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

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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

AbstractAlthough 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 ex-pansion to today’s 2,000-t/d operation, transforming “Chacua” into the largest primary silver producer in SouthAmerica.

The ores occur in fractures and faults, as well as in pipes, irregular replacement bodies, and mantos hostedby Late Cretaceous limestone. Porphyritic dacite bodies are probably pre-, syn-, and postore. Most of the oreoccurs in distal manganiferous exoskarn and limestone and is mineralogically diverse, consisting mostly of thefollowing.

rhodonite rhodochrosite sphalerite pyrargyrite- quartzbustamite kutnahorite wurtzite proustite pyrite

alabandite galena argentiteThe grade of the ore mined varies between 16 and 20 oz/t Ag combined with about 10 percent Mn, 1.5 per-

cent 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 corre-lations 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 viceversa. The corresponding longitudinal vein sections can generally be contoured unambiguously, showing thatthe bands of highest grades of lead and zinc coincide very well. The highest silver grades can be contoured con-vincingly 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 highestsilver 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 aresult, 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 prox-imity to ore. Underground, multiple calcite veinlets constitute a guide to nearby orebodies. Geochemicalanomalies of 60 to 80 ppm Ag have been documented up to 15 m from an orebody. By extrapolation, 10 ppmAg anomalies may extend 25 m from ore, and 1 ppm Ag anomalies may attain 40 to 45 m. Ore continues to befound at depth as well as laterally and between known ore zones.

ResúmenSi 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 eldescubrimiento de cuerpos de mena en profundidad permitió expansiones hasta llegar a la producción actualde 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, ymantos en calizas del Cretásico Superior. Los intrusivos dacíticos probablemente se emplazaron antes, durantey después de la mineralización. La mayoría de la mena está en exoskarn manganífero distal y en caliza. Sudiversa mineralogía incluye:

rodonita rodocrosita esfalerita proustita- cuarzobustamita kutnahorita wurtzita pirargirita pirita

alabandita galena argentitaLa 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, mientrasque un 7 porciento tiene altas leyes de zinc y plomo con sólo moderada cantidad de plata y poco manganeso.

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

† Corresponding author: e-mail, [email protected]

Introduction

THE UCHUCCHACUA mining district is about 170 km north-northeast of Lima, near the continental divide. Surface eleva-tions range from 4,200 to 5,100 m above sea level (Fig. 1).

The mineralization of Uchucchacua (“little old lady” inQuechua) is close to lake Colquicocha (“lake of silver” inQuechua). According to Torrico and Mesa (1901), there were

already old mine workings up to 90 m deep in 1897. Theseworkings were in oxide ore and stopped upon encounteringsulfides, which, at that time, could not be treated satisfacto-rily. The district did not attract the interest of major miningcompanies because at surface there were only narrow veinswith minor ore. Toward the end of the 1950s, Alberto Bena-vides became interested in Uchucchacua, acquiring andamplifying the available claims for Compañía de Minas

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Los gráficos logarítmicos de leyes muestran buenas correlaciones lineales con pendientes positivas para zincversus plomo, moderadas correlaciones para plata versus manganeso, y arcos de correlación para plata ómanganeso versus plomo ó zinc. Estas relaciones indican que la secuencia zonal es de plomo-zinc a plata-manganeso ó vice-versa. Las secciones longitudinales correspondientes generalmente pueden contornearseconvincentemente de manera que las bandas de leyes altas de plomo y zinc coinciden mayormente. Las leyesaltas de plata pueden contornearse convincentemente formando una banda externa y/ó a mayor altura que lasbandas de leyes mayores de plomo y zinc. Sin embargo, las leyes de manganeso a veces requieren dós bandasde leyes altas: una banda principal que coincide con la banda de leyes altas de plata y una banda secundaria amayor altura que podría representar un enriquecimiento supergeno.

Los intervalos de mena en las vetas, chimeneas y cuerpos de reemplazamiento abarcan hasta unos 200 mverticales. Sin embargo, las elevaciones de estos intervalos cambian progresivamente, reflejando la geometríageneral del sistema hidrotermal responsable de la mineralización. Además, los intervalos de mena han sidodesplazados por fallas. Como resultado, se ha encontrado mena sobre un intervalo de 600 m, entre 4,730 y4,040 m.

En la superficie, la proximidad de la mena puede reconocerce por la presencia de manchas de óxidos demanganeso en la caliza y por limonita en fracturas y fallas. Bajo tierra, la presencia de múltiples venillas decalcita constituye una guía hacia cuerpos de mena cercanos. Además se han documentado anomalíasgeoquímicas de 60 a 80 ppm Ag hasta 15 m de un cuerpo de mena. Extrapolando esta información, puedeinferirse que probablemente se tengan anomalías de 10 ppm Ag hasta 25 m de la mena, y anomalías de 1 ppmAg hasta 40 a 45 m. Todavía se está encontrando mena en profundidad, así como lateralmente y entre laszonas con mena conocida.

LIMA

UCHUCCHACUA

Cerro de Pasco

OC

EA

N

Santander

Minas Ragra

RauraColquipucro

Chanca

PA

CI

FI

C

Huarón

Chungar

Morococha

Casapalca

Antamina

77° 76° 75°

77° 76° 75°

10°

11°

12°

10°

11°

12°

170

km.

0 30 60 km.

Iscaycruz

La Oroya

Atacocha - Milpo

Colquijirca

Yauricocha

FIG. 1. Location of the Uchucchacua mine, Peru.

Buenaventura and carrying out modern exploration and de-velopment by driving about 10 km of tunnels. In 1975 this ledto mining narrow veins at a scale of 200 t/d containing 14 to16 oz/t Ag. The discovery of orebodies at greater depth en-couraged successive production increases to 500, 1,200, andfinally 2,000 t/d containing 14 to 18 oz/t Ag. This transformed“Chacua” into a major South American silver producer, re-covering about 10 million oz of silver in 2003. Ore continuesto be found both at depth and laterally, between and beyondthe known orebodies.

The general geology and mineralization of the Uchuccha-cua district were studied intensively during the first 30 yrs ofoperation by Compañía de Minas Buenaventura, culminatingin a comprehensive paper by Bussell et al. (1990). However,that paper did not describe the geochemical studies byMartínez (1986) and, since then, ore has been found moreabundantly in large orebodies and at greater depths than ex-pected as well as laterally. In addition, the vertical zoning ofthe economically valuable metals has been better docu-mented and understood by plotting grade and metal contentcontours on longitudinal vein sections and verified using log-arithmic graphs. This information is relevant for selecting al-ternative exploration, mining, and concentrating strategies.Extensive electron microprobe analyses (Petersen, 1995,2000, 2001) also identified several new minerals and clarifiedthe sulfur fugacity existing during the various mineralizationstages. Documentation of two sphalerite populations withcontrasting zinc and iron plus manganese contents has im-portant metallurgical implications. More of a curiosity was thediscovery of apparently hypogene native silver. Finally, thesetting of Uchucchacua in the context of Andean magmatismwas further clarified by the studies of Noble and McKee(1999) and Petersen (1999). In order to discuss these new de-velopments in their proper context and to provide an inte-grated picture of this important ore deposit, which may rep-resent a separate silver-manganese model, we summarize thepertinent information provided by Bussell et al. (1990) andadd the new findings.

General GeologyThe regional geology of Uchucchacua (Fig. 2) was de-

scribed by Cobbing and Garayar (1971), Cobbing (1973), Ro-maní (1982), and Bussell et al. (1990). The rocks that host themineralization are mostly limestone and marl of Late Creta-ceous age. These are followed, above a slight unconformity,by Santonian red beds. The Cretaceous sedimentary rockswere strongly folded and faulted prior to the deposition ofTertiary volcanic rocks.

Figure 2 reveals the north-trending axis of the Cachipampaanticline. In addition, there appear to be faults trendingnorth, northeast, east, and southeast that radiate from a cen-ter between the concentrator and the Plomopampa camp.These faults may have been generated by an unexposed in-trusion underlying the center.

Figure 2 also shows segments of thrust faults which do notcause major displacements of stratigraphic contacts. How-ever, the northern radial faults cut the Calipuy volcanic rocks,indicating that the former are younger and possibly related tothe igneous and tectonic processes that gave rise to the min-eralization of the district.

Figure 2 further reveals a small outcrop of dacite with skarnin the Casualidad area. Figure 3 of Bussell et al. (1990) alsoshows two areas of “dacite with skarn in mine” (i.e., projectedto the surface) adjoining the Socorro fault. These three daciteoccurrences may be parts of a single intrusion that was cutand dextrally displaced by the Socorro fault.

Noble (1980) considered that a K-Ar age of 25.3 Ma fordacite from Uchucchacua was unreliable because of the ef-fects of argon metasomatism, but Soler and Bonhomme(1988) thought that this age does reflect the timing of daciteemplacement. Bussell et al. (1990) pointed out that the intru-sions at Uchucchacua are probably 8 to 15 Ma old becausethe intrusions in the nearby Raura deposit gave ages between10.2 and 7.8 Ma (Noble, 1980), and because the gravimetricmodeling of Bussell and Wilson (1985) suggests that theCordillera Blanca batholith continues southward at depth,forming stocks above it at Raura and Uchucchacua. ButNoble and McKee (1999, table 5) prefer an age of 24.5 Ma forUchucchacua based on the age of a relict sanidine phenocrystin a premineral dike. In map 7 of Petersen (1999) Uchuccha-cua lies within a northeast-trending alignment of 25 to 35 Maages, but in his map 3 a 5 to 10 Ma age for Uchucchacua cor-responds 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 Maage of the premineral dike corresponds to either the north-east-trending magmatic alignment or to the Cordillera Blancamagmatism, whereas the 5 to 10 Ma age reflects the positionof the main Cordillera Blanca magmatic belt. More radio-metric ages are needed for fresh and altered intrusive rocks,as well as for the various mineralization stages and their al-teration halos, in order to decipher the magmatic and hy-drothermal chronology of Uchucchacua.

Veins, Orebodies, and MantosThe known intrusions in the Uchucchacua area have re-

lated endoskarn and adjoining exoskarn, but the majority ofthe mineralization occurs in fractures and faults (veins) andin replacement bodies within distal exoskarn or in limestone.In this context, the term distal indicates that the exoskarndoes not directly adjoin an intrusion and has no mineralogicimplications.

The outcrops of mineralized fractures and faults (veins) haveevident black manganese oxide stains (Fig. 3a) that attractedthe initial explorers. Underground proximity to ore is indicatedby multiple calcite veinlets in the limestone (Fig. 3b).

Some orebodies are tubular, subvertical pipes that areovoid in horizontal section. Examples of these are the RosaNorte, Irma, and Viviana bodies discovered because of theirassociation with the Rosa vein. These bodies may have irreg-ular forms but typically have a maximum horizontal width ata certain elevation. For example, Paz and Pamo (1983) pro-vided horizontal measurements for Rosa Norte and Viviana(Table 1), and Martínez (1986) provided vertical ranges forthree orebodies (Table 2). The numbers of Martínez (1986)differ somewhat from those of Paz and Pamo (1983) but re-veal that the vertical ranges of the orebodies are on the orderof 200 to 250 m.

The recently discovered Rubí and Verónica orebodies mea-sure 20 × 70 × 150 and 20 × 15 × 120 m, respectively. It is

UCHUCCHACUA MINING DISTRICT, SOUTH AMERICA 245

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FIG. 2. Geology of the Uchcchacua mining district (after Bussell et al., 1990). A-A' section line for Figure 12.

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 4Jumasha

Marker succession Formation

Member 2

estimated that these orebodies contain about 6 and 3 Moz ofsilver, respectively. Commonly the orebodies are associatedwith veins, faults, or fractures. Locally, ore occurs in mantosthat are concordant with the limestone stratigraphy (Paz andPamo, 1983) and are connected occasionally with the orebodies.

In places, veins and orebodies are truncated by bedding-parallel faults. In most such cases, their displaced parts havenot been located.

Mineralogy and ParagenesisThe mineralogy and paragenesis of the Uchucchacua min-

eralization was described by Alpers (1980), Paz and Pamo(1983), Bussell et al. (1990), and Petersen (1995). Uchuccha-cua stands out for having an unusually varied mineralogy(with numerous silicates, carbonates, sulfides, and sulfosalts),for its abundance of manganese, silver, arsenic and antimonyminerals, and for being the type locality for several rare min-erals, such as uchucchacuaite and benavidesite (Oudin et al.,1982; Moëlo et al., 1984).


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Socorro 3 Vein

a b

TABLE 1. Rosa Norte and Viviana Orebodies Horizontal Measurements(from Paz and Pamo, 1983)

Rosa Norte VivianaLevel m Level m

730 45 × 8 590 30× 15680 50 × 12 565 60 × 14630 62 × 19 550 66 × 18590 40 × 30 500 55 × 8550 15 × 7

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

Orebody Vertical range Maximum width(m) (m) (m)

Rosa Norte 4,765–4,520 245 25?Viviana 4,745–4,530 215 40Irma 4,800–4,550 250 15

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

The following simplified paragenetic sequence is based ondescriptions by Alpers (1980a, b), Paz and Pamo (1983), Bus-sell et al. (1990), and Petersen (1995), as well as on more re-cent observations:

I. Formation of an Mn-bearing exoskarn: rhodonite, fer-roan tephroite, johannsenite, quartz, and calcite.

II. Base metal stage: deposition of galena, sphalerite,wurtzite, chalcopyrite, tetrahedrite, pyrite, pyrrhotite, ar-senopyrite, rhodonite, kutnahorite, rhodochrosite, bustamite,friedelite, alabandite, manganpyrosmalite, manganaxinite,quartz, calcite, fluorite, and magnetite-jacobsite.

III. Silver stage: deposition of pyrargyrite-proustite, argen-tite, miargyrite, polybasite, pyrite, calcite, manganaxinite,kutnahorite, alabandite, stibnite, realgar, orpiment, and somegalena, 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 in-dicates that this stage precipitated from fluids with a rela-tively low sulfur fugacity (Barton and Skinner, 1967). In con-trast, the presence of enargite in stage III shows that itprecipitated from fluids with a relatively high sulfur fugacity(Barton and Skinner, 1979). This difference in sulfur fugaci-ties probably explains why about half of the sphalerite-wurtzite 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 <8percent Fe + Mn (Barton and Skinner, 1967, 1979). The nu-merous microprobe analyses of sphalerite by Petersen (2000)from the Rubí, Alison, and Lisa replacement orebodies, aswell as from the Ramal Cachipampa and Tina veins, revealranges 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). Thesphalerite analyses of Petersen (2000) further indicate thatthe full range of sphalerite compositions occurs in all threereplacement orebodies and in one of the two veins studied.Only the Veta Ramal Cachipampa shows a more limitedrange of sphalerite compositions. Petersen (2001) also docu-mented the variation in zinc, iron, and manganese concen-trations in sphalerite by means of a comprehensive study ofconcentrates from the Luz, Tina, and Vanessa veins. The lasttwo veins contain sphalerite with >50 percent Zn and mostly<12 percent Fe and <4 percent Mn, whereas the Luz veincontains sphalerite of both types, but predominantly with<50 percent Zn but >12 percent Fe and >4 percent Mn. Thesphalerite poor in zinc but rich in iron plus manganese prob-ably corresponds to the low sulfur fugacity stage II, whereasthe one rich in zinc but poor in iron plus manganese is fromthe high sulfur fugacity stage III. These findings have im-portant implications for the production of high-grade zincconcentrates.

The microprobe analyses of Petersen (1995) indicate thatthe galena contains about 85 percent Pb and 0.17 to 4.1 per-cent Ag. The 4.1 percent Ag value may have inadvertedly in-cluded 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 andcorresponds to about 2 oz Ag per 1 percent Pb. This galena is

more silver rich than that typically found in lead-zincdeposits, which contains about 1 oz Ag per 1 percent Pb. Theonly tetrahedrite analyzed by Petersen (1995) contains 21.7percent Ag, 20.6 percent Cu, 25.4 percent Sb, and 0.1 per-cent As. The microprobe analyses also show that most of thesulfosalts contain much more antimony than arsenic.

In terms of the metallurgical treatment of the ore, it makesa difference if the manganese is in rhodochrosite orrhodonite, which were deposited during stage II and do notcause metallurgical problems, or in alabandite, which was de-posited during stages II and III and causes metallurgicalproblems. Rhodochrosite and rhodonite appear to dominatein the Carmen section of Uchucchacua, whereas alabandite ismore abundant in the remaining sections.

Fluid Inclusion and Isotopic StudiesIn a preliminary investigation, Alpers (1980) studied two

fluid inclusions in sphalerite from Uchucchacua, obtaininghomogenization temperatures of 280° and 292°C. One ofthem contained halite and had a salinity of 31.5 wt percentNaCl equiv. He also described secondary fluid inclusions inquartz with homogenization temperatures between 244° and290°C.

Bussell et al. (1990) carried out an extensive study of pri-mary and secondary fluid inclusions in 13 samples of calciteand quartz from stages II and III of the Irma and Rosa Nortereplacement orebodies and the Rosa vein. Disregarding theinclusions that are evidently secondary, this range narrows to156° to 320°C for quartz. The homogenization temperaturesfor calcite are 185° to 322°C. Given that there are no vapor-rich inclusions and that the inclusions did not homogenize toa vapor phase, they concluded that the fluid did not boil dur-ing the deposition of calcite and quartz. This conclusion isprobably also valid for the economically valuable minerals be-cause the homogenization temperatures for calcite and quartzare in the same temperature range as the few determinationsavailable for sphalerite.

The 112 fluid inclusion salinity determinations of Bussell etal. (1990) vary between 0.5 and 29.7 wt percent NaCl equivand indicate up to 20 wt percent CaCl2 equiv. This variabilityin salinity may indicate mixing of saline and dilute fluids.Inasmuch as the ore is not located at an intrusive contact andthat 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 couldhave acquired calcium from the Jumasha limestone. Bussellet al. (1990) measured the 87Sr/86Sr ratios of vein calcite buthad to infer these ratios for potential source rocks. Their esti-mates are summarized as: Sandstone and shale beneath theJumasha limestone = 0.709 to 0.722, calcite in ore = 0.707 to0.711, Jumasha limestone = 0.707 to 0.708, and intrusiverocks = 0.705.

The range of the strontium isotope ratios for the 11 hy-drothermal calcite samples analyzed matches that inferred forthe Jumasha limestone, so it is reasonable to assume thatlimestone dissolution contributed predominantly to the com-position of the fluid that deposited calcite. However, therange for hydrothermal calcite exceeds somewhat that in-ferred for the Jumasha limestone, so it is possible that thisfluid interacted previously with the sandstone and shale

248 PETERSEN ET AL.

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underlying the Jumasha limestone. The available data do notrequire interaction with igneous rocks, but the latter may wellhave acted as heat engines and could have supplied hydro-thermal fluids that deposited sulfide minerals.

The high salinity and NaCl/CaCl2 ratios of the fluid inclu-sions led Bussell et al. (1990) to speculate that the hydrother-mal fluids of Uchucchacua were similar to those that formedMississippi Valley-type deposits, although the Uchucchacuafluids have slightly higher KCl/NaCl ratios. However, thehigh salinities are also compatible with a magmatic-hy-drothermal fluid component. Ore deposition took place at aminimum depth of 1,600 m, based on a geologic reconstruc-tion. This would require 15° to 40°C corrections to the ho-mogenization temperatures.

Grade Distributions and ZoningDuring the late 1970s and the 1980s one of us (UP) and col-

laborators conducted studies to interpret grade distributions

and zoning in hydrothermal ore deposits. These investigationsgenerated grade, metal content (grade × width), and metalratio distributions in maps and longitudinal vein sections, aswell as logarithmic grade and metal content graphs. This workwas comprehensively illustrated by Murdock (1989) and cul-minated in the model presented by Petersen (1990). Uchuc-chacua was one of the deposits studied (Petersen, 1979;Alpers, 1980a, b; Paz and Pamo, 1983; Moore, 1985; Petersenet al., 1985; Martínez, 1986; Bussell et al., 1990). In retro-spect, figures 16 through 22 in Bussell et at. (1990) are notentirely convincing because they do not show the data pointsin the longitudinal vein sections, they portray metal ratios,which provide less direct evidence than grade and metal con-tent, and they show closure of contours in areas where thereare insufficient data points. In light of these concerns, it is ap-propriate to present in Figure 4a and b the current concep-tual model of an “ore band,” in Figures 5 to 8 examples ofgrade contouring (for the Luz vein), and in Figures 9 to 11


Cutoff

50 - 250m

Marginal grade

Ore

Marginal grade

Low grade

Low grade

Cutoff

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

Ore island

Envelope of

Antishoot

marginal grade

Ore band

Generaltendency

Low grade

a

b

examples of logarithmic-grade graphs (for ore reserves ofUchucchacua).

The dark sinuous band in Figure 4a represents the axis ofmaximum grades above a given cutoff grade, i.e., the ore bandif this representation is for an economically valuable elementor the mineral band if the element is part of the gangue. Thisband may be locally discontinuous and eventually pinch out atboth ends. An ore band may contain high-grade areas (notshown), referred to as “ore shoots” or “bonanza ore” if thepertinent element is of economic interest. Ore bands maycontain areas below the cutoff grade, designated as “anti-shoots.” Grades diminish laterally from the ore and/or min-eral band, passing from the grade interval represented by theblack band to the grade interval indicated by “×” symbols,and to even lower grades shown by the dotted pattern. Lo-cally, there may be high-grade areas outside of the mineralband, which are referred to as “islands.” The sinuous oreand/or mineral band can generally be envisaged as meander-ing between two roughly parallel lines (dashed in Fig. 4a),which define its general tendency or trend. This trend can beuseful in guiding the exploration for extensions to the oreband. The cross section of the vein in Figure 4b shows the oreinterval, which is commonly 50 to 250 m but may be zerowhere the ore band is discontinuous or terminates. It can beup to 400 m in exceptionally large and rich veins. This crosssection implies that vein structures are wider in the ore inter-val, which is often observed but not necessarily so. Thereshould be one such longitudinal section for every relevant el-ement assayed, but complications arise because an elementcan occur in various minerals (e.g., silver in argentite, sulfos-alts, tetrahedrite, and galena or copper in chalcopyrite andtetrahedrite), and any mineral can consist of various ele-ments. Hence, grade contours generally reflect a compositepicture. In most cases, one of the minerals greatly predomi-nates over the others, thus simplifying the interpretation. Asa precaution it is generally advisable to study both the distri-bution of grades and metal contents because both approachesshould give similar results (or reveal unusual local circum-stances).

One reason for contouring grade and metal content inter-vals is to determine the shape, position, and general tendencyof an ore or mineral band in order to follow it efficiently andavoid unproductive exploration in both of its low-grade sides.Another reason is to determine if the high-grade bands forother elements coincide with the ore band or are zoned rela-tive to it. If they coincide, this provides an opportunity to de-tect erratic values; if they are zoned, this presents an oppor-tunity to diagnose if a given low- grade vein intercept is onone side or the other of the ore band, thus deciding if the nextintercept should be aimed lower, higher, or laterally to eitherone side or the other. For this purpose and assuming that thehydrothermal fluids flowed essentially perpendicular to theore 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 theore band that presumably is closest to the source of the hy-drothermal fluids, and “distal” refers to the side of the oreband toward which the hydrothermal fluids were flowing.

For the Luz and Rosa veins, Moore (1985) produced 12logarithmic-grade and metal content graphs. Of these, the

Pb-Zn metal content graphs showed good to excellent linearcorrelation bands that are narrower than half an order ofmagnitude. This means that in both veins the bands of high-est lead and zinc metal contents coincide spatially. In general,this seems to be true throughout the Uchucchacua district, asit is in most but not all mining districts. For the pairs Ag-Mn,Ag-Pb, and Ag-Zn the correlations were poor, resembling thecorrelation arcs expected when the maximum-grade bands oftwo elements are zoned relative to each other. For Mn-Pband Mn-Zn the correlations were very poor, indicating thatthe maximum manganese grades are clearly zoned relative tothe bands of maximum lead and zinc grades.

Figures 5 and 6 depict contours for zinc and lead grades inthe Luz vein. Note that the values chosen for the grade con-tours differ for both metals. This is because their correlationband in the pertinent logarithmic graph (not reproducedhere) indicates that for every percent lead there is, in general,about 1.3 to 1.5 percent Zn. In Figure 5 there are three anti-shoots and two ore shoots between the 1.5 and 4 percent Zncontours, and in Figure 6 there are three antishoots betweenthe 1 and 3 percent Pb contours.

In Figures 5 and 6 the 1.5 percent Zn and 1.0 percent Pbcontours are dome shaped and the 4 percent Zn and 3 per-cent Pb contours are quite continuous in the vicinity of the450 level, i.e., the zinc and lead grades increase downward.However, a major crosscut on the 360 level cut very low zincand lead grades. Of the four lower intercepts one encoun-tered 6.0 percent Zn with 5.2 percent Pb, indicating that thebands of highest zinc and lead grades pass through this inter-cept. Another deep intercept cut 3.1 percent Zn and 2.4 per-cent Pb, indicating that it is probably close to the bands ofhighest zinc and lead grades. The exact geometries of thebands of highest zinc and lead grades remain to be deter-mined below the 450 level, but on the basis of this informa-tion they may turn sharply, as indicated by the dotted lines.

Figure 7 shows the contours for 10 and 15 oz/t Ag in theLuz vein. Both contours are close to each other because theydo not differ appreciably on a logarithmic scale (log 10 = 1.0,log 15 = 1.2). Nevertheless, a dome-shaped silver ore bandwith four antishoots can be envisaged. It is debatable, how-ever, whether the low silver area in the central part of level450 is the lower (proximal) side of silver ore band or if this isan antishoot. Considering that the intercept by the crosscuton the 360 level was low grade, it seems likely that the lowerlimit of the high-grade silver band is indeed crossed by the450 level. Thus, the high-grade silver band is at a higher ele-vation than the bands of highest zinc and lead grades. In otherwords, the silver band is zoned with respect to the zinc andlead bands.

In Figure 7 the axis of the band of high silver grades ap-pears to plunge to both the left and right. However, with anaverage width of 100 to 150 m it could well have turned andpassed close to the lower two left intercepts because they cut14 and 8 oz/t Ag. This interpretation (shown with dottedlines) would be consistent with the interpretations based onzinc and lead grades.

Figure 8 shows four contours for 5 percent Mn in the Luzvein. The two lower ones clearly define a robust antiformalband of >5 percent Mn. A comparison with Figure 7 showsthat 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.

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FIG. 7. Silver grade contours for Luz vein.

FIG. 8. Manganese grade contours for Luz vein.

upper part of the high-grade silver band. Hence, the formeris zoned relative to the latter and more so relative to the high-grade zinc and lead bands. However, the upper two 5 percentMn contours define another high-grade manganese band.Double bands for one element are rare but not impossible orunknown (e.g., separate bands for copper in chalcopyrite andtetrahedrite were documented for the Cananea-Duluth ovalvein by Bushnell, 1982). Inasmuch as the older upper levelsare no longer accessible, one can only speculate that the twomanganese bands correspond to different manganese miner-als or that the upper band corresponds to a near-surface en-richment 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 thispaper shows that there is no relationship between vein widthand ore grade.

Martínez (1985) made an independent interpretation of theRosa vein and concluded that the higher grade bands for sil-ver and manganese rise progressively westward and are cut bythe Socorro fault, with their continuation being vein 3, 200 mnorth and 200 m higher.

Martínez (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 arewidest) and decrease both upward and downward, as well asfrom the center toward their peripheries. The richest nucleican 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 withtwo nuclei at different elevations.

Inasmuch as the higher grade bands for the various metalsare somewhat irregular and not perfectly parallel, it is notsurprising that Uchucchacua ore has significant but variableconcentrations of manganese, silver, zinc, and lead. This is re-flected in Table 3, which summarizes the percentages of thetotal tonnage of the 2002 ore inventory that fall into varioustypes of ore (A through F) in different mine sections (Casual-idad, Socorro, and Carmen). The ore types are defined on thebasis of grade ranges (i.e., > or <10 oz/t Ag, > or <2% Mn, >or <5% Pb + Zn) and are arranged in the inferred zonal se-quence. Thus, ore types A through C have >2 percent Mn,whereas types D through F have <2 percent Mn (except typeE, which may not be representative because it involves a rel-atively small tonnage). Ore types B through D have >10 oz/tAg, whereas types A and E-F have <10 oz/t Ag. Ore types Cthrough F have >5 percent Pb + Zn, whereas A and B have<5 percent Pb + Zn. All three mine sections contain the A


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TABLE 3. Ore Types, Grades, and Widths1

Ag Mn Zn + Pb Tons Ag equiv Ag Zn Pb Mn WidthType 10 2 5 Mine section (%) (oz/t) (oz) (%/t) (%) (%) (m)

A < > < Casualidad 0.5 11.8 9.7 1.5 0.6 2.5 1.43A < > < Socorro 4.0 12.8 8.4 3.0 1.4 2.3 1.39A < > < Carmen 10.5 12.6 8.0 2.2 2.8 6.5 12.02

Total 15.0

B > > < Carmen 55.8 20.3 17.4 1.3 1.9 10.8 6.28B > > < Socorro 11.1 17.4 15.2 1.4 0.9 9.0 4.38B > > < Casualidad 5.5 15.7 13.6 1.3 0.8 7.5 1.58

Total 72.4

C > > > Carmen 2.3 26.0 16.5 6.3 4.1 10.5 20.30C > > > Carmen 2.0 23.0 16.4 3.3 3.9 7.1 6.25C > > > Socorro 0.2 19.1 12.8 2.8 3.4 4.6 1.83

Total 4.5

D < > Socorro 0.6 24.1 11.7 7.2 5.1 1.2 1.74D > < > Socorro 0.0 16.1 10.0 5.6 0.62 1.6 1.57

Total 0.6

E < > > Carmen 1.2 13.0 6.9 3.3 3.4 4.13 5.36

Total 1.2

F < < > Socorro 0.4 13.7 6.6 3.5 3.7 1.9 1.30F < < > Casualidad 0.7 14.0 4.2 5.8 4.0 1.2 1.79F < < > Casualidad 0.1 15.1 4.9 4.5 5.6 0.5 0.90F < < > Socorro 1.3 17.4 7.7 6.6 4.7 1.5 1.71F < < > Socorro 3.8 14.3 6.1 6.4 1.8 1.9 1.75

Total 6.3

Notes: See text for explanation of symbols < and >1 Source: 2002 ore inventory2 Nonrepresentative value due to low local tonnage3 Possibly nonrepresentative value

type ore (low in silver and lead + zinc but >2% Mn), the sil-ver-rich B, C, and D types of ore, and the relatively lead-zincrich ores of types C through F.

In Table 3, widths >2 m generally involve replacement ore-bodies, whereas narrower widths correspond to veins. It is ap-parent that most of the ore (72.4% of type B) has high silverand manganese grades but low zinc and lead grades, whichimplies that it consists mostly of stage III mineralization. Incontrast, only 6.3 percent of the ore (type F) has high lead-zinc

grades and low silver-manganese grades, consisting mainly ofstage II mineralization. The other ore types (21.3%) consist ofmixtures of these two stages. The fact that both mineralizationstages are present in Carmen, Socorro, and Casualidad sup-ports the idea that probably both stages belong to a single hy-drothermal cell rather than to separate cells.

Figures 9, 10, and 11 are logarithmic plots of the averagegrades in Table 3. Figure 9 confirms the linear correla-tions of silver with manganese and of zinc with lead if the

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FIGS. 11. Logarithmic-grade graphs for the ore types listed in Table 1. White squares = types A and B, crosses = types Cand E, and black rhombs = types D and F.



nonrepresentative 0.6 percent Pb grade identified in Table 3is omitted. The linear correlation band for zinc versus leadhas a slope close to 45°, as expected for elements precipitatedduring the same paragenetic stage. The slope of the linearcorrelation band for silver-manganese differs from 45° be-cause these elements have different dispersions. These linearcorrelation bands have widths of about one-half of an order ofmagnitude, which contrasts with widths of two-thirds of anorder of magnitude (or more) commonly observed when plot-ting grades of individual samples.

Figures 10 and 11 are graphs of manganese and silver ver-sus zinc and lead and show the arcuate correlation bands dueto zoning. In all these graphs there is a general separation be-tween white and black symbols, with the crosses falling in be-tween. The white squares correspond to high silver and man-ganese grades (ore types A and B), the black rhombscorrespond to high zinc and lead grades (ore types D and F),and the crosses imply intermediate compositions (ore types Cand E).

Districtwide Ore Distribution and ExplorationAt Uchucchacua, the ore intervals in the veins and associ-

ated replacement orebodies and mantos are generally at aboutthe same elevation. There may be exceptions due to post-ore faulting, but many of the replacement orebodies were dis-covered by following veins. The assumption is that tensionfractures adjoining faults and veins, as well as cymoid loopsand vein junctions or intersections (vein wedges), enhancefluid flow and hence the chances of finding a replacementorebody. For this reason, following veins continues to be a fa-vored exploration tactic. Exploration also focuses on the anti-clinal axis shown in Figure 2, on the assumption that this areawas more intensely fractured prior to mineralization.

Figure 3b illustrates the numerous calcite veinlets that ad-join orebodies at Uchucchacua. Martínez (1986) showed thatsuch calcite veinlets occur above, below, and alongside ore-bodies at distances from zero to 40 m. The veinlet halos areused consistently and successfully to position drill holes.Martínez (1986) also showed that on surface, at an elevationof 5,050 m, there are calcite veinlets adjoining an apparentlybarren intrusion. Could there be an orebody below them?

Martinez (1986) also studied the possibility of detectingorebodies by means of their geochemical halos. His study wasrestricted to silver anomalies along workings on the 590 levelthat radiate from the Rosa Norte orebody toward the north,south, east, and west. Three of the four samples from the ore-body returned 360 ppm Ag (10–12 oz/t Ag) and can, there-fore, be considered marginal ore. In two of his profiles, thesilver grades first decrease away from ore and then increaseat greater distances, possibly upon approach to another ore-body, because a veinlet was sampled or because of analyticalerror. Plotting the remaining data on semilogarithmic graphssuggests that the ore could be detected up to 30, 35, 50, and55 m (avg 42.5 m), using routine geochemical analyses with adetection limit of 1 ppm Ag. The detection limit is now 0.05ppm Ag. In addition, by choosing other elements, such aslead, zinc, antimony, arsenic, iron, calcium, and manganese,or isotopic signatures, such as δO18 and δC13, it may be possi-ble to further increase the distance and reliability of detectionof replacement orebodies and veins.

As illustrated by Figures 5 to 8, the ore interval may varyconsiderably within a given vein as a consequence of spatialchanges in permeability and hydrothermal cell geometry atthe time of ore deposition. In addition, the elevations of theore intervals vary from vein to vein (Fig. 12). In general, onecan use the mirror-image strategy to explore for ore in nearbyveins. According to this strategy, the most efficient way to ex-plore for ore in a neighboring structure is to aim crosscuts ordrill holes to intersect the unexplored structure at an eleva-tion that corresponds to the middle of the ore interval in thealready-known vein. Using this procedure there is a goodchance of intersecting the upper, central, or lower part of itsore interval.

Figure 12 shows that the ore intervals are at high elevationsin the northwestern part of Mina Carmen, decreasing to thesoutheast and northwest (in Mina Socorro). In Mina Carmen,the ore intervals pass from member 2 to member 1 of the Ju-masha Formation, suggesting that there is no significantstratigraphic control. It remains unclear whether the ore atMina Carmen and Mina Socorro belongs to two separate hy-drothermal cells or whether the two are parts of a singlemajor cell.

Figure 12 also shows the intrusive bodies that are inferredto exist at depth on the basis of igneous rock intercepts in thedeeper mine workings and in drill holes. This pattern sup-ports the concept of separate hydrothermal cells. However,Figure 12 also suggests that the ore intervals of the varioussectors at Uchucchacua could be part of a single major hy-drothermal system reminiscent of the Tayoltita and Pachuca-Real del Monte districts in Mexico. Such a view is supportedby indications that the hydrothermal fluids responsible forUchucchacua may have been sedimentary brines that flowedthrough extensive aquifers. However, ascending hydrother-mal fluids from any source could have mixed with meteoricwater to produce the observed undulating ore intervalswithin veins and replacement orebodies, as well as the varia-tions in the elevation of ore intervals from vein to vein (as in-dicated by the shaded band that connects the ore intervals inFig.12). Recent exploration is finding silver ore in Lucrecia,between Carmen and Huantajalla, and in Huantajalla; silver-zinc ore was intersected at depth between Carmen andHuantajalla; and lead-zinc ore was cut at depth between Lu-crecia and Socorro. Consequently, districtwide explorationnow seeks to document and understand the broader miner-alization geometry.

ConclusionsAn important lesson provided by Uchucchacua is that in

many respects it resembles the famous mining districts withveins, pipes, replacement bodies, and mantos in limestone inMexico and in the western United States. It is probable thatother such deposits can still be found in Peru, where theAndes have a greater proportion of limestone in the strati-graphic column than in countries to both north and south.The limestone has been folded, faulted, and invaded by mul-tiple magma bodies capable of supplying and/or mobilizinghydrothermal fluids for generation of veins and replacementbodies, as Atacocha, Milpo, Cerro de Pasco, Colquijirca, Mo-rococha, and Yauricocha (Fig.1). These districts were discov-ered because they had already been partially eroded, thus


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exposing oxidized ore. At Uchucchacua, however, the ore in-dications are unimpressive at surface because erosion barelyreached the uppermost limits of the ore intervals. This is thereason that major mining companies showed no interest inthe prospect. It was the progressive deepening of mine work-ings on economic veins that led to discovery of large tonnage,high-grade orebodies. In the final analysis, this success mustbe credited to the optimism, foresight, and perseverance ofAlberto Benavides and to the support he received from themanagement and directors of Compañía de Minas Buenaven-tura, who initially started this mine at a very modest scale. Fu-ture discoveries will face similar challenges.

AcknowledgmentsThe authors thank Compañía de Minas Buenaventura for its

support of the research studies reported in this paper, as wellas for its permission to publish the results. In addition, wethank the reviewers of this paper, L. Fontboté, F. Graybeal,and R. Sillitoe for their thoughtful and constructive commentsthat led to substantial improvements of the manuscript.

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Compañía de Minas Buenaventura S.A., private report, 7 p.——1980b, Mineralogy, paragenesis, and zoning of the Luz vein, Uchuccha-

cua, Perú: Unpublished Honors dissertation, Cambridge, Harvard Univer-sity, 138 p.

Barton, P.B., Jr., and Skinner, B.J., 1967, Sulfide mineral stabilities, inBarnes, 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 ofhydrothermal ore deposits, 2nd ed.: New York, Wiley, p. 278–403.

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——2000, Esfaleritas de Uchucchacua: Lima, Peru, Compañía de MinasBuenaventura, S.A., private report, 17 p.

——2001, Uchucchacua zinc concentrates: Lima, Peru, Compañía de MinasBuenaventura, S.A., private report, 31 p.

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——1990, Ore distribution, zoning, and exploration of hydrothermal ore de-posits: Economic Geology, v. 85, p. 424–435.

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