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Chapter 15

Porphyry-Epithermal Transition, Cajamarca Region, Northern Peru

LEWIS B. GUSTAFSON,†

5320 Cross Creek Lane, Reno, Nevada 89511

CÉSAR E. VIDAL,Compañía de Minas Buenaventura S.A.A., Carlos Villarán 790, Santa. Catalina, Lima 13, Perú

RITA PINTO,Minera Yanacocha S.R.L., Jirón Puno 255, Cajamarca, Perú

AND DONALD C. NOBLE

3450 Rolling Ridge Road, Reno, Nevada 89506

AbstractAt least 14 porphyry copper-gold deposits and 19 epithermal gold deposits are known within 60 km of Caja-

marca. The partly explored porphyry deposits vary in grade, Cu-Au-Mo proportions, and depth of erosion. As-sociated epithermal mineralization occurs at Perol, Peña de las Águilas, Kupfertal, Yanacocha Norte, MaquiMaqui, and Pampa Verde but not at Michiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sorpresa, Col-payoc, and Chamis. These deposits are associated with Miocene magmatic activity, northwest-trending foldsand thrusts, and northeast-trending faults.

In the porphyry deposits, granular A quartz veins, associated with K-feldspar-biotite alteration and dissemi-nated chalcopyrite-magnetite with bornite or pyrite, are typically present within and about multiple coeval por-phyry intrusions. Banded quartz veins occur near the tops of some shallowly eroded systems, and late sericite-pyrite ± chalcopyrite is superimposed on most. Epithermal mineralization is mostly of high-sulfidationcharacter, with pyrite-enargite-covellite typically underlying oxide Au zones leached of Cu. Epithermal Au-Cuis associated with multiple stages of brecciation and intense silicification, zoned outward and downward withdecreasing SiO2 and Au through quartz-pyrophyllite-diaspore-alunite-dickite to quartz-alunite and kaolinite.Structurally controlled, high-grade Au is apparently late and associated locally with intermediate-sulfidation as-semblages, barite, and chalcedony.

The transition between porphyry and epithermal environments is exposed at Perol and Huaylamachay, LaZanja, and especially Tantahuatay and Yanacocha. At Perol and Huaylamachay, porphyry gold-copper depositsare adjacent to generally contemporaneous volcanic vents altered to quartz-alunite with minor structures con-taining quartz-pyrophyllite-alunite-Au. At Perol, the dacitic vent is intruded by a later mineralized porphyry,whereas at Huaylamachay the vent breccia contains clasts with quartz-molybdenite veins and is cut by bandedquartz veins, which we interpret as indicating a second, deeper porphyry Au system.

At Tantahuatay, an andesitic dome complex is pervasively brecciated and altered to quartz-alunite-pyrophyl-lite-diaspore ± dickite, with extensive pyrite-enargite-covellite-(bornite) veins and disseminations beneath Au-rich oxide mineralization. A gusano texture of soft, round patches of pyrophyllite-diaspore and/or alunite in asilicified matrix is widespread and associated with anomalous concentrations of Mo. Only one of several drillholes to 600-m depth encountered A quartz veins and minor porphyry intrusions. This hole provides evidencefor prograde advance of quartz veining associated with one or more porphyry intrusions into the epithermal en-vironment and subsequent retrograde collapse.

At Yanacocha, the most abundant evidence of direct, albeit complex, spatial and temporal relationships be-tween multiple centers of epithermal mineralization and porphyry intrusion and mineralization has been par-tially deciphered. At Kupfertal, the matrix of gusano alteration above the top of the porphyry becomes in-creasingly silicified and patchy downward, developing very contorted wormy quartz veins that overlap the topof A quartz veins. Intense pyritic quartz-pyrophyllite-diaspore-alunite and underlying sericite alteration is su-perimposed on K-feldspar-biotite alteration of the early stage. Fluid inclusions in quartz are vapor dominant,with downward-increasing proportions of high-salinity inclusions and amounts of minute relict chalcopyrite ±bornite grains “locked” in A vein quartz. A-veined and advanced argillic-altered xenoliths in pyroclastic rocksintruded by porphyries and hosting gold mineralization demonstrate multiple generations of porphyry and ep-ithermal mineralization. Early Cu and Au of the porphyry event appear to have been remobilized and incor-porated into the overlying epithermal system.

©2004 Society of Economic GeologistsSpecial Publication 11, 2004, pp. 279–299

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

Introduction

PORPHYRY Cu-Au-Mo and epithermal Au-Ag orebodies aremajor sources of these metals and have been the focus of ex-ploration efforts for several decades. Based on the commonoccurrence of each in the same districts and indirect evi-dence, many geologists have proposed a genetic connectionbetween the two (e.g., Wallace, 1979; Asami and Britten,1980; Sillitoe, 1983, 1988, 1989; Heald et al., 1987; Rye,1993). It has been well established that at upper levels por-phyry Cu systems commonly evolve from potassic and inter-mediate-sulfidation–state alteration assemblages to superim-posed advanced argillic and high-sulfidation–state assemblages(Meyer and Hemley, 1967; Gustafson and Hunt, 1975; Ein-audi, 1977, 1982; Brimhall, 1979; Arribas, 1995; Einaudi etal., 2003). More recently, Arribas et al. (1995) and Heden-quist et al. (1998) have documented a close temporal and ge-netic relationship between the Lepanto epithermal Cu-Au

deposit, with pyrite-luzonite and advanced argillic alteration,and the subjacent Far Southeast porphyry Cu-Au deposit.Muntean and Einaudi (2001), in the most detailed and com-plete study of the subject published to date, have docu-mented the porphyry-epithermal transition in three districtswithin the Maricunga belt of Chile. Recently, Rohrlach(2002) has established temporal and genetic relationships be-tween the Tampakan epithermal Cu-Au deposit superim-posed on a slightly older porphyry system, all within an evolv-ing volcanic edifice.

The historic provincial capital of Cajamarca is located innorthern Peru, about 570 km north-northwest of Lima and120 km inland from the Pacific Ocean. Within 60 km to thenorth of Cajamarca, there are at least 14 known porphyry Cu-Au-Mo deposits and 19 epithermal Au-Ag deposits (Fig. 1).The partly explored porphyry deposits vary in grade, Cu-Au-Mo proportions, and depth of erosion, and not all are asso-ciated with known epithermal mineralization. Associated

280 GUSTAFSON ET AL.

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ResumenEn la región de Cajamarca se han reportado por lo menos 14 yacimientos del tipo pórfidos Cu(Au) y 19

yacimientos epitermales. Los yacimientos porfiríticos han sido parcialmente explorados y varían en leyes, pro-porciones de Cu-Au-Mo y en el nivel de erosión. La mineralización epitermal ocurre asociada a los pórfidos enprospectos tales como Perol, Peña de las Aguilas, Kupfertal, Yanacocha Norte, Maqui Maqui y Pampa Verdemas no aparece en Michiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sorpresa, Colpayoc, y Chamis. Re-gionalmente, todos estos depósitos están relacionados con actividades magmáticas del Mioceno, con pliegues ysobreescurrimientos de rumbo Noroeste y fallas o fracturas transandinas de rumbo Noreste.

Los yacimientos del tipo pórfido presentan vetillas tipo A con alteración asociada de feldespato potásico y bi-otita secundaria con diseminación de calcopirita-magnetita y con bornita o pirita dentro y alrededor de intru-siones porfiríticas, múltiples y coetáneas. Cerca de los techos en algunos sistemas poco erosionados, ocurrenvetillas bandeadas de cuarzo, la sobreimposición tardía de sericita-pirita ± calcopirita es muy frecuente. Lamineralización epitermal es predominantemente de carácter alta sulfuración, con pirita-enargita-covelita-bajoencapes lixiviados de cobre con óxidos auríferos. Esta etapa epitermal está asociada con múltiples etapas debrechamiento y con intensa silicificación; la misma que está zonificada hacia fuera y en profundidad con menosSiO2 y Au a través de cuarzo-pirofilita-diáspora-alunita-dickita, luego cuarzo-alunita y finalmente caolinita.Ocasionalmente aparecen altas leyes de Au en paragénesis tardías, controladas estructuralmente y asociadas aensambles de sulfuración intermedia con baritina y calcedonia.

La transición entre los ambientes epitermales y porfiríticos está expuesta en Perol y en Huaylamachay, LaZanja y especialmente en Tantahuatay y Yanacocha. En Perol y en Huaylamachay, los pórfidos de Au-Cuestán adyacentes a chimeneas volcánicas alteradas a cuarzo-alunita con estructuras menores que contienencuarzo-pirofilita-alunita-Au. En Perol, la chimenea dacítica está intruida por un stock mineralizado tardío.En Huaylamachay la chimenea de brecha contiene clastos con vetillas de cuarzo-molibdenita y, a la vez, esrecortado por vetillas de cuarzo bandeado, lo cual interpretamos como indicios de un segundo sistema por-firítico en profundidad.

En Tantahuatay, el complejo de domos de composición andesítica está profusamente brechado y alteradoa cuarzo-alunita-pirofilita-diáspora +/- dickita con abundante mineralización de pirita-enargita-covelita (bor-nita) en venas y como matriz de brechas por debajo de óxidos auríferos de baja ley. Una textura “agusanada”o tipo “gusano” consiste en parches redondeados de pirofilita-diáspora y/o alunita en una matriz silicificadacon fuerte anomalía de Mo. Uno de los sondajes diamantinos profundos encontró vetillas tipo A y manifesta-ciones de intrusivos porfiríticos menores. Este sondaje presenta evidencia para un vetilleo de cuarzo progradoasociado a una o más intrusiones porfiríticas en el ambiente epitermal con un subsecuente colapso retrógradodel sistema.

En Yanacocha, tenemos la evidencia más clara y abundante de las relaciones directas, aunque complejas,entre múltiples centros de mineralización epitermal. En Kupfertal, la matriz de las alteraciones tipo “gusano”por encima del techo de los pórfidos se torna más silicificada y con mayor proporción de parches que en pro-fundidad desarrollan venillas de cuarzo contorsionadas, las cuales parecen ser transicionales a las vetillas tipoA. Una intensa alteración piritosa con cuarzo-pirofilita-diáspora-alunita y alteración sericítica en profundidadestán sobreimpuestas en zonas de alteración con feldespato potásico y biotita de la primera etapa. Las inclu-siones fluidas en cuarzo están dominadas por vapor con algunas inclusiones de alta salinidad que aumentan enprofundidad y granos pequeños de calcopirita +/- bornita atrapadas en vetillas de cuarzo tipo A. Notamosmúltiples generaciones de mineralización porfirítica y epitermal en xenolitos de roca piroclástica intruida porpórfidos con vetillas tipo A y alteracilón argílica avanzada. La mineralización porfirítica temprana de Cu y Auparecería estar removilizada y haber sido incorporada en el sistema epitermal.

epithermal mineralization occurs at Perol, Peña de lasÁguilas, Kupfertal, Yanacocha Norte, Maqui Maqui, andPampa Verde. The absence of epithermal mineralization atMichiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sor-presa, Colpayoc, and Chamis is possibly related to depth oferosion. This study describes the time-space relationships be-tween porphyry and epithermal deposits within three districtsin this region, Minas Conga (CG and PL in Fig. 1), Hual-gayoc-Tantahuatay (HG, CC, and TA), and Yanacocha, wherethey are well displayed. This is an area of active explorationand mine development, particularly at Yanacocha, where newfeatures are continually being discovered. This paper pre-sents descriptive evidence on the nature of the transitionfrom porphyry to epithermal environments. We leave to oth-ers the explanation of chemical mechanisms.

Regional GeologyThe deposits under consideration are part of the extensive

Miocene metallogenic belt of central and northern Peru.Noble and McKee (1999) gave a comprehensive list of refer-ences to individual deposits. The oldest rocks in the regionare Paleozoic to Mesozoic platform sedimentary rocks (Fig.1). References to the stratigraphy and structure include Be-navides-Cáceres (1956, 1999), Mégard (1978, 1984, 1987),Cobbing et al. (1981), Cobbing (1985), and Noble et al. (1990).

In the Cajamarca region, there is more than 2 km of Lowerto Upper Cretaceous strata, beginning with arenites of theGollarisquizga Formation, overlain by increasingly calcareousstrata of the Inca, Chulec, Pariatambo, Yumagual, and Mu-jarrun Formations, and ending with the Cajamarca Forma-tion (Benavides-Cáceres, 1956; Wilson, 1984). These areoverlain by a thick and extensive sequence of volcanic rocksof Eocene to late Miocene age. The area of volcanic rocks inFigure 1 includes several volcanic fields of pyroclastic, flow,and domal rocks of rhyolitic to andesitic composition andprobably mainly of Miocene age. The best documented is theroughly 25- by 20-km Yanacocha volcanic complex, dated at12.5 to 11.8 Ma, which covers most of the Yanacocha district(Longo, 2000). Most of these volcanic rocks are altered andmineralized in the vicinity of the ore deposits. Pyroclasticrocks postdating mineralization (Fraylones Formation) arescattered across the area, covering altered rocks. These all ap-pear to be younger than the 9 Ma Quechua II orogenic event(Noble and McKee, 1999). All of the mineral deposits in thisregion that have been isotopically dated were formed duringthe Miocene, from about 19 to 10 Ma (Noble and McKee,1999; Noble et al., 2004).

Both the Mesozoic sedimentary and all but the youngestCenozoic volcanic rocks were progressively folded andfaulted during several tectonic episodes, starting with the

PORPHYRY-EPITHERMAL TRANSITION, CAJAMARCA, N. PERU 281

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COASTAL BATHOLITH

CAJAM ARCA

LZLZ

SPSP

CCCC

PLPL

MQMQYANACOCHA

0

km

40

N

CHCH

COCO

CGCGGAGA

TATAHGHG

LCLC

KUKU

PACIFICOCEAN

73°81°

PUCALLPA

HUANUCO

HUARAZ

CAJAMARCA

TRUJILLO8°

CHICLA YO

B R A S I L

E C U A D O R C O L O M B I A

PIURA

MOYOBAMBA

IQUITOS

0°

Paleozoic-Mesozoicsedimentary rocks

EPITHERMAL DEPOSITS

Andean parallel structures

Trans-Andean structures

Tertiaryvolcanic rocks

LZ - La Zanja

SP - Sipan

TA - Tantahuatay

YA - Yanacocha - various

CC - Cerro Corona

KU - Kupfertal

PL - Perol

CG - Chailhuagón

GA - El Galeno

MQ - Michiquillay

CO - Colpayoc

CH - Chamis

LC - La Carpa

PORPHYRY Cu-Au-Mo

HG - Hualgayoc

POLYMETALLIC Ag

Cretaceous & Tertiaryintrusive rocks

79°

6°

79°

6°YAYA

FIG. 1. Generalized geologic map of the Cajamarca region, northern Peru, showing location of mineral deposits.

Paleocene Incaic I event and subsequent middle Eocene In-caic II, Oligocene to Miocene Incaic III and IV, and the mid-dle and late Miocene Quechua I and II events (Canchaya,1990; Noble et al., 1990; Benavides-Cáceres, 1999; Noble andMcKee, 1999). The oldest volcanic rocks were deformed bythe middle Eocene Incaic II event (≥45 Ma, Noble et al.,1990; D.C. Noble and C.E. Vidal, unpub. data) and are in-truded by dioritic stocks of slightly younger age in the easternand northern parts of the region (Macfarlane et al., 1994;Llosa et al., 2000). The trend of the Andean fold belt changesfrom northwest to west-northwest at about the latitude of Ca-jamarca and is also cut by a series of northeast-trendingtransandean faults, some with sinistral movement (Vidal andNoble, 1994; Petersen and Vidal, 1996; Fig. 1). A northeast-trending belt 30 to 40 km wide and 200 km or more long thatencompasses the mineral deposits in the Cajamarca regionhas been named the Chicama-Yanacocha structural corridor(Quiroz, 1997). Intersections of the Andean and transandeanstructures are thought by many geologists to have localizedmany of the deposits, and both trends are clearly reflected indetailed patterns of gold grade within the Yanacocha deposits(Vidal et al., 1997; Longo, 2000; Teal et al., 2002).

Minas CongaThe Minas Conga district contains two thoroughly explored

porphyry copper-gold deposits, Chailhuagón and Perol, 5 kmto the north. These lie 15 to 20 km north of Michiquillay and10 to 12 km east of the Maqui Maqui deposit in the Yana-cocha district. Defined resources are 428.5 million metrictons (Mt) of 0.31 percent Cu and 0.78g/t Au for Perol and 190

Mt of 0.28 percent Cu and 0.77 g/t Au for Chailhuagón (Cía.de Minas Buenaventura S.A.A., annual report 2000). Huyla-machay Sur is an incompletely explored prospect 2 km westof Perol (Fig. 2). The history and methods of exploration andthe geology of the deposits have been documented by Llosaet al. (1996, 2000). A Cretaceous section of calcareous sedi-mentary rocks, from the Inca to the Quilquiñan Formations,is strongly folded with a N 65° W trend across the district.These rocks are unconformably overlain by Miocene daciticvolcanic rocks and intruded by late Eocene diorite dated at43.6 ± 3.7 Ma (Llosa et al., 2000), as well as by mineralizedMiocene porphyries of intermediate composition. Part of thedistrict is hidden beneath postmineral pyroclastic rocks of thelate Miocene Fraylones Formation.

Three hydrothermally altered rocks from Minas Congahave been isotopically dated using standard incremental-heating 40Ar/39Ar methods at the Nevada Isotope Geo-chronology Laboratory, Department of Geosciences, Univer-sity of Nevada, Las Vegas (Table 1). Incremental-heatingspectra and isochron plots are given in the Appendix. Theisochron ages for each of these three samples agree closelywith plateau and total gas ages, are preferred, and are takenas the basis for subsequent discussion. They are coarse hypo-gene alunite from the Cocañes zone at 16.06 ± 0.11 Ma, or-thoclase from potassic-altered porphyry in the Perol ore zoneat 15.80 ± 0.09 Ma, and biotite from potassic-altered por-phyry in the Chailhuagón ore zone at 15.58 ± 0.12 Ma.

There are no known epithermal ore deposits in the vicinityof these porphyry deposits, which have been relatively deeplyeroded, as evidenced by the extent of exposure of porphyry at


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7920

00

9234000

9236000

Perol

HuaylamachaySur

7900

00

Strike and dipLithologic contact

Anticlinal axis: overturned

Porphyry Cu-Au mineralization

Dacitic volcanic rocks/volcanic vent

Picota diorite (± 44 Ma)

Quaternary

Breccia pipe

1 km

Hydrothermal alteration (phyllic, argillic, advanced argillic; ± 16 Ma)

Mujarrún Formation:Cretaceous carbonate rocks

Early fine-grained dioriteporphyry

Quartz-feldspar porphyryassociated with mineralization (± 16 Ma)

Fraylones Formation:postmineral pyroclastic rocks

Andesite

Skarn / hornfels

Marble

FIG. 2. Generalized geologic map of the Perol-Huaylamachay area, Minas Conga district; modified from unpublishedCEDIMIN reports (1999).

present surface. However, advanced argillic alteration andlocal weak gold mineralization do affect rocks within two vol-canic vents that are adjacent to Perol and to the Huayla-machay Sur porphyry prospect.

Styles of alteration and mineralization

Although differing in many details, the size, metal content,and types and sequence of intrusion, alteration, mineraliza-tion, and veining make Chailhuagón and Perol fairly typical ofporphyry Cu-Au deposits (Gustafson, 1978; Llosa et al., 2000;Sillitoe, 2000).

Chailhuagón: This deposit has at least two intramineralporphyries that are potassic altered, with early biotite-mag-netite veins and A quartz veins, and magnetite-chalcopyriteaccompanied by bornite or pyrite (Fig. 3a). There is onlyminor superposition of late-stage sericite-pyrite and even lesssupergene alteration and enrichment. In the highest gradezones of the deposit there is an unusual and poorly studiedbody of quartz-magnetite rock, which lacks K-feldspar andaverages 0.34 percent Cu and 1.17g/t Au (Fig. 3a). Chalcopy-rite with trace amounts of pyrite occurs with the magnetite,which is partially altered to hematite. Granular quartz in thistype of alteration contains only moderately abundant fluid in-clusions, predominately polyphase brine inclusions withanisotropic daughter crystals of undetermined mineralogy ac-companied by only a few vapor-dominant inclusions. AtChailhuagón, Mo is weakly anomalous (most samples <15ppm) and essentially coincident with the Cu and Au zones,which fade out at the limits of potassic (biotite-K-feldspar) al-teration. A broad halo of marble surrounds the deposit, butthere are only very small amounts of nearly barren skarn atthe porphyry contacts.

Perol: This deposit had an early alteration and mineraliza-tion history very similar to Chailhuagón, with the exceptionthat it lacks quartz-magnetite alteration. However, it looksquite different because of a much stronger late-stage over-print of sericite-pyrite (Fig. 3b). Chalcopyrite-bornite is con-verted to chalcopyrite-pyrite, typically with no reduction inthe magnetite content, although commonly with minor con-version to hematite. Locally this overprinting has producedpyrite-bornite-chalcopyrite without hematite and magnetite.Fluid inclusions in quartz throughout this deposit are charac-terized by much more abundant vapor-dominant inclusionsthan at Chailhuagón. The highest Cu (>0.3%) and Au (>0.5g/t)grades are largely coincident with greater abundances of earlyA quartz veins in the central zone of potassic alteration of theearly quartz feldspar porphyries and premineral fine-grained

diorite. The greatest amount of superimposed pyrite (>4% S)also is found in this zone. Although it appears that thesericite-pyrite overprint has locally decreased the Cu and Augrades and zonal differences are hard to resolve, there is anoverall increase in mean values of Cu, Au, and Au/Cu ratiothroughout the deposit as a result of the sericite-pyrite over-print on the potassic assemblage (Table 2). Supergene sulfideenrichment and accompanying argillic alteration andhematite alteration of magnetite are moderately well devel-oped near the top of the deposit. In contrast to Chailhuagón,at Perol Mo is peripheral to the zone of Cu-Au and potassicalteration and is present in higher concentrations. Skarn ismuch more widely developed and mineralized; an innerquartz-magnetite-Cu-Au zone and an outer garnet-pyroxene-epidote-Zn-Pb-Ag-Au-Cu zone locally carry grades of poten-tial economic significance.

Huaylamachay Sur: This is a relatively small and incom-pletely know porphyry center (Fig. 2), which is more similarto Perol than to Chailhuagón. Like Perol, there is a strongsericitic and pyritic overprint on the early A quartz veins andpotassic alteration.

Porphyry-volcanic relationships

A stock of fine-grained diorite is clearly intruded and min-eralized on its western margin by the Perol porphyries. Al-though the time of intrusion has not been determined byisotopic dating, it is interpreted to be the first intrusivephase of the Perol porphyry system, based on its texturalsimilarity with intrusions in many porphyry copper districtsaround the world. Directly north of the Perol orebody, adacitic volcanic vent complex underlies Cerro Cocañes. Al-though largely composed of pyroclastic material (Fig. 3c), italso includes flow-banded rock. Drilling confirms that thevent has a steep contact extending to depth. The dacitic ventrock contains abundant xenoliths of altered fine-graineddiorite and intensely silicified rock, but there are no frag-ments of quartz-feldspar porphyry or of any rock cut byquartz veins. At the northern end of the Perol porphyry, thedacite is altered to quartz-andalusite-(sericite), which in


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TABLE 1. Summary of 40Ar/39Ar Age Determinations on Samples from Minas Conga

Sample Locality Mineral Isochron Plateau Total gasno. dated age (Ma) age (Ma) age (Ma)

O-34/96M Cocañes Alunite 16.06 ± 0.11 16.11 ± 0.18 16.09 ± 0.18H-46/535M Perol Orthoclase 15.80 ± 0.09 15.86 ± 0.10 15.96 ± 0.10N-21/235M Chailhuagón Biotite 15.58 ± 0.12 15.35 ± 0.04 15.46 ± 0.12

Notes: All analytical uncertainties are at the 1σ level; isochron ages are preferred; for the Cocañes and Perol samples the isochron, plateau, and total gasages are identical well within the limits of analytical uncertainty; for the Chailhuagón sample, the plateau and total gas ages are slightly lower than the pre-ferred isochron age but still within the limits of analytical uncertainty; the difference in the ages of the Cocañes alunite and the Perol orthoclase is meaning-ful, and the Chailhuagón biotite is clearly younger than the other two samples

TABLE 2. Mean Values of Assays at Perol through 1999

Alteration Number oftype samples Au (g/t) Cu (%) g Au/%Cu Ag (ppm)

Potassic 2,003 0.539 0.243 2.22 2.30Sericitic 2,441 1.401 0.393 3.56 1.89

turn is cut by pyrophyllite-diaspore-alunite alteration mar-ginal to pyrite-covellite veinlets. Here also, symmetric transi-tional-stage quartz-molybdenite veins (B veins) locally cut thedacite. At least one body of late breccia flanking the north endof the Perol deposit contains fragments of dacite altered toadvanced argillic assemblages, rarely with gusano texture (see

subsequent sections). The breccia contains minor amounts ofsphalerite-galena-pyrite rimmed by arsenopyrite, but the rel-ative timing of this association has not been determined. Ad-vanced argillic assemblages affect most of the dacitic vent andextend southward into fine-grained diorite at the easternflank of the Perol deposit, with the altered rocks locally con-


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e f

dc

a b

FIG. 3. Photographs of the Minas Conga and Hualgayoc districts. a. Chailhuagón porphyry; feldspar porphyry with strongbiotite-alkali feldspar-magnetite-chalcopyrite and cut by early A quartz veins and 1-cm-wide igneous breccia dikelet (left);younger intramineral porphyry, less texture-destructive biotite alteration, cut by quartz-magnetite and biotite-magnetiteveinlets (center); intense texture-destructive quartz-magnetite alteration, cut by drusy quartz-chalcopyrite veinlets (right). b.Perol ore, with strong sericite-pyrite-chalcopyrite overprinted on early potassic-altered and quartz-veined porphyry. c. Flat-tened pumice clasts dipping into center of Cocañes vent. d. Huaylamachay vent breccia (nearby clasts of A and B quartz-molybdenite veins) cut by parallel, banded quartz veins. e. Looking west-northwest from Hualgayoc village (bottom), withCerro Jesús and old Spanish silver vein workings (right), across Cerro Corona with drill roads, to the Pozos Ricos workingson polymetallic mantos (top left). f. Quartz vein stockwork, Cerro Corona porphyry Cu-Au-Mo deposit.

taining veinlets of pyrite-enargite-covellite with anomalousAu values.

It is clear that the porphyries were emplaced after forma-tion of the dacitic vent. If, however, the interpretation of thefine-grained diorite as the first of the Perol porphyry intru-sions is correct, both intrusion of the porphyry and dacitic vol-canism were closely linked in time as well as space. Advancedargillic alteration accompanied by the introduction of largeamounts of quartz occurred before or during the formation ofthe porphyry system, as well as in very late stages when it wassuperimposed around the edges of the porphyry. Althoughthe isochron and plateau ages of the primary alunite in Co-cañes dacite are slightly older than those of the alteration or-thoclase at Perol, the ages are identical within the limits of an-alytical uncertainty and show that the advanced argillicalteration in the volcanic vent was essentially coeval withpotassic alteration in the porphyry.

Huaylamachal Sur lies directly south of another steep-walled dacitic vent (Fig. 2). Although partially covered by thepostmineral Fraylones Formation, the vent appears to consistentirely of pyroclastic material that has been altered to ad-vanced argillic assemblages. The dacite contains abundantfragments of quartz-veined and mineralized porphyry, includ-ing relatively late quartz-molybdenite vein material close tothe contact with the adjacent mineralized porphyry. It is inturn locally cut by banded quartz veins (Fig. 3d), pyrite veins,and thin tuffisite dikes. These banded veins are megascopi-cally similar to those in the Maricunga belt of Chile (Vila andSillitoe, 1991; Muntean and Einaudi, 2000, 2001).

Tantahuatay

Hualgayoc-Tantahuatay district

Discovered in 1771 during Spanish Colonial times, theHualgayoc district was one of the most important silver pro-ducers in Peru. The geology and ore deposits have been dis-cussed by Málaga Santolalla (1904), Ericksen et al. (1956),Vidal and Cabos (1983), Canchaya (1990), Macfarlane andPetersen (1990), Macfarlane et al. (1994), and Paredes(1997), among others (see Noble and McKee, 1999, for amore complete list of references). Geologic relationships aregreatly simplified in Figure 4. The central part of the Hual-gayoc district is pictured in Figure 3e.

A thick section of largely carbonate sedimentary rock ofCretaceous age was folded with minor faulting during severalpulses of deformation that extended at least until the middleMiocene. During the Eocene, dioritic stocks and sills were in-truded, and during the middle Miocene a series of dacitic toandesitic domes and associated pyroclastic rocks were em-placed, along with subvolcanic stocks, sills, and dikes. Manytypes of mineralization and alteration are found in the district.

Most historic production came from silver-rich veins in theCerro Jesús and Cerro San José domes of Miocene age; mostrecent production has been from veins and replacement man-tos in Mesozoic siltstone-carbonate strata. Three major sul-fide associations are pyrite-sphalerite-galena-chalcopyrite-tennantite-tetrahedrite and pyrite-pyrargyrite-proustite-otherminor sulfosalts-sphalerite, both associated with quartz-kaoli-nite ± pyrophyllite (Vidal and Cabos, 1983). To the northwest,


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20°

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28°

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30°

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C° Tingo

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C° Jesus

C° San Jose

Yanacancha Sill

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Alluvial deposits(Quaternary)Postmineral tuff(Miocene)

Postmineral dacite dome(Miocene)Andesite dome(Miocene)

Pyroclastic rocks(Miocene)

Quartz-phyric dacite dome(Miocene)

Quartz-diorite porphyry stock(Miocene)

Porphyritic diorite(Eocene)

Limestone(Cretaceous)Sandstone(Cretaceous)

24° Bedding

Fault

Veins (Ag, Cu)

Massive pyrite-enargite

4 km

Porphyry Cu-Au-Mo

Oxide Au

Hualgayoc village

FIG. 4. Simplified geology of the Hualgayoc-Tantahuatay district; after S. Canchaya, J. Paredes and R. Tosdal, unpublishedmaps.

cutting the San Miguel diorite and at the eastern edge of theTantahuatay volcanic complex, veins and replacement bodiesof pyrite-enargite have been mined. The andesitic volcaniccomplex at Tantahuatay hosts a number of high-sulfidationepithermal Au-Ag deposits, with advanced argillic alterationand small oxide gold resources overlying extensive pyrite-enargite-covellite mineralization. A porphyry Cu-Au deposithas been explored at Cerro Corona, and at least five other oc-currences of porphyry-style mineralization are known else-where in the district.

Mineralization in the district appears to range in age be-tween 14.3 and 12.4 Ma (Macfarlane and Petersen, 1990;Macfarlane et al., 1994; Noble and McKee, 1999). Broadzonation in the distribution of the different types of mineral-ization has been noted (Vidal and Cabos, 1983), but in detailthe patterns are complex and poorly documented. It is not atall clear how the various vein and replacement bodies are re-lated to the various intrusive centers, and some authors haveeven interpreted a syngenetic origin for the mantos (e.g.,Canchaya, 1990).

Cerro Corona and other porphyry copper centers

Directly west of the town of Hualgayoc is Cerro Corona(Fig. 3e). Although the porphyry character of the system haslong been appreciated (e.g., Paredes, 1981), it was only rela-tively recently that the presence of potentially economicamounts of gold was recognized. In the mid-1990s, a reserveof about 90 Mt of 1.0 g/t Au and 0.5 percent Cu was estab-lished by drilling, but the deposit has not yet been developed.Mineralization occurs primarily in the first phase of apolyphase quartz diorite porphyry and is associated with K-feldspar-biotite alteration that is overprinted by pyrite-sericite (James, 1995). Gold occurs with magnetite-chalcopy-rite accompanied by variable amounts of bornite, pyrite, andhematite. James (1995) classified a sequence of vein types as(1) biotite, (2) K-feldspar, (3) magnetite, (4) quartz-oxide-sul-fide, (5) quartz-pyrite, (6) pyrite, and (7) calcite veins. Thestockwork in the main ore zone contains quartz veins similarto A and B veins of the typical porphyry suite (Fig. 3f), andmolybdenite is abundant in the core. A thin supergene cop-per enrichment blanket developed under a gold-rich leachedcapping. Although there are only sparse Mo data, molybden-ite is relatively abundant in drill core, and the deposit is prob-ably richer in Mo than those at Minas Conga and elsewherenear Cajamarca, with the exception of Michiquillay. Skarnand sulfide mineralization in the surrounding PariatamboFormation is restricted to within 20 m of the diorite-lime-stone contact, although marble may extend to about 70 m.Any epithermal mineralization that may have been related tothis deposit has been removed by erosion.

Two other areas with outcropping porphyry copper-stylequartz veins are present east and west of Cerro Corona (Fig.4). The porphyry Cu-Au-Mo zone west of El Molino has beendrilled and found to be extensive but of very low grade. It hasmoderately strong biotite-alkali feldspar alteration and earlyA quartz veinlets, with low-intensity chalcopyrite-pyrite-mag-netite mineralization. The small exposure of intense quartzveining at the Quijote mine, a few hundred meters southeastof Hualgayoc village, has apparently very low values at surfaceand has not been explored at depth. Drilling in the Manto

Lola area, northeast of Quijote, has encountered a blind zoneof quartz veining with anomalous Mo contents, local skarn, andbodies of hydrothermal breccia, but without potassic alterationor anomalous Cu values. This occurrence has close affinities toporphyry Cu-Mo mineralization but lacks the abundance ofsulfur and metals to form a porphyry-style deposit.

There is no clear pattern indicating that any of these por-phyry centers were responsible for the polymetallic veins andreplacement mineralization in the district. Nevertheless, themultiple and variably mineralized intrusions and dome cen-ters so closely associated with the polymetallic veins and man-tos are indicative of a magmatic origin for all the mineraliza-tion. Lead isotope data also lead to this same conclusion(Macfarlane and Petersen, 1990).

At Peña de las Aguilas, in the southwestern part of Tanta-huatay, banded quartz veinlets similar to those described atRefugio, Chile, by Muntean and Einaudi (2000) occur locally.These veinlets suggest the top of a buried porphyry Cu-Audeposit, although with very low Cu and Au values. A more in-teresting porphyry system occurs under Tantahuatay 2, in thenortheastern part of the dome complex (Figs. 4–5).

Tantahuatay, geologic framework

Mapping by R.H. Tosdal (Cia. Minera Coimolache S.A.,unpub. map and report, 1996) outlined the geologic frame-work of the Tantahuatay volcanic field, which forms thenorthwestern part of the Hualgayoc-Tantahuatay district (Fig.4). Small-volume pyroclastic eruptions, of andesitic to daciticcomposition, preceded and accompanied emplacement of aseries of volcanic domes. These domes are calc-alkaline,clinopyroxene-bearing, fine-grained hornblende- and plagio-clase-phyric andesite. Stratigraphic relationships evidence asouthwest to northeast growth of the dome field, and theeroded shape of the complex reveals a control on emplace-ment by northwest- and northeast-trending structures. Fig-ure 6a shows the dome complex of Cerro Tantahuatay, at thenortheastern end of the volcanic complex, and the spectacu-lar ferricretes formed by acid drainage from the extremely al-tered and pyritic rocks. Alunite from a hydrothermally altereddome was dated at 12.4 ± 0.4 Ma, and biotite in a postmineraldike at 8.6 ± 0.3 Ma (Noble and McKee, 1999), making theTantahuatay dome field and associated mineralization slightlyyounger than the mineralization at Hualgayoc.

Quartz-alunite alteration is pervasive across most of thedome field, and multiple stages of brecciation with intensesilicification, quartz-alunite-pyrophyllite-diaspore-zunyite al-teration, and sulfide mineralization are concentrated in theindividual dome centers. Although glacial erosion has, unfor-tunately, removed most of the oxidized capping, enoughresidual oxide remains for oxide gold resources to have beendelineated at two of these centers, La Cienaga and Tantahu-atay 2 (Figs. 4–5). Although similar in many ways to Yana-cocha, where there has been less erosion, at Tantahuataythere is less silicification relative to advanced argillic alter-ation. Yanacocha also has much more gold mineralization.

Tantahuatay 2

The largest, most complex and best mineralized centerwithin the dome complex appears to be Tantahuatay 2 (Fig.5). Intense silicification and advanced argillic alteration, as


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well as brecciation, completely obliterate igneous textures ex-cept in peripheral and early quartz-alunite alteration. All ofthe rocks are silicified, but zones of total silicification areshown in Figure 5, with textures ranging from massive tovuggy to sandy. Zones of massive to vuggy quartz invariablycarry anomalous gold values but typically not >0.1g/t Au un-less there is superimposed brecciation and sulfide mineraliza-tion. Multiple, crosscutting stages of breccia (Fig. 6b) arecommonly characterized by somewhat different dominant al-teration mineralogy. Multiple stages of alunite are typicallymost easily recognized. Breccias were differentiated duringthe mapping as crackle breccia (little matrix or rotation offragments), tectonic breccia (fault breccia), hydrothermalbreccia (mineralized fragmental matrix, polymict clasts), andpebble breccia (fragmental matrix and relatively roundedpolymict clasts). Only the largest, most coherent breccia bod-ies are shown in Figure 5, but brecciation is more or less per-vasive across this area. In detail, late-stage gold mineralizationis best developed in areas most strongly affected by multiplestages of silicification and brecciation. Pyrite, enargite, andother sulfides are associated with the highest Au values belowthe base of oxidation.

A large area of gusano texture is shown in Figure 5. Illus-trated by Figure 6c-d, this alteration texture completely oblit-erates original rock texture. Gusano texture is characterizedby segregation of soft, white patches of pyrophyllite, typicallywith minor diaspore and as much as 100 percent alunite, in ahard siliceous matrix. In thin section, the siliceous matrix con-sists of granular quartz containing approximately 5 to 30 per-cent interstitial aluminosilicate minerals similar to the softpatches that generally contain less than 10 percent quartz.The siliceous matrix is hard but generally scratchable with asteel blade. Table 3 summarizes defining characteristics ofgusano texture and other features discussed below. At Tanta-huatay 2, pervasive gusano-textured rock covers an area of atleast 500 by 600 m and extends in drill holes to a depth of atleast 200 to 300 m. Smaller amounts are present in the othermineralized and advanced argillic-altered dome centers in thedistrict. In places it crops out only in structural zones severalmeters or less in width.

Gusano-textured rock is typically dense and contains dis-seminated pyrite and generally <0.1 g/t Au. More intenselysilicified breccia and vein zones cut this rock, are more brit-tle, and have localized more intense sulfide mineralization


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TUNEL ISABELNiv. 3,960

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SYMBOLS

Pebble dike

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Crackle brecciaGusano textureGeologic contactFault, inferred

Exploration adit

Quartz - pyrophyllite - aluniteQuartz - pyrophylliteQuartz - alunite

Vuggy silicaMassive silica

SILICIFICATION

ADVANCED ARGILLIC

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FIG. 5. Alteration and structure of the Tantahuatay 2 deposit; after M. Miranda T. (1999), unpublished map for Cía. Min-era Coimolache S.A.

(Fig. 6d). This relatively late assemblage is mostly pyrite-enargite-covellite. Copper and Au contents vary directly withthe abundance of enargite, which is by far the most abundantcopper mineral. Enargite-pyrite alone rarely contains morethan 0.2 g Au/percent Cu, although some zones with semi-massive pyrite reach 0.4 g Au/percent Cu. The highest grades

of Au with grams Au/percent Cu ratios reaching 2 or more in-variably occur where bornite, digenite, covellite, and/or spha-lerite, and rarely barite are present with, and apparentlyyounger than, the enargite. These intervals are typically in thesame siliceous zones of breccia and veins that contain themost enargite. Weak supergene chalcocite enrichment locally


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e f

dc

a b

FIG. 6. Photographs of the Tantahuatay district: a. Looking southeast, across terraced ferrocrete in drainage, to CerroTantahuatay; Tantahuatay 2 at center skyline. b. Breccia dike cutting more silicified breccia. c. Gusano texture; texture-de-structive alunite-pyrophyllite-diaspore-quartz alteration. d. Enargite-pyrite follows permeable zones of intense silicificationcutting denser gusano-textured rock with weakly disseminated mineralization. e. Gusano texture quartz-pyrophyllite-aluniteconverted to more irregular patchy quartz texture at appearance of first A quartz vein (arrow); drill hole T41-±53.9 m. f. Twosides of same piece of core near (e), with patchy quartz grading into linked wormy quartz veinlets; drill hole T41-51.3 m.

extends in fracture zones to depths of hundreds of meters.The presence of unusually high concentrations of secondarychalcocite locally distorts the Au/Cu ratio. In the overlyingoxide zone the Au remains, with local enrichment very closeto surface, whereas Cu has been strongly leached during sur-ficial oxidation.

Substantial sulfide Cu-Au resources exist at Tantahuatay 2and probably at other centers within the volcanic complex,but exploration has been insufficient to quantify them. Mostdrilling has been confined to the oxide zone, where a resourceof 11.3 Mt at 0.78 g/t Au and 17.86 g/t Ag has been defined.At the La Ciénaga center, there is an additional indicated andinferred oxide resource of 12.6 Mt at 0.97 g/t Au and 1.2 g/tAg (Compañía de Minas Buenaventura S.A.A., annual report2002).

Hidden porphyry system

The potential for one or more porphyry Cu-Au orebodiesbeneath the alteration and epithermal gold-silver mineraliza-tion at Tantahuatay has long been recognized, supported bythe abundance of Mo (commonly >100 ppm) and enargite inthe system, and made more compelling by the analogy withLepanto-Far Southeast in the Philippines (Arribas et al.,1995). A deep porphyry orebody was the major objective ofan extensive drilling campaign by Companía Minera Coimo-lache S.A. in the early 1990s. Although several deep holeswere drilled at Tantahuatay, no potentially economic por-phyry-style mineralization was recognized. Two holes didpenetrate below the unconformity at the base of the volcanicrocks and encountered skarn in the limestone that hosts mas-sive pyrite-enargite mineralization a few hundred meters far-ther east (Paredes, 1981). The skarn contains pale diopside,epidote, and clay with minor amounts of pale sphalerite,galena, and chalcopyrite. In the dome complex with only raredikes, only advanced argillic- to sericitic-altered volcanic rockand pyrite-enargite mineralization with minor amounts ofpyrite-bornite-chalcopyrite and pyrite-covellite were seen at500 to 700 m below surface.

Only in one drill hole, T41 at the northeastern edge of Fig-ure 5, has clear evidence for a deeper porphyry deposit beenencountered. Granular quartz veins, similar to A veins atMinas Conga and porphyry deposits throughout the world,occur from 50 m to the bottom of the hole at 560 m. Onlyvapor-rich fluid inclusions occur above the top of halite-bear-ing inclusions at 180 m, and anisotropic daughter minerals arecommon in polyphase brine inclusions from there downward.The quartz contains traces of minute chalcopyrite inclusions,although the bulk sulfides are pyritic, high-sulfidation assem-blages with sharply diminishing grades of gold and copper atdepth. Although potassic alteration is invariably associatedwith A veins in porphyry systems, any preexisting potassic al-teration has been completely obliterated by the overprintedadvanced argillic and sericitic alteration.

Within a few meters of the top of A quartz veins in thishole, gusano texture, which is dominant in quartz-pyrophyl-lite-(alunite)–altered rock in the upper part of the hole, un-dergoes an abrupt change. As shown in Figure 6e, the high-est A vein cuts a more silicified rock in which the siliceousmatrix to the rounded patches in the surrounding gusano tex-ture is harder and segregated into irregular, locally contortedveinlike masses (Fig. 6f). This modified texture is much moreabundant at Yanacocha, where it has been termed patchyquartz and wormy veins, which are described below and inTable 3.

Almost all the Mo at Tantahuatay occurs as “smears” offine-grained, anhedral molybdenite in pyritic advancedargillic-altered rock rather than in quartz veins typical of por-phyry deposits. Tantahuatay 2 is only one of several centerswith strongly anomalous Mo, but the values are scattered withlittle consistent pattern at surface within these centers. How-ever, when all Mo assays from surface and drill holes at Tan-tahuatay 2 are plotted together, it can be seen that there is acrudely arcuate shape within which values are consistently>50 ppm (Fig. 5). This zone overlaps the southern and west-ern edge of the 0.5 percent Cu zone of the sulfide resource,as it is currently known. Beneath this area are rare occur-rences of euhedral molybdenite in quartz veins, which furthersupport the interpretation of a porphyry system at depth.

YanacochaThe Yanacocha district is the central and economically most

important in the Cajamarca region (Fig. 1). With 2.29 Moz ofgold produced in 2002 from multiple open pits, it is thelargest gold producer in South America. With more than 10Moz already produced and 32.6 Moz in oxide reserves (1,100Mt at 0.90g Au/t, Cía. de Minas Buenaventura S.A.A., annualreport, 2002), and a cash cost still under $130/oz, Yanacochais one of the greatest gold districts in the world. Various as-pects of the discovery and geology of Yanacocha have beenpresented by Turner (1997), Harvey et al. (1999), Longo(2000), Myers and Williams (2000), and Teal et al. (2002).

The district contains at least ten distinct near-surface, high-sulfidation epithermal deposits formed in the middleMiocene (12.5–11.8 Ma) Yanacocha volcanic complex, whichis about 24 km in diameter. The complex evolved in five majoreruptive and/or intrusive cycles, with late-stage formation ofmultiple bodies of breccia and main-stage gold mineralizationassociated with the emplacement of shallow-level porphyries


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TABLE 3. Defining Characteristics of Types of Veins and Alteration Textures Discussed in the Text.

A veins: Granular quartz, locally with disseminated alkali feldspar, sulfide,magnetite, and/or anhydrite, filling generally planar fractures and asso-ciated with potassic alteration in porphyry copper systems; typicallymultiple sequences with earliest veins less planar and less continuous

Wormy veins: Granular quartz with discontinuous and contorted shapes as-sociated with advanced argillic alteration and with gusano to patchy al-teration texture; apparently restricted to the top 100 m of occurrence ofA quartz veins

Gusano texture: Soft, white, rounded blobs or patches of alunite ± pyro-phyllite ± diaspore in a moderately hard matrix of granular quartz withinterstitial pyrophyllite ± diaspore ± alunite; destroys original rock tex-ture

Patchy texture: Irregular, more or less angular, hard granular quartz withina generally interconnected matrix of pyrophyllite ± diaspore ± alunite;destroys original rock texture

D veins: Sulfide veins with sericitic alteration halos and typically with littlequartz and pyrite-rich intermediate-sulfidation assemblages

Note: These are end-member types that show gradational characteristicsand locally may be difficult to distinguish with certainty

that intruded the volcanic pile (Teal et al., 2002). Epithermalgold mineralization is associated with large volumes of mas-sive to vuggy quartz and lesser advanced argillic clay alter-ation. Associated alunite has been dated at 11.5 to 10.9 Ma(Turner, 1997; Noble and McKee, 1999). The oxidized goldmineralization is underlain by multiple centers of porphyry-associated Cu-Au sulfide mineralization, which have been in-completely explored. At least two distinct periods of mineral-ization are recognized: an early, premain-stage gold event atKupfertal, Yanacocha Sur, and Maqui Maqui, and a later post-main-stage gold event contemporaneous with the emplace-ment of the Yanacocha diatreme (Teal et al., 2002). Interme-diate-sulfidation mineralization with high gold grades isperipheral to central high-sulfidation Cu-Au mineralization.

The porphyry Cu-Au system that crops out at Kupfertal,between the Yanacocha Sur and San José open pits (Figs.7–8a), was first drilled as a copper target in the 1960s. From1999, exploration of deep sulfide resources has expandedknowledge of the Kupfertal porphyry system (Pinto, 2002), aswell as other occurrences below Yanacocha Norte and MaquiMaqui. Porphyry Cu-Au alteration and mineralization atKupfertal affect rocks of the early eruptive cycle of Longo(2000), whereas most of the epithermal gold orebodies are athigher elevations in overlying explosive-cycle rocks. The basalSan José ignimbrite of the early explosive cycle contains a fewxenoliths with quartz veins (Fig. 9d) as well as fragments withintense silicification and advanced argillic alteration. Thesexenoliths are tentatively interpreted as having been derivedfrom the Kupfertal system, which is considered to be earlierthan the main Au mineralization in the district (Teal et al.,2002).

Alteration and quartz vein patterns at Kupfertal

Collared in the bottom of the Encajón valley, diamond drillhole CLL-5 (Fig. 10) extends 842 m at –70o, passing from ep-ithermal to porphyry environments. The most distal indica-tions of the proximity of the porphyry are seen in the southwall of the valley, roughly 200 m laterally from and 75 mabove the collar of drill hole CLL-5. In rock altered to alu-

nite-pyrophyllite with volcanic texture still visible, a quartz-alunite vein has a texture-destructive alteration halo (Fig. 8b).In the halo, soft white clots of alunite-pyrophyllite are sur-rounded by siliceous matrix similar to the gusano texture atTantahuatay. This alteration becomes pervasive closer to drillhole CLL-5 and in one outcrop is cut by a discontinuous fine-grained dike (Fig. 8c). The dike is altered to pyrophyllite, ap-pears to have been an aphanitic igneous dike rather than tuff-isite, and was apparently intruded after formation of thegusano texture. Within several meters of the drill collar, thequartz-pyrophyllite alteration is more ragged, with irregularquartz patches harder than typical gusano matrix that aremore or less isolated in pyrophyllite and locally linked to formvery irregular, sinuous veinlike shapes (Fig. 8d). This textureis called patchy by Yanacocha geologists (Table 3), who con-sider our gusano texture to be a variety of patchy texture. Therock is also cut by quartz veins that are both sinuous and pla-nar. Particularly in the underlying drill core (Fig. 8e), themore planar quartz veins, which are typical of A veins recog-nized in many porphyry deposits (e.g., Gustafson and Hunt,1975; Sillitoe, 2000), are clearly younger than the sinuous anddiscontinuous veins, called wormy quartz veins by Yanacochageologists (Table 3). These wormy quartz veins abruptly de-crease and disappear at about 90 m in this drill hole (Fig. 11),whereas the A veins continue to depth.

A similar transition in alteration textures and shape ofquartz veins occurs in nearby drill hole KUP-6 (Fig. 8f). Rockat the top of the hole has well-rounded gusano texture.Wormy quartz veins appear at 27 m and A veins near 50 m.Wormy veins do not extend below 85 m, whereas A veins con-tinue to depth. Some A veins are irregular and discontinuous(similar to the earliest A veins formed at El Salvador;Gustafson and Hunt, 1975), making the distinction betweenA and wormy veins locally ambiguous. However, in this andother drill holes at Kupfertal wormy veins are consistently re-stricted to an interval of 100 m or less that overlaps the top ofrelatively continuous and planar A veins. Wormy veins alsoconsistently form only within patchy or gusano-textured alter-ation. Note that the wormy veinlet indicated with the arrowin Figure 8f at 46.0 m seems to mimic the sinuous distribu-tion of the gusano siliceous groundmass. The dark wormyvein is composed of hard, granular quartz with only traces ofinterstitial pyrophyllite (Fig. 9a), compared to the gusano ma-trix, which is softer because of abundant interstitial pyrophyl-lite. Granular quartz in both patchy alteration and wormyveins is essentially identical to that of A quartz veins in theserocks and contains the same high concentrations of fluid in-clusions, which are overwhelmingly vapor dominant at this el-evation (Fig. 9b). In different drill holes, the abundance anddistribution of patchy quartz in the vicinity of the top of Aquartz veins varies considerably, and because of gradationalcharacteristics, it is not always possible to distinguish patchyquartz from gusano and wormy veins.

In drill hole CLL-5 (Fig. 11), A veins are present, with theirabundance gradually decreasing downward through the top520 m of volcanic host rock (possibly including some intrusiveporphyry), in which original texture has been moderately toextremely obliterated. A vein quartz contains trace inclusionsof relict anhydrite, chalcopyrite, and bornite, apparently relicsfrom an earlier stage of mineralization. Present alteration


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km

FIG. 7. Location of the Kupfertal porphyry deposit relative to the goldorebodies at Yanacocha. From Pinto (2002).

assemblages are pyrophyllite-alunite-kaolinite, passing down-ward to sericite with decreasing amounts of pyrophyllite andincreasing kaolinite. The kaolinite may well be supergeneafter chlorite, which is abundant at deeper levels. Unlikesericitic alteration in most other porphyry districts, which doesnot obliterate texture, at least as seen under the microscope,

original texture in this generally fine-grained sericite-kaolin-ite–altered rock appears to have been obliterated down to atleast 595 m. At this depth, texturally unmodified feldspar por-phyry with only trace amounts of small A veins is present.Weak chlorite-sericite is superimposed on weak potassicalteration, manifested by biotite replacing hornblende


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e f

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a b

FIG. 8. Photographs of the Yanacocha district. a. Looking southeast in 1995 across Yanacocha Norte (left) and Sur (right)orebodies to Carachugo (center) and San José (right); the Kupfertal porphyry system crops out in the valley between Yana-cocha Sur and San José. b. Alunite vein with irregular, siliceous gusano-textured alteration halo in volcanic rock, about 50 mabove (c). c. Gusano-textured volcanic rock cut by discontinuous fine-grained dike, about 150 m laterally and 50 m abovequartz veining of the Kupfertal porphyry system; both rocks altered to pyrophyllite. d. Outcrop at collar of drill hole CLL5;patchy quartz and large wormy quartz vein, quartz-pyrophyllite alteration. e. Wormy quartz veins cut by more planar A quartzveins in top 21 to 77 m of drill hole CLL-5. f. Transition from gusano texture to patchy quartz and wormy quartz veins at topof A quartz veins in drill hole KUP-6, 13.7 to 61.2 m downhole. This is about 350 m from drill hole CLL5 (Fig. 10); notewormy quartz veinlet just below 46.0 label (arrow).


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dc

a b

FIG. 9. Photographs of the Yanacocha district. a. Thin section with cross-polarized light, showing granular quartz of wormyquartz vein in drill hole KUP6-46.0 m (Fig 7e), which is very similar to that in patchy quartz and A quartz veins; quartz isfilled with minute, mostly vapor-filled fluid inclusions; bar is 0.2 mm. b. Very abundant fluid inclusions, overwhelmingly low-density vapor-rich, with only traces of high-salinity inclusions, some of which have anisotropic daughter crystals; these aretypical of high-level quartz in wormy, banded, and A quartz veins and also patchy quartz; thin section, bar is 0.1 mm. c. Rel-atively large (1.3 cm) and complex banded quartz vein above top of A quartz veins, below Yanacocha Norte oxide gold ore-body. d. Xenolith with A and wormy quartz veins, in the San José ignimbrite unit; collected by Peter Mitchell in the Yana-cocha Sur pit; sample about 20 cm across.

m

Advanced argillic

Intermediate argillic

Sericitic

Potassic

Propylitic

0 250 m

FIG. 10. Cross section, looking northwest, across the Kupfertal porphyry deposit, showing drill holes and the alterationpattern; the propylitic alteration corresponds to a late intramineral porphyry intrusion. From Pinto (2002).

phenocrysts. Unambiguous evidence of A veins truncated atan intrusive contact is not preserved but this is evidently a lateintramineral intrusion. A well-mineralized porphyry responsi-ble for the mineralization has yet to be recognized.

Banded quartz veins, such as those described in the Mar-icunga belt, occur in other porphyry centers at Yanacochabut have not been seen at Kupfertal. Figure 9c shows abanded vein from below Yanacocha Norte. This vein has

dark bands full of minute vapor-filled fluid inclusions and isa complex vein formed by multiple reopening and consider-able recrystallization of the dark bands. As reported in theMaricunga belt (Muntean and Einaudi, 2000), banded veinsoccur within a narrow vertical range above and overlappingthe top of A veins and generally cut A veins. At Yanacocha,there does not seem to be a consistent age relationship be-tween the two.


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00

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

20%

W A

15%

pyencv

pycv

(cc)en

pycven

(cc)sl

pyp

10%

5%

10%

2%

2%

1%

alkao

ser(py p)

kao(py p)

ser

kaoser

kaoser

chlser

chlser

Fspar

Fsparchl

ser

pycp

(cc)

cp(mg )(cv)

py

cppy

mg

cppy

mg

cp(mg )

py

(cp)(mg )

py

(gn)(tn )

(sl )

(cp)(mg )

(py)

A

A

A

A

Rock

Alte

ratio

n

Qtz

Vns

Mag

Sulfi

des

Au g

/t

Cu %

0 1.00 1.0en/t

n

1%

tr

tr

tr

tr

tr

Hydrothermal brecciaTexture obliterated - coarse granularTexture obliterated - f.g. andesiteLate intermineral porphyry

Advanced argillic over potassicSericite-kaolin over potassicChlorite-sericite over potassicChlorite-sericite over weak potassic

pypal

A

tn tennantiteen enargite

Qtz Veins w = wormy veinsA = A veins

CLL-5, KupfertalRock

Alteration

Dept

h(m

)0 1.00 1.0Ro

ck

Alte

ratio

n

Qtz

Vns

Mag

Sulfi

des

Au g

/t

Cu %

en/t

n

Dept

h(m

)

A

A

A

A

en

tn

NO CORE

sl

FIG. 11. Alteration and mineralization in drill hole CLL-5, Kupfertal. Abbreviations: (minerals are listed in order of de-creasing abundance, with minor phases in parentheses.) pyp = pyrophyllite, al = alunite, ser = sericite, kao = kaolinite, chl= chlorite, fspar = feldspar, py = pyrite, en = enargite, cv = covellite, cc = chalcocite, sp = sphalerite, cp = chalcopyrite, mg= magnetite, tn = tennantite, gn = galena.

Mineralization patterns

Erosion has exposed sulfides at the surface, and coincidentwith the advanced argillic to sericitic alteration in the upper350 m of drill hole CLL-5 are pyrite-rich enargite and covel-lite. The relatively coarse grained covellite is evidently hypo-gene, based on textural criteria. Sulfides are both disseminatedand in veinlets, with local pyrophyllite extending deepest instructural zones accompanying pyrite-enargite veinlets. Lo-cally there is weak supergene chalcocite replacing primarysulfides, but this contributes only a small proportion of thetotal Cu, and the higher Cu grades in the top 100 m of thehole mainly reflect hypogene enargite and covellite (Fig. 11).Sphalerite is a minor accessory in different assemblagesthroughout the hole. Enargite is not seen below about 325 m,but copper grade remains steady at about 0.25 percent Cu,mostly as chalcopyrite with pyrite. Magnetite is absent, withno evidence of sulfidization of magnetite to pyrite, above 420m, whereas magnetite is abundant as disseminations and rareveinlets below 475 m. Bornite is absent from the sulfide as-semblage but is fairly common as minute traces locked in veinquartz. In the late feldspar porphyry below 530 m, dissemi-nated chalcopyrite-pyrite is 1 vol percent or less, with variableamounts of magnetite.

Gold grades are erratic and mostly 0.2 to 0.5 g/t to justbelow the bottom of the enargite, increasing downward toerratic values mostly >0.5 g/t. Within the zone of chalcopyrite-pyrite-magnetite below 475 m, comparable Au values are some-what less erratic. In the 50 m with minor residual magnetiteabove 475 m, more or less sympathetic variation in detailed Augrades with magnetic susceptibility suggests that Au was re-moved as magnetite was destroyed. In the late porphyry, ±0.1g/t Au is associated with trace amounts of chalcopyrite-pyrite.However, in small D veins (Table 3) with pyrite-tennantite-galena-sphalerite and sericitic alteration halos, Au spikes toover 1 g/t but Cu values are greatly reduced (Fig. 11).

Fluid inclusions in our samples of quartz from near surface,in all of the different kinds of quartz veins and in patchy al-teration, are very abundant and overwhelmingly vapor filled.With depth, saline inclusions with halite and less commonunidentified anisotropic daughter crystals increase, as do two-phase liquid-vapor inclusions. The sparse A veins in the lateporphyry have only moderately abundant fluid inclusions con-taining the usual suite of vapor-dominant, liquid-vapor, andbrine inclusions. A and wormy quartz veins near surface andincreasingly in depth contain traces of “locked” chalcopyrite-(bornite) and rare anhydrite. In contrast, relatively finegrained granular quartz in the matrix of gusano texture abovethe top of wormy veins and patchy alteration contains manyfewer fluid inclusions, all vapor dominant, and only extremelyrare relict chalcopyrite grains.

Strongly anomalous Mo values, >50 to 250 ppm, are wide-spread at Yanacocha. An irregular zone of anomalous Mo gen-erally correlates with gusano alteration and covers the surfacenorthwest of the drilled copper zone Kupfertal and overlap-ping the eastern edge of that zone (Pinto, 2002). As at Tanta-huatay, molybdenite occurs typically as anhedral smear grainsin pyritic advanced argillic-altered rock but as subhedral toeuhedral crystals in quartz veins within the porphyry systemat depth.

Discussion and Interpretations

Time-space relationships

Porphyry and epithermal deposits within the Cajamarca re-gion present a wide range of characteristics. In the three dis-tricts discussed there is unusually good exposure of the timeand space relationships between the deposit types, reflectingin part differences in level of erosion, degree of telescoping,and sequence of intrusion.

The Minas Conga district lends insights primarily into thedeeper seated porphyry regime. Quartz-magnetite without K-feldspar at Chailhuagón is of a type of alteration rarely foundin porphyry copper deposits but it appears to be somewhatsimilar to the quartz-magnetite ± K-feldspar assemblage atBajo de la Alumbrera, Argentina (Sillitoe, 1979; Ulrich andHeinrich, 2001). The fluid inclusions in this type of alteredrock at Chailhuagón also are at least qualitatively similar topolyphase brine inclusions in Bajo de la Alumbrera (Ulrich etal., 2001). Although this quartz-magnetite assemblage lackingK-feldspar has not been reported in any other porphyry de-posit in this region, it has been noted qualitatively thatanisotropic daughter crystals in brine fluid inclusions aremuch more abundant in the Au-rich porphyry systems in thisdistrict than in most Cu-Mo porphyries we have studied else-where in the world. There is only indirect evidence to suggestthat the stronger sericite-pyrite overprint at Perol, relative toChailhuagón, is at least partially related to shallower erosion.Perol is nearer to flat-lying Miocene volcanic rocks and is im-mediately adjacent to a dacitic pyroclastic vent, which mayhave been a feeder for some of the extrusive rocks. In addi-tion, the vent rocks and fine-grained diorite at the east marginof the Perol mineralization are also advanced argillic altered.

The most important contribution of this district to under-standing relationships between porphyry and epithermalmineralization and volcanism is the clear evidence that atboth Perol and Huaylamachay explosive dacitic volcanism wasclosely related to porphyry copper-gold mineralization in bothtime and space. Volcanism preceded formation of the Perolporphyry system but followed the porphyry mineralization atHuaylamachay. The occurrence of banded quartz veins cut-ting the vent breccia at Huaylamachay is suggestive of a stillyounger porphyry system superimposed at a slightly greaterdepth and is consistent with the volcanic vent closely postdat-ing formation of the Huaylamachay porphyry. The bandedquartz veins are megascopically similar to banded veins in theMaricunga district of Chile (Muntean and Einaudi, 2000,2001), which there occur near the top of porphyry systems, al-though Au values in these veinlets at Huaylamachay appear tobe low.

Although only encountered in one drill hole at Tantahuatay2, a porphyry system with well-developed A quartz veins wasemplaced in a lava dome complex, which also hosts well-de-veloped epithermal Au-Cu mineralization. The A veins haveshapes, textures, and characteristic fluid inclusions andlocked chalcopyrite grains that leave no doubt that they wereformed in a porphyry copper system and originally associatedwith potassic alteration. The chalcopyrite in vein quartz isrelict within the high-sulfidation bulk assemblage but giveslittle indication of how well developed the porphyry mineral-ization may be at depth. Any significant copper introduced


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with the A quartz veining was reworked to low-grade (±0.2%Cu) pyrite-enargite, and any magnetite was completely de-stroyed. The overprinting of advanced argillic and deepersericitic alteration is similar to occurrences in many other dis-tricts and represents late retrograde collapse of the hy-drothermal system. The recrystallization of gusano to patchytexture at the top of A quartz veins, indicating that the por-phyry system encroached upward onto presumably contem-poraneous advanced argillic-altered rock before subsequentdownward collapse, is discussed below together with similarevidence at Yanacocha.

Yanacocha is the least eroded of the three districts underconsideration, and with several drill holes in well-developedporphyry mineralization directly beneath and overprinted bythe actively mined epithermal mineralization, offers the bestopportunity to prove relationships between the two types ofmineralization. Advanced argillic and sericitic alteration inthe upper portions of the Kupfertal system were superim-posed on potassic alteration, as evidenced by abundant earlyA quartz veins that are invariably associated with potassic al-teration. The accompanying superposition of pyritic, high-sul-fidation mineralization on chalcopyrite-bornite is shown bythe traces of chalcopyrite and bornite locked within A quartzveins, also a typical feature of porphyry systems. Such super-position, or retrograde overprinting, of early by late assem-blages is nearly universal in porphyry systems. The prior exis-tence of biotite-K-feldspar alteration probably explains whytextures are obscured in the fine-grained sericite-kaolinite al-teration between 70 and 460 m in drill hole CLL-5 (Fig. 11).Fine-grained sericite is not normally associated with muchtextural modification, but very intense potassic alteration typ-ically tends to obliterate rock texture.

Much less common than evidence for retrograde overprint-ing of epithermal assemblages in porphyry environments inmost parts of the world, is evidence for prograde advance ofthe high-temperature porphyry environment onto rock af-fected by earlier lower temperature epithermal alteration. AtTantahuatay 2 and Kupfertal, such an advance is demon-strated by A veins cutting wormy veins and patchy alteration,and by locally altering gusano texture to patchy texture. Lessfirm evidence is provided by the small dike at Kupfertal cut-ting gusano alteration, as correlation of this dike with any por-phyry body related to the deeper mineralization is uncertain.The contorted shapes of wormy veins was initially confusing,because they give the impression of having been plasticallydeformed, as are the earliest A veins at El Salvador, Chile,which formed in recently solidified and still ductile porphyry(Gustafson and Hunt, 1975). As explained by Burnham(1967) and Fournier (1999), the brittle-ductile boundarymoves downward to zones of higher temperature during veryshort-lived episodes of very high strain rate, to enable the for-mation of fractures filled by the earlier A quartz veins, whichin turn were deformed plastically. At both Tantahuatay 2 andKupfertal, however, wormy veins occur in volcanic rock tensto hundreds of meters above the top of any porphyry. As de-scribed above (Figs. 6e-f, 8f), there is textural evidence thatwormy veins are formed by selective silicification of contortedmatrix of gusano texture. This occurs close to the uppermostextent of formation of A quartz veins and is associated withpatchy recrystallization of the gusano texture. Planar A veins

cut earlier formed wormy veins and patchy alteration for tensof meters, and these A veins in drill hole CLL-5 contain tinyinclusions of chalcopyrite and bornite. We conclude thatquartz veins, potassic alteration, and chalcopyrite-bornite as-sociated with porphyry intrusion prograded onto overlyingadvanced argillic alteration and subsequently was in turnoverprinted by gold-bearing pyrite-enargite-covellite miner-alization with pyrophyllite and underlying sericitic alteration.

One of the major questions remaining concerns the timerelationship between late-stage D veins of the porphyry sys-tem and the overlying pyrite-enargite-covellite veins—arethey contemporaneous zonal variations or is the overprintedhigh-sulfidation system distinctly later than the D veins? Al-though Teal et al. (2002) believe that pyroclastic volcanismand the major period of gold mineralization postdated forma-tion of the porphyry Cu-Au systems, multiple intrusive andmineralizing events are recognized and correlations acrossthe district are continually being improved. Ongoing work atthe mine and exploration of deep sulfide resources shouldeventually produce a clearer understanding of the relation-ships reported here.

Figure 12 is an attempt to illustrate schematically our in-terpretation of the time-space evolution of porphyry and ep-ithermal features in the Cajamarca region. It is an extensionof figure 17 in the paper on the Maricunga belt in Chile byMuntean and Einaudi (2001), where the two deposit types donot overlap as much as in the deposits discussed herein. AtMinas Conga and Yanacocha, we have evidence of explosivevolcanism separating episodes of porphyry intrusion and min-eralization. At Tantahuatay and Yanacocha, we also have bet-ter evidence for advanced argillic and vuggy quartz alterationformed contemporaneously above porphyry mineralization,


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Vuggy quartz

Advanced argillic top of gusano texture

Banded quartz veins

Wormy quartz veins

A quartz veins-Kspar-biotite

D veins-sericite

Brittle-ductile transi tion

Porphyritic intrusion

Volcanic/pyroclastic extrusion

km

TIME

Ap

pro

xim

ate

dep

th (k

m)

FIG. 12. Schematic depth-time diagram for the porphyry-epithermal tran-sition zone in the Cajamarca region.

with both prograde and retrograde transgression of one uponthe other. D veins are poorly represented at Kupfertal, andthe possibility of vertical zonation of these into pyrite-enar-gite-covellite with advanced argillic alteration is still unclear;the latter is both older and younger than D veins but may notbe contemporaneous. The brittle-ductile transition is skill-fully explained by Muntean and Einaudi (2001), following thearguments of Fournier (1999), and the brittle-ductile bound-ary in Figure 12 is taken from them.

Remobilization

Remobilization of early-formed copper in porphyry de-posits by late hydrothermal veins, D veins at El Salvador,Chile, and Main-stage veins at Butte, Montana, has been welldocumented (Gustafson and Hunt, 1975; Brimhall, 1979).Copper was leached while coarse sericite/muscovite andpyrite were formed in vein halos and was redeposited athigher elevations with sericite or advanced argillic alterationassemblages as pyrite-bornite and ultimately covellite andchalcocite. There is much less information on the behavior ofAu and Mo in these situations.

Diamond drill hole CLL-5 at Yanacocha (Fig. 11) offers ev-idence, albeit inconclusive, for the importance of remobiliza-tion of metals from underlying porphyry mineralization into asuperimposed epithermal system. As discussed above, a deepchalcopyrite-magnetite assemblage, situated above the con-tact with late intramineral porphyry, grades upward with in-creasing pyrite and removal of magnetite. Copper content re-mains fairly constant but Au is more erratic. Within thetransition zone (410–470 m), where there are only scatteredresidual zones with reduced magnetite content and no evi-dence of its sulfidation, there is an imperfect systematic vari-ation of gold values with magnetic susceptibility. The upwardappearance of enargite and hypogene covellite in place ofchalcopyrite correlates generally with an increase in sericiticalteration and a decrease in gold content with fewer erratichighs. The copper content is little changed passing from chal-copyrite- to enargite-covellite–bearing rock but does increasesome 200 m higher up as the enargite and covellite contentsincrease. The overprinting of sericite-pyrite-chalcopyritebelow the zone of advanced argillic alteration on the earlyporphyry assemblage appears to have remobilized Au moreeffectively than Cu. We speculate that remobilization of Aufrom the underlying porphyry systems into the epithermalregime is a major reason Yanacocha is such a large, productivedistrict.

At Perol, within the heart of the porphyry orebody, there isa significant increase in Au values with little increase in Cucontents where early potassic alteration has been over-printed by sericite-pyrite (Table 1). The source of this rela-tively late gold is unknown. At both Tantahuatay and Yana-cocha, highly anomalous Mo occurs as anhedral smears ofmolybdenite without quartz, scattered throughout the multi-ple centers of advanced argillic alteration and anomalous Aucontents. At both Yanacocha and Minas Conga, quartz vein-lets with euhedral molybdenite like those typical of porphyrycopper deposits do occur, although not abundantly, at depth.This and similar occurrences at the MM deposit in theChuquicamata district, Chile (L.B. Gustafson, pers. observa-tion, 1993) suggest that much or all of the molybdenite in the

high-sulfidation epithermal regime is also remobilized fromunderlying porphyry systems.

Significance for exploration

The empirical relationships reported here have direct sig-nificance for exploration. Gusano and patchy textures seem tobe rare outside a few deposits in the Cajamarca region, oneexception being the Arco Punco prospect in the Huachocolpadistrict, south-central Peru, which has been demonstrated tooverlie a buried porphyry system (L.B. Gustafson, 1998,unpub. report for Cia. de Minas Buenaventura S.A.A.). In theCajamarca region, pyrophyllite-alunite–altered rock with gu-sano texture is taken as strong evidence of relatively high tem-perature extreme base leaching within centers of high-sulfi-dation Cu-Au mineralization (Meyer and Hemley, 1967). Therecrystallization and silicification of gusano to patchy quartz isindicative of close proximity to subjacent A quartz veins andporphyry copper mineralization. The demonstration of buriedporphyry copper systems enhances the size and perhapsgrade potential of epithermal targets higher in the systems.Elevated concentrations (50–X00 ppm) of Mo in the high-sul-fidation epithermal environment are a strong indication of apossible porphyry Cu-Au system at depth. Porphyry coppercenters are, of course, targets in their own right. Just as re-mobilization of Cu and Au from the porphyry into the ep-ithermal regime may be important in enhancing tonnage andgrade of the epithermal deposits, it can also convert at leastthe top of a potentially good porphyry deposit into pyriticwaste.

We have also seen what looks like incipient gusano tex-ture—an enlargement and rounding of plagioclase sites withdisseminated pyrite—accompanying advanced argillic alter-ation in several porphyry copper deposits in Chile and Mex-ico. We would not be surprised to see these features reportedfrom other districts.

AcknowledgmentsThe studies reported here were conducted for Compañía

de Minas Buenaventura S.A.A. and joint-venture partnersover a 12-yr period beginning in 1991. We are indebted to themany geologists at the different districts who developed theframeworks within which the relationships described herehave significance, especially Fernando Llosa T., Javier VelizM., and Omar Rodríguez P. at Minas Conga; Miguel MirandaT., Doug Pryor, Andy Swarthout, and Dick Tosdal at Tanta-huatay; Peter Mitchell, Lew Teal, Cindy Williams, and PeteRogowski at Yanacocha; and Omar Rodríguez P. and NestorCcasa A. at La Zanja. Don Alberto Benavides de la Quintanaof Buenaventura has been a constant supporter, to whom weare very grateful. We are also grateful for permission to pub-lish from joint-venture operators Minera Yanacocha SRL andCEDIMIN. Reviews of the manuscript provided by AntonioArribas, Marco Einaudi, Jeffrey Hedenquist, and Lewis Tealwere very helpful in improving the presentation, although thereviewers are not responsible for any errors in fact or inter-pretation that may remain.

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APPENDIXIncremental-Heating Spectra and Isochron Plots for Isotopically Dated Samples from Minas Conga

(sample numbers indicate drill hole number and depth in meters)

2004 gustafson, l.b., vidal, c.e., pinto, r., and noble, d.c

Documents

porphyry deposits

porphyryepithermal transition

quartzmolybdenite veins

banded quartz veins

mineralized porphyry

epithermal aucuis

epithermal gold deposits

porphyry coppergold