band boundaries, and continues as the fine grained quartz spreads into the large grains;

polygoniza-

tion of the large grains into slightly misoriented subgrains takes place in the northern area. 3. Recrystallization ensues by growth (enlargement) of the small grains (or subgrains) with the most stably oriented lattices, which in this area was apparently those whose c-axes had the largest east-west directional component.

These grains grow at the expense of those with

unfavourably oriented c-axes, and thus produce an aggregate of grains comparable in size to the original grains but in which the c-axes are now preferentially oriented east-west.

It is presumably the movement

of material during this period of selective grain growth that accomplishes the flattening of the grains. 4. The cycle then repeats, so that eventually the final aggregate of grains can have quite a pronounced degree of preferred orientation of the lattices.

The large paddle-shaped grains probably formed by success-

ive accumulations of quartz on to grains whose c-axes were oriented east-west from the start. In the Atnarpa Range, the single girdle with two strong maxima in the quartzite from near thrust; faults is thought to have arisen by plastic deformation, whereas the weaker crossed girdles or single oblique girdle in the northeastern area have formed by recrystallization. The northwards increase in grade of the retrogressive assemblages in the basement rocks implies a northwards increase in temperature, and the observed mechanisms of emplacement are consistent with this.

The

early part of the White Range Nappe was emplaced under cool conditions

Ia9 by thrusting and sliding of a slab of basement plus the overlying cover rocks; brecciation and cataclasis occurred in the rocks near the faults, and mild intragranular flow affected them generally, though nowhere was it strong enough to cause measurable deformation. As the temperature and associated retrogression in the root zone increased, faulting diminished and further emplacement was achieved by penetrative deformation.

The major processes during this stage were

cataclasis of feldspar an4 syntectonic recrystallization of quartz, resulting in the formation of a new foliation and lineation in both basement and cover rocks.

In the Atnarpa Range, warming of the rocks

apparently took place later than in the White Range area, and so the bulk of the nappe emplacement was achieved by imbricate sliding of thrust sheets;

penetrative deformation became important only in the

final stages of the emplacement.

TWO EXAMPLES OF THE INTERACTION AND DEFORMATION OF BASEMENT AND COVER ROCKS FROM THE NORTHERN MARGIN OF THE AMADEUS BASIN, N.T. R.W. Marjoribanks and J.L. Funk, Geology Department, Australian National University General Statement: Two contrasting areas of thrust-nappe development are compared.

In the Ormiston Pound-Mount Razorback area thrusts and

fold nappes affect the Arunta basement and its immediately overlying cover in a simple manner, thought to represent the earliest stages in the development of such thrust-nappe complexes.

One hundred and

eighty kms to the east, in the Georgina Gap area a more prolonged and intense deformational history involving multi-phase folding is superimposed on these early thrust nappes producing identical structures in both basement and cover rocks. I

Ormiston Pound-Mount Razorback Area (R.W.M.)' B.M.R. mapping (Forman et al. 1967) indicated that the Heavitree

Quartzite and Bitter Springs Formation were involved in two large recumbent fold nappes, each nappe having basement rocks in its core. Detailed mapping confirms the presence of the uppermost of these two nappes (the Razorback Nappe).

Beneath this nappe is a fold-

thrust complex involving basement and cover rather than a major fold nappe.

This structural interpretation is presented in figures 1(A)

and 1(B). The lower limb of the Razorback Nappe is the Razorback Thrust,

IalO along which Arunta basement has moved southwards over cover rocks for' at least 9.6 kms to where it is preserved in the nose of a klippen within the Amadeus Basin Sequence.

The Ormistpn Thrust may similarly represent

the lower limb of another nappe but it is not yet clear whether the nose of this nappe outcrops.

Within the parautochthonous zone under-

neath these major thrusts the basal unconformity is preserved and only slightly modified by the deformation. Isoclinal similar-style folds with a near horizontal plunge and steep north-dipping axial planes in the Heavitree Quartzite immediately south of Ormiston Thrust change progressively southwards to more open, overturned folds with shearing on their lower limbs.

These folds in

their turn pass southwards into open upright flexural-slip folds of the autochthon. The changes in fold style are associated with fabric changes.

The

quartzite of the 1/2 km wide belt of isoclinal folds shows a well developed axial plane foliation caused by the orientation and regrowth of quartz grains within that plane and parallel to the fold axes, giving a weak B-lineation.

Two nkrrow zones, a few meters thick only,

have a protomylonitic fabric of brecciated microscopic quartz grains; larger quartz grains are elongated to define a north-south lineation normal to the axes of the adjacent folds and parallel to the assumed movement direction.

No folds were observed in this direction.

With

the change to more open folds to the south the amount of refoliation and recrystallization rapidly decreases and (as far as can be judged in the field) disappears.

Quartzite of the autochthon shows no cata-

clastic or recrystallization effects. The foliation developed in the quartzite immediately south of Ormiston Thrust is parallel to the thrust and largely parallel to the bedding of the quartzite.

Muscovite has recrystallized along with

quartz to define the foliation.

This foliation is also developed

within the basement rocks for a few tens of meters to the north of the thrust where it obliterates earlier basement structures.

Within this

zone gneisses and schists of the Arunta complex show some retrogression from their regional amphibolite facies grade. At Ormiston Pound, north-trending upright folds of Arunta age adjacent to the parautochthonous cover rocks are refolded by overturned folds of Alice Springs Orogeny age, and develop an associated crenulation cleavage.

Within the Ormiston Thrust zone the Arunta has deformed

I a 9(a)

::::::::::::::

RANGE

Fig. 1;

Geological map of irltunga Nappe Complex, modified from Forman, 1971, J . geol. Soc. A.ustf> 18, 173-192. Blank basement rocks; sparse dots - autochthonous oover i?ocks (Heavijrree Quartzite and Bitter Springs Formation); vertical ruling = allochthorpus cover rocks; circles = Upper Proterozoic and Palaeozoic sedimentary rocks. Close dots » area of greenschist metamorphism that caused retro*grade effects in basement rocks and progracje effects in allochthonous cover rocks.

Iall by fault movement and by a strong, penetrative new fabric development, associated with retrograde metamorphism.

North of the thrust zone the

basement rocks appear to have moved south as a rigid block and show no signs of the Alice Springs Orogeny deformation. In the zone of displacements between Mount Razorback and Ormiston Pound there is a progressive change from north to south in the style of folding, degree of recrystallization and in the development of new penetrative fabrics.

These factors are genetically related to the

degree of southwards movement.

The tightest folding and most intense

deformation are associated with the basal thrusts of the Razorback Nappe and (?) Ormiston Nappes, structures which have had the greatest movement across them.

Lesser southwards translation in the parautochthonous

zone have correspondingly less associated deformation. II

The Georgina Gap Area of the White Range Nappe, Arltunga Nappe Complex (JLF) Detailed structural and stratigraphic mapping of both basement

(Arunta Complex) and unconformably overlying cover (Heavitree Quartzite and Bitter Springs Formation) in the western portion of the White Range Nappe (Fig 2) demonstrate that the model for the structural evolution of the Arltunga Nappe Complex as proposed by Forman (1971) is too simple. Five phases of deformation belonging to the Alice Springs Orogeny are now recognized, of which the first three represent superimposed co-planar deformations of regional importance.

The first phase comprises

thrust faults with inclined to recumbent folds followed by second phase, isoclinal reclined folds both with north-dipping axial planes.

The

third phase folds are tight to isoclinal, east-west upright folds. The earliest recognized structural event is the emplacement of basement-cored thrust nappes.

Virtually undeformed, fault-bounded

basement slices were forced through the Heavitree Quartzite into the Bitter Springs Formation on a series of imbricate thrusts to be translated southwards up to 5 km on glide horizons of highly ductile shales. After thrusting the intimately intercalated pile of basement and cover rocks was deformed producing a field of reclined folds with an axialplane schistosity and a mineral-elongation lineation developed parallel to the fold axes.

This schistosity penetrates the entire tectonic pile

and affects the adjacent basement rocks.

A later upright fold system

refolds the reclined folds producing a crenulation cleavage that is virtually co-planar with the pre-existing schistosity.

There are

Ial2 marked variations in the plunge of fold axes within axial planes of both reclined and upright phases.

The variability appears to be sinu-

soidal involving periodic constriction and is probably related to the map-scale extensional features and boudinage seen along the strike of the nappe.

Deformed quartz grains in the Heavitree record high strains

in the flattening field producing oblate elipsoids.

Two a4ditional

generations of small folds of minor and local importance complete the deformation picture. The interaction of basement and cover is marke4 by the distortion of the unconformity and the development of a convergent mylonite across the contact.

The Arunta basement contains at least three sets

of superimposed folds in rocks of amphibolite facies that locally are obliterated by the new structures associated with the Alice Springs Orogeny.

The effects of this latest deformation are confined to a

narrow zone up to 3 km

wide that cross-cuts earlier structural trends.

Within this zone, the basement was remobilized and refoliated and deformed together with the allochthonous cover rocks.

At a high

structural level in the cover rocks, lumps and blobs of basement rock are preserved as tectonic fish, or klippen, within the Bitter Springs shale along the core of the nappe.

Deformation was accompanied

by hydration and possibly retrograde metamorphism producing a strong, slaty schistosity that penetrated both basement and cover during the reclined phase of folding.

Virtual similarity in the style and

geometry of the structures in both the basement and cover rocks indicate a uniform material response to the deformation. Ill

Structural Correlation The Arltunga Nappe Complex is similar in structural style to the

Razorback Nappe and underlying fold-thrust complex, but it is developed on a much larger scale and with a more complex fold history and associated new fabric development.

The Razorback Nappe occupies a

similar structural position to the White Range Nappe; parautochthonus zones of folding and thrusting.

both overlie

The structures seen

between Mount Razorback and Ormiston Pound can illustrate on a small scale an earlier phase of the structural history preserved in the White Range Nappe.

Isoclinal folding and related thrusting in the

Ormiston area bring basement rock southwards to overlie and interpenetrate the Heavitree Quartzite.

These structures are equivalent

to the first phase of deformation in the White Range Nappe.

Ial3 Owing to the superimposed deformations in the White Range Nappe there is only indirect evidence for this first phase of folding; namely stratigraphic inversions implying thrusting and the existence of a lineation refolded by the reclined fold system.

Evidence from the structures

of the Ormiston area indicate that the interaction of basement and cover occurred by means of essentially simultaneous folding, thrusting and development of a new penetrative foliation. In the White Range Nagpe the current structural evidence indicates that a large mass of basement rock was driven into the overlying cover prior to any folding.

The mechanical paradox presented by this mode

of emplacement is yet unresolved.

A similar interpretation has been

given by Sutton and Watson for the "Scardroy Sheet" of Lewisan rocks interpenetrating the Moine series in the Scottish Highlands. References: FORMAN, D.J., 1971

The Artunga Nappe Complex, Macdonnell Ranges,

Northern Territory, Australia.

Journ. Geol. Soc. Aust.,

Vol 18 (2), 173-182. FORMAN, D.J., MILLIGAN, E.N. and McCARTHY, W., 1967 Regional Geology and Structure of the North-Eastern Margin of the Amadeus Basin, Northern Territory.

B.M.R. Report No. 103.

SUTTON, J. and WATSON, J., 1962

An interpretation of Moine-Lewisan

Relations in central Ross-shire:

Geol. Mag., 19: 527-541.

PRELIMINARY INTERPRETATION OF THE BASEMENT STRUCTURES NORTH OF THE ARLTUNGA NAPPE COMPLEX 1

M.J. Rickard , and *R,D. Shaw

1 Geology Department9 Australian National University 2 Bureau of Mineral Resources, Canberra The rocks in the northeastern part of the Alice Springs 1:25,GOOl000 sheet are gneisses and metasediments.

Pioneer work

by Joklik (1955) subdivided these into several units, and the triple junctions, arrowhead and refolded fold patterns shown on his map clearly indicate double fold geometry * Detailed mapping of selected areas, together with reconnaissaince traverses and air photo interpretation show that at least three phases of folding have occurred accompanied by extreme attenuation and disruption under high

Presented with the permission of The Director, Bureau of Mineral Resources

Ial4 amphibolite fades conditions.

Some progress with correlation of the

lithological units has been achieved (see Fig 1), although the structural geometry is riot yet fully understood and recourse to plasticine models is made in an attempt to represent the structures. Lithological sequence.

Joklik's units (1 Entia gneiss; 2 Bruna gneiss;

3 Irindina gneiss - containing the Riddock amphibolite, a marble layer and the Naringa calcareous quartzite as important members; gneiss;

4 Brady

5 Cadney gneiss) are disposed in a simple dome-like arrange-

ment about the Inkamulla granite to the east of the Alice Springs sheet. In the southern part of the area Cadney gneiss lies beneath the Bruna and Irindina gneisses although the formations are separated by i

slides.

The Cadney gneiss can be divided into six units which from

the base read:1 Calc-silicate rock with minor quartzo-feldspathic (garnet biotite) gneiss and biotite gneiss. 2 Calc-silicate rock with minor biotite (sillimanite) gneiss 3 Lenticular quartzo-feldspathic gneiss, thickly foliated with pegmatite segregations. 4 Amphibolites, calcareous amphibolites and some biotite gneiss. 5 Fine-grained quartzo-feldspathic gneiss with some amphibolite and calc-silicate rock* 6 Porphyritic quartzo-feldspathic gneiss and leucocratic^^oMte gneiss with a few calc-silicate layers. A prominent porphyritic feldspar biotite gneiss (Bruna) is followed upwards by a thick amphibolite (Riddock) and the dark schistose biotite garnet (sillimanite) Irindina gneiss.

The latter formation con-

tains several amphibolite, marble, quartzite and calc-silicate layers. A thicker flaggy calc-silicate (Naringa) passing upwards into marbles forms the uppermost part of the formation. In the Upper Florence and Maude Creeks the Naringa formation outlines a large reclined fold that terminates northweistwards in a complex arrowhead and passes southeastwards into large recumbent folds (Fig 1).

Riddock amphibolite forms the core of the structure

surrounded by Irindina gneiss.

The quartzit^s and calc-silicates of

the Naringa Member become more calcareous outwards passing into massive bedded marbles and then interlayered gneisses, schists, calcsilicates and marbles.

These thicken greatly south of localities

A arid C and form complex tight interfolds in the arrowhead, locality B. Traces of grading and ripple cross Redding in the Naringa Member

I a 13(a) Figure 1(A)

Map of Ontiiston Pound - Mount Razorback Area west of Alice Springs, N.T.

90 Km

Figure 1(B) Sketch Sections to illustrate structure (not

Figure 2

Map 0 f

Gap Area,

Arltunga Nappe Complex

GEORGINA GAP A* FOLDED THRUSTS A—fiZZSCSSJBCT •

^^ 6 FC O0 C

Basement Retaliated

Basement

Autochthonous

Cover

Parautochthonous Allochthonous

Cover

scale)

Ial5 indicate that the sequence quartzite-marble is a natural one;

thus

the arrangement in the antiform at locality C suggests that the Bruna gneiss is younger than the Irindina.

A similar big feldspar

gneiss possibly representing attenuated Bruna gneiss occurs in finegrained quartz-feldspathic gneiss above the Naringa Member along the northern limb of the reclined fold (locality A). Although most of the mappable lithological units represent stratigraphic units, locally sequences appear in different order complicated by extreme attenuation and metamorphism; major slides also interrupt the sequence. i n addition some of the interlayered gneiss, calc-silicate and marble units clearly result from tectonic rather than stratigraphic interfingering. Structure

Three or four phases of folding are apparent.

The first,

and in places possibly also the second, are expressed as small isoclines in the gneissic layering; larger folds influencing the map pattern can also be recognized.

These were probably N-S recumbent

folds now tilted into reclined positions (Fig 2)• reclined fold hinges at localities A, D and E.

Examples are the

The repetition of the

Naringa Member in the arrowhead at locality B must also be due to early folding, although it is impossible to determine the fold closure. There is no obvious axial-plane schistosity and gneissic layering is generally parallel to the lithological layering;

early mineral

lineations are however strongly developed locally. The second-phase folds have a general E-W trend, overturned to the south.

They vary greatly from tight to open structures with

rarely developed axial-plane schistosity.

Small folds show wide

variations in plunge from reclined to horizontal within the axial plane (Fig 2).

In the Maude Creek arrowhead (locality B) cascades

of near-recumbent folds with disharmonic relations between isoclines and more open folds are complicated by attenuation, boudinage, and diapiric intrusion of the marble.

A£ locality C the Naringa flags

outline large near-recumbent folds. Third-phase structures are more open upright folds, and in places are associated with a rough fracture cleavage.

At localities

C and D they trend gently northeastwards, in the Maude Creek arrowhead they have an E-W trend and gently warp the recumbent folds. In the Tug and Muller Creek districts the fold sequence is somewhat different.

Two sets of isoclinal folds are shown by the

Ial6 distribution of quartzo-feldspathic gneisses within the calcsilicates (Fig 1).

Strong sliding was associated with these folds.

W.N.W.-trending reclined folds swing the earlier axial traces sharply but all the folds have the same plunge.

Occasionally, an axial-

plane schistosity is developed in tight fold cores.

This phase is

probably equivalent to the second,phase to the northeastf and the later gentle folding has not been observed. The lack of characteristic schistosity, the variability of fold plunge and style, and the addition of a third phase of folding considerably impedes structural analysis.

Examples of typical

structures and a generalized sketch of the structural arrangement are shown on figure 2.

Stereo plots of minor structural elements indicate

a remarkably uniform total geometry.

Poles to bedding and gneissosity

all lie in a broad girdle about a gentle to moderate northeasterly axis.

Secondary spreads of poles form a partial girdle about an

E-W axis representing the second-fold generation.

Where distinguish-

able F1 and F2 axes tend to correspond to these two directions.

All

fold axes and lineations, irrespective of age or type, lie in a broad girdle.defining a moderate to gentle northward - dipping plane that probably represents the axial-plane of the second phase folds (Fig 2). Reference JQKLIK, G.F., 1955:

The geology and mica fields of the Harts

Range, Central Australia. Bull. 26.

Bur. Miner. Resour. Aust.

la 15U)

Quartzo-feldspathic gneiss (Cadney in part) Big feldspar gneiss (Bruna) Marbles,gneisses & caic-silicates Calc-quartzi te flags (Naringa) Biotite, garnet gneiss (Irindina) Calc-silicate rock & gneiss( Cadney) Ampfaibolites

1 mile

FIG1

HARTS v RANGE

J1 GEQCHMICAL CHARACTERISTICS OF DIFFERENTIATED DYKES. AMD GREENSTONE SUITES OF THE YILGARN BLOCK WESTERN AUSTRALIA G.J.H. McCall - Research & Exploration Management Pty. Ltd. Geochemical results representing 225 full major element analyses, mostly by XRF methods, are evaluated. This study is complementary to papers presented by the author and colleagues at the Archaean Symposium of the GSA in Perth in May, 1971. Only certain new analyses are given in tabulation, but the variation diagrams presented are b&sed on my many additional analyses published in works referred to in the text. The emphasis of this study is a consideration of the relationship of the premetamorphic Archaean sills to the volcanics of the same greenstone belts:

to consider the original nature of the magma

differentiated after emplacement, and to more closely define the premet amorphic intrusive s and post-met amorphic (earliest Proterozoic) dyke rocks. A contrast is seen between magnesia-rich basic and ultrabasic eruptives represented amongst both sills and volcanics, and the abyssal-type K-poor tholeiitic suites of the monotonous pillowed lava sequences, studied in detail by Hallberg (in the press). A similar contrast is evident, though less emphatically, among the post-metamorphic dyke rocks. Residually enriched iron and alkali-rich differentiation suites are recognised among both premetamorphic and post-metamorphic eruptives. The possibility of some gradation between magnesia-rich and tholeiitic suites cannot be discounted: they are intimately intermingled in Space and Time, and amongst intrusive and volcanic sequences. Alkali/ Silica plots after Macdonald and Katsura show that, whether or not the magnesian rocks are regarded as tholeiitic, or "komatiitic11 (following Anhaeusser, Viljoen and Viljoen), no alkalic igneous rock suites are represented.

Spilites are.of no more than extremely

rare occurrence, according to Hallberg (op.cit). M A plots indicate that among the premetamorphic volcanics and intrusives, and also among the post-metamorphic dykes there are eruptives suites and differentiated bodies that resulted from accession of already differentiated melts or magmas from deep below the surface,

J2 possibly to suffer further differentiation after emplacement, in situ. That is, the parental stems to differentiated and undifferentiated sills and volcanics were products of an earlier, deepseated differentiation process. "What the nature of such a process was is unknown. A diagram based on solidification indices illustrates this. These results are discussed in relation to the "komatiites" of the Barberton Mountain Land, South Africa. The resemblance between komatiite analyses and those of the olivine tholeiites of Nockolds is stressed, and the suggestion is made that the highly magnesian suites of South Africa and Western Australia might well be regarded as an extension of the abyssal tholeiite association, rather than as a discrete compartment in petrology. Doubts are raised concerning the anti-uniformitari&n one burst connotation given to these highly magnesian suites, although it is accepted that these early eruptives ascended under very different conditions of geothermal background gradient* Figure 1

M A diagram (Total Iron as FeO; MgO; total alkalis) for the Yilmia Sills (McCall & Doepel, 1969) and host rocks of the Yilmia greenstone belt. The sills are highly magnesian, as are many of the host volcanics.

Figure 2

Alkali-silica diagram for the Yilmia sills and representative basalts from the Yilmia greenstone belt.

Figure 3

A comparison of solidification indices (100 x MgO/FeO + Fe2C>3 + MgO + Na20 * K2O) for various intrusions and greenstone belts; values from 35-40 are supposed to be primary un differentiated basalts according to Kuno (1967)*; above 40 are supposed to be derived from ferromagnesian cumulatesj low values represent considerable quantities of late differentiate material. The horizonral dimension is proportional to the number of rocks analysed.. The Golden Mile Dolerite and Binneringie Dyke contain predominantly late differentiate material, whereas the *In Hess, H.H. and Poldervaart, A.A. "Basalts11 (John Wiley, New York).

J3 m m & m . and Yilmia Sills, Marshall Pool Greenstone Belt (McCall & Leishman, 1970) and Jimberlana Dyke largely consist of material showing values indicative of accumulation of early crystal separations, according to theory. The Carnilya Belt (McCall & Leishman, op. cit.) contains similar material, as well as residually differentiated tholeiite sills. The discreteserpentinites of the type associated with nickel mineralisation have the highest solidification indices, exceeding 80.

J4 PRIMARY GEOCHMICAL PATTERNS OF ARCHAEAN IGNEOUS SUITES AND A MODEL OP EARLY CRUSTAL EVOLUTION A.Y. Glikson - Bureau of Mineral Resources A widespread retention of primary geochemical features by lowgrade metamorphosed Archaean igneous, suites in Western Australia, South Africa and Canada, is indicated "by recent independent research of these systems. The preservation of original characteristics issues from the water-deficient

low-pressure and low-grade type

of metamorphism, and is expressed by highly consistent chemical levels and ratios, as well as by systematic variations with time shown by volcanic and plutonic series. A synthesis of available geochemical data on Archaean systems is attempted, with the aim of elucidating the evolution of the Archaean geotectonic pattern in the light of recent studies of the geotectonic distribution of igneous provinces and the origin of magmas. The.principal observations afforded by the comparative study follow. The primordial crust consisted of an oceanic ultrabasic to basic assemblage segments of which are retained at basal stratigraphic levels of early Precambrian sequences in South Africa and Western Australia. Metabasalts

incorporated in these assemblages

compare with oceanic tholeiites, but have higher Mh, Ni, Cr, Co, Pe/Pe + Mg and Rb, and lower Al, Ti, Sr, Zr, Y, K/Rb and Pe+3/Pe+2 than recent oceanic tholeiites

(Table 1, Pigs 1, 2), In comparison

metabasalts incorporated in Archaean calc-alkaline volcanic associations, which overlie the oceanic ultrabasic-basic units through major unconformities, are more highly differentiated, as is shown by their higher K, Ba, Sr, Zr, and Pe/Pe+Mg values. The low alumina of the metabasalts of.the basal oceanic assemblages suggests their derivation through partial melting of the upper mantle at depths shallower than 15kn. The calc-alkaline sequences, on the other hand, are more likely products of a two-stage cycle, namely the partial melting of subsiding zones of the oceanic ultrabasic-basic assemblage;

this possibilityJLs consistent with the more differ-

entiated state of Archaean.basalts of calc-alkaline suites compared to those of oceanic suites. The high Ni, Cr, Co and Mn levels of the latter may possibly reflect high levels of siderophile and transition elements in the Archaean mantle;

it may be further

inferred that this has been related to a lesser degree of development of the core in the Archaean, a possibility consistent with the commonly high Pe/Mg ratios of Archaean basic rocks.

J3(a) HALLSERS'S AVCRACC TMOLEHTE OF MONOTONOUS PILLOWED SEQUENCES

YILMIA

volcanic breccias

BASALTS BETWEEN THE SILLS (im«a»oaiotiM* ond waked In ppwophyrwej BASALTS norite sab0ros amd GA8BR0S of upper

STRATIFORM SILL ROCKS

discrete . . s i l l rocks

SILL NORITE QAB0ROS AND GAB9ROS OF LOWER sax NORITES OF LOWER SILL FELSWATMIC BRONZITITE OF LOWER SILL HARZSURSITE OF UPPER SILL HARZBUROfTE OF LOWER SIU. TREMOLITE-* CHLORITE ROCKS (picnta basalts ?) SERPENTINITE DIFFERENTIATION TREND FOR SILLS

AfcWAfci/snacA nil

. HALLSCWGS AVERAGE BASALT FROM MONOTONOUS PILLOW LAVA SCQUtltttS ROCKS FROM THE STRATIFORM

SILLS

BASALT FROM YILMIA (ty*c«l MramtynmWb variety) I BASALT FROM YILMIA ( anomef ou«, tWD YWWa summit- nephelliw mrmctvf)

SILICA

S1TIS

THOCtllTIC UNKC

SiLXS

Iff •ASALTS

61115

TNEMOLITlC U LTIt AMAFlC 5 ( OLIVINE PTWOXENlTES , *C«tTE »ASAlTS SEHPEHTIMITE x

NORITE

6ABBR0

a F S R BROH2ITITE

l»e»lOOTtTES AMD WCRITES

STIIS

snis

•ASA.TS

NO PTHOXENlTE ANALYSES

S3MA0

6ABBR0

BASALTIC

OLIVINE

OOLERITE

K T » t t » SILLS

1*A0

S«Ai30PWY«e MATTE PWeOWTE

BASALTS

3*AQ S3XA0

bkmzite gasbag

•owitic

olivwc

oounrrt

TREMOLITIC

wmuroee UK* U*TMT MACS

ULTRAMAFICS

(UMITI MSALTSf)

J5 The earliest Preeambrian granites include granodiorites^ tanalites and soda granites,, Granitic cycles shoring a progressive decrease of the Na/lC arid K/Hb ratios represent an increase in the degree of.differentiation related to a thickening crust with time (Table 2)0

The high Na/K9 Hi and Or of early

sodic granites and porphyries of the K&lgoorlie System* suggests a derivation throu^i low degrees of partial melting of eclogite* a possibility supported by the occurrence of quarts phenocrysts and possible pseudomorphs after garnet in the porphyries * The earliest event following the formation of the oceanic crust was the subsidence and partial melting of linear zonesf and th© emergence of nuclei of sodic granite emplaced in the oceanic eras to Interbatholithic &ones of the oceanic crust subsequently evolved i^to the volcanic-sedimentary * greenstone belts % the pattern and development of which were controlled by the granitic nuclei* The 5greenstone belts9 are therefore not analogous to Alpine geosynclines or island-arc belts* The subsidence of the greenstone belts into warmer regions of the crust ensued in folding and low~grade metamorphism* following which the early sodic granites were aggregated into shields* The boundaries of these shields with the surrounding oceanic crust evolved into high-grade mobile zones9 possibly overlying subduction zones and analogous to island-arc systems* This abstract is adapted from a paper by the author (Glikson^ 1971)« References, Baragar WoRoA* and Goodwin A0M0 (19&9) Andesites and Archaean volcanism of the Canadian Shield* Ins Andesite Symposium (editor AeR* Mcbiraey), pp0 121-142* Oregon Dept0 Geol0 Min* lndustry0 Bliss NoWe and Stidolph P0A0 #(19^9) The Rhodesian Basement . Complex? A review0

In Upper Mantle Project, Geol* Soc* S*

Africa spec* publ* 2f 305-333* Cann JoRo (1971) Major element variations in ocean floor basalts* Roy* See, London Philo Trans* 268, 495-506~ Engel A*E* J* 9 Engel C a n d Havens R*G* (19^5) Chemical characteristics of oceanic basalts and the upper mantle* Geole Soc* Amer0 Bull, 76* 719-734

J6 Glikson A0Y0 (19?0) GeosynclinaX s o l u t i o n and geochemical a f f i n i t i e s of early Precambrian systems0

TectQnophysies

9? 397-433. Glikson AoYo (1971) Primitive Archaean element distribution patterns? Chemical evidence and geotactanic significance© Earth Planet 0 Sci Lett 0 12, 309-320* Hallberg J«A* (1970) The petrology and geochemistry of metamorphosed Archaean basic vetieanic rocks between Goolgardie and Norseman, Western Australia® PhoD* thesis 9 UniYofo Australia,, Joplin GoAo (1963) Chemical analyses of Australian- rocks 9 1s. Igneous and met amorphic rocks f BurolKneroResoiar^ Bullo 65 o Kuno Ho (i960) High alumina basalts* J» Petrols 19 -121.-445• Manson Y0 (19^7) Geochemistry of basaltic rockss Major elementse In Basalts (editors H0EL Hess and A0 Poldervaart) f pp* 21 269o

Interscienee*

Melson WoGe and Thompson Gc (1971) Petrology of a transfopa f a u l t zone and adjacent ridge segments0

RoyoSoc^ London

Philo Transo 268, 423-441*

lockolds SoRo (1954) Average chemical compositions of som@ igneous rocks 0

Geol0 Soc0AaierQBull0 65* 947os>950*

Q^Beirne W0Re (19^8) The acid pfiiiphyrietf and porphyroid rocks of the Kalgoorlie area*

Ph0D0 thesis f , Univ0W0Australiao

In Physics and Chemistry of the

Earth (editors L.E« Ahernb9 F 0 Press* S«K« Rimcorn and H0Ge Urey) YI* ppft 133°*2t3»

Pergamon Press*

Yiljoen M0J0 and Yiljoen ReP« (1969a) The geology and geochemi s t r y of the lower ultramafic unit of the Gnverwacht Group and a proposed new class of igneous rocks 0

Ins Upper Mantle

Project, G e o l * S o c A f r i c a spec* Publ* 2* 55-86* Tiljoen R 0 P. and Yiljoen McJo (1969b) Geological and geochemical significance of the upper formations of the Gnverwacht Group* Ins Upper Mantle Project^ Geol*Soc*S*Africa spec* publ 0 2 9 113-152o Yiljoen M0J0 and Yiljoen ReP0 (1969c) The geochemical evolution - of granitic rocks of the Barberton region 0

Ins Upper Mantle

Project, GeoloSoCcS0Africa spec0 publ 0 2 f 189-220*

J7 Pig. 1

: Relationships between iron (total as FeO) and magnesia in Archaean metabasalts of ultrabasicbasic assemblages (solid circles), Archaean metabasalts of calc-alkaline volcanic sequences (centred circles), and recent basalts (open circles). A- average of 10 oceanic basalts; B- average of 94 oceanic tholeiites;

C- average

of 33 mid-Atlantic Ridge basalts; D- average of 460 tholeiites from oceanic regions; E- average high-alumina basalt from Japan; F- average Hawaiian tholeiite. Fig. 2

: Cr-Ni relationships in Archaean and average basic and acid igneous rocks.

Solid circles - average

metabasalts from Archaean ultrabasic-basic assemblages.

Centred circles - average metabasalts

from Archaean calc-alkaline volcanic sequences. Open circles - average basalts. Crosses - average Archaean granites and porphyries. average granites. tholeiite;

Circled crosses-

1 - Average olivine-normative

2 - average quartz-normative tholeiite;

3 - average of 10 oceanic tholeiite; 4 - average of 33 mid-Atlantic Ridge basalts; granite;

6 - average granodiorite.

5 - average

J8 Table 1: Average compositions of metabasalts from Archaean ultrabasic-basic (Oceanic) assemblages^ and Archaean calcalkaline volcanic sequences, compared to averages of recent basalts and high-alumina basalts. Archaean ultrabasic-basic suites

S±02</o

Archaean calc-alkaline volcanic suites

average basalts

(1)

(2)

(3)

(4)

(5)

(6)

(A)

(B)

(c)

51.2

51.9

52.13

50.0

50.07

50.47

49.9

49.2

50.19

Ti02i°

0.9

1.01

1.09

0.97

1.22.

0.89

1.51

1.39

0.75

A1 2 03$

13.7

14.1

13.33

14.7

13.34

15.45

17.2

15.8

17.58

Fe 2 0 3 $

2.9

2.3

2.24

2.70

2.31

1.63

2.0

2.2

2.0

8.96

9.10

9.62

6.9

7.2

7.19

0.21

0.19

0.15

0.17

0.16

0.25

FeO$>

9.3

8.9

9.94

MnOio

0.15

0.18

0.21

MgO io

7.95

7.1

6.35

6.11

6.10

6.05

7.2

8.5

7.39

CaO$

10.4

10.3

8.98

8.93

9.47

11.76

11.8

11.1

10.5

Na 2 0io

1.7

2.3

2.97

2.21

3.34

1.68

2.7

2.75

0.14

0.26

0.39

0.54

0.57

0.16

0.26

0.40

2.01

K 2 0 io

0.15

H20

1.6

1.1

1.97

H2O - i

0.1

0.11

C02io

0.1

Total

99.95 99.53 99.62

Ba ppm

Cu ppm

2.95

1.1

0.06

0.12

0.23

0.2

99.00 98.92

99.96

1.81

Rb ppm

Sr ppm

217 133

Zr ppm

Y ppm

Co ppm

Cr ppm

407

Ni ppm

150

99.54 98.51 99.84

122

130

123

100

;300

144

297

:296

I350

110

123

Fe/Fe4Mg<- 0.6

0.6

0.65

0.64

O.64

0.54

0.52

0.56

0.31

0.25

0.22

0.31

0.25

0.16

0.28

0.30

0.39

Ca/Al

0.75

0.73

0.67

0.60

0.70

0.76

0.68

0.70

0.59

Na/K

11.5

16.4

11.4

5.6

6.2

2.9

16.9

10.4

6.9

*Fe = total iron as FeO (1) Average of 7 metabasalts from Coolgardie, Western Australia (Joplin 1963; Glikson, 1970) (2) Average of 13 metabasalts from Coolgardie, Western Australia (Hallberg, 1970) (3) Average of 4 metabasalts from the lower Onverwacht Series, Transvaal (Riljoen and Yiljoen, 1969a)

• ©

•

H O

<®

0 o

41 o

• • ©

—

K O F O

E O

1000

Cr ppm

100

I ,I

i I II i ion

Ni ppmi i i ,

J9 (4) Average of 134 metabasalts from Lake of the Wo ods -Wab i go on, Superior Province, Canada (Baragar and Goodwin, 1969). (5) Average of 4 metabasalts of the Kromberg Formation, Upper Qnverwacht Series (Viljoen and Viljoen, 1969b). (6) Average of 11 metabasalts of the Bulawayen Group, Rhodesia (Bliss and Stidolph, 1969). (A) Average of 10 oceanic tholeiites (Engel et al., 1965) (B) Average of 33 mid-Atlantic Ridge tholeiites (Melson and Thompson, 1971) (C) Average high-alumina "basalt from Japan (Kuno, 196o)# Table 2: Average chemical values of Archaean acid igneous rocks

(1)

(2)

(3)

(4) (5)

Si02

Na20

K20

Na/K

K/Eb

68.04 72.21 72.38 70.42 71.20

4.88 4.37 3.76 4.65 3.38"

1.78 3.57 4.67 3.61 5.10

2.7 1.2 0.8 1.3

488 278 134 486 293

434 282 205 280 242

—

7.4

67 465

298 281 882 91

239

5.68 4.09

0.49 0.74 2.28 4.44

6 22 19 16

9 23 19 14

3.90 3.84 3.35 3.08

1.42 3.07 4.58 5.46

150 120

285 450

(6) 74.6

(7)

74.0

(9)

74.28

(A) (B) (C)

66.15

(8) 71.28

(D) (E) (F)

66.88 69.15 72.08 .

6.03

10.7 15.1 8.1 2.5 0.9

105 189

107

175 17 -

251 79

2.74

1.25 "0.73

0.56

4 o 5 20 20

(1)

Average of 19 tonalites (3400-3200 m.y.) Kapvaal Shield, Transvaal (Viljoen and Viljoen, ^^6^o). Average of 3 gneiss-migmatite rocks (3000'm.y.). Kapvaal Shield, Transvaal (Viljoen and Viljoen, 1969c). (3) Average of 8 potash granites (3000tfuy«)>Kapvaal Shield, Transvaal 0£p.joen and Viljoen, 19&9c) (4) Average of 3 porphyritic high-level granites, Kapvaal Shield, Transvaal (Viljoen and Viljoen, 1969c). (5) Average of 4 porphyritic high-level granites, Kapvaal Shield, Transvaal (Viljoen and Viljoen, 1969c) (6) Average of 7 sodic granite pebbles from the Kurrawang Conglomerate, Kalgoorlie System, Western Australia (Glikson, unpublished results), (7) Average of 10 sodic*porphyry granite pebbles from the Kurrawang Conglomerate, Kalgoorlie System, Western Australia (Glikson, unpublished results) (8) Average of 50 sodic porphyries occurring as flows and sills in the Kalgoorlie System (about 2700 m.y.) (O'Beirne, 1968) (9) Average of 8 adamellites, granites and aplites intruded into the Kalgoorlie System about 2615 m . y , (O'Beirne, 1968) (2)

(A) (B)

Average of 58 tonalites (Nockolds, 1954) Average of 137 granodiorites (Nockolds, 1954) Average of 121 adamellites (Nockolds, 1954) Average of 22 calc-alkaline granites (Nockolds, 1954) Average granite (Taylor, 19^5) Average granodiorite (Taylor, 1965)

J10 THE PETROLOGY OF THE STRANGWAYS RANGE CARBONATITE D.C. Gellatly, Max, Western Australia The Strangways Range carbonatite, located in the Northern Territory near Alice Springs, is a lenticular complex of banded rocks that are concordant with the schistosity of enclosing Precambrian rocks of the Arunta Complex. The predominant rock type is a foliated biotite-calcite rock containing small amounts of sodic amphibole, apatite, and zircon. This is interlayered with dolomitic carbonatite con-> taining dolomite phenocrysts and with biotite-bearing calcitic carbonatite*

Country rocks adjacent to the complex have been

metasomatized resulting in the production of acid gneisses characterized by sodic pyroxene and amphibole, and of plagioclaseamphibolite characterized by a hastingsitic amphibole depending on original composition. Albite pegmatites containing minor sodic amphibole and pyroxene cut the complex. Nodules of serpentinite are present locally, and adjacent dolomitic carbonatite contains olivine and phlogopite. No feldspathoidal rocks are known in the area. Contents of Nb, Y, La, Ba, and Sr are within the ranges that are characteristic of carbonatite. Nb values, although lower than in most carbonatites, exceed those found in sedimentary limestone and are apparently derived from Nb-baaring ilmenite. Rare earths, particularly lanthahum, are contained mainly in apatite. The complex has possibly been formed by a phase of lit-parlit injection of carbonatite magma and associated metasomatic activity, and may be the deep level expression of a normal high level eruptive carbonatite. ARSENIC AS A GUIDE TO GOLD MINERALIZATION AT THE WATTLE GULLY AND SAMBAS MINES - E.G. Bowen, Mines Department, Victoria By 1967, after over 100 years of gold production only three gold mines the Wattle Gully Mine Chewton, Sambas Mine Harrietville and A1 Mine Gaffneys Creek were operating in Victoria. Although arsenic is widely quoted in the literature as a pathfinder for gold no information was available on the association or distribution of

J11 arsenic in relation to Victorian gold, deposits.

The results

presented here are of work carried out at the Wattle Gully and Sambas Mines, but they may have a wider application. The purpose of the investigation was to determine the presence or otherwise of arsenic haloes of sufficient size to be of value in the location of further ore bodies or reef systems.

Consequently the investigation may have failed to

locate phenomena within a few feet of the ore veins. WATTLE GULLY MINE The Wattle Gully Mine has produced 341,074 ozs of gold from quartz reefs associated with a complex 4 5 ° west dipping shear zone y/hich intersects east dipping Lower Ordovician slates and sandstone.

It consists of a main hanging wall component and

subsidiary footvrall components which split off at successively lov/er levels down the northerly pitch of the main component. The arsenic content of the samples do not show a regular decrease with increasing distance from ore.

All values however

lie within a well defined envelope with a lower limit of 5 PP^ and an upper limit decreasing from 1000 ppm to 15 V V

450 ft

from ore. Statistical interpretation of the results using Student's n

test shows that there is no significant difference in the

arsenic content of slate compared with sandstone (sets 1 and 2 Table 1) but the mean arsenic content within 300 ft of ore is significantly higher than that more than 300 ft from ore (sets 3 and 4 Table 1, p <.05 for a two-tailed test). Table 1:

Summary of Mean and 90fo Confidence Limits for Sets of samples from Wattle Gully Mine.

Lithology of set

Slate (1) Sandstone (2) Slate and Sandstone Slate Sandstone Slate and Sandstone(3) Slate Sandst one Slate and Sandstone(4)

90$ Confidence Limits (ppm As) Lower Upper

N o , of samples

Mean (ppm As)

0+ 0+

53 35

34 46

20 20

o-3bo

0-300

39 27

41 58

22 24

60 91

0-300

300+ 300+

14 8

14 6

11 5

18 8

300+

Distance f yre

J12 SAMBAS MINE Production from the Sambas Mine is estimated at 40,000 oz obtained from narrow quartz reefs associated with

a branching

system of steeply dipping bedded faults or walls. The faults represented a possible zone of entry or leakage of ore bearing fluids and dispersion haloes may be preserved in the fault pug in addition to the wall rocks. Because of the method of prospecting in the mine the arsenic content of the fault pug is more useful as an aid to prospecting than that of the wall rocks. No correlation exists between the arsenic content of the fault pug and distance from ore. There is however a highly significant difference (p<.001 for a two-tailed test) between the main arsenic content of pug from faults associated with ore (1310 ppm) and those not associated with ore (238 ppm). Table 2. Summary of Mean and 90$ Confidence Limits for arsenic content of Sets of fault pug samples from Sambas Mine. 90io Confidence Limits of Mean (p-pm As) Lower Upper

Set

No.

Mean (ppm As)

Faults associated with ore

1,310

866

1,753

Faults not associated with ore

238

106

370

APPLICATION OF RESULTS Analysis of wall rocks for arsenic should be carried out as a matter of routine in any exploratory work at Wattle Gully. Comparison of these results with those given above for the halo zone can be made using normal statistical techniques or the nomograph of Pig. 1. The curves represent the 90 percent one sided lower limit of the mean of samples within 300 ft of ore and the 90 percent one sided upper limit of the mean of samples more than 300 ft from ore and are plotted against number of samples. The use of the nomograph is illustrated with fictitious and real data. In general at least eight samples are required for a meaningful result. Although these results apply specifically to Wattle Gully, haloes may also exist in other goldfields in Western Victoria

J13 such as Ballarat* Daylesford, St. Arnaud and Bendigo which in many aspects are similar to Wattle Gully. Testing of fault pug samples from the Sambas Mine to determine whether they are anomalous can be carried out by similar methods outlined for Wattle Gully. In the use of the nomograph (Fig. 2) which is illustrated with real data at least five samples are required for a meaningful result. It seems likely that in many vein type deposits there is a similar difference in the arsenic (or some other pathfinder) content of the fault pug between faults associated with ore, and those not associated with ore although spatially the two may be close together. Routine fault pug sampling and analysis for the mineral mined or suitable pathfinders is probably a worthwhile aid to exploration therefore in many vein deposits of gold, antimony, silver, mercury and possibly also copper, lead and zinc. Fig. 1

Nomograph for determining anomalous samples, Wattle Gully Mine.

Fig. 2

Nomograph for determining anomalous samples, Sambas Mine.

J14 THE Bi-Pb-S-Se MINERAL IN THE JUNO OREBODT, TEMANT CREEK K.J. Henley-and P.K. Schultz, AMDEL, Adelaide The Juno mine operated by Peko Mines N,L. at Tennant Creek Northern Territory, produces a gold-bismuth-lead-copper ore with a significant content of selenium. Typical head assays are 55 dwt/ ton Au, 0.5$ Bi, 0.1$ Pb, 0.2$ Cu and 0.05$ Se. Detailed mineral ogical investigation of mill products and underground samples has shown that virtually the whole of the bismuth, lead and selenium is present in a complex solid solution mineral, termed "bismuthinite". Copper is present predominantly as chalcopyrite;

gold is present as native metal, although some

may also be present in other forms. The main gangue mineral is magnetite, through which the "bismuthiriite" is irregularly disseminated. Minor amounts of hematite, pyrite, chlorite, quartz, carbonate and talc are also present. Electron probe analysis of a large number of grains of "bismuthinite" has established the following approximate range of element contents : Bi Pb S Se

34 - 82$ ' 0 - 40$ 5-18$ 0-25$

Lead and bismuth vary antipathetically, as do sulphur and selenium, and there appears to be complete solid solution within the above ranges. Reference is made to previously recorded occurrences of "bismuthinite" and to experimental work, mainly on the system Bi-Pb-S. MINERAL LAYERS IF NORTH-EAST TASMANIAN GRANITES J.D. Cocker, Geology Department, University of Tasmania In the high level, Devonian, granitic rocks of northeastern Tasmania several intrusive phases have characteristic mineral layers containing up to 70$ biotite, hornblende, and garnet. These layers are subhorizontai, are usually composite with an upper felsic portion, and occasionally occur as rhythmic and cross-cutting sequences.

Ji3(a) W A T T L £ GVJLLY

P.gl 100

Anomalous samples within 300 f t . of ore

Arsenic content (ppm)

Upper limit 300'+ 10

Background samples more than 300ft from ore

Moving Average Indeterminate

Bore

No.73

Interval

B-C

Fig.9

Interval

A-B F/g.9

Chewton

(further sampling required)

Number of samples

SAMBAS

Faults with ore

1000

Indeterminate Indeterminate (further sampling required)

Upper limit faults without ore

Arsenic Content (ppm)

500

Faults without ore

100

Moving Averages Crosscut

420S

Crosscut

180S

East Wolff

101

Number of Samples

Wall

100N

Reef

400 S

J15 Extremely well developed rhythmically layered and crosscutting sequences define a well exposed basin shaped structure in the Binalong Bay Pluton, on the south-eastern side of the Blue Tier Batholith.

Strike lengths of groups of mafic layers exceed 60 m

and rhythmic sequences

over thicknesses of 3 m are common. Many

layers are cross-cutting, there is little evidence of mineral alignment and there is evidence for minor faulting and folding of the layers during and after their formation. The upper felsic portion of several layers contain numerous miarolitic cavities. In several plutons red garnets are a common accessory (less than 2<fo) and on Cape Barren Island the garnets are partially replaced by biotite clusters. Euhedral garnets form layers parallel to biotite rich layers in several exposures, and the whole structure indicates that flow processes and gravitational accumulation may have been involved in their segregation. Studies of the composition of the various layers, the component minerals, and their host rocks are underway. Field studies suggest that gravitational accumulation, combined with flow processes may be involved, possibly as a result of lowered magma viscosities within restricted areas or at certain times during crystallizat ion of the magma. Widely distributed fragmentary layering possibly indicates that the segregation process was widespread and that later flow processes disrupted the layering. It is hoped that detailed laboratory studies of these layered granites may further elucidate the crystallization history of large granitic plutons. COINCIDENCE IN GRAIN BOUNDARIES G.S. Lister, Department of Geophysics, Australian National University Suppose that two atomic models of a crystalline structure are held in an arbitrary relative orientation, and that additions are made, to these models until they meet to form a mutual grain boundary.

Some atoms in this' boundary might belong to both

structures if their locations are appropriate, and if bond angle requirements can be met. The locations common to both structures are termed coincidence sites, and the atoms coincident atoms. Should the density of coincident atoms exceed an arbitrary value

J16 that has been chosen as significant, then the above grain boundary model is termed a coincidence site grain boundary. The amount of coincidence in the grain boundary is related to the amount of coincidence that exists in a superimposition of the structures in the same relative orientation.

For certain

relative orientations the pattern of coincidence assumes a significantly higher density, and this result applies also to coincidence in the grain boundary.

These special relative orien-

tations are KROEBERG-WILSOU relations. Relative orientation decides.whether a particular boundary may fit the coincidence site model.

It is therefore important to

calculate complete lists of K-W relations if the applicability of the model is to be tested. Previous work has not been general, and there is no certainty that existing lists for cubic lattices are complete.

Important relations may not be evident in an

inspection of point group symmetry, especially in non cubic lattices.

Although BOLBIAOT has developed interesting concepts

his numerical methods are not suited to the task described, nor have they application to non primitive crystal structures such as hexagonal close packing, or quartz. Computational procedures have been developed that allow calculation of complete lists of these relations up to an arbitrary density and repeat distance chosen as significant, for any lattice, and ydth less generality (because of motif considerations) for any crystal structure.

Lists for the PC, BCC, FCQ lattices and the

quartz structure are in the initial stages of computation. Experiments with metals indicate that during recrystallization new grains grow fastest if they are oriented relative to the old grain matrix by approximately a K-W relation.

The coin-

cidence site grain boundary plays an. important part in the development of metallurgical fabrics.

This may be due to less

interaction with impurities because of better fit, or due to lower free energy, but whatever the cause K-W relations may also be important in the petrofabric evolution of metamorphic rocks. Their computation is therefore a prerequisite to exploring the relevance of this important metallurgical concept to studies of recrystallization of rock forming minerals.

J17 Selected References. K.T. AUST and J.W. ROTTER in "Recovery and Recrystallization of Metals". Interscience, New York, 1963, p 131. P. GORDON and R.A, VAWDERMEER in "Recrystallization, Grain Growth and Textures11. ASM, Metals Park, Ohio, 1965, p 205. W. BOLLMANN "Crystal Defects and Crystalline Interfaces". Springer Verlag, Berlin, 1970. THE RELATION OF PETROLEUM COMPOSITION TO SOURCE, MATURATION AND MIGRATION IN AUSTRALIA T.G. Powell and D.M. McKirdy Bureau of Mineral Resources Oil discoveries have "been made in a number of widely separated Australian sedimentary basins yet by world standards the oils are all uniformly light. This feature prompted a geochemical investigation in an attempt to assess the factors which might have contributed to their present composition. Thirty oils from ten sedimentary basins have been distilled and fractions collected at 50°C intervals. The specific gravity of each fraction was measured and the data used to calculate U. S.B.M. correlation indices. These correlation indices enable oils to be classified in terms of their dominant hydrocarbon constituents. Thus values up to 30 indicate a paraffinic base, 30-40 a napthenic base and values over 40 an aromatic base. The cuts boiling above 250° were combined and the asphalt content determined. The combined fractions were separated into saturated hydrocarbons, aromatic hydrocarbons and compounds containing oxygen, nitrogen and sulphur (0NS) using liquid chromatography. The saturated hydrocarbons were analysed by capillary gas chromatography. In general Australian oils have API gravities greater than 35, low sulphur and asphalt values and are of paraffinic-napthenic base. With the exception of the Conloi crude, the oils from the Bowen-Surat Basin show similarities in the composition of the fractions boiling above 275°C. The correlation indices for these fractions fall in the range 27-31 and the composition of the oil boiling above 250°C is constant at 81 fo saturated hydrocarbons 13ft aromatics and &fo 0NS compounds.

In contrast the fractions boiling

above 275°C have a correlation index of 50 indicating a napthenic aromatic base, yet the results of the column chromatography show a

J18 composition similar to the other oils (82$ saturated hydrocarbons, 10io aromatics,

OSS compounds).

Gas chromatographic analysis

of the saturated hydrocarbons indicates that they comprise almost entirely a complex mixture of branched and cyclic alkanes with only small amounts of straight-chain alkanes. This feature probably accounts for their high correlation indices. The lower-boiling fractions of the Bowen-Surat Basin oil& show considerable variation. Thus, of the fractions boiling between 125°C and 175°C, the Duarran oil has a correlation index of 46 (napthenic-a^omatic)

whilst the Dirinda oil has a correl-

ation index of 17 (highly paraffinic), Hence although these oils may have a similar source, considerable differences must have arisen during migration. The phenomenon of 1 separation migration* would affect the lighter fractions to the greatest extent and may account for the results observed. The oil samples from the Cooper, Gippsland and Gtway Basins are all paraffinic. The gas chromatograms of the saturated hydrocarbons obtained from the Marl in (Gippsland) and Yanpurra (Cooper) oils are remarkably similar particularly since they both have a very high isoprenoid content. BROOKS AND SMITH (1969) have attributed the formation of oil in the Gippsland Basin to generation from terrestrial organic material. The non-marine character of the host sediments and the similarity in hydrocarbon distribution of the Yanpurra oil to the Marlin oil suggests a terrestrial source for the Cooper Basin oils. The distribution of oil and gas in the Cooper Basin seems partly to be controlled by regional 'eometamorphism'.

Thus, in the Moomba area, dry gas only is encountered

in association with high rank coals. In the Gidgealpa area gas and condensate are encountered together with slightly lower rank coals (BROOKS 1971)• Presumably maturation has not proceeded to the same extent to the north east where oil is found (e.g. Yanpurra). The two oils from the Otway Basin, Port Campbell and Plaxmrns, each show a slight odd predominance of n-alkanes in the range c

20~c27o

This feature is associated with immature oils derived

from terrestrial sources (MARTIN et al. 1963), In the Carnarvon Basin, the Barrow-Windalia crude differs markedly from the deeper Jurassic oils from the Barrow and Legendre Wells, The Jurassic oils are primarily napthenic whilst the

J19 Windalia crude is distinctly aromatic. The Windalia crude contains a higher proportion of ONS compounds and its saturated hydrocarbons comprise a complex mixture of branched and cyclic hydrocarbons.

In the Jurassic crudes the n-alkanes are major

constituents. The relatively high content of GETS compounds in the Windalia oils is perhaps indicative of a lower level of maturation or may indicate a more euxinic environment for its source rock. The Uramu oil from the offshore part of the Papuan Basin is slightly rich in sulphur (0.6$) compared to the other oils studied so far. This is consist ©n't with, its occurrence in a limestone reservoir. The correlation indices show that the Uramu oil is aromatic, whilst a seepage sample from the Papuan Highlands is highly paraffinic. The Mereenie crude is the only Lower Paleozoic oil so far found in Australia and is highly paraffinic in nature. There is a pronounced odd predominance in the n-alkane distribution below C20

This feature has been reported previously for Lower

Palaeozoic oils. A major control of the composition of Australian crude oils appears to be -the depositional environment of the source rock* Many of the oils show evidence of having been generated, at least in part, from terrestrially derived organic matter*

The location '

of all but one of the reservoirs within sequences dominated by the sandstone - shale association is consistent with the likely contribution of land plant detritus to their source environment. However the paraffinic nature of many of the oils may also to some extent be due to relatively high levels of maturation. On the other hand the low sulphur and asphalt values reflect the scarcity of favourable carbonate-evaporite source and reservoir situations in Australia. REFERENCES BROOKE J.D. and SMITH, J.W. (19^9)

diagenesis of plant lipids -4

during the formation of coal, petroleum and natural gas IL Coalification and the formation of oil and. gas in the Gippsland Basin. Geochimet Cosmochim. Acta 33> 1183-94* BROOKS, J.D. (1971) The natural conversion of oil to gas in sediments of the Cooper Basin. J.Aust. Pet. Exploration Assoc. 11, 121-125

J20 MARTIN, RoLo WINTERS, J.C. and WILLIAMS, J.A. (1963) Distribution of n-paraffins in crude.oils and their implications to the origin of petroleum Nature 199, 110-113. EOMETAMQRPHISM OF ORGANIC MATTER IN PRECAMBRIAN AND CAtCBRIAN SEDDJEMTS D.M, McKirdy, Bureau of Mineral Resources To date, all major organic geochemical studies of the Precambrian have been marked by a lack of metamorphic control, In comparing the 'organic geochemical facies1 of different ancient sediments one would normally expect to find similarities or differences according to the types of precursor organisms. Since 1

chemical fossils1 are susceptible to thermal alteration, such

comparisons are unlikely to be very instructive without prior knowledge of the relative metamorphic grades of the host rocks. The hydrocarbons and insoluble organic matter (kerogen) from twenty-five sedimentary rocks (13 shales, 10 carbonates) of Archaean to Middle Cambrian age, were examined. Shales and carbonates containing small but significant amounts of organic matter are of widespread occurrence throughout.the Precambrian and Cambrian sedimentary records of Australia*

Carbonaceous shales date from

the Archaean' and comprise perhaps the most common organic-rich lithofacies in sediments older than 500 m.y. Carbonates are rare in the Archaean but become increasingly important throughout the Proterozoic until, in the Cambrian, they are a major rock-type. Their total organic carbon content (0.01 - 0.12$), however, is at least one order of magnitude lower than that of shales (0.4-1.7 of similar age. Illite 10°A peak sharpness ratios (Weaver, i960) were used to establish the relative eometamorphic rank of the sediments. Changes in the elemental composition and the structure of the kerogens isolated are indicative of their progressive graphitization with r." advancing eometamorphism. Both the hydrogen to carbon atomic ratio and the <3qq2 repeat distance of the incipient graphite lattice appear to have potential as quantitative indices of organic metamorphism0

Comparison of the Cambrian and Precambrian samples with

less mature Tertiary kerogens similarly derived from marine micro- . organisms (as distinct from higher land plants), demonstrates their apparent structural and compositional continuity0

J21 Study of the hydrocarbons revealed important differences in the responses of the soluble organic content of shales and carbonates to diagenesis and subsequent eometamorphism.

It is suggested

that the majority of the shales originally contained montmorillonite, in which case flushing of hydrocarbons from the compacting argillaceous sediment may have occurred upon expulsion of inter-layer water from the montmorillonite lattice as it altered to illite (Burst, 1969). The early lithification and incompressibility of carbonate mud (Bathurst, 1970) is thought to have prevented initiation of this dehydration and consequent loss of hydrocarbons from the ..carbonates, thus explaining their higher extraction ratios, Montmorillonite was found in the majority of the carbonates. The greater proportion of aromatics in the hydrocarbon fraction of the carbonates is attributed to the generally higher free sulphur content of these sediments. Elemental sulphur is a powerful dehydrogenating agent. With increasing eometamorphism, the total hydrocarbons decrease, both in absolute concentration and as a percentage of the total organic matter. In carbonates, alkanes and aromatic hydrocarbons are destroyed at approximately the same rate; in shales, aromatics are lost more rapidly, possibly as a result of claymineral-catalysed reactions. Thus, the sedimentary features which lead to the differentiation of organic metamorphism in shales from that in carbonates seem to be: (i)

lower free sulphur content;

(ii)

conversion of primary montmorillonite to secondary illite;

(iii)

greater compressibility and slower rate of lithification; and

(iv)

higher concentration of clay mineral catalysts.

These factors to be taken into account in any comparative study of hydrocarbon patterns in ancient sediments of different lithofacies. Gas chromatographic analysis of the alkanes isolated from representatives of both rock types revealed further evidence of their eometamorphic alteration. Progressive early metamorphism of the shales appears to have resulted in generation of branched and cyclic alkanes by catalytic cracking of precursor hydrocarbon chains. Carbonium ions were probably intermediates in the cracking process•

In the absence of high concentra-

tions of dispersed clay minerals, as in the carbonates, organic maturation is more likely to have been mainly a thermal process involving free radicals. In contrast to catalytic cracking, thermal cracking of hydrocarbon chains is characterized by a lack of alkyl isomerization so that

J22 the preferred products are normal alkanes (Jurg and Eisma, 19&9)* This accords with the observed increase in concentration of normal alkanes relative to branched and cyclic alkanes with increasing eometamorphic rank of the carbonates•

In general, both types of

cracking may be expected to result in alkanes of lower molecular weight. However, bimodal alkane distributions in the Skillogalee Dolomite, the Urquhart Shale and a carbonaceous shale from the northern sector of the Kalgoorlie greenstone belt, suggest that, in some cases, chain-lengthening reactions may have occurred* The relative concentration of the isoprenoid alkanes, pristane and phytane, appears to be a function of the lithofacies of the host sediment, and possibly also of its eometamorphic rank*

It is poss-

ible to explain the higher pristane to phytane ratios in shales in terms of a greater exposure of the parent organic matter to oxidizing conditions en route to final lithification*

The observed

variation of pristane to phytane ratio with increasing eometamorphic rank in each lithofacies is similar to that reported in coals (Brooks et al., 1969)* REFERENCES BATHURST, R.G.C., 1970 : Problems of litMfication in carbonate muds. Proco Geol. Assoc., 8l, kZ9-kkO. BROOKS, J.Da, GOULD, K. & SMITH, J.W., 1969 : Isoprenoid hydrocarbons in coal and petroleum* Nature, Lond., 222, 257-259• BURST, J*F*, 1969 ? Diagenesis of Gulf Coast clayey sediments and its possible relation to petroleum migration*

Bull* Am* Assoc*

Petrol* Geolo, 53, 73-93* JURG, J*W*, & EISMA, Ee, 1969 s Fundamental aspects of the generation of petroleum*

In Eglington, G* & Murphy, M*T*J* (Eds*)* Organic

Geochemistry: methods and results, pp. 676-698*

Springer-

Verlag, New York. WEAVER, C*E., i960 t Possible uses of clay minerals in search for oil* Bullo Am* Assoc* Petrol* Geol., kk^ 1505-1518*

J23 PSEUDOMOBPHOUS REPLACEMENT OF TOURMALINE BY PREHNITE, AND KYANITE-ANDALUSITE BY TOPAZ. G 0 Derrick,

Bureau of Mineral Resources.

The study of pseudomorphs allows greater understanding of mineral structures, the nature of the metasomatic transfer of elements and the metamorphic history of rocks in which the pseudomorphs occur* Prehnite after tourmaline. The replacement of tourmaline by prehnite has been noted at Mary Kathleen, in coarse-grained segregations of epidote which occur on the top benches of the open cut* host rocks.

Amphibolite and schist are the

Tourmaline forms anhedral masses and e&hedral trigonal

crystals 1 to 15 mm diameter; the former are brecciated and veined by prehnite, while the latter show all stages of pseudomorphous replacement by coarse-grained prehnite® Tourmaline (schorlite) has the formula Na(Fe, Mn^AlgB^Si^y in which F and OH radicles substitute for one another, the basic structure is that of SiO^ tetrahedra arranged in a ring, and in which there is no substitution of A1 for Si» as Ca2Al2Si^010(0H)29 with Fe

Prehnite can be written

substituting for Ale

The structure

of prehnite is complex, consisting of SiOg octahedra linked by paired tetrahedra with a ring of eight SiO4 tetrahedra«

Aluminium may sub-

stitute for silicon in one or both of the paired tetrahedral link* » Elements gained by or lost to the prehnite-tourmaline system may be determined by comparison of chemical analyses and corrected mineral formulae * The chemical analyses of prehnite from Mary Kathleen and an average schorlite are listed below: Since the replacement process is entirely pseudomorphous, equal volumes of tourmaline and prehnite can be considered in the correction of the formulae, using the method of Poldevaart (1953)•

The mole

volumes of both tourmaline and prehnite are found using molecular weight and specific gravity, and the two volumes are made equivalent by use of a certain factor*

This factor has been calculated to be 1*9> and the

revised formula of prehnite can be written 1e9(Ca2Al2Si301C)0H2) Ca 3 t 3 Al 3 e 8 Si 5 o 7 0 1 9 (Oh^ o 8 .

J24 Schorlite

Prehnite

Gain/Loss

Si02

3 4 . 66

^5.77

+11.11

Ti02

.21

0.07

B2O3

9.48

Alo0, 2 3 (Fe 2 03 (Fe 0

32.90

MnO

.14

- 9.48 25.57

- 7.33

14.05

1.03

-14.02

0.63

0.02

MgO

1.97

0.65

- 1.32

CaO

0.61

26.54

+25.93

- 2.30

2.30

2°

.61

k2o F

0.09

H2O+

2.96 Total

99.86

99 <>65

Thus, in the replacement of tourmaline by prehnite elements lost completely to the system include sodium, manganese, boron and most of the iron; elements lost from tourmaline include aluminium and probably oxygen in tetrahedral sites, and the element gained by the system is calcium.

The corrected chemical formulae

suggest that OH radicies and silicon have remained constant in the replacement, although the analyses show a relative gain of silica. The precise nature of the replacement process is beyond the scope of this paper.

However, one of the fundamental structural

differences between the two minerals is the Al-Si replacement in the tetrahedral site, which is possible in prehnite but not possible in tourmaline.

It is suggested that the prehnite structure couldc

develop from tourmaline if A1 could be induced to substitute in the Si sites in tourmaline.

A mechanism by which this could occur'"Is*""

outlined below. Deer, Howie & Zussman (1962) note that the boron-oxygen bond in tourmaline is responsible for tjie lack of A1 substitution in Si sites. Only a Si-O-Si bond is strong enough to exist in its presence, and the bond Al-O-Si, if it formed, would be too weak to exist in the presence of co-ordinated boron.

Thus it is suggested that the removal of boron

is fundamental to the collapse of the tourmaline structure.

J25 Differential thermal analysis indicates that boron is lost from tourmaline near 1000°C.

Since it is unlikely such temperatures

were attained in the Mary Kathleen area, it is suggested that stress, as indicated by shattered tourmaline grains and microfaulting in the epidote gangue, may have been a factor in the removal of boron. Topaz after kyanite after andalusite Kyanite and topaz pseudomorphs after andalusite have been recorded from the West Kimber^ey area, Western Australia, in a finegrained, chert-like topaz matrix. The pseudomorphed andalusite appears to be a product of low-pressure regional metamorphism; kyanite has developed from it during subsequent Barrovian dynamothermal metamorphism, possibly accompanied by local alumina metasomatism. The origin of the topaz in the kyanite deposit is obscure, but derivation from locally fluorine-rich epizonal granite is thought most likely. REFERENCES DEER, W., HOWIE, R., and ZUSSMAN, J., 1962 - Rock Forming Minerals V.1. Longmans, London. POLDEVAART, A., 1953 - Petrological calculations in metasomatic processes.

Am. J. Sci. V. 251 •

BURIAL METAMORPHIC HYDR0G1RNET Keith A.W. Crook & G.R. Kelly, Geology Dept., A.N.U. V.Z. Bliskovskii (1) in an authoritative review of hydrogarnets, noted that they had not been encountered as authigenic minerals in sediments. nized.

He predicted that this mode of occurrence vtould be recog-

We now describe an unequivocal occurrence of hydrogarnet as an

authigenic mineral in a burial metamorphosed plagioclase arenite interbedded in mudstones of the Namoi Formation (L.Carb.) at Merlewood, N.S.W.

(Precise location: creek north of Pidgeon Hill, Por. 62, Par.

Babbinboon, 8/2 chains west of eastern boundary - vide (2), pi. XVII). Mineralogy: The hydrogarnet occurs as euhedras 0.2 mm.

with transparent rims.

Anomalous gray-blue birefrigence is irregularly

developed.

The euhedras

from 0.06 - 0.32 mm

diam. with a mode at

Physical parameters are:

are colourless, commonly

J26 Table 1: measured

calculated Sample B Sample A

11.870^0.007

1.753-0.005

1.758

1.75*+

SG 3.66 3.671 3.651 Electron microprobe analyses, obtained with the assistance of M.N. Ware, gave: Table 2: Ti02

Sample A 37.57 0.26

AI2O5

18.42

Si02

Cr203 Fe

Sample B 38.39 0.33 19.08

< 0.03

0.03

6.70

5.19

tot 2°3 Mn02

2.15

2.29

MgO

0.18

0.22

CaO

32.52

Na20

<0.04

34.31 <0.04

K2O

97.80

99.84

Analysis of volatiles on a..concentrate slightly contaminated with calcite, by J. Wasik and M. Kaye gave; Table 3i H2O

uncorrected 1.00

H 2 O"

0.22

C02

0.88

corrected 0.98 0.216

Recalculation of the results in Table 2 using a value of 1.00% for H 2 0

and a "Garnet Structural Analysis Program" designed by M.N

Ware gave values for FeO and Fe20y SG,, and n D , and distribution of cations between x, y, and z sites. Of the various possible results, those which gave SG and n^ values closest to measured values (Table 1) are reproduced below:

J27 Table 4s

Sample , 5.31

3.02

FeO

1,26

0.16

Total (incl.H 0 + )

98.67

Ionic proportions Fe

Total

Sample B

Fe 2 0 3

100.82 remainders

0.081

remainders 0.010

0.141

0.146

0.021

0.025

2.690

2.771

0.0

2.9331

2.9317

A1(VI)

1.676

0.030

1.696

0.013

0.019

0.309

0.284

0.0

0.002

2.0000

0.0446

2.0000

0.0322

2.901

-0.032

2.894

-0.058

Al(VI)

0.0

H/4

0.129

0.126

3.0297 -0.0321

3.0194 -0.0580

In terms of garnet end-members, compositions are as follows: Sample A Mol % Wt % 4.390 2.495

Table 5: Hydrogrossular

Sample B Mol % Wt % 4.258 2.429 0.091

0.100

Andradite

15.784

17.658

14.456

16.231 74*708

Uvarovite

Grossular

71,551

70.955

75.068

Pyrope

0.706

0.627

0.837

0.746

Almandine

2.775

3.041

0.336

0.369

Spessartine

4.794

3.225

4.953

5.417

Paragenesis:

The framework of the hydrogarnet-bearing arenite

consists of plagioclase ( $ = 1•536: An^) with minor volcanic rock fragments and biotite* comprise the cement• rock*

Abundant calcite, some chlorite and laumontite Veins of laumontite and stilbite transect the

The hydrogarnet euhedras occur in the cement, often in clusters

at the contacts between albite grains, against which they show sharp contacts.

The hydrogarnet is generally in contact with calcite and

sometimes with laumontite as well, There is no evidence of disequilibrium between phases*

J28 This paragenesis indicates burial metamorphism in the zeolite facies, with low ratio o f / ^ Q Q

0 ^^'

REFERENCES: 1. BLISKOVSKII, V.Z. (1968). Akad. Nauk SSSR, Min. Muz, im. A.E. Fersman, Trudy 18, 2k-kO. 2. CAREY, S.W., (1937). Proc. Linn. Soc., N.S.W. 62, 3^1-376. 3. COOMBS, D.S., etal. (1970). Am. J. Sci., 268, 142-136.

AUTHOR INDEX Allisori GB Ayres DE

D15 112,, H12

Baker WE Barry J Beevers JR Bettenay E Bloom H Bo land. JN Bowen KG Brown MC Bums KL Butt CRM

B8, D9 H3 B17 D39 D12 A15,, 119 J10 C1 125

Clema JM Cocker JD Conybeare CEB Cook PJ Cooper W Crook KA¥

B12 J14 C15 C20 B5 F1, G5, J25

Daily B Dalgarno R Davies HL Davies PJ Derrick GM Dimmock GM Dodson RG Doutch HF

C10 B12

C14, D2J B7, H16, 18, J23 D46 C19 E10

Eadingxon PJ Eggleton RA Etheridge MA

H12 A16 122

Flood RH Forsythe DL Franzen N Funk JL

A1 H10 A2 Ia9

Gellatly DC Gibbons GS Giblin AM Gibson DL Gilkes RJ Glikson AY Goldbery R Green DO' Green R Green TH Gregory P Griffiths JR Grubb PLC Gunthorpe RJ

J10 126 H11 G5 D32, D46 J4 D29 B5 11 A7 B5 F6, 16 D35 F12

12, 15

Hallberg J Hancock W Hausen DM Heidecker E Henley £J van den Heuvel HB Hill RM Hingston FJ Holmes JW Hutchison DS Hutka J

HI B10 A2 F7 J14 H16 D39 D15 15 G3

Johns RZ

C15

Keays RR Kelly GR Khan MY

B11 J25 Ia5

Lacy WC Lambert IB Lister GS Loughnan FC Lovering JF

HI G7, H14 J15 D29 A7

Marjoribanks RW Mayer ¥ McCall GJH McKay KG McKirdy DM McLaren AC Moore PB van Moort JC Moss AJ Mulcahy MJ

Ia9 G14 J1 H3 J17,, J 20 A12 A19 D19 G1 D39

Nesbitt RW Norvick M

A2, H1, H3 15

O'Driscoll EST Oliver RL O'Reilly SY Oversby BS

19 A2 A9 F4

Paterson MS Plumb K Pontifex IR Powell C McA Powell TG Purvis AC

116 E15 B16 114 J17 H1, H3

Radke B Reinson GE Richards JR

D23 G6 H10

Richards SM Rickard MJ Ring-wood AE Roy PS Ryburn RJ

018 Ia15 A7 C21 15

Saxby JD Scheibner E Scholz G Schultz PK Senior BR Shaw RD Shaw SE Shepherd MS Smith JW Solomon M Stevens-Hoare NP Stewart AJ Swaine DJ

D5 E1, F12 DA 6

J14 D43 Ia1, Ia13 A1 C25 H7 C9, P6 B12 Ia5 D1, H11

Sweet IP Sydenham PH

15 11

Taylor Gr Taylor GF Taylor GH

G7 01.2 C2

Yallance TG

Walker KR Walker PH Ware NG Whittle AWG Wilmshurst J . Wilson IH

A7 G3 A7 B1 B10 H16

Yates KR Young R

B15 D32