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©2004 Society of Economic Geologists Special Publication 11, 2004, pp. 243–257

Chapter 13 Uchucchacua: A Major Silver Producer in South America ULRICH PETERSEN,† Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138

OSCAR MAYTA, LUIS GAMARRA, CÉSAR E. VIDAL, AND ANGEL SABASTIZAGAL Compañía de Minas Buenaventura, S.A.A., Carlos Villarán 790, Lima 13, Perú

Abstract Although known since at least 1897, Uchucchacua was first explored on a major scale by Compañía de Minas Buenaventura since 1960. Narrow vein mining started in 1975, but orebodies discovered at depth enabled expansion to today’s 2,000-t/d operation, transforming “Chacua” into the largest primary silver producer in South America. The ores occur in fractures and faults, as well as in pipes, irregular replacement bodies, and mantos hosted by Late Cretaceous limestone. Porphyritic dacite bodies are probably pre-, syn-, and postore. Most of the ore occurs in distal manganiferous exoskarn and limestone and is mineralogically diverse, consisting mostly of the following. rhodonite rhodochrosite sphalerite pyrargyritequartz bustamite kutnahorite wurtzite proustite pyrite alabandite galena argentite The grade of the ore mined varies between 16 and 20 oz/t Ag combined with about 10 percent Mn, 1.5 percent Zn, and 0.9 percent Pb. Between 75 and 80 percent of the reserves are high in silver and manganese, whereas about 7 percent contain high zinc and lead grades with only moderate silver and low manganese. Logarithmic-grade graphs show very good positive linear correlations for zinc versus lead, moderate correlations for silver versus manganese, and arcuate correlation bands for silver or manganese versus zinc or lead. These relationships indicate that the outward zoning sequence is from lead-zinc to silver-manganese or vice versa. The corresponding longitudinal vein sections can generally be contoured unambiguously, showing that the bands of highest grades of lead and zinc coincide very well. The highest silver grades can be contoured convincingly as a band that is zoned outward and/or at a higher elevation than the lead and zinc bands. However, the manganese grades often require two high-grade bands: a main band that mostly coincides with the highest silver grades and a thinner upper band that may represent near-surface manganese enrichment. Ore intervals in individual veins, pipes, and replacement bodies are up to 200 m in vertical extent. However, the elevations of these intervals change progressively, reflecting the overall geometry of the hydrothermal cell (or cells) responsible for the mineralization. In addition, postore faulting has displaced the ore intervals. As a result, ore has been found to date over a vertical interval of 600 m, between 4,730 and 4,040 m. At surface, manganese oxide stains in the host limestone and limonite in fractures and faults indicate proximity to ore. Underground, multiple calcite veinlets constitute a guide to nearby orebodies. Geochemical anomalies of 60 to 80 ppm Ag have been documented up to 15 m from an orebody. By extrapolation, 10 ppm Ag anomalies may extend 25 m from ore, and 1 ppm Ag anomalies may attain 40 to 45 m. Ore continues to be found at depth as well as laterally and between known ore zones.

Resúmen Si bién yá se conocía por lo menos desde el año 1897, Uchucchacua recién fué explorada a mayor escala por la Compañía de Minas Buenaventura desde 1960. El minado de vetas angostas comenzó en 1975, pero el descubrimiento de cuerpos de mena en profundidad permitió expansiones hasta llegar a la producción actual de 2,000 toneladas/día, transformando a “Chacua” en el mayor productor primario de plata en Sudamérica. La mena ocurre en fracturas y fallas, así como en chimeneas, cuerpos de reemplazamiento irregulares, y mantos en calizas del Cretásico Superior. Los intrusivos dacíticos probablemente se emplazaron antes, durante y después de la mineralización. La mayoría de la mena está en exoskarn manganífero distal y en caliza. Su diversa mineralogía incluye: rodonita rodocrosita esfalerita proustitacuarzo bustamita kutnahorita wurtzita pirargirita pirita alabandita galena argentita La ley del mineral explotado varía entre 16 y 20 oz/t Ag con unos 10 porciento Mn, 1.5 porciento Zn, y 0.9 porciento Pb. Entre el 75 y el 80 porciento de las reservas tiene altos contenidos de plata y manganeso, mientras que un 7 porciento tiene altas leyes de zinc y plomo con sólo moderada cantidad de plata y poco manganeso. † Corresponding

\

author: e-mail, ulrichp@aol.com

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Los gráficos logarítmicos de leyes muestran buenas correlaciones lineales con pendientes positivas para zinc versus plomo, moderadas correlaciones para plata versus manganeso, y arcos de correlación para plata ó manganeso versus plomo ó zinc. Estas relaciones indican que la secuencia zonal es de plomo-zinc a platamanganeso ó vice-versa. Las secciones longitudinales correspondientes generalmente pueden contornearse convincentemente de manera que las bandas de leyes altas de plomo y zinc coinciden mayormente. Las leyes altas de plata pueden contornearse convincentemente formando una banda externa y/ó a mayor altura que las bandas de leyes mayores de plomo y zinc. Sin embargo, las leyes de manganeso a veces requieren dós bandas de leyes altas: una banda principal que coincide con la banda de leyes altas de plata y una banda secundaria a mayor altura que podría representar un enriquecimiento supergeno. Los intervalos de mena en las vetas, chimeneas y cuerpos de reemplazamiento abarcan hasta unos 200 m verticales. Sin embargo, las elevaciones de estos intervalos cambian progresivamente, reflejando la geometría general del sistema hidrotermal responsable de la mineralización. Además, los intervalos de mena han sido desplazados por fallas. Como resultado, se ha encontrado mena sobre un intervalo de 600 m, entre 4,730 y 4,040 m. En la superficie, la proximidad de la mena puede reconocerce por la presencia de manchas de óxidos de manganeso en la caliza y por limonita en fracturas y fallas. Bajo tierra, la presencia de múltiples venillas de calcita constituye una guía hacia cuerpos de mena cercanos. Además se han documentado anomalías geoquímicas de 60 a 80 ppm Ag hasta 15 m de un cuerpo de mena. Extrapolando esta información, puede inferirse que probablemente se tengan anomalías de 10 ppm Ag hasta 25 m de la mena, y anomalías de 1 ppm Ag hasta 40 a 45 m. Todavía se está encontrando mena en profundidad, así como lateralmente y entre las zonas con mena conocida.

Introduction

already old mine workings up to 90 m deep in 1897. These workings were in oxide ore and stopped upon encountering sulfides, which, at that time, could not be treated satisfactorily. The district did not attract the interest of major mining companies because at surface there were only narrow veins with minor ore. Toward the end of the 1950s, Alberto Benavides became interested in Uchucchacua, acquiring and amplifying the available claims for Compañía de Minas

THE UCHUCCHACUA mining district is about 170 km northnortheast of Lima, near the continental divide. Surface elevations range from 4,200 to 5,100 m above sea level (Fig. 1). The mineralization of Uchucchacua (“little old lady” in Quechua) is close to lake Colquicocha (“lake of silver” in Quechua). According to Torrico and Mesa (1901), there were 77°

76°

75°

Antamina

10°

10°

Raura

Colquipucro

Chanca UCHUCCHACUA

Atacocha - Milpo

Cerro de Pasco Colquijirca Minas Ragra Huarón

Iscaycruz

11°

11°

170 km.

C FI CI PA

Chungar Santander

O

Morococha

La Oroya

C

Casapalca

E A N

Yauricocha

12°

LIMA

0

77°

76°

FIG. 1. Location of the Uchucchacua mine, Peru. 0361-0128/98/000/000-00 $6.00

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30

12° 60 km. 75°


UCHUCCHACUA MINING DISTRICT, SOUTH AMERICA

Buenaventura and carrying out modern exploration and development by driving about 10 km of tunnels. In 1975 this led to mining narrow veins at a scale of 200 t/d containing 14 to 16 oz/t Ag. The discovery of orebodies at greater depth encouraged successive production increases to 500, 1,200, and finally 2,000 t/d containing 14 to 18 oz/t Ag. This transformed “Chacua” into a major South American silver producer, recovering about 10 million oz of silver in 2003. Ore continues to be found both at depth and laterally, between and beyond the known orebodies. The general geology and mineralization of the Uchucchacua district were studied intensively during the first 30 yrs of operation by Compañía de Minas Buenaventura, culminating in a comprehensive paper by Bussell et al. (1990). However, that paper did not describe the geochemical studies by Martínez (1986) and, since then, ore has been found more abundantly in large orebodies and at greater depths than expected as well as laterally. In addition, the vertical zoning of the economically valuable metals has been better documented and understood by plotting grade and metal content contours on longitudinal vein sections and verified using logarithmic graphs. This information is relevant for selecting alternative exploration, mining, and concentrating strategies. Extensive electron microprobe analyses (Petersen, 1995, 2000, 2001) also identified several new minerals and clarified the sulfur fugacity existing during the various mineralization stages. Documentation of two sphalerite populations with contrasting zinc and iron plus manganese contents has important metallurgical implications. More of a curiosity was the discovery of apparently hypogene native silver. Finally, the setting of Uchucchacua in the context of Andean magmatism was further clarified by the studies of Noble and McKee (1999) and Petersen (1999). In order to discuss these new developments in their proper context and to provide an integrated picture of this important ore deposit, which may represent a separate silver-manganese model, we summarize the pertinent information provided by Bussell et al. (1990) and add the new findings. General Geology The regional geology of Uchucchacua (Fig. 2) was described by Cobbing and Garayar (1971), Cobbing (1973), Romaní (1982), and Bussell et al. (1990). The rocks that host the mineralization are mostly limestone and marl of Late Cretaceous age. These are followed, above a slight unconformity, by Santonian red beds. The Cretaceous sedimentary rocks were strongly folded and faulted prior to the deposition of Tertiary volcanic rocks. Figure 2 reveals the north-trending axis of the Cachipampa anticline. In addition, there appear to be faults trending north, northeast, east, and southeast that radiate from a center between the concentrator and the Plomopampa camp. These faults may have been generated by an unexposed intrusion underlying the center. Figure 2 also shows segments of thrust faults which do not cause major displacements of stratigraphic contacts. However, the northern radial faults cut the Calipuy volcanic rocks, indicating that the former are younger and possibly related to the igneous and tectonic processes that gave rise to the mineralization of the district. 0361-0128/98/000/000-00 $6.00

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Figure 2 further reveals a small outcrop of dacite with skarn in the Casualidad area. Figure 3 of Bussell et al. (1990) also shows two areas of “dacite with skarn in mine” (i.e., projected to the surface) adjoining the Socorro fault. These three dacite occurrences may be parts of a single intrusion that was cut and dextrally displaced by the Socorro fault. Noble (1980) considered that a K-Ar age of 25.3 Ma for dacite from Uchucchacua was unreliable because of the effects of argon metasomatism, but Soler and Bonhomme (1988) thought that this age does reflect the timing of dacite emplacement. Bussell et al. (1990) pointed out that the intrusions at Uchucchacua are probably 8 to 15 Ma old because the intrusions in the nearby Raura deposit gave ages between 10.2 and 7.8 Ma (Noble, 1980), and because the gravimetric modeling of Bussell and Wilson (1985) suggests that the Cordillera Blanca batholith continues southward at depth, forming stocks above it at Raura and Uchucchacua. But Noble and McKee (1999, table 5) prefer an age of 24.5 Ma for Uchucchacua based on the age of a relict sanidine phenocryst in a premineral dike. In map 7 of Petersen (1999) Uchucchacua lies within a northeast-trending alignment of 25 to 35 Ma ages, but in his map 3 a 5 to 10 Ma age for Uchucchacua corresponds to the Raura and Cordillera Blanca ages. Actually, the Cordillera Blanca ages span from 2 to 25 Ma (Petersen, 1999, maps 2–6). Therefore, it is possible that the 24.5 Ma age of the premineral dike corresponds to either the northeast-trending magmatic alignment or to the Cordillera Blanca magmatism, whereas the 5 to 10 Ma age reflects the position of the main Cordillera Blanca magmatic belt. More radiometric ages are needed for fresh and altered intrusive rocks, as well as for the various mineralization stages and their alteration halos, in order to decipher the magmatic and hydrothermal chronology of Uchucchacua. Veins, Orebodies, and Mantos The known intrusions in the Uchucchacua area have related endoskarn and adjoining exoskarn, but the majority of the mineralization occurs in fractures and faults (veins) and in replacement bodies within distal exoskarn or in limestone. In this context, the term distal indicates that the exoskarn does not directly adjoin an intrusion and has no mineralogic implications. The outcrops of mineralized fractures and faults (veins) have evident black manganese oxide stains (Fig. 3a) that attracted the initial explorers. Underground proximity to ore is indicated by multiple calcite veinlets in the limestone (Fig. 3b). Some orebodies are tubular, subvertical pipes that are ovoid in horizontal section. Examples of these are the Rosa Norte, Irma, and Viviana bodies discovered because of their association with the Rosa vein. These bodies may have irregular forms but typically have a maximum horizontal width at a certain elevation. For example, Paz and Pamo (1983) provided horizontal measurements for Rosa Norte and Viviana (Table 1), and Martínez (1986) provided vertical ranges for three orebodies (Table 2). The numbers of Martínez (1986) differ somewhat from those of Paz and Pamo (1983) but reveal that the vertical ranges of the orebodies are on the order of 200 to 250 m. The recently discovered Rubí and Verónica orebodies measure 20 × 70 × 150 and 20 × 15 × 120 m, respectively. It is

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Karst breccia Dacite with skarn in mine Dacite with skarn in outcrop TERTIARY Calipuy Group volcanics Angular unconformity SANTONIAN Capas Rojas Formation CONIACIAN - EARLY SANTONIAN Celendin Formation TURONIAN - LATE ALBIAN Members 3 and 4 Marker succession

Jumasha Formation

Member 2

FIG. 2. Geology of the Uchcchacua mining district (after Bussell et al., 1990). A-A' section line for Figure 12. 0361-0128/98/000/000-00 $6.00

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a

247

b

Socorro 3 Vein

FIG. 3. a. Outcrop area of the Socorro 3 vein. Note the widespread manganese oxide stains. b. Calcite veinlets surrounding an orebody.

TABLE 1. Rosa Norte and Viviana Orebodies Horizontal Measurements (from Paz and Pamo, 1983) Rosa Norte Level 730 680 630 590 550

m 45 × 8 50 × 12 62 × 19 40 × 30 15 × 7

Viviana Level

m

590 565 550 500

30× 15 60 × 14 66 × 18 55 × 8

TABLE 2. Vertical Ranges for the Rosa Norte, Vivana, and Irma Orebodies (from Martínez, 1986) Orebody Rosa Norte Viviana Irma

Vertical range (m)

(m)

Maximum width (m)

4,765–4,520 4,745–4,530 4,800–4,550

245 215 250

25? 40 15

0361-0128/98/000/000-00 $6.00

estimated that these orebodies contain about 6 and 3 Moz of silver, respectively. Commonly the orebodies are associated with veins, faults, or fractures. Locally, ore occurs in mantos that are concordant with the limestone stratigraphy (Paz and Pamo, 1983) and are connected occasionally with the orebodies. In places, veins and orebodies are truncated by beddingparallel faults. In most such cases, their displaced parts have not been located. Mineralogy and Paragenesis The mineralogy and paragenesis of the Uchucchacua mineralization was described by Alpers (1980), Paz and Pamo (1983), Bussell et al. (1990), and Petersen (1995). Uchucchacua stands out for having an unusually varied mineralogy (with numerous silicates, carbonates, sulfides, and sulfosalts), for its abundance of manganese, silver, arsenic and antimony minerals, and for being the type locality for several rare minerals, such as uchucchacuaite and benavidesite (Oudin et al., 1982; Moëlo et al., 1984). 247


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The following simplified paragenetic sequence is based on descriptions by Alpers (1980a, b), Paz and Pamo (1983), Bussell et al. (1990), and Petersen (1995), as well as on more recent observations: I. Formation of an Mn-bearing exoskarn: rhodonite, ferroan tephroite, johannsenite, quartz, and calcite. II. Base metal stage: deposition of galena, sphalerite, wurtzite, chalcopyrite, tetrahedrite, pyrite, pyrrhotite, arsenopyrite, rhodonite, kutnahorite, rhodochrosite, bustamite, friedelite, alabandite, manganpyrosmalite, manganaxinite, quartz, calcite, fluorite, and magnetite-jacobsite. III. Silver stage: deposition of pyrargyrite-proustite, argentite, miargyrite, polybasite, pyrite, calcite, manganaxinite, kutnahorite, alabandite, stibnite, realgar, orpiment, and some galena, sphalerite, enargite, uchucchacuaite, benavidesite, jamesonite, and bournonite. IV. Supergene oxidation: formation of cerussite, siderite, marcasite, orpiment, goethite, and manganese oxides. The presence of pyrrhotite and arsenopyrite in stage II indicates that this stage precipitated from fluids with a relatively low sulfur fugacity (Barton and Skinner, 1967). In contrast, the presence of enargite in stage III shows that it precipitated from fluids with a relatively high sulfur fugacity (Barton and Skinner, 1979). This difference in sulfur fugacities probably explains why about half of the sphaleritewurtzite grains analyzed by Bussell et al. (1990, table A9) from the Luz vein have <47 percent Zn but >19.5 percent Fe + Mn, whereas the other half have >58.5 percent Zn but <8 percent Fe + Mn (Barton and Skinner, 1967, 1979). The numerous microprobe analyses of sphalerite by Petersen (2000) from the Rubí, Alison, and Lisa replacement orebodies, as well as from the Ramal Cachipampa and Tina veins, reveal ranges of 47.5 to 58.8 percent Zn and 16.0 to 7.8 percent Fe + Mn, corroborating the results of Bussell et al. (1990). The sphalerite analyses of Petersen (2000) further indicate that the full range of sphalerite compositions occurs in all three replacement orebodies and in one of the two veins studied. Only the Veta Ramal Cachipampa shows a more limited range of sphalerite compositions. Petersen (2001) also documented the variation in zinc, iron, and manganese concentrations in sphalerite by means of a comprehensive study of concentrates from the Luz, Tina, and Vanessa veins. The last two veins contain sphalerite with >50 percent Zn and mostly <12 percent Fe and <4 percent Mn, whereas the Luz vein contains sphalerite of both types, but predominantly with <50 percent Zn but >12 percent Fe and >4 percent Mn. The sphalerite poor in zinc but rich in iron plus manganese probably corresponds to the low sulfur fugacity stage II, whereas the one rich in zinc but poor in iron plus manganese is from the high sulfur fugacity stage III. These findings have important implications for the production of high-grade zinc concentrates. The microprobe analyses of Petersen (1995) indicate that the galena contains about 85 percent Pb and 0.17 to 4.1 percent Ag. The 4.1 percent Ag value may have inadvertedly included some tetrahedrite, but 7 (22%) of his 32 analyses have >0.5 percent Ag. This is >5,000 ppm or about 160 oz/t Ag and corresponds to about 2 oz Ag per 1 percent Pb. This galena is 0361-0128/98/000/000-00 $6.00

more silver rich than that typically found in lead-zinc deposits, which contains about 1 oz Ag per 1 percent Pb. The only tetrahedrite analyzed by Petersen (1995) contains 21.7 percent Ag, 20.6 percent Cu, 25.4 percent Sb, and 0.1 percent As. The microprobe analyses also show that most of the sulfosalts contain much more antimony than arsenic. In terms of the metallurgical treatment of the ore, it makes a difference if the manganese is in rhodochrosite or rhodonite, which were deposited during stage II and do not cause metallurgical problems, or in alabandite, which was deposited during stages II and III and causes metallurgical problems. Rhodochrosite and rhodonite appear to dominate in the Carmen section of Uchucchacua, whereas alabandite is more abundant in the remaining sections. Fluid Inclusion and Isotopic Studies In a preliminary investigation, Alpers (1980) studied two fluid inclusions in sphalerite from Uchucchacua, obtaining homogenization temperatures of 280° and 292°C. One of them contained halite and had a salinity of 31.5 wt percent NaCl equiv. He also described secondary fluid inclusions in quartz with homogenization temperatures between 244° and 290°C. Bussell et al. (1990) carried out an extensive study of primary and secondary fluid inclusions in 13 samples of calcite and quartz from stages II and III of the Irma and Rosa Norte replacement orebodies and the Rosa vein. Disregarding the inclusions that are evidently secondary, this range narrows to 156° to 320°C for quartz. The homogenization temperatures for calcite are 185° to 322°C. Given that there are no vaporrich inclusions and that the inclusions did not homogenize to a vapor phase, they concluded that the fluid did not boil during the deposition of calcite and quartz. This conclusion is probably also valid for the economically valuable minerals because the homogenization temperatures for calcite and quartz are in the same temperature range as the few determinations available for sphalerite. The 112 fluid inclusion salinity determinations of Bussell et al. (1990) vary between 0.5 and 29.7 wt percent NaCl equiv and indicate up to 20 wt percent CaCl2 equiv. This variability in salinity may indicate mixing of saline and dilute fluids. Inasmuch as the ore is not located at an intrusive contact and that there is no evidence of significant thermal gradients, Bussell et al. (1990) assumed that a hot fluid rich in NaCl, KCl, and CaCl2 mixed with meteoric water. Both fluids could have acquired calcium from the Jumasha limestone. Bussell et al. (1990) measured the 87Sr/86Sr ratios of vein calcite but had to infer these ratios for potential source rocks. Their estimates are summarized as: Sandstone and shale beneath the Jumasha limestone = 0.709 to 0.722, calcite in ore = 0.707 to 0.711, Jumasha limestone = 0.707 to 0.708, and intrusive rocks = 0.705. The range of the strontium isotope ratios for the 11 hydrothermal calcite samples analyzed matches that inferred for the Jumasha limestone, so it is reasonable to assume that limestone dissolution contributed predominantly to the composition of the fluid that deposited calcite. However, the range for hydrothermal calcite exceeds somewhat that inferred for the Jumasha limestone, so it is possible that this fluid interacted previously with the sandstone and shale

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underlying the Jumasha limestone. The available data do not require interaction with igneous rocks, but the latter may well have acted as heat engines and could have supplied hydrothermal fluids that deposited sulfide minerals. The high salinity and NaCl/CaCl2 ratios of the fluid inclusions led Bussell et al. (1990) to speculate that the hydrothermal fluids of Uchucchacua were similar to those that formed Mississippi Valley-type deposits, although the Uchucchacua fluids have slightly higher KCl/NaCl ratios. However, the high salinities are also compatible with a magmatic-hydrothermal fluid component. Ore deposition took place at a minimum depth of 1,600 m, based on a geologic reconstruction. This would require 15° to 40°C corrections to the homogenization temperatures. Grade Distributions and Zoning During the late 1970s and the 1980s one of us (UP) and collaborators conducted studies to interpret grade distributions

and zoning in hydrothermal ore deposits. These investigations generated grade, metal content (grade × width), and metal ratio distributions in maps and longitudinal vein sections, as well as logarithmic grade and metal content graphs. This work was comprehensively illustrated by Murdock (1989) and culminated in the model presented by Petersen (1990). Uchucchacua was one of the deposits studied (Petersen, 1979; Alpers, 1980a, b; Paz and Pamo, 1983; Moore, 1985; Petersen et al., 1985; Martínez, 1986; Bussell et al., 1990). In retrospect, figures 16 through 22 in Bussell et at. (1990) are not entirely convincing because they do not show the data points in the longitudinal vein sections, they portray metal ratios, which provide less direct evidence than grade and metal content, and they show closure of contours in areas where there are insufficient data points. In light of these concerns, it is appropriate to present in Figure 4a and b the current conceptual model of an “ore band,” in Figures 5 to 8 examples of grade contouring (for the Luz vein), and in Figures 9 to 11

a

Ore island Low grade General tendency Envelope of marginal grade Ore band

Antishoot

b

Low grade Marginal grade Cutoff Ore 50 - 250m Cutoff Marginal grade

Low grade

FIG. 4. Diagrammatic representation of a sinuous ore band in a longitudinal section (a) and in a cross section (b) of a vein.


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PETERSEN ET AL.

examples of logarithmic-grade graphs (for ore reserves of Uchucchacua). The dark sinuous band in Figure 4a represents the axis of maximum grades above a given cutoff grade, i.e., the ore band if this representation is for an economically valuable element or the mineral band if the element is part of the gangue. This band may be locally discontinuous and eventually pinch out at both ends. An ore band may contain high-grade areas (not shown), referred to as “ore shoots” or “bonanza ore” if the pertinent element is of economic interest. Ore bands may contain areas below the cutoff grade, designated as “antishoots.” Grades diminish laterally from the ore and/or mineral band, passing from the grade interval represented by the black band to the grade interval indicated by “×” symbols, and to even lower grades shown by the dotted pattern. Locally, there may be high-grade areas outside of the mineral band, which are referred to as “islands.” The sinuous ore and/or mineral band can generally be envisaged as meandering between two roughly parallel lines (dashed in Fig. 4a), which define its general tendency or trend. This trend can be useful in guiding the exploration for extensions to the ore band. The cross section of the vein in Figure 4b shows the ore interval, which is commonly 50 to 250 m but may be zero where the ore band is discontinuous or terminates. It can be up to 400 m in exceptionally large and rich veins. This cross section implies that vein structures are wider in the ore interval, which is often observed but not necessarily so. There should be one such longitudinal section for every relevant element assayed, but complications arise because an element can occur in various minerals (e.g., silver in argentite, sulfosalts, tetrahedrite, and galena or copper in chalcopyrite and tetrahedrite), and any mineral can consist of various elements. Hence, grade contours generally reflect a composite picture. In most cases, one of the minerals greatly predominates over the others, thus simplifying the interpretation. As a precaution it is generally advisable to study both the distribution of grades and metal contents because both approaches should give similar results (or reveal unusual local circumstances). One reason for contouring grade and metal content intervals is to determine the shape, position, and general tendency of an ore or mineral band in order to follow it efficiently and avoid unproductive exploration in both of its low-grade sides. Another reason is to determine if the high-grade bands for other elements coincide with the ore band or are zoned relative to it. If they coincide, this provides an opportunity to detect erratic values; if they are zoned, this presents an opportunity to diagnose if a given low- grade vein intercept is on one side or the other of the ore band, thus deciding if the next intercept should be aimed lower, higher, or laterally to either one side or the other. For this purpose and assuming that the hydrothermal fluids flowed essentially perpendicular to the ore and mineral bands (thus generating the observed zoning), the senior author (UP) has for many years used the terms “proximal” and “distal”: “proximal” refers to the side of the ore band that presumably is closest to the source of the hydrothermal fluids, and “distal” refers to the side of the ore band toward which the hydrothermal fluids were flowing. For the Luz and Rosa veins, Moore (1985) produced 12 logarithmic-grade and metal content graphs. Of these, the 0361-0128/98/000/000-00 $6.00

Pb-Zn metal content graphs showed good to excellent linear correlation bands that are narrower than half an order of magnitude. This means that in both veins the bands of highest lead and zinc metal contents coincide spatially. In general, this seems to be true throughout the Uchucchacua district, as it is in most but not all mining districts. For the pairs Ag-Mn, Ag-Pb, and Ag-Zn the correlations were poor, resembling the correlation arcs expected when the maximum-grade bands of two elements are zoned relative to each other. For Mn-Pb and Mn-Zn the correlations were very poor, indicating that the maximum manganese grades are clearly zoned relative to the bands of maximum lead and zinc grades. Figures 5 and 6 depict contours for zinc and lead grades in the Luz vein. Note that the values chosen for the grade contours differ for both metals. This is because their correlation band in the pertinent logarithmic graph (not reproduced here) indicates that for every percent lead there is, in general, about 1.3 to 1.5 percent Zn. In Figure 5 there are three antishoots and two ore shoots between the 1.5 and 4 percent Zn contours, and in Figure 6 there are three antishoots between the 1 and 3 percent Pb contours. In Figures 5 and 6 the 1.5 percent Zn and 1.0 percent Pb contours are dome shaped and the 4 percent Zn and 3 percent Pb contours are quite continuous in the vicinity of the 450 level, i.e., the zinc and lead grades increase downward. However, a major crosscut on the 360 level cut very low zinc and lead grades. Of the four lower intercepts one encountered 6.0 percent Zn with 5.2 percent Pb, indicating that the bands of highest zinc and lead grades pass through this intercept. Another deep intercept cut 3.1 percent Zn and 2.4 percent Pb, indicating that it is probably close to the bands of highest zinc and lead grades. The exact geometries of the bands of highest zinc and lead grades remain to be determined below the 450 level, but on the basis of this information they may turn sharply, as indicated by the dotted lines. Figure 7 shows the contours for 10 and 15 oz/t Ag in the Luz vein. Both contours are close to each other because they do not differ appreciably on a logarithmic scale (log 10 = 1.0, log 15 = 1.2). Nevertheless, a dome-shaped silver ore band with four antishoots can be envisaged. It is debatable, however, whether the low silver area in the central part of level 450 is the lower (proximal) side of silver ore band or if this is an antishoot. Considering that the intercept by the crosscut on the 360 level was low grade, it seems likely that the lower limit of the high-grade silver band is indeed crossed by the 450 level. Thus, the high-grade silver band is at a higher elevation than the bands of highest zinc and lead grades. In other words, the silver band is zoned with respect to the zinc and lead bands. In Figure 7 the axis of the band of high silver grades appears to plunge to both the left and right. However, with an average width of 100 to 150 m it could well have turned and passed close to the lower two left intercepts because they cut 14 and 8 oz/t Ag. This interpretation (shown with dotted lines) would be consistent with the interpretations based on zinc and lead grades. Figure 8 shows four contours for 5 percent Mn in the Luz vein. The two lower ones clearly define a robust antiformal band of >5 percent Mn. A comparison with Figure 7 shows that this high-grade manganese band generally overlaps the

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FIG. 5. Zinc grade contours for Luz vein.

FIG. 6. Lead grade contours for Luz vein. 0361-0128/98/000/000-00 $6.00

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FIG. 7. Silver grade contours for Luz vein.

FIG. 8. Manganese grade contours for Luz vein. 0361-0128/98/000/000-00 $6.00

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upper part of the high-grade silver band. Hence, the former is zoned relative to the latter and more so relative to the highgrade zinc and lead bands. However, the upper two 5 percent Mn contours define another high-grade manganese band. Double bands for one element are rare but not impossible or unknown (e.g., separate bands for copper in chalcopyrite and tetrahedrite were documented for the Cananea-Duluth oval vein by Bushnell, 1982). Inasmuch as the older upper levels are no longer accessible, one can only speculate that the two manganese bands correspond to different manganese minerals or that the upper band corresponds to a near-surface enrichment of manganese oxide minerals. Alpers (1980) also contoured vein widths for the Luz vein. A comparison of his section D-12 with Figures 5 to 8 in this paper shows that there is no relationship between vein width and ore grade. MartĂ­nez (1985) made an independent interpretation of the Rosa vein and concluded that the higher grade bands for silver and manganese rise progressively westward and are cut by the Socorro fault, with their continuation being vein 3, 200 m north and 200 m higher. MartĂ­nez (1985) also showed that in the Rosa Norte, Irma, and Viviana replacement orebodies the highest silver grades

are in their middle sections (where their cross sections are widest) and decrease both upward and downward, as well as from the center toward their peripheries. The richest nuclei can be outlined with contours of 30 oz/t Ag for Rosa Norte, 20 oz/t Ag for Viviana, and 15 oz/t Ag for Irma, the last with two nuclei at different elevations. Inasmuch as the higher grade bands for the various metals are somewhat irregular and not perfectly parallel, it is not surprising that Uchucchacua ore has significant but variable concentrations of manganese, silver, zinc, and lead. This is reflected in Table 3, which summarizes the percentages of the total tonnage of the 2002 ore inventory that fall into various types of ore (A through F) in different mine sections (Casualidad, Socorro, and Carmen). The ore types are defined on the basis of grade ranges (i.e., > or <10 oz/t Ag, > or <2% Mn, > or <5% Pb + Zn) and are arranged in the inferred zonal sequence. Thus, ore types A through C have >2 percent Mn, whereas types D through F have <2 percent Mn (except type E, which may not be representative because it involves a relatively small tonnage). Ore types B through D have >10 oz/t Ag, whereas types A and E-F have <10 oz/t Ag. Ore types C through F have >5 percent Pb + Zn, whereas A and B have <5 percent Pb + Zn. All three mine sections contain the A

TABLE 3. Ore Types, Grades, and Widths1 Type

Ag 10

Mn 2

Zn + Pb 5

A A A

< < <

> > >

< < <

Mine section

Tons (%)

Ag equiv (oz/t)

Ag (oz)

Zn (%/t)

Pb (%)

Mn (%)

Width (m)

Casualidad Socorro Carmen

0.5 4.0 10.5

11.8 12.8 12.6

9.7 8.4 8.0

1.5 3.0 2.2

0.6 1.4 2.8

2.5 2.3 6.5

1.43 1.39 12.02

20.3 17.4 15.7

17.4 15.2 13.6

1.3 1.4 1.3

1.9 0.9 0.8

10.8 9.0 7.5

6.28 4.38 1.58

26.0 23.0 19.1

16.5 16.4 12.8

6.3 3.3 2.8

4.1 3.9 3.4

10.5 7.1 4.6

20.30 6.25 1.83

24.1 16.1

11.7 10.0

7.2 5.6

5.1 0.62

1.2 1.6

1.74 1.57

13.0

6.9

3.3

3.4

4.13

5.36

13.7 14.0 15.1 17.4 14.3

6.6 4.2 4.9 7.7 6.1

3.5 5.8 4.5 6.6 6.4

3.7 4.0 5.6 4.7 1.8

1.9 1.2 0.5 1.5 1.9

1.30 1.79 0.90 1.71 1.75

Total B B B

15.0 > > >

> > >

< < <

Carmen Socorro Casualidad

Total C C C

72.4 > > >

> > >

> > >

Carmen Carmen Socorro

Total D D

< <

> >

Socorro Socorro

Total

0.6 0.0 0.6

<

>

>

Carmen

Total F F F F F

2.3 2.0 0.2 4.5

>

E

55.8 11.1 5.5

1.2 1.2

< < < < <

< < < < <

> > > > >

Socorro Casualidad Casualidad Socorro Socorro

Total

0.4 0.7 0.1 1.3 3.8 6.3

Notes: See text for explanation of symbols < and > 1 Source: 2002 ore inventory 2 Nonrepresentative value due to low local tonnage 3 Possibly nonrepresentative value 0361-0128/98/000/000-00 $6.00

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type ore (low in silver and lead + zinc but >2% Mn), the silver-rich B, C, and D types of ore, and the relatively lead-zinc rich ores of types C through F. In Table 3, widths >2 m generally involve replacement orebodies, whereas narrower widths correspond to veins. It is apparent that most of the ore (72.4% of type B) has high silver and manganese grades but low zinc and lead grades, which implies that it consists mostly of stage III mineralization. In contrast, only 6.3 percent of the ore (type F) has high lead-zinc

grades and low silver-manganese grades, consisting mainly of stage II mineralization. The other ore types (21.3%) consist of mixtures of these two stages. The fact that both mineralization stages are present in Carmen, Socorro, and Casualidad supports the idea that probably both stages belong to a single hydrothermal cell rather than to separate cells. Figures 9, 10, and 11 are logarithmic plots of the average grades in Table 3. Figure 9 confirms the linear correlations of silver with manganese and of zinc with lead if the

FIGS. 9. Logarithmic-grade graphs for the ore types listed in Table 1. White squares = types A and B, crosses = types C and E, and black rhombs = types D and F.

FIGS. 10. Logarithmic-grade graphs for the ore types listed in Table 1. White squares = types A and B, crosses = types C and E, and black rhombs = types D and F.

FIGS. 11. Logarithmic-grade graphs for the ore types listed in Table 1. White squares = types A and B, crosses = types C and E, and black rhombs = types D and F. 0361-0128/98/000/000-00 $6.00

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nonrepresentative 0.6 percent Pb grade identified in Table 3 is omitted. The linear correlation band for zinc versus lead has a slope close to 45°, as expected for elements precipitated during the same paragenetic stage. The slope of the linear correlation band for silver-manganese differs from 45° because these elements have different dispersions. These linear correlation bands have widths of about one-half of an order of magnitude, which contrasts with widths of two-thirds of an order of magnitude (or more) commonly observed when plotting grades of individual samples. Figures 10 and 11 are graphs of manganese and silver versus zinc and lead and show the arcuate correlation bands due to zoning. In all these graphs there is a general separation between white and black symbols, with the crosses falling in between. The white squares correspond to high silver and manganese grades (ore types A and B), the black rhombs correspond to high zinc and lead grades (ore types D and F), and the crosses imply intermediate compositions (ore types C and E). Districtwide Ore Distribution and Exploration At Uchucchacua, the ore intervals in the veins and associated replacement orebodies and mantos are generally at about the same elevation. There may be exceptions due to postore faulting, but many of the replacement orebodies were discovered by following veins. The assumption is that tension fractures adjoining faults and veins, as well as cymoid loops and vein junctions or intersections (vein wedges), enhance fluid flow and hence the chances of finding a replacement orebody. For this reason, following veins continues to be a favored exploration tactic. Exploration also focuses on the anticlinal axis shown in Figure 2, on the assumption that this area was more intensely fractured prior to mineralization. Figure 3b illustrates the numerous calcite veinlets that adjoin orebodies at Uchucchacua. Martínez (1986) showed that such calcite veinlets occur above, below, and alongside orebodies at distances from zero to 40 m. The veinlet halos are used consistently and successfully to position drill holes. Martínez (1986) also showed that on surface, at an elevation of 5,050 m, there are calcite veinlets adjoining an apparently barren intrusion. Could there be an orebody below them? Martinez (1986) also studied the possibility of detecting orebodies by means of their geochemical halos. His study was restricted to silver anomalies along workings on the 590 level that radiate from the Rosa Norte orebody toward the north, south, east, and west. Three of the four samples from the orebody returned 360 ppm Ag (10–12 oz/t Ag) and can, therefore, be considered marginal ore. In two of his profiles, the silver grades first decrease away from ore and then increase at greater distances, possibly upon approach to another orebody, because a veinlet was sampled or because of analytical error. Plotting the remaining data on semilogarithmic graphs suggests that the ore could be detected up to 30, 35, 50, and 55 m (avg 42.5 m), using routine geochemical analyses with a detection limit of 1 ppm Ag. The detection limit is now 0.05 ppm Ag. In addition, by choosing other elements, such as lead, zinc, antimony, arsenic, iron, calcium, and manganese, or isotopic signatures, such as δO18 and δC13, it may be possible to further increase the distance and reliability of detection of replacement orebodies and veins. 0361-0128/98/000/000-00 $6.00

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As illustrated by Figures 5 to 8, the ore interval may vary considerably within a given vein as a consequence of spatial changes in permeability and hydrothermal cell geometry at the time of ore deposition. In addition, the elevations of the ore intervals vary from vein to vein (Fig. 12). In general, one can use the mirror-image strategy to explore for ore in nearby veins. According to this strategy, the most efficient way to explore for ore in a neighboring structure is to aim crosscuts or drill holes to intersect the unexplored structure at an elevation that corresponds to the middle of the ore interval in the already-known vein. Using this procedure there is a good chance of intersecting the upper, central, or lower part of its ore interval. Figure 12 shows that the ore intervals are at high elevations in the northwestern part of Mina Carmen, decreasing to the southeast and northwest (in Mina Socorro). In Mina Carmen, the ore intervals pass from member 2 to member 1 of the Jumasha Formation, suggesting that there is no significant stratigraphic control. It remains unclear whether the ore at Mina Carmen and Mina Socorro belongs to two separate hydrothermal cells or whether the two are parts of a single major cell. Figure 12 also shows the intrusive bodies that are inferred to exist at depth on the basis of igneous rock intercepts in the deeper mine workings and in drill holes. This pattern supports the concept of separate hydrothermal cells. However, Figure 12 also suggests that the ore intervals of the various sectors at Uchucchacua could be part of a single major hydrothermal system reminiscent of the Tayoltita and PachucaReal del Monte districts in Mexico. Such a view is supported by indications that the hydrothermal fluids responsible for Uchucchacua may have been sedimentary brines that flowed through extensive aquifers. However, ascending hydrothermal fluids from any source could have mixed with meteoric water to produce the observed undulating ore intervals within veins and replacement orebodies, as well as the variations in the elevation of ore intervals from vein to vein (as indicated by the shaded band that connects the ore intervals in Fig.12). Recent exploration is finding silver ore in Lucrecia, between Carmen and Huantajalla, and in Huantajalla; silverzinc ore was intersected at depth between Carmen and Huantajalla; and lead-zinc ore was cut at depth between Lucrecia and Socorro. Consequently, districtwide exploration now seeks to document and understand the broader mineralization geometry. Conclusions An important lesson provided by Uchucchacua is that in many respects it resembles the famous mining districts with veins, pipes, replacement bodies, and mantos in limestone in Mexico and in the western United States. It is probable that other such deposits can still be found in Peru, where the Andes have a greater proportion of limestone in the stratigraphic column than in countries to both north and south. The limestone has been folded, faulted, and invaded by multiple magma bodies capable of supplying and/or mobilizing hydrothermal fluids for generation of veins and replacement bodies, as Atacocha, Milpo, Cerro de Pasco, Colquijirca, Morococha, and Yauricocha (Fig.1). These districts were discovered because they had already been partially eroded, thus

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FIG. 12. Section A-A' through the Uchucchacua mining district (see Fig. 2 for location of section), showing the general stratigraphic section, the known silver orebodies and intervals, the general trend of these intervals, the recent intercepts of silver and of silver-zinc ore (circles labeled DDH), and the known or inferred intrusions at depth.

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exposing oxidized ore. At Uchucchacua, however, the ore indications are unimpressive at surface because erosion barely reached the uppermost limits of the ore intervals. This is the reason that major mining companies showed no interest in the prospect. It was the progressive deepening of mine workings on economic veins that led to discovery of large tonnage, high-grade orebodies. In the final analysis, this success must be credited to the optimism, foresight, and perseverance of Alberto Benavides and to the support he received from the management and directors of Compañía de Minas Buenaventura, who initially started this mine at a very modest scale. Future discoveries will face similar challenges. Acknowledgments The authors thank Compañía de Minas Buenaventura for its support of the research studies reported in this paper, as well as for its permission to publish the results. In addition, we thank the reviewers of this paper, L. Fontboté, F. Graybeal, and R. Sillitoe for their thoughtful and constructive comments that led to substantial improvements of the manuscript. REFERENCES Alpers, C.N., 1980a, Mineralización de la veta Luz, Uchucchacua: Lima, Compañía de Minas Buenaventura S.A., private report, 7 p. ——1980b, Mineralogy, paragenesis, and zoning of the Luz vein, Uchucchacua, Perú: Unpublished Honors dissertation, Cambridge, Harvard University, 138 p. Barton, P.B., Jr., and Skinner, B.J., 1967, Sulfide mineral stabilities, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits: New York, Holt, Rinehart and Winston, p. 236–333. ——1979, Sulfide mineral stabilities, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits, 2nd ed.: New York, Wiley, p. 278–403. Bushnell, S.E., 1982, Paragenesis and zoning of the Cananea-Duluth breccia pipe, Sonora, Mexico: Unpublished Ph.D. dissertation, Cambridge, Harvard University, 455 p. Bussell, M.A., and Wilson, C.D.V., 1985, A gravity traverse across the Coastal batholith of Peru: Journal of the Geological Society [London], v. 142, p. 633–641. Bussell, M.A., Alpers, C.N., Petersen, U., Shepherd, T.J., Bermudez, C., and Baxter, A.N., 1990, The Ag-Mn-Pb-Zn vein, replacement, and skarn deposits of Uchucchacua, Perú: Studies of structure, mineralogy, metal zoning, Sr isotopes, and fluid inclusions: Economic Geology, v. 85, p. 1348–1383. Cobbing, E.J., 1973, Geología de los cuadrángulos de Barranca, Ambar, Oyón, Huacho, Huaral y Canta: Boletin del Servicio de Geología y Minería del Perú, v. 26, 172 p. Cobbing, E.J., and Garayar, J., 1971, Mapa geológico del cuadrángulo de Oyón: Servicio de Geología y Minería del Perú, Hoja 22j.

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Martinez, P., 1986, Controles y guías en la exploración de cuerpos argentíferos en skarn del yacimiento de Uchucchacua, Cajatambo-Lima: Unpublished Tésis de Maestría, Lima, Universidad Nacional de Ingeniería, 96 p. Moëlo, Y., Oudin, E., Picot, P., and Caye, R., 1984, L’uchucchacuaïte, Ag MnPb3Sb5S12, une nouvelle espèce minérale de la série de l’andorite: Bulletin Minéralogique, v. 107, p. 597–604. Moore, J.C., 1985, Documentation of the absence of significant zoning in two Peruvian hydrothermal deposits: Unpublished Honors dissertation, Cambridge, Harvard University, 117 p. Murdock, G.P., 1989, Application of element distribution analysis to hydrothermal ore deposits: Unpublished Ph.D. dissertation, Cambridge, Harvard University, 550 p. Noble, D.C., 1980, Potassium-argon age determinations on rocks from Raura and Uchucchacua: Lima, Compañía de Minas Buenaventura, private report, 2 p. Noble, D.C., and McKee, E.H., 1999, The Miocene metallogenic belt of central and northern Perú: Society of Economic Geologists Special Publication 7, p. 155–193. Oudin, E., Picot, P., Pillard, F., Moëlo, Y., Burke, E., and Zakrzewski, A., 1982, La benavidesite, Pb4(Mn,Fe)Sb6S14, un nouveau mineral de la série de la jamesonite: Bulletin Minéralogique, v. 105, p. 166–169. Paz, F., and Pamo, G., 1983, Mineralización de plata en cuerpos en la mina Uchucchacua: Lima, Compañía de Minas Buenaventura S.A., private report, 17 p. Petersen, E.U., 1995, Solid-solution compositions of sulfide and sulfosalt minerals from Uchucchacua, Perú: Sociedad Geológica del Perú [Lima] Volúmen Jubilar Alberto Benavides, p. 243–260. ——2000, Esfaleritas de Uchucchacua: Lima, Peru, Compañía de Minas Buenaventura, S.A., private report, 17 p. ——2001, Uchucchacua zinc concentrates: Lima, Peru, Compañía de Minas Buenaventura, S.A., private report, 31 p. Petersen, U., 1979, Exploración y desarrollo de Uchucchacua: Lima, Perú, Compañía de Minas Buenaventura, S.A., private memo, 12 p. ——1990, Ore distribution, zoning, and exploration of hydrothermal ore deposits: Economic Geology, v. 85, p. 424–435. ——1999, Magmatic and metallogenic evolution of the Central Andes: Society of Economic Geologists Special Publication 7, p. 109–153. Petersen, U., Alpers, C., Helmericks, M., and Moore, J., 1985, Metal distribution in the Uchucchacua and Atacocha vein and carbonate replacement districts, Perú [abs.]: American Institute of Mining, Metallurgical and Petroleum Engineers (AIME) 114th Annual Meeting, New York, 1985, Abstracts, p. 74. Romaní, M., 1982, Geologie de la région minière Uchucchacua-Hacienda Otuto, Pérou: Unpublished Ph.D. dissertation, Grenoble, L’Université Scientifique et Medicale de Grenoble, 177 p. Soler, P., and Bonhomme, M.G., 1988, Oligocene magmatic activity and associated mineralization in the polymetallic belt of central Peru: Economic Geology, v. 83, p. 657–663. Torrico y Mesa, J., 1901, Memoria acerca de las riquezas minerales de la provincia de Cajatambo y especialmente de los Cerros de Chanca: Lima, Anales de la Escuela de Construcción Civil y Minas, 70 p.

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2004 Petersen, U., Mayta, O., Gamarra, L., Vidal, C.E. and Sabastizagal, A.  

"Uchucchacua: A major silver producer in South America". In: “Andean metallogeny: new discoveries, concepts, and updates”. Eds: R.H. Sillito...

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