teal benavides 2010

0361-0128/10/3918/1173-18 1173

History and Geologic Overview of the Yanacocha Mining District, Cajamarca, Peru

LEWIS TEAL1,† AND ALBERTO BENAVIDES2

1Newmont Peru Ltd., Calle 41, No. 894, Urb. San Isidro, Lima, Peru2Minas Buenaventura S.A.A., Ave. Carlos Villaran 790, Urb. Santa Catalina, Lima, Peru

AbstractDiscovery and recognition of the Yanacocha district as a belt of northeast-trending, near-surface, high sulfi-

dation-type epithermal gold deposits with strong supergene oxidation was made over a four-year period(1984–1988) through a joint-venture alliance operated by Newmont Mining Corporation with partners Com-pañia de Minas Buenaventura and CEDIMIN (BRGM). Open-pit mining began in the district in late 1993with initial full-year gold production in 1994 of 300,000 oz and a reported end-of-year reserve of 4 Moz. Theproduction growth curve at Yanacocha over 15 years (1994–2008) has been about 10 times the first full year ofproduction in 1994. In February 2007, Minera Yanacocha (Newmont 51%; Buenaventura 44%; InternationalFinance Corp 5%) poured its 20-millionth gold ounce life-of-mine. Reported 2008 end-of-year oxide reserveswithin the operating district are >13 Moz gold. Since exploration began in the district, over 20 hard-rock deposits and two gravel fan deposits (La Quinua, La Quinua Sur) have been developed. Cumulative historicaldiscovery-to-reserve conversion costs average <$10/oz Au. The estimated oxide plus sulfide drill-indicated goldendowment within the district exceeds 70 Moz (2,170 tonnes).

The high-sulfidation epithermal gold deposits are hosted by volcanic rocks that occur at the southern termi-nus of the northern Peruvian volcanic belt, a continuous sequence of a north-northwest–trending Miocene-Pliocene suite of bimodal andesite to rhyolite volcanic rocks that extend into southern Ecuador. In the Yana-cocha district, the volcanic pile has been subdivided into three groups: (1) the lower andesite sequence,consisting of an intercalated sequence of block and ash flow tuffs, flow sequences with rare, associated flowdomes, and an upper zone dominated by ignimbrites and fine-grained, laminated epiclastic sequences thatshow a transition into the overlying Yanacocha pyroclastic sequence; (2) the Yanacocha pyroclastic sequence, avariable sequence of lithic to lithic crystal tuffs, extensively altered in the central portion of the district and pri-mary host to the majority of gold deposits within the district; (3) the upper andesite-dacite sequence, consist-ing of intercalated units of andesite to dacite flows, dominated by multiple flow dome complexes in its upperportion. Ar-Ar age dating within the district has yielded ages ranging from 19 Ma (basal lower andesite) to >12Ma (upper andesite sequence). The entire volcanic pile has been crosscut by multiple phases of phreatic (vaporphase dominant), phreatomagmatic (intrusive component) and hydrothermal breccias, and intruded by multi-ple late-stage phases of andesite dikes and dacite to quartz dacite plugs, dikes and stocks (10–8 Ma), the latterof which are associated with shallow Au-Cu porphyry-style mineralization that underlies the high-sulfidationepithermal deposits.

Alteration in the district consists of multiple stages of advanced argillic assemblages, typical in nature, thatare zoned outward from a central core of massive to vuggy quartz to broader envelopes of advanced argillic alteration (alunite-pyrophyllite) to still broader zones of intermediate argillic and propylitic alteration halos. Ona district scale, a generally southwest to northeast progression of multiple alteration centers with various tim-ing has been documented through Ar-Ar dating of hydrothermal alunite, with ages ranging from 11.5 to 8.5 Ma.In places the advanced argillic alteration assemblage is overprinted by a later event consisting of creamy col-ored cryptocrystalline chalcedony plus barite associated with intermediate sulfidation type feeders. Gold min-eralization is a late-stage event that was superimposed over all alteration types, most commonly occurring inlate-stage fractures with massive, vuggy, and granular quartz, and especially where higher grade gold mineral-ization (>3.0 g/t Au) is associated with indurated replacement zones of creamy chalcedony. Mineral depositsare commonly localized within pyroclastic host rocks and phreatic breccias that envelop the margins of less per-meable phreatomagmatic and within hydrothermal breccia pipes and andesite-dacite dome margins.

Geophysical surveys have been employed as part of an integrated approach from the earliest explorationstages in the district. Resistivity and magnetic and potassium radiometric response anomalies are spatially associated with gold on a district-wide and deposit scale. Gravity surveys have added to understanding of

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

©2010 by Economic Geology, Vol. 105, pp. 1173–1190 Submitted: February 22, 2010Accepted: July 21, 2010

Economic GeologyBULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 105 November NO. 7

IntroductionSINCE THE initial discovery of outcrop mineralization in 1984,Yanacocha has grown to become one of the largest gold min-ing districts in the world (Fig. 1). Production from 1993 to2008 has totaled 27.1 Moz gold all from oxide-classified ores.Single-year production peaked in 2005 at 3.33 Moz gold and1.8 Moz was produced during 2008. Average annual life-of-mine (LOM) production grades have varied, ranging from1.84 to 0.65 g/t Au during this period. Operating costs frominitial production in 1993 through 2008 have ranged from alow of $97 (1997) to $358 per ounce (2008). Historically, lowoperating costs at Yanacocha have been aided by low waste-to-ore strip ratios, averaging less than 0.5 during much of itsoperating life, due primarily to many of the original quartz-dominant outcrop deposits forming topographic highs.

Most ore is classified as run-of-mine, indicating that nocrushing circuit is needed in the recovery process. An addi-tional recovery characteristic is the friable nature of most ofthe ore, with the majority of gold present as micron-size in-clusions within limonitic fractures. Life-of-mine (LOM) re-covery has exceeded 70 percent, whereas by-product silverrecovery averages less than 10 percent. Silver to gold ratios in

the oxide ores LOM have averaged approximately 20:1. Thefirst reported oxide-classified resource at Yanacocha in 1988consisted of 1.97 Moz gold, averaging 2.6 g/t. Discovery andgrowth curves during the 1990s reflected the phenomenalmetal endowment and exploration success in the district,where gold reserves of oxide-classified ores within the districtpeaked in 2000 at 36.6 Moz. Reported remaining oxide re-serves at the end of 2008 stood at 13.5 Moz.

As a world-class mining district, Yanacocha remains one ofthe most intensely explored areas globally. In this paper, wesummarize the knowledge and understanding of the Yana-cocha district at this point in its history, as a companion to theMap Series of the district (Plates I to III).

Discovery and Development History

Prediscovery history

As with all large mining camps, the discovery and develop-ment of the Yanacocha district has an illustrative history thatinvolved multiple contributors to what became, and contin-ues to be, an evolving body of knowledge. Although the oc-currence of gold at Yanacocha was not documented before

1174 TEAL AND BENAVIDES

0361-0128/98/000/000-00 $6.00 1174

basement geometry and have outlined volcanic subbasins. The east-northeast alignment of the Yanacocha golddeposits is interpreted to have been caused by the intersection of a northeast-trending trans-arc crustal break(Chicama-Yanacocha structural corridor). This is superimposed over older, north-northwest to northwest An-dean-parallel folded and thrusted Cretaceous-age sedimentary basement terrane (Paleocene-Eocene). Bothstructural orientations dominate in the Yanacocha district, controlling emplacement of breccias and intrusionsas well as formation of gold mineralization. Discontinuous east-west–oriented faults and fracture zones are in-terpreted to be extensional and control gold mineralization on a deposit scale.

The transition from near-surface, gold-dominant mineralization to underlying copper-dominant sulfide min-eralization associated with shallow quartz eye-bearing monzonite to tonalite intrusions has been documentedby deep (500–1,200 m) diamond drilling beneath the Yanacocha complex (Sur-Norte-Oeste), the largest singlegold deposit in the district with an endowment exceeding 18 Moz.

0°

4°

6°

8°

10°

82° 78° 74° 70°

Yanacocha

P E R U

E C U A D O R

C O L O M B I A

B R A Z I L

Pa

ci

fi

c Oc

ea

n

B O L I V I A

L i m a

C H I L E

General Geology of Peru

Intrusive Rocks (Various ages)

Volcanic Rocks (Miocene-Cretaceous)

Sedimentary Rocks (Mesozoic)

Sedimentary Rocks (Paleozoic-Precambrian)

0 200 400km

NORTH

FIG. 1. Schematic map showing the geology of western Peru and general location of the Yanacocha mining district.

the modern exploration era, its existence was presumablyknown, as indicated by documented pre-Columbus workingsat what are now the Carachugo, Maqui Maqui, Cerro Negro,and Pabellon deposits, which are interpreted to have been ex-ploited for mercury, hematite, native sulfur, and gold. Thehistorical name of the drainage northwest of the Cerro Quil-ish deposit is Corimayo, which in the native Quechua lan-guage of the region literally translates to “gold river.”

The first documented report of mineralization at Yanaco-cha came from Ramondi (1913, p. 72), reporting on his expe-dition of 1859 in northern Peru; he described historical nativeworkings at what is now the Carachugo deposit: “El camino deCajamarca á Yanacocha tiene la dirección casi a sur ánorte…Estas minas se conocen con el nombre de Carachugo,y están en un roca cuarzosa porosa, sus socavones son muylargos…” (“The road from Cajamarca to Yanacocha has al-most a north-to-south direction….These mines they know bythe name of Carachugo, and they are in a porous quartz rock,their tunnels are very long…”).

The first modern era exploration at Yanacocha was con-ducted by the Guggenheim Brothers (Asarco), who stakedthree mineral concessions in 1962. These efforts followed thediscovery of the Michiquillay porphyry copper deposit byAsarco in 1957, located 20 km east-southeast of Yanacocha(Hollister, 1978). Hollister (1962, p. 2) described five mineraloccurrences consisting of silicified breccias and the occur-rence of “breccia pipes.” A proposal for “5 holes totaling 2200feet of drilling” was never undertaken.

In 1968 Nippon Mining completed 13 exploration holes de-signed to test for porphyry copper mineralization within anortheastern extension of Encajon Creek, an incised drainagethat separates the San Jose and Yanacocha Sur gold deposits.This prospect area, situated some 300 m vertically below thequartz cap that forms the top of Cerro Yanacocha, was laterthe site of the discovery of the Kupfertal copper-gold por-phyry system by Yanacocha geologists Thomas Klein and RitaPinto in 2000.

In 1970 to 1971, acting on behalf of a Peruvian governmentand United Nations mandate, the British Geological Surveyundertook a reconnaissance regional stream sediment surveycovering an extensive area in northern Peru, including what isnow the Yanacocha district. The survey was designed to stim-ulate private interest in exploration and development in theregion. Samples were analyzed for a suite of elements includ-ing copper, lead, zinc, and silver, but not gold. Results of thesurvey clearly outlined the district as a significant silver (>10ppm) anomaly, and where incised drainages intersected themassive quartz replacement cores (Cerro Yanacocha complex,Cerro Carachugo, Cerro Quilish), stream sediment silver val-ues exceed 20 ppm.

In 1973, St. Joe Minerals Company completed the firstknown induced polarization (IP) survey within what was tobecome the Yanacocha district. Follow-up exploration con-sisted of completing three diamond drill holes between whatis now the Cerro Quilish and Cerro Negro gold deposits(Turner, 1997) (Plate III).

As a follow-up to the British Geological Survey stream sed-iment results, CEDIMIN (a joint-venture alliance betweenthe BRGM and Compañia de Minera Buenaventura) ac-quired the original mineral concessions within the Yanacocha

district (Paverd and Bowerman, 1994). Recognition of the ini-tial potential of what would eventually become the Yanacochadistrict is credited in part to Jacques Boulanger, South Amer-ican manager for the BRGM at the time. Initial surfacetrenching in 1982 at Cerro Yanacocha yielded significant sil-ver (>20 ppm) and lead (>300 ppm) anomalies.

In August 1983, a meeting was held in Lima betweenBoulanger of BRGM, Alberto Benavides, chairman of Bue-naventura, and Aubrey Pavard, then South American man-ager of Newmont Mining, during which a Newmont repre-sentative was invited for a site visit to the Yanacocha prospect.In September 1983, Anthony Bowerman, exploration man-ager for Newmont-Peru, completed an initial site visit anddocumented the presence and extent of an outcrop epi-thermal system with bulk tonnage potential (Paverd and Bow-erman, 1994; Paverd, 2003, 2004).

Exploration work in 1984, under a letter of intent, consistedof completing 25 percussion drill holes near Cerro Yanacocha.Turner (1997) reported an initial hand-calculated cross sec-tional inventory at the end of the first year’s program thatyielded 3.125 Mt, averaging ~ 90 g/t Ag. The conclusionreached after the first year of systematic exploration was thatYanacocha was a bulk-tonnage, low-grade silver system. It isimportant to note that the best gold intercept from the firstround of scout drilling at the time consisted of 7 m averaging9.6 g/t.

A joint-venture agreement that included an initial equity of40 percent Newmont Mining-40 percent CEDIMIN-20 per-cent Minera Buenaventura was signed in September 1985, atwhich time Newmont became operator of the core mineralconcession and CEDIMIN remained operator over the ad-joining and surrounding area of interest (AOI) concessions.This agreement was later modified in 1999 by unitization ofthe Yanacocha and adjoining Minas Conga mineral conces-sions at which at the time the current equity of the MineraYanacocha S.R.L. partnership was established: 51.35 percentNewmont Mining, 43.65 percent Minera Buenaventura, 5percent International Finance Corporation (World Bank).

Initial discovery and preproduction phase discoveries,1985–1993

Exploration concepts during the initial discovery period of1985 to 1993 evolved rapidly, including recognition of Yana-cocha as an approximately 18-km-long by 6-km-wide, north-east-trending belt of gold-bearing, high-sulfidation (HS) sys-tems in outcrop. Drilling at Cerro Yanacocha in 1986confirmed the gold-bearing potential of the district (Fig. 2).Discoveries resulted from systematic chip and channel sam-pling of outcrops, project-scale (1:10,000–1:1,000) geologicmapping and IP geophysical surveys. This approach was usedconcurrently over a number of prospects that resulted in thediscovery of a succession of outcrop to near-outcrop deposits,including Carachugo, San Jose, Yanacocha Norte, MaquiMaqui, Yanacocha Sur, Cerro Negro Este, and Cerro Quilish(Plate III). The work of the original Minera Yanacocha explo-ration team under the initial direction of A. Paverd, A. Bow-erman, M. Cardozo, and A. Quiroz was responsible for theseinitial successes. By the time initial production began in Sep-tember 1993 with a modest reported oxide reserve of 4.0 Mozgold at the Cerro Carachugo, San Jose, and Yanacocha Norte

HISTORY AND GEOLOGIC OVERVIEW OF THE YANACOCHA MINING DISTRICT, CAJAMARCA, PERU 1175

0361-0128/98/000/000-00 $6.00 1175

deposits, Minera Yanacocha geologists had already begun torecognized a belt of world-class high sulfidation systems anddelineate a mineral district of vastly larger dimensions(Turner, 1997; Harvey et al, 1999) (Fig. 3).

Concurrent production growth cycle, 1994–2001

A second generation of discoveries during the 1994 to 2001period included a series of buried and blind high sulfidationoxide gold deposits and a large, gold-bearing, gravel-hostedfan deposit (La Quinua) that continued the Yanacocha growthcycle. These successes were aided by (1) improving the un-derstanding of structural controls on mineralization, both atthe deposit and district scale; (2) an evolving understanding ofthe volcanic stratigraphy and recognition of favorable miner-alized horizons resulting from regional-scale petrologic andpetrogenetic studies and age determinations by Turner (1997)in his doctoral dissertation in the district; (3) internal studiesof deposit-scale volcanostratigraphic and structural controlsadvanced by detailed documentation by the mine develop-ment geology team; (4) recognition of the multiple styles andtypes of deposits, including diatreme and flow-dome marginsthat served as traps for gold deposition; and (5) and charac-terization of high sulfidation-type alteration assemblages andzoning patterns.

Minera Yanacocha geologists who contributed to significantdiscoveries during this period include the following: S. Moore

and P. Mallette, 1997 (La Quinua: Mallette et al., 2004); A.Longo, 1998–1999 (Chaquicocha Sur: Longo, 2000); S. Myersand M. Rutti, 1998 (El Tapado: Harvey et al., 1999); M.Goldie and J. Gomez, 2000–2001 (Corimayo: Goldie, 2000;Gomez, 2002) under the successive direction of B. Harveyand S. Myers.

Transition to sulfide feeders and Au-Cu porphyry environs,2000–present

The discovery of the Kupfertal Au-Cu porphyry deposit(Pinto, 2002; Bell et al., 2004; Gustafson et al., 2004) usheredin a new chapter of deeper exploration beneath the gold-bearing high sulfidation-type (now oxidized) caps, into thehigher grade, sulfide-dominant feeder zones and, concur-rently, into the transitional environment to shallow Au-Cuporphyry deposits. The initial phase of this program began in2000–2001 and focused on porphyry targets at Kupfertal andalong the margins of the Yanacocha diatreme, beneath theYanacocha Norte and Yanacocha Sur deposits. Initial resultsdelineated a supergene sulfide blanket of covellite + chal-cocite intermixed with hypogene enargite, grading >1 wt per-cent total copper.

Beginning in late 2001, an expanded phase of what was thencalled the Yanacocha sulfides project refocused its efforts ondiscovery of higher grade (>3 g/t Au) sulfide orebodies thatform the roots to the gold-bearing oxidized caps (Moore, 2003).


0361-0128/98/000/000-00 $6.00 1176

NORTH

0

Approximate Scale

~100m

4

DDH Au GM/MT

CUT-OFF 1 GM AU/METRIC TONSilicified Zone With Assay Results To Date

Ag OZ/ST PROB.WOM

13

14

10

11

12

2.0

1.5

3.0

2.7

1.4

1.9

2.0

2.8

0.3

1.8

2.5

5.9

1.3

1.4

2.7

1.4

2.9

0.2

1.6

0.4

1.0

2.0

1.4

1.2

1.5

1.1

4.2

2.4

4.1

3.9

4.1

0.1

1.2

0.3

2.1

3.6

1.2

6.8

3.2

9 3.7

1.4

3.4

2.1

1.5

1.2

2.5

0.8

1.6

0.4

0.3

0.2

0.2

0.2

0.70.1

5.3

32.7

17.5

2.3

14.8

15.2

3.2

4.2

6.7

22.1

47.4

1.8

4.2

4.2

21.2

3.1

4.3

28.9

3.9

13.9

10.3

1.1

10.6

9.5

3.9

19.1

Silicified Zone

YanacochaSur

YanacochaNorte

DDH-14

DDH-13DDH-12

DDH-10

DDH-1

DDH-5

DDH-2

DDH-9

DDH-15

YanacochaLake

Fault Zone

DDH-11

DDH-4

FIG. 2. Original drill hole location map and assay results from initial Newmont-Peru scout drilling campaign at YanacochaNorte and Yanacocha Sur, November 1986.

Minera Yanacocha geologists who contributed to significantsulfide discoveries during this period include Williams et al.(2002), P. Reyes, Finn (2003), and E. Flores, 2002–2003(Chaquicocha sulfides); E. Flores, J. Trujillo and P. Zamora,2003–2004 (Maqui Maqui sulfides); R. Pinto and J. Qusipe,2004–2005 (Antonio deposit) (Fig. 4).

In 2004, following drilling of the Perol and Chalhagon Au-Cu porphyry deposits in the Minas Conga district 20 km tothe northeast, exploration was redirected again to the centralportion of the Yanacocha district, focused beneath the giantYanacocha Sur and Yanacocha Norte deposits. This efforteventually led to the discovery in 2006 of what is interpretedas a system of high-level, nested Au-Cu porphyry deposits be-neath the Yanacocha Sur oxide deposit that extends from thesouthern margin of the Yanacocha diatreme ~1 km southwestbeneath the high sulfidation cap to the La Quinua fault zone;this concept was put forth 10 years earlier by Sillitoe (1996,2000). Minera Yanacocha geologists who contributed to sig-nificant discoveries during this period include C. Loayza, P.Reyes, R. Pinto, R. Patchas, P. Bell, and P. Rowgoski,2005–2006 (Yanacocha Verde deposit); and C. Schnell and J.Martinez, 2005 (Yanacocha Norte diatreme margin) (Fig. 5).

Regional Geologic SettingThe N50°–60°E trend of deposits that form the Yanacocha

district occur within the central Andean orogenic belt, anorthwest-trending region of deformation that encompassesthe entire western half of Peru (Fig. 1). This orogenic beltconsists of folded and thrusted Ordovician to Cretaceous

sedimentary basement rocks overlain by early Tertiary toHolocene volcanic rocks and early to late Tertiary intermedi-ate to felsic intrusive rocks.

In northern Peru, Cretaceous sedimentary rocks are inter-preted to have been subjected to two major periods of east-northeast–directed compressive deformation referred to asIncaic I (Paleocene, 59–55 Ma) and Incaic II (mid-Eocoene,43–44 Ma) orogenesis. Volcanism within this orogenic beltbegan in the early Eocene (45 Ma) following Incaic I defor-mation and continued intermittently through the succeedingperiod of Incaic II deformation. In the Cajamarca region, In-caic II deformation corresponds to a regional scale deflectionof earlier, dominant northwest-directed fold and thrust IncaicI deformation to more west-northwest– to east-west–trendingfold axes and thrust faults in basement Cretaceous sedimen-tary rocks. This deflection has been referred to as the Caja-marca curvature and is interpreted to have formed as a resultof increase in convergence rate and a change in subductionmotion of the underlying Nazca plate (Benavides-Cáceres,1999). The east-northeast alignment of the Yanacocha belt isinterpreted to occur along this transecting cross-arc break, re-ferred to by Turner (1997) as the Chicama-Yanacocha struc-tural corridor (Fig. 6).

Yanacocha District-Scale Geology

Cretaceous sedimentary basement rocks

The oldest exposed rocks within the Yanaocha district con-sist of folded and thrusted sequences of Farrat Formation


0361-0128/98/000/000-00 $6.00 1177

1994

4

400

0

5

10

15

20

25

30

35

40

1995

4.9

400

1996

6.1

400

1997

13.9

400

1998

20.6

350

1999

32.9

325

2000

36.6

300

2001

34.2

300

YearGoldPrice

Moz

2002

32.6

300

2003

31.7

325

2004

26.5*

375

2005

21

400

*4M oz from Cerro Quilish extracted from reserve 2004

2006

17

500

2007

16

575

2008

13

725

FIG. 3. Yanacocha district end-of-year gold reserves 1994–2008; note that all Yanacocha gold reserves shown are classi-fied metallurgically as oxide.


0361-0128/98/000/000-00 $6.00 1178

CHQ-602

CHQ-550

CHQ-555

CHQ-605

Looking North

CHQ-557

CHQ-601

CHQ-599

Cham

pa Fault

4,000v

3,800v

3,600v

17,8

00E

18,0

00E

18,2

00E

0 100m

$425 SulfideCone 2005(Projected

onto Section)

$400 Oxide ReservePit 2005 (Projected

onto Section)

64m at15.7 g/t Au

88m at14.8 g/t Au

Tft

Teut

Teut

La

La

TisjTisj

bxa

Tud

Gold Shape 2.50 g/tFaultDrillholes (2005)

0.000.150.301.00

2.505.00

>10.00

Drillhole Intercepts (g/t) Au

Montura DomeSan Jose IgnimbriteEutaxitic Tuff

Fine Grain TuffLower Andesite

Teut

Tisj La

Tud Tft

FIG. 4. Chacquicocha sulfide deposit showing >3 g/t Au sulfide zone beneath the 2005 hypothetical oxide pit design basedon a 2005 $400 gold price (red), and a hypothetical $425 sulfide pit (green). Note that the late-stage, fresh to weakly alteredMontura dome formed a rheological boundary to gold-bearing hydrothermal fluids, which caused precipitation of gold in thefavorable host Yanacocha pyroclastic sequence.

Modeled Limit of +3 g/t Au Oxide

Sulfide – Au and Ag at +5 g/t Au C/O

+3% Sulfide Cu (Ag, Au)

Central Yanacocha Diatreme

Hypobyssal Yanacocha Diorite (Yp)

Breccia Envelopes (Bx2)

Yanacocha Pyroclastic Sequence

28,000N

27,000N

14,0

00E

15,0

00E

Yp

Yanacocha Sur

Yanacocha Norte

Yanacocha Oeste

Ys Sulfide Au-(Ag)Ys Supergene Cu

YN Main Gold Zone-Sulfide Au-(Ag)YN Cu Zone-Hypogene Cu-Au-(Ag)

Oeste Supergene Cu-(Ag)

Yp

Bx2

DiatremeComplex

0 150 300m

NORTH

FIG. 5. Yanacocha mine showing footprint of the central Yanacocha diatreme, hypobyssal Yanacocha diorite (Yp), andbreccia envelopes crosscutting the Yanacocha pyroclastic sequence (white). Shown are projected composite models of high-grade oxide (red) and sulfide gold mineralization (blue), plus high-grade sulfide copper mineralization (green). Note howgold and copper mineralization types are focused around the diatreme complex. At Yanacocha Sur gold is hosted in the Yana-cocha pyroclastic sequence and is controlled along a rheological boundary that extends beneath the eastward dipping con-tact of the Yanacocha diorite.


0361-0128/98/000/000-00 $6.00 1179

Cha

ilhua

gon

La Z

anja

Yan

acoc

ha

La Q

uinu

a

Cer

ro C

oron

a

Sip

an

Cer

ro Q

uilis

h

La C

arp

aG

alen

o

Per

ol

Mic

hiq

uilla

y

Tert

iary

vol

cani

c ro

cks–

und

iffer

entia

ted

Tert

iary

intr

usiv

e ro

cks–

und

iffer

entia

ted

Cre

tace

ous

sed

imen

tary

roc

ks–u

ndiff

eren

tiate

dY

anac

ocha

alte

ratio

n en

velo

pe

HS

; Au-

Cu

por

phy

ry d

epos

its

Faul

ts (d

ashe

d w

here

unc

erta

in)

Fold

axi

sTh

rust

faul

tN

orm

al fa

ult

NO

RTH

010

km5

FIG

.6.

Yana

coch

a re

gion

al g

eolo

gic

sett

ing

alon

g no

rthe

ast-

tren

ding

Yan

acoc

ha-C

hica

ma

tran

s-ar

c br

eak.

Dep

osit

foot

prin

ts a

re s

how

n to

sca

le (

red)

with

in a

lter-

atio

n en

velo

pe (

red

hatc

hed

patt

ern)

. Not

e th

at t

he w

est-

nort

hwes

t to

eas

t-w

est

defle

ctio

n of

fol

d ax

es in

Cre

tace

ous

sedi

men

tary

bas

emen

t ro

cks

is in

terp

rete

d as

a re

gion

al m

anife

stat

ion

of th

is tr

ans-

arc

stru

ctur

e. D

epos

it fo

otpr

ints

of t

he A

ndea

n pa

ralle

l nor

ther

n Pe

ru p

orph

yry

belt:

Cer

ro C

oron

a–Pe

rol-C

haul

huag

on–L

a C

arpa

-G

alen

o-M

ichi

quill

ay s

how

n in

the

nor

thea

st q

uadr

ant

of t

he m

ap. A

lso

show

n ar

e sm

alle

r hi

gh s

ulfid

atio

n-ty

pe g

old

depo

sits

, inc

ludi

ng S

ipan

and

La

Zanj

a. M

odifi

edaf

ter

Riv

era

(198

0) a

nd W

ilson

(19

85).

sandstones of the Lower Cretaceous Goyllarisquiza Groupexposed in the far southwestern part of the district. In thenortheast, the Yumagual Formation (Cy) consists of region-ally extensive Middle Cretaceous Puilluicana Group. The Yu-magual Formation consists of thick bedded limestone withminor intercalated shale partings that was subjected to north-east-directed compressional deformation from prevolcanic,early Tertiary Incaic I and Incaic II orogenesis (Rivera, 1980;Wilson, 1985; Benavides-Cáceres, 1999) (Yanacocha districtinterpretive geology, 1:25,000, Plate I).

Yanacocha Tertiary volcanic pile

Identifying and mapping a traceable and predictable vol-canostratigraphic sequence within the Yanacocha district de-veloped over a number of years as the result of observationsby numerous Yanacocha exploration and mine developmentteams, aided by an ever-increasing database of surface andsubsurface information that now includes over 10,000 drillholes. In addition, these efforts were advanced by the de-tailed geologic and petrographic studies combined with dat-ing by Turner (1997) and Longo (2005) (Yanacocha district in-terpretive geology, 1:25,000, Plate I).

Efforts to unravel the volcanic stratigraphy were made ex-ceedingly more difficult by two factors: (1) the degree of tex-tural and compositional destruction in the alteration envelope

that encompasses the district—approximately 18 km northeastby 6 km northwest; and (2) the interpreted mosaic of paleo-volcanic topographic highs and basins that resulted in abruptthickness changes in portions of the volcanic pile. A break-through in this process occurred internal to the Yanacocha ge-ology team in early 2001, with the recognition of a capping se-quence what came to be referred to informally as the upperandesite sequence. Before its recognition, the upper andesitehad been misinterpreted as the much earlier recognizedlower andesite sequence. This misinterpretation was due tosimilar megascopic compositional and textural characteristicsalong with similar flow and flow-dome horizons (now recog-nized as containing diorite) within both sequences. As a re-sult, the Yanacocha team recognized three distinct volcanicpackages (Teal et al., 2002; Bell et al., 2004). It became clearthat it was possible to drill through a capping sequence of un-altered flows and flow-dome margins and back into the dom-inant mineralized host sequence, the Yanacocha pyroclasticsequence (Fig. 7).

Beginning in 2001, this new understanding led to a com-plete district-wide re-assessment, particularly of the easternhalf of the district, which up until that time had been inter-preted outside of the Maqui Maqui deposit to consist of theless favorably mineralized lower andesite sequence (Longoand Teal, 2000). As a direct result, discovery of a series of


0361-0128/98/000/000-00 $6.00 1180

HydrothermalBreccia

PhreaticBreccia

Phreato-magmatic

Breccia

Mio

cene

Low

er A

ndes

ite S

eque

nce

Cre

tace

ous

Bas

emen

t

Mio

cene

Yan

acoc

haP

yroc

last

ic S

eque

nce

Mio

cene

Up

per

And

esite

Seq

uenc

e

Generalized Stratigraphic Column

12.3-11.2 Ma

? +12 Ma

13.7-19 Ma

San JoseChaquicochaYanacocha Complex

CorimayoCerro NegroTapado

San JoseCarachugoYanacocha Complex

Cerro QuilishCerro Negro

La Quinua

Host Units For Gold Mineralization

Maqui Maqui NorteAntonio

AntonioMaqui Maqui

CorimayoTapado

Diatreme Ypq QuaternaryGravels

FIG. 7. Yanacocha district schematic stratigraphic column showing mineralized horizons by deposit and volcanic se-quences. Modified after Bell et al. (2004).

significant new buried and blind deposits was made duringsucceeding years (e.g., Corimayo, Antonio, and ChaquicochaSur Sulfide-Montura dome margin). An important contribu-tion in this process was the first detailed district-scale volcanicstratigraphic study completed by Moore and Saderholm(2002), in which different sectors and gold deposits were cor-related using this volcanic nomenclature.

The lower andesite sequence is exposed predominantlywithin the western portion of the district and consists of an in-tercalated sequence of block and ash flow tuffs, flow se-quences with rare associated flow domes, and an upper zonedominated by ignimbrites grading upward into a transitionalfine-grained, laminated epiclastic horizon. The lower an-desite lies unconformably on folded early Cretaceous base-ment sedimentary rocks and grades upward into the overlyingYanacocha pyroclastic sequence. Longo (2005) subdividedand dated the sequence using Ar39/Ar40 to include a basal py-roxene andesite lahar (19.53 Ma), a lower, biotite-rich tuff(Tlbp) (15.5 Ma) and an upper pyroxene-hornblende andesite(Tlpba) (13.2 Ma) (Yanacocha district interpretive geology,1:25,000, Plate I).

The Yanacocha pyroclastic sequence consists of a variableprogression of lithic to lithic crystal tuffs. This sequence is in-terpreted to include the Maqui Maqui ignimbrite, a distinctfragmental lapilli tuff confined to the western portion of thedistrict, for which Longo (2005) reports an Ar-Ar date of 12.4to 12.6 Ma. Because of their inherent porosity, these rockswere more susceptible to moderate to intense, pervasive ad-vanced argillic alteration, which ranges from moderate tocomplete textural destructive and as a result altered theiroriginal chemical compositions. Consequently, accurate dat-ing for most of these rocks is not possible, but they can beconstrained to a range of <13.2 to ~12.4 Ma, based on datingby Longo (2005) of underlying and overlying sequences. TheYanacocha pyroclastic sequence is exposed primarily in thecentral portion of the district and is the primary host to themajority of gold deposits. Moore and Saderholm (2002) sub-divided the sequence into a series of cohesive, traceable unitsincluding the following: lower fine tuff with interbedded lam-inated epiclastic sediments (Tl), a transitional eutaxitic tuff(Teut), an upper lithic tuff facies (Tilt) that includes theMaqui Maqui ignimbrite (Tim), and the San Jose ignimbrite(Tisj).

The upper andesite sequence consists of intercalated an-desitic to dacitic flows and ignimbrites dominated by multipleflow dome complexes in its upper portion. The unit is subdi-vided into a lower hornblende porphyry andesite flow (Tupha,11.8–12.3 Ma), Shascha ignimbrite (Tutx, 11.25 Ma); andesiteporphyry flows (Typ, 11.6–12.3 Ma) and flow-dome com-plexes (Tud, 11.2–11.6 Ma) (Turner, 1997; Longo, 2005)(Yanacocha district interpretive geology, 1:25,000, Plate I).

Intrusive rocks

Intrusions into the volcanic pile consist of multiple phasesof late-stage dikes, plugs, and stocks, which are spatially andtemporally associated with deeper Au-Cu porphyry-style min-eralization that occurs beneath the gold-dominant high sulfi-dation epithermal deposits. Classification includes two broadsuites of early hornblende diorite or andesite porphyry (Tcp)and a dacite-to-tonalite quartz-eye porphyry (Typq). Ages of

these units range from 12 to 8.4 Ma (Turner, 1997; Longo,2005; Fig. 7).

Breccias

The entire volcanic pile has been cut by multiple phasesand types of breccias that are associated both spatially and ge-netically with episodes of gold mineralization. On a districtscale, they have been classified into three genetic categories,including phreatic, phreatomagmatic, and hydrothermalbreccias, although each style commonly grades into another(Fig. 8). Phreatic breccias form the most common type volu-metrically and consist of monolithc subangular to subroundedclasts in an abraded sandy matrix. Their textures vary frommatrix supported to clast supported and clast-matrix sup-ported. They are interpreted to have formed early in theeruptive volcanic cycle, a result of vapor-dominated explo-sions in the volcanic pile.

Phreatomagmatic breccias consist of rounded to subangu-lar heterolithic clasts in a crystal-rich, felspathic matrix. Thesebreccias are interpreted to have formed as a result of near-surface eruptions of partially solidified magmas intrudedalong structural conduits and volcanic throats in which theconfining pressure in the magma column overcame lithostaticload of the volcanic pile. Consequently, they consist domi-nantly of porphyry-derived clasts and matrix.

Hydrothermal breccias constitute a third class. In contrastto phreatic breccias, hydrothermal breccias contain roundedto angular heterolithic clasts in a dominantly cryptocrystallineto fine-grained quartz matrix. In the oxide environment, thematrices are flooded with gossanous limonite, chalcedony,alunite, and minor barite. In the sulfide environment, matri-ces are flooded by cryptocrystalline quartz + fine-grainedpyrite ± alunite ± barite. These breccias are associated spa-tially and genetically with higher grade gold mineralization,ranging locally upward to bonanza grades of >30 g/t Au. Theirgeometries range from stratiform to structurally controlled,high-angle feeders, breccias, and disseminated mineralizedshoots. Heterolithic brecciated clasts within a crosscuttingenvelope of younger hydrothermal breccia are common alongflow-dome and older phreato-magmatic margins.

AlterationWhen asked of his impressions from his first site visit to

Yanacocha in 1984, Aubrey Paverd responded without hesita-tion, “mountains of siliceous alteration…I walked for threedays over the length and breadth of the property and re-mained in massive and vuggy silicificied rock the vast major-ity of the time” (A. Paverd, pers. commun., 2003). Alterationwithin the district consists of mineral assemblages that arezoned outward from a central, generally gold-bearing core ofmassive to vuggy quartz, to broader envelopes of advancedargillic alteration (alunite-pyrophyllite) to still broader zonesof intermediate argillic and propylitic halos. This overlappingmosaic is repeated in multiple alteration centers to form abroad envelope within the volcanic pile along a major axis ap-proximately 18 km northeast by 6 km northwest (Yanacochadistrict alteration map, 1:25000, Plate II).

On a deposit scale, pervasive massive siliceous cores gradeabruptly outward into intense, acid-leached vuggy quartz en-velopes, and where preserved, caps of granular quartz, which


0361-0128/98/000/000-00 $6.00 1181

are interpreted to represent the near-surface, vapor-domi-nated horizons. Broader envelopes of advanced argillic alter-ation, alunite + kaolinite ± pyrophyllite ± diaspore, form lateral and vertically zoned assemblages around massivequartz-dominant replacement cores (Fig. 9). The outermostlower temperature portions of the hydrothermal systems arerepresented by envelopes of montmorillonite ± illite-sericite,grading to propyllitic alteration. The outermost expression ofthe system consists of volcanic and intrusive rocks with minorchalcedony + opal replacement horizons. This alteration zon-ing pattern has been summarized by Harvey et al. (1999)(Yanacocha district alteration map, 1:25000, Plate II).

In one of the most significant advances in understandingthe geologic evolution of the Yanacocha district, Longo (2005)documented a general west-southwest–east-northeast pro-gression of multiple alteration centers using Ar39/Ar40 datingof hydrothermal alunite, with ages of mineralized centersranging from 11.5 to 8.5 Ma. The hydrothermal systems thatspan the district are interpreted to have formed initially at theCerro Negro Oeste deposit from earliest dates at 11.5 Ma inthe southwest margin of the district. The focus of hydrother-mal activity migrated 18 km east-northeast to the MaquiMaqui deposit (10.2 Ma) and, finally, a late major pulse pro-duced a late phase of the central Yanacocha Norte and Sur

deposits (8.5 Ma). In essence, the epithermal system(s) form-ing the Yanacocha deposits endured for a minimum of ~3m.y., contemporaneous with the span of porphyry intrusions(Longo et al., 2010).


0361-0128/98/000/000-00 $6.00 1182

Hydrothermal Breccia

Phreato-magmatic Breccia

Phreatic Breccia

HydrothermalBreccia

PhreaticBreccia

Phreato-magmatic

Breccia

Mio

cene

Low

er A

ndes

ite S

eque

nce

Cre

tace

ous

Bas

emen

t

Mio

cene

Yan

acoc

haP

yroc

last

ic S

eque

nce

Mio

cene

Up

per

And

esite

Seq

uenc

e

Generalized Stratigraphic Column

San JoseChaquicochaYanacocha Complex

CorimayoCerro NegroTapado

San JoseCarachugoYanacocha Complex

Cerro QuilishCerro Negro

La Quinua

Host Units For Gold Mineralization

Maqui Maqui NorteAntonio

AntonioMaqui Maqui

CorimayoTapado

Diatreme Ypq QuaternaryGravels

FIG. 8. Yanacocha district schematic column showing breccia subclassifications and general occurrences within the vol-canic sequences.

Looking Northeast

130m/1.02 g/t160m/2.20 g/t

97m/0.90 g/t

165m/3.42 g/t

Leached GranularSilica Cap

Massive/VuggyTextured Silica Core

104m/1.30 g/t

0 100m

Granular QuartzMassive-Vuggy Quartz

Quartz Alunite Propylitic

Drillholes with Au Intercepts

FIG. 9. Schematic cross section of the Chaquicocha Sur deposit showinga high sulfidation-type alteration assemblage of massive to vuggy siliceouscore and with preserved granular quartz cap. Note buried gold mineraliza-tion beneath barren granular quartz cap.

StructureThe alignment of the Yanacocha gold deposits are inter-

preted to have been controlled by the intersection of a mid-dle Tertiary age, northeast-trending trans-arc crustal breakreferred to as the Chicama-Yanacocha structural corridor(Turner, 1997), superimposed over an older Paleocene-Eocene, Incaic I and Incaic II north-northwest to northwest-erly Andean parallel deformational fabric in the form offolded and thrusted Cretaceous-age sedimentary basementterrain (Benavides-Cáceres, 1999). The lattice pattern of in-tersecting northwestern Andean parallel and northeasterntrans-arc fault systems dominate the structural domain at adistrict and deposit scale, controlling the localization of brec-cias and emplacement of shallow intrusions into the volcanicpile, and subsequently localizing multiple phases of gold min-eralization. This intersecting pattern resulted in a mosaic pat-tern of horst and grabben subdomains that have controlledthe distribution of volcanic units within the overall volcanicpile. The most prominent of these features is the northwest-striking La Quinua basin that down faulted the volcanic pileto the southwest and separates the Yanacocha district geo-graphically into east and west subdistricts. During the periodof volcanism this resulted in abrupt internal volumetric andthickness changes and localization of volcanic facies and units,in some cases over short distances. On a deposit scale, dis-continuous east-west–oriented faults and fracture zones areinterpreted as purely extensional and are important in con-trols to higher grade gold mineralization.

During 2000–2001, Rehrig and Hardy (2001) worked inconjunction with the Yanacocha geology team to complete adistrict-wide structural synthesis of the district. This involvedcollection, classification, and analysis of >30,000 exposedfractures, joint sets, and faults. Their conclusions (Fig. 10) aresummarized as follows: (1) The primary σ1 direction of neareast-west compression rotated counter-clockwise 10° to 20°over time, creating complementary northeast translational

shears and extensional faults and splays (Yanacocha parallel)generated along and between sinistral northwest-strikingtranspressional faults; (2) formation of pure tensional, gener-ally east-west–striking faults and joint systems were wide-spread and measured in every exposed mining pit.

Geophysical ResponseThe use of geophysical surveys as a constant, and in some

cases, primary tool in drill hole targeting has been a part ofthe integrated exploration methodology used in the districtfrom the earliest exploration stages. With IP, airborne elec-tromagnetic (EM), ground, and airborne magnetic and air-borne radiometric surveys covering >15,000 line km, plus3,000 gravity stations, Yanacocha is one of the most intenselysurveyed districts worldwide (Wright, 2003; Bolin, 2006). Un-derstanding and interpretation of the geophysical responsehas improved progressively in concert with the understandingof the surface and subsurface geology of the district.

Resistivity response

The genetic and spatial association of gold deposits withmassive and vuggy quartz is conducive to application of resis-tivity as a direct targeting tool. Resistivity surveys evolvedover time beginning with (1) pole-dipole IP for near-surfacetargeting, (2) application of time domain EM (TDEM)ground surveys targeting intermediate depths (150–300 m),and (3) progressing into airborne TDEM surveys in order toidentify intermediate to deeper targets (>300 m) (Fig. 11).The discovery of the Corimayo deposit in 2000–2001, a blindsystem beneath 150 to 200 m of barren cover, was a direct re-sult of the TDEM survey completed in 1999 (Goldie, 2000).

Magnetic response

As a direct result of magnetite destructive alteration, the di-mensions of the alteration system have produced a magneticresponse that forms a broad relative low over the Yanacocha


0361-0128/98/000/000-00 $6.00 1183

Regional Compression

West-Northwest Striking Fault

NORTH

FIG. 10. Schematic of the Yanacocha district structural kinematics: primary northwest-striking left-lateral transpressionalfabric with superimposed northeast translational fabric (from Rehrig and Hardy, 2001).

district. This relative low corresponds spatially with the outerdimensions of the mapped alteration system. This response isreflected in both the reduced to the pole and analytical signaldata sets but shows punctuated patterns of relatively anom-alous highs, localized particularly within the central portionsof the district (Yanacocha Norte, Sur deposits) and within theLa Quinua grabben. These anomalous highs are interpretedto reflect intermediate- to shallow-level intrusive rocks, someof which have formed productive porphyry systems in thebase of the volcanic pile. Of particular note, apparent in thereduced to the pole image is a northeast-trending, relativelyintermediate to higher response, interpreted as the Chicama-Yanacocha cross-arc break that reflects the northeast-trend-ing expression of porphyry intrusions beneath the volcanicpile (Fig. 12).

Radiometric potassium response

The radiometric potassium response yields a broad relativepotassium low over the entire Yanacocha district, interpretedto reflect potassium feldspar destructive alteration within theadvanced argillic envelope. Relatively anomalous highs be-tween the Yanacocha and San Jose deposits correspond to theprojected footprint of the Kupfertal Au-Cu porphyry deposit.The embayment between the Yanacocha Norte and Sur de-posits is interpreted to reflect the Yanaocha diatreme (Fig.13).

Gravity response

As a total field measurement of mass and indirectly of den-sity, the gravity response over the Yanacocha district is inter-preted to reflect in part the geometry of underlying basementrock. Two broad, northwest-trending relative gravity lows areinterpreted to reflect structural volcanic subbasins within thedistrict. The northern relative low corresponds with thenortheast Quebrada Colorado graben and the southwest rela-tive low within the La Quinta graben (Fig. 14).

Surface Geochemical AnomaliesBefore the discovery of the first deposits, multiple geo-

chemical surveys were used within the Yanacocha district aspart of an integrated exploration approach. It is commonlyoverlooked that the original 1970–1971 British GeologicalSurvey stream sediment reconnaissance geochemical pro-gram was a significant early contribution in the discoveryprocess. While, ironically, the Survey program did not includegold analysis, it can be argued that anomalous silver (>2 ppm)along with pathfinder zinc and copper outlined the dimen-sions of the high sulfidation mineralized envelope.

Given the nature of gold deposits in outcrop within theYanacocha district, gold has predictably been the best geo-chemical indicator element. Systematic gold (>50 ppb) andsilver (>4 ppm) rock chip reconnaissance grid and channel


0361-0128/98/000/000-00 $6.00 1184

5.24.64.23.93.73.53.33.12.92.72.62.42.22.11.91.71.41.20.90.5

-0.2

log ohm-m

Chaquicocha

San Jose

CorimayoEl Tapado

CerroQuilish

Cerro Negro

Yanacocha

Carachugo

MaquiMaqui

0 4km

NORTH

FIG. 11. Yanacocha district image of airborne time domain EM response (ohm-m) in which the signal has been upwardcontinued to create a hypothetical depth slice response of 100m with isopach footprints of the deposits superimposed. Notespatial relation of gold deposits with higher resistivity response. In the case of Corimayo and Tapado deposits, these are blindlode deposits discovered beneath a >150-m barren cap; consequently, the 100-m depth sliced resistivity response is not re-flected (modified after Bolin, 2006).

surveys across continuous outcrop were the single most effec-tive method used to delineate the first of outcrop deposits, in-cluding Cerro Yanacocha (Norte, Sur), Carachugo, San Jose,Maqui Maqui, Cerro Negro, and Cerro Quilish. Associated in-dicator trace elements for outcrop and near-surface gold de-posits include silver, arsenic, barium, bismuth, antimony, and,to a lesser extent, mercury. Zoned anomalous silver (>4 ppm)in the central portion of the district is interpreted to be relatedto a younger hydrothermal pulse (~8.5 Ma) associated withshallow porphyry systems intruded into the base of the vol-canic pile beneath Cerro Yanacocha (Yanacocha district Au,Ag, Cu geochemistry, Plate III) (Figs. 15, 16).

An example of vertical geochemical zonation occurs alongthe southwestern margin of the La Quinua grabben, where achalcedony replacement cap (400 × 100 m) forms the sub-dued crest of Cerro Corimayo and contains highly anomalous(>10 ppm) mercury. A resistivity survey and later drilling con-firmed the blind surface occurrence of >2 g/t oxide gold min-eralization of the Corimayo deposit 150 to 200 m below themercury anomaly (Gomez, 2002).

Controls on MineralizationGold mineralization occurs as multiple pulses as a latest stage

event superimposed on all alteration types, most commonly in

fractures in massive, vuggy, and granular quartz, and highergrades (> 3 g/t Au) are especially associated with the inter-mediate-sulfidation event that accompanies creamy silica.Deposits are commonly localized around the margins of lesspermeable phreatomagmatic and phreatic breccia bodies aswell as around andesite and dacite dome margins.

Lithologic controls

Gold mineralization has been shown to occur throughoutthe Tertiary volcanic column, and at depth hosted in gold-copper–bearing intrusive quartz-eye porphyries. However,due to higher porosity and permeability, ignimbrites of theYanacocha pyroclastic sequence were the preferred hostrocks of the dominantly stratiform gold deposits (Fig. 6).

Rheological contrasts along flow-dome margins in theupper andesite sequence, shallow dioritic intrusions and late-stage diatreme breccia margins created structural traps forhypogene gold + silver ± copper sulfide and sulfosalt precip-itation, with deposits having both stratiform and discordantfeeder and/or contact margin geometries (Figs. 4, 5).

Paragenesis

Hydrothermal activity spanned a period of ~3 m.y., with mul-tiple periods of gold and gold + copper deposition accounting


0361-0128/98/000/000-00 $6.00 1185

San Jose

Corimayo

El Tapado

Carachugo

Chaquicocha

CerroQuilish

Cerro Negro

Yanacocha

MaquiMaqui

00.0

0.2

0.4

0.6

0.8

1.0

-200 200 400 600-400

0 4km

NORTH

FIG. 12. Yanacocha district image of reduced to pole airborne magnetic response (nT) with gold deposit footprints. Notebroad magnetic low with elevated northeast-trending response interpreted to reflect the Turner’s (1997) Yanacocha-Chicamacross-arc structural corridor (modified after Bolin, 2006).


0361-0128/98/000/000-00 $6.00 1186

2.92.72.52.32.11.91.71.51.41.21.00.80.60.40.20.0

-0.2-0.3-0.5-0.7-0.9

K %

Chaquicocha

San Jose

CorimayoEl Tapado

CerroQuilish

Cerro Negro

Yanacocha

Carachugo

MaquiMaqui

0 4km

NORTH

FIG. 13. Yanacocha district image of radiometric potassium response (K percent). Note deposits correspond to relativepotassium lows, interpreted to reflect potassium feldspar destruction within the advanced alteration envelope (modified afterBolin, 2006).

-238.4-239.6-240.9-242.2-243.4-244.7-245.9-247.2-248.5-249.7-251.0-252.2-253.5-254.8-256.0-257.3-258.5-259.8-261.1-262.3-263.6

mgal

Chaquicocha

San Jose

CorimayoEl Tapado

CerroQuilish

Cerro Negro

Yanacocha

Carachugo

MaquiMaqui

0 4km

NORTH

FIG. 14. Yanacocha district image of gravity response (mgal). Note northwest-trending relative lows that correspond tothe Quebrada Colorado graben (northeast) and the La Quinua graben (southwest) (modified after Bolin, 2006).

for the size and metal endowment in the Yanacocha district.Bell et al. (2004) recognized and interpreted five distinct pe-riods of mineralization:

1. Stage 1: Characterized by a pervasive silicification event,contemporaneous with the deposition of fine, disseminatedAu-bearing pyrite. At deeper levels, this stage resulted in theformation of patchy textured silicification, grading to wormyand at deeper levels A-type veinlets, some banded, suggestinga transition from high sulfidation epithermal to a copper-goldporphyry system (Pinto, 2002). Clasts of this style of mineral-ization are incorporated into deeper levels of the Yanacochadiatreme. Fluid inclusion data indicate temperatures thatrange from 200° to 500°C and salinity ranges to >40 wt per-cent NaCl equiv in some samples, supporting this interpreta-tion (Loayza, 2002). Secondary biotite from potassic alter-ation at the Kupfertal deposit yielded an age of 10.72 ± 0.09Ma (Ar39/Ar40: Longo, 2005).

2. Stage 2: The main gold event; postdates pervasive sili-cification. Mineralization is characterized by fine-grainedpyrite with minor enargite and covellite. Sulfides occur asdisseminations and void and fracture fillings. Gold in thisstage occurs as submicron grains usually closely associatedwith Fe oxides along fracture networks (Turner, 1997;Bersch, 1999).

3. Stage 3: A higher grade (intermediate sulfidation?) goldevent (avg >1 g/t Au) is recognized by the occurrence of coarse


0361-0128/98/000/000-00 $6.00 1187

$400 Ultimate Pit

3,750v

3,500v

3,250v

4,250v

EW

0 250m

4,000v

Andesiticintrusion

Lowerandesite

Teut tuff

Ypq intrusion

Cu Mineralization+1% Cu0.5 - 1% Cu0.3 - 0.5% Cu

Phreatomagmatic brecciaPhreatic brecciaYpq breccias

FIG. 16. Copper sulfide mineralization of the Yanacocha Verde deposit beneath the Yanacocha Sur gold deposit. Min-eralization consists of a supergene sulfide assemblage of covellite + chalcocite intermixed with hypogene enargite. Miner-alization grades downward into hypogene covellite and eventually into a chalcopyrite dominant-assemblage in the underly-ing porphyry.

FIG. 15. Core from Kupfertal deposit, illustrating transition from the baseof the high sulfidation system exposed in Encajon Creek; this transition con-sists of a patchy textured, amorphous silica-pyrophyllite (top), to wormy tex-tured A-type veining (middle), to A-type veining. This transition occurs overa vertical column of 100 to 150 m; after Pinto (2002).

gold associated with late-stage barite or creamy chalcedony.The creamy quartz crosscuts previously silicified pyroclasticrocks and phreatic breccias, and occurs as the matrix in somehydrothermal breccias. Stage 3 mineralization occurs in alldeposits, and is especially important at the Chaquicocha Alta,El Tapado, and Corimayo deposits.

4. Stage 4: Late copper-(gold) mineralization is closely as-sociated with dacite to tonalite intrusions and phreatomag-matic breccias. It is characterized by the presence of enargite+ covellite + gold-bearing pyrite with quartz-alunite alter-ation at shallow levels and pyrophyllite-diaspore alteration atdepth. Alunite related to this stage yielded a radiometric ageof 9.12 ± 0.32 Ma (Ar39/Ar40: Longo, 2005). This stage hasbeen observed at the Cerro Yanacocha deposits.

5. Stage 5: Represented by sparsely distributed veinlets ofrhodochrosite-dolomite and base metal sulfides, this stage isinterpreted to represent a transition from acidic fluids to amore neutral pH fluid, accompanied by a decrease in thesulfidation state of the system. This latest stage has been ob-served at the Cerro Yanacocha deposits. The last mineral-ization stage is supergene, related to a district-wide weath-ering profile ranging from 50 to 300 m in depth; outcrop andnear-surface gold deposits were subjected to supergene oxi-dation that resulted in leaching of copper-bearing sulfide as-semblages, principally covellite + enargite ± trace chalcopy-rite; in the cases of the Yanacocha Sur and Maqui Maquideposits, the copper was redeposited as supergene, strati-form blankets containing ~1 wt percent Cu as secondarycovellite + chalcocite.

Transition from High Sulfidation to Porphyry Environment

The best exposures within the district of the epithermal-porphyry transition occur in a northeast-trending window atthe base of Encajon Creek, a deeply incised (>300 m)drainage that separates the San Jose and Yanacocha Sur de-posits. This occurrence reveals in outcrop the transition fromthe upper high sulfidation lithocap into the Au-Cu porphyrysystem. As observed in drill core, this transition grades down-ward over ~100 to 150 m from a mosaic stockwork of amor-phous, patchy textured cryptocrystalline quartz + clay towormy textured quartz veinlets, to a pyritic shell with A-type(Gustafson and Hunt, 1975) veins (Pinto, 2002; Gustafson etal., 2004; Proffett, 2006) (Fig. 15).

Progressively deeper drilling beneath the base of the highsulfidation caps has exposed the transition into shallow por-phyries beneath a series of outcrop gold deposits, includingthe connection zone between La Quinua and Yanacocha Sur,Yanacocha Sur-Norte, and Maqui Maqui.The discovery of theKupfertal deposit in 2000 (Pinto, 2002; Gustafson et al., 2004)led to a change in the exploration and development strategyof the entire Yanacocha district, from near-surface oxide gold-dominated high sulfidation systems to transitional, shallowgold + silver + copper-dominated sulfide systems. The dis-covery in 2006–2007 of the Yanacocha Verde deposit demon-strates the potential of the next generation of development(Fig. 16). The construction of the Yanacocha gold mill in late2008, dedicated to processing higher grade and more com-plex ores, represents the next generation in this continuallyevolving district.

AcknowledgmentsThe authors would like to thank Carlos Loayza, Jamie

Gomez, Rita Pinto, and Jose Quispe for their support andcontributions to the Yanacocha district map series; withoutthem this product would not have been possible. Specialthanks are to due to Dr. Aubrey Paverd for his contributions,ideas, inspiration, and his perseverance as an explorationist,without whom Yanacocha would not have become what it is.Our sincere thanks to the Yanacocha exploration team, in-cluding Bruce Harvey, Steve Moore, Carlos Loayza, JaimeGomez, Rita Pinto, Elmer Flores, Jose Trujillo, ThomasKlein, Tony Longo, and Stan Myers, for their dedicated con-tributions. Thanks are also due to the Yanacocha explorationand mine development teams—in particular, Cindy Williams,Pedro Reyes, Peter Rogowski, Peter Bell, Brian Arkell, andPeter Mitchell for their ideas and early contributions to theadvancement of understanding into the transitional environ-ments of the deeper sulfide porphyry. We extend our appre-ciation to César Vidal of Buenaventura for his many years ofguidance, support, and inspiration to all geologists who haveworked in the district. Lastly, the authors would like to thankJeffrey Hedenquist, Noel White, and Lew Gustafson for theirconstructive edits and suggestions.

REFERENCESBell, P., Loayza C., Gomez, J. and Pinto, R., 2004, Geology of the gold deposits

of the Yanacocha district, northern Peru: Australasian Institute of Miningand Metallurgy (AusIMM), Carlton, PACRIM 2004 conference, 17 p.

Benavides-Cáceres, V., 1999, Orogenic evolution of the Peruvian Andes: So-ciety of Economic Geologists Special Publication 7, p. 63–107.

Bersch, M., 1999, Yanacocha high grade gold characterization study: MineraYanacocha internal report, Cajamarca, Perú, 157 p.

Bolin, B., 2006, Newmont airborne electromagnetic survey at Minera Yana-cocha, northern Peru: Minera Yanacocha internal report, Cajamarca, Peru,17 p.

Finn, D., 2003, Chaquicocha sulfide project 2002–2003 development pro-gram: Minera Yanacocha internal report, Cajamarca, Peru, 27 p.

Goldie, M., 2000, A geophysical case history of the Yanacocha gold district,northern Perú: Society of Exploration Geophysicists, Annual Meeting, Ab-stracts with Program, p. 1–4.

Gomez, J., 2002, Corimayo, deposito aurifero de alta ley en un sistima epit-hermal tipo acido sulfato, distrito minero de Yanacocha, Cajamarca, Perú:Unpublished Ingeniero Geólogo thesis, Peru, Universidad Nacional de In-geniería, 90 p.

Gustafson, L., and Hunt, J., 1975, The porphyry copper deposit at El Sal-vador, Chile: ECONOMIC GEOLOGY, v. 70, p. 875–912.

Gustafson, L., Vidal, C., Pinto, R. and Noble, D., 2004, Porphyry-epithermaltransistion, Cajamarca region, northern Perú: Society of Economic Geolo-gists Special Publication 11, p. 279–300.

Harvey, B., Myers, S., and Klein, T., 1999, Yanacocha gold district, northernPeru: PACRIM ’99, Bali, Indonesia, Conference Guidebook, p. 445–459.

Hollister, V., 1962, Summary report of the J & A mine area, Cajamarca, Peru:Guggenheim Brothers Internal Report, 6 p.

——1978, Geology of the porphyry copper deposits of the Western Hemi-sphere: New York, American Institute of Mining, Metallurgical and Petro-leum Engineers, 219 p.

Loayza, C., 2002, Geologic study of Cerro Yanacocha gold-silver deposit,Yanacocha district, northern Peru: Unpublished master’s thesis, Reno, Uni-versity of Nevada, 94 p.

Longo, A., 2000, The San Jose-Carachugo-Chaqicocha gold trend, Yanacochadistrict, northern Peru: Geological Society of Nevada, Geology and ore de-posits conference, Abstracts, 1 p.

——2005, Evolution of volcanism and hydrothermal activity in the Yana-cocha mining district, northern Peru: Unpublished Ph.D. thesis, OregonState University, 469 p.

Longo, A., and Teal, L., 2000, Interpretive geologic map of the Yanacochadistrict, Peru: Minera Yanacocha unpublished folio map, scale 1:25,000.


0361-0128/98/000/000-00 $6.00 1188

Longo, A.A., Dilles, J.H., Grunder, A.L., and Duncan, R., 2010, Evolution ofcalc-alkaline volcanism and associated hydrothermal gold deposits at Yana-cocha, Peru: ECONOMIC GEOLOGY, v. 105, p. 1191–1240.

Mallette, P., Rojas, R., and Gutierrez, A., 2004, Geology, mineralization, andgenesis of the La Quinua gold deposit, Yanacocha district, northern Peru:Society of Economic Geologists Special Publication 11, p. 301–312.

Moore, S., 2003, Yanacocha sulfide targeting 2003: Minera Yanacocha inter-nal report, Cajamarca, Peru, 59 p.

Moore, S., and Saderholm, E., 2002, Yanacocha district stratigraphic sum-mary, Minera Yanacocha internal report, Cajamarca, Peru, 59 p.

Paverd, A., and Bowerman, A., 1994, Yanacocha project chronology: New-mont Perú, Limited, internal report, 9 p.

——2003, Technical review of the Minera Yanacocha exploration and devel-opment program: Minera Yanacocha internal report, Cajamarca, Peru, 31p.

——2004, Technical review of the Minera Yanacocha exploration programMay-June 2004: Minera Yanacocha internal report, Cajamarca, Peru, 25 p.

Pinto, R., 2002, Transicion de un sistema de alta sulfuracion a un sistema por-firitico de alto nivel en deposito Kupfertal, distrito minero de Yanacocha,Cajamarca, Perú: Unpublished Ingeniero Geólogo thesis, Lima, Peru, Uni-versidad Nacional Mayor de San Marcos, 62 p.

Proffett, J., 2006, Review of porphyry exploration projects in the Yanacocharegion, northern Peru: Minera Yanacocha internal report, Cajamarca, Peru,46 p.

Ramondi, A., 1913, El Perú, estudios mineralógicos y geológicos, segundaserie: Sociedad Geográfica de Lima y Cuerpo de Ingenieros de Minas,Peru, v. 2.

Rehrig, W., and Hardy, J., 2001, Structural study of the central Yanacochadistrict, Perú: Minera Yanacocha internal report, Cajamarca, Peru, 156 p.

Rivera, L., 1980, Mapa geológico del cuadrángulo de Cajamarca: SectorEnergía y Minas, Instituto Geológico Minero y Metalúrgico, Republica delPerú, Bulletin no. 31, 67 p.

Sillitoe, R., 1996, Comments on geology and exploration of the Yanacochagold district, Perú: Minera Yanacocha internal report, Cajamarca, Peru, 22p.

——2000, Comments on geology and exploration of porphyry copper-goldmineralization in the Yanacocha district, Perú: Minera Yanacocha internalreport, Cajamarca, Peru, 10 p.

Teal, L., Harvey, B., Williams, C., and Goldie, M., 2002, Geologic overviewof the Yanacocha district gold deposits, northern, Peru: Society of Eco-nomic Geologists, Global Exploration 2002, Integrated methods for dis-covery, Abstracts, p. 43–44.

Turner, S., 1997, The Yanacocha epithermal gold deposits, northern Peru:High sulfidation mineralization in a flow dome setting: Unpublished Ph.D.thesis, Golden, Colorado, Colorado School of Mines, 342 p.

Wilson, J., 1985, Mapa geológico del cuadrángulo de Cajamarca: SectorEnergía y Minas, Instituto Geológico Minero y Metalúrgico, Republica delPerú, Bulletin no. 38, 104 p.

Williams, C., Reyes, P., Flores, E., Manyari, C., and Ugarte, E. 2002, Cha-quicocha sur sulfide project exploration summary report, Yanacocha dis-trict, Peru: Minera Yanacocha internal report, Cajamarca, Peru, 41 p.

Wright, J., 2003, Yanacocha regional geophysics review and targeting report:Minera Yanacocha internal report, Cajamarca, Peru, 55 p.


0361-0128/98/000/000-00 $6.00 1189

List of PlatesPlate I. Yanacocha District - Cajamarca, Perú, Interpretive Geology, 2010,

L. Teal, J. Gomez, R. Pinto, C. Loayza, and J. Quispe, Compilers, scale1:25,000.

Plate II. Yanacocha District - Cajamarca, Perú, Alteration Map, 2010, L.Teal, J. Gomez, R. Pinto, C. Loayza, and J. Quispe, Compilers, scale1:25,000.

Plate III. Yanacocha District – Cajamarca, Perú, 2010, Surface Geochem-istry Gold, Silver, Copper and Gold Isopach Contour Image Maps and Dis-trict Cross Sections, L. Teal, J. Gomez, R. Pinto, C. Loayza, and J. Quispe,Compilers.


0361-0128/98/000/000-00 $6.00 1190

teal benavides 2010

Documents

yanacocha district

yanacocha mining district

operating district

year gold production

alteration types

moz gold

type epithermal gold

year reserve