Abstract 39 - Contentious_issues_in_Tasmanian_geology_Hobart_Nov_1994 by GSAustralia

Geological Society of Australia

ABSTRACTS Number 39

Contentious issues in Tasmanian geology Edited by David R. Cooke and Paul A. Kitto A symposium presented by the Geological Society of Australia, Tasmania Division Hobart 3-4 November 1994

Geological Society of Australia

ABSTRACTS Number 39

Contentious issues in Tasmanian geology Edited by David R. Cooke and Paul A. Kitto

A symposium presented by the Geological Society of Australia, Tasmania Division Hobart 3 - 4 November 1994

Generously supported by: Aberfoyle Resources AusIMM CODES Key Centre, University of Tasmania CRA Coldstream Mining Mineral Resources Tasmania Pasminco Exploration Placer Exploration RGC Exploration Utan Resources

Published by the Geological Society of Australia Challis House, 10 Martin Place, Sydney 1995

ISBN 0 85901 5882

Ill

Geological Society of Australia Abstract Series Number 39

Contents page

Preface D.R. Cooke and P.A. Kitto

vii

Tectonic history of Tasmania from the Proterozoic to the Devonian

Thin-skinned deformation and aliochthonous models for the tectonic evolution of northern Tasmania D.R. Gray and N.B. Woodward Tectonics of western Tasmania: Late Precambrian-Devonian R.R Berry Discussion

3 6 9

Tectonic models for the Mount Read Volcanics

Tectonostratigraphy of the Dundas Trough Terry Lees and John Wright Tectonic setting of the Mount Read Volcanics: crustal and geophysical implications David Leaman A geochemical approach to a tectonic model for the Mount Read Volcanics Tony Crawford Discussion

15 17 23 27

Re-interpretation of the Mount Read Volcanic succession

Stratigraphic mapping, Tyennan connections, Cambrian orogenies, the Arthur Lineament, and the tectonic context of the Mount Read Volcanics. A fresh look at Western Tasmania. Keith Corbett 35 Recent advances in volcanic facies analysis of the Cambrian Mount Read Volcanics, western Tasmania J. McPhie 38 Volcanic facies analysis indicates large pyroclastic eruptions, sill complexes, synvolcanic grabens, and subtle thrusts in the Cambrian "Central Volcanic Complex" volcanic centre, western Tasmania. Rodney L. Allen 41 Discussion 44 Geological Society ofAustralia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Consequences of recent geochronology for Tasmanian geology Geochronologic constraints on the Mount Read Volcanics, and the Hellyer, Rosebery and Mount Lyell massive sulphide deposits, Tasmania Caroline Perkins Pre-Middle Cambrian stratigraphy, orogenesis and geochronology in western Tasmania N.J. Turner, L.P. Black and M. Kamperman Discussion

49 51 57

Geology, geophysics and mineralization of NE Tasmania The role of Devonian granodiorites and the influence of Tabberabberan tectonism in Tasmania-North Victoria Land (Antarctica) correlations R.Vame and R.Fulton The characterization of quartz and gold in the Beaconsfield goldfield David Russell New geophysical data for northeastern Tasmania Robert Richardson Relationships between Devonian thrusting and gold mineralization in northeastern Tasmania Richard A Keele, Bruce Taylor and Garry J. Davidson Nature and origin of gold mineralization, Mangana-Forester area, northeastern Tasmania J. Taheri and R. Bottrill Discussion

61 63 65 69 73 75

Geophysical misinterpretations and realistic applications Realistic applications of gravity and magnetics David Leaman EM conductors in western Tasmania Jovan Silic FM signatures at Hercules and Rosebery Neil Hughes Discussion

79 85 86 87

The sources of sulphur in the cassiterite-sulphide deposits of western Tasmania Meteoric fluids and sulphur in tin-tungsten deposits with special reference to carbonate replacement types Mike Solomon 91 The groundwater model for the formation of the cassiterite-sulphide deposits of western Tasmania John L. Walshe 93 Structural and geochemical controls on metal zonation at the Renison tin mine, western Tasmania Paul Kitto 95 Discussion .97

Genetic models for Cambrian volcanic-hosted massive sulphide deposits Formation of the Rosebery and Hercules ore deposits, Tasmania by syntectonic mobilization of metals and wallrock replacement about structural traps Domingo G.A.M. Aerden Synvolcanic, subseafloor replacement model for Rosebery and other massive sulphide ores Rodney L. Allen The magmatic connection for VHMS deposits Ross R. Large, Mark Doyle, David Cooke and Ollie Raymond Discussion

101 107 109 112

Student posters Mineral paragenesis, fluid inclusion and oxygen isotope studies of the magnetite-scheelite skam deposit, Kara, northwestern Tasmania Blackwell Singoyi and Khin Zaw Gravity and magnetics of the Scamander Mineral Field Mark Duffett Character and setting of the Anthony Road Andesite, Mount Read Volcanics, western Tasmania. Andrew Jones The depostional setting of basal Dundas Group sediments, western Tasmania David Selley Geochemistry and tectonic implications of magmatism in northern Hunter Ridge — Kadavu Island Group (Fiji) Alicia Verbeeten, Anthony J. Crawford, Patrick Maillet and Steve.M Eggins Textures and origins of carbonate associated with the Rosebery VHMS deposit Karin Orth and Anthea R Hill Geophysical interpretations from the north flank of the South Tasman Rise Andrew Wellington Stratigraphy and palaeovolcanology of the Cambrian Tyndall Group, Mount Read Volcanics, western Tasmania Matthew J. White Apatite fission track thermochronology of northeastern Tasmania and the southern Bass Basin Andrea J. O'Sullivan Polyphase metamorphism and fluid flow in the Corella Formation, Mary Kathleen, Queensland Cathryn C. Gifkins Topographic lineaments: Unlocking buried mineralization in southeastern Tasmania Peter J. Rice

131

Symposium participants

133

Index of authors

136

119 121 122 123

124 125 126

127 129 130

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

VII

Preface David R. Cooke and Paul A Kitto

The Contentious Issues in Tasmanian Geology symposium was held on Thursday the 3rd and Friday the 4th of November, 1994, in the Centenary Lecture Theatre, University of Tasmania. A total of 130 participants enjoyed 22 invited and one unscheduled presentations that covered aspects of the Paleozoic and Precambrian geology, geophysics and mineralisation of western and northeastern Tasmania. A key feature of the conference was the half horngeneral discussions at the end of each session. These were the scene for much lively debate, with speakers and members of the audience enjoying the opportunity to espouse pet theories, seek further clarification of key points, and suggest key issues that need further attention. The student poster display attracted keen interest, with the prize for best poster being awarded to David Selley for his presentation entitled The depositional setting of basal Dundas Group sediments, Western Tasmania. Abstracts from the poster session are presented in the final section of this volume. Many people and organisations contributed to the successful running of the symposium. The generous financial and in-kind support of Aberfoyle Resources, AusIMM, CODES Key Centre, CRA, Coldstream Mining, CSH (Tas. Division), Mineral Resources Tasmania, Pasminco Exploration, Placer Exploration, RGC Exploration and Titan Resources was crucial to the success of the conference. The initial organising committee (Paul Kitto - chairman, Fergus Fitzgerald, David Wallace, Bruce Cemmell, Ron Berry, Bill Baker) are thanked for their organisational and promotional efforts — they should take pride in the success of the symposium. Bill Baker is also thanked for manfully trying to keep

some semblance of order with the finances, despite the chaos that ensued at the registration desk on the opening morning. The tireless efforts of June Pongratz in producing both abstract volumes have been greatly appreciated. Rohan Hine, Nilar Hlang, Bianca Manzi, Andrew Jones and Alicia Verbeeten are thanked for their efforts in helping to run the registration desk, book stall and slide projectors. Garry Davidson and Ron Berry are thanked for operating the tape recorder. Peter Cornish's efforts in searching for poster boards at the last minutes were much appreciated, and Jeanette Harris and Sue Hinksman are thanked for a countless variety of administrative matters. The session chairpeople kept excellent control of the discussion sessions, and are thanked for their labours in producing transcripts for this volume. Geoff Green and John Bishop in particular are thanked for producing their transcript/ summaries despite the failure of the tape recorder during their sessions. Finally, the speakers and poster presenters are thanked, for they were ultimately responsible for making the conference a stimulating learning experience. The first version of the conference abstract volume was provided directly to conference participants in November 1994. This second edition has been compiled and distributed in July, 1995, in response to the keen interest by the participants in the discussion sessions. Transcripts of each discussion session have mostly been prepared by the session chairman, and have been included in this volume immediately after the relevant abstracts. We hope that you find this a useful reference volume, and that it stimulates new research ideas and exploration activity in Tasmania. We look forward to your participation in the next symposium.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

VIII

Tectonic history of Tasmania from the Proterozoic to the Devonian

Thin-skinned deformation and allochthonous models for the tectonic evolution of northern Tasmania D.R. Gray^ and N.B. Woodward^

Re-examination of mapped relationships on the Sheffield-Devonport-Bumie-Beaconsfield sheets, plus the incorporation of new structural element/ fabric data has led to a revised structural and tectonic synthesis for northern Tasmania (see Woodward et al 1993; Elliott et al 1993). The basis of the interpretation is the application of typical thrust-belt geometries and section balancing techniques (see Woodward et al. 1989). The approach is based firstly on the recognition of faults and their increased importance in section reconstruction, secondly the hypothesis that these faults are linked in some way to a major detachment fault which underlies the region, and thirdly that the exposed structures at the surface are a function of the geometry of the structures at depth. Previous regional sections across northern Tasmania (Figure la) had assumed a thick skinned interpretation of the surface geology where the Precambrian basement was autochthonous without crustal detachments and with only minor warping of the Ordovician through Devonian cover sequence. This is despite geophysical interpretation (Leaman et al 1973) that argued the smaller Precambrian massifs were thin slices and therefore probably allochthonous (cf. Leaman 1992; Leaman et al. 1994). Section construction involving a thin-skinned interpretation (Figure lb) requires utilization of a simplified stratigraphy as a template and a depth to basal detachment determined by a simple downwards extrapolation of reasonable stratigraphic thicknesses of the same units in relatively well exposed areas. The implications of this regional cross-section are that:

^ Department of Earth Sciences, Monash University, Melbourne, Victoria 3168, Australia. 2 25522 Coltrane Drive, Damascus, MD 20872-2603, USA.

• Both west-directed Cambrian and Devonian thrusting has caused imbrication of Precambrian basement, the Cambrian oceanic suite and the Ordovician continental margin overlap sequences of northern Tasmania. • Total shortening across the region is on the order of 80 km with at least half of this displacement attributable to the Late Cambrian deformation. • The smaller Precambrian massifs (U1 vers tone/ Forth and Badger Head) are allochthonous and interpreted to be the bases of major west-vergent thrust sheets which were largely emplaced in the Late Cambrian (cf. Leaman et al. 1994). • Previously recognised overprinting regional fold generations within the area are considered to be the result of Devonian thrusting and reflect sequencing of thrust movement. East—west folding followed by southwest-vergent thrusting dominated Devonian deformation in the Fossey Mountain region. •The balanced cross-section (Figure lb) suggests a depth of 5-10 km to autochthonous non-imbricated Precambrian basement which must extend beneath the Tamar zone. • Juxtaposition of the West and East Tasmania terranes was by Devonian westward thrusting of the East Tasmania terrane over the West Tasmania terrane (cf. Powell & Baillie 1992; Leaman et al 1994). • Subthrust basement, once present in the eastern hinterland of the Late Cambrian thrust belt, has probably been tectonically removed during Devonian deformation, giving the basement a geophysical grain which is not parallel to surficial structural trends. • The Devonian event redeformed the earlier Cambrian thrust structures, emplaced the Badger Head, Golden Valley, Mersey River and Forth Basement massifs, and carried the East Tasmania terrane onto the underlying West Tasmania Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Liena - Mayberry Syncline

Standard Hill anticlinorium

Mersey River

Tyennan Massif

Badger Head Massif -

Dolcoalh Granite

A' 5b

Precambrian

Figure 1: (a) Thick-skinned interpretative regional cross section across northern Tasmania (from Williams, 1989, fig. 14b). (b) Thin skinned interpretative cross section across northern Tasmania (after Woodward et al, 1993,fig.ll)

terrane basement in a piggyback fashion above the Devonian thrust sheets. • Later ESE-vergent Devonian thrusting within the Dial Range requires that the Rocky Cape Basement massif is also somewhat allochthonous, perhaps in the manner of the foreland basement uplifts of the Rocky Mountains of the United States. • Juxtaposition of west-vergent and east-vergent thrust sytems has produced a triangle zone to the southwest of the Forth massif. The Cambrian thrusting event originally defined by Berry and Crawford (1988) is related to an episode of failed subduction with west-directed (cf. Berry 1989) obduction of oceanic crust as a series of mafic-ultramafic slices. Associated with this were the emplacement of slices of Precambrian basement (formerly thinned and extended craton), accretionary complex materials (cf. Elliott et al 1993) and forearc materials (cf. Berry 1994, this volume). The Middle Devonian deformation is more complex and enigmatic particularly with respect to tectonic

setting, and how the structure and geometry of northern Tasmania relates to the mainland Lachlan Fold Belt (cf. Powell & Baillie 1992; Elliott & Gray 1992). Linkage of the structural elements in the north with those in the west, particularly those of the West Coast ranges, may require continuation of a major detachment fault beneath the Tyennan Massif (cf. Leaman et al. 1994; Berry 1994, this volume). Despite these problems, geophysical interpretation linked to the observed surface structural relationships can be best explained by a thinskinned interpretation of northern Tasmanian geology References Berry, R.F., 1989: Microstructural evidence for a westward transport direction during Middle Cambrian obduction in Tasmania. Geological Society of Australia Abstracts 24: 8-9. Berry, R.F. & C r a w f o r d , A.J., 1988: The tectonic significance of Cambrian allochthonous maficultramafic complexes in tasmania. Australian Journal of Earth Sciences 35: 523-533.

Elliott, C.G. & Gray, D.R., 1992: Correlations between Tasmania and Tasman-Transantarctic orogen: Evidence for easterly derivation of Tasmania relative to mainland Australia. Geology 20: 621624. Elliott, C.G., Woodward, N.B. & Gray, D.R., 1993: Complex regional fault history in the Badger Head region, northern Tasmania. Australian Journal of Earth Sciences 40:155-168. Leaman, D.E., 1992: The Tasmanian Precambrian and basement involved thrusting. Geological Society of Australia Abstracts 32: 28-29.

Leaman, D.E., Baillie, P.W. & Powell, C.McA. 1994: Precambrian Tasmania: a thin-skinned devil. Exploration Geophysics 25:19-23. Powell, C.McA. & Baillie, PW., 1992: Tectonic affinity of the Mathinna Group in the Lachlan Fold Belt. Tectonophysics 214,193-209. Williams, E., 1989: Summary and synthesis. Geological Society of Australia Special Publication 15: 469-499. Woodward, N.B., Gray, D.R. & Elliott, C.G. 1993. Repeated thrusting and allochthoneity of Precambrian basement, northern Tasmania. Australian Journal of Earth Sciences 40:297-311.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Tectonics of western Tasmania: Late PrecambrianDevonian R.F. Berry^

The mesoscopic structural history of western Tasmania is very complex. Throughout the area, the structural history is dominated by non-cylindrical folding with strong evidence of multiple reactivation of early fold and fault structures. This complexity has led to a plethora of local unconformities and apparently contradictory structural relationships. Many of the fold trends that have been considered as Devonian structures are open Cambrian folds which have been tightened in the Devonian. These structures all need to be assessed in the regional context and with regard to both structural and stratigraphic analysis. A synthesis of the structural history determined from this analysis is given below. Event 1. Late Proterozoic shallow water sedimentation on the eastern margin of the East Antarctic shield Event 2.

Penguin Orogeny

The oldest demonstrated event in Tasmania is recorded as the syn-kinematic intrusion of the King Island Granitoid at 760 ± 12 Ma (Turner, this volume). D^ to D^ apparently predate event 3 (see Cox, in Turner 1989). The Rocky Cape Group also shows evidence of extensive deformation before the emplacement of the 600 Ma Rocky Cape dyke swarm. This deformation has been recognised as the Penguin Orogeny and is usually considered to have occurred at 700 ± 50 Ma. on the basis of K/Ar dating of the Cooee Dolerite. This age for the dolerite must be called into serious doubt, on the basis of the new

1 Key Centre for Ore Deposit and Exploration Studies, Geology Department, University of Tasmania, GPO Box 252C, Hobart 7001, Australia

zircon U-Pb dating (Turner, this volume; Black 1994). The early deformation in the Rocky Cape Group and Bumie Formation may correlate with the 760 Ma folding on King Island. The Arthur lineament at this stage was the major detachment between the weakly folded upper thrust sheet (Rocky Cape Group) and the strongly deformed footwall (Bumie Formation). Event 3.

Passive margin formation

A two stage rift phase is recognised in most of western Tasmania. An early shallow water sequence is followed by a broad zone of extensive tholeiitic volcanism with a nominal age of 600 Ma to early Cambrian (Crawford & Berry 1992). This phase correlates with the later part of the Adelaidean sedimentation history in the Adelaide fold belt. Event 4. 525-510 Ma; late Early to early Middle Cambrian The passive margin of eastern Australia collided with an oceanic arc, and major slices of fore-arc lithologies were thrust over Tasmania. The only allochthonous element specifically identified by Berry and Crawford (1988) was the mafic/ultramafic complexes. Woodward et al (1993) argued that the Forth Metamorphic Complex (MC), Badger Head MC and other small metamorphic inliers were also allochthonous. New radiometric dating (Turner et al., 1992; Turner, 1993; Black, 1994; Turner, this volume) makes it almost certain that all these, plus the high grade metasediments of the western part of the Tyennan Complex are allochthonous elements. The association with serpentinite and similarity to the Forth MC suggests the Settlers Hills and Simmonds Hills schists are allochthonous. The metamorphism and age dating of the Bowry

Formation (Turner et al 1992; Black 1994) also suggests that this is part of the allochthon. The early part of the thrust emplacement as recorded in high temperature mylonites indicates thrusting towards the southwest (Berry 1989). A major phase of thrusting to the S is recorded in widespread cataclasites in western Tasmania (Findlay 1993; Findlay & Brown 1992; Berry et al 1990). This event is correlated here with late stages of the obduction process. The Arthur Lineament was probably reactivated at this stage with a sinistral west side down motion. An alternative is that this thrusting event correlates with 5 part 2 below (Selley, in prep). Event 4 coincides with the rapid sag phase which initiated the Kanmantoo cycle of deposition. A possible interpretation is that tectonic loading of the continental margin produced the new cycle of active deposition in a foreland basin setting (Turner et al 1994; Coney et al 1990). An early Middle Cambrian unconformity (Rowell et a/.1992) in west Antarctica probably correlates with this event. Event 5.

Delamerian Orogeny 510-490 Ma

The Delamerian Orogeny in Tasmania is a complex event with rapidly changing stress patterns. Deposition continued throughout this event but the locus and style of deposition change very rapidly. The geometry of the associated folding has more in common with the Delamerian folding in the Olary region than with the intense high temperature history in the Kanmantoo region. Part 1. Middle Middle Cambrian-extensional phase: The first event in this area is a middle Cambrian extensional phase with rapid subsidence, active syn-orogenic deposition and major postcollisional felsic-dominated volcanism (Mt Read Volcanics). Extension stresses are recorded in the hydrothermal activity and in presence of the Henty dyke swarm. The geometry of extension was approximately east-west. Part 2. Late Middle to early Late Cambrian: The extensional phase was closely followed, and may overlap with, a north-south compressional event which has produced east-west trending folds in the Fossey Mountain Trough, at Adamsfield and near Bathurst Harbour Part 3. Late Cambrian: The last phase of the Delamerian orogeny reactivated earlier extensional faults as reverse faults (Henty Fault). Major

reverse faults and upright open north-trending folds were formed in western Tasmania. This phase also caused major uplift of the Tyennan block with syn-orogenic sediments (Owen Conglomerate) accumulating in synclinal cores. Event 6. Ordovician-Silurian sag phase: platform sedimentation in shallow water. Event 7. The Silurian deformation phase: recognised in eastern Victoria and further north but only represented as a hiatus in deposition in Tasmania. Event 8. Early Devonian cycle of deposition: renewal of shelf sedimentation Event 9.

Middle Devonian orogenesis

The Devonian deformation throughout Tasmania is characterised by the complexity of fold orientations (e.g. Williams 1989), explained, in large part, by the complexity of the pre-existing geometry. In many areas, the fold geometry is controlled by the existing Cambrian fold trends which were tightened during the Devonian. This led to Devonian cleavage orientations which are not parallel to the axial plane of the folds with which they are associated. In the Fossey Mountain 'trough,' east-west Cambrian folds are tightened (D^,D2 at Wilmot (Seymour 1980, 1989). In the Dundas Trough, north-trending Cambrian folds are tightened with an associated NNW-striking Devonian cleavage (D3 at Wilmot, D^ at Mt Lyell). NNE-trending folds north of Tullah are controlled by the reactivation of the Henty Fault. Most western Tasmania granites were intruded late syn- to post- kinematic with respect to the NNWtrending cleavage and associated folding and faulting. The subsequent north- to NNE-trending compression produced WNW-trending folds and thrusts in the south, and northwest-striking thrusts and associated folds in the north (D^ at Wilmot; D2 at Mt Lyell, Duck Creek). On the west coast, there is an associated phase of brittle wrench faults dominated by NNE-striking sinistral movement on the Henty Fault. A return to east-west compression is found at a few isolated localities. The regional metamorphic grade associated with the Devonian orogeny is prehnite-pumpellyite with local zones of greenschist facies in the vicinity of

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

late syn- to post-orogenic granites. This indicates there was limited crustal thickening during the Devonian orogeny. Woodward et al (1993) emphasised the southwest-directed thrusting in northern Tasmania. The equivalent structure in western Tasmania is the Linda Zone which has a late history (post- Devonian folding). There is some evidence that a detachment surface continues back under the Tyennan Block to link with the thrusts in northern Tasmania, but only a small movement is possible on this structure in the Devonian.

References

Berry, R.F., 1989: Microstructural evidence for a westward transport direction during Middle Cambrian obduction in Tasmania. Geological Society of Australia Abstracts 24: 8,9. Berry, R.F. & Crawford, A.J. 1988: The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Australian Journal of Earth Sciences 35: 523-533. Berry, R.F., Elliott, C.G. & Gray, O.K., 1990: Excursion guide E3: Structure and tectonics of western and northern Tasmania. 10th Australian Geological Convention, Hobart: 53 pp. Black, L.R, 1994: The significance of current and proposed SHRIMP dating. Mineral Resources Tasmania unpublished report 1994/16: 20 pp. Crawford, A.J. & Berry, R.F., 1992: Implications of Late Proterozoic-early Palaeozoic igneous rock associations for the tectonic evolution of Tasmania. Tectonophysics 214: 37-56. Coney, P.J., Edwards, A., Hine, R., Morrison, F. & Windrim, D., 1990: The regional tectonics of the Tasman orogenic system, eastern Australia. Journal of Structural Geology 12: 519-543. Findlay, R.H., 1993: Summary of structural and stratigraphic observations on the Proterozoic/ Eocambrian/Cambrian units of the Zeehan 1:50000 quadrangle. Mineral Resources Tasmania unpublished report 1993/29: 27pp.

Findlay, R.H. & Brown, A.V., 1992: The 10th Legion Thrust, Zeehan district. Distribution, interpretation, and regional and economic significance. Mineral Resources Tasmania unpublished report 1992/02: 27pp. Rowell, A.J., Rees, M.N. & Evans, K.R., 1992: Evidence for a major Middle Cambrian deformation in the Ross orogen, Antarctica. Geology 20: 31-34. Seymour, D.B., 1980: The Tabberabberan orogeny in northwest Tasmania. PhD thesis. University of Tasmania (unpubl.). Seymour, D.B., 1989: Geological Survey Explanatory Sheet 34 , St Valentines. Tasmanian Department of Mines. Tumer, N.J., Bottrill, R.S., Crawford, A.J. & Villa, I., 1992: Geology and prospectivity of the Arthur Mobile Belt. Bulletin of the Geological Survey of Tasmania 70: 226-232 Tumer, N.J., 1989: Precambrian. In Burrett, C.F. & Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Special Publication of the Geological Society of Australia 15: 5-46. Tumer, N.J., 1993: K-Ar geochronology in the Arthur Metamorphic Complex, Ahrberg Group and Oonah Formation, Corinna district. Mineral Resources Tasmania unpublished report 1993/27: 23 pp. Turner, S., Sandiford, M., Flottman, T. & Foden, J., 1994: Rb/Sr dating of differentiated cleavage from the upper Adelaidean metasediments at Hallett Cove, southern Adelaide fold belt. Journal of Structural Geology 16: 1233-1241. Williams, E., 1989: Mid Palaeozoic deformation. In Burrett, C.F. &: Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Special Publication of the Geological Society of Australia 15: 239-253. Woodward, N.B., Gray D.R. & Elliott, C.G., 1993: Repeated Palaeozoic thrusting and allochthoneity of Precambrian basement, northern Tasmania. Australian Journal of Earth Sciences 40: 297-311.

Tectonic history of Tasmania from the Proterozoic to the Devonian Discussion session — chaired by David Leaman

MIKE SOLOMON (MS) TO RON BERRY (RFB): How do you know that the Linda Fault Zone was a transform at that time (Cambrian)? RFB: There has been a long history on this structure which has affected both fault geometry, mineralisation and structure generally. There are different fold histories north and south of the zone for structures sitting there during extension of the main rifts. Cambrian folds of different wavelengths are involved. Mineralisation is stopping some gaps. I feel that the zone truncates the main Mt Lyell alteration system. CAROLINE PERKINS (CP) TO RFB: How did you estimate the youngest age for the Delamerian at 490 Ma? What is the youngest age Owen Conglomerate from fossil evidence? RFB: Pioneer Beds are the limiting tie and scattered dates of just less than Middle Ordovician are implied. But there is no direct ability to date these. CP: Does deformation extend through to the top of this horizon? RFB: Essentially right through to base of the limestones. CP: So deformation could extend to 460 or 470 Ma? RFB: Yes, if that fits the age of the limestone. A discussion followed on the problems of dating the boundaries, the changes in dates that have occurred frequently of late. CP suggested that the CambroOrdovician boundary might be at 480-490 and the Mid Ord/Late Ord at 460-470. RICHARD KEELE (RAK) TO DAVID GRAY (DRG): Concerning the Tabberabberan Orogeny. We believe that Tasmania went east in this event yet your papers indicate movements to the west. Is this a problem? Perhaps back thrusting is an explanation. DRG: An E-vergent solution is consistent with everything seen in the Mathinna Beds and this view is more consistent with the mainland. Baillie and Powell's ramp idea is also a bit unusual and special. Tasmania

remains an enigma as to where and when it fits. This is not an easy problem to resolve given there are probable transfer zones in Bass Strait and even if the Bass Basin system is restored there still remains the problem of thrusts in different directions. CHAIRMAN (DEL): Both systems are present in Tasmania, but between Penguin and George Town the thrusts are predominantly west directed. West of Penguin the situation is more balanced and the youngest thrust system (which brings in the Rocky Cape and Temma zones) is east directed. So we may be looking at different stages and ages in the orogeny across Bass Strait. There may be no problem at all. NIC TURNER (NT) TO RFB: Why do you see the need for Cambrian deformation as two events; Delamerian and one earlier? Could they not be one event which developed and adjusted from the earlier collisional event and the entire process extended to the end of the Cambrian? RFB: It is based on South Australia. The Delamerian is defined. NT: I accept the names but the issue is the Cambrian history of Tasmania. An evolving situation, not one then the other. RFB: I am happy to have a series of post collisional adjustments but we do not know enough about what causes those to know if they are separate or ongoing. DEL: All this presumes there was a collision in the first place. I argue later that this may not be an established fact. There are other ways to explain the ultramafics and exotic blocks. KEITH CORBETT (KDC) TO RFB: You suggest that the Bowry Formation in the Arthur Lineament zone might be exotic. Do you have any feel for where it might have come from or its affinities with other possibly exotic lithologies? RFB: Most of the metamorphic pieces may be fragments of an old passive margin. So the source could be any Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

10 part of that. The Crimson Creek Formation has many chemical similarities and alteration or metamorphism of that could he enough. It is hard to explain the blueschist grade and amphibolite overprint ifyou do not get it from somewhere else. Geochemically you could leave it where it is. NT: The Bowry Formation is not a distinct unit unlike its surroundings. The schists and amphibolites are like adjacent rocks. It is not a relict blueschist unit sitting in a prehnite-pumpellyite terrane. In fact as you come from the west you find gradation. There may be arguments for its being exotic but many aspects are similar to adjacent rocks. DEL: Many of these extreme variations now labelled exotic, and most sites of established open Cambrian folding correlate to the approximate positions of the major trough boundaries which controlled Burnie-Oonah deposition. If these boundaries have been wrenched — and continue to be moved — then simple inversions and folds will result in the cover sequences locally and this process will ultimately lead to local changes in metamorphic grade as well. JOHN FODEN (JF) TO RFB: How do you regard the sedimentational history of the Owen in your syntectonic model? It looks to me very like an isostatic rebound and stripping condition into a basin which is developing. In your model, due to deformation, how do you get rapid erosion? RFB: Just about everything I see is compression. So uplifts and folding in compression. I think the unit is in fold depressions and the Tyennan is an anticline and thus the source. I have looked hard for extensions through this time. JF: So you see this as an alpine type molasse. RFB: Yes. DEL: Geophysical and some exposure evidence (eg. near Tullah) is not consistent with simple folding forms. Abrupt basins are implied and severe local erosion. The asymmetric shape of these opposes the form of the Cambrian basins but the change occurs above and near the older trough edges mentioned earlier and which I will discuss in my presentation. I agree with John; these are extensional structures with local imposed compression elements due to the underlying motion. TONY YEATES (TY) TO THE MEETING: AGSO is pursuing deep seismic this summer and the Pieman, Cradle Link and Fingal sections will be traversed. The Rig Seismic will traverse offshore. Airborne magnetics is underway at the moment. The

project is being undertaken in partnership with Mineral Resources Tasmania as part ofNGMP. He commented that we are fortunate — we will get our models tested.... FRANK LEE (FL) TO RFB: In the Cambrian you have proposed a collision with an island arc. Have you considered the plate tectonics idea of a collision with North America at this time. The continents met N-S and rebounded and I believe this fits better with what you have described. Any thoughts? RFB: We seem to be dealing with passive margin extension from the Adelaidean and there is no evidence of a near US after about 760 Ma. So any proposal like that is too old. I think the oceanic arc was unrelated to any continental fragment and it came 200 Ma later. MEL JONES (MJ) TO DRG: What happened to the granites shown in the Williams 1989 section? DRG: We did not show them. We argue that they postdated the transport and have omitted them for simplicity. Our section is purely indicative and interpretive and directed at tying surface structures into a style of deformation. MJ: Even if they were later couldn't their geometry give you any support for your interpretation? DRG: I do not believe so. I don't know enough about Tasmania but emplacement in the Lachlan Fold Belt includes intrusion pre-, syn- and post-deformation. Age patterns and shapes are complex but they can tell you something about stress states in the upper crust. In central Victoria large late Devonian granites are oriented E-W and all around are Silurian-Mid Devonian bodies which are smaller and blob like. The Omeo belt has large bodies inlaid parallel to the grain and this pattern extends into NSW, N-S. My impression of Tasmania is that they are mainly post deformation. DEL: I think the shape ofgranites is important and agree with Mel. But we cannot talk about either the granites, structure or control without geophysics, a description of their actual shape and the third dimension. It is no good just looking at surface exposure distributions or gravity maps and concluding we know the shape. Neither process is good enough; the shapes have to be calculated and extracted. Then relationships can be determined or evaluated. In my experience there will be features related to various aspects of the form so defined. On the matter of balanced sections. While I do not disagree with the general conclusions about E- and Wdirected thrusting in N Tasmania I would comment that the Rocky Cape zone is involved in this. More important

11 though is the recognition that we have been given an Occam solution. It is the simplest possible. It ignores the granites as Mel has pointed out but it also assumes that the sequence was of constant thickness across the region, there were no early inversions, no basinal irregularities or other structures. Any pre-existing structure, especially one still active, would bias the outcome. Many of the reverse arrays from the root thrust are probably caused this way. On the Dial Range example shown. Diagram attached showing the best current interpretation of the Housetop Granite. You don't need seismic to back this up. There are some interesting relationships between surface structures and granite form and perhaps we should both discuss and include these in any evaluation. The interpretation fully supports your conclusions east of the granite itself, including the main basement-involved thrusting. But we must have the actualform of the granite described and included. DRG: The granite is part of the story. One of my concerns has been the volume of granite. lam surprised that we do not see more thermal effects given your models. Still most of it is 2 km down. In Victoria we see about 500 m of aureole. Should we expect more? DEL: Tasmanian experience is 500-1000 m at steep contacts but the actual width depends on where the fluids go, what the rocks were like, how conductive, porous etc. DRG: Our sections are interpretive but we are certain that surface relationships are telling us something. Involving the granite tends to embroil us in arguments about granite intrusion mechanisms and what goes on around them. TY TO RFB: The big bend at the north end of the Tyennan Block dominates the geology. Is it an artifact ofDundas Trough and Fossey Mountain structures or a genuine orocline? RFB: The orocline concept was killed off years ago, so no. (Chairman s editorial note: I was unaware of this since it appears in the encyclopedia of plate tectonics and regret missing the funeral). I see it as a primary basin feature controlled by a major transform which offset the original rift structures rather than anything which happened later. TY: I am not doubting, just asking, but why is it so neatly curved? RFB: An illusion. In detail it is quite complex and variable and I suggest you discuss this with David Seymour for details. KDC: Mapping tells us many things in the Mt Reads, including porphyries wrap around and disappear. An original depositional/eruptional distribution is indicated

rather than an imposed fold from later on. DEL: In my paper I give the North Sea as an example. Rift patterns there also include irregularities of extent and form and include complex junctions and near 90° bends even at an early stage which are controlled by other elements. Once these early features get covered it is easy for projections to appear smooth and all the while the cover will be controlled and variable around the arms, as here. Exhumation only tends to enhance this effect. I would agree that most of the effect is primary and ancient. RFB TO DRG: (Much amusement as Ron took the opportunity to challenge the "opponent''). On the basis of thicknesses used most people in the field would say that your theory is basically wrong. Especially with regard to the Barrington Chert and the Motton SpilliteMsalt. The Dial Range stratigraphy is very suspect. Indeed you could be out by 100% which may be "quite serious". DRG: As the senior author I guess I am responsible but it was not my part of the work and I cannot remember all the details. David Leaman has already commented on our assumptions by way of discussion in AJES. So we are already in trouble. We did no detailedfieldwork there. There then followed a general discussion of the fades interdigitations and correlations of the chert and basalt with DRG eventually asking RFB to say what he thought. RFB indicated that he thought a correlation of the Motton Spillite with Crimson Creek, and Barrington Chert with Success Creek, but both are known to be variable so the balancing problem remains. RFB then drew attention to the derivation of material in many units in the Dial Range which showed that the Forth Metamorphics had to be there during deposition before the Middle Mid Cambrian, not just in the Late Cambrian when DRG was implying Cambrian thrust insertion. RFB pointed out there were all kinds of difficulties, including involvement of ultramafics and sourcing. DRG: Agreed there were problems but that he was trying to test variations and were moving some of the timing around. Agreed that his model was oversimplified. OTHER COMMENTS FROM FLOOR: Concerning the Beulah Formation and Gog Range Greywacke. To DRG: You said that the Beulah Formation overlies the Gog Range Greywacke in a syncline but this overturns your stratigraphic column and thus disturbs the balancing equation. DRG: I accept this. I repeat we offered the simplest possible solution. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

12 GEOFF GREEN (GG) TO RFB: About your upset rift model for the Middle Cambrian deposition. First I would challenge your idea that the Cambrian rocks of the Smithton zone are simply synclinal rather than in a trough and agree with Keiths comments earlier. So what is the evidence for a western margin to your rift system? RFB: The western margin is not easily constrained. All we can do is give a limit, either near the Rosebery Fault or the next structure to the west. Then the whole story is lost in cover of various types and the ultramafics. We know this was in extension to the Delamerian and then we appear to have lost that margin so I have assumed a boundary buried by thrusting in compressional events. This is all a contentious issue. DEL: Absolutely. It is not that simple. I think we are looking at pull apart and extension above a major marginal wrench and will say more later. And there are various rifts within this zone with transforms between oriented rift sets which are in half graben form. I disagree about the so-called western limits. The real limits are just east of Rosebery in that part of the area, not west of it. The structures feather to the west.

Thus ended the debate with the Chairman supposed to make a summation. He declined to do so given the content of the following session but managed to indicate his general disagreement with most of the bases of the presentations given; namely that there was a need for dominance of compression, for massive obduction, and that all the stages of an early Cambrian "orogeny" could be fitted into the now very compressed time scale. He noted that this would require complete extension, breakage of the crust to ocean, some spreading, a collision, rebound, uplift and further extension (all the while with much compression) leading finally to the Delamerian episode — all in the time available between deposition of the Crimson Creek Formation and the Pioneer Beds. He frankly disbelieved this given any ancient or modem analogues and time scales, announced that he intended to offer an alternate solution in the next session based on a direct basin evolution extension model with underlying wrenching of older trough limits, and suggested that given the contentious nature of the topic we might all be better off at morning tea.

Tectonic models for the Mount Read Volcanics

Tectonostratigraphy of the Dundas Trough Terry Lees^ and John Wright

In western and northern Tasmania, there are intimate and complex relationships between stratigraphy and structure/ tectonics. To interpret these relationships and simplify the complex stratigraphy of western Tasmania, seven informal megasequences have been erected, each representing a major phase in basin evolution: I -Piecambrian regions of low- to high-grade metamorphics; n An "Eocambrian" rift succession (Success Creek Group, Crimson Creek Formation and equivalents) of clastic sediments, carbonates, and tholeiitic basalts and intrusives; III - Mafic-Ultramafic Complexes (MUC); IV - Flysch-facies fill of the Dundas Trough (Dundas Group), volcanics, epiclastics and intrusives of the Mount Read Volcanic Belt and Cambro-Ordovician granitoids; V Molassic conglomerates, generally Owen Conglomerate but includes an older succession (Sticht Range Formation); VI - Shallow marine sands and limestone (Pioneer beds, Gordon Limestone and Moina Sandstone) and Vn - Siluro-Devonian sediments. The distribution of all megasequences is a result of structural and tectonic controls operating during basin formation and two subsequent periods of deformation. Rifting of Precambrian crust during the "Eocambrian" was immediately followed by a compressional phase with MUC thrust over Rocky Cape crust mantled by Megasequence II, but we contend the Tyennan region was not at this stage covered by MUC and was an approaching fragment of continental crust. The Dundas Trough was initiated as a foreland basin immediately following thrusting of MUC, perhaps even as piggy-back sub-basins atop MUC, by further depression of the Rocky Cape crust now

1 Pasminco Exploration, 116 Fullarton Road, Norwood, SA 5067, Australia

mantled by Megasequences II and III. Megasequence IV is the major basin fill phase, comprising flysch-facies sediments and Mount Read Volcanics, erupted above and adjacent to the thrust-thickened suture between the foreland basin and Tyennan region. The Tyennan Fault Zone occurs at this contact, a locus of syn-volcanic granitoid and porphyry intrusion. This major mylonite zone is also a major control on the distribution of Megasequence V, thick molasse shed mainly from the emergent Tyennan region. Megasequence VI marks collapse and stabilisation of the Delamerian Orogen with deposition of mainly shallow marine limestones. A thrust at the base of Megasequence VI marks the final compressive phase. Megasequence VQ is an extensive marine basin succession, which was terminated by the Tabberrabberan Orogeny, which comprises several fold and thrust episodes (Woodward et al 1993). Structural control on the distribution of megasequences is emphasised by the Henty Fault ZoneTyennan Fault Zone corridor, in which relationships are often more complex and variable than outside the corridor. Within the overall compressive regime operating during the Delamerian Orogeny — emplacement or deposition of Megasequences ED, IV and V — minor extensional phases are evidenced by growth faulting on the western margin of the Mount Read Volcanics near Hercules, and emplacement of the Henty dyke swarm in the Cambrian (Crawford et al 1992). A continuing problem in western and northern Tasmania is the differentiation of Delamerian from Tabberrabberan structure. In most cases, field evidence is insufficient to be definitive, but the linking of particular Cambro-Ordovician successions to the structural framework supports the foreland basin model.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

16 In the Adamsfield and surrounding areas, equivalents of all megasequences must be allochthonous, and transported a considerable distance by Devonian thrusting to their present position, if the hypothesis is correct.

References Crawford, A. J., Corbett, K. D. & Everard, J. L., 1992: Geochemistry of the Cambrian volcanic-hosted massive sulfide-rich Mount Read Volcanics, Tasmania, and some tectonic implications. Economic Geology 87: 597-619. Woodward, N.B., Gray, D.R. & Elliott, C. G., 1993: Repeated Palaeozoic thrusting and allochthoneity of Precambrian basement, northern Tasmania. Australian Journal of Earth Sciences 40: 297-312.

Tectonic setting of the Mount Read Volcanics: crustal and geophysical implications David Leaman^

I have previously suggested (Leaman 1988, 1992) that a comprehensive and continuous tensional environment existed from the late Precambrian to Devonian (indeed I would add to the Present) across all of Tasmania. No exclusion is made for the short interval during which the Mount Read Volcanics were accumulated in western Tasmania. Clearly this view must be reconciled with the compressive and uplift activity which punctuated this period — especially the Early and Late Cambrian, and the Late Devonian/Early Carboniferous. The apparent contradiction or paradox is resolved by application of tensional-wrench tectonics and its dynamic interplay. We should not ignore crustal or geophysical information while forming an opinion. No concept can be evaluated from surface in a complex environment and the geophysical input must be based in quantitative appraisal. All sources of information must be blended, ranked and filtered in terms of fundamental character and implied features properly described. The reliability of any setting or history assessment depends on the amount of knowledge available. Geochemical, stratigraphic, volcanic environment and structural concepts may appear to offer a clear line of argument about the setting of any rock suite but, unless credibly dove-tailed with primary structural elements (rarely obvious) and appropriate descriptions of volumes and relationships, these may mislead or confuse because we may p r e s u m e too much. Any argument which is chemically biassed and based on assumptions derived from modern earth without geometric support should be treated with caution.

1 Leaman Geophysics, GPO Box 320D, Hobart, Tasmania 7001, Australia

My approach is from description (what is actually there), not ideology. Let the direct inferences guide the theory. So, what can we say about the setting and distribution of the Mount Read Volcanics? 1. They are not restricted to a narrow belt near the type area — but occur Tasmania-wide at least. 2. They occur in relatively narrow belts which flank, and are external to, deep, narrow, relatively recent (in their terms), deformed trough sequences. In western Tasmania these troughs include the Bumie-Oonah Formations. 3. The troughs are blanketed by Success Creek and Crimson Creek correlates. These cover sequences are also widespread and occur in half grabens (now) but may originally have been elements of a sag phase cover. 4. The MRV also occur in half grabens, usually of opposing facing with thickest piles adjacent to the marginal uplift zones of the primary extension limit (Figure 1). 5. The relationships indicate some basin inversion between rifting involving the old sag blanket and the new deposition. Dundas Group rocks were deposited in these developing, highly active sub basins. 6. The MRV piles, which are up to 4 km thick, occur on, or close to, siliceous basement just clear of the penetrative first phase rifts. They do not overlie any substantial Cambrian or Late Precambrian sequence although they may distally interfinger with it and have been subsequently partially overthrust across them. 7. Rifting was irregular, with hinges and transforms, and all was focussed near older trough margins. These margins extend at least 7-10 km into the crust. 8. Late Cambrian granitoids are variable in composition and character and small in volume. All are located inboard of the volcanics and closer to basement. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

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KEY Fig. 5.4. T h e m a j o r characteristics in cross-sectional view of an idealized strike-slip fault. T h e upward branching f r o m a near-vertical fault-is termed a ; flower structure (after Christie-Blick j ^ d Biddle 1985).

MAJOR CHARACTERISTICS • Basement-involved • P D Z Sub-vertical at d e p t h • U p w a r d s diverging and rejoining splays JUXTAPOSED ROCKS • C o n t r a s t i n g b a s e m e n t type • A b r u p t variations in thickness a n d facies in a single stratigraphic unit S E P A R A T I O N IN O N E P R O F I L E • Normal and reverse-separation faults in s a m e profile • Variable m a g n i t u d e a n d s e n s e of s e p a r a t i o n for different horizons offset by the s a m e fault SUCCESSIVE PROFILES

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Figure 2 — Aspects of rift basins and wrench systems. A, B.from the North Sea. Diagrams courtesy bH Petroleum. Note the stratigraphic patchiness of the active phases of extension and the inclusion of volcanics. In this basin Triassic units are often thick and within troughs but the active extension begins with the Lower Jurassic. This finishes within the Middle Jurassic and normal sag covers follow. The rift pattern is also irregular with some deep core grabens. Volcanics are associated only with critical transferred structures and are neither universal nor within the deep axes. The lower diagram suggests the possible combinations of stratigraphic and structural character associated with transferance of an underlying wrench into a covering blanket. Geological Society of Australia. Tasmania Division Contentious issues in Tasmanian geology: a symposium

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Figure 4 Example of the use of magnetic data to infer the site of Cambrian rift structures. The application depends on use of upward continued magnetic data (to allow focus on primary structures) courtesy of Pasminco Exploration, and the association of primary anomalies to zones in which ultramafics and early mafic associations are either dominant or shallow. Rifted zones carry these units at greater depth (if at all) and the patterns can be linked to Dundas Group style deposition and felsic volcanic piles. A possible early Cambrian restoration is indicated which reassembles the now apparently dispersed or distinct primary structures. Ceolof'uixl Society of Australia, Tasmania Division Contentious i.\.\ue\ in Ta.\manian geology: a symposium

22 9. The rift sequence appears to have migrated eastward from the original trough margin during the Cambrian. Activity declined from the Early Ordovician but the Late Cambrian extension leading to deposition of the massive conglomerates was associated with significant inversions and reversals of structures which previously controlled deposition of the volcanics. Note that these elements are typical of large extensional basins of all ages and that the compressive elements are subsidiary overall but normal. Active rift zones are marked by partial and local uplifts which are dynamically linked to subsidence and the rotation of blocks. An overall uplift is likely if thermal aberration is significant. Peak temperatures, lowest viscosities and highest rates of isostatic adjustment are often asymmetrically focussed — on the stabler margin wall. Regional extensional basin evolution is always phased due to interplay of thermal, chemical, rotational, loading and gravitational forces. It is common to find deep narrow gaps initialising the basin with perhaps two phases of active rifting and then a multiphase sag stage. Just how active and how prolonged depends on local variables. Volcanism is focussed — spatially and temporally — near the penetrative elements of the active rift stage where piles are common, and basin wide in the late sag phases. It may be observed that the most variable, and siliceous volcanism, in modem analogs are associated with active local rifts and underlying wrenching (Japan, New Zealand). I believe these conditions describe Tasmania during the Cambrian period. The volcanics could only have been produced and introduced at that point in the cycle. Notice: No abnormal or special evolution is called for, no opening or closing of island arcs, no collisions and no massive subduction. And, as argued by Leaman (1992), we must take a much wider view than a single "arc" in western Tasmania if we are to understand the ruling environment.

Much of this geophysical view depends on the particular magnetic properties of key units — including the ultramafics and Crimson Creek Formation. But what about the ultramafics? These occur as rare lumps or thin slices and involve all Tasmania and the basement to northeastern Tasmania. These can be introduced in a tensional setting with only two requirements — deep crustal penetration, as in narrow gape troughs, and a major flowering wrench system. The wrench system will generate local amplification of extension and matching compressions, sequence inversions and discontinuity, and pull slices of the deep crust into the basin structures by 'intrusion' or obduction. All volumes will be relatively small and the effects will be highly localised. Such active features are crucial to generation, release and transfer of melts, mineralisation and fluid control and critical sites must involve them and the intra rift system penetrative transforms. But finding them is another story ... And the Mount Read Volcanics? These were formed in an active rift setting marginal to the deep action and the accumulation has occurred on the marginal shoulders of the most active rifts. The onset of rifting is directly related to wrenching of an older trough margin and the original trough may be the result of earlier wrenching. The process was not restricted to a single narrow belt and we should expect a number of gapes and irregular accumulations across a wide area deep in the early Tasman/Lachlan basin (geosyncline). Most will now be buried beneath more widespread sag coverage. These primary elements may only be found, or confirmed, geophysically References Leaman, D.E., 1988: Late Precambrian evolution. Mount Read Project Report. Mines Department of Tasmania. Leaman, D.E., 1992: Cambrian keys. Bulletin of the Geological Survey of Tasmania 70: 124-148.

A geochemical approach to a tectonic model for the Mount Read Volcanics Tony Crawford^

It is sometimes still claimed (i.e. pockets of margin arc. A number of contrasting, hypothetical ignorance persist) that the task of identifying plate tectonic models for the geological evolution modem analogues of Palaeozoic volcanic suites in of westem Tasmania were proposed, with the MRV foldbelts is compromised by the possibility that forming above either west-dipping, or east-dipping global tectonic processes then may not have been subduction zones. Here, I show that some major advances in our like those operative today. That is, the concept of actualism ('the present is the key to the past') is understanding of the MRV have resulted from either wrong, or inappropriate for Palaeozoic rocks. geochemical studies, especially with respect to This may be true for the Archaean and older interpretation of the tectonic significance of the MRV Proterozic Earth, where products are urdike those within the Lachlan Foldbelt. I use our compreon modem Earth. However, an overwhelming body hensive compositional database for the MRV to of evidence from Palaeozoic to Tertiary foldbelts the argue that these volcanics did not form in an world over indicates that the nature, relative continental margin arc necessarily associated with dispositions, and timing of tectono-stratigraphic active subduction. Rather, they are a post-collisional units constituting major foldbelts, are exactly as magmatic suite emplaced in relaxation rifts and half those in active (or future) collision zones on the graben following an arc-continent collision in the modern Earth. If the composition of the pro- Middle Cambrian. ducts,their temporal sequence and their spatial Most of the MRV, including the bulk of the realtionships are the same, then it is reasonable to Central Volcanic Complex (CVC), the Que Footwall infer that the processes that produced them were sequence, and the Tyndall Group and correlates, are the same. I know of no evidence that refutes this medium-K calc-alkaline lavas and shallow intmassertion. sives. This magma series is not uniquely diagnostic of any particular tectonic setting, and occurs in mature arcs (e.g. Japan) and active continental Affinities of the Mount Read Volcanics margin magmatic belts (e.g. Andes and Central The basalt-andesite-dacite-rhyolite magmatic America), as well as in collisional and postsuite(s) constituting the MRV led most workers to collisional settings such as in the Aegean and Papua consider that the MRV are a typical 'orogenic' New Guinea. Crawford et al (1992) referred to this magma series, and thus related in some way to series within the MRV as Suite 1. South of the Henty Fault, intmsive and extrusive subduction. Since they are clearly tied to Tyennan region Precambrian metamorphics, and appear to homblende andesites and dacites formed during the wrap around the westem and northem margins of latter stages of, or after, CVC magmatism (Corbett the Tyennan, the MRV were thus considered to be & Solomon, 1989). These have high-K calc-alkaline part of a Middle to Late Cambrian continental affinities, and stronger LREE- and P205-enrichment than the medium-K CVC lavas. We have classified these as Suite 2 (Crawford et al 1992). Suite 3 includes the massive volumes of often primitive 1 Geology Department, University of Tasmania, basalts and andesites (e.g. Hellyer Basalt) that GPO Box 252C, Hobart, Tasmania 7001, Australia dominate the Que-Hellyer hangingwall section in Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

24 the north of the main belt of MRV, but which also occur around Lynchford in the Queens town area. Suite 3 basalts show a remarkable compositional range, from medium-K basalt approaching arc tholeiite in composition (e.g. Sock Creek South) to very strongly LREE- and P203-enriched shoshonitic basalts, often with more than 12% MgO. Suite 3 shoshonitic basalts are critical to a tectonic interpretation of the MRV, for several reasons. First, similar primitive (i.e. unfractionated), strongly LREE-enriched shoshonites are unknown in continental margin arcs, and they are unlike the rare, high-K, undersaturated basalts that occur over the deeper part of the Benioff Zone in some arcs (e.g. Muriah in Java). Second, better geochemical analogues for the Hellyer-type shoshonites are found in collisional and post-coUisional settings, for example as dykes in the North Italian Alps and Karakorum, and as volcanoes in the Highlands of Papua New Guinea. Third, the temporal transition from mainly felsic lavas in the CVC, through mainly andesites in the Que footwall sequence, to primitive magnesian basalts at Hellyer, suggests that the crust was actively rifting at this time. Mantle-derived parental magmas to the CVC and Que footwall andesites were unable to rise relatively unimpeded through the crust, and thus suffered fractionation and contamination. With time, and active crustal attenuation during rifting, increasingly primitive lavas were able to pass through the crust to eruption with minimal ponding, heat loss and fractionation. Is there any other supporting evidence for a collisional or post-collisional origin for the MRV? In both the north (Que-Hellyer area) and the south (Miners Ridge-Lynchford) of the main MRV belt, quartz+mica-rich greywackes occur stratigraphically below Suite 3 lavas. These greywackes (Animal Creek Greywacke, Miners Ridge Sandstone and correlates) are derived in large part from pelitic metamorphics, yet usually contain >400ppm Cr. Thin sections always show several large, detrital, deep-red chromite grains, and microprobe analysis shows these to have distinctively high Cr/(Cr+Al) values, usually > 0.8. Only boninitic lavas and their cumulate complement have such Cr-rich chromites. Boninitic rocks dominate the ophiolitic maficultramafic complexes of the west coast (Brown, 1986), and are the only possible source for these detrital chromites. Clearly, the ophiolites were emplaced, and were being eroded prior to, and/or during deposition of these greywackes, and before eruption of the Suite 3 shoshonites and associated

rocks. If emplacement of ophiolitic rocks records a collision (see later, and Crawford & Berry 1992), then the Suite 3 lavas are unambiguously a postcollisional magmatic suite, and are not likely to have been associated with active subduction. Are MRV Suites 1 and 2 post-collisional? The Que footwall andesites are the key to answering this question. Since they occur stratigraphically above the Animal Creek Greywacke, they are also a postcollisional suite of lavas. Importantly, they are compositionally identical at 60% Si02 to CVC andesites from just further south, around Tullah. If the Que footwall andesites are post-collisional, then there is no petrological or geochemical argument against a similar origin for the CVC (and hence the entire MRV). It is highly unlikely that two compositionally identical suites, stratigraphically separated by less than a few hundred metres, might have formed in contrasting tectonic environments. A Tectonic Model for the Mount Read Volcanics According to Berry and Crawford (1988) and Crawford and Berry (1992), the west Tasmanian ophiolites formed as the forearc section of a Middle Cambrian intra-oceanic volcanic arc. Collision of this arc with an attenuated passive continental margin in the late Middle Cambrian emplaced allochthonous sheets of forearc crust onto the passive margin. How did the MRV form within and upon this crustal collage? Emplacement of this Cambrian ophiolite onto the passive margin reset mineral ages in the underthrust passive margin rocks. The plate boundary locked, and continued compression on the jammed suture forced upthrusting of blocks or slices of the underthrust, attenuated passive margin rocks. These eventually 'popped-up' as the Tyennan region, and much of the ophiohtic cover may have either slid, or been eroded off the rising Tyennan region at this time. Exhumation of part of the underthrust passive margin led to structuring of the new collisionrelated foldbelt; grabens and relaxation rifts formed along margins of the uplifted blocks. Detritus from the actively-rising exhumed passive margin, and also from the ophiolite, washed into these localized troughs, and formed the Sticht Range Beds and their correlates on the northern margin of the Tyennan region. A key element in the tectonic evolution of the MRV is extension. It has long been recognized that

25 the MRV accumulated in troughs and grabens. This claim is now supported by evidence from the geochemistry of the MRV themselves (see above) that the crust was rapidly thinning during the short time (-500-495 Ma, Perkins et al 1992) interval over which the MRV were being erupted. Collapse of, and extension within foldbelts shortly after their formation during collision events is now well proved (see Burchfiel's Ballarat GSA keynote address for a summary). I argue, then, that crustal extension around 500 Ma in the newly assembled foldbelt led to passive ascent of the convecting asthenosphere beneath this region. This, in turn, led first to generation and eruption of the parental magmas of the medium-K andesites that formed Suite 1 of the CVC and associated rocks. Thinned subcontinental lithospheric mantle was the source of these magmas, not the rising asthenosphere. With a relatively thick crust to traverse, these magmas had plenty of opportunity to pool and fractionate within the crust, so that mafic magmas were rarely able to ascend to eruption, and felsic lavas and pyroclastics dominate. Continued extension and crustal thinning led to further ascent of the asthenospheric diapir, and generation of lower-degree partial melts at shallower levels in the subcontinental lithospheric mantle. These high-K to shoshonitic basalts, isotopically essentially identical to the Suite 1 parent magmas (Whitford & Crawford 1993, in prep.), were erupted through thin crust in an actively rifting environment, forming the unusually primitive Suite 3 basalts at Hellyer and Lynchford. Another vital ingredient of this model is the Henty E>yke Swarm, and compositionally analogous lavas in the Henty Fault Wedge. These are isotopically quite different from the MRV (Whitford & Crawford 1993, in prep.), and require a source in the depleted asthenospheric upper mantle. I believe that they were generated late in the history of the MRV, when the passively ascending asthenospheric mantle diapirs rising beneath the zone(s) of rifting and extension eventually themselves partially melted, yielding characteristic rift-type tholeiitic basaltic magmas. The unusual Sock Creek South basalts are compositionally transitional between the Suite 3 shoshonites and Henty Dyke Swarm-type rift tholeiites. The large sill-like quartz-feldspar porphyrites along the western and northern margins of the MRV have isotopic compositions indicative of crustal sources (Whitford & Crawford 1993 in prep.). They may have been produced by pooling

of mafic magma (Suite 3 or tholeiitic) at the base of, or within, the continental crust, following cessation of rifting and extension. If, as seems likely, the Henty Fault Wedge sequence includes a small oceanic crustal section (basalts, dolerites, gabbros, ultramafics), then the rifting event that triggered MRV magmatism appears to have proceeded to break-up stage. However, the limited nature of the oceanic crustal section, and the rapid cessation of MRV-type magmatism, suggests that rifting was aborted at this stage (-495 Ma?). Regional plate kinematics demand that relative plate motions were adjusted. It is possible that, as has happened in many Tertiary collision zones in the western Pacific, plate motion switched from compressional during collision, through transtension, to transform (or wrench faulting). A final point worth noting is that most modem or Tertiary post-coUisional suites (e.g. PNG, NE Iran and E Turkey, Italian Alps, Karakorum) are erupted hundreds to thousands of meters above sea-level, through thickened crust in new collisional mountain belts. In contrast, the MRV were mainly erupted below sea-level, as indicated by pillowed Hellyer basalts, fossil evidence within and just above (Que River Shale) the MRV, and the 'background' sedimentation of Animal Creek Greywacke-type washing into the rift during pauses in MRV magmatism. How could arc-continent collision produce so little crustal thickening in the collage of crustal elements constituting the new foldbelt? A simple answer to this is that passive margins are now known to vary enormously in their crustal architecture and thickness. Several major modem passive margins have cmstal sections composed of crystalline cmst thinned to only 5-15 km over hundreds of kilometres width. Collision of such an attenuated passive margin with a Mariana arc-type forearc in the Middle Cambrian set the stage for production of the post-collisional Moimt Read Volcanics. References Berry, R.R & Crawford, A.J., 1988: The tectonic significance of Cambrian allochthonous mafic-ultramafic complexes in Tasmania. Australian Journal of Earth Sciences 35: 523-533. Brown, A.V., 1986: Geology of the Dundas-MtLindsayMt Youngbuck Region. Geological Survey of Tasmania Bulletin 62: 221 pp. Corbett, K.D. & Solomon, M., 1989: Mount Read Volcanics and associated deposits. In: C.F. Burrett (Ed.): THE GEOLOGY OF TASMANIA. Geological Society of Australia Special Publication 15: 84-153. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

26 Crawford, A.J. & Berry, R.F., 1992: Tectonic implications of Late Proterozoic-Early Palaeozoic igneous rock associations in western Tasmania. Tectonophysics 214: 37-56. Perkins, C. & Walshe, J.L., 1993: Geochronology of the Mount Read Volcanics, Tasmania, Australia. Economic Geology 88:1176-1197.

Tectonic models for the Mount Read Volcanics Discussion session — chaired by Ross Large Transcribed by Rohan Mine, Bianca Manzi and David Cooke

John Fodon (JF) to Tony Crawford(AJC): / m interested and somewhat perturbed by the thermal explanations for the post-collisional magmatism. I'm quite willing to accept that it occurs, but I still think that the explanation you give of rebound on collision is insufficient to actually exhume the asthenosphere through sufficient vertical distance to develop decompressional melting of the kind that produced the Henty Dyke Swarm. The other problem I have is that the Mount Read Volcanics has been largely taken from the lithospheric mantle, but elsewhere you see rather different geochemical signatures from the lithospheric mantle — things like relative Nb enrichment, and characteristics that you see in the kimberlites are more often associated with enrichment associated with the lithospheric mantle. Those are not the characteristics of the Mount Read Volcanics — they've got arc-like negative Nb anomalies. There are two questions here (a) do you think your scenario says enough about how you can exhume the asthenosphere through enough vertical extent, and (b) do you have any doubts about the source of the Mount Read Volcanics? AJC: Starting with the second question, suitable analogies for the lithospheric mantle providing Nb- and Ti-enriched magmas and so on are based on stuff mainly on the mainland. If you recall the diverse high-K magmas of the Papuan highland volcanoes, or the Romon Province, each has substantial negative Nb-Ta anomalies, and each occurs in a manifestly post-collisional setting, so you can see that my claim has modern counterparts. Also the Jurassic dolerite of Tasmania has huge Nb-Ta negative anomalies. I don't see that as a problem. I don't know that it is fair to say that the sub-continental lithospheric mantle must be incredibly homogeneous because of what we see coming from it. The first point is a good and valid point, but the thing is that we do see an asthenospheric melt coming out right at the end. The Henty dyke swarm has eNd = -i- 6 that we think is close to Cambrian depleted mantle composition.... (inaudible) ... +5, +6 for the Henty dykes, the Henty pillow lavas and the rocks underneath (gabbros and

dolerites) are very oceanic-looking in an ocean crustal sense. I think they record that the rifting got as far as actually tapping the top of the asthenosphere, and started to trigger the whole Mount Read episode so that the earlier diapiric ascent of the asthenosphere caused in-situ melting of the lithospheric mantle, and just at the last gasp ... (inaudible) ... the asthenosphere itself, it managed to yield the Henty dyke swarms. I should mention also that exactly the same sequence of rocks occurs in the Cambrian Devil River volcanics in New Zealand and is cut by exactly the same sort of dyke swarms, and I think that's a reasonable story. I agree with what you say; scalewise, you want something like the Red Sea, but it's very hard to judge how far the asthenosphere rises up in these passive rift situations Glen van Kerkvoort (GVK) to David Leaman (DEL): I'm just wondering how your model copes with the island arc signatures for the volcanics? DEL: The short answer is "who cares?". Tony has made an affinity interpretation, whereas I have made a geophysical interpretation around the sides. Now with what Tony has just said, I don't have any problems with all. (Chairman — Oh well, we can all go to lunch!) While you're talking about what's actually in the rocks, and about the chemistry, I don't have a problem at all. I don't have a problem with the asthenosphere rising. I don't have a problem with the mantle undergoing changes as it's rising. I don't have a problem with the first melt being a certain composition and a later melt being different in composition. What I do have a problem with is when you say that melt A or melt C is this affinity or that affinity. While you use the word affinity, fine, you don't get a problem from me. The moment you say that suite C,for example, is an Island arc, then I say you are on dicey ground, and that's the distinction I'll draw. I don't say that Tony's chemical sequence is in any way wrong, I don't have a problem with that. I don't have a problem fitting it in , but I don't need an arc either.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

28 AJC: The ruling out of the analogies, the way it works and what we're trying to do (which, by the way, isn't restricted to the University of Tasmania, but is generally a global way of doing this sort of work), it's equivalent to me claiming that the magnetic susceptibilities you (DEL) stated for your serpentinite of300 and greywacke of 4 is wrong, and is the other way round, with the serpentinite having no magnetisation at all, and the greywacke is probably really magnetic. Of course it's ridiculous to say that! Rocks have a geochemical signature based upon a set number of processes and sources that form them. Fancy drawing an analogy between western Tasmania and the North Sea. There are no oceanic rocks, there is no Mount Read Volcanic correlates, there is no post-collisional molasse. I have great trouble finding any point of analogy whatsoever! If the North Sea continues to develop, then an ocean will open a la the Wilson cycle, subduction will start and then you might get a Tasmanian setting after another 150 million years, but I can't see that here. It seems to me that the North Sea analogy is forced and unreal. DEL: I think the point here is when we look at the modem Earth, we are making judgments of what constitutes an arc, what constitutes what is actually happening in a so-called collision zone. I think we have to be very careful about whether we interpret the modem Earth in the right way tectonically, and I will disagree with some of the interpretations. The chemistry that you assign to what appears to be an arc is one thing. To assume that an arc will behave in a certain way and subsume in a Wilson cycle is something else. I have been criticised for my use of the North Sea; I could have picked a number of other basins, some of which have been exhumed, but I unll point out here that you don't know what is actually happening, or has happened, deep within the basin of the North Sea; it is not exhumed yet. But if you go to basins that have been through that cycle, I pointed that out as a simple thing, if we are going back to the Cambrian, I was asking the question "what was it like in the Cambrian"? Remember now, we are dealing with the basement to whatever was going on in the Cambrian. Mike Solomon (MS): / always feel sorry for geophysicists. They wrestle with what's in front of them. The geologist has the much greater pleasure of taking a somewhat wider view, so that we can be interested in whether we are looking at a margin, or a island arc, etc.. We use everything we've got to get at the answer, and everybody knows they're just plugging in little ideas to try and make a story. So I'm going to plug in another idea. I think that Tony is absolutely spot-on with the approach (and he is aware more than any of us of the difficulties). For example, if you go to Porgera, you'll

find that all this is obviously totally wrong — the chemistry of the rocks doesn't fit at all. Fortunately, though, that's an isolated case, and the geochemical approach generally works. So the ore deposit guy, he says "well I'm certain these ore deposits occur all over the place. They occur from the Archean right up to modem times." In modem. Tertiary and Cretaceous times we can see these things. In the westem Pacific, we have the most fantastic workshop to go and study, and we can actually put together a detailed history that covers down to hundreds of thousands of years. If there is one thing that stands out, it is that these massive sulphide deposits occur in back arc basins. There is only really good one, I admit, that is very much like our own and that is the Okinawa Trough which is a post-collisional extensional basin with arc-style volcanism shifting from the arc to the back arc basin. So we have a continuous process, and we can actually see volcanoes on the arc giving up and shifting to the back arc basin, and there you are looking at thin continental crust undergoing extension. It's quite soon (1 Ma or so) after a major collision with Taiwan, and there, low and behold, right where the asthenospheric basaltic dykes are occurring, we've got these VMS type deposits. This looks to me like a fantastic way of saying to everybody "Let's try that model out." Of course, in Solomon and Groves (1994), we said that's the story, but what we really need is another way of looking at it, which exactly fits in with everything Tony said. The fact that those two approaches tie together, it immediately gives each of us common support. DEL: I don't think that Tony's approach apart from terminology disagrees with mine. MS: Really? I don't think so. AJC: I agree. I have no problem whatsoever with the structural detail and I admire immensely the geophysicists that come through. An interesting point that was nmde by Dave (DEL), and by Ron Berry and by David Gray was that everyone was pointing out low angle thrusting in 1970-1975. Something happened for 20 years in Tasmanian geology, and that idea died. Its rebirth was two years ago, when DG, RFB and DEL have drawn it out again. Something really suspect happened twenty years when all these ideas died out. MS: John Griffiths in 1971 was trying to persuade everybody that west Tasmania has to be full of major thrust faults because it seemed obvious that it was a continental margin of some kind. AJC: I agree with all the geophysical evidence. I find it totally nihilistic and fatalistic and dissatisfying not to try and make that into some modern framework that I find aids my understanding. And also, I think that it aids communication. We have got to look beyond

29 Tasmania with what we see. In the first paragraph or two in my abstract is my attempt to justify my approach. I think it is really important to play around with models, you learn a lot about what's happening. Ron Berry (RFB) to Terry Lees (TL): Terry, you were suggesting that you were going to explain why 700 °C,16kb rocks line up along pretty small faults of the Tyennan Margin. The problem is at those pressures, you're talking about rocks that started below the base of the crust. I do not think you want us to assume that on your section you would have 45 km of uplift. Would you like to suggest 45 km of uplift in the Tyennan Shear Zone? TL: No not on that one, but I imagine that in a microcontinental block such as that envisagedfor the Tyennan region, during a major collision you would have marginal and internal major structures. I'm suggesting there are some major internal structures and ... (inaudible)... Nick Direen (ND) to AJC: What mechanism do you and RFB propose for reversing the extraordinary extension of the continental crust, and what time frame that was taken under. AJC: First of all, Ron is the structural geologist, but I will try to answer. In Oman, Papua New Guinea, Taiwan — anywhere where there has been an arc-continent collision, the big sheet of fore-arc crust emplaced onto the passive margin in the style that I have showed (in each case they are Mesozoic and Tertiary examples), for whatever reason, you can call it back-thrusting, pop up or whatever, the under-thrust margin has suddenly reappeared. It is not really a core complex in the extensional sense of the word, but they're there, and the ongoing collision is forcing them to pop up. I'll defer to Ron for the mechanisms of this. RFB: What is the great extension that you are talking about? ND: To thin the continental crust to within 1-2 km and then prevent it from rifting away entirely. RFB: You are actually talking about a passive margin formation which is older than the collision, which we see in a large number of places. AJC: It's just evidence, Nick. I just had a paper on my desk last night that goes into a whole lot of detail about super deep fancy geophysics on a cross section from the Red Sea. I might as well have photocopied that diagram as well, in fact it is in the course notes for the next structure and tectonics of ore deposits course at CODES. The diagram I'm talking about shows that if you stretch it, it's like toffee — at some stage it will break. You have to get it very thin before it breaks, and it's the same with

the continental crust. It is just whether it starts to break very sharp — if the extension is distributed over a small area quickly, that might produce a different architecture in the crust, and if it's a long time frame, and I would say it is like the Red Sea in that area. From Give Calver's work, it may be forty to fifty million years for that sort of rifting stage. ND: Given the time frame for all that stretching, what caused it to reverse? AJC: That's global plate kinematics, and it's is like asking why subduction is occurring on the Western Pacific margin and not in the Atlantic. It's just how it is at the moment, at that instant in time, as a function of the global interactions of all of the plates. When you 're rifting open an ocean, for some reason, subduction starts, subduction produces an oceanic arc, and perhaps eventual closure of the remainder of the basin, and arc-continent collision occurs. Andrew McNeill (AMcN) to AJC, DEL and TL: How do the Connor Peak and Glenorchy Bore hole volcanics fit into your models? They're the only volcanics that occur that far to the east. AJC: The Connor Peak volcanics are Crimson Creek Formation rift tholeiite correlates. The one sample that I have analysed certainly looked similar to Crimson Creek. As for the Glenorchy bore hole, I've said that the Mount Reads are formed in structural traps that formed during this emergent phase of exhumation of the Tyennan. The Tyennan ages, by the way, have been reset by that collision, which is why they record significantly younger ages than the Rocky Cape Group.

Michael Raitz (MR) to AJC: Would you comment on your slide where the epsilon Nd values for 500 are anomalously close to zero for the suite 3 rocks (high-K Hellyer basalts), and are distinct from the crustally contaminated rocks. Would you like to speculate on how you would get a rock like that tectonically, and on its economic significance? It strikes me that if these are mantle-affinity rocks, there should be copper deposits rather than Pb-Zn deposits in that setting. AJC: The eNd values around zero record the composition of the source mantle from which they were derived at that time, and it's absolutely not the composition at that time of convecting Earth's asthenosphere, so in my opinion it has to be that it records the composition of that block oflithosphere that is underneath it, and that's had a long and complex history. Who knows the previous history of what's under the Rocky Cape Block? It's Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

30 anomalous only in the sense that it is not asthenosphere. If you were to actually analyse a lot of those sorts of things from around the place, in more modem settings you'll find similar sorts of provinces that are not recording asthenospheric values. For the second point, as to what is the exploration significance, I have two points to make. One is that I would be very impressed by the association of these interesting ore deposits here, and not to mention Porgera and Lihir — Lihir is not a normal island arc — it has some very strange lavas. The whole Tabar-Feni Group has some of the biggest Au deposits in the world, and it may turn out that this postcollisiorml setting, although it is certainly bizarre, seems to provide some interesting and very useful ingredients for mineralisation, be they tectonic, metals, sulfur, water, whatever, or the setting — Fm not so sure. The key point I want to make is that the tenor of the VHMS deposits in western Tasmania, is abnormally high, while across the way in Victoria in the Lower Ordovician, we've got vast reef gold deposits. Where's the gold coming from? It's a really interesting thing that underneath this whole area we have the ophiolites that we've talked about. These are dominantly composed of what were glassy, brecciated, easily alterable and accessible to fluids, boninites. Boninites happen to be ten to one hundred times more enriched in PGE's and gold than normal crustal rocks. So it may not just be chance that these gold deposits occur where they do. It's worth considering so that you 're exploring in areas where there have been collisions involving thrust and forearc-type ophiolites. The key thing about the Mount Read Volcanics is that this belt was largely produced, and stayed underwater the whole time — that's so bizarre!! The volcanoes in New Guinea are 3000 m above sea level — they're not under water. To keep it underwater, the underthrust passive margin crust must have been extraordinarily thin. RRL to DEL: Do you agree that there are ultramafics underneath the Mount Read Volcanics sitting on Precambrian basement? DEL: There are slices of them. The magnetic basement is essentially the Success Creek or Crimson Creek Group. That's been the previous cover sequence. If we can just go back a step to the discussion about Glenorchy. I was raised with the idea that the whole of what was now continental Tasmania and things beyond it were all stretched at this time. Something like Glenorchy is another one of these half-graben positions. It's again on the eastern margin of an older trough. So that's something else that's been there and has been taken apart again. That sequence again is covered by magnetic rocks like the Crimson Creek Formation and is only seen in troughs. We've had everything sliding over the top, and

most of that's been peeled offfortunately. Each of those margins appears to have had lateral movement. Tony raised this — I was accused of picking the wrong thing in the North Sea. He's used Oman, and in all of those kind of situations — I don't care whether you call them arcs, or continental this's or that's, if you go to what is actually there in all of those places, Oman is a wrenchstyle of emplacement. If it's a collision, fine, have your collision, but thefundamental thing is the lateral motion, and that was the point I was making. Identify what is there, and what is important to exploration, and worry about the terminology later. I believe it would be whole lot simpler if treated as a natural progression of some rotation dynamics and extension rather than other things. Mel Jones (MJ) to AJC: You compared the source of the Hellyer basalt with the source of Tasmanian dolerites, but stopped short of suggesting a mantle plume source. Is that a possibility and does it have any implications for tectonics in the light of a Campbell-Hill plume tectonics model. AJC: I am not a real big fan of all the plume story. The vast combined volume of Jurassic dolerite and volcanics in Antarctica, Kangaroo Island, western Victoria and Tasmania indicates a fair size event that could have been produced by the impact of a plume on the base of that lithosphere. That's if you like the Campbell-Hill type of story, where the plume was insulated and cooled, and did nothing other than drive out the melt. The plume may have caused partial melting of the sub-continental lithosphere, forming the Ferrar-Tasmania dolerites, and then organise itself into a normal spreading system which formed the Southern Ocean. I certainly haven't followed up this at all, but I just thought that it was an interesting point to make. RRL to TL: Most of the major faults in the Mount Reads have mineralisation along them. Did you find any mineralisation along the Tyennan fault. TL: The Tyenrmn Fault is only exposed in a few places. It has been the focus of the intrusion of the CambroOrdovician granitoids. If you're interested in those granitoids and the mineralisation that's surrounding them, then there is potential, because there are large systems, usually west of the Tyennan margin in the Tyennan fault. Bob Close (BC) to AJC: Can you comment on the variation in the Mount Read Volcanics in the Fossey Mountain Trough compared to the west coast, and also south of Macquarie Harbour?

My second question concerns the Noddy Creek and Mainwaring volcanics in the Sorell Peninsula area — do you regard them as Mount Read Volcanics, and if not, would they have different potential mineralisation settings?

incompatible element ratios. This makes it very hard to work on felsic rocks (rhyolites and dacites) compared to mafic rocks. I think Dave Whitford and I can convincingly show with isotope data that the felsic rocks have a much larger component of crustal melt in them. AJC: From my work around the Hellyer area to Black Yet I showed from the Placer holefrom west ofMurchison Bluff, Mount Cattley all the way across to Golden Valley, Highway that those dacites interbedded in the Hellyer all of the rocks that I have analysed from there, were what basalts are not much different from the Hellyer basalts I call Suite one. I would correlate them certainly, and — they're co-magmatic and crystallise in a magma the rocks underneath them, with the Animal Creek chamber from the Hellyer basalt. The big porphyrites Greywacke-Que Footwall Andesites in my opinion. The down on the western side of the Mount Reads have Noddy Creek Volcanics down on the Sorell Peninsula entirely different isotopic signatures that presumably area have much the same story — they look much more reflect heterogeneous crustal melts. A lovely project for like Que footwall andesites than the Tullah to somebody in due course will be to link up with someone Queenstown type sequence in my opinion. They are who can provide Nd isotope data to look at subdividing undoubtedly part of the Mount Reads. The Mainwaring the felsic rocks in the Mount Read Belt. After all, Volcanics look more like Crimson Creek to me. volumetrically they're dominant! BC: No, the ... (inaudible) ...is Miner's Ridge, that's RRL: There was supposed to be time for me to sum up, about as far north as we can take them and give an overview but thankfully the time is just about AJC: They're Crimson Creek tholeiites in my opinion. up (laughter). What this second session has shown is There are Mount Read Volcanics that go way beyond that either you believe in the use of geochemistry to the major N-S belt. fingerprint tectonic environments, or you believe in the use of geophysics to give a three-dimensional picture. I Roger Poltock (RP) to AJC: think we can probably believe both of them, and we can It seems to be relatively easy to distinguish four suites use the geophysics to give us a much better threein the mafic to intermediate rocks but only one suite in dimensional picture down to ten or so kilometres. We the felsics. Is it possible that the felsics are made up of can then interpret that in terms of current tectonics. We several suites and it hasn't been possible as yet to detect can also use the geochemistry as much as possible, but them? with reservations (and I think we've got to be aware of AJC: An extremely important point. Silica takes up more these reservations) to categorise certain tectonic events and more of the total rock volume when you go from that may have occurred within that geophysical basalt to rhyolite, and things like apatite and zircon start framework. crystallising, which causes enormous changes in

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Re-interpretation of the Mount Read Volcanic succession

Stratigraphic mapping, Tyennan connections, Cambrian orogenies, the Arthur Lineament, and the tectonic context of the Mount Read Volcanics. A fresh look at Western Tasmania Keith Corbett^

Mapping of the Mount Read Volcanics is a difficult business, however you approach it. The MRV Project required rapid, belt-wide coverage using good field geologists not particularly trained in volcanology. The stratigraphic approach emphasised those criteria considered to be useful for correlation between regions, without necessarily understanding the processes involved. Names were established for the units as we went. The welter of research and company work which has followed (probably more intense than in any other comparable area in Australia) suggests the maps have filled a definite need. However, it is not surprising that some of the interpretations and correlations have been modified by further work and by a better understanding of facies and processes. I regard the two approaches as being entirely complementary. Eight years ago, at the 1986 Burnie MRV Symposium, I reviewed knowledge of the mineralised horizons in the volcanic sequence and suggested that there were six main ones: RoseberyHercules, Que-Hellyer, Pinnacles, Chester, Howards Anomaly and Red Hills. RoseberyHercules was considered to lie in the lower part of the Central Volcanic Complex, and to be older than that at Que-Hellyer, lying in a younger sequence above the CVC. It is now known that the Que-Hellyer sequence extends via Sock Creek to Burns Peak and the Pinnacles, and that it is probably equivalent, or virtually so, to the Rosebery-Hercules horizon (on the basis of equivalence of the hangingwall sequence with the Southwell Subgroup, and the Rosebery

1 Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018, Australia.

shale with the Que River Shale — McPhie & Allen 1992). The Howards Anomaly horizon is now known to extend between Henty and Comstock and also south to Lynchford and possibly to the Garfield River. It might also be equivalent to the Que-Hellyer horizon (on the basis of the andesite connection), but critical assessment suggests it could be slightly younger, lying right at the base of the Tyndall Group. This raises the question of where the Southwell Subgroup equivalents are in the Queenstown sequence. Lithological correlations suggest they could be represented by the Yolande River Sequence, which has large intrusive-extrusive felsic porphyry bodies and associated porphyry-like sedimentary units like the Southwell Subgroup. If we make this correlation, then the southern CVC rocks are seen as lying at the top of the Southwell Subgroup (and partly interfingering with it), in a position matched by some minor felsic lavas in the Cradle Mountain Link Road area (MRVP Maps 7,8). The extrusive andesites in the Queenstown area sit above the CVC, possibly uncomformably, and contain the Comstock exhalative mineralisation with its associated cherts, limestone, and jasper. They are followed by the Tyndall Group volcaniclastics. So in this case, the main exhalative mineralisation is associated with andesites which could be stratigraphically above the Southwell Subgroup. In the broader tectonic context, it should be noted that the Central Volcanic Complex rocks are an integral part of the MRV (being partly equivalent to Que-Hellyer Volcanics and Southwell Subgroup), and need not be ascribed a different origin (e.g. part of the allochthon — Berry & Crawford (1988), or generated above a temporary westward-dipping subduction zone — Crawford & Berry (1992)). If other parts of the MRV are post-collisional in origin, then so are the CVC rocks. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

36 An old question which remains unanswered is why the MRV belt is located along the western margin of the Tyennan Massif — what is the connection? The boundary between the MRV and Tyennan Precambrian has now been mapped most of the way between Elliott Bay and Cradle Mountain. Three boundary types are evident. In most areas the boundary is marked by a west-facing sequence, up to several hundred metres thick, of Precambrian-derived siliciclastic conglomerate and sandstone — the Sticht Range Beds and correlates — which underlies and grades into the MRV. There are a few localities (Fleece Creek, east of Lake Spicer, Wanderer River) where there is actual exposure of the unconformable contact or where such a contact can be confidently assumed. The second type is an intrusive boundary where one of the large porphyry bodies (e.g. Bonds Range Porphyry) transgresses the Cambrian sequence and directly intrudes the Precambrian rocks (e.g. Fury Gorge). The third type is a faulted contact, seen in sections up to several kilometres long (e.g. on east side of Murchison Granite, in Lewis River area), with a mylonitic fabric developed in some areas. I conclude that the contact is essentially a depositional unconformity which has been faulted in places, and that the Tyennan Massif was in place and being eroded at the beginning of MRV time in the Middle Cambrian. This is not a Johnny-come- lately thrust block. However, its western boundary beneath the MRV may well be a thrust boundary. Whether the Tyennan represents genuine basement or a large thrust block or series of thrust blocks emplaced prior to MRV time is uncertain. However, the evidence seems compelling for a period of large-scale emplacement of allochthonous blocks, including the arc-derived mafic-ultramafic complexes, into western Tasmania in the pre-MRV period, as deduced by Berry and Crawford in their arc-continent collision model, and it seems likely that the Tyennan was involved in this activity. But for what reason did the western margin of this largest of the basement blocks become the site of intense volcanic activity in the post- collisional stage? Finding the explanation for this could have important implications for understanding the volcanics and the formation of the orebodies. The collisional event in the Early to Middle Cambrian may have been the largest tectonic event to have affected western Tasmania (Figure 1), at least in terms of amount of tectonic transport. It involved only three of the major West Coast rock units — the

Precambrian ^basement', the sediments and associated rift tholeiites (Crimson Creek Formation, Success Creek Group and correlates) which had been deposited on that basement, and the nappes of mafic-ultramafic rocks and other "exotic" lithologies. It seems quite likely, as suggested by Crawford and Berry (1992), that various segments from the leading edge of Tasmania's passive margin were also back-thrust over western Tasmania, so that telescoping of disparate continental fragments would be expected. A general westerly direction of transport seems likely. Most of the thrust- bounded Precambrian blocks now being identified (e.g. Badger Head, Forth, Mt Bischoff, Dundas, Cape Sorell) probably relate to this event and were part of the transported package. One of the outstanding problems to be resolved in relation to this event, and to Tasmanian Lower Palaeozoic geology generally, concerns the identification and characterisation of the exotic arcderived and related 'oceanic' sequences which came with the ultramafic-mafic rocks. The chert-rich 'Cleveland-Waratah Association' and Ragged Basin Complex (Adamsfield area) are likely candidates, but others could include the Port Sorell sequence and some of the units in southwest Tasmania (A.V. Brown, pers. comm). These exotic melanges are probably a major component of the pre-MRV basement of Tasmania. The deformation and metamorphism in the Arthur Lineament are associated with the appropriate pre-MRV rock types, and could well be related to this collision event. Turner's (1991) radiometric ages from the amphibolites (510 Ma and younger) are in about the right position (latest Early Cambrian) according to the new time scale, and Brown's (1986) 520 Ma date for the ultramaficrelated tonalite gives an older age limit. It is suggested that the extraordinary Arthur Lineament zone, in which the Penguin Orogeny reaches its maximum expression, represents a leading edge effect from emplacement of this great pile of nappes, while the Smithton area to the west remained relatively unaffected. The major collision and nappe-emplacement event seems to have triggered the most intense geological activity in Tasmania's history, in the brief Middle to Late Cambrian period. This 20 my period (509-490 Ma) saw eruption of the Mount Read Volcanics, deposition of the various fossiliferous Cambrian sequences (partly from erosion of the chert- rich nappe complexes), uplift and erosion of

AGE AND NAME 380

EVENTS AND EFFECTS • Thrusting of Mathinna terrane over West Tasmania terrane

MIDDLE DEVONIAN

• Fracturing of Tyennan Massif and limited over-thrusting to west

Tabberabberan O r o g e n y

420

• Widespread folding and cleavage development (deep burial) • Reactivation of many faults, some thrusting A Continuous deposition during period of tectonic "meta-stability"

• Deposition of shallow-water siliciclastic sequences

MIDDLE TO LATE CAMBRIAN

490

509

POST'COLLISIONAL

OROGENIC

PERIOD

J u k e s i a n or Delamerian Orogeny

• Intermittent uplift and erosion of Tyennan Massif • Local folding and cleavage development; continuous faullting • Deposition of coarse flysch sequences, partly from erosion of nappe complexes • Eruption of Mt Read Volcanics along Tyennan margin yjy® Rebound thrusting and other fault adjustments Continuous tectonism

? 510

EARLY TO MIDDLE CAMBRIAN MAIN

COLLISION

EVENT

Penguin O r o g e n y ?

Figure

1 — Suggested

• Deformation and metamorphism of Arthur Lineament belt (leading-edge effect?) • Large-scale thrusting and telescoping of Proterozoic fragments in melange matrix • Deformation of in-situ Proterozoic rocks and Crimson Creek Formation • Emplacement of nappes of arc-derived mafic-ultramafic rocks and associated chert-rich "oceanic" lithologies to form complex melanges

orogenic development

for Palaeozoic

the Tyennan Massif and various other units, and infiUing of fault-controlled basins with the thick Owen Group-Denison Group siliciclastic sequences. It is notable that virtually all of this intense activity of eruption, deposition and erosion occurred on the disrupted nappe-covered area east of the Arthur Lineament, while the Smithton-King Island area to the west remained relatively unaffected and apparently uncovered. There appears to have been continuous faulting and tectonic adjustment throughout this period, and some local folding. Only minor cleavage development is apparent, however, probably because of the shallow burial. Significant thrusting may have occurred at this time, although probably not on the same scale as previously. This 'Delamerian' or 'Jukesian' episode died out gradually in the Ordovician as the basins were filled and covered by a marine carbonate transgression. Many radiometric ages relate to this period, but the compression of the time scale suggests there will be blurring of the record of individual events and relationships, and that confusion with the earlier collision event is possible.

Tasmania.

The Tabberabberan Orogeny, in the MidDevonian, was also a major event involving widespread thrusting and reactivation of earlier faults. However, the relative integrity of large elements such as the Mount Read belt suggests that the amount of tectonic transport may have been much less than for the Cambrian collision event. Recementing of the disrupted metastable area of Palaeozoic Tasmania into relatively stable cratonic crust was finally achieved by the widespread intrusion of granites in the Late Devonian-Early Carboniferous. References Berry, R . E & C r a w f o r d , A.J., 1988: Australian

Journal

B r o w n , A . v . , 1986: Tasmanian Geological Survey Bulletin

62.

Earth Sciences

35: 523-533.

C r a w f o r d , A.J. & Berry, R.F., 1992: Tectonophysics,

214:37-

56. M c P h i e , J. & A l l e n , R . L . , 1992: Economic

Geology, 87: 5 8 7 -

596. Turner, N.J., 1 9 9 1 : Gondwana

8 Abstracts, Hobart.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Recent advances in volcanic facies analysis of the Cambrian Mount Read Volcanics, western Tasmania J. McPhie^

In the beginning, there were lavas, agglomerates and tuffs! With these categories, even the most complicated volcanic sequences, whether subaerial or submarine, could be mapped and subdivided. Direct observation of modem sea-floor volcanoes and studies of ancient, uplifted, submarine volcanic sequences over the past 10-15 years have greatly augmented knowledge and understanding of subaqueous volcanism. We now recognise diverse volcanic textures and structures that can be powerful tools in unravelling the internal structure and facies architecture of submarine sequences, such as the Mount Read Volcanics in western Tasmania. An appreciation of the potential complexity is crucial in effective exploration for volcanic-hosted massive sulfide (VHMS) ore deposits. One of many, important recent advances in volcanology is the recognition that significant additions to the architecture of submarine volcanic sequences may remain subsurface, in the form of syn-volcanic sills, crypto-domes and other intrusions. Positive identification of these elements depends heavily on upper contacts that are demonstrably intrusive (locally cross-cutting stratigraphy; induration or alteration of the host) and evidence that the host sequence was unconsolidated at the time of intrusion (presence of peperite; deformation or destruction of bedding). Syn-volcanic sills and peperite undoubtedly play a role in initiating or modifying hydrothermal systems and can also dismember unrelated pre-existing sulfide deposits intercalated in volcaniclastic sequences. Models for VHMS deposit genesis require refinement in order

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

to include the additional styles and sites of mineralisation associated with syn-volcanic intrusions, and the effects of such intrusions on oreforming processes. Other developments relevant to volcanic facies analysis of the Mount Read Volcanics are that: (1) pumice (and scoria) can be created in appreciable volumes in association with the extrusion of submarine lava flows and need not always imply explosive volcanism; (2) hot syn-eruptive pumice probably behaves differently from cold pumice and may rapidly become water-logged; (3) some deep submarine, basaltic volcaniclastic deposits are generated by mildly explosive fountaining; (4) concentrated, poorly sorted, particulate suspensions (such as syn-eruptive megaturbidity currents) may undergo self-fluidisation during sedimentation, resulting in loss of fine and light components (pumice and shards) and forming markedly crystaland lithic-enriched, massive to diffusely bedded deposits; (5) textures in volcanic rocks evolve, beginning with features imprinted by eruption and emplacement processes that are subsequently modified by devitrification, hydration, and diagenesis; in many cases these textures are also overprinted by hydrothermal alteration and metamorphism. Industry and ARC-supported research underway at CODES is focussed on applying careful textural interpretation and volcanic facies analysis to the Mount Read Volcanics. Our research builds on the understanding developed by Keith Corbett and colleagues (Department of Mines, Tasmania) in the course of 1:25 000 regional mapping of the Mount Read Volcanics. Some of the highlights are reported briefly below. Correlations: Two widespread and distinctive volcaniclastic sandstone-breccia units interbedded with black mudstone in the younger parts of the

39 northern Mount Read Volcanics allowed correlations extending about 13 km along strike (Units A and B; McPhie & Allen 1992). Although the lower unit is limited to north of Sock Creek South, the upper unit appears to continue to Bums Pinnacles. Furthermore, sections at Bums Pinnacles include crystal-rich sandstone containing crystal populations similar to those in some Tyndall Group sandstones. This correlation supports temporal equivalence of sequences now separated by the Henty Fault. The potential for extending correlations into other parts of the Mount Read Volcanics using mafic (pyroxene) and oxide (ilmenite and magnetite) crystals in sandstone units is very promising. Hellyer Basalt: Contact relationships together with marked lateral changes in thickness and stratigraphic position of basalt intervals in the QueHellyer Volcanics indicate that most, if not all, the Hellyer Basalt is intmsive. Basalt emplacement post-dates deposition of most of the Que River Shale and was evidently controlled by the marked density contrast between the wet, poorly consohdated mud and the older andesitic and dacitic volcanics. Because several, compositionally discrete sheets are present, emplacement probably involved multiple injections, as suggested by models for the development of sill-sediment complexes (e.g. Einsele 1985). Palaeotopography: A cross-section from the Cradle Mountain Link Road to Sock Creek shows that the Que-Hellyer Volcanics formed considerable positive topography relative to the Bulgobac-High Point areas to the west and southwest. The Que River Shale thickens markedly from about 100200 m near Hellyer to more than 450 m west of the Murchison Highway (Figure 1). Volcaniclastic units A and B (McPhie & Allen 1992) also thicken in the same direction, but only slightly, implying that the Que River Shale had effectively buried the older topography by that stage. Similarly, the Sock Creek area must have been elevated relative to BulgobacHigh Point during or after the accumulation of the Que-Hellyer Volcanics because equivalent parts of the stratigraphy at Sock Creek are much condensed (Figure 1). The positive topography was probably created by a combination of constmctional relief and syn-volcanic faulting. Sock Creek South: A distinctive coherent dacite and breccia lithofacies association occurs in the lower part of the Sock Creek South sequence. The association is interpreted to represent resedimented

BRD 4

SCS 3

BRD 1

HL 541

HL 345

LEGEND:

l*iic-4rMnn«li«« lava volcanic brecda I

sandstone

100 m

Figure 1 — Simplified stratigraphic logs of drill core in the northern Mount Read Volcanics. Holes HL 541 and HL 345 are east of the Murchison Highway in the Hellyer area. BRD 4 is in the western part of the Bulgobac area. SCS 3 is located at Sock Creek South. BRD 1 is in the central part of the Bulgobac area. Sandstone near the base of BRD 4, SCS 3 and BRD 1 have dominantly Precambrian basement provenance. Units A and B are widespread, rhyolitic, syneruptive volcaniclastic mass-flow deposits (McPhie and Allen 1992).

hyaloclastite and mud-matrix peperite derived from a partly extrusive felsic cryptodome complex. Very similar lithofacies occur in the westernmost part of the Bulgobac area (BRD 4). Both sequences include units not present in the Hellyer-Bulgobac-High Point stratigraphies and evidently developed separately. Anthony Road Andesite: The Anthony Road Andesite in fact comprises several relatively small volume, andesitic and dacitic, porphyritic intrusive and extrusive units consisting of coherent and autoclastic facies (hyaloclastite and peperite). Subdivision into separate emplacement units has been achieved mainly by mapping phenocryst mineralogy (types and proportions) and is supported by geochemistry. The lithofacies association is typical of a submarine, intermediate composition, dominantly effusive volcanic centre and potential sites for hydrothermal mineralisation occur both at

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

40 (exhalative) and below (replacement) the palaeoseafloor. Tyndall Group and Mount Black Volcanics: The Tyndall Group overlies a non-conformity above the Darwin Granite and includes lithofacies (eg. welded ignimbrite) not present in other parts of the Mount Read Volcanics. The possibility that the Tyndall Group is significantly younger than the rest of the Mount Read Volcanics is supported by recent U/Pb zircon dating (Perkins & Walshe 1993). Furthermore, the dates suggest that the Mount Black Volcanics (the younget part of the Central Volcanic Complex) are coeval with the Tyndall Group. A link between these two lithostratigraphic units can be inferred from relationships in the Howards Road-Newton

Dam area where quartz-rich volcaniclastic sandstone of the Tyndall Group overlies rhyolitic lavas and volcanic breccias similar to those in the Mount Black Volcanics. References

Einsele, G., 1985: Basaltic sill-sediment complexes in young spreading centers: Genesis and significance. Geology 13: 249-252. McPhie, J. & Allen, R.L., 1992: Facies architecture of mineralised submarine volcanic sequences; Cambrian Mount Read Volcanics, western Tasmania. Economic Geology 87: 587-596. Perkins, C. & Walshe. J,L,. 1993: Geochronology of the Mount Read Volcanics, Tasmania, Australia. Economic Geology 88: 1176-1197.

Volcanic facies analysis indicates large pyroclastic eruptions, sill complexes, synvolcanic grabens, and subtle thrusts in the Cambrian "Central Volcanic Complex" volcanic centre, western Tasmania Rodney L. Allen^

Facies analysis is a system of observation and documentation that steers us toward those features required to interpret processes and environments of eruption and emplacement. The basic philosophy behind the research reported here is that a better understanding of processes and environments should help us resolve the complex stratigraphy and structure of the "Central Volcanic Complex" (CVC) and the setting of its ores. Early steps in the analysis are documentation of rock texture, alteration and deformation overprints, composition, bedform, contact relationships, emplacement unit, facies association. Subsequent steps, which are the aims of the studies, include interpretation of processes of fragmentation, transport and deposition, relationship of mineralization to host facies, environment of emplacement, syneruptive versus posteruptive emplacement, relative proximity to vent, two- or three-dimensional reconstruction or summary model, setting of mineralization, prediction. Choosing critical areas to do fades analysis has become possible recently because of excellent stratigraphic, structural, exploration and mine mapping by the "Mines Department", University and mining companies. Like many volcanic terrains, the Mount Read Volcanics contain a great range of volcanic facies types. However, facies analysis indicates that depositional environments were mainly submarine, below wave base, and this environment was dominated by just three groups of emplace-ment

1 Volcanic Resources, Tastagata 43, Stavanger 4007, Norway

unit: (1) ambient suspension sediment, (2) volcaniclastic mass flow units, and (3) shallow sills, cryptodomes, partially extrusive domes, and lava flows. These are the main buiding blocks of the Mount Read Volcanics stratigraphy, and for that matter, the stratigraphy of many other subaqueous volcanic successions. Groups (2) and (3) are by far most abundant, and group (1) mainly occurs as layers alternating with mass flow units of group (2). Emplacement units of group (3) form a continuum in emplacement style, and intrusions appear to be more abundant than extrusions. Building on these initial results, the following themes are developed in order to try and unravel the internal geology and regional relationships of the CVC)

Shallow intrusions and a stratigraphic revolution Most of the intrusions are lava-like in texture, were emplaced into water-saturated, unlithified strata of groups (1) and (2) from a few metres to a kilometre or more below the seafloor, and are characterized by viscous fingering contacts or encasing intrusive hyaloclastite facies (peperite). The interpretation that many texturally lava-like units are intrusions means that care must be taken to distinguish extrusive and intrusive units. For units emplaced on the sea floor (lavas, mass flow units and suspension deposits), lithostratigraphic position can be used directly to interpret the sequence of volcanic events and to make strati graphic correlations. However, texturally and compositionally similar lava-like intrusions related to one magmatic event are commonly scattered irregularly like raisins in a

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

42 pudding through up to a kilometre or more of stratigraphy, i.e. they produce a complex thick lithostratigraphy, but relate to a simple thin chronostratigraphic interval above the uppermost intrusion (e.g. Rosebery-Hercules footwall, the Mount Black volcanics, Newton Creek area). For these intrusions it is useful to define the position of the seafloor at the time of intrusion as the time-stratigraphic level of intrusion, and simplify intrusion complexes (the raisins in the pudding) to one time-stratigraphic level (or interval) for the purposes of volcanological interpretation and correlation.

Compaction foliation: a possible structural revolution still to be realized Many pumiceous subaqueous mass flow units have either pervasive phyllosilicate-quartzifeldspar alteration or contain phyllosilicate-rich lenses set in a more feldspar-quartz-rich matrix. In both cases relatively phyllosilicate-rich altered pumice clasts and aggregates of pumice clasts commonly have a stylolitic S^ foliation that predates regional cleavage, and is interpreted as a diagenetic compaction foliation or very early tectonic foliation. This Si foliation occurs only in pumiceous rocks and limestones. Mapping of the foliation in outcrop and drill core throughout the Rosebery-Hercules area shows that the fohation is bedding-parallel and can be used to define bedding orientation, folds and fault discordances in areas of monotonous massive volcanics where it was not previously possible to determine detailed structure. Mapping the S^ fabric is incomplete and its full value is still to be realized. However, an example of its use occurs along the enigmatic contact between the CVC and overlying White Spur Formation (Dundas Group) in the Hercules-Howards Road area. Here, a combination of S^ foliation and bedding data show that both stratigraphic units are concordant and are separated by a subaqueous paraconformity or disconformity attributed to synvolcanic block faulting. The nature of this contact and the structure of the area imply that the Rosebery-Hercules hangingwall (within the CVC) correlates with the White Spur Formation. This alternation of CVC and Dundas Group equivalents can be explained by two alternatives: (1) the Dundas Group overlies the CVC and the units are repeated by reverse faults, or (2) the CVC and Dundas Group abut and interfinger in the

Rosebery-Hercules area across a series of major synvolcanic faults.

Catastrophic pyroclastic eruption-induced mass flow packages: the ?only stratigraphic markers Thick, juvenile, pumiceous mass flow packages consisting of one or more mass flow units of identical composition, represent syneruptive deposition from large pyroclastic eruptions. These units should be widespread, geologically instantaneous deposits, and consequently are ideal timestratigraphic marker "horizons". No other comparable regional stratigraphic marker units have been located in the CVC or Dundas Group. The thickest and most extensive syneruptive mass flow package identified is the Rosebery-Hercules footwall (> 800 m thick, > 14km strike). Several other parts of the CVC also resemble this unit and it is possible that the unit extends throughout the CVC.

Detailed stratigraphy defines synvolcanic and later tectonic faults; lots of them A detailed stratigraphy has been constructed in the Rosebery-Hercules area by characterizing and correlating individual mass flow emplacement units according to their (1) lithic clast types and relative abundances, (2) ratio of volcanic feldspar to quartz crystals, and (3) pumice-rich versus crystal-rich character. Abrupt lateral changes in this stratigraphy define both synvolcanic and later tectonic faults. Synvolcaruc faults are abundant and were especially active during deposition of the Rosebery-Hercules footwall and lower part of the hangingwall. These faults can be related to caldera subsidence and postcaldera adjustments.

Old ideas revisited: the Central Volcanic Complex nested volcanic centre The CVC is massive, poorly stratified, and mainly consists of thick, syneruptive, rhyolitic-dacitic, pumiceous, subaqueous mass flow units intruded by swarms of ?comagmatic shallow sills and lavas. Extremely thick sequences of this syneruptive facies association, such as the Rosebery-Hercules footwall, are interpreted as proximal subaqueous caldera-fill

sequences. No original lateral margins to the Rosebery-Hercules caldera sequence have been located, and I suggest that the entire CVC could be one, tectonically dismembered, nested caldera complex. In the Rosebery area, depending on the preferred structural interpretation, the Mount Black volcanics either represent a second proximal caldera forming association, or are a fault repetition of the Rosebery-Hercules footwall caldera sequence. The overlying or interfingering White Spur Formation is a medial to distal, syneruptive to post-eruptive, mass flov^-mudstone fades association derived from adjacent and as yet undefined volcanic centres.

Future directions The logical next step is to combine facies analysis and geochemistry to fingerprint the thick syneruptive mass flow units, and correlate them through the Mount Read Volcanics. This combined approach would be useful to test whether the Rosebery-Hercules footwall is equivalent to other thick pumiceous units elsewhere in the CVC.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Re-interpretation of the Mount Read succession Discussion session — chaired by John Waters

David Leaman (DEL) to Keith Corbett (KDC): A comment first. It was interesting to notice your allochthonous section looked very much like my Midlands section. I would suggest that most of the displacements shown in your diagram were Devonian. The Rocky Cape group I think you also have to conclude is facing the other way, and I think you need to draw a distinction between what is the Arthur Lineament and what are the edges of those structures. I urge you to consider the suggestion I made about trough boundaries, because I think that the volcanics, your one type section of the Mount Reads, occupies the zone over one of the trough boundaries. My question is you have got that critical site for the Mount Read Volcanics, yet in the diagram you have several other sites that you could argue that are equivalent. Effectively you have panned the future State Governments expenditure on Midlands exploration by suggesting that that's the only one. Why do you feel that there is only the one, why even if you had that story, which I disagree with, you couldn't have more Mount read Volcanics elsewhere? KDC: Well I suppose you could. I don't know why their were they are, and I can't say that they wont be anywhere else as well. What I'm saying is that from the information that we have(inaudible). Whether there is any other Mount Read belts, I don't know.(inaudible) DEL: Well I would suggest that there are other troughs that may have undergone the same history rather than that. The whole state width, shall we say, is a set of similar types of structures. Why not have more of them (Mount Read Volcanics)? Roger Poltock (RP) to Jocelyn McPhie (JMcP): Jocelyn, the topography pre Que River Shale, what has caused that? Is there any other evidence, other than the thickness of the Que River Shale, and the plain on top of it being the Southwell Subgroup contact? JMcP: It's not easy to constrain what has caused that palaeotopography because the units that have formed it

principally comprise lavas and related breccias. There's no wide spread volcaniclastic units that you can use to tell relative positions in one drill hole compared to another, its more a less a matter of those blocks not allowing us to see their internal structure at the moment. So my feeling is that there is almost certainly a contribution from volcanic construction, as in eruption and building of dome like lava flows on the seafloor that's creating some of the topography. Secondly, but I can't prove it with those rocks, there is the likelihood of synvolcanic faults along the lines of what Rod Allen been talking about. But where Rod Allen he has been working he has continuous units that he can trace that actually tell him how the correlations can be arranged, but without those marker units there is nothing that can be done. John Waters (JCW): I agree with Jocelyn about the formation of volcanic topography, but I'm am a bit worried about the location of the mineralisation on a volcanic high. It's a problem if you look at the volcaniclastics around the ore body, they all thicken toward the ore body indicating that they flowed into a depression, so its unlikely that they flowed up hill and therefore it is more likely that the mineralisation sits in a depression. JMcP: I think that's a matter of scale. In particular the topography that I was showing was greatly exaggerated because my vertical scale is not equal to the horizontal scale and I only showing one drill hole. Its almost certain that even on that 'Hellyer hill' there would have been considerable smaller scale topography which would be on the locally derived volcaniclastics units and containing what ever venting hydrothermal system you want to have there. So I think that your observation (JCW) is obviously quite true and this has been published and proven by Gary McArthur in his analysis of the thickness of the sulphide mound and the isopacs drawn on that showing quite clearly that there was some sort of micro topography, or some sort of basin, that constrained the ore and volcaniclastics.

45 Ron Berry (RFB) to KDC: Keith, you were making the point that King Island and the far north west of the state missed out on even/thing. But if you look at the metamorphic grade there it is recorded as prehnite-pumpelly it e grade, so all of that has been covered by 4-6 km of rocks sometime since it was deposited. Do you wish to comment on that? KDC: Sometime, but when? My point is that what happened after the Middle Cambrian essentially missed King Island. RFB: My point is that you have deposited 5 km of rock sometime and eroded it back off sometime. The fact that you can't recognise it in the structures now doesn't mean that the area missed all that action. JCW: You have to be careful using prehnite-pumpellyite because the pressure and temperature constraints for this fades are poorly known. So we don't know the depths it was buried to from the metamorphic fades. RFB: Well we certainly know its over zeolite grade, because those basalts should have zeolites all through them. JCW: You can stand on basalts and get zeolites formed in them RFB: Exactly that's why they should have zeolites in them, but they don't! Geoff Green (GG) to JMcP: Jocelyn, is there a possibility that the Hellyer Basalt is more than one unit and emplaced at different times? I have a problem at Hellyer in that there is clear evidence that the hydrothermal system kept going after the Hellyer ore body hadformed and in fact altered the Hellyer Basalts and you can see that the higher temperature alteration is centred on the locus of the underlying Hellyer ore body itself. Yet I don't know of any evidence to suggest that the Que River Shale in the Hellyer mine area is altered. It may be that it was just an impermeable capping, but is the alternative a possibility? JMcP: Yes it is, and I don't think the model of a sill complex is incompatible with multiple sheets being emplaced. As Tony Crawford showed this morning in one of his drill holes, which is also one of the Placer holes that I showed, he has actually distinguish quite considerable geochemical differences within units of the Hellyer Basalt so there must be a stratigraphy within that stack of sills. It wouldn't surprise me at all if they were emplaced over a period of time. We basically, given from the modem work, have no idea what is a reasonable time for such complexes to form. One thing that's impressive is that you see basically no basalt derived volcaniclastic detritus of any description in the Que River

Shale. To my mind that telling you that the basalt was seeing the seawater — sediment interface, it was below it at the time the shale was accumulating. GG: But you don't see very much coarse detritus of any description. JMcP: If there basalt lavas being erupted on to a mud substrate you would expect to be seeing related resedimented hyaloclastite at those horizons. Khin Zaw (KZ) to Rod Allen (RLA): Rod, there's a black slate sequence at the Rosebery and Hercules hangingwall. Is that a very distinct black shale /mudstone horizon? RLA: In my stratigraphic nomenclature the black slate is hangingwall volcaniclastic unit 1. I don't see it as anything of major difference to the rest of the hangingwall. You can prove that there are several black slate horizons in the hanging wall this one just appears to be the thickest one. Its just the sedimentation style in between the mass flow units. David Gray (DRG) to RLA: Rod I accept that your ... inaudible ... Mount Black Fault is conjecture. But as I understood it in your opinion the Mount Black Fault is significant because the Mount Black Volcanics are so similar to the Rosebery footwall rocks. In my opinion you might be pushing your fades mapping to its limit in saying something like that. How confident are you that its the same unit? RLA: I don't believe that I or anybody else can distinguish pumiceous mass flow units from the Mount Black Volcanics from the Rosebery footwall units. And that goes for the amount of crystals, character of the pumice and right down to the textural and compositional character of the lithic fragments. Rosebery footwall has very distinctive occasional small blob shaped intermediate composition lithic clasts which are almost aphyric, with a few feldspar crystals. I think the blob shape is due to the fact that they were injected into the magma chamber during eruption, they're quenched magma blobs. You find these in the hangingwall at Rosebery and in the Mount Black Volcanics. So texturally and fades wise they're more or less identical units. (Rod then refers to cross section, without which the diagram doesn't make much sense) The critical thing is we have this footwall sequence, black mudstone, bedded sequence, quartz phyric mass flows and then the repetition ofthefeldspar-phyric units, what I believe those people might argue is that there are faults steeper than bedding at that contact and over and over again you can see down dip stretching lineations and

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

46 reverse sense shear indicators on thefault. From the onset I wasn't trying to push these as being an identical unit, but your pushed into that by structural geology, A combination of having a reverse fault on the inside of that anticline, ami having a unit deposited on the other side of that fault that is identical to the footwall above a reverse fault that is steeper than bedding your pushing me to the conclusion that it should be part of this sequence. It could be a coincidence, but to get a coincidence like this would be extremely rare. Secondly how do you then go back and explain the structure if the unit was deposited at some time latter? DRG: Once you establish one style of faulting, say at Hercules, then I will be quite happy in extending it north to Rosebery. JCW to RLA: Rod, with respect to these syn-eruptive mass flows, do you have any indication whether they are erupted subaerially or subaqueously? Secondly is the pumice within them explosively generated or non-explosive pumice? RLA: We don't have any real diagnostic criteria as far as I know for saying whether the eruption was subaqueous or subaerial, but what I would say is that the character of these pumiceous mass flows units within the Central Volcanic Complex, they have the textural characteristic of normal large volume subaerial ignimbrites, there's nothing different about them, texturally. The question then arises whether you can erupt in deep water an of material identical to a subaerial ignimbrite. In my mind I find it difficult to have an eruption completely underwater and not see some evidence of the water-magma interaction, even evidence for something phreatomagmatic or quench clasts. The characteristic of say the Rosebery footwall or Mount Black Volcanics is quitefrothy, irregular, very highly vesicular pumice clasts and almost no dense clasts. The amount oflithic material is extremely minor, its almost all pumice. So to me its just an aggregate of normal subaerial ignimbrite material, so without other evidence I would suggest that the vent areas has probably domed up and it is essentially a subaerial vent. It doesn't mean that all the material wasn't deposited under water, we don't have any evidence otherwise.

For the second question in the area I worked on its most likely pyroclastic because you almost never see any gradations from the mass flows units back into something insitu brecciated pumiceous rock, as you would expect to see if tit was a lateral equivalent of a pumiceous lava flow. The mass flows therefore do not show any insitu fragmentation characteristics, they have been transported. Mike Solomon (MS) to RLA: Rod, If you look as you have done at the Hokuroku Basin volcanics, you say they look pretty similar, do they have any evidence for subaqueous eruption. RLA: Yes there is evidence for submarine eruptions in the Hokuroku Basin, but they are very minor in volume. There is an enormous volume of volcanics material but most of the pyroclastic material can be reasonably interpreted to comefrom the basin margin, but the pyroclastics associated with the ores were actually erupted insitu within the deep water basin. But those pyroclastics are extremely minor in volume and nothing like the Rosebery units. The biggest volume of footwall tuff breccia to a Kuroko deposit is something in the order of 30 cubic kilometres. The Rosebery ones are an order of magnitude greater, its more in the scale of a big caldera forming eruption. Nick Direen (ND) to KDC: Keith, you characterised in your presentation some rocks at Cleveland as an oceanic melange, consisting of cherts, sandstones mudstones and tholeiitic volcanics. I would have thought ifyou had a oceanic melange that you would have had island arc basalts in that melange which would be calc-alkaline and not tholeiitic? KDC: Yes but I was just talking about the oceanic part of the melange, which consists of oceanic sediments, basalts plus various forearc sequence and these other odd volcanic rocks which we are yet to establish were they might have come from. ND: Could they be rift volcanics from deeper in the crust somewhere? If they are tholeiitic? KDC: Well the Crimson Creek basalts are reasonably tholeiitic. ... (inaudible) ... The problem is we still haven't got a lot of data.

Consequences of recent geochronology for Tasmanian geology

Geochronologic constraints on the Mount Read Volcanics, and the Hellyer, Rosebery and Mount Lyell massive sulphide deposits, Tasmania Caroline Perkins^ The late Middle Cambrian Mount Read Volcanics Tasmania, Australia, comprise submarine rhyolite to basalt lavas, intrusives and volcaniclastic rocks which host significant polymetallic massive sulphide deposits. Isotopic U-Pb and ÔAr/^Âr dating of magmatic zircons and hornblendes, respectively, from different parts of the Mount Read Volcanics yields a concordant age of 502.6 ± 3.5 Ma (Is), which constrains the time of volcanogenic massive sulphide mineralization (Perkins & Walshe 1993). The Comstock Tuff from the Tyndall Group in the upper part of the volcanic sequence, and the Mount Black Dacite from the Central Volcanic Complex, have weighted mean 206pb/238U ages of 494.4 ± 3.8 Ma and 494.9 ± 4.3 Ma, respectively, and may be slightly younger than the other rocks. Fauna in the Mount Read Volcanics is diagnostic of a late Middle Cambrian (Mid to Late Menevian stage) age, and the isotopic dates therefore imply that the Late Cambrian is younger than 500 Ma. Geological evidence, including the presence of massive sulphide clasts in the volcaniclastic rocks in the Mount Read Volcanics, suggests that mineralization was contemporaneous with the host volcanic sequence. The ÔAr/^Âr dating of alteration minerals from the Hellyer base and precious metal volcanogenic massive sulphide deposit has, however, revealed a complex alteration history. A well developed alteration zone and stringer vein system underlies the massive sulphide (Gemmell & Large 1992), and sericite from the stringer zone, stringer envelope zone, and veins in the footwall and fuchsite from alteration of the hangingwall

^ Department of Geology and Research School of Earth Sciences, The Austrahan National University, ACT 0200, Australia

basalt were dated by the 40Ar/39Ar technique (Perkins et al., in press). The oldest sericite, from the stringer envelope zone associated with quartz and pyrite alteration, yields a maximum age of -477 Ma. Sericite from the stringer zone yields a maximum age of -452 Ma, and vein sericite yields maximum ages of -447 Ma, and -441 Ma respectively. Fuchsite from the hangingwall basalt yields a maximum age of -447 Ma. The isotopic dates may suggest that alteration, which may in part be associated with deformation events, continued intermittently for up to 60 Ma after the deposition of the host volcanic rocks and massive sulphides at about 500 Ma. Banks (1989), summarizing dates from the Mount Read Volcanics, noted that clusters existed around 475 Ma and 455 Ma, groupings which are broadly consistent with some of the Hellyer dates and which may suggest Ordovician deformation events at these times. Though Ordovician deformation is noted in a number of localities in the Mount Read Volcanics, little is known about the nature of the Ordovician in the vicinity of the Hellyer deposit. Alternatively, the range in ages on Hellyer alteration phases may be due to mixtures of Devonian metamorphic and Cambro-Ordovician hydrothermal or deformationrelated micas. The 40Ar/39Ar dating of alteration muscovite from the Rosebery Zn-Pb-Cu and Mount Lyell Cu deposits, Mount Read Volcanics, has established a continuum of deformation events which occurred from - 400-378 Ma, and comprises the Devonian Tabberabberan Deformation. The dates from Rosebery range from - 400-390 Ma, are a minimum age for mineralization, indicate the time of shearing, and are a maximum age for granitoid emplacement in the vicinity of the deposit. The ages from the Mount Lyell field range from -400-378 Ma, are a minimum age for mineralization, and date cleavage development. The North Lyell Cu mineralization. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

50 which was broadly coeval with deformation, probably formed at - 4 0 0 Ma. All pre-Devonian alteration micas in the Rosebery and Mount Lyell areas have been recrystallized. The Tabberabberan Deformation in western Tasmania was broadly contemporaneous with deformation in southeastern A u s t r a l i a , as e s t a b l i s h e d f r o m the d a t i n g of alteration minerals associated with deformationrelated precious and base metal deposits.

References Banks, M.R., 1989: Late Cambrian and Ordovician movements. In Barrett, C.F. & Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Geological Society of Australia Special Publication 15:197-200 Gemmell, J.B. & Large, R.R., 1992: Stringer system and alteration zones underlying the Hellyer volcanichosted massive sulfide deposit, Tasmania, Australia. Economic Geology 87: 620-649. Perkins, C. & Walshe, J.L., 1993: Geochronology of the Mount Read Volcanics, Tasmania, Australia. Economic Geology 88:1176-1197. Perkins, C., Walshe, J.L. & Morrison, G.: Metallogenic episodes of the Tasman fold belt system, eastem Australia. Economic Geology, in press.

Pre-Middle Cambrian stratigraphy, orogenesis and geochronology in western Tasmania NJ. Turner\ LP. Black^ and M. Kamperman^

New gecKhronological data relevant to the tectonics of western Tasmania have been obtained through a joint project between Mineral Resources Tasmania and the Australian Geological Survey Organization employing U-Pb ion probe zircon dating (Turner 1993; Black 1994). In central to northwestern Tasmania (Figures la, b) the rock units with contentious ages include the variably metamorphosed sedimentary and igneous rocks that occur in the Tyennan and Forth regions, in the Arthur Metamorphic Complex and in western King Island. Also included are the less metamorphosed rocks of the Rocky Cape Group and correlates, the Bumie/Oonah Formation and the Ahrberg Group together with the Success Creek Group, Crimson Creek Formation, the ultramafic complexes and the pre-Middle Cambrian rocks in the Smithton synclinorium. Most previous interpretations (e.g. Turner 1989) have treated the main deformation and metamorphism in the Tyennan and Forth regions, the Rocky Cape Group and correlates, the Arthur Metamorphics and the Bumie/Oonah Formations as being due to Precambrian events. These events are the Frenchman orogeny in the Tyennan and Forth regions and the Penguin Orogeny in the others. The Penguin Orogeny has been regarded as older than the Success Creek Group and Crimson Creek Formation in the middle Pieman River (Brown 1986) and older than the correlates of these units in the lower part of the succession in the Smithton synclinorium. The view taken here is that the Frenchman orogeny and the Penguin Orogeny are parts of the Delamerian Orogeny

1 N.J. Turner Geological Services, 65 Lochner Street, North Hobart, Tasmania 7000, Australia. 2 AGSO, PO Box 378, Canberra, ACT 2600, Australia. 3 Geology Department, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

Stratigraphy in NW Tasmania In northwestern Tasmania (Figure lb), stratigraphy and basalt chemistry support correlation of the Ahrberg Group at Corinna which displays Penguin deformation and metamorphism with littledeformed rocks in the lower part of the Smithton synclinorium which have been regarded as postPenguin orogeny (Crawford 1992; Turner & Crawford 1993). The implied relatively young age for the Penguin Orogeny is consistent with three essentially concordant K-Ar hornblende ages that group around an age of 500 ± 10 Ma for Penguin metamorphism in the Bowry Formation of the Arthur Metamorphic Complex (Turner et al 1992; Turner 1993). Further correlations of formations in the Ahrberg Group with the Burnie/Oonah Formation, the Success Creek Group and the Crimson Creek Formation support the reconstruction of northwestern Tasmania as part of a continental shelf and margin setting during the latest Proterozoic with shallow-water carbonates then rift tholeiites near the top of the succession (Gee 1967; Turner et al 1992). The basal, shallow-water, siliceous clastics near Smithton are thought to pass laterally southeast through proximal turbidites in the basal Arhberg Group into the turbiditic Bumie/Oonah Formation. The Penguin Orogeny postdates this entire shelf and margin assemblage.

Limitations of the geochronological data set Before applying the existing geochronological data to an analysis of the western Tasmania rocks it is necessary to review the data itself. The full set of data consists of whole-rock K-Ar shale and dolerite ages; whole-rock Rb-Sr metasediment and metabasite ages; a whole-rock Pb-Pb isochron; and Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

OLDEST TASMANIAN ROCKS K - A r Tr dolerite c.600Ma K - A r Tr dolerite c.725Ma Very low greensch/st fades sub-greenschist fades

A r - A r amph 510Ma K - A r amph 493Ma K - A r mica 493Ma

Greenschist

fades

and

to edogite

fades

ARTHUR UNEAMEm-

SUCCESSION K - A r amph K - A r amph K - A r amph

510Ma 499Ma 494Ma

FORTH REGION TYENNAN REGION TP cjgto Pb-Pb 1 3 0 0 — 1 7 0 Ma 3Rb-Sr

IIOOMa

c. 486-C.502 Ma

UPUFT HP. METAMORPHISM ALLOCHTHON

C.502-C.510 Ma

MARC VOLCANISM

c.600-7725 Ma

JUBILEE REGION K - A r ph 515Ma Rb-Sr ph-amph-cpx 505Ma

SEDIMENTATION

K - A r ph 496Ma U - P b Zr 502 ± 8 Ma

GRANITE + MET. (Kl)

760 ± 1 2 Ma

SEDIMENTATION c.1100Ma MINOR MARCS

Figure la—Distribution of Tasmania's oldest rocks. 1. Continental shelf and margin deposits with rift tholeiites near top — not all occurrences shown; 2. Sandy turbidite fades; 3. Quartzarenite, siltstone, minor dolomite; 4. Forth, Tyennan and Cape Sorell regions largely similar to 3. Note: Arthur Lineament contains 2 plus metabasaltic rocks and pelites. Ages as for Figure 3. (After Turner et al. 1992)

PROTEROZOIC ROCKS IN NW TASMANIA

-I-

DCVWIAN

I OUNOZOIC

TO CMBOMFUfOUS

TO knODLE

GRAMTVIOS,

CAUBRIAN.

. 509 Ma Delarnerian Orogeny OCEANIC

510 Ma [ C.580r C.600 Mai

AmNmES-TAUOCmONOUS.

COmWEmAL SHEW AND UAPCtN DEPOSITS MTH RtTT T>K)L£mES NEAP TOP. UHAJJOPPHOSED

>711 ±16 Ma

SANtrr

/ /

TUffBtOfTE

DOLERITE

DYKES

TTiANSmONAL

yETAUORPHfC

FAULT

40km

Figure lb—Proterozoic rocks in northwestern Tasmania. Ages as for Figure 3. (After Turner et al. 1992)

BOUHOAFf

53 mineral ages derived by K~Ar, Rb-Sr, Ar-Ar, conventional U-Pb and ion probe U-Pb. In this review the whole-rock K-Ar and Rb-Sr ages are discounted for the following reasons: 1. Shales in the Rocky Cape Group and its correlates, in the Ahrberg Group and in the Bumie/ Oonah Formation are usually siltstones or silty mudstones which contain not one, but several generations of mica in varying proportions. This variation in mica type is thought to account for the wide spread of results in Adams et al (1985). 2. Metasedimentary rocks in the Tyennan region usually show the effects of several episodes of mineral growth. Sampling by Raheim & Compston (1977) was carried out with great care in this respect. However, the wide spread of their results suggests that the Rb-Sr whole-rock method as applied in the Tyennan region needs further refinement. 3. A coherent, readily interpretable set of data (Figure 3) is formed if only mineral ages are considered Perhaps the whole-rock K-Ar ages of 725 ± 35 Ma for the Cooee Dolerite (Crook 1979) and 580600 Ma for dolerite in the Rocky Cape Group (Adams et al 1985) are more indicative of age of formation than the shale and metasediment ages though argon loss is still a problem.

Interpretation of the geochronological data Figure 3 shows that at 510 ± 6 Ma the ion probe zircon age of the tonalite from the Heazlewood ultramafic is indistinguishable from, though probably older than, the 500 ± 10 Ma K - A r hornblende age of the Bowry Formation, the ion probe zircon age of 502 ± 8 Ma for eclogite in the Franklin Metamorphics, the 510 Ma Ar-Ar hornblende age for the Forth metamorphics, Perkin's & Walshe's (1993) 501.5 ± 5.7 Ar-Ar hornblende age and 503.2 ± 3.8 Ma ion probe zircon age for the Mount Read Volcanics and their 501 ± 5.7 Ma ArAr hornblende age for the Murchison Granite. Crystallization of the Heazlewood tonalite and presumably the rest of the ultramafic complex appears to have been followed very quickly by a continent-ocean collision that saw emplacement of the ultramafic complex as an allochthon (Berry & Crawford 1988). This began the Delamerian Orogeny during which continental material was subject to very high P, T conditions (Kamperman 1984) at burial depths of greater than 30 km, then rapidly rebounded. The rising metamorphics were the major alternative source of sediments in the Mount Read Volcanics (Corbett 1990) and completely dominated large parts of the Middle Cambrian

60 km opprox tz

J< K

CONTINENTAL SHELF AND MARGIN DEPOSITS WITH RIFT THOLEIITES 509 Ma Delamerian orogeny C.580-

Ahrt>«fg Cp.

Iy VI

Baton and

ndirrMnti

StromotoHtK

Oolomitt

yoinJy

Most intenM zona of i

aataloot/mudwton*

Sehhtot* mmtOM9dimfttM

• QI

>711±16 Ma [•.•/.•.]

Turtidrtic Mondsion* and

congiomTatt

TurtidHic aandMton^ Unconformity Bosmirwft of mainly ailtston*.

quartiarwvtm,

Figure 2—Above: Much simplified composite cross-section designed to illustrate stratigraphy. It includes the eastern ed^e of the Smithton synclinorium then is offset in order to pass through Corinna to Middle Pieman River Below: Reconstruction of the late Proterozoic shelf and margin setting. Ages as for Figure 3. (After Turner et al. 1992)

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Tectonic setting in Adelaide Geosyndine

Gentfe fotding Faulting

Riphean

Marginal marine and basinal

Broad subsidence

Delamerian Orogeny

Vendian

Sturtian

1 — r " 545 Darwin Granite'

Mersey River Granite '

Bi, Hb

Murchison Granite

Hb, Zr

Dove Granite '

Hb. Bi

434

509 490

I 354

410

K-Ar Rb-Sr U-Pb (conventional) U-Pb (ion probe) Ar-Ar

Mount Read Volcanics'^ « '° Hb, Zr Arthur Metamorphics ®

Forth Metamorphics ' ^

Hb. Mu

Franklin Metamorphics ^

mu, Hb, Cpx, WR. Zr

Settlers Metamorphics '

Heazlewood Tonalite

Cooee Dolerite '

Oonah Formation detritus" Mu W King Island granites ' ^

•

Bi. Mu,Zr 800 1

•

• •

•••• • •

•

400 1

500

600 1

700

Figure 3—Mineral ages. Sources are 1. McDougall & Leggo (1965); 2. Raheim & Compston (1977); 3. Adams et al. (1985); 4. Perkins & Walshe (1993); 5. Turner et al. (1992); 6. Turner (1993a); 7. Turner (1993b) and Black (1994); 8. Kimbrough & Brown (1992) and Everard & Villa (1994); 10. Baillie & Green (1984).

•

Timbs Group felsic porphyry ^

••

•• •

•

t 503.2±3.8

Mount Read Volcanics * Black River Dolomite/Sturlian glacials (correlated by C.R. Caiver, in prep.)

appfox

Bowry Formation metasediment' 501 ±8 Franklin metamorphics (eclogite) ^ Pb-Pb Isochron Tyennan metasiltstones. Lyell Highway Rb-Sr model ages

Strathgordon Phyllite ^

510±€ Heazlewood Tonalite ^ K-Ar 711116 BI

Cooee Dolerite ^

King Island Granite '

••

•

Archean

Palaeoproterozoic

^••Jl

•••

•

760±12

••

Mesoproterozoic

Neoproterozoic

€

1 3000 Ma

2000 Ma

1000 Ma

. igure 4— Inherited zircon ages. Sources are 2, 4, and 7 of Figure 3. Pb-Pb isochron from Gulson et al. (1990). Time divisions are from Jones (1994).

55 to Ordovidan succession. Uplift was sporadic rather than continuous with the generation of localised unconformities through the succession until tectonic stability was re-established in the early Ordovidan. In general terms, this Middle Cambrian to early Ordovician evolution is of the style envisaged by Carey and Banks (1954) for their major Cambrian orogeny. Uplifting metamorphic rocks from depths of 1230-f km would have involved substantial lateral movement as well as vertical movement. Such movement is indicated by a large shear zone with sub-horizontal pre-Devonian dip that occurs between the high-grade Franklin Metamorphics and the subjacent, apparently lower grade Mary Metamorphics (Spry & Gee 1964). Other large, lowangle structures have been interpreted in the north of Tasmania (Leaman et al. 1973; Woodford et al 1993). A substantial pre-Delamerian orogenic event is represented by granites and polydeformed metasedimentary rocks in western King Island (Cox 1989). The ion probe zircon (crystallization) age of a foliated part of the granite suite at Cape Wickham is 760 ± 12 Ma and the same rock gave a Rb-Sr muscovite age of 728 ± 16 Ma. The Rb-Sr age is regarded as an uplift age and compares with the 708 ± 6 Ma K-Ar age of detrital muscovite in distinctive muscovite-rich beds fairly high in the Bumie/Oonah Formation (Turner 1993). Granite pebbles occur in the same part of the Bumie/Oonah succession. Thus, it seems clear that Wickham-type granites were being uplifted as the Bumie/Oonah basin formed, ultimately breaking surface after sedimentation was well advanced. There is also good agreement between the granite uplift age and the 711 ± 16 Ma K-Ar biotite age of the Cooee Dolerite (McDougall & Leggo 1965). The dolerite was intruded into the Bumie/Oonah Formation during or shortly after sedimentation (Crook 1979). Its age should be further tested by Ar-Ar biotite and homblende determinations. Although a substantial event on King island, the Wickham orogeny was mild in mainland Tasmania. The sum of its stmctural effects are marked by the gentle unconformity beneath the Smithton synclinorium and beneath the Ahrberg Group. Protolith sediments in the Tyennan region were deposited well before the Wickham orogeny. Phyllite near Strathgordon has Rb-Sr model ages of 1100-1150 Ma which probably correspond to the age of deposition (Raheim & Compston 1977). A Pb-

Pb isochron derived from metasiltstones on the Lyell Highway gives an indistinguishable age of 1300 ± 170 Ma (Gulson et al 1990).

Inherited zircon ages The Tyennan protolith ages roughly correspond to the young end of the age ranges of inherited zircons (Figure 4) in a Devonian granitoid near Corinna, in the Proterozoic WTickham granitoid and in a preMiddle Cambrian metasediment in the Arthur Metamorphics. In the granitoids the inherited zircons represent the rock(s) from which the granite melt was derived. It is not clear whether the source rocks ranged in age from late Palaeoproterozoic to late Mesoproterozoic or were late Mesoproterozoic sediments with a provenance of that range. The very similar range of provenance ages in the pre-Middle Cambrian metasediment suggests the latter. In the Cooee Dolerite correlative, the inherited zircon ages provide good evidence that the alkali basaltic rock was derived by melting of a 17001800 Ma source. Since the source contained seemingly common zircon, it may have been a mafic granulite. Of the rocks investigated so far, only this Cooee Dolerite correlative provides strong evidence of Palaeoproterozoic material in the cmst below Tasmania. The two markedly older zircon ages in Figure 4 are probably from recycled grains. The interval 870-1100 Ma is unrepresented in the iiiherited zircon data for most rocks and there are only two ages in the range 760-870 Ma. This suggests that there may have been no new zircons introduced in the 1100-870-7760 interval in Tasmania. However, data for the Mount Read Volcanics may provide better evidence of activity in this period. References Adams, C.J., Black, L.P., Corbett, K.D. & Green, G.R., 1985: Reconnaissance isotopic studies bearing on the tectonothermal history of Early Palaeozoic and late Proterozoic sequences in Western tasmania. Australian Journal of Earth Sciences 32: 7-36. Baillie, PW. & Green, D.C., 1984: A radiometric age from the Crown Hill Andesite, western Tasmania. Tasmanian Department of Mines report 1984/16. Berry, R.R & Crawford, A.J., 1988: The tectonic significance of the Cambrian allochthonous maficultramafic complexes in Tasmania. Australian Journal of Earth Sciences 35(4): 161-171. Black, L.R, 1994. The significance of current and proposed SHRIMP dating. Mineral Resources Tasmania report 1994/16. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

56 Brown, A.V., 1986: Geology of the Dundas-Mt LindsayMt Ramsay area. Geological Survey of Tasmania Bulletin 62. Calver, C.R., Turner, N.J., McClenaghan, J. & Brown, A.V., 1989: Pedder, Tasmania. Tasmanian Department of Mines Geological Atlas 1:50 000 Series, Sheet 8013N. Calver, C.R., in prep.: PhD thesis. University of Tasmania. Carey, S.W. & Banks, M.R., 1954: Lower Palaeozoic unconformities in Tasmania. Papers and Proceedings of the Royal Society of Tasmania 88: 245-269. Corbett, K.D., 1990: Cambro-Ordovician stratigraphy. West Coast Range to Black Bluff. In GEOLOGY IN TASMANIA: A GENERALISTS INFLUENCE. Geological Society of Australia (Tasmania Division): 8-13. Cox, S.F., 1989: Cape Wickham. In Burrett, C.F. & Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Geological Society of Australia Special Publication 15 Everard, J.L. & Villa, I.M., 1994: Argon geochronology of the Crown Hill Andesite, Mt Read Volcanics, Tasmania. Australian Journal of Earth Sciences 41: 26S-272. Crawford, A.J., 1992: Geochemistry and implications of mafic metavolcanics in the Corinna-^vage River area. In Turner, N.J. (Ed.): Corinna 1:50 000 geological Map. Field guide to selected rock exposures. Department of Mines report 1992/06. Crook, K.A. W., 1979: Tectonic implications of some field relations of the Adelaidean Cooee Dolerite, Tasmania. Journal of the Geological Society of Australia 26: 353-361. Gee, R.D., 1967: The Proterozoic rocks of the Rocky Cape Geanticline. In Geology of Western Tasmania — A Symposium. University of Tasmania, Hobart. Gulson, B.L., Large, R.R., Mizon, K.L. & Turner, N.J., 1988: Is the Precambrian the source of metals and rocks in the Mt Read Volcanic belt of western Tasmania. Centre for Isotopic Studies (CSIRO) Research Report 1988-90. Jones, RJ., 1994: AGSO GEOLOGICAL TIME SCALE. AGSO, Canberra. Kamperman, M., 1984: The Precambrian metamorphic geology of the Lyell Highway-Collingwood River area. BSc Honours thesis. University of Tasmania (unpubl.).

Kimbrough, D. & Brown, A.V., 1992: Zircon Pb/U age of 520 Ma for a tonalite associated with the Heazlewood Ultramafic/Mafic Complex, western Tasmania. Tasmania Department of Mines report 1992/24. Leaman, D.E., Symonds, PA. & Shirley, J.E., 1973: Gravity survey of the Tamar region, northern Tasmania. Geological Survey of Tasmania Professional Paper 1. McDougall, I. and Leggo, P.J., 1965: Isotopic age determination on granitic rocks from Tasmania. Journal of the Geological Society of Australia 12: 295-332. Perkins, C. & Walshe, J.L., 1993: Geochronology of the Mt Read Volcanics, Tasmania, Australia. Economic Geology. Raheim, A. & Compston, W., 1977: Correlations between metamorphic events and Rb-Sr ages in metasediments and eclogite from western Tasmania. Lithos 10: 271-289. Seymour, D.B., 1992: Woolnorth, Tasmania. Geological Survey of Tasmania Geological Atlas 1:50 000 Series, Sheet 7816S. Spry, A.H., & Gee, R.D., 1964: Some effects of Palaeozoic folding on the Precambrian rocks of the Frenchman's Cap area, Tasmania. Geological Magazine 101: 385-396. Turner, N.J., 1989: The Precambrian rocks. In Burrett, C.F. & Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Geological Society of Australia Special Publication 15: 5-46. Turner, N.J., 1993a: National Mapping Accord programme geochronology component: 1993-94 batch of samples. Mineral Resources of Tasmania 1993/12. Turner, N.J., 1993b: K-Ar geochronology in the Arthur Metamorphic Complex, Ahrberg Group and Oonah Formation, Corinna district. Mineral Resources Tasmania report 1993/27. Turner, N.J., Bottrill, R.S., Crawford, A.J. & Villa, I., 1992: Geology and prospectivity of the Arthur Mobile Belt. Tasmania Department of Mines Bulletin 70:227233. Turner, N.J. & Crawford, A.J., 1993: General features and chemical analyses of mafic and other rocks, Corinna geological map quadrangle. Tasmania Development and Resources report 1993/23. Woodward, N.B., Gray, D.R. & Elliott, C.G., 1993: Repeated Palaeozoic thrusting and allochthoneity of Precambrian basement, northern Tasmania. Australian Journal of Earth Sciences 40: 297-311.

Consequences of recent geochronology for Tasmanian geology Discussion session — chaired by Ron Berry

Michael Raitz (MR) to Caroline Perkins (CP): Would you like to comment on the differences in ages between the Mount Read Volcanics and the alteration ages. Much of the sericite may have more to do with deformation than with alteration! CP: In terms ofHellyer there is a difference. Clearly this a difference which ... (inaudible) ... evidence of Ordovician deformation. Clearly there is a structural complication.

alterations micas are continually changing during continuing crystallisation. It is not contrary to anything you have said, but I was just wondering if you would expect to see original micas. CP: In the Ordovician of NSW, alteration micas are very well preserved. ... (inaudible) ... alteration micas in shoshonites. So certainly in some circumstances we can find evidence for this preservation.

Ross Large (RRL): Alteration zones around the massive sulphides seem to attract structure. So you get structure concentrated in these zones. So it is not surprising that you get reset ages.

Tony Crawford (AJC) to Nic Turner (NT): The Cooee dolerite ages are bizarre. I can't imagine, the rocks I have analysed from that area are highly alkaline dolerites and should certainly have crystallised zircon. It's got 400-500 ppm Zr. I would not expect a date like you are recording there (or like Lance Black is recording). What do you think about that? NT: Well, you have got two markedly different rock types. You get exceedinglyfinegrained porphyritic things, very rapidly cooled, and there are the ones with the frothy tops which make them look like they intruded into wet sediments, and then there is quite a range of other types which are generally coarser grained. We sampled the coarser grained rocks.

Domingo Aerden (DA) to CP: You probably know that I believe that at least the Rosebery orebody was emplaced in the Devonian, and I was wondering if your data for Rosebery are absolutely inconsistent with this model. Your ages for sericite are Devonian even where the sericite is unoriented. CP: Sure! I dont think my dates can tell one way or the other. They certainly date the deformation. I believe ... (inaudible) ... reset. So if there were any early sericite at Rosebery they have been reset to this deformation age. DA: You agree that is one interpretation. You also argue the early alteration of the volcanics produce sericite which you may pick up at Hellyer. Which you do not pick up at Rosebery and that the actual alteration that is associated with the orebody is Devonian. An equally valid model? CP: Certainly the ages cannot distinguish these models. Khin Zaw (KZ): I would just like to say that all the samples Caroline analysed are too strongly overprinted. They are all from the south end of J lens. We have to go to the least metamorphosed and least overprinted samples that might tell you something about the age of mineralisation. Rod Allen (RLA) to CP: Just a comment. Would you expect to find original micas in any Palaeozoic alteration systems? If you even consider sea floor alteration where during the diagenetic

Geoffrey Green (GG) to CP: That sample you dated from the Mt. Black volcanics, is there any evidence that it was intrusive, or was it extrusive? CP: I believed at the time that it was extrusive but subsequent work ... (inaudible)... RLA: It has intrusive peperitic upper margins. I believe it is intrusive, but into an unlithified host. The question remains as to how long a rock remains unlithified in a sea floor situation. I can't answer that. I think, in general, that we should start to get away from doing just the chemistry and dating of coherent intrusives and lavas, and go for the laterally extensive mass flow units that Jocelyn and I have been talking about. To date any of these sequences, it would be much better to determine ages from the pumice-rich monomict juvenile mass flows. It is much less ambiguous to interpret. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

58 Jan van Moort (JCVM) to CP: Technically, it is possible to fish out odd muscovites that are enclosed in quartz veins that are either Devonian or Cambrian age, or in barite or carbonate nodules of different ages. Did you attempt to isolate muscovites of specific ages from such enclosed protected sites? CP: Yes, I did attempt to do that ... (inaudible) ... especially at Hellyer. Certainly ... (inaudible) ... resolving crystals... (inaudible)... its veryfine grained material. Certainly, I looked for coarser grained material. At Hellyer, I did try to sample material that formed at the time of mineralisation. RRL to CP: Caroline, you got two dates from the Tyndall Group — onefrom the Comstock Tuff, and onefrom a volcaniclastic unit. The slide you showed of the latter material looked very like the Comstock Tuff, yet the dates are very different. Could you comment? CP: The older section is from the Anthony Road area. There are a number of possibilities. The first is that the zircon from that part of the succession were brought in from older sequences, and we are actually dating the older sequence. Another is that there are breaks in the Tyndall Group. Matt White (MW): I understand that the Anthony Road sample is from near Tyndall Creek. Is that correct? CP: I guess so. MW: I would put that in the same stratigraphic position as the Comstock Tuff. So I dont understand why there is such a vast difference in the ages. CP: Okay then, the material in the Anthony Road sample must have been washed in from the older volcanic sequence. Ron Berry (RFB) to NT: You suggest that in the section from the Oonah Formation through the Success Creek Group to the Crimson Creek Formation, there is a continuum in deformation, but in this section, some important deformations are not found in the Success Creek Group and Crimson Creek Formation. In fact, the two early deformations associated with recumbent folding are missing from these two units. Given that the definition of the Penguin Orogeny is the deformation in the Bumie Formation, you would have to say that there is a major unconformity under the Success Creek Group. NT: The Penguin deformation includes the Arthur Lineament deformation. I see that succession in the Pieman River, where there is a major fault equivalent to the Tenth Legion Fault, which brings the Crimson Creek Formation and Success Creek Group, relatively

undeformed rocks into juxtaposition with rocks which are more deformed, but which are not necessarily separated by a major unconformity, just as if you juxtaposed rocks of the Arthur Metamorphic Complex with rocks out to the west in the basin which are not as strongly deformed. The age relationships is still that the deformation is younger than all the rocks. David Leaman (DEL): The support for my great trough idea was in your section, and ifyou take my major lateral structure in the nominal position of the Arthur Lineament as a flower, you get all the deformations you want. They will be localised, and throughout the deposition of the sequence. You will have a whole range of juxtapositions, so I am with you. Mel Jones (MJ) to CP: Alteration rutiles from Kidd Creek are about 100 Ma younger than the deposit itself I dont really like the idea, but is it conceivable that these major deposits occur in an area of high geothermal gradient that persists for 10 s of millions of years. In the case of Hellyer, perhaps until such time as a closure temperature is reached for your sericites. CP: That is a possibility. You would need very slow cooling. It would explain the range in ages. Without more detailed micro-structural control, it is very hard to say what the spread in ages is. MJ: Does the microstructural argument apply to the younger rutiles at Kidd Creek? CP: In that case, there are a whole set of events which you can see in the deposits. It is possible for the ages to be reset. RLA: There is very good work by the Japanese that shows the Kuroko systems continue for ^ 2-2.5 Ma after formation of the ores. They have beautiful micropalaeontological control on the ages of the formations, and they date the alteration by mapping the alteration zones in 3D up into the hangingwall, which they can date. They show it dies out after 2 Ma. A lot of big ore deposits form on structures that are reactivated many times, so do these younger rutiles need to be exactly the same alteration event, or are they just the next event on the same set of structures 100 Ma later? John Foden (JF): You could test the cooling hypothesis by dating some other minerals with very different closure temperatures. So if you had biotite with 200X1 lower closure temperatures, with a prolonged cooling history they would give very different ages. CP: This is certainly possible, but you would have to consider the microstructural setting of the biotite — if any could be found.

Geology, geophysics and mineralisation of NE Tasmania

The role of Devonian granodiorites and the influence of Tabberabberan tectonism in Tasmania-North Victoria Land (Antarctica) correlations. R.Varne^ and R.Fulton^

Australia-Antarctica reconstructionists find the piece marked "Tasmania" difficult to fit into their jigsaw puzzles. Most agree that 55 My ago Tasmania was not far north of North Victoria Land. This first step backwards in the reconstruction process, when the southeast Indian Ocean is closed by reversal of its spreading history, is well-constrained by dated oceanic magnetic anomalies and prominent fracture zones south of Tasmania. Yet by Cretaceous times, most Gondwanan reconstructions have removed Tasmania to the east of North Victoria Land. Chappell et al/s (1988) concept of granite source terranes, that granites are the image of the source rocks which melted deep in the crust to yield the magmas, can be used to test the idea that the continental fragment which became Tasmania shared the same basement rocks with North Victoria Land in Devonian times. Geochemical and Sr isotopic comparison shows that granodiorites of the Scottsdale Batholith of northeastern Tasmania are remarkably similar to the Admiralty Intrusives of North Victoria Land, Antarctica, consistent with the idea that both were derived from the same Gondwanan granite source terrane. So far so good. However, the Admiralty Intrusives are dominantly granodiorites, and the granites which make up about half of the Blue Tier and most of the Eddystone Batholiths of northeastern Tasmania have no known counterparts in North Victoria Land. Conversely, the youngest rocks intruded by the Admiralty Intrusives are the late Cambrian to early Ordovician quartz-rich turbidites of the Robertson Bay Group of North Victoria Land whereas in northeastern Tasmania the batholiths are

1 Geology Department, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

emplaced within Mathinna Group turbiditic sequences which range from Ordovician (Arenig) to Devonian in age. These differences might be explained if the Tasmanian granites were shallow intrusions into the younger Mathinna Group sequences: similar rocks might at one time have been present in North Victoria Land, but erosion of the North Victoria Land crust might have cut deep enough to remove them, leaving only the lower Palaeozoic equivalents of the older Mathinna Group sequences. There are good reasons to suppose this might have happened. Both sets of granodiorites were intruded within the Gondwanan continental crust at about the same time (390-350 Ma ago) and erosion had exposed some of the intrusions at the surface by late Carboniferous times. Permian sedimentary rocks were laid down on this erosion surface both in Tasmania (Parmeener Supergroup) and in Victoria Land (Beacon Group). Substantial Cainozoic uplift of North Victoria Land has formed mountains from which the uplifted Beacon Group sedimentary rocks have been almost completely eroded away, whereas their Parmeener Supergroup correlates are still extensively preserved in Tasmania, consistent with the idea that deeper crustal levels are exposed in North Victoria Land. We have tested this speculation. Recent experimental studies have shown that the aluminium content of hornblende in the typically granodioritic assemblage quartz + plagioclase + K-feldspar + hornblende + biotite + sphene + magnetite or ilmenite can be used to assess the pressure at which the assemblage equilibrated. The compositions of hornblendes in Admiralty Intrusives granodiorites suggest they crystallized at pressures corresponding to depths in the crust ranging from 12 km up to Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

8 km, whereas the granodiorites of northeastern Tasmania, although now at the same level in the crust, i.e. exposed at the surface, crystallized originally at depths ranging from 12 km up to nearsurface conditions. It is notable that granodiorites with hornblende compositions suggestive of crystallization at or near the surface are associated in the field with the part-extrusive St Marys Porphyrite, thereby providing an independent check of the methodology (see map). This work supports the speculation that different crustal levels have been exposed by erosion in the two areas, and that equivalents of shallow granites still preserved in northeastern Tasmania may have been eroded away in North Victoria Land, together with any younger Mathinna Group equivalents which might have originally been present in the uppermost few (4-8) kilometres of the Robertson Bay Group. However, it is implicit in the original idea that the deep erosion in North Victoria Land occurred because the area was uplifted relatively recently. It now seems that in northeastern Tasmarua (and by inference also in North Victoria Land), there were substantial vertical movements following shortly after the intrusion of the granodiorites, and long before the rifting of Gondwanaland. The hornblende data are preliminary but for the Scottsdale Batholith, our data and data of Goscombe et al. (1994) are consistent with emplacement of the granodiorites at shallow levels (see map and section), with an earlier higher-pressure (2.8- 4.5 kb) history preserved in enclaves. Other granodiorites, particularly the Pyengana Granodiorite of the western Blue Tier Batholith, crystallized at much greater depths (-12 km) within the crust. Notably, this also implies that Mathinna Group rocks were deeply buried at the time of intrusion, and gives a minimum pressure of generation of about 3.6 kb for the granodioritic magmas. The Scottsdale Batholith and Pyengana granodiorites are 20 km apart (see map) today. Hornblende compositions suggest that about 10 km of vertical movement has occurred between the two granodiorites (see map and section), and shear zones in Mathinna Group rocks between Scottsdale and Pyengana are the location of gold deposits believed to have been deposited from deep-seated fluids about the time of granodiorite intrusion. Yet Parmeener Supergroup sedimentary rocks now rest on a near-horizontal erosion surface which cuts both granodiorites. This has allowed us to

Figure 1 — Sketch map and section, modified from Williams (1976), shomng hornblendegeobarometry (pressure in kilobars) for granodiorites of northeast Tasmania. Granite: crosses; granodiorite: no ornament; Mathinna Group: stipple; postCarboniferous cover: diagonal lines. Line of section located on map by arrows.

place constraints on the timing of the relative vertical displacement between the granodiorites: it must have occurred before the deposition of the Parmeener Supergroup, and might have been before emplacement of the granites. The hornblende pressure data (see map and section) also suggest that markedly differing crustal levels are exposed along transects westwards and southwards from St Helens. Studies of illite crystallinity in Mathinna Group rocks may provide a test of these preliminary conclusions, which may have profound tectonic implications. References

Chappell, B.W., White, A.J.R. & Hine, R., 1988: Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 35: 505-521. Goscombe, B.D., Findlay, R.H., McClenaghan, M.R & Everard, J., 1994: Multi-scale kinking in northeast Tasmania: crustal shortening at shallow crustal levels. Journal of Structural Geology 16:1077-1092. Williams, E., 1976: Structural map of pre-Carboniferous rocks of Tasmania. Tasmanian Department of Mines.

The characterization of quartz and gold in the Beaconsfield goldfield David Russeir

The quartz veins of the Tasmania Reef (Beaconsfield) are zoned and consist of auriferous microcrystalline quartz with a central core of carbonate. Sulphides are distributed throughout the two to six metre wide reef, which has average grades ranging from 14g/t to 38g/t gold. The Ordovician quartzose sandstones/conglomerates of the Cabbage Tree Formation host the reef within a pre-existing fault zone. Approximately 150 analyses of the fineness of gold grains were conducted using an electron probe microanalyses Beaconsfield gold has an average fineness of 934, both the high fineness values and the narrow fineness range (900-950) indicate that the deposit is hypothermal in origin. A study of thin section slides of the Tasmania Reef suggested that there were at least eight phases of mineral deposition, with the gold being deposited during phase 3 (quartz, pyrite and gold) and phase 4 (quartz and gold). There appears to be four generations of quartz, with the finer grainer aquartz being associated with gold. The gold has several forms ranging from very small inclusions in pyrite, through to coarse grains completely replacing euhedral pyrite crystals. This study proposes a model for the deposition of gold at Beaconsfield. During the Devonian, the Tabberabberan Orogeny provided a deep geothermal system through which seawater (high 5^S) and metamorphic waters (low 5^45, high circulated through a fracture system. As the waters mixed, either a deep seated magmatic plume interacted with the system or a fractionation of the parent fluids occurred. The resultant fluids precipi-

1 The Don College, Watkinson Street, Devonport, Tasmania 7310, Australia.

tated ore minerals (5^45 mean = +8.9 %o, gi^Owater , of

vein quartz = + 1 2 . 8 to + 1 8 . 5 % o ) at temperatures between 3 5 0 ° - 4 1 0 ° C (arsenopyrite geothermometry) in a fault zone. In order to characterise the quartz of the vein systems in the Beaconsfield area, 44 trace elements were determined in 327 samples of vein quartz by Proton Induced X-ray Emission (PIXE) and Proton Induced Gamma Ray Emission (PIGME). The analyses from these 100 mg spot samples of vein quartz were compared with the gold fire assays of one metre length diamond drill core splits. In the auriferous reef quartz, the mean concentrations of Au, As, Ag, Li, Ge, Mn, Cr, Al, and Fe were higher than in the barren vein quartz, while Na, CI, Ni and S had higher mean concentrations in the barren quartz. A statistical Cluster Analysis was used to identify elements correlating with auriferous vein quartz with confidence (i.e. Au, Ag, Ge, Li, As, and Zr). A more sophisticated system of numerical data evaluation of the chemical analyses was developed to aid in the identification of auriferous quartz. These methods are based on significant statistical correlation (Spearman Rank) between pairs of elements for the auriferous and non-auriferous populations. These pairs of elements were classified as indicators for auriferous vein quartz or indicators for barren quartz. Further statistical processing using a series of "screens" enabled the nonmineralised samples to be removed, retaining the majority of the mineralised population. Successful screens developed were mol{Al); mol{Ge+Li}; mol{Na) and mol{Al+Ti+ K+Rb); mol{Ge+Li}; mol{Na}. A study of the intensities of Electron Paramagnetic Resonance (EPR) spectra was useful in distinguishing between the auriferous and nonauriferous quartz samples. The EPR signal intensity

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

had a significant correlation with the auriferous quartz as determined by Au fire assay. For the whole sample population (N=71), the EPR signal intensity also correlated with the concentration of Ti, Li, K, F, and Ge (in decreasing order) in vein quartz. Within the auriferous population only (N=37), the EPR signal intensity correlated even more strongly with the concentration of Ti, F, Au(fire), A1 and Li (in decreasing order) in vein quartz. Three screens were developed to differentiate between auriferous and non-auriferous quartz. The efficiency of these screens was measured in two ways; by the mean Au value (the higher the mean Au value, the greater the efficiency) and by the ratio between the number of auriferous samples (NR) and

the number of barren samples (NB) in the final screened population (the higher the NRiNB ratio, the more efficient the screening process). The initial screen used EPR signal intensity alone, producing a Au mean of 47 ppm and a NR:NB ratio of 3.00. The other screens used were EPR intensity; mol| Al) and EPR intensity; molfK), both producing a Au mean of 51 ppm (for this population, the maximum value would be 52 ppm Au) and a NR :NB ratio of 4.13. These EPR and trace element techniques for the characterisation of vein quartz and the subsequent identification of auriferous reefs, provide a tool useful in exploration geochemistry.

New geophysical data for northeastern Tasmania Robert Richardson^

Northeastern Tasmania is the major gold-bearing province of the State, with an estimated total production of 54 tormes of gold. As part of the NETGOLD project, high-resolution aeromagnetic and in-fill reconnaissance gravity data were acquired to supplement existing data and provide a basis for stimulating exploration. The residual gravity field is dominated by negative granitoid-related anomalies in the east. The major west-east gradient in the data passes through the Scottsdale Batholith and marks the boundary between the low density granites and adamellites to the east and more dense graniodioritic and Mathinna Group rocks to the west. The regional magnetic data are characterized by high-frequency anomalies sourced by Jurassic dolerite in the south and broader, more subdued anomalies in areas of Mathinna Group and granitoid outcrop. The broad magnetic high south of West Sandy Cape is interpreted to result from an east-dipping body of ultramafic rocks at depths in excess of 3 km beneath the Mathinna Group and rocks of the Scottsdale Batholith (Richardson & Roach 1994).

Fingal Area The known primary gold mineralization, with the exception of that at Golden Ridge, lies predominantly within a large north to NNW-trending relative positive residual gravity anomaly. This anomaly is within an area of Mathinna Group rocks but is more restricted than the mapped outcrop. At the northern end it terminates against a negative anomaly linking the Mt Paris Mass and the Tombstone Creek Pluton. The gold belt between 1 Mineral Resources Tasmania. PO Box 56, Rosny Park Tasmania 7018, Australia

Mathinna and New River is approximately along the axis of the positive anomaly and appears to be above the region of greatest thickness of Mathinna Group rocks. The aeromagnetic data (Figure 1) show that the Mathinna Group rocks are variably magnetic, and subtle northwest-trending features marking lithological variations are visible in all areas. West of Mathinna, near Alberton and in the Golden Ridge area the Mathinna Group materials are strongly magnetic (30 x 10-3 SI) but elsewhere they exhibit lower levels of magnetization. A number of linear and curvi-linear features which predominately trend northeast and northwest have been highlighted on Figure 1. With the exception of features within the Pyengana granodiorite, most north- to northwest-trending linears terminate against northeast-trending features. The valleys of the South Esk River and Tower Rivulet have narrow, often braided, high-frequency positive anomalies that are interpreted to be sourced by dolerite boulder beds. The main gold belt lies along a magnetic low and this is particularly apparent between Alberton and New River where the anomaly is narrow. Further south the magnetic low is broader and the gold occurrences have a greater scatter. A cross section from the Scottsdale Batholith, south of Ben Nevis, passing along the South Esk Valley north of Mathinna and terminating at Scamander has been modelled (Figure 2). The model has a granitic basement for the entire length and except where surface mapping shows granodiorite, has been calculated using only Mathinna Group materials above the granitic basement. There is a maximum thickness of approximately 2 km of Mathinna Group rocks in the Mathinna area with the thickness decreasing steadily to the east. The shallower Mathinna Group materials are nonmagnetic with the exception of a highly magnetic

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

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570000E

580000E

590000E

Figure 1 — Aeromagnetic image, Fingal area showing selected linear features.

600000E

Figure 2 — Two dimensional gravity and magnetic model of the Mathinna area from 550 150 mE, 5 412 850 mN to 605 100 mE, 5 404 100 mN. Values in curved brackets are densities (t/m3), values in square brackets are magnetic susceptibilities [xlO-

Figure 3 — Aeromagnetic image, Northern Coastal Area showing selected linear features.

Geological Society of Australia. Tasmania Division Contentious issues in Tasmanian geology: a symposium

68 (30x10-3 SI) lense at a depth varying from 400 m in the west near the margin of the Scottsdale Bathohth to 1100 m approximately 4 km east of Mathinna. The gold belt crosses the section at approximately 23 000 m.

The Northern Coastal Area The northern coastal area includes the Back Creek, Lefroy, Golconda, Lisle, Lyndhurst, Mt Horror, Warrentinna and Gladstone goldfields. The relationship between the known primary gold mineralization and the residual gravity anomaly is more complex than in the Mathinna area. The extension of the Mangana-Alberton gold belt through Lyndhurst follows a NNW-trending gravity high which corresponds to outcropping Mathinna Group rocks. Northeast of Gladstone the mineralization is in an area of relative positive gravity anomaly corresponding to the Mathinna Group rocks which separate the Blue Tier and Eddystone Batholiths. At both Lisle and Denison the mineralization occurs near relative negative anomalies. The Back Creek and Lefroy goldfields are in Mathinna Group rocks on a large positive gravity anomaly (the Stony Head anomaly). The aeromagnetic data (Figure 3) fall into three main areas. The first is an area of low background east of Croppies Point; the second is an area of generally positive anomalies south of West Sandy Cape; and the third is an area of low background between Weymouth and the western boundary of the survey area. Areas of basalt show as both high-

frequency positive anomalies, indicating either no or slight remanent magnetization, and highfrequency negative anomalies, indicating reverse remanent magnetization. North- to northwesttrending linear anomalies occur in areas of Mathinna Group outcrop and these are interpreted as lithological features and are locally parallel to the strikes of the Mathinna Group. In common with the Fingal data the predominant direction of the linears (Figure 3) is NW to NNW and NNE to NE. There are north-trending features in the Gladstone area, some of which are truncated by northwest-trending linears. The known primary gold mineralization lies predominantly on the flanks of positive magnetic anomalies.

Synthesis The new high-resolution aeromagnetic and reconnaissance gravity data delineate a number of subtle features that have not been identified by existing regional mapping. There is however no consistent correlation between known gold mineralization and any one set of geophysical criteria. The data provide a good basis for the selection of areas for detailed exploration, including ground geophysical and geochemical methods and detailed structural mapping. Reference

Richardson, R.G. & Roach, M.J., 1994: An interpretation of recent geophysical surveys, northeastern Tasmania. Report Mineral Resources Tasmania 1994/08.

Relationships between Devonian thrusting and gold mineralization in northeastern Tasmania Richard A Keele\ Bruce Taylor^ and Garry J. Davidson^

The northeast Tasmania terrane can be conceived of as a gently west-dipping thrust wedge, comprising early to mid-Palaeozoic strata which had experienced crustal thickening during the MidDevonian orogeny.

East of the Scottsdale Batholith The Devonian strata at the front end of the wedge (at Scamander) were folded into open style kink folds typical of high crustal levels. Immediately west of this, a major east-directed thrust (subsequently intruded by the Catos Creek Dyke) juxtaposed Silurian strata against Devonian strata with an estimated 3 km uplift on the western block (Figure 1). At the back end of this thrust system, a zone of limited extension and collapse is suggested by the presence of an inferred normal fault east of Mathinna. Gold mineralization at Golden Gate, and other deposits on the metallogenic linear known as the Main Slide, lie on strike slip faults which are inferred to intersect the west-dipping thrust at a depth of 5-7 km. The Mathinna-Alberton lineament lies within a region of back steepening within the thrust wedge which suggests that the mineralised structures may be reactivated thrust tips. Regional scale upright folds west of Mathinna and east of the Scottsdale Batholith are floored by a granitic substratum at depths of 2.5 km, or less (Roach 1992).

1 Key Centre for Ore Deposit and Exploration Studies, Geology Department, University of Tasmania, GPO Box 252C, Hobart 7001, Australia 2 North Flinders Mines, 24 Greenhill Road, Wayville, SA 5034, Australia

West of the Scottsdale Batholith The Cambrian to Devonian-aged strata west of the Scottsdale Batholith are folded into upright to recumbent folds (Drown 1984; Powell & Baillie 1992). These folds occur above a region of major back thrusting that originated from a point where the main thrust wedge ramped up through the strong middle crust (e.g. Coward 1994). The occurrence of east-dipping reverse faults at mid to high crustal levels on either side of the Scottsdale Batholith represents a late stage within the orogenic episode, i.e. a D3 event (Keele 1994). These faults, which post date the wrench event P j ) ' represent a return to the far field stress conditions of the earlier part of the orogenic event and probably coincided with the waning stages of batholith emplacement. Cambrian sediments are inferred to lie at depth above the shallow E-dipping detachment surface in the Tamar Valley area (Leaman et al 1973; Elliott et al 1993); this detachment breaks surface near the Beaconsfield gold deposit. The Pipers Brook recumbent fold structure is here interpreted to be the lower side of a shallowly dipping east-directed thrust, or a 'pop up' zone, that brought Middle Cambrian ultramafics and sediments to the surface. This thrust may also be linked to the west-directed thrust which lies on the underside of the Precambrian Badger Head Block. The region centred on the Tamar Valley marks the point of maximum crustal thickening and tectonic uplift in the wedge, since it can be shown that the Devonian strata have risen upwards by as much as 10 km when compared to strata of the same age in the front part of the wedge (Figure 1). These geometric relationships suggest an overall eastward tectonic transport in northeastern Tasmania, which is consistent with the eastward vergence of structures within the Melbourne Zone of central Victoria (Fergusson et al. 1986).

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

Fluid Pathways Likely pathways for the deep-seated auriferous C O-H metamorphic fluid (Taheri & Bottrill 1994) are suggested by the gross symmetry of the doubly tapering thrust wedge (Figure 2). The ultimate source region for the fluid is likely to be the point where the major east-dipping back thrust meets the main thrust wedge near the base of the strong middle crust at a depth of approximately 15 km. The broad fan-like distribution of thrusts (about a central point corresponding to the "granite wall" ) suggests that this fault array may have acted as the main control on the ascent of mineralising fluids. Subsidiary channelways in the western part of the wedge, which are yet to be defined, may account for the occurrence of the deposits at Lisle and Lefroy. Whilst the presence of only small deposits east of Mathinna could be accounted for by: (1) short circuiting of the fluids via steeply dipping wrench faults within the Mathinna-Alberton lineament, and (2) local derivation from granitoid sources (e.g. Golden Ridge). Short circuiting would have siphoned off much of the auriferous fluid before it had a chance to reach the distal parts of the thrust wedge. Oxygen isotope data from Beaconsfield and Lefroy (Russell & van Moort 1992) do not preclude a component of mantle-derived fluid in the hydrothermal system; if this is the case, the base of the doubly tapering wedge is the most likely point of entry for such a fluid into the crust.

References

Coward, M.P., 1994: Continental collision. In P.L. Hancock (Ed.): CONTINENTAL DEFORMATION. Pergamon Press, Oxford: 264-288. Drown, C.G., 1984: The sedimentology and structure of the Mathinna Beds, Double Sandy Cape. BSc Honours thesis. University of Tasmania (unpubL): 72 pp. Elliott, C.G., Woodward, N.B. & Gray, D.R., 1993: Complex regional history of the Badger Head region, northern Tasmania. Australian Journal of Earth Sciences 40:155-168. Fergusson, C,L., Gray, D.R. & Cas, R.A.F, 1986: Tectonostratigraphic terranes, fold-thrust zones and regional metamorphics in the Palaeozoic of central-eastern Victoria. Geological Society of Australia SGTSG. Field trip guide Mt Buffalo Conf: 124 pp. Keele, R.A., 1994: Structure and Veining in the E^evonianaged Mathinna-Alberton Gold Lineament, northeast Tasmania. Mineral Resources Tasmania report No. 1994/06, Tasmania Development and Resources: 27. Leaman, D.E., Symonds, P.A. & Shirley, J.E., 1973: Gravity Survey of the Tamar Region, northem Tasmania. Geological Survey of Tasmania Paper 1. Powell, C.McA. & Baillie, RW., 1992: The tectonic affinity of the Mathinna Group in the Lachlan Fold Belt. Tectonophysics 214:193-209. Roach, M.J., 1992: Regional geophysics of the AlbertonMangana goldfield, northeast Tasmania. Geological Survey of Tasmania Bulletin 70:199-207 Russell, D.W. & van Moort, J.C., 1992: Mineralogy and stable isotope geochemistry of the Beaconsfield, Salisbury and Lefroy goldfields. Geological Survey of Tasmania Bulletin 70:208-226 Taheri, J. & Bottrill, R.S., 1994: The nature and origin of the gold mineralisation, northeast Tasmania — Mangana-Forester area. Mineral Resources Tasmania Report 1994/10. Taylor, B., 1992: Structural traverse across the Mathinna Group, northeastern Tasmania. BSc Honours thesis. University of Tasmania (unpubL): 47 pp.

Nature and origin of gold mineralization, ManganaForester area, northeastern Tasmania J. Taheri^ and R. BottrilP

Gold-bearing quartz veins occur in a NNW-trending belt and are hosted by the Mathinna Beds in northeastern Tasmania. The Mathinna Beds are turbidite-deposited sedimentary rocks which have been metamorphosed to lower greenschist facies and comprise mainly slates (carbonaceous in part) and quartz wackes. The gold mineralization is of mesothermal type and is typical of many other mesothermal gold-lode deposits. Paragenetically the quartz veins may be divided into three broad categories, viz.: 1. Early veins which include minor barren fibre veins and gold-bearing grey laminated and/or brecdated veins. Grey quartz veins occur in most of the gold prospects and contain the most consistent gold values. The gold-bearing grey quartz veins contain abundant fine grained arsenopyrite with lesser pyrite, carbonates, base metal sulphides, chlorite, mica, rutile and graphite. Some of the vein components were derived from the wallrocks. 2. Intermediate veins which are white in colour, erratic in sulphide and gold content and may show comb, buck, breccia and ladder textures. They are the most common vein type in the field, and 3. Late veins including spider quartz veinlets and narrow veinlets of sulphides or carbonates ± quartz. They are volumetrically quite insignificant and are not known to contain gold. Hydrothermal wall rock alteration is mainly limited to silicification and carbonate alteration. The latter appears to be related to the gold mineral-

1 Mineral Resources Tasmania, PO Box 56, Rosny Park Tasmania 7018, Australia.

ization and occurs as small rounded porphyroblasts of Mg-siderite in pelites in close proximity to many gold mines, mainly in the southern part of the field. Gold grains range in size from sub micron to about 1 mm and occurs mainly as : (a) small inclusions in arsenopyrite and quartz, (b) micro-veinlets in coarse, brecdated arsenopyrite, quartz and slates, (c) intergrowths with secondary lead-aluminum arsenates, and (d) intergrowths with goethite. The gold grains generally show a narrow range in composition and are high in fineness (920 to 990) with the exception of some electrum grains from the Linton deposit showing fineness ranging from 654 to 661. The gold deposits are structurally controlled and show no obvious correlation with stratigraphy or host-rock composition. Fluid focusing appears to be the most important factors in the formation of high grade gold mines. Early sulphide and gold-bearing quartz veins were formed episodically by opening and closing of the extensional veins due to cyclic fluid pressure fluctuations ranging from superlithostatic to hydrostatic along shear zones. This process also produced hydrothermal breccias of possibly different generations. The gold-bearing quartz veins were formed from two distinct types of fluids viz.: (a) H2O-CO2 (CH4)-rich, low salinity fluids at temperatures of around 300°C. The ore-forming fluids are characterised by isotopically heavy and consistent values for the early and intermediate quartz veins, ranging from 9.2 to 11.4%o. The data are compatible with metamorphic fluids derived from devolatization of deep-seated metasediments at the greenschist-amphibolite facies boundary Deeply convecting, chemically modified meteoric fluids probably also played a role in the formation

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

74 of the ore forming solutions. The metamorphic fluids appear to be responsible for the formation of the vast majority of the gold-lode deposits, which are characteristically low in Sn and W, generally <10 ppm. (b) C02-poor fluids of possibly lower formation temperatures showing isotopically lighter oxygen values (6.9-9.0%o), similar to the isotopic composition of fluids responsible for the greisenization of granites. The quartz veins exhibits different mineralogy in containing wolframite and tourmaline and are anomalous in Sn and W (10-990 ppm). The results are indicative of the involvement of granitic intrusions in the formation of some particular gold-bearing deposits.

Reduction in activity of sulphur, along with decrease in temperature, caused by boiling of oreforming fluids, appear to have been the most effective mechanisms in precipitating the gold and sulphides in the early-formed quartz veins. Rapid deposition is evidenced by the presence of very finegrained sulphides and chert in many gold-bearing veins. Wallrock-fluid interaction appears to have played a less important role in the formation of gold deposits along the gold belt. Fluid inclusion studies, oxygen isotope, and Sn and W analyses of the quartz lodes may effectively be used as three different techniques in differentiating the quartz veins of metamorphic origin from those formed as a result of granite intrusions along the belt.

Geology, geophysics and mineralization of NE Tasmania Discussion session — chaired by Garry Davidson

Editors Note: Due to the number of speakers and the diverse nature of the topics in this session^ speakers zveve only allocated five minutes of question time at the end of each talk. Due to a technical error, the questions relevant to Richard Keele's and Bob Richardson's presentations were not taped, and there was no time for questions at the end of Rick Varne's talk. The transcripts of questions from David Russell and Jaffar Taheris talks are provided below. David Gray (DRG) to David Russell (DR): In your abstract, you mentioned that you had oxygen isotope data on quartz. Did you do analyses of auriferous versus barren quartz, and was there any difference in the isotopic signature of the two? DR: There was a bit ofa difference, but I was not confident in terms of the samples I had DRG: The numbers you quote ranged from 12.8 to 1 8.5% how many samples are involved here? DR: About five samples. Mel Jones to DR: What is it that you are actually measuring with this EPR? Is it lattice discontinuities in the quartz, or what? DR: The EPR we were looking at relates to two things. I think there are distortions, but part of the distortion relates to substitution for some elements... (inaudible)

Bill Baker (WB) to DR: Have you extended your work into quartz veins with minor sulphides? Given that Ge is a chalcophile element; wouldn't you expect to see similar things caused by the introduction of sulphides, quite independent of whether gold is present or not? DR: The veins I describe as barren are samples where there is no indication of gold, because the holes did not hit the reef, but sulphides are still present in veins. In

terms of blanks, I have also looked at them, and these have no sulphides in them. Nick Green? (NG) to DR: How many drillholes did you sample? DR: It would have been eight or nine drillholes. Garry Davidson (GD) to DR: You noted that Zr was positively correlated in your veins with gold. Did you find any microscopic evidence of this ? DR: This is probably a problem with the limited number of samples and spearman ranking. It is probably not real Ken Morrison (KM) to DR: Dave, a lot of the NE Tasmania reefs are characterised by high grade variability, very spotty gold grades. Do you see evidence of fertile veins turning into barren veins along strike? DR: I haven't noticed that, no... (inaudible) ... Mike Solomon (MS) to Jaffar Taheri (JT): That was very interesting. May I make one or two comments and a question? You refer on the second page of your abstract to convectingfluids. Did you really mean that, or wouldn't it be more likely that they were driven by tectonic pressures? That's the first point. I've been thinking about the Victorian gold veins. They too have insignificant wallrock reaction, and a much more likely mechanism seems to be reduction of the fluid by fluid mixing in the wall rocks. They will have water in them that has equilibrated with carbonaceous matter. These waters will be CH^-bearing, and mixing and reduction could account for the very loose relationship that exists between carbonaceous matter and gold. It doesn 't actually occur where the rock meets the fluid, it just commonly occurs higher up in the vein. The fluid mixing looks as if it could be quite important. JT: Yes, fluid mixing is important, but sulphidation looks important too.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

MS: But you don't really have enough wallrock sulfur for sulphidation to be important.... and my third question was, do your really have evidence that you reach hydrostatic pressure? JT: Fluid inclusions indicate this, and extensive hydrothermal brecciation does occur ... (inaudible) ... Jan Van Moort (JcVM) to JT: Hooking in on Mike Solomon's question, I think the extensive cracking that goes together with this type of gold mineralisation precludes mixing with extraneous fluids, because the system is expanding, and the cracks crack after crack alter crack causes the system to break open and doesn't allow very much mixing with extraneous fluids. Then we have of course the peculiar oxygen isotope composition that we observe over at Beaconsfleld and the quartz chemical composition with enormous amounts of Ge, that also are typical for the Ordovician and typical for the Victorian deposits. The

thing is that there is an active deep-seated metamorphic fluid of god knows what origin. JT: There are two types of indication of the two fluids fluid inclusions and the hydrogen isotopes... (inaudible)

Dave Gray (DRG) to JT: We've done extensive isotopic analysis on a regional scale in Victoria and found basically that the oxygen isotopes in veins within a given zone are fairly uniform, and we believe that with the Victorian gold deposits that we have auriferous and nonauriferous veins. Certainly in terms of oxygen isotopes the indication is that the fluids were buffered by the rocks, and any evidence of the source of the fluid is basically gone. There are very distinct signatures for each of the structural zones in Victoria. Bendigo-Ballarat fluid transport distances are of the order of a stratigraphic stage, about 1 km.

Geophysical misinterpretations and realistic approaches

Realistic applications of gravity and magnetics David Leaman^

This paper addresses two issues: the ubiquitous use of one method (magnetics) which often generates an under-utilised data base, and the very cautious use of the other (gravity) with the result that many options for geological and prospect discrimination are lost. Some common misconceptions also exist: especially that gravity is of limited prospect use and therefore not generally applicable in a primary way, which misses the point that its essential use is structural. Realistic applications of gravity and magnetic methods — indeed any geophysical method — depend on rational evaluation/definition of the geological/target problem, the existence of an appropriate physical contrast and a reliable and adequate data set. Satisfaction of these elements does not guarantee success; it may depend more on how the data is processed, or interpreted, or regionally incorporated. Note that null solutions may be viable and important - if properly defined. Many explorers use geophysical methods as target finders, i.e. some ores may have magnetic components or associates,and many are presumed conductors. This explains the widespread use of magnetics followed by EM-IP variants for base metal exploration. The fact that this is rarely realistic or beyond rational economic argument is not often faced. I have always found it rather ironic that the same people who will accept these assumptions / presumptions for magnetic and electrical methods will not do so for gravity. The reason is not hard to find. The theory is the simplest yet the results are often complex and geologically involved. Analysis of the requisite interactions is often called "over-

1 Leaman Geophysics, GPO Box 320D, Hobart, Tasmania 7001, Australia.

interpretation". This actually suggests that the method might yield more information than the others. None of this is meant to imply that speculative applications are not viable nor lacking in value; just that we should be more general method users. If we will not use gravity in this way then we should review the use of other techniques and certainly not use EM on ground in a regional way What is really cost effective? Perhaps there is also an abuse in "no conductor, no target..."! The crucial point is that gravity and magnetic (and seismic) methods are structural-geological methods par excellence. Their use will always yield usable geological information and they will never fail in this given appropriate analysis. Any specific prospect targeting applications may also be viable and may sometimes evolve from such primary usage (cream on the cake). Most mineral explorers have an appreciation of magnetic methods. Their low cost has created an almost unquestioning usage and even petroleum explorers are belatedly re-discovering that it may have cost effective uses. Most common applications are related to mapping of units or structures (especially under cover) or depth to magnetic basement/sources. Modem image processing has made the data presentation more attractive visually and enhanced the mapping character role. This has not altered the common view that many spiky characters have an economic association although most are related to geometric irregularities of local geology The methods are not basic ore finders. We should also appreciate that any combination of methods is much stronger than the simple possibilities of either, or their sum. Realistic applications assume high quality data and proper correction. Some of these issues are Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

80 beyond the scope of this paper but most magnetic survey presentations omit critical information (including base level data) which restricts quantitative analysis and many gravity surveys carry gremlins in the form of poor elevation control and omission of local terrain corrections. Garbage in, garbage out. Applications may proceed, and succeed, on two levels. Direct response and extracted response. Magnetic methods tend to be more direct than gravity methods.

interpretation. Filter or gridding distortions may also lead to invalid results. Criteria are available for the assessment of such problems and the choice of interpretation techniques (Leaman 1994). The methods are well suited to dipping sources/ structures, i.e. those with irregular geometry and vertical relief. Contrast seismic methods where the opposite is true. But recent work has shown them to be very sensitive to low angle variations common to basins where the large horizontal scale leads to distinctive anomaly characteristics.

Evaluations restricted to direct responses represent an enormous under-utiiisation of the data set. Direct techniques are generally based on simple displays and obvious character. Gravity methods could achieve the same result but are rarely acquired in sufficient detail. Mapping, fracture and pattern tracing and perhaps limited alteration responses will be recognised. Images assist, but this form of display can never constitute more than an introduction to complete interpretation. Some magnetic responses, like those associated with banded iron formations, may cloud other details. Adequate survey specification may be crucial. Extracted responses result from complete use of the data and may involve more processing, filtering, regional separation and modelling. Every one of these is a risky procedure and quantitative evaluation/induction requires recognition of regional features and interference and geological input. It is here that gravity data both shine and complicate. Where magnetic data reflect only a few components of the geology of the place this is rarely true of the gravity field which is more affected by lateral and deeper sources and most of the geology. This means that some elements may be evident but most are not and all modify the others in some way. Direct inspections and conclusions from a gravity map are fraught with risk. We also need to consider more data than those of the immediate area of concern. Gravity data, by responding to large volume mass changes, also allow discrimination of scale which may select critical structures from the set defined profusely by magnetic data. This is important information since elephants can only be expected near mammoth fracture/circulation systems in crustal terms. Both methods suffer from anomaly interference where confused gradients or even 'false' anomalies may deceive automated analytic methods of

Most texts focus on the use of magnetics for depth of source, or other local geometry, and gravity for complex excess mass targets. The latter is far from a simple issue, but these applications barely do the methods justice. Some Tasmanian examples of wider use include magnetic mapping (magnetic, western Tasmania ultramafics; non magnetic units, Mathinna Gladstone; units under basalt/water, Waratah/ Derwent; other sequences, southeastern Tas), form of granites western Tasmania (g) and mafics Fingal feeders (g, m), trends, ore setting Rosebery/Cygnet, ore bodies (g, m) Rosebery/Que, mag responses, ore constitution (m, g) Rosebery, aquifers Scottsdale and deep leads northeastemTasmania, Tertiary (g), granitoid composition (m, g) Lisle, vein systems and alteration (m) Gladstone/Tower Hill and regional structure (m, g). Midlands, southeastern Tasmania and western Tasmania etc. Some other examples include basin architecture and content studies Sydney, ore constitution Isa, complex structures and mineralisation settings (m, g) Isa, aquifer alteration (m, g) Hilton, responses in built up areas (m) Isa. Such applications are both realistic and practical but stretch both the data set and the interpreter and the established geological background. Many suggest these are overinterpretations but the results speak for themselves. So I will join Voltaire " I'll defend their right to suggest it — but they are wrong, and it is no game for amateurs". So be warned. There is no quick solution in such applications and all involve geological/geophysical treatments far removed from simple sources and analyses. If you have gravity and magnetic data, pose geological questions or tests and demand answers. You will get them — and there will be surprises as well.

FlC. 7 — Disiribution of inferred basement rock types at the base of the Parnteener Supergroup. The distribution of post-Cambrian Palaeozoic units is not welt established and these may be present only as small patches. Possible regional fold trends and thrusts indicated in interpretation are shown.

FlC. < — Typical magnetic and gravity interpretation models. (A. B) for E-W line at 5220000 niN. (C. D) at 52JOOOO mS. Symbols: Ts. Tertiary sediments: Ksy, Cretaceous syenite: Jd. Jurassic dolerite: Sms, Silurian Maihinna Beds: Dg. Devonian granite: Ev. Cambrian volcanic suites: pE^ Jubilee region style Precambrian: Tyennan region style Precambrian.

iigurc I — South East Tasmania: A, B, C from Leaman (1990), Pap. Proc. Roy. Soc. Tasm. 124:1-12. These show how gravity and magnetic data can be integrated to yield structural and formational information beneath variable and complex cover sequences. B shows the inferred distribution of rock types at the unconformity at cover base. C indicates the distribution of Jurassic dolerite feeders in the area, all trends, and the thickness of late Precambrian and Cambrian basin sequences. Volcanics form a significant component. Gladstone, North East Tasmania. D from Leaman (1992). Bull. geol. Surv. Tasm. 70: 149-160. The relatively non magnetic Mathinna Beds, deeply weathered and covered by sand, yield up their secrets and can be mapped as lithological elements. Cross structures and faults are apparent.

• lU.. •OUCUCII . lull l u i r ^ i T ^ Ju - «\M« Will I 1

FAULT

contractor

^2Dsensitive position flexible }l

dip range

LGEO^ k:0005

/ 7 S 3 D dip range

Fig. 2 Model illustralini the effect of incorrect choice of rock property contrast and interpretation method upon the nature and reliability of magnetic results along line 15 in the Stone Axe area, northeast of Mt Isa ( - 3 4 6 000 mE, 7 725 000 m N A M C ) . Higher magnetic contrasts based on local exposures force a higher relief solution (LGEO) which is compatible with exposed structures, while lower contrasts based only on data from Mt Isa Mine led to incorrect solutions (contractor). Threedimensional methods allow significant refinements.

Fig. 3 Structural evaluation of the Mt Isa Mine area options based on a single aeromagnetic data line and simple methods prior to completion of regional surveys and rock property studies. Letters indicate the position of the surface of the Eastern Creek Volcanics (ECV) for each option. Model C (Young 1984) is shown to be most unlikely, A and D are similar preferred conclusions supported by subsequent complete analysis and hole De300. Limits of options within the lined zone are imperfectly resolved using simple analysis and tr.csc (pre-full survey) data.

^ -

lillltMIHl , V

FiC. 9. N a t u r e o f gravity a n d m a g n e t i c a n o m a l i c j at 34 000 anc 36 0 0 0 m N . T h e s t r u a u r a l interpretation is also s h o w n . The differentia] between observed a n d caJculatcd gravity p r o H l o rcHccu w e a t h e r i n g or alteration not i n c l u d e d in the 3 - D m o d e l s . N o i c that the rocks near 15 0 0 0 m E at 34 0 0 0 m N are u n w c a i h c r s d T h e s t r o n g m a g n e t i c r e s p o n s e s reflect this a n d s h a l l o w i n g of v o l c a n i c s . T h e bulk o f the m a g n e t i c s response is due to ihc d i s p o s i t i o n a n d contrast o f the volcanics, w h i c h is locally d i m i n i s h e d near the mine, a n d the fact that the local ano.T.aiy at 12 0 0 0 m E , 36 0 0 0 m N h a s its sources w i t h i n the Pc>-Zn m i n e r a l i z a t i o n . T h i s is s h o w n as a d o t t e d v a r i a t i o n of the calculated p r o f i l e N o similar response is observed at 34 ( m j in a s s o c i a t i o n with C u ores. S y m b o l s refer to Mt. Isa g r o - > \ a n d Breakaway shale ( P i b ) .

Figure 2 — Mt Isa region, Queensland: r om Leaman (1991 a, b). A J E S 38: 457-472; Geophysics, 56, 542-549. Surface gravity and magnetic fields near Isa Mine are shown in A, B. D indicates elements of the structure at Mt Isa. The anomalous character of the mine site is immediately evident due to interaction of many effects related to the ore package. Part C shows how important it is to properly assess rock properties and methods. See also Leaman (1994), First Break, 12, 181-191.

Fig. 12 V a r i a t i o n in b u l k m a g n e t i c c o n t r a s t of t h e E a s t e r n Creeic Y o l c a n i c s e x p r e s s e d as s u s c e p t i b i l i t y m e a s u r e m e n t s (cgs u n i t s ) a s d e r i v e d by 3 D a n a l y s i s of b o t h s t r u c t u r e a n d p r o p e r t i e s . N o r m a l v a l u e s a r e in t h e o r d e r of 0 . 0 0 4 - 0 . 0 0 5 cgs o r a b o v e . N o t e t h e l o c a l i z e d loss of c o n t r a s t n e a r N a t i v e Bee a n d M t Isa M i n e . S i m i l a r p a t t e r n s m a y be r e c o g n i z e d in d e n s i t y a s s e s s m e n t s , b u t t h e v a r i a t i o n s a r e m o r e s u b t l e a n d v o l u m e s a r e n o t as r e l i a b l y d e f i n e d . N o t e t h a t v a l u e s a p p l y to v o l c a n i c r o c k v e r t i c a l l y beneath the labelled position a n d m a y derive f r o m u p p e r o r l o w e r s u r f a c e s (Figs 4 - 1 0 ) .

(HIM)

Fig. 11 S t r u c t u r e c o n t o u r s u m m a r y o f t h e cn for the Eastern Creek Volcanics between 1 4 0 0 0 0 m N ( M I M g r i d ) . It is n o t p o s s i b l e l o s c r i b e t h e m u l t i - l a y e r e d c h a r a c t e r o f t h e Sybi B e e - C r y s t a l l e n a - I s a slices b u t t h i s is suggesi dotted and broken lines for both u p p e r C r o lower P i c k w i c k s u r f a c e s . T h e L e n a Q u a r t z i t e N Deen p a t t e r n e d .

Bdthurst

Batholith LACHLAN

FOLD

BELT

Figure 3 — Mt Isa region, Queensland: from Leaman (1991a), AJES 38: 457-472. A, B, C. An example of the potential of the methods. Detailed extraction of structure, prospect setting and relationships at depths up to 5 km and evaluation of variations in magnetisation within the structure leading to assessment of the volume of alteration near the mine. Geometry and magnetisation are neither independent factors nor deducible by inspection from the raw data (e.g.. Figure 2). The solutions allow development of a concept as a basis for further exploration. Sydney Basin, NSW: from Leaman (1990, 1994). AJES 37: 107-108; First Break 12: 181-191. The example illustrates why regional effects must be properly evaluated and how it is possible to judge the validity of an interpretation.

)

/ //

.$J7*00®«H

/ j)7$ooe«N

5371000" y

;

S)T)000.

I/-FIGURE 3 B o u g u t r Anomalies n « a r R o i t b t r y MIn* b a » « d on r t s u l U of orlginaJ IriaJ »urv«y (1987). RtducUon dtnsJty. t/cu m. Contour Interval O J mGal. Preclalon about 0.1 mGal.

FIGURE 4 Residual Bouguar Anomalies n e a r R o s e b e r y Mine. Realonal aeparallon based on approximate estimates of the effect of local granites. General validity has been confirmed after more complete coverage and allowance lor crustal and oranlte m o d e l s . The large central positive anomaly la a s s o c i a t e d with the s o u t h e r n lens s y s t e m . Contour Interval 0 . 2 5 mGal.

FIGURE 6 Olflcrentlal b e t w e e n residual anomaly and mine attraction calculated lor known density distribution In R o s e b e r y Mine. Contour Interval 0 . 2 5 mGal. Note the e x c e s s m a s s Implied at the northern end of the s o u t h e r n l e n s e s and within the footwail.

Figure 4 — Tivo examples of near mine usage. Upper: Rosebery, Tasmania: Leaman (1991), Exploration Geophysics 22; 231-2^4. The three maps present the observed Bouguer anomalies which are domirmted by local granitoids, the Bouguer anomalies after removal of the granite effect, and the anomaly differential after removal of the known density distribution at the mine. This has pinpointed zones in which anomalous material occurs. Lower: Hilton, Queensland: Leaman (1994), Exploration Geophysics 24: 615-622. This gravity application was directed at definition of aquifer volume for dewatering evaluation and pump design. This was achieved and the armlysis also defined anomalous mass nearby leading to accelerated review of the northern lens system. Neither aspect is obvious in the raw data.

EM conductors in western Tasmania Jovan Silic^

Electrical techniques have been used in the Mount Read Volcanics as direct targetting tools in the search for conductive/polarizable orebodies. The purpose of this paper is to comment on the effectiveness and complications in the application of these techniques, which have played an important role in Aberfoyle's exploration strategy. The discovery of the Que River mineralization in 1974 was partly due to identification of an airborne EM target in an area of anomalous geochemistry. Following the discovery of the P / Q and S lens mineralization, application of the 1970s ground EM and IP technology resulted in conclusions that the S lens mineralisation was unequivocally conductive, but the major P / Q lens was considered to be non-conductive and polarizable. Some eight years later, this was proven to be incorrect, with 1980s technology detecting P / Q mineralization, which was 'T:>lind" to the 1970s technology due to limitations of the technology and structural complications at P / Q lens which were not recognized in the 1970s. The discovery of the Hellyer orebody was largely a result of highly rating an "unusual" EM response, which could have been dismissed as being caused by something other than significant accumulations of conductive sulfides. Subsequent analysis of this response resulted in identifying the geometry of the conductive Hellyer orebody as being the cause of the fast decaying or "unusual" EM response over the deposit.

In the essentially geo-electrically clean Mount Read Volcanics environment, where lithological conductors detectable by EM techniques are rare, EM cannot be used effectively as a geological mapping tool, however they can play a very effective role in directly detecting conductive massive sulfides. As a result, during the last 20 years, Aberfoyle Resources has conducted a number of surface and downhole EM surveys throughout the Mount Read belt, and has successfully identified conductive targets, some as much as 250 m away from the drill holes. For a number of years it was believed that the only EM targets with sulfide-based mineralogies are the VMS deposits. However, recent experience has shown that, in some instances, stringer systems which contain 5-15% pyrite can electrically interconnect in a sheet-like geometry and produce EM responses which may be confused with VMS effects. Aberfoyle's experience has shown that in essentially geo-electrically clean envimments, such as the Mount Read Volcanics, EM techniques can be very effective in detecting conductive VMS mineralisation, even at distances greater than 250 m from drill holes or the surface. However, care and understanding needs to be apphed in rating the importance of EM responses, as a body's geometry, mineralogy and structural complications can produce "non-classical" responses which are not commonly associated with VMS deposits.

1 Aberfoyle Resources Ltd, 123 Camberwell Road, Hawthorn East, Victoria 3123

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

EM signatures at Hercules and Rosebery Neil Hughes^

Successful use of the UTEM system in locating the economic mineralisation at Que River (P lens) and in the discovery of Hellyer has drawn attention to the potential of this type of system for detecting blind massive sulphide deposits. Testing of the UTEM system by Pasminco Exploration at South Hercules led to its widespread use on the Rosebery mine lease and extensions. Numerous anomalies have been detected during these surveys. To date no economic massive sulphides have been drilled

1 Pasminco Exploration PO Box 703, Broken Hill, NSW 2880, Australia

based on these results. Problems encountered during interpretation include not being able to adequately separate the "ore" response from that of the overlying black slate at Rosebery and being able to detect with confidence the small zinc-rich pods at Hercules. Successful use of EM in this environment requires a better knowledge of the physical properties of not only the ore but of the host and adjacent lithologies.

Geophysical misinterpretations and realistic applications Discussion session — chaired by John Bishop

Editor's Note: Due to a technical error, the discussion for this section was not recorded. The following summary of the talks and the discussion session has been provided by John Bishop, Three papers were given in the session on geophysics. The first paper, by David Leaman, dealt with the Realistic Applications of Gravity and Magnetics. Leaman stated that the prime application of magnetics and gravity was to assist in the structural and geological understanding of an area and that their role as direct ore finders was usually secondary ("cream on the cake"). He also pointed out that both methods will yield much more information than is usually obtained if (1) the data is properly collected and processed and (2) it is then properly, quantitatively, interpreted. The second paper, by Jovan Silic, was entitled EM Conductors in Western Tasmania. Silic showed that the Mt Read host rocks for the Western Tasmanian volcanichosted massive sulphides (VHMS) were very resistive and that in this "geoelectrically clean environment'', Aberfoyle had successfully used the electromagnetic technique (airborne, survey and drillhole), leading to the discovery of two orebodies (Que River and Hellyer). Recent experience with drillhole EM had shown that sheet-like stringer systems containing 1-5% pyrite, may be well connected electrically and give similar responses to VHMS lenses. The third paper also concentrated on EM, with Neil Hughes giving a talk entitled EM Interpretations at Hercules and Rosebery. EM, and particularly the litem method, has been extensively used on the Rosebery mine lease. Most of the known mineralisation has been shown to occur within conductive zones and numerous anomalies have been defined, though none have yet resulted in the discovery of further economic massive sulphides. Hughes also showed some of the problems associated with interpretation on the mine lease. In

particular, the separation of the black shale response from the ore at the Rosebery Mine and the very low conductance of the zinc-rich lenses at the Hercules Mine. For the discussion following the papers, the audience were invited to question the speakers on their exploration philosophies for VHMS deposits in Western Tasmania, as well as on specific points arisingfrom the papers. Some lively discussion ensued, both between the audience and the panel and within the panel; with Leaman the protagonist for potential field methods, and Silic and Hughes, the electromagnetic experts. Unfortunately, the taping of this session (together with Hughes' talk) did not work and thus the comments given below are from the chairman's imperfect memory with some assistance from the speakers and members of the audience. One of the first questions was addressed to Silic by Gerald Purvis who asked why Aberfoyle had used EM so extensively and apparently to the exclusion of other geophysical methods. Silic replied that Aberfoyle had drilled only five EM targets in Tasmania: two of these had turned out to be orebodies and two were stringer zones. The fifth was due to a fault, but it was understood at the time that there had been only a small chance of the source being sulphides. Hugh Skey asked David Leaman whether the short wavelength of one of the residual gravity responses at Rosebery was in keeping with its deep source, and why the calculated excess mass was so high. Leaman replied that the sharp gradients were largely reflecting the surface alteration, both leaching (giving a negative residual) and pyritic alteration (giving a high). This was superimposed on a longer wavelength anomaly which was associated with the whole sulphide system. The large excess mass value was due to the total sulphide system at Rosebery which was in excess of 50 million tonnes. Neil Hughes was asked "whether, given that the IP response at Rosebery was so strong, would you see it with EM?" Hughes was honest enough to reply that he did not know the answer to that question. (In fact, the IP

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

88 surveys at Rosebery only cover the northern and southern ends of the deposit and the strong responses are probably largely due to the pyritic footwall, rather than the orebody.)

The sources of sulphur in the cassiterite-sulphide deposits of western Tasmania

Meteoric fluids and sulphur in tin-tungsten deposits with special reference to carbonate replacement types Mike Solomon^

Two contentious issues relating to the origin of SnW deposits in general concern the role of nonmagmatic water in ore formation, and the source(s) of the sulphur, particularly in carbonate replacement types like Mount Bischoff, Renison and Dachang.

The role of non-magmatic fluids Halley (1987) and Halley and Walshe (in press) have proposed for Mt Bischoff that because the fluids involved in the main cassiterite-rich phases of ore formation in Greisen Face are cooler (330-360°C) and less saline (12-14 wt % NaCl equiv) than those that formed the earlier barren chondrodite- and serpentine-bearing phases (400-460°C and 30-36 wt % NaCl equiv), and because the values of the former (2.2 to 3.0 %o) are somewhat higher than the latter (0.0 to -0.2 per mil), then the Sn-depositing fluids are of meteoric origin and have derived part of their sulphur content from country rocks. At Aberfoyle and Lutwyche, Halley (1982) and Solomon et al (in prep., reviewed in Solomon & Groves 1994) have shown that the primary fluids in cassiterite are not unlike the later fluids at Mount Bischoff (250-400°C, up to 7 wt % NaCl equiv), and that hotter (280-490°C), more saline (30-45 wt % NaCl equiv) fluids are also present but only in fractures. The following history of vein growth is proposed: The cassiterite-forming fluids, which like those at Mt Bischoff were boiling and C02-bearing, appear to have been trapped at pressures of 150500 bars at a likely depth of 4 km. The Aberfoyle veins grew in hydraulic fractures developed above a cupola during expansion of the crystals-melt-

1 Key Centre for Ore Deposit and Exploration Studies, Geology Department, University of Tasmania, GPO Box 252C, Hobart 7001, Australia

aqueous fluid complex on cooling following pluton emplacement. The fractures remained open while magmatic fluid escaped, and (partially) collapsed when the fluid reached the hydrostatic regime. Vein minerals were precipitated largely from the low salinity fluid, the only buoyant fluid during the collapse phase. Repeated opening/collapse cycles decrepitated earlier fluid inclusions (first observed by Ron Wilkins, CSIRO) and fractured earlier cassiterite. S^^Q^^ values (from quartz at known Th) suggest a magmatic fluid; similar results could be derived from groundwater but only at low water/rock values incompatible with significant quartz vein formation. 534S values for sulphides range from -3.3 to 3.6 %o. The two fluid-vapour types may have formed as a result of: (a) separation of the immiscible components of the magmatic "vapour" at high pressure during their rise toward the granite surface, and (b) subsequent unmixing of these components into second-order liquidvapour fluid systems, one Uquid component being highly saline and one relatively dilute. Although the more saline components can carry orders of magnitude more Sn than the dilute components, cassiterite appears to have been deposited largely from the latter. The processes and components outlined above were essentially magmatic, but the presence of biological marker hydrocarbons in the fluid inclusions at Aberfoyle (Hoffmann et al 1988) indicates that groundwater in the host sedimentary rocks was drawn into the veins. Indeed, the resulting dilution could be largely responsible for cassiterite deposition, other mechanisms not being particularly effective in this environment (Heinrich 1990). At Mount Carbine also, 87/86Sr and eNd data for apatites in the ore veins indicate the presence of a fluid previously equilibrated with the country rocks (Higgins et al 1987).

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

92 Applying these findings to Mount Bischoff could

margins have been attributed to groundwater

account for the two fluid types recorded by Halley

circulation through adjacent sedimentary rocks

and Walshe (in press), and repeated cycles account

(Taheri 1985). However, the scarcity of data for host

for post-pyrrhotite ore veins such as the Giblin and

rock S (e.g. only two reliable numbers in Proterozoic

North Valley lodes. Some groundwater involvement

rocks, Taheri 1985) has prevented quantification of

seems inevitable during escape of the low density

this process. Incorporation of local sedimentary S

fluid because the reactive carbonates lie 1 km or

is clearer in distal veins at Mt Carbine, Sundown

more from the source granites. Indeed, the total Sn

and above the Mole Granite where local S in

could be generated by groundwater circulation,

sedimentary rocks has negative 5 ^ S values.

provided this was mostly within the granite (the Sn source). However, generating the relatively saline fluids by convective circulation in the Renison and Mt Bischoff situations would appear difficult, and more importantly, wherever adjacent granites are exposed in Tasmania and worldwide, Sn and S n - W deposits are spatially and temporally related to s p e c i a l i s e d tin g r a n i t e s , i m p l y i n g a g e n e t i c connection far beyond the latter acting solely as heat

Conclusions Non-magmatic groundwater, of mixed origin, is probably common in the fluids responsible for forming extra-granitic, orogenic Sn-W deposits, and in more distal deposits is in part at least responsible for the addition of S to the ore-forming fluids.

sources. Detailed D / H isotpic studies might help

However, the spatial and temporal ties between the

in identifying the significance of groundwater in the

deposits and tin granites sensu stricto suggests that

Aberfoyle and Mt Bischoff situations.

the Sn and W contents of the deposits (and almost certainly other metal contents) are essentially of magmatic origin and that exsolution of aqueous

The sulphur problem

fluid from the cooling tin granite magma is the most potent driving force behind ore deposit formation.

The low solubility of S in felsic magmas (perhaps no more than 10 ppm, Poulson & Ohmoto 1990) is clearly a difficulty in generating deposits like Renison with 15 or more million tonnes S, unless the melts contain immiscible S. Some S in country rock may be dissolved directly by magmatic fluids passing from granite source to the site of deposition (e.g. at Aberfoyle and Renison) but introduction of S from host rocks by groundwater is likely, and supported by the apparent shifts in S^^S from magmatic S to local sedimentary values. Near-zero values in the Mt Bischoff dykes, in the Federation ores at Heemskirk, and in most East Tasmanian deposits appear to be magmatic signatures. The increase in 5 ^ S values from Mt Bischoff to Cleveland to Renison has for many years been taken to indicate an increased component of local S, and the very high values in those Heemskirk deposits close to granite

References Halley, S.W., 1982: BSc Honours thesis. University of Tasmania (unpubl.). Halley, S.W., 1987: PhD thesis, Australian National University (unpubl.). Halley, S.W. & Walshe, J.L., in press: Economic Geology. Heinrich, C.A., 1990: Economic Geology 85: 457-481. Higgins, N.C., Forsythe, D.L., Sun, S.S. & Andrew, A.S., 1987: Proc. Pacific Rim Congress 87:173-177. Hoffmann, C.F., Henley, R.W., Higgins, N.C., Solomon, M. & Summons, R.E., 1988: Chemical Geology 70: 287-299. Poulson, S. R. & Ohmoto, H., 1990: Chemical Geology 85: 57-75. Solomon, M. & Groves, D.I., 1994: Geology and Geophysics Monograph 24. Oxford University Press. Solomon, M., Jaireth, J., Higgins, N.C., Heinrich, C.A., Halley, S.W., Hellsten, K. & Etheridge, M.A., in prep. Taheri, J., 1985: PhD thesis. University of Tasmania (unpubl.).

The groundwater model for the formation of the cassiterite-sulphide deposits of western Tasmania John L. Walshe'

The cassiterite-sulphide deposits of western Tasmania have long been regarded as having formed from magmatic fluids emanating from highly fractionated alkali feldspar granites. A problem with this interpretation has been the origin of the large amounts of sulphur, occurring mostly as pyrrhotite, in these deposits. Reduced felsic melts have limited sulphur solubility and it is difficult to source the tens of millions of tonnes of sulphur in the Renison deposit from a reduced, fractionated felsic melt. Possible mechanisms which have been suggested are: 1. Incorporation of sulphur into the felsic magmas as an immiscible sulphide phase at a late stage by partial melting of the country rocks (Ohmoto, 1986). 2. Mixing of sulphur-deficient magmatic fluids with sulphur-rich groundwaters in the up-flow zone of the hydrothermal system above the granite and/or dissolution of rock-sulphur in the up-flow zone by a magmatic fluid (Solomon & Groves, 1994). 3. The ore fluid is essentially a sulphur-rich groundwater which leaches tin from the upper parts of the granite at a late stage in the hydrothermal system (Halley & Walshe, 1994). Recent studies of Nd isotope characteristics of latestage Sn granites and a reassessment of their S isotope characterishcs suggests a mantle component is present in these melts and that the magmatic sulphur isotope signature is close to zero (Sun & Wybom 1994; Walshe et al in press). It appears that these conclusions may hold for all of the Sn

Geology Dept, Australian National University

provinces in the Tasman Fold Belt System. If this is true then mechanism 1 is effectively eliminated. The range of sulphur isotope data observed in the Mount Bischoff and Cleveland deposits, in the Zeehan Field, and the Heemskirk Granite in western Tasmarua appears to reflect a mixture of a magmatic sulphur source, that has an isotopic composition around zero or slightly negative, and country rock sulphur source(s). Sulphur sources from the Precambrian appear to provide the very positive signatures that range up to +20 %o. At Renison sulphur isotope values fall in a narrow isotope range around +5 to +7%o in the Federal-Bassett Fault down to the granite and in the dolomite replacement deposits (Patterson et al 1981; Kitto et al 1994). While there is as yet no strong evidence from the Renison granite and environs for a magmatic sulphur isotope signature close to zero, the regional assessment is compelling. It suggests that the narrow sulphur isotope signature seen in the Renison deposit is not a magmatic signature but rather a well mixed signature from igneous and sedimentary sources with the sedimentary component being dominant to account for the amount of sulphur present in the deposit. Lack of evidence for fluid mixing in the Federal-Bassett Fault and mass balance constraints make incorporation of this country rock sulphur into the ore fluids in the upflow zones in the Federal-Bassett Fault (mechanism 2) an implausible process. Evaluation of mechanism 3, which is regarded as the most plausible explanation of the available data, provides a basis for future research and a little speculation. It is suggested that the petrological and geochemical study by Bajwah et al (in press) of the granite provides evidence for two granites, the approximate outlines of which may be recognized in the gravity data of Leaman and Richardson (1989). The older, and slightly more mafic phase

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

94 (here termed the Renison Granite), is the granite high to the northwest of Pine Hill and southwest of the Renison Mine. It was intruded by a more felsic phase (here termed the Pine Hill Granite) that lies along the prominent NNE-trending ridge of granite inferred from the gravity. The Pine Hill Granite sericitised and tourmalinized the Renison Granite along their mutual contact, generating the broad northeast-trending zone of alteration documented by Bajwah et al (in press). The tourmaline alteration appears to mark the magmatic stage of the hydrothermal system at the present level of exposure. The Federal-Bassett Fault lies along the northeastern margin of the Renison Granite (Kitto, pers. comm.) and appears to truncate the Pine Hill Granite. Early normal fault activity on this structure is related to the emplacement of the Renison and Pine Hill(?) Granites (Kitto 1994) and tourmaline alteration within the structure (Kitto, pers. comm.) suggests the presence of a magmatic fluid component at this time. The Federal-Bassett Fault acted as a wrench fault during the main stage of cassiterite-sulphide deposition (Kitto 1994). As it appears to truncate the Pine Hill Granite there may be a significant time break between the magmatic stages of the hydrothermal system and subsequent groundwater stages. A northwest-trending line of base metal sulphide deposits and prospects along the line of the Federal-Bassett Fault attests to the importance of the Federal-Bassett structure in controlling regional fluid flow during the main stage of deposit formation at Renison. (The Hercules deposit in the Mount Read Volcanics appears to fall on this line.) It is suggested that groundwater flow was essentially constrained within these northwest-southeasttrending structures and that the residual heat from the Pine Hill Granite drove fluid circulation. Magmatic volatiles emanating from depth in the Pine Hill Granite may have acidified the reduced groundwater and promoted leaching of the granite. First-order mass balance arguments suggest that the amounts of sulphur required to form the Renison deposit could readily be acquired in groundwater systems with aspect ratios around 2.5 to 1. The very constant isotope signature in the deposit is consistent with a large-scale homogenization process operating over a sustained period. In longdimension the patterns of fluid circulation could be

considered analogous to those obtained by Solomon et al. (1987) in their Hele-Shaw cell experiments of fluid flow around cooUng plutons. A strong up-flow zone occurs near the margin of the cooling pluton. The siting of the Renison deposit is consistent with such a zone on the margin of the Pine Hill Granite. The very large size of the Renison deposit is accounted for by a long-lived groundwater system maintained by the high heat flow from a fractionated granite and a stable regional stress regime, that followed the decay of the local stresses associated with granite emplacement. Acknowledgements I am indebted to Scott Halley and Paul Kitto for many stimulating conversations on the origins of the Sn deposits of western Tasmania. References

Bajwah, Z.U., White, A.J.R., Kwak, TA.P. & Price, R.C., 1994: The Renison Granite, westem Tasmania: a petrological, geochemical and fluid inclusion study of hydrothermal alteration. Economic Geology, in press. Halley, S. W. & Walshe, J.L., 1994: A re-examination of the Mount Bischoff cassiterite-sulphide skarn, Westem Tasmania. Economic Geology, in press. Kitto, PA., 1994: Structural controls on the hydrothermal paleoflow and metal zonation at the Renison Tin Mine, westem Tasmania, Australia. In the Footsteps of Solomon, Western Tasmanian Field Trip, Abstracts. University of Tasmania. Kitto, PA., Cooke, D.R. & Large, R.R., 1994: Mechanisms for cassiterite deposition at Renison, Western Tasmania. Geological Society of Australia Abstracts 37: 217-218. Leaman, D.E. & Richardson, R.G., 1989: The granites of west and northwest Tasmania. A geophysical interpretation. Tasmanian Department of Mines Bulletin 66. Ohmoto, H., 1986: Stable isotope geochemistry of ore deposits. In Stable Isotopes in high temperature geological processes. Reviews in Mineralogy 16. Patterson, D.J., Ohmoto, H. & Solomon, M., 1981: Geological setting and genesis of the cassiteritesulfide mineralization at Renison Bell, Western Tasmania. Economic Geology 76: 393-438. Sun, S. S. & Wyborn, 1994: Source character and magmatic processes in some Au, Cu and Sn provinces: A trace element and isotope approach. Geological Society of Australia Abstracts 37: 421. Solomon, M. & Groves, D.I., 1984: THE GEOLOGY AND ORIGINS OF AUSTRALIA'S MINERAL DEPOSFTS. Oxford University Press: 864 pp. Solomon, M., Walshe, J.L. & Eastoe C.J., 1987: Experiments on convection and their relevance to the genesis of massive sulphide deposits. Australian Journal of Earth Sciences 34: 311-323.

Structural and geochemical controls on metal zonation at the Renison tin mine, western Tasmania Paul Kitto^

Renison Tin Mine is located at Renison Bell, on the west coast of Tasmania. It is Australia's largest primary tin producer, with an identified mineral resource totalling 9.5 mt at 1.4 % Sn and an annual production in 1993 of 580,0001 at 1.6 % Sn (Thomas & Roberts 1994). Total Sn recovery since the commencement of large scale underground mining operations in the 1960s is over 115,000 t. Renison is hosted by subaerial to shallow marine. Late Precambrian to Early Cambrian dolomitic and clastic sediments of the Early Palaeozoic EHmdas Trough. The deposit occurs on the northeast limb of a broad, southeast-plunging Devonian anticline which constitutes a major fault-bounded horst. Major brittle structures associated with tin mineralization at Renison formed during the forceful emplacement of the Pine Hill Granite, include the Federal-Bassett Fault, Argent Fault, Blow Fault and a series of east-west trending interconnected Transverse Faults. The Pine Hill Granite forms a buried 'spine' that connects the Heemskirk and Granite Tor Batholiths. The Pine Hill granite is classified as ilmenite-series, and is reduced (Fe3+/Fe2+ + Fe^^ ratio of 0.14), peraluminous, and has corundum normative values between 0.8 and 1.5. Plots of major, trace and REE analyses of the unaltered Pine Hill Granite show well-developed fractionation trends indicating approximately 60 % Rayleigh fractionation during crystallization. Beneath Renison, an apophysis of late stage quartz-feldspar porphyry granite generated a high temperature boron and fluorine-rich fluid, which caused in-situ sericitization, albitization and tourmalinization.

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Australia 7001

Detailed study of kinematic indicators on mineralized faults has revealed four phases of brittle deformation (Devonian to Tertiary), based on style and relative ages of fault striations. Initial brittle deformation associated with the forceful emplacement of the Pine Hill Granite formed the FederalBassett Fault, with up to 700 m of normal-dextral dip-slip movement, which allowed magmatichydrothermal fluids access to the dolomitic host sequence. A high temperature oxide-silicate vein stage (qz-asp-cass) formed d u r i n g this fault movement. Fluid inclusions associated with this event have homogenization temperatures ranging from >400°C at the base of the fault (3000 m beneath the Devonian palaeosurface), to 300°C near the top of the mine workings. These early NaCl-KCl-H20 brines had average salinities between and 12 eq. wt. % NaCl, and fluid pressures of 250 bars (hydrostatic). fluid values of 9%o are clearly magmatic, consistent with a fluid which ascended and cooled with the Federal-Bassett Fault before interacting with the wallrocks in the higher mine levels. S^Sfl^jj values from the oxide-silicate stage are < 6%o and indicate the probable source of sulphur is magmatic. As the granite-related stress field decayed, a regional Taberraberran-related dextral wrench reactivated earlier fault structures, and produced a dilational jog in the Federal-Bassett Fault. This fault reactivation released a second generation of magmatic-hydrothermal fluids, that ascended within the Federal-Bassett Fault and infiltrated the overlying dolomite horizons. Main sulphide stage mineralization (pyrrhotite+cassiterite-quartzfluorite-stannite-chalcopyrite ± arsenopyrite and minor base metals) produced the stratabound carbonate replacement orebodies that characterise the Renison deposit. During this stage of mineralization, mineral deposition in the Federal-Bassett

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

96 Fault occurred over a temperature range from <350°C, immediately above the Pine Hill Granite, to ~200°C at the top of the mine workings. The deeplevel NaCl-KCl-H20-rich magmatic-hydrothermal brines evolved to CaCl2-MgCl2-NaCl-H20-rich fluids during fluid-rock reactions with carbonates in the upper mine levels. Salinities averaged between 8 and 12 eq. wt % NaCl throughout the sulphide stage. Contoured tin values and homogenization temperatures from fluid inclusions clearly outline two high temperature tin-rich dilational jogs on the Federal-Bassett Fault, as do variations in values. 5^45 values remained constant at throughout the sulphide stage, which is consistent with a homogeneous magmatic sulphur source. Minor uneconomic base metal veins (rhodochrosite-galena-sphalerite-quartz), associated with late stage fault reactivations, overprint the earlier vein stages, as do vug-fill carbonate-quartz veins (quartz-carbonate±fluorite± pyrite). The late stage veins were associated with low temperature (150° to 200°C), bimodal salinity (<2 and --10 eq. wt. % NaCl), NaCl-KCl-H20 brines that formed via mixing of contemporary meteoric groundwaters with magmatic-hydrothermal fluids. fluid values (~5%o) remained unchanged, indicating that magmatic fluid continued to supply sulphur to the Renison system over a protracted period. The only fluid inclusion evidence for phase separation at Renison occurs in these late stage veins. Fluid inclusion results from the oxide-silicate stage, in association with thermodynamic modelling, provide the following estimates for initial magmatic-hydrothermal fluids at Renison: -250 bars fluid pressure, -'350°C temperature, salinity -12 eq. wt. % NaCl, pH 3.8 to 5.4, I S = 0.05 molal, log /O2 between -32.0 and -33.8, log fH^S between -0.5 and -2.5, aH3As03 between 10-5 and

10-^ aNa+ - 0.1742, aK+ - 0.085, mMg2-^ between 1.6 X 10-5 and 6.2 x IO-2, mCa2+ between 7.96 x 10-3 and 12.61, mF- between 1.05 x 10-5 and 4.16 x 10-4, and Sn solubility = 20 ppm. Numerical simulations for this Renison-type oxide-silicate stage fluid predict that boiling, cooling, and mixing with pure water (25°C) are inefficient depositional mechanisms for precipitating cassiterite. In contrast, fluidrock interaction appears to be crucial for cassiterite deposition. Based on numerous simulations of fluidrock interaction, reaction with dolomite provides the closest approximation of the actual oxide-silicate, sulphide stage and carbonate replacement mineral assemblages. Sn transport in the Renison-type fluid was dominated by SnCl^- and Sn(OH)2Cl2 complexes at 350°C and logfO^ =33.5, allowing the hydrothermal fluid to carry = 20 ppm LSn. At lower temperatures, Sn(OH)2Cl2 complexes became dominant. The most effective mechanism for cassiterite deposition, as predicted by numerical modelling, was by redox and pH changes induced by arsenopyrite deposition and carbonate dissolution, respectively. In conclusion, distal skam deposits like Renison, associated with carbonate replacement mineralization are intimately related to shallow mesothermal granitoids. The highly fractionated and reduced ilmenite series granites acted as fertile sources for tin, which is transported by magmatichydrothermal fluids from volatile-rich (B, F, CI) apophyses in the roof of the intrusion. Thermal metamorphism and forceful granite emplacement assisted brittle deformation of the overlying host sequences allowing high temperature (300°-400°C) acidic ore fluids access to reactive carbonate hosts. Cassiterite deposition is controlled by fluid-rock interaction associated with redox changes induced by arsenopyrite deposition, and by increased pH due to carbonate dissolution.

'..la

The sources of sulfur in the cassiterite-sulfide deposits of western Tasmania Discussion session — chaired by Geoffrey Green

Chairman's Comment: Unfortunately the tape recorder did not work for the discussion part of the session, so the report of this part of the session relies on a sketchy memory plus a few personal enquiries. Malcolm Bendall made a comment on the apparent contradiction between the low temperatures implicit in some of the organic components in inclusion fluids, recorded in the latter stages of mineralisation at Renison Bell and elsewhere and in biomarkers in Early Palaeozoic rocks and the higher temperatures suggested by conodont colour indices in the Gordon Limestone. This statement failed to draw a response from any of the contributors. The Chairman queried John Walshe's conclusion that SD values measured by Patterson et al. (1981) on inclusion fluids obtained by crushing represented a better estimate of ore fluid SD values during main stage mineralisation than measurement of SD values of paragenetically constrained gangue minerals and calculating fluid hydrogen isotope composition knowing the temperature of mineralisation. Patterson employed a technique of crushing quartz samples under vacuum and measuring the SD values of extracted water which would have incorporated fluids of later generations including known fluids of meteoric water derivation. Other questions and comments were made by John Elliston, Ray Roberts, Scott Halley and others, but unfortunately my memory is too vague to attempt to record these accurately. Both presentations provided much more than the titles of the papers suggested and attempted to erect unified genetic models of tin mineralisation. John Walshes thesis represents an extension of work on Mount Bischoff carried out with Scott Halley, for which Scott received a PhD and which is in press in Economic Geology. Both Mike Solomons and John Walshes models accept (and invoke a need for) a nonmagmatic component in

the process of tin deposit formation. Where they differ is the role of this component in ore formation and the stage at which it performs this role. Mike Solomon's model suggests that the fluid responsible for tin mineralisation is exsolved from a crystallising, geochemically specialised granitic magma. This fluid separates into saline and less saline components, both of which can migrate away from the granite when fractures are open. However, when the fractures are partially closed, only the less saline, more buoyant fluid can escape and it is from this fluid that the bulk of the vein minerals, including cassiterite, are precipitated. The incursion of a low density fluid phase into the environment above the pluton produces a density gradient which drives groundwater into that regime and provides a very effective mechanism for tin mineralisation. This groundwater also provides the means by which country rock sulphur can be introduced into the ore forming environment accounting for the difference between the near zero S^^S values for sulphides forming at Mount Bischoff during the early skam stages of mineralisation and the slightly higher values associated with the later main pyrrhotite-cassiterite ore-forming event. At Mount Bischoff the pyrrhotite-rich replacement ore is cut by later veins containing highly saline fluids of magmatic derivation suggesting a continuity of process and the involvement of magmatic fluids in ore formation. John Walshe's model requires groundwater circulation into the granitic pluton, with derivation of ore components from both inside and outside the pluton. An implication of the ''magmatic'' ore forming fluid S^^O values is that the ore forming system must be rockdominated, but there appears to be no overwhelming reason why this should not be so on mass balance considerations. The source of sulphur in the deposits is a critical element in the "leaching" model. Western Tasmanian cassiterite-sulphide deposits typically display non-zero S^^S values, but for individual deposits these arefairly homogeneous. According to Walshe, this process

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

98 ofhomogenisation most likely occurred during large scale convective circulation of fluid involving both the country rocks and granite during ore formation. He suggests that the tight cluster of S^'^S values of5-7%o of sulphides from the Federal-Bassett Fault (FBF) precludes the incorporation of sulphur by mixing of the ore fluid with groundwater as the former ascended the structure. In a novel interpretation of the Renison deposit based on gravity data and the zonation ofhydrothermal alteration assemblages, he suggests that there were two separate phases of granite emplacement with the major phase of tin mineralisation postdating the latter. Paul Kitto relates the Renison mineralisation to fluids sourced from the Pine Hill Granite during and after dextral wrench movement on the FBF. His oxygen isotope work nicely demonstrates two main paths of fluid movement up the fault during the early, pre-main sulphide, stage of mineralisation. He describes a systematic increase in S^^S values from 5 to 7%o from the bottom to the top of the FBF which parallels a decrease in fluid inclusion temperatures from 300° to about 200X1 upwards (and away from the main foci of high temperature fluid input and tin mineralisation; Paul Kitto, pers. comm.). Interestingly, this increase in S^S values cannot be the product of cooling alone. At a constant value for fluid H2S a temperature decrease from 300° to 200X1 will only result in an increase of pyrite S^S values of

and a quarter of that amount for pyrrhotite (using the fractionation factors of Ohmoto & Rye (1979)), clearly inadequate to account for the observed distribution. Reservoir effects (Rayleigh fractionation) also cannot account for the increase and would drive fluid d^^S values lower during sulphide precipitation. This suggests that a combined process of assimilation of country rock sulphur by the granite melt prior to mineralisation producing an initial S^^S value of 57oo followed by incorporation of some heavy country rock sulphur from groundwater incorporated during fluid movement up the FBF is at least an equally valid interpretation of the data. 0.57%o,

In summary, the classical magmatic hydrothermal model of tin mineralisation appears to have survived the intense scrutiny of late twentieth century technology updated, but essentially intact. However, the challenging ideas proposed by John Walshe can be tested by applying other techniques such as Nd isotopes to the large tin mineralising systems of western Tasmania and open up exciting new possibilities for further research. Reference

Ohmoto, H. & Rye, R.O., 1979: Isotopes of sulfur and carbon. In Barnes, H.L. (Ed): GEOCHEMISTRY OF HYDROTHERMAL ORE DEPOSITS, 2nd Edition. Wiley-Interscience.

Genetic models for Cambrian volcanic-hosted massive sulphide deposits

100

101

Formation of the Rosebery and Hercules ore deposits, Tasmania by syntectonic mobilization of metals and wallrock replacement about structural traps Domingo G A M . Aerden^ The Rosebery and Hercules Pb-Zn-Ag-Au massive sulphide deposits of the Middle-Cambrian Mount Read Volcanics, western Tasmania, are hosted by tuffaceous slate lenses, surrounded by felsic volcanics. The volcanic belt and its Ordovician cover were deformed and metamorphosed during the Early to Middle Devonian Tabberabberan orogeny During this orogeny, D3 east-west shortening was partially accommodated by reverse beddingparallel shearing within the slate lenses and immediately underlying volcanic rocks. At Rosebery this led to asymmetric foliation-boudinage structures, developed both in the XY and XZ plane (chocolate tablet boudinage). At Hercules this resulted in the partial unfolding of previous F2 folds. The Rosebery mineralization is localized within necks and along extensional shear-planes of foliation-boudinage structures. At Hercules, a series of massive-sulphide pods align parallel to cleavage and are localized in the short-limb zones of parasitic F2 folds at different scales. These structural positions are attributed to localized dilation and fracturing, infiltration by metalliferous hydrothermal-fluids and progressively replacement of wall rock (Figures 1, 2).

Structural analysis of folds in bedding in the host rock, as well as direct observations, indicate a completely discordant position of both ore deposit (e.g. Figure 3). This is difficult to match with earlier interpretation of the deposits as tightly-folded exhalative sediments or subsurface replacement bodies. Conformable massive-sulphides can obtain discordant positions by intruding the host rock

^ Laboratoire de Tectonique et Geophysique, case postal 060, Universite de Montpellier II, Place E. Bataillon, 34095 Montpellier, France

during deformation, but such mechanical remobilization is commonly superimposed on a largescale conformable ore geometry. In fact, this is observed in the Rosebery Mine from the mm to m scale. With no traces of conformable ore, there is no evidence for a precursor orebody that intruded the host rock as a whole. Strain localization in the ore could have lead to an apparent discordance by juxtaposition of high and low-strain zones. However, in that case, bedding and cleavage immediately above and below ore lenses would be affected by throughgoing high-strain zones, which is not observed (Figure 2). Down-dip and lateral zoning-trends in wall rock, orebodies and within individual ore-lenses correlate remarkably well with the inferred sites of structurally controlled ore deposition. For example, at Rosebery dilational bends in shears associated with foliation-boudin-necks are marked by relatively Curich mineralization, whereas ore lens extremities contain most of the barite (Figure 1). In contrast, the mineral zoning is not related to a pre-cleavage stockwork or other potential syngenetic source structure. Mineralization occurred in a paragenetic replacement sequence (pyrite, quartz-chloritebarite, pyrite, sphalerite-galena-chalcopyrite), in which the early minerals show signs of early-D3 deformation (pressure solution features and D3 extension fibres in pyrite) and the later minerals (e.g. sphalerite-galena) are mostly undeformed. Replacement relationships were established by means of the following criteria in combination (Figure 4): (1) highly irregular contacts with scalloped ingressions of the late minerals truncating crystallographic and textural elements in the early minerals, (2) a texture of "enclaves" with matching crystallographic orientations, unrelated to the crystal structure of the host material and (3) dendritic

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

102 MINE LEVEL 15 N

high-grade Pb-Zn

-rSO-

figure 1: (a) 15th mine-level map of the Roseben/ orebody. (b) syntectonic replacement interpretation of the orebody. A Curich assemblage (solid) marks dilational areas and principle shear zones associated with cleavage-parallel extension and shearing, (c) Kinematic sketch illustrating the envisaged process offoliation-boudirmge controlled mineralization: Dilation on extensional cleavage ramps causes brecciation of the wall-rock. Fluid-access to the fractured zone results in progressive replacement along shear zones and cleavage. This is reflected by the mineral zoning. The ore geometry resembles a folded layer, but it in fact crosscuts folded bedding (dashed lines).

patterns of the replacing mineral into the host. The fact that replacement textures are widely preserved disfavours a syngenetic replacement origin (cf. Khin Zaw & Large 1992) as deformation gradually transforms mixed ores into an alternation of sheared and boudinaged, respectively incompetent and competent mineral components with smoothed contacts. Dynamic or static (annealing) recrystallization of deformed mineral assemblages potentially explains the absence of foliations within monomineralic grain aggregates (equant grain texture and growth twins in sphalerite), but not replacement relationships between different minerals. Recrystallization involves a rearrangement of grain boundaries into low energy configurations, but not redistribution of material or creation of replacement textures. The total extent of replacement textures implies progressive bulk compositional changes in the orebody that cannot be attributed to metamorphic recrystallization. Widely preserved growth textures in pyrite and carbonate further indicate that at least these minerals did not recrystallize. A transition exists from patchy sulphide replacement gradually becoming more massive until incorporating cleaved wallrock inclusions

(Figure 5). Wallrock cleavage is sharply crosscut by the irregular sulphide boundaries without its orientation being influenced. This passive truncation texture is independent of sulphide composition and rheology, whereas microstructures resulting from deformation would strongly depend on the deformational properties of the material involved. The detailed geometry of the ore-host rock contacts follows irregular networks controlled by cleavages and D3 fractures in the matrix further indicating that these textures resulted from replacement of cleaved wallrock (Figure 5). Slight relative rotations between S3 in adjacent inclusions can be attributed to a dynamic fracturing/dilation process controlling sulphide replacement. A syn-D3 timing of associated massive chlorite replacement is evidenced by sericite schist inclusions containing crenulated S2, whereas S2 is not present in the chlorite, except as inclusion trails. Syntectonic dissolution of primary sulphides and local redeposition by replacement has been shown to mask primary ore-host rock contacts in syngenetic deposits. Evidence for such a process are textures indicating synmetamorphic dissolution along cleavage-parallel ore contacts and redeposition (by fracture filling and/or wall rock

103

Figure 2 — (a) Interpretation of the Hercules orehody based on surface data and all available underground data from old mine plans, (b) Microscopic Hercules ore lens, discordant to bedding and localized in an F^ short limb. The mineralization sharply truncates bedding, S^ and S3. Its shape is controlled by the orientations of S^ and S3, (c) Dilation in short-limbs of F^ folds, induced during reverse D3 shearing, is proposed to have controlled sulphide replacement at Hercules. Stippled line outline's future ore zones.

Figure 3 — East-west underground cross-cut (16-mezzanine level; approx. 122mN, 325mE) in the Rosebery Mine showing folded bedding dipping lower and discordant to massive sulphide lenses. The orientation of fold axes or bedding-cleavage intersection are indicated.

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

104

•

1 Figure 4 — (a) Schematic illustration of the relationships between macroscopic ore lens geometry, wall rock cleavage, wallrock fragments in the ore, ore texture and mineral zoning. The ore is simplified to contain only two minerals, (b) Texture indicating replacement of quartz by a sphalerite-pyrite assemblage (black). Quartz grains with matching crystallographic orientations have the same ornament pattern and are interpreted as remnants of partially replaced single grains, (c) Replacement of metasomatic albite by sphalerite-pyrite (black) and probably quartz (stipple). Albite twin lamellae are truncated and can be followed between different ''islands". The geometry of the sulphide front suggests that replacement was guided along fractures and grain boundaries.

Opposite: Figure 5 — Different degrees of sulphide replacement controlled by cleavage-parallel extension. In (a), fracturing and boudinage of sericite schist layers between pyrite-bearing layers (coarse stippled) lead to sphalerite replacement from the boudin necks outward. In (b) only remnants of host rock are preserved in massive pyrite. S2 and S3 cleavages are truncated by pyrite. The irregular geometry of the pyrite front and alignment of wallrock fragments reflect replacement controlled by fracturing and guided along cleavage, (c) S3 in wallrock fragment, truncated by surrounding sphalerite and minor pyrite (length of photograph: 2.5 mm).

105

^ S

5mm

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

106 replacement) in localized areas of extension and fracturing. Such a segregation of the ore deposit into primary and remobilized components is absent in the studied ores (Figure 4). That is, no evidence for pre-Dg base-metal sulphides exists. The Rosebery ore lenses possess a compositional banding that roughly parallels their margins. Microstructural evidence indicates that individual bands formed successively by replacement of adjacent cleavage-parallel zones. Progressive D3 deformation at Rosebery involved heterogeneous cleavage-parallel extension, with microfracturing and selective replacement localized in the zones of maximum strain rate. Continuous shifts in the strain-rate distribution during paragenetic mineralization explains the formation of new bands progressively richer in late minerals (Figure 4b). Rare fold geometries in the banding can be attributed to mimicking of pre-existing folds in S2 and/or bedding. Published Pb-isotope data point towards a Cambrian source rock for the orebody. This suggests a metamorphic origin by regional-scale dissolution

of dispersed volcanogenic metals, focused solution transfer and concentrated redeposition in structural traps. Sulphur isotope values compare with those of both Devonian and Cambrian considered deposits, giving further support to ore genesis by regional synmetamorphic mobilization. For further reading Aerden, D.G.A.M., 1991: Foliation-boudinage control on the formation of the Rosebery Pb-Zn orebody, Tasmania. Journal of Structural Geology 13:759-775. Aerden, D.G.A.M., 1993: Formation of massive sulfide lenses by replacement of folds; theHercules PbZn Mine, Tasmania. Economic Geology 88:377-396. Aerden, D.G.A.M., 1994:. Microstructural timing of the Rosebery massive sulfides, Tasmania; evidence for a metamorphic origin through mobilization of disseminated base metals, journal of Metamorphic Geology 12: 505-522.

Reference

Khin Zaw & Large, R.R., 1992: The precious metal-rich South Hercules mineralization, westem Tasmania: A possible sub-seafloor replacement volcanichosted massive sulfide deposit. Economic Geology 87: 931-952.

107

Synvolcanic, subseafloor replacement model for Rosebery and other massive sulphide ores Rodney L. Allen^

I interpret the Rosebery and Hercules massive sulphide deposits as synvolcanic, submarine, hydrothermal ore deposits, in which at least part and possibly the major part of the ores replaced permeable strata up to 200 m below the sea floor. The ore deposits formed within the proximal (near vent) region of a marine silicic caldera volcano, directly after a major caldera forming eruption. Subsurface replacement dominated over exhalation due to the extremely porous permeable character and glass-rich composition of the footwall and host strata. This interpretation for the Rosebery region is based on detailed graphic logging of 20 000 m of drill core from 60 drill holes, the construction of five cross sections through the Rosebery deposit, construction of stratigraphic correlation diagrams for the region, and mapping. I have also considered, reassessed, and incorporated information from many other geologists who have worked in the area. However, they cannot be blamed for my interpretation! The stratigraphy at Rosebery from base to top comprises: (1) the Rosebery-Hercules Footwall Volcanics, which are capped by a discontinuous upper member termed the Transitional Stratified Volcaniclastics (TSV), (2) the Rosebery-Hercules Hangingwall Volcaniclastics, and (3) the Mount Black Volcanics. The Rosebery-Hercules Footwall Volcanics and Mount Black Volcanics are thick, poorly stratified rhyolitic-dacitic sequences mainly of pumiceous subaqueous mass flow units, intruded by sills. However, the TSV and Rosebery-Hercules Hangingwall Volcaniclastics are well stratified 1 Volcanic Resources, Tastagata 43, Stavanger 4007, Norway

subaqueous mass flow and suspension sediment sequences. These well stratified sequences have with the aid of graphic logging been subdivided into individual emplacement units (beds) ranging from 10 cm to 100 m in thickness. Several beds can be correlated for kilometres throughout the Rosebery mine and beyond, and have been used to define the structure. The continuity of stratigraphy indicates that post-depositional tectonic deformation is localized and has not destroyed stratigraphic relationships. The Rosebery and Hercules ore deposits occur within the lowest of three main TSV units and in the top of the underlying massive footwall. Diagnostic evidence for synvolcanic origin

(1) Mineralization is closely related to zoned alteration commonly comprising a silicified mineralized core zone overlain, underlain or surrounded by a manganiferous carbonatesericite marginal zone. Within the silicified core zone and in some carbonate nodules, delicate pumice textures are superbly preserved. The pumice has been protected from subsequent deformation by the competant nature of the silicified or carbonate altered rock. This indicates that silicification, carbonate alteration and mineralization occured prior to sedimentary diagenetic compaction and subsequent cleavage development. Outside the silicified zones and carbonate nodules, in the incompetant phyllosilicate-rich parts of the carbonate-sericite zone and other areas, pumice has been compacted parallel to bedding and then strongly crenulated and transposed by the regional cleavage. This data indicates that alteration and mineralization pre-date diagenetic compaction and subsequent tectonic deformation. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

108 Therefore post-volcanic deformation is not relevant to the fundamental location of the Rosebery ore deposit, and at most is responsible for dislocation and local concentration of an already existing ore deposit. (2) Massive sulphide clasts (pyrite, sphalerite mainly) occur within the lowest thick mass flow unit of both the Rosebery-Hercules Hangingwall Volcaniclastics and the White Spur Formation, and both of these mass flow units can be shown to have eroded down into the Rosebery-Hercules mineralized stratigraphic interval. Massive sulphide clasts occur at Bastyan Dam, West Hercules, White Spur, and probable sulphide clasts occur in the GIO pit at Hercules. These sulphide clasts cannot be proven beyond all doubt to come from the Rosebery and Herciiles ore deposits, but that conclusion is most likely.

Evidence most consistent with synvolcanic origin (3) The largest ore deposits, Rosebery and Hercules, and the massive sulphide prospect at Rosebery lodes, all occur in the same stratigraphic position and volcanic setting: in a proximal volcanic facies association at the top of a major felsic volcanic eruptive cycle, and within syndepositionally downfaulted structural blocks filled with thick hangingwall successions. This volcanotectonic setting is similar to the setting of other volcanic-associated massive sulphide deposits in less deformed terranes> and indicates a regional and local volcanic and stratigraphic control on the location of the ore deposits. (4) At the north end of the Rosebery mine, the two main ore lenses, B lens and K lens, both contain mineralization within the lowest TSV subunit and one or more levels within the top of the footwall pumice unit. Ore grade mineralization does not occur above the lowest TSV subunit. The apparent discordance of the ore zone to stratigraphy (progressive down dip divergence of the ore zone toward the footwall and away from the hangingwall units) is due to thickening and downward rotation of stratigraphy in a syn-depositional half graben. The actual stratigraphic position of the ore does not change greatly down dip.

Evidence for subseafloor replacement (1) Numerous examples occur of sulphide and barite mineralization impregnating and replacing TSV crystal-rich sandstones and footwall pumiceous mass flow rocks. The instantaneous mass flow emplacement process of most of the host units to mineralization precludes synchronous deposition of host rocks and exhalation of mineralization as an alternative explanation. (2) The position and style of mineralization is locally influenced by a large rhyolitic sill within the TSV at the north end of the Rosebery mine. Where the sill is more than 2 m thick, primary mineralization occurs directly below the sill. Where the sill is thin or absent, the main mineralization occurs higher in the stratigraphy within the lowest TSV subunit. The sill is interpreted to have intruded just below the sea floor, prior to mineralization, and to have ponded mineralization below it. Locally sphalerite-galena rich massive sulphide ore has been mechanically remobilized up fractures in the sill during later deformation. This remobilized mineralization has a different texture to the primary mineralization. (3) Spotty and blebby ore textures are common at Hercules and parts of Rosebery. These sulphide spots occur within tuffaceous pumiceous rocks and are interpreted as replacement spots that grew at scattered nucleation sites (commonly the feldspar phenocrysts) within the pumiceous rock. The occurrences of massive sulphide clasts in hangingwall units suggest that replacement occured sufficiently close to the sea floor that the first major mass flow unit of the hangingwall could erode down into the top of the mineralization. Other massive sulphide deposits with similar evidence for synvolcanic replacement of pumiceous strata include Tulsequah Chief (British Columbia), Renstrom (Sweden), LSngsele (Sweden), and Hanaoka-Shakanai Qapan).

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The magmatic connection for VHMS deposits Ross R. Large\ Mark Doyle\ David Cooke^ and OIlie Raymond^

There has been considerable debate on the role of granitic magmas during the generation of volcanic hosted massive sulphide deposits; are they simply heat engines driving seawater (e.g. Ohmoto & Rye 1974; Solomon 1976) or do they directly supply magmatic components to ore-forming solutions (e.g. Henley & Thomley 1979; Stanton 1985)? Pioneering research by Solomon and his students in the Mount Read Volcanics (e.g. Solomon 1976, 1981; Polya et al 1986; Eastoe et al 1987) clearly demonstrated a relationship between hydrothermal alteration and sulphur isotope zonation around the granites, indicating that the granites acted as heaters for the ore-forming convective fluid. In this paper we provide evidence to suggest that the Cambrian granites may have also provided important metal contributions to the ore-forming fluid, especially Fe, Cu,Au,P,F±Tiand Zr. The following factors indicate a relationship between Cambrian granites and VHMS mineralization in the Mount Read Volcanics. Distribution: Two narrow bodies of Cambrian granite (Murchison Granite and Darwin Granite) intrude the eastern margin of the Central Volcanic Complex (CVC) in the Mt Read Volcanics. Interpretations based on magnetic and gravity data indicate that the two granite bodies form a semicontinuous narrow vertical sheet of granite 65 km long and about 2 km wide. A series of copper-gold and basemetal prospects occur along the margins of the granite sheet (e.g. Prince Darwin, Jukes Pty., Lake Selina). The Mount Lyell Cu-Au VHMS

^ Key Centre for Ore Deposit and Exploration Studies, Geology Department, University of Tasmania, GPO Box 252C, Hobart 7001, Australia 2 AGSO, GPO Box 378, Canberra, ACT 2601, Australia

deposits are located immediately west of the projected continuation of the subsurface granite. Timing: Previous mapping by Corbett (1989) suggested that the Murchison granite intruded the Tyndall Gp volcanics (which unconformably overlie the CVC) and is therefore younger than the VHMS deposits. However, later work (e.g. Corbett 1992) has revised this interpretation, and recent dating by Perkins and Walshe (1993) has confirmed that the Murchison granite has an age of 501 ± 5.7 (Ar/Ar), the same age as the host rocks to the massive sulphide deposits. Composition: Both the Murchison and Darwin grarutes are high-K, magnetite series granites which show anomalous enrichment in barium and potassium. The Murchison granite varies in composition from granodiorite to granite (58- 78% Si02. Abbott, 1992), while the Darwin granite is composed of two highly fractionated granite phases (74-78% Si02. Jones 1993). K2O varies up to 8.5% and Ba up to SOOOppm; however, some of this enrichment is related to alteration. Alteration: Well developed zones of hydrothermal alteration have been mapped around the margins of the granites (e.g. Polya et al 1986; Eastoe et al 1987; Hunns 1987; Doyle 1990). An extensive zone (Z^) of pink K-feldspar alteration extends from the outer part of the granites into the surrounding volcanics. An overlapping shell (Z^) of chlorite ± pyrite ± magnetite alteration overprints and extends outwards from the K-feldspar zone. Sericite-chlorite ± pyrite forms a distal alteration zone (Z3). At both Jukes Pty and Lake Selina, Cu ± Au mineralization occurs in the chlorite ± pyrite ± magnetite zone (Z^). Magnetite-apatite association: The strongest link between the granites and VHMS Cu-Au mineralization is provided by the common occurrence of magnetite-apatite-Cu ± Au vein style and disseminated mineralization both within the Z2 alteration Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

110

Comstock Rosebery Hellyer

to15%o

seafloor sulphides

pyrite, no magnetite

Figure 1 — Model relating granites, mineralisation and S-isotope distribution in the Mount Read Volcanic belt.

halo of the granites and within the centre of the Prince Lyell ore deposit in the Mount Lyell VHMS district. A good linear correlation exists between Cu and P2O5 and Fe and P2O5 both within the mineralized alteration halo of the granites and in the Prince Lyell ores. Oxygen isotopes indicate that the magnetite veins within the granite halo and the Prince Lyell deposit have b^^O values that are consistent with a magma tic source (Doyle 1990; Raymond 1993). Apatite, which is commonly intergrown with magnetite, pyrite and chalcopyrite, has consistently high F/Cl ratios, with a mean of about 6 wt% F.

Relationship of Cu-Au to Pb-Zn-Cu VHMS deposits The Mount Lyell field contains both stringer-style copper-gold deposits such as Prince Lyell and separate stratiform lead-zinc-copper deposits such as Comstock and Tasman & Crown Lyell Extended. Most previous workers (e.g. Solomon 1976; Walshe & Solomon 1981) consider that the Cu-Au and Pb-

Zn-Cu deposits formed as part of the same hydrothermal system; the Cu-Au stringer-style forming by subsurface replacement and the Pb-ZnCu massive sulphides by contemporaneous seafloor exhalation. Although our work suggests a source for Cu and Au from the Cambrian granites, the source for Pb, Zn, Ag and S remains unresolved and may be either magmatic or related to seawater leaching.

Conclusions Cambrian granites in the Mt Read Volcanics form a thin linear discontinuous sheet 65 km long which is spatially related to Cu-Au mineralization, including the VHMS deposits at Mount Lyell. The highly fractionated, oxidized, magnetite series granites have overlapping alteration shells of Kfeldspar, chlorite-magnetite and sericite. Preliminary evidence suggests that the VHMS copper-gold mineralization at Mount Lyell may be associated with fluids enriched in Fe-Cu-Au-P205-F-Zr-Ti released directly from the granite magma.

111 References

Abbott, RD.B., 1992: Geology of a barite-galena occurrence exposed in the Anthony Power Development Tunnel, western Tasmania. BSc Honous thesis. University of Tasmania (unpubL): 62 pp. Corbett, K.D., 1989: Stratigraphy, palaeogeography and geochemistry of the Mount Read Volcanics. In Burrett, C.R & Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Geological Society of Australia Special Publication 15: 86-119. Corbett, K.D., 1992: Stratigraphic-volcanic setting of massive sulfide deposits in the Cambrian Mount Read Volcanics, Tasmania. Economic Geology 87: 564-586. Doyle, M.G., 1990: The geology of the Jukes Proprietary prospect, Mt. Read Volcanics. BSc Honours thesis. University of Tasmania (unpubL): 114 pp. Eastoe, CJ., Solomon, M. & Walshe, J.L., 1987: Districtscale alteration associated with massive sulfide deposits in the Mount Read Volcanics, western Tasmania. Economic Geology 82:1239-1258. Henley, R.W. & Thomley, P., 1979: Some geothermal aspects of polymetallic massive sulfide formation. Economic Geology 74: 1600-1612. Hunns, S.R., 1987: Geology and geochemistry of the Lake Selina prospect, western Tasmania. Masters Qualifying thesis. University of Tasmania (unpubL). Jones, A.T., 1993: The geology, geochemistry and structure of the Mount Darwin-South Darwin Peak Area, western Tasmania. BSc Honours Thesis, University of Tasmania (unpubL): 120 pp. Ohmoto, H. & Rye, R.P, 1974: Hydrogen and oxygen isotopic compositions of fluid inclusions in the Kuroko deposits, Japan. Economic Geology 69:947953.

Perkins, C. & Walshe, J.L., 1993: Geochronology of the Mount Read Volcanics, Tasmania, Australia. Economic Geology 88: 1176-1197. Polya, D.A., Solomon, M., Eastoe, C.J. & Walshe, J.L., 1986: The Murchison Gorge, Tasmania — a possible cross-section through a Cambrian massive sulfide system. Economic Geology 76:1341-1355. Raymond, O.L., 1993: Geology and mineralisation of the Southern Prince Lyell Deeps, Queenstown, Tasmania. MSc thesis. University of Tasmania (unpubL): 160 pp. Solomon, M., 1976: "Volcanic" massive sulphide deposits and their host rocks — a review and an explanation. In Wolf, K.A. (Ed.): HANDBOOK OF STRATABOUND AND STRATIFORM ORE DEPOSTTS, IL Regional studies and specific deposits. Elsevier, Amsterdam: 21-50. Solomon, M., 1981: An introduction to the geology and metallic mineral resources of Tasmania. Economic Geology 76: 194-208. Stanton, R.L., 1985: Stratiform ores and geological processes: Royal Society of New South Wales 118, 77-100. Walshe, J.K. & Solomon, M., 1981. An investigation into the environment of formation of the volcanichosted Mount Lyell copper deposits using geology, mineralogy, stable isotopes, and a sixcomponent chlorite solid solution model. Economic Geology 76, 246-284.

Acknowledgements: Permission to use geochemical data for Cambrian granites from the Mineral Resources Tasmania data-base, and from Steve Hunns and Andrew Jones for unpublished analyses on the Lake Selina prospect and Mount Darwin area, is gratefully acknowledged.

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Genetic models for Cambrian volcanic-hosted massive sulphide deposits Discussion session — chaired by Fergus Fitzgerald Transcript by Bianca Manzi and Paul Kitto Paul Basford (PB) to Ross Large (RRL): How can you be sure that the magnetic signature, ranging from south to north is all the Cambrian granites, and how do you know that the Cambrian granites are all magnetic? If you've got magnetic veining, a mass populous of magnetic veining, can't you say that this feature is causing the magnetic anomaly, and not the granite beneath? RRL: Sure, the magnetite veining is contributing to the magnetic anomalies, and it's contributing in a very big way. However, the susceptibility data that Dave Leaman and Bob Richardson have collected over many years indicates that when you pick the least altered granites away from magnetite veins, they have the characteristic susceptibility of magnetite series granites. Now, the Darwin granite has a very consistent geochemistry. It's quite an evolved granite with high silica contents, whereas the Murchison granite has quite a range in silica content. There is obviously a lot ofzonation within the Murchison Granite. I would expect that there would be a change in magnetic susceptibility through our granites, but the data we have still indicates that they are magnetite series, high magnetic susceptibility compared with the Devonian granites. Ron Berry (RFB) to Domingo Aerden (DA): Domingo, given that the stretching lineation at Rosebery has always been recognised as being directed down-dip, why you should have a combination boudinage parallel to that? DA: You do have that, but... RFB (interjects): Your main concentration of mineralisation stratigraphically, the one you showed there? DA: ... inaudible ... stretching in two directions ... RFB (interjects): But there's no stretching in the other direction! DA: Yes, there is. The first direction, the dominant direction is down dip so that the overall strain ellipse has ... inaudible ... geometry.

RFB to DA: My next question is why is it that the propagation lineation ... inaudible ... horizontal? DA: Because you have two axes (refers to slide), one down here and two, I expect comes from somewhere down there, and move into the vertical of the downward axis, replacing most of the net direction. ANON: Have you got some data from these horizontal ones? DA: Yes. I showed you the ... Ron Berry (interjects): While we're on that, why is the host sequence at its thickest point, where that combination boudinage is occurring. Just where you want this "thing" to be thin, dramatically in order to have your foliation boudinage, is actually where the host sequence reaches its maximum thicknesses. DA: That's assuming the host sequence is a perfectly straight layer. We are not ... I would first like to see a schematic representation of how bedding actually runs in the area. Khin Zaw (CODES) to Rod Allen (RA): We've documented a sub-seafloor origin for South Hercules in 1992, but concerning Rosebery, I have a problem with a sub-seafloor deposition, particularly the barite-rich lens, that chalcopyrite-pyrite-rich zone and the Pb-Zn zone at Rosebery. H lens (the barite-rich lens), that might be exhalative part of the system as well. RA: So what's you're problem? Khin Zaw: Well, sub-seafloor ... RA: Why do you have a problem? FF to Geoff Green (GG): You want to comment on that? GG: Yes. A couple of points about that. The mass-flow deposits with the sulphide clasts, you have to, its where sulphur and lead isotopes are very useful actually, because you can use these as techniques to trace them to the source, and at least turbidity currents, as I know, the one on White Spur road pyrite-clasts there cannot come from Rosebery or Hercules and the ones on the south shore of

113 Lake Rosebery near Bastyan dam, are very unlikely to have come from Rosebery. The challenge is to find their source, if they haven't been eroded away.

DA: Couldn't you explain the ... Like if you look at all the deposits in the Mount Reads for example, you have a big range of lead isotope values. Not by two groups a

Anon (3) to GG: Is it true that all these ore deposits have totally homogenous isotope values ? GG: No they don't. Rosebery has a range and in fact that's very good evidence that the accumulation of the Rosebery deposit was diachronous. I don't have a problem with some of that ore being sub-seafloor replacement but I do have a problem, as Ross does, with it all being subseafloor replacement and in fact, according to crosssection evidence its sitting on a perfect place for direct exhalation onto the seafloor at a major lithological contact within the sequence. Can you comment on that? DA: Well, going back to what Khin Zaw said before. I myself haven't actually studied in detail H-lens, but I've studied the fringes ofD-lens. I know you can argue that they're not the same part of the stratigraphy, but anyway the fringe ofD-lens is very prolific. It clearly grades out from impregnation mineralisation and massively replaced units. I would say that barite mineralisation doesn't need to be exhalative. But, H-lens could be, I haven't looked at it, I mean I haven't looked at everything to be 100% sure.

Cambrian and a Devonian one but all deposits show a diverse mixture. RRL: There is a trend but that trend is related to the uranium-lead ratio in the source areas and because we've got the tin ore-bodies close by that have a typical Devonian signature and then the Cambrian ores have a typical Cambrian signature, and they're sitting virtually sideby-side which most of them are its very hard to argue from your point of you. DA: I can always see lead isotopes are in the first place, the lead is too young for the ore deposits, so you are never sure which came first. So this is a problem, I don't know! Aung Pwa (AP) to RA: I'm studying the geology of the Rosebery mine area, I'm studying zonation and alteration geochemistry. I already published my results in the 1992 Tasmanian—an island of potential volume. What I found in the Rosebery area there was a distinct geochemical zonality associated with the Rosebery deposit. The zonality sequences from top to bottom antimony, silver, then there's barium, lead, zinc and copper. This kind of zonality is generally similar to the epigenetic deposits such as gold or any vein-type

But, its all replacement, that wasn't really my point. My point was to open everyone's eyes to the fact that it does exist. I don't really care if its not all sub-seafloor replacement, but I think in some ore deposits, not all, in some ore deposits it can be all sub-seafloor replacement. Roger Poltock (RP) to DA: When do you see the metals being introduced in the Rosebery system? DA: The lead ages indicate the lead is derived from the volcanics. So I am arguing that during the Devonian, magmatic and/or metamorphic fluids have derived the lead from the volcanics and have transported this material and redeposited it in a structural trap. The question is still open if this material was disseminated in a large area or if there was a precursor ore-body not very far away that was remobilised and redeposited.

deposits because the zonality totally depends on the chemical properties of an area, and doesn't depend on the type of deposits. This indicates there was an upwelling movement of ore solutions during the mineralisation. You cannot have such a zonality in an area, the zonality what I study is the in the rocks not the ore body, the geochemical zonality in the rocks. RA: Do you have a question? AP: Just a comment. ...One question, how can you tell such kind of zonality, if you have this kind of upper movement of ore solution. How is it plausible to get such zonality? RA: I think yourfirst point is correct. You can't interpret the ore zoning in various genetic models, and it is one reason why I didn't discuss it. I don't think it is diagnostic of the ore bodies, or that the zonation documents the path of fluid, chemical changes and so on. That could be

RRL: The lead isotopes indicate, ifyou're going to have that model, that there really had to be a precursor ore there, because if there wasn't , if the lead was just disseminated at very low levels, it would have fractionated and therefore would then give a Devonian age instead of a Cambrian age. Because it has a Cambrian signature it means there must have been a lot lead in the area, cause the fluids then moved to a deposit. There doesn't have to be ore grades of lead but there has to be a hell-of-a lot of disseminated lead.

occurring in a seafloor system, or sub-seafloor replacement or even may be tectonic. The fluid still has to come from somewhere and go somewhere, DA: It could go up or along stratigraphy, AP : That's OK. if you are dealing with stratigraphic zonality, but this is not spherical zonality, this is along the dip direction, upward movement! FF: Yes. What Rod's saying that can also move laterally and have the same effect within the stratigraphy. I think we might have some more questions.

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114 ANON to DA: Domingo, obviously from your sketches and photomicrographs the rocks are quite strongly deformed, I just wondered, obviously the deformation is heterogeneously developed through the sequence. Have you preferentially sampled the lower part or, how representative are your rocks? DA: Of course I cant prove this but I've worked from almost all the core logs, except the recent discovery ones. I tried to show the typical character of the massive sulfide ore. I showed several examples. ANON: So is there an increase in deformation around —just associated with the ore bodies and as you go away you start to see the volcanic textures that Rod described? DA: I haven't really concentrated on the primary volcanic textures. The structural maps in particular have mapped out the fine detail, and also the shear zone which can come to mind—that's one of the problems. The shear zones are steeper than the ore lenses, which causes repetition. John Foden (JF) to DA: Domingo, do you also regard the barium as being transported along with the lead and the zinc during your replacement process and if so, do you regard it as odd that it seems to be concentrated stratigraphically high in the sequence? DA: It's not really stratigraphically high, its at the termination point, particularly towards higher levels (higher mine levels), and also laterally towards the extremities. In the literature Ifound examples of the faultbreccia related main faults in which identification of zoning has been observed. Also copper-rich in the lower part, zinc-rich in the centre part and a barite-rich top. JF: So you do have the barium coming in as well. RRL: Can I comment on that? The amount of barite processed is not insignificant, there's as much barite (barium) in the orebody as there is Cu-Pb-Zn. In fact there's more barium than Cu-Pb-Zn. The critical thing about getting barite deposition is you need sulphate, and the best way to get sulphate is out ofseawater. Therefore that connection of barium and sulphate, I think is critical and I would argue that to get beautifully laminated massive barite like we see at Rosebery, I think you've virtually got to be in a seafloor position. Geoff Green (GG) to DA: I think the fact that you've got Cu lower in the system, Pb-Zn in the middle, and barite towards the top of the system is quite consistent in the grossest sense, with an east facing sequence and the reverse dip-slip movement on the major faults within the unit. Any comment on that?

DA: Tell me why it was so obvious? What relationship do you see between the thrusting and the zonation? GG: The units towards the east have been moved up dip the furthest, therefore barite is going to be, if it was initially furthest to the east, its going to be in its present position at the highest elevation. And, Cu which was initially furthest west will be relatively low, topographically after that sort of movement. DA: You have to first assume that you know how the pre-deformation zonation was developed. It could be that the reconstruction has pieced the Cu and barite the wrong way around. Also in the barite rich lens, there is a Curich bottom and a barite-rich top, and if you want to thrust, you have this thrust repetition, you would expect the top of one ore lens would connect with the bottom of the next ore lens. You don't see that, if you don't continue with barite at the next lens, you start again with Curich... RFB: I just want to comment on that. Zoning is nowhere near as simple as that single section. If you start looking around the mine you can find Cu in the middle of lenses and the zoning is certainly not always Cu at the base and barite at the top. There is tremendous complexity, I don't think any "simple-minded" single-source is ever going to explain the complex distribution of minerals in the mine. So, you might well find your simple section is going to require a much more complicated model. DA: ....(inaudible)... section? RFB: The one which shows zoning and that's the one from north to about SOON where you get Cu-replaced. If you go to F-lens, you'll see that same sort of pattern and I think you'll find the B-lens can't be explained by that simple pattern. They are much more complicated metal distributions than that. DA: Surely the complexity of the ore lenses are related to foliation ... inaudible ... RA: I think I showed a rather fantastic slide on the right side at one stage of blocks of host rock within the ore. The rocks had cleavage truncated by the ore, and in some of the recent drillcore, tectonically remobilised pods of ore have been found, including one which occurs within the massive ore as well, and they are texturally quite different from the strongly deformed ore. They are massive, siderite-rich, pyrite poor and they have clasts of host rock in them. I think there is tectonically remobilised ore that is texturally distinct to the earlier ore it is derived from. The other problem I have is that in any tectonic boudinage model, I never quite got to the point of understanding how you end up with a such a pervasive alteration and replacement as at Rosebery. Why don't do get extensional veins for instance, why don't you get veining? There's almost no veins at Rosebery.

115 DA: That's because you normally get them with more brittle deformation ? When you more ductile deformation you can have microfracturing actually concentrated in a cleavage-parallel lens — that's why mineralisation started to pinch. Of the advanced stages of brittle deformation, I expect veins and they actually occur. There's local remobilisation of the ore associated with those veins. Maybe there will always be extension, and (of course) it's ductile deformation, and the fracturing is concentrated in these parallel layers. Garry Davidson (GD) to RRL: I wanted to ask you about the enrichment in potassium in the Hellyer fluid inclusions. We know that when we see seawater altering volcanics it generally results in albitisation so there's obviously a lot of sodium being transferredfrom the seawater and presumably there's still a lot of conservative elements K and Ca retained in the seawater and that trend that you showed could just as well have been caused by evaporation during boiling of that Ca and K. RRL: I agree there are many ways you can probably explain those high K-inclusions. I guess I included that cause its new data, its quite interesting and it could well support the model, but I agree that you could argue another direction. We need more data and I believe Khin Zaw is going to be collecting it, which will contribute to the story.

you would expect at a contact between massive epiclastics and ore. DA: I don't think you could prove it was an erosional contact consequently ... Angela Lorigan (AL): Do you have a contact somewhere, not with the ore and the epiclastics but with the host rocks and the epiclastics which appear — where the black slates are missing, which appear not to have faults in them. DA: I'm talking about the contact between

the

hangingwall epiclastics and the orebody. MQ: It can be seen at the base of the epiclastics you have rip-up clasts of black slate directly above the black slate so that does indicate that's its eroding black slate at that point. FF: I think we might close at this time, I know that this debating could go on forever, certainly between the geologists who know the contacts of the outcrops underground. I know people want to move onto other things.

Mike Quayle (MQ) to DA: I think the most convincing piece of evidence for synsedimentary deposition for the Rosebery-Hercules ore is the sulphide clumps. How do you account for those? DA: If I advocate the model for Rosebery and Hercules, then I think the material has to be derived from the volcanics, so I believe there has also been a case of Cambrian sulphideformation and probably the formation of ore deposits. I'm not totally convinced that those things are clasts, they can also be other things. If they are clasts they could be derived from ore deposits in the MRV. I find it a bit strange that the clasts occur directly above the orebody could be derived from that orebody. MQ: It can be seen that the epiclastic that contains those clasts has eroded into the orebody, that can be clearly seen at Rosebery.

The chairman of each session is asked to give a group wrap-up at the end of the question and answer times and they all managed to duck it. Probably because its too hard to succinctly wrap up the diverse range of ideas and comments that have been made so I don't intend to break that tradition. However, I would like to conclude this session with a persorml observation. I've been underground at Rosebery many times with different geologists with quite a spectrum of geological models that they're pushing and we can always find a piece of evidence to support their particular model. As an explorer in the MRV I'm only to well aware of the danger of trying to use specific characteristics of a deposit to try and find the next one. I think the classic example of this was the early work at Hellyer, following the discovery drill intersection. At that time everybody knew that massive sulphides were narrow, tabular, steeply dipping orebodies. I think it was sometime after hole number 10 or 11 that the project geologist rather tentatively suggested to senior management that Hellyer was perhaps a flat dipping cigar shaped orebody. He actually 'boldly' suggested that the company allow him a couple of drill holes to test this 'heresy'. The Aberfoyle management , they were still sceptical but fortunately they weren't like the clergy of old, Doug was, of course, not burnt at the stake and the rest is history.

DA: I've seen the contact between the orebody and the hanging wall and its always very sheared and faulted. I would very much doubt if you can recognise an erosional contact. MQ: No. It is always faulted, but that is exactly what

There are two things I am quite certain of in this uncertain world. First is that the next orebody discovered in the Mount Read Volcanics will be different to the ones that have gone before. The second is that the controversy about the origin of these ore deposits will continue.

Khin Zaw: It would take me half an hour to explain that one!

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Student posters

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Mineral paragenesis, fluid inclusion and oxygen isotope studies of the magnetite-scheelite skarn deposit, Kara, northwestern Tasmania Blackwell Singoyi^ and Khin Zaw^

The Kara magnetite-scheelite deposit (latitude 41°18' Stage I — Gamet-clinopyroxene ± wollastonite ± S and longitude 145° 48' E) is situated at the southern quartz. end of the Devonian Housetop Granite in northStage II — Magnetite-vesuvianite ± g a r n e t ! western Tasmania. Total mineable reserves include scheelite ± pyrite ± chalcopyrite. 1.7 mt magnetite @ >30% Fe and 0.3 mt scheelite @ Stage in — Epidote-quartz ± fluorite ± sphene ± 0.52 % WO3 (1992 estimates of Tasmania Mines N. hematite ± pyrite. L. Limited). Literature on the Kara skarn includes Stage IV — Amphibole-magnetite-scheeliteReid (1924), Barrett (1980), Plumridge (1986), Collins fluorite ± calcite ± epidote ± vesuvianite. (1989) and Whitehead (1990). The deposit is one of Stage I is dominated by anhydrous minerals of a series of carbonate replacement orebodies garnet and clinopyroxene. These minerals are developed within Cambrian to Lower Devonian generally fractured and partially replaced or altered. sedimentary sequence adjacent to Devonian Minerals of later stages pervasively replace and fill Granitoids in northwestern Tasmania. The aim of fractures or veins in early mineral phases. Magnetite this study is to clarify skarn formation and ore and scheelite occur in Stages II and IV where deposition at Kara from mineral paragenesis, fluid scheelite shows a close association with hydrous inclusion and oxygen isotope investigations. minerals (vesuvianite in Stage II and amphibole in The major orebodies at Kara are hosted by the Stage IV). Scheelite is more abundant in Stage IV Ordovician Gordon Limestone, adjacent to the where it forms very coarse grains averaging 5cm. housetop Granite, or separated from it by the Microprobe analyses indicate that garnet in Ordovician Moina Sandstone. The skarn mineralogy Stage I is andradite-rich and clinopyroxene is is dominated by magnetite, garnet, vesuvianite, diopside-rich. Microprobe data shows that the clinopyroxene, epidote, amphibole and quartz with amphiboles are calcic, plotting mostly in the subordinate amounts of fluorite, calcite, scheelite, hastingsite and magnesian hastingsite fields (Leake hematite chlorite and sericite. The skarn displays a 1978). The scheelite contains very little molybmineral zonation away from the granite contact. The denum, typically <2%, compared with 15% in proximal to distal zonation based on dominant scheelite from the King Island deposit, northwestern skarn assemblages range from epidote-quartz to Tasmania (Wesolowski et al., 1986). garnet, magnetite-amphibole, vesuvianite and Fluid inclusion microthermometric measureclinopyroxene zones. ments have been undertaken for garnet, vesuvianite, Mineral paragenetic studies based on macro- and quartz, calcite, fluorite and scheelite. Four major micro-textures reveal at least four stages of skarn inclusion types in the skarn minerals are recorded; formation and ore deposition: Type I: primary liquid-vapour inclusions with low liquid/vapour ratios; Type II: primary liquidvapour inclusions with high liquid/vapour ratios; Type III: primary liquid-vapour-unidentified daughter minerals; and Type IV: secondary liquid1 CODES Key Centre, University of Tasmania, GPO vapour inclusions with variable liquid/vapour Box 252C, Hobart, Tasmania 7001, Australia ratios. The measurements reported for the different stages of skarn formation are from Type I-III fluid Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

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inclusions. Stage I had homogenisation temperatures of 464°-585°C (mode 525°C) from garnet and clinopyroxene. Stage II gave homogenisation temperatures ranging from 349° to 600°C (mode 500°C) from vesuvianite and early sc±ieelite. Stage m minerals formed at temperatures of 326°- 359°C (mode 350°C) from quartz. Homogenisation temperatures for Stage IV (as revealed by scheelite, fluorite and calcite) varied from 233° to 342°C (mode 300°C) with a corresponding salinity of 10.0-11.0 eq. wt % NaCl. Quartz in the granite reveal homogenisation temperatures above 565°C. Oxygen isotope measurements on Stage II scheelite (n = 2) yielded 5^80 values of 5.6 %o while those of the stage IV scheelite (n = 1) had a S^^O value of 5.8 %o. The Stage n and IV magnetite grains (n = 2) had S^SQ values of 3.5 %o and 4.7 %o respectively. Oxygen isotopic composition of ore fluids calculated using a theoretical scheelite-H20 fractionation equation of Wesolowski et al (1986) gave 518O value = 9.6 %o (500°C) and value = 7.2 %o (300°C) for the Stage II and IV ore fluids respectively The ore values of 7.2 to 9.6 %o from scheelite compare well with scheelite ore values of 6 to 10 %o for the King Island scheelite deposit (Wesolowski et al 1986) and 7 to 10 for the Glenorchy Au-W-Sb mineralisation. New Zealand (Paterson 1982). These values are consistent with a magmatic source for the ore fluids. Mineral paragenesis, fluid inclusion and oxygen isotope studies indicate that magnetite-scheelite mineralisation at Kara was formed as a proximal skarn assemblage in carbonate host rocks from

magmatic hydrothermal fluids derived from the nearby granitoid source. Skam formation and ore deposition occurred in stages. The garnet-clinopyroxene skam assemblages deposited at >525°C, representing early anhydrous prograde metasomatism, and were overprinted by hydrous mineral assemblages containing magnetite and scheelite at temperatures of 500°-300°C and salinities of 10.0-14.0 eq. wt % NaCl. References Barrett, D.E.,1980: Geology, mineralogy and conditions of formation of the Kara scheelite skarn. BSc Honours thesis, Geol. Dept., Univ. Tasmania (unpubL). Collins, RL.R,1989: Kara tungsten deposit. In Barrett, C.F. & Martin, E.L. (Eds): GEOLOGY AND MINERAL RESOURCES OF TASMANIA. Geological Society of Australia Special Publication 15: 282. Leake, B.E., 1978: Nomenclature of amphiboles. American Mineralogist 63: 1023-1058. Paterson, C.J., 1982: Oxygen isotopic evidence for the origin and evolution of scheelite ore-forming fluid, Glenorchy, New Zealand. Economic Geology 77: 1672-1687. Plumridge, C., 1986: Notes of the Kara deposit and surrounding area and its potential for gold mineralisation: Unpublished report, Tasmania Mines N. L. Ltd., Bumie. Reid, A.M., 1924: Deposits of iron ore at Hampshire: Unpublished report. Mines Department, Tasmania. Wesolowski, D. & Ohmoto, H., 1986: Calculated oxygen isotope fractionation factors between water and the minerals scheelite and powellite: Economic Geology 81: 471-477. Whitehead, C.H.,1990: Kara scheelite-magnetite deposit. In Excursion guide E2, Tin and tungsten deposits related to Devonian granitoids. 19th Aust. Geol. Conv., Hobart.

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Gravity and Magnetics of the Scamander Mineral Field Mark Duffett^

The Scamander Mineral Field is an area of approximately 250km2 containing scattered mineral deposits, all currently sub-economic. These deposits clearly define a mineralogical zonation from W-Mo through Sn and Cu to Pb-Zn-Ag, away from the Constable Creek granite. The Constable Creek granite is a marginal zone differentiate of the Mount Pearson Adamellite, a member of the Blue Tier Batholith. It is believed responsible for the introduction of metal-bearing hydrothermal fluids into turbiditic sediments of the Mathinna Group, which host most of the mineral deposits. Decreasing temperature of fluids with distance from the granite intrusion is thought to be the primary control on mineralogical zonation (Ruxton & Plummer 1984). However, mineralisation extends well away from Constable Creek granite outcrop. Previous workers have thus hypothesized that the Constable Creek granite is present at relatively shallow depths beneath the Scamander Mineral Field. A regional gravity survey was undertaken to test this hypothesis. Aeromagnetic data from an exploration survey was also reprocessed and reinterpreted. Interpretation of the two data sets was performed in conjunction, on five interlocking profiles using MODEL2D, a 2D forward modelling program. Most of the magnetic anomalies are attributable to localized occurrences of magnetite and pyrrhotite in veins and fractures. The largest anomalies are associated with known prospects and have been intersected by drilling. Other, smaller anomalies, also probably due to magnetite veins, are scattered

throughout the hornfelsed contact zone of the Constable Creek granite intrusion, and do not necessarily correspond to known mineralized sites. All modelled magnetic bodies are constrained almost entirely within the inferred extent of Mathinna Group occurrence. Linear northeast-trending features extending across the aeromagnetic image are the signature of ?Devonian quartz-dolerite dykes containing significant magnetite and pyrrhotite. One of these has intruded along a 1 km wide shear zone, on which there has been hundreds of metres of sinistral wrench movement. A complex series of highfrequency anomaUes in the vicinity of the Yarmouth Pb-Zn-Ag prospect may be due to a combination of magnetite and pyrrhotite in veins and dolerite dykes, or possibly the contact zone of a second phase of mineralising granite intrusion. A relatively large density contrast between the highly differentiated mineralising granite and the denser Mathinna Group metasediments enabled the contact between the two units to be confidently delineated. The mineralising Constable Creek granite is modelled as an irregular sheet up to a kilometre thick, marginally less dense than its underlying parent body, the Mount Pearson Adamellite. The contrast between these two units is not sufficient to determine whether subcrop of the Constable Creek granite continues all the way to the coast, or terminates at the Scamander Tier granodiorite dyke.

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

122

Character and setting of the Anthony Road Andesite, Mount Read Volcanics, western Tasmania. Andrew Jones^

The Anthony Road Andesite (ARA) outcrops in the central part of the Cambrian Mount Read Volcanics, 12 km north of Queenstown. Stratigraphic relationships in the region (examined in outcrop and drillcore) indicate that the ARA post-dates and is also partly coeval with deposition of the Yolande River Sequence, has been emplaced partly coeval with the Central Volcanic Complex and pre-dates deposition of the Tyndall Group. These relative ages are supported by results of recent U / P b and AT/AT dating of the ARA and Tyndall Group (Perkins & Walshe 1993). Texturally and chemically the ARA comprises a compositionally diverse (basaltic andesitic, andesitic, dacitic) sequence of coherent lavas, shallow intrusions and associated units, intercalated with lesser carbonate and volcanogenic sedimentary units. The ARA is the second largest sequence (second to the Que-Hellyer Volcanics) of dominantly intermediate volcanics in the Mount Read Volcanics and comprises a true stratigraphic thickness in excess of 1 km. Six lithofacies have been identified in the ARA on the basis of initial mapping and petrography: 1. 2. 3.

Feldspar-pyroxene-phyric basaltic andesite Feldspar-homblende-pyroxene-phyric andesite Feldspar-homblende-quartz-phyric andesite/ dacite 4a. In situ andesitic breccia (sandy andesitic matrix) 4b. In situ andesitic breccia (mudstone matrix) and 5. Volcaniclastic breccia/sandstone (undifferentiated).

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

Lithofacies 1 is interpreted to represent a lava, lithofacies 2 and 3 a combination of lavas and shallow intrusions and lithofacies 4a and 4b shallow intrusions. The presence of volcaniclastic turbidites and mass-flow deposits stratigraphically above and below the ARA and of hyaloclastite and intrusive hyaloclastite facies within the ARA indicate a subaqueous below storm-wave base depositional setting. The stratigraphic thickness, the predominance of coherent units and paucity of intercalated sedimentary units all suggest that the ARA represents a submarine, dominantly effusive (and shallow intrusive) volcanic centre active in the late Middle Cambrian. Reference Perkins, C. & Walshe, J.L., 1993: Geochronology of the Mount Read Volcanics, Tasmania, Australia. Economic Geology 88:1176-1197.

123

The depostlonal setting of basal Dundas Group sediments, western Tasmania David Selley^

The unfossiliferous Red Lead Conglomerate (RLC) represents the lower most formation of the Dundas Group and lies with erosional contact upon a basement of intercalated basaltic pillow lavas and volcanic breccia. The underlying volcanic succession occurs as part of the Serpentine Hill Complex, which is considered to be an allochthonous fragment of intra-oceanic forearc crust, tectonically emplaced during the Middle Cambrian (Berry & Crawford 1988; Crawford & Berry 1992). In the type section of the Dundas Group, the RLC is overlain by a sequence of interbedded mudstone, volcaniclastic sandstone and pumiceous mass-flow units which yields fossils of Undillan age (mid-Middle Cambrian) Qago 1979). Although this upper contact is unexposed, intense deformation within portions of the overlying sedimentary package, involving recumbent, tight to isoclinal folds, numerous slide surfaces and ubiquitous soft sediment deformation suggests that it may be structurally emplaced. A minimum age for the RLC is thus poorly constrained. For the most part, the RLC is deposited in a system of subaqueous fans and comprises a succession of channellised, crudely stratified, clastsupported conglomerate intercalated with muddominated interchannel deposits. Basaltic clasts derived from underlying basement rocks together with volcanic, volcaniclastic and sedimentary detritus from the adjacent Mt Read volcanic arc, indicate a period of relative uplift during deposition. Syn-depositional tectonism is further evidenced by pervasive soft sediment deformation throughout the finer grained portion of the package.

In the Ring River section, the fan deposits are punctuated by thick, unstratified, matrix-supported conglomerate units. These conglomerates may be subdivided into two broad facies types based on distinct compositional and textural differences: (1) talus breccia, largely redeposited in debris flows and composed almost entirely of basement-derived detritus, and (2) intrabasinally-derived debris flow facies, involving reworked, unlithified fan sediments and talus breccia. The matrix-supported conglomerates are interpreted as localised mass failure deposits resulting from gradually increasing slopes along fault-controlled basin margins. The style of deformation within sequences which overlie the RLC, reworking of the adjacent Mt Read volcanic arc, localised uplift and erosion of basement and the significant thickness of debris flow units (>50 m), are features which are characteristic of foreland basin deposition. References

Berry R. F. &c Crawford A. J., 1988. The tectonic significance of Cambrian allochthonous maficultramafic complexes in Tasmania. Aust. J. Earth Sc. 35: 523-533. Crawford A. J. & Berry R. F, 1992. Tectonic implications of Late Froterozoic-Early Palaeozoic igneous rock associations in westem Tasmania. Tectonophysics 214: 37-56. Jago, J. B., 1979. Tasmanian Cambrian biostratigraphy — a preliminary report.}. Geol Soc. Aust. 26(5): 223230.

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

124

Geochemistry and tectonic implications of magmatism in northern Hunter Ridge — Kadavu Island Group (Fiji) Alicia Verbeeten\ Anthony J. Crawford\ Patrick Maillet^ and Steve.M Eggins^

The Kadavu island group (Fiji) is the northern most exposed tip of the Hunter Ridge and is situated within the southwest Pacific region. The southwest Pacific is a tectonically complex region, where there has been reorganization of plate boundaries and the development of various island arcs, back-arc basins, troughs and ridges. The tectonic setting and geochemical evolution of the Hunter Ridge and the Kadavu island group (Fiji) may be a clue to understanding the problems in the early stages of subduction, and hence, help in understanding the complex tectonic history of the southwest Pacific The Fiji Islands are remnants of a once continuous Solomon-Vanuatu-Fiji-Tonga volcanic arc (the Vitiaz arc), located at the boundary between the Indian-Australian and Pacific plates. They record a history of (i) arc volcanism from the late Eocene to the late Miocene, (ii) an extensive magmatic response to arc rifting (5.5-3.0 Ma) during which volcanism was dominated by shoshonitic to tholeiitic basalts, and as rifting continued, (iii) a transition from arc to intraplate volcanism. The Hunter Ridge-Hunter Fracture Zone, which links Fiji and Vanuatu forms the boundary between the North Fiji Basin and the South Fiji Basin. The Hunter Ridge-Hunter Fracture zone has been considered to be a transform zone, almost parallel to the motion vector of the Australian plate. However rocks dredged during the recent 'Alize' cruise from along the northern part of the Hunter Ridge are typical arc tholeiites, and therefore demand that subduction has occurred beneath the Hunter Ridge sometime during the last ~ 5 Ma. The Kadavu island

1 Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tasmania, 7001, Australia 2 Centre ORSTOM, BP 70, 29280 Plouzane, France 3 Research School of Earth Sciences, The Australian National University, Canberra 0200, Australia

group therefore possibly results from oblique subduction of South Fiji Basin oceanic crust, which may have accompanied the anticlockwise rotation (arc rifting) of Fiji during opening of the North Fiji Basin. These islands consist dominantly of 3.4-0.5 Myr old medium-high K andesites and dacites, with many traits typical of subduction-related volcanics, i.e predominance of intermediate members (55-65% SiOj), Uttle Fe enrichment (FeOVMgO<2), Ti02<l, and negative Nb anomalies. These features contrast strongly with volcanics from elsewhere in Fiji around this time, where volcanic centres produced OIB-like intraplate olivine basalts. Aside from the main island of Kadavu, which is the fourth largest in Fiji, the group includes the island of Ono and a number of smaller islands lying within the Astrolabe reef. The Kadavu islands become gradually younger to the southwest. This temporal change is accompanied by a change in the composition of the lavas, the most notable of which is the significant decrease in K2O with time. The northeastern-most islands (Astrolabe islands) containing the oldest rocks have shoshonitic affinities and are considered to be of similar age and composition to the youngest shoshonites found elsewhere in Fiji. The presence of these shoshonites possibly represents a specific tectonic regime for Kadavu. In other island arcs, the more K-rich rocks are younger and occur above the deeper part of the Benioff zone, i.e further away from the trench. One geochemically distinct lava group (Ngaloa group) on Kadavu has affinities to OIB (1.4-2.0% Ti02; 90-200ppm Zr). We aim (i) to evaluate petrogenetic scenarios for the high-K shoshonites and the OIB suite in the Kadavu Group and (ii) to compare these compositionally with similar suites erupted elsewhere in Fiji during the last 5 Myr.

125

Textures and origins of carbonate associated with the Rosebery VHMS deposit Karin Orth^ and Anthea P. HilP

Carbonate is asscxriated with the volcanic-hosted Cathodoluminescence shows concentric commassive sulphide deposit at Rosebery and occurs positional zonation in spheroid rims. Two episodes immediately along strike with and interfingers with of carbonate alteration replace spheroids and blebs, and is occasionally replaced by the sulfides. forming massive carbonate. Each episode of carbonSpheroidal textures common in the carbonate units ate deposition could occur concurrently, but at and are concentrically zoned with radial euhedral different positions within the deposit. Euhedral carbonate overgrowing cores of anhedral grains. rhombs and rims on spheroids post-date replaceSpheroids vary from 2 mm to 20 mm in diameter ment of pumice clasts, but formed prior to the onset and are close-packed to dispersed in a sericite- of sulphide mineralisation. Ore fluids partly quartz-chlorite-albite matrix. Small carbonate replaced the carbonate altered host rocks, indicating rhombs are interstitial to spheroids. Massive to that Rosebery is a subsea-floor replacement deposit poddy carbonate forms in response to variations in rather that an exhalite. the spacing and number of nucleation sites. Recent volcanological studies (Allen & Cas 1990; References & Cas, R.A.F., 1990: The Rosebery controversy: McPhie & Allen 1992) demonstrated that the Allen, R.L. Distinguishing prospective submarine ignimbriteRosebery deposit occurs in the upper part of a like units from true subaerial ignimbrites in the Rosebery-Hercules Zn-Cu-Pb massive sulphide graded pumiceous mass-flow deposit. Arcuate and district, Tasmania. Geological Society of Australia circular textures in the core of carbonate spheroids Abstracts 25: 31-32. are similar to pumice textures in unaltered host McPhie, J. & Allen, R.L., 1992: Facies architecture of mineralized submarine volcanic sequences: rocks along strike from the orebody. Together with Cambrian Mount Read Volcanics, western altered feldspar crystals in the core of some Tasmania. Economic Geology 87: 587-596. spheroids, the preserved pumice textures suggest that carbonate growth was initiated by nucleation on pumice clasts and feldspar laths. Uncompacted pumice textures in the carbonate indicate that pyroclasts were replaced while still porous and permeable either immediately below the sea floor position, or at some shallow depth beneath the sea floor during the accumulation of the overlying mudstones ("black slate"). Carbonate probably precipitated as C02-rich fluids mixed with seawater, and replaced bubble walls and infilled spaces within pumice clasts.

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

126

Geophysical interpretations from the north flank of the South Tasman Rise Andrew Wellington^

The researcJi vessel L'Atalante was c±iartered in early 1994 by the Australian Geological Survey Organisation, AGSO, to survey large areas of the South Tasman Rise and the west Tasmanian Margin. Gravity, magnetic, seismic reflection, bathymetric and 3.5 kHz echogram data were collected. This thesis presents interpretations of part of this data, with a focus on an 20,000 km^ area of the north flank of the South Tasman Rise. The area shows complex bathymetry including raised blocks bounded by fault scarps and flat depositional areas. The complex structures present are interpreted as being due to the interaction of two major tectonic regimes; left-lateral strike slip motion along a major northwest/southeast-trending fault system, and oblique extension associated with activation of the Tasman Fracture Zone. This shows that the original position of the South Tasman Rise was adjacent to the West Tasmanian Margin, as a part of the Antarctic continent. A displacement by strike-slip faulting of around 300-500 km from the original position is implied. The interaction of the two tectonic regimes has formed deep basins in the study area. These formed between the mid and late Cretaceous and filled with sediments of a type similar to those seen in the Otway and west Tasmanian basins. These early basin filling sediments were deposited in a subaerial environment. A low-stand systems tract was deposited over the Cretaceous sediments during periods of low sea-level, correlated with incised canyons in basal Tertiary sections in the Sorell basin of westen Tasmania.

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

The north flank of the South Tasman Rise is composed of continental crust, around 18 km thick in the west grading to around 16 km thick in the east of the area. Tasmanian rock units continue into the study area, with Permo-Triassic sediments, discrete granite plutons and Pre-Cambrian basement all common components on the crust. A large volcano, 1.6 km high and 25 km across, has been identified along the axis of the wrench system. Mafic volcanics are common close to the surface, and are both normally and reversely polarised. Pelagic accumulation dominates the sedimentation currently occurring in the study area, as shown by sediment samples cored during the 1985 Sonne cruise. Pelagics, however, do not remain in situ but are invariably modified by either strong bottom currents or are re deposited as turbidites. Surficial sediments are essentially confined to deeper waters, with basement highs

127

Stratigraphy and palaeovolcanology of the Cambrian Tyndall Group, Mount Read Volcanics, western Tasmania Matthew J. White'

The Tyndall Group comprises dominantly felsic to intermediate redeposited volcaniclastic mass-flow deposits, accompanied by minor felsic pyroclastic flow deposits, felsic to intermediate coherent lavas a n d / o r intrusions and non-volcanic sedimentary rocks. The Group extends from near Mount Darwin in the south, to near Mount Read. Correlates of the Tyndall Group have been identified in the upper part of the Mount Charter Group, around the Cradle Mountain Link Road, km northeast of Hellyer (Corbett 1992). This sequence correlates well with Tyndall Group sequences further south. The Tyndall Group constitutes the upper part of the Mount Read Volcanics, and in places is conformably overlain by the Late Cambrian to Early Ordovician O w e n C o n g l o m e r a t e . The Tyndall G r o u p has b e e n moderately affected by low grade greenschist to prehnite-pumpellyite facies regional metamorphism a n d / o r diagenetic alteration and compaction. Although the internal stratigraphy of the Tyndall Group is complex, a three-fold informal subdivision is generated, consisting of the basal Comstock Tuff, the upper Comstock Tuff, and the overlying upper Tyndall Group sequences. The subdivisions are based on contrasting lithology and composition, which largely reflect different provenance characteristics. The subdivisions are regionally extensive in the central Mount Read Volcanic belt. The basal Comstock Tuff consists of dominanfly syn-eruptive, andesitic to dacitic, crystal±lithic volcaniclastic mass-flow units and turbidites, together with minor laminated mudstone, carbonate and volcaniclastic lithic breccia. The upper Comstock Tuff is rhyolitic to dacitic in character, and is dominated by syneruptive, crystal±lithic volcaniclastic mass-flow

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

units, with minor felsic welded ignimbrite and felsic lavas (and/or intrusives). The upper Tyndall Group consists of post-eruptive, polymict volcaniclastic conglomerate and sandstone units. The volcaniclastic units that dominate the Tyndall Group are interpreted as resedimented s u b a q u e o u s m a s s - f l o w units and t u r b i d i t e s , indicating transportation and deposition by low- to high-density turbidity currents and/or debris flows, in a below-storm-wave-base submarine environment. However, the local presence of an in situ limestone unit at Comstock containing abundant shallow marine fossils Qago et al 1972) and an in situ welded ignimbrite unit at Zig Zag Hill (Corbett et al. 1974; White et al. 1993) suggest that part of the g r o u p w a s d e p o s i t e d in shallower water, in proximity to subaerial environments. Sources of the volcaniclastic components in the Tyndall Group are not exposed or have been eroded away. However, syn-eruptive resedimented volcaniclastic deposits and primary pyroclastic flow deposits (welded ignimbrite) in the basal and upper Comstock Tuff sequences, provide a record of the character and setting of volcanic activity in the source. The high proportion of juvenile pyroclasts (e.g. crystals, crystal fragments, shards, pumice) within the syn-eruptive volcaniclastic facies, and occurrences of welded ignimbrite, indicate explosive magmatic a n d / o r phreatomagmatic eruptions in the source, and suggests that the source volcanic centres were subaerial to shallow marine. Pyroclasts were probably transported to the marine basin by pyroclastic flows which transformed into watersupported mass flows on entry into water, and finally deposited below storm wave base. During transportation fine ash was driven off into secondary ash plumes generated at the shoreline, and into ash-rich suspensions associated with the ensuing subaqueous volcaniclastic mass flows (cf. Cas 1983). The initial eruptions were andesitic to dacitic. Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

128 leading to deposition of the plagioclase-clinopyroxene-magnetite dominated units of the basal Comstock Tuff. Rhyolitic to dacitic eruptions followed leading to quartz-plagioclase ric±i massflow deposits of the upper Comstock Tuff. Ignimbrite was deposited in subaerial to shallow marine parts of the basin from the subaerial pyroclastic flows. Some of the ignimbrite deposited in shallow water environments contributed clasts (some up to several tens of metres across) that were subsequently transported downslope, and together with crystal and lithic components, formed giant subaqueous mass flows (White et al 1993). The upper Tyndall Group volcaniclastic facies contains abundant rounded volcanic lithic (and other) clasts, indicating erosion and reworking in high energy environments (prior to redeposition by subaqueous mass flows), and that the source areas were subaerial to shallow marine. Tectonic uplift may have increased erosion rates in the source, producing the large proportion of epiclasts present in this facies. This uplift eventually exposed Precambrian basement rocks, feeding Precambrian quartzite clasts into the basin, as seen in the upper parts of the upper Tyndall Group. The overall palaeogeographic setting of the Cambrian Tyndall Group is similar to that in the eastern to central parts of the North Island of New Zealand and the adjacent marine basins (e.g. Harve

Trough). Active subaerial volcanic centres in the Taupo Volcanic Zone, produce enormous volumes of volcanic debris, and lie in proximity to the sea. Both primary pyroclastic processes (flows, fallout) and secondary processes (fluvial, etc.) have delivered volcanic debris to the sea, depositing large volumes offshore. References Cas, R.A.R,1983: Submarine 'crystal tuffs': their origin using a lower Devonian example from southeastem Australia. Geological Magazine 120:471^86. Corbett, K.D., 1992: Stratigraphic-volcanic setting of massive sulfide deposits in the Cambrian Mount Read Volcanics, Tasmania. Economic Geology 87: 564-586. Corbett, K.D., Reid, K.O., Corbett, E.B., Green, G.R., Wells, K. & Sheppard, N.W., 1974: The Mount Read Volcanics and Cambrian-Ordovician relationships at Queenstown, Tasmania. Journal of the Geological Society of Australia 21:173-186. Jago, J.B., Reid, K.O., Quilty, RG., Green, G.R & Daily, B., 1972: Fossiliferous Cambrian limestone from within the Mount Read Volcanics, Mount Lyell Mine area, Tasmania. Journal of the Geological Society of Australia 19: 379-382. White, M.J., McPhie, J., Corbett, K.D. & Pemberton, J., 1993: Welded ignimbrite emplaced below wave base: Cambrian examples in Tasmania. lAVGEI General Assembly, Canberra Abstracts: 121.

129

Apatite fission track thermochronology of northeastern Tasmania and the southern Bass Basin Andrea J. O'Sullivan^

Tasmania is an ideal site to study continental extension tectonics as it is a small island with rifted margins on three sides. This study is particularly focussed on the interaction of northeastern Tasmania and the adjacent Durroon and southern Bass Basins, in the southern part of Bass Strait between Tasmania and mainland Australia. Fission track data from northern Tasmania indicate two regional phases of uplift, probably associated with Early Cretaceous Otway rifting and Late Cretaceous Tasman rifting respectively. There is evidence for a third, more localised phase of uplift in the early Tertiary. Apatite fission track analysis of the rifted continental margins of southeastern mainland Australia has shown that uplift and erosion was caused by the rifting and separation of Australia from Antarctica (-120-90 Ma) and the Lord Howe Rise (-90-80 Ma). Continental rifting initiated major subsidence in the sedimentary Otway, Bass and Gippsland basins of Bass Strait. This was accompanied by at least 1.5-3 km of uplift and erosion along the Tasman Sea and part of the Bass Strait coasts, with negligible uplift -100 km inland. The Durroon Basin, in the southeastern comer of the Bass Basin, preserves the thermal sag phase sediments of the Otway rifting and a distinct Tasman rift sequence, mappable on seismic and recorded in the Durroon-1 well. Apatite fission track data from the central plateau region of Tasmania indicate that this area has not been greatly affected by either phase of rifting, although the Permo-Triassic sediments may have been partially reset by earlier Jurassic dolerite

emplacement. Samples from the elevated areas of northeastern Tasmania generally exhibit ages of 90110 Ma with moderately long lengths, while 70-80 Ma ages frequently occur in the region at lower elevations, especially near the eastern coast. Isolated samples yield ages as young as 56 ± 6 Ma with relatively long confined track lengths, but are generally in the range 60-70 Ma with shortened lengths. Samples from the thinned onshore extension of the Durroon Basin, in northeastern Tasmania, exhibit mixed provenance ages of 150-180 Ma. The youngest single grain ages recorded, 80-90 Ma, represent the maximum possible age of this southeastern extension of the Durroon Basin. Confined fission track lengths decrease downhole, indicating partial resetting due to increased temperatures during nearby rifting, and possibly involving more than 1 km of denudation. Data from the Durroon-1 well indicate that the Cretaceous to Pliocene sediments and volcanics intersected in the well are experiencing maximum temperatures at the present time. In conclusion, data from northern Tasmania indicate three phases of uplift. Ages of 90-110 Ma recorded from the tops of the highlands and from north-central Tasmania are probably associated with the Otway rifting event, while data from many samples in Northeastern Tasmania are consistent with rapid cooling and denudation at approximately 70-80 Ma, during the Tasman rifting event. Isolated indications of the third early Tertiary event may be due to localised fault reactivation, possibly related to the increased rate of sea floor spreading in the Southern Ocean.

^ School of Earth Sciences, LaTrobe University, Bundoora, Victoria 3083, Australia

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

130

Polyphase metamorphism and fluid flow in the Corella Formation, Mary Kathleen, Queensland Cathryn C. Gifkins^

The poly-deformed, metamorphosed and altered rocks of the Proterzoic Corella Formation (1780-1740 Ma) crop out east of the Overlander granite, in the Mary Kathleen Fold Belt. The Corella Formation is dominated by calc-silicate granofels, impure marbles, meta-sediments, and meta-basites. The Corella Formation has undergone three phases of deformation associated with contact and regional metamorphism. The earliest deformation, Dj (1740-1730 Ma), was a major period of tectonic extension during which a bimodal complex of granites and dolerites were intruded, resulting in contact metamorphism of the adjacent calcareous and meta-sedimentary sequence. Two phases of east-west compression and crustal thickening post dated D^. The first of these, D2 (1600-1550 Ma), produced the dominant structural fabrics throughout the sequence. These fabrics were associated with high temperature mineral and vein growth indicating that peak regional metamorphism was synchronous with D2 deformation. The final phase of deformation, D3 (1550-1480 Ma), is characterised by east-west strike-slip faults spatially associated with retrograde and metasomatic mineral assemblages. Contact Metamorphism Contact metamorphism pre-dated regional metamorphism and is well preserved along the eastern margin of the Overlander granite, where the rocks were metamorphosed to pyroxene homfels grade. In close proximity to the granite, woUastonite veins, skarns, monomineralic banding and the absence of calcite in the metamorphic assemblage were caused by the infiltration of H20-rich magmatic fluids into

1 Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia Currently; CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

the country rock during contact metamorphism. Further away from the granite, the only evidence for contact metamorphism is the high ratio of clinopyroxene to calcite. The coexistence of clinopyroxene and calcite attests to the internal buffering of the fluid composition and low F / R ratios during metamorphism. F/R ratios decreased away from the intrusion as fluid flowed down temperature gradients, outwards from the granite. Prograde Metamorphism Peak regional metamorphic assemblages in the metasediments and metabasites are typical of lower to mid amphibolite facies metamorphism or sillimanite-andulasite grade. The temperature of regional metamorphism is inferred to have reached 500-700°C at 3.5 kbars and the initial pore fluid composition was C02-poor. The paradox that CO2 concentration increased as metamorphic decarbonisation reactions took place in the calcsilicates, while in more micaceous lithologies H2Orich fluids were generated, is evidence for heterogenous metamorphism where low F/R ratios and internal buffering predominate. The resultant CO2and H20-rich fluids were mostly confined to indiviual lithological units rather than infiltrating and interacting with the surrounding rocks. Fluid generated during regional metamorphism essentially flowed along layers or away through fractures, rather than across lithological boundaries. Retrograde Metamorphism Retrograde metamorphism is characterised by greenschist facies metamorphism which occurs between 300-500°C. The retrograde assemblages are commonly spatially associated with D3 faults, which are inferred to have acted as fluid conduits during deformation.

131

Topographic lineaments: Unlocking buried mineralization in southeastern Tasmania Peter J. Rice

Digital contours at 100m intervals and the drainage network of Tasmania at 1:250 000 scale have been processed into a digital elevation model (DEM) of the state. By applying an artificial illumination or hill shading to the image from the northwest, northeast, southeast and southwest, a set of topographic lineaments have been defined. Lineaments in this case are linear features in the digital elevation model that cut across primary topographic features, e.g. conspicuous breaks in mountain chains that form a linear pattern or stretches of major rivers that are noticeably linear. At this scale only features which are greater than 10 km in length are recorded or considered significant. The DEM is a representation of the geomorphological response which is the product of environmental conditions, active geological processes and the u n d e r l y i n g geology. Thus the DEM is a continuous and unbiased image of the geomorphological response to these conditions which can be interpreted in a qualitative sense in a similar fashion to a gravity or magnetics image. O'Driscoll (1980) and Campbell (1989) have described many geophysical, geological and topographic lineaments on a continental and subcontinental scale. Some of these trends such as O'Driscoll's Bendigo-Broken Hill Corridor and Campbell's Port Campbell-Netherby Corridor have been observed in this study. In addition to these, new trends have been identified. A m a p of Tasmanian gold deposits shows a distinct clustering of deposits about topographic lineaments or lineament intersections and many major deposits ,of all types, display a spatial association to lineaments. Topographic lineaments defined from

1 CODES Key Centre, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia

higher resolution data (1:25 000, 10m contours) around the southeastern margin of the Housetop Granite are the expression of faults which controlled the intrusion of this Devonian granite and testify to the ancient origins of lineaments. A pair of lineaments form a major corridor with a WNW or Tethyan trend (O'Driscoll 1980) that extends from the northwest to the southeast coast of Tasmania. The northern-most lineament extends from Mt Cameron West on the far northwest coast and forms the southern limit of the Tamar Graben. The southern element extends form Trial Harbour on the west coast to Marion Bay on the southeast coast and exhibits sinistral fault offsets throughout. Mt Lyell lies adjacent the southern lineament which also coincides with the dramatic thinning in the outcrop of the Cambrian Dundas Trough sequence. Faulting in the same orientation as this lineament has been described as the Linda Disturbance by previous authors and is coincident with a major trai\sform structure that sinistrally offsets Devonian fold axes. All these aspects point to a ancient and deep seated structure that has the potential to tap deep crustal or mantle fluids for formation of large Cambrian mineral deposits and exert control over a major Devonian deformation event. Volcanic rocks similar to Mount Read Volcanics have been drilled below post Carboniferous cover and inferred from geophysics (Leaman 1994) in south east Tasmania. The passage of the prominent Trial Harbour-Marion Bay lineament through this area is a good starting point to explore for buried mineralization. Faults in the Broadmarsh to Marion Bay area show a major sinistral offset across the lineament suggesting reactivation in the same sense as in the Devonian. Sulphides along the contact of a Jurassic dolerite dyke at Dysart reported by Leaman (1992), sulphides including chalcopyrite associated with a Jurassic dolerite dyke in the Coal River Valley

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

132 (Lewis, pers. comm.),

and a gold prospect at

Gunning's Sugar Loaf could be indicative of buried mineralization. The Trial Harbour-Marion Bay lineament may have influenced the location of a major sulphide deposit in pre-Carboniferous times. Continual reactivation of the lineament and Jurassic intrusions have transported traces of these sulphides to the present day surface.

References CampbeU, I.B., 1989: The Port Campbell-Netherby NorthNorthwest Structural Corridor in Southeastern Australia. In R.W. LeMaitre (Ed.): PATHWAYS IN GEOLOGY, ESSAYS IN HONOUR OF EDWIN SHERBON HILLS. Blackwell Scientific Publications. Leaman, D. E. 1994: Geological Note. The Tamar Fracture System in Tasmania: Does it exist? Australian Journal of Earth Sciences 41: 73-74. Leaman, D. E. 1992: Finding Cambrian Keys: An essay in controversy, prospectivity and tectonic implications. Geological Survey of Tasmania Bulletin 70:124-148. O'Driscoll, E.S.T., 1980: The double helix in global tectonics. Tectonophysics 63: 397-417.

133

Symposium participants

AERDEN, DOMINGO ALLEN, ROD BAKER, DR BILL BASFORD, PAUL BEESON, BOB BENDALL, MALCOLM BENTLEY, CHRIS BERRY, RON BISHOP, JOHN BORTON, DAVID BOTTRILL, RALPH BRADBURY, JASON BROWN, TONY BULL, STUART BURRETT, CLIVE CALVER, CLIVE CATHRYN GIFKINS CLOSE,BOB COOKE, DAVID CORBETT, KEITH CRAWFORD,TONY DAVIDSON, GARRY DEAN, ALISON DICKSON, TOM DIREEN, NICK DRAVANIS, THEO DUFFETT, MARK DUHIG, NATHAN DUNCAN, DAVID DUNHAM, SCOTT ELLIS, PETER ELLISTON, JOHN EVERARD, JOHN EWINGTON, DOUGLAS FITZGERALD, FERGUS FODEN, JOHN FORSYTHE, STEVE

Laboratoire de Tectonique et Geophysique, case postal 060, Univesite de Montpellier II, Place E. Batallion, 34095 Montpellier, France Volcanic Resources Ltd, C/O Boliden Mineral AB, Prospektering, Boliden, Sweden Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Pasminco ExpL, Old Bumie Railway Station, Bumie, Tas, 7320 Acacia Resources Condor Oil Investment, 84 Wells Parade, Blackmans Bay, Tas. Dept. Applied Science, Uni. South Australia, 5095 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mitre Geophysics, PO Box 974, Sandy Bay Tas., 7006 Acacia Resources Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas GPO Box 44A, Hobart, 7001 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Plutoruc Resources, Level 37,100 Miller St., North Sydney, 2060 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mineral Resources Tasmai\ia, PO Box 56, Rosny Park, 7018, Tas Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Dept. of Earth Sciences, Monash University, Clayton, 3168 CRA Exploration, PO Box 8093, Northland Centre, Vic, 3072 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 CRA Exploration, PO Box 8093, Northland Centre, Vic, 3072 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas RGC Ltd., 24th Floor, Goldfields House, Sydney Cove, 2000 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 lOB the Bulwark, Castlecrag, NSW, 2068 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas PO Box 362, Launceston 7250 Pasminco ExpL, Old Bumie Railway Station, Bumie, Tas, 7320 Dept. Geology & Geophysics, Uni. Adelaide, GPO Box 498, Adel Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

134 FULTON, RUSSELL GRAY, DAVID GREEN, DAVID GREEN, GEOFF GREEN, NICK HALLEY, SCOTT HANNAN, DAVID HARRISON, EMMA HARVEY, KEN HARWOOD, MAL HAYDON, BOB HERLEC, UROS HERMAN, WALLY HILL, ANTHEA HILLS, PETER HINE, ROHAN HOLDEN, ROD HUGHES, NEIL JAGO,JIM JONES, ANDREW JONES, MEL KEELE, RICHARD KHIN ZAW KITTO, PAUL KUIPERS, GERRIT LARGE, ROSS LEAMAN, DAVID LEAR, GREG LEAVER, BRUCE LEE, FRANK LEES, TERRY LEWIS, ROB LORRIGAN, ANGELA MAHER, SIMON MATTEWS, W. L. McARTHUR, GARY McCLENAGHAN, JEAN McCLENAGHAN, MARCUS McDONALD, GRANT McGOLDRICK, PETER MCNEILL, ANDREW McPHIE, JOCELYN McQUITTY, BRUCE MOORE, C. LEAH MORRISON, KEN MROCZEK, CHRIS NAND, ALIND NEWNHAM, LINDSAY O'SULLIVAN, ANDREA PARKINSON, ROB PARKINSON, W. DUDLEY PEMBERTON, JOHN

Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Dept. of Earth Sciences, Monash University, Clayton, 3168 Pasminco Mining, Rosebery, Tas., 7470 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Normandy Expl. Ltd., 103 King William St., Kent Town, S.A. RGC Exploration, PO Box 62, Zeehan, Tas. University of Tasmania - Launceston Campus Pasminco Expl, Old Bumie Railway Station, Bumie, Tas, 7320 MIM Exploration, GPO Box 1042, Brisbane, QLD, 4001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Pasminco Expl, Level 7, 380 St. Kilda Rd., Melbourne, 3004 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 23C Baker St, Charters Towers, QLD Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Plutonic Resources, Level 37,100 Miller St., North Sydney, 2060 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 South Mine Operations, Eyre St., Broken Hill, NSW, 2880 Dept. Applied Science, Uni. South Australia, 5095 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Pancontinental Mining Ltd., Level 36,1 MacQuarie PL, Sydney Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Public Land Use Commission, GPO Box 2036 Hobart, 7001 102 Mill Street, Ballarat, Victoria, 3350 Pasminco Ex., 116 Fullarton Rd., Norwood, SA Aberfoyle Resources Ltd., PO Box 952, Bumie, 7320 Pasminco Expl., Old Bunue Railway Station, Bumie, Tas, 7320 CRA Exploration, PO Box 8093, Northland Centre, Vic, 3072 Mineral Resources Tasmarua, PO Box 56, Rosny Park, 7018, Tas Aberfoyle Resources Ltd., PO Box 952, Bumie, 7320 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Renison Ltd., PO Box 20, Zeehan, Tas. Dept. of Earth Sciences, Monash University, Clayton, 3168 4 Westringa Road, Femtree, 7054 Renison Ltd., PO Box 20, Zeehan, Tas. Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 PO Box 40, Legana, Tas., 7277 School of Earth Sciences, La Trobe University, Bundoora, 3083 CRA Exploration, PO Box 8093, Northland Centre, Vic, 3072 68 Risdon Rd., Newtown, Tas., 7008 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas

135

PERKINS, CAROLINE POLTOCK, RCX3ER PURVIS, GERALD PWA, AUNG QUAYLE, MIKE RAETZ, MIKE RICE, PETER RICHARDSON, BOB RICHARDSON, STEVE ROACH, MICHAEL ROBERTS, RAY RORSCH, RUSSELL RUSSELL, DAVE SAXON, MARK SELLEY, DAVID SEYMOUR, DAVID SINGOYI BLACKWELL SKEY, HUGH SMTTH, STUART SOLOMON, MIKE STEVENSON, PETER TAHERI,JAFFER TUNKS, ANDREW TURNER, NICK VAN MOORT JAN VARNE,RICK VERBEETEN, ALICL\ VICARY MICHAEL WALDRON, HELEN WALLACE, DAVID WALSHE,JOHN WATERS, JOHN WELLINGTON, ANDREW WHITE, MATT YEATES, TONY

RSES, Australian National University, Canberra, ACT, 0200 C/O PO Box, Wilmot, Tas., 7310 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Pasminco ExpL, Old Bumie Railway Station, Bumie, Tas, 7320 BHP Minerals, 801 Glenferrie Rd., Hawthorn, Vic., 3122 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Aberfoyle Resources Ltd., PO Box 952, Bumie, 7320 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Renison Ltd., PO Box 20, Zeehan, Tas. AGSO, GPO Box 378, Canberra, 2601 Pasminco ExpL, Old Bumie Railway Station, Bumie, Tas, 7320 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Aberfoyle Resources Ltd, 123 Camberwell Road, East Hawthorn, Vic 3123 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 11 Wandella Ave., Taroona, Tas. Mineral Resources Tasmania, PO Box 56, Rosny Park, 7018, Tas Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 65 Lochner St., West Hobart, Tas., 7000 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Urn. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 RGC Exploration, PO Box 62, Zeehan, Tas. Becquerel Laboratories, PMB 1 Menai, NSW Aberfoyle Resources Ltd., PO Box 952, Bumie, 7320 Geol. Department, Australian National University, Canberra 16/39 Raymond Rd., Neuh-al Bay NSW, 2089 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 Geology Dept., Uni. of Tasmania, GPO Box 252C, Hobart, 7001 AGSO, GPO Box 378, Canberra, 2601

Geological Society of Australia, Tasmania Division Contentious issues in Tasmanian geology: a symposium

136

Index of authors

Aerden, D.G.A.M Allen, R. A Berry, R.F. Black, L.P. Bottrill, R Cooke, D.R Corbett, K.D Crawford, A.J Davidson, G.J Doyle, M Duffett, M Eggins, S.M Fulton, R Gifkins, C.C Gray D.R Hill,A.R Hughes, N Jones, A Kamperman, M Keele,R.A Khin Zaw Kitto,R Large, R.R Leaman, D.E

;

101

Lees, T.

41,107

Mamet,R

124

McPhie,J

51 73 109 35 23,124 69 109 121 124 61 130 3 125 86 122 51 6? 119 95 109 17,79

Orth, K 0'Sumvan,A.J Perkins, C Raymond, O Rice, P.J Richardson, R Russell, D. SeUey,D.' SiUc,J Singoyi, B Solomon, M Taheri,J Taylor, B Turner, N.J Vame,R Verbeeten, A Walshe,J.L Wellington, A White, M.J Woodward, N.B Wright, J

38 125 129 49 109 131 65 63 123 85 119 91 73 69 51 61 124 93 126 127 3 15

¥>

Figure 1 — Regional cross section through the northeast Tasmania terrane The section line comprises two segments joined along the Scottsdale Bathohth - a northern segment between Badger Head and Bridport and a southern segment between Mathinna and Scamander. The data was compiled from various sources and attribution is made next to the relevant area on the section. Correlation of sequences east of the Scottsdale BathoUth is mainly based on lithologies and is therefore considered to be tenuous.

Figure 2 — Interpreted Devonian thrust weage in the northeast Tasmania terrain based on data from previous figure. Likely pathways for the auriferous fluids are shown with solid arrows. The granite terrain has been omitted, with the exception of the Scottsdale BathoUth, in order to emphasise the point that the granites are largely post-kinematic in age and therefore overprint the thrusts.