copper skarn deposits

8/17/2019 Copper Skarn Deposits

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Copper Skarn Deposits, Perú

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To J. Isabel…

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Ore-Deposits II

Student: ESPIRITU PUJAY, Junior

School of Geology Teaching: Flores Coaguila, SaturninoEighth Semester Uniersi!a! "acional #aniel $lci!es Carri%n

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The Antamina mine is located at approximately 9 32! S and "" #3! $, 2"# kmnorth o% &ima and 13# km %rom the Paci'c coast, in Ancash Department in north(centralPer)* +t lies in the eastern part o% the Cordillera ccidental, east o% the Cordillera -lancaand .est o% the /0o aran alley* The skarn is exposed 5et.een approximately 4,2##and 4,6## m a*s*l*, at the head o% a so)th.est(drainin7 7lacial alley, 5)t prior to minin7m)ch o% the ore5ody .as coered 5y &a7o Antamina, a 7lacial tarn 8i7* 2:* The history o% exploration at Antamina and the 7eneral 7eolo7y o% the deposit are s)mmari;ed 5y/ed.ood 81996, 1999:* :* The Antaminadeposit %ormed at 9*6= to 1#*16 a 84#Ar?39Ar step(heatin7 data o% &oe et al*, 2##3:aro)nd a small mon;o7ranitic porphyry intr)sion* +t is hosted 5y @pper Cretaceo)scar5onate strata .ithin the aran thr)st and %old 5elt, %ormed 5y the late ocene +ncaicoro7eny 8Bo5le et al*, 19"9 7ard, 1964:* The .estern Andes o% Per) .ere the site o% episodic arc ma7matism %rom the &ate Triassic to the late iocene 8Co55in7 et al*, 1961:*Copper minerali;ation .as kno.n at Antamina 8anta: copper in E)ech)a: in pre(Colonialtimes, 5)t only modest amo)nts o% P5 and A7 are kno.n to hae 5een prod)ced in thedistrict prior to 2##1 8/ed.ood, 1999:* The skarn contains proen and pro5a5le resereso% >=1 t .ith an aera7e 7rade o% 1*24 percent C), 1*#3 percent Fn, 13*"1 7?t A7,and#*#29 percent o 8calc)lated at a #*"G C) eH)i c)toI 7rade:*

TJ CPP/ SKA/B deposit at Coroccoh)ayco lies " km so)theast o% the Tintaya openpit mine in the Tintaya district, C)sco Department, Per) 814 >"! 21L S, "11>!1"L $ i7*1:* This deposit is one o% seeral in the Andah)aylas(Ma)ri copper 5elt, a late ocene(li7ocene 5elt o% i7neo)s rock .hich extends %or 3># km %rom Andah)aylas in the north toso)theast o% Ma)ri 8Bo5le et al*, 1964 Perell et al*, 2##3:* Copper deposits in this 5elt arehosted 5y ocene(li7ocene i7neo)s and Cretaceo)s sedimentary rocks and incl)de theC) 8NA): porphyry and skarn deposits at Katan7a, Charcas, and the &as -am5as and

Tintaya districts* Perell et al* 82##3: proided a ran7e o% K(Ar a7es %rom 5iotite andhorn5lende o% 3# to 4# m*y* %or intr)sie rocks o% the Andah)aylas(Ma)ri 5atholith* This7eneral a7e ran7e is contemporary .ith i7neo)s actiity o% the ocene to early li7oceneolcanic arc in central Per) 8Bo5le et al*, 1964: and ma7matic actiity related to lar7eporphyry copper deposits in northern Chile 8e*7*, Ch)H)icamata, &a scondida Clark et al*,199#:* A thoro)7h re7ional reie. o% the tectonic context, str)ct)re, strati7raphy,petro7raphy, and minerali;ation o% the Andah)aylas(Ma)ri 5elt has5een s)mmari;ed 5y Perell et al* 82##3: and .ill not 5e repeated here* ther p)5licationshae o)tlined re7ional

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Pre%ace


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strati7raphy, a7e constraints, and tectonic and str)ct)ral aspects o% the 5elt 8Bo5le et al*,1964 De &a Cr); -*, 199> -enaides(Caceres, 1999 Oaillard et al*, 2###:*

The St)dent

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Pre%ace************************************************************************************************************** *>

Antamina Copper-in! Skarn Deposit, An!as"************************************************6

/e7ional Qeolo7ic Settin7************************************************************************************************1#

Stratigra&hic relationshi&s********************************************************************************************1#

Tectonic relationshi&s***************************************************************************************************13

Stratigra&hic ariations across the cross'stri(e structural !iscontinuity:*********************13

Igneous actiity along the cross'stri(e structural !iscontinuity:***********************************1=

&ocal Qeolo7ic Settin7*****************************************************************************************************1"

Se!imentary host roc(s************************************************************************************************1"

$lteration of se!imentary roc(s a!)acent to the s(arn*************************************************19

Jumasha Formation:******************************************************************************************************2#Celen!*n Formation:*****************************************************************************************************21

Structural geology of the mine area*****************************************************************************24

+ocal fol!s an! faults:***************************************************************************************************24

Structural control of mineraliation:*****************************************************************************26

Disc)ssion************************************************************************************************************************29

-egional stratigra&hic an! tectonic relationshi&s*********************************************************29

$ mo!el for the geologic history of the $ntamina region*********************************************3#

-egional eects on local structure an! mineraliation*************************************************32Im&lications for local e/&loration an! ore genesis*******************************************************3>

0etallogenic an! geotectonic im&lications*******************************************************************3>

Concl)sions*********************************************************************************************************************3"

/e%erences************************************************************************************************************************** 36

Copper Skarn Deposit at Coro!!o"ua#!o, Tinta#a Distri!t******$Error% &ar!ador node'nido(42

Deposit Settin7**************************************************************************************************************** 42

Stratigra&hy *****************************************************************************************************************42Structural geology ********************************************************************************************************43

Igneous roc(s***************************************************************************************************************43

Deposit Alteration************************************************************************************************************46

Igneous roc(s***************************************************************************************************************46

1ornfels************************************************************************************************************************49

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Contents


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0arble an! s(arn**********************************************************************************************************>#

-etrogra!e an! lo2'tem&erature alteration*****************************************************************>1

inerali;ation******************************************************************************************************************>2

Igneous roc(s***************************************************************************************************************>2

S(arn**************************************************************************************************************************** >2

3re &etrogra&hy ***********************************************************************************************************>2

0agnetite*********************************************************************************************************************>4

+7neo)s /ock Qeochemistry******************************************************************************************** >4

ineral Chemistry************************************************************************************************************>4

Garnet **************************************************************************************************************************>>

4yro/ene********************************************************************************************************************** >=

Sul5!e an! &recious metal mineraliation chemistry ***************************************************>"

l)id +ncl)sions****************************************************************************************************************>9

Disc)ssion************************************************************************************************************************=#

-ole of igneous roc(s***************************************************************************************************=#

S(arn onation**************************************************************************************************************=1

S(arn'forming con!itions*********************************************************************************************=2

0ineraliation***************************************************************************************************************=3

0agnetite*********************************************************************************************************************=3

Concl)sions*********************************************************************************************************************=4/e%erences************************************************************************************************************************** ==

ProRects and minin7 prospects

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Antamina Copper(Finc Skarn Deposit,


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The $ntamina !e&osit

The Antamina deposit comprises endoskarn and exoskarn, .ith s)5ordinate 5reccia5odies that c)t 5oth skarn and intr)sion .ithin the perimeter o% skarn* The minerali;edskarn is dominated 5y 7randite 7arnet, .hich 7rades %rom 5ro.n to 7reen nearer the hostlimestone 8Petersen, 19=>:* Th)s the deposit con%orms to the oxidi;ed calcic clan o%

ina)di et al* 81961:* The 7eolo7y o% the deposit and its immediate s)rro)ndin7s iss)mmari;ed in i7)re 3a on three north.estso)theast drill(hole cross sections, spaced >#m apart, thro)7h the middle o% the ore5ody* The simpli'ed s)r%ace 7eolo7y map in i7)re35 .as constr)cted 5y proRectin7 the litholo7ic 5o)ndaries de'ned on these and thirtyother cross sections*

The area delimited 5y the minerali;ed skarn %ront is approximately 1*16 km 2 oerall* The skarn ;one straddles the ori7inal intr)sie contact and s)rro)nds a circa #*24 km2 coreo% porphyry that %orms a cr)dely parallelo7ram(shaped prism .ith a ertical axis 8i7* 35:*

The o)ter 5o)ndary o% the minerali;ed skarn is more elon7ated northeast(so)th.est thanthis core 5eca)se it expands aro)nd %a)lts and dikes extendin7 to the east, northeast, andso)th.est* +t is there%ore ro)7hly elliptical in plan and has a north.est(so)theast .idth o% )p to 1,### m, and a northeast(so)th.est len7th o% more than 2,>## m* The lon7 axis

parallels the Antamina alley and is perpendic)lar to the re7ional str)ct)ral 7rain o% thede%ormed car5onate host rocks* As reco7ni;ed 5y Petersen 819=>: the o)ter limit o% skarnis 7enerally s)5ertical* The skarn narro.s .ith depth as the core o% porphyry .idens 8i7*3a:, 5)t there is no si7ni'cant chan7e in the C) and Fn 7rades to depths o% at least 4## m5elo. the ori7inal alley oor*

xcept at hi7h eleations in the eastern part o% the deposit, the exoskarn is almosteery.here minerali;ed* +n detail, the indiid)al skarn %acies and 5reccia 5odies arecomplexly shaped and discontin)o)s, 5)t the deposit can 5e simpli'ed as comprisin7 aninner shell o% endoskarn, stock.ork, 5reccia, and 5ro.n 7arnet exoskarn that contains thecopper moly5den)m ore and an o)ter shell o% 7reen 7arnet exoskarn comprisin7 thecopper(;inc ore* oly5denite is disseminated in irre7)lar ;ones .ithin and at the mar7ino% the intr)sion* Chalcopyrite is the dominant copper mineral except at shallo. depths inthe so)th.estern part o% the deposit, .here 5ornite predominates in .ollastoniticexoskarn, .hich %orms an enclae in 7reen 7arnet C)(Fn exoskarn* Jydrothermal 5reccia iscommon at or near the endoskarn(exoskarn contacts alon7 the north.est and so)theastsides o% the deposit* -reccia also c)ts the porphyry core as anastomosin7 sheets and pipesthat are commonly eneloped 5y 'ne(7rained maroon 7arnet endoskarn 8i7* 3:* The5reccia ;ones contain minor chlorite and .ere ori7inally descri5ed 5y Petersen 819=>: aschlorite skarn* The 5reccias are 7enerally poorly sorted and comprise an7)lar to ro)nded%ra7ments o% the skarn and metallic minerals s)pported in a sand(si;ed matrix o% similarcomposition* As in skarn, sphalerite and moly5denite do not commonly occ)r to7ether inthe 5reccias sphalerite is %o)nd in 5reccias in or near exoskarn, 5)t oly5denite occ)rs in5reccias in endoskarn* The common metallicminerals in the 5reccias are pyrite, chalcopyrite, and ma7netite, .hich are mostlycommin)ted 5)t also occ)r as eins, massie 5odies, and lar7e %ra7ments* This mineralassem5la7e is rare in exoskarn 5)t %orms .idespread stock.orks and sheeted ein s.armsin 'ne(7rained maroon 7arnet endoskarn, in many places 7radin7 into crackle, mosaic, andmatrix(dominated 5reccia* $e estimate that approximately onethird o% the ore atAntamina may hae %ormed d)rin7 this late 5recciation, einin7, and endoskarn(%ormin7sta7e* $idely spaced, late calcite(tetrahedrite sphalerite 7alena einlets are commonand c)t all skarn and 5reccia types, altho)7h they are also locally dismem5ered in 5reccia,

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pro5a5ly 5eca)se o% settlin7* Scarce real7ar einlets occ)r in calc(horn%els a5oe andperipheral to the skarn*

+Q* 1* &ocation map o% the Antamina deposit and 7eneral 7eolo7y o% part o% Ancash and &a &i5ertad Departments,Per)* Compiled and modi'ed a%ter 7eler and De -ooy 819>=:, Coss0o 819=4:, $ilson and /eyes 819=4:, Coss0o and Oan 819=":, $ilson et al* 819=", 199>:, yers 819"=, 196#:, /eyes 8196#:, SUnche; 8199>:, Allende 8199=:, Co55in7et al* 8199=:, Oacay 8199=:, SUnche; et al* 81996:, +BQT 81999:, and Str)sieic; et al* 82###:* The Tapacochaaxis delineates the .estern ed7e o% the Cretaceo)s shel% 8yers, 19"4, 19">:* ther deposits 8Pierina and Pasto-)eno: and prospects 8a7istral: mentioned in the text are sho.n, as are the areas ill)strated in i7s* > and =* T-V aran thr)st and %old 5elt*

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2* Premine physio7raphy o% the Antamina area, .ith place names re%erred to in the text, ill)stratin7 the cl)sterin7 o% A7(5earin7

Re)iona* +eo*o)i! Settin)

The @pper Cretaceo)s strata enclosin7 the Antamina deposit are part o% ametallo7enically important Al5ian and @pper Cretaceo)s packa7e o% car5onate rocks, theachay Qro)p, that hosts many ore deposits in the polymetallic skarn and car5onate(

replacement 5elt o% central Per) 8Soler et al*, 196=:* These rocks %ormed d)rin7 the later o% t.o Permian to Paleocene episodes o% 5asin deelopment that deposited a s)ccession o% alternatin7 siliciclastic and car5onate %acies in .estern So)th America 8Sempere et al*,2##2:* This &ate O)rassic to Paleo7ene s)5sidence %ormed the $est Per)ian tro)7h, .hichseparated the ma7matic arcs to the .est %rom the eastern 7eanticline no. represented 5ythe aran metamorphic complex 8-enaides, 19>= $ilson, 19=3 Atherton et al*, 19637ard, 196":* The Cretaceo)s sedimentary rocks that crop o)t in the Antamina areaacc)m)lated in the shallo.(.ater portion o% this tro)7h, the Ma)li shel% 8S;ekely, 19=":*

The Tapacocha axis, no. a north(north.estY trendin7 hi7h strain ;one, separates the

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.estern, deeper(.ater portion o% the tro)7h %rom the shel% sedimentary rocks 8i7* 1yers, 19"4, 19">:*

Stratigra&hic relationshi&s

The host achay Qro)p 8i7s* 1 and 4: incl)des all Al5ian to mid(Campaniancar5onate rocks so)th o% 9 S and east o% the Tapacocha axis in north(central Per)* S;ekely819=": introd)ced the achay Qro)p in central Per) and Samam(-o77io 8196#: applied itthro)7ho)t Per)* These rocks also hae 5een re%erred to in%ormally as the WmiddleCretaceo)s limestone seriesX 8Jarrison, 194#:, the W)pper Cretaceo)s and Al5iancar5onate seriesX 87ard, 1964:, and the W)pper car5onate seH)enceX 8anriH)e, 1996:*

The 7ro)p is )nderlain 5y a predominantly siliciclastic seH)ence comprisin7 @pper O)rassicmarine 5lack shales o% the Chicama Qro)p and &o.er Cretaceo)s 8-erriasian to Aptian:continental to shel% sandstones, shales, and minor limestones o% the QoyllarisH)is7a Qro)p8i7* 4:* +t is .idely oerlain, con%orma5ly or sli7htly )ncon%orma5ly, 5y red 5eds 8$ilson,19=3:, .hich are mainly Campanian to Paleocene 5)t as old as Santonian in central Per)8Oaillard, 196":* Bot presered in the immediate Antamina area, these nonmarine, coarse

clastic rocks hae 5een ario)sly descri5ed as the Poca5am5a ormation, 2> km so)theasto% Antamina at &a @nin 8$ilson, 19=3, a%ter c&a)7hlin, 1924:, the Chota ormation, 2#km north o% Antamina 8-enaides, 19>=Z a%ter -ro77i, 1942:, and the Casapalca Qro)p, 2>km so)th.est o% Antamina in the Cordillera J)ayh)ash 8Coney, 19"1, a%ter c&a)7hlin,1924:*

The achay Qro)p contains t.o trans7ressie seH)ences separated 5y adiscon%ormity ascri5ed to late(middle Al5ian )pli%t and erosion related to the ochicaoro7eny 87ard, 1964:* +n the lo.er part o% the 7ro)p, the s)ccessie Pariah)anca,Ch)lec, and Pariatam5o ormations 8i7* 4: record a transition %rom near(shore, calcareo)ssandstone and massie, shelly limestone, thro)7h thin(5edded limestone and marl, todeep(.ater, thin(5edded, 5it)mino)s, dark 7ray marl and limestone 8-enaides, 19>=,1999 $ilson, 19=3 Oaillard, 196":*

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+Q* 3* Simpli'ed cross sections 8a: and proRected s)r%ace 7eolo7y map 85: o% the Antamina deposit, ill)stratin7 thecr)dely elliptical, ertical ;ones o% endoskarn and exoskarn deeloped 5et.een a core o% lar7ely skarn(%ree porphyryand the limestone host rocks* Simpli'ed s)r%ace 7eolo7y map is 5ased on drill(hole 7eolo7y proRected to s)r%ace andon s)r%ace mappin7 5y D*A*&* and O*K*Q*, com5ined .ith that 5y &* Jatha.ay 8+nmet: and * $)nder 8Boranda:*Prominent north.eststrikin7 +ncaic %olds and the le%t(steppin7, transerse [alley %a)lt 8[: are indicated* The location

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o% the proposed [alley lateral ramp 8[&/:, in%erred to 5e responsi5le %or the apparent dextral oIset o% the Antaminaanticline 8AA:, is also sho.n*

ollo.in7 the late(middle Al5ian hiat)s, car5onate sedimentation on the plat%orm res)med.ith the deposition o% the shallo.(.ater, )pper Al5ian to )pper T)ronian O)mashaormation 8Oaillard, 196":, ori7inally de'ned 5y c&a)7hlin 81924: in central Per)* This%ormation is oerlain 5y the m)ddier, deeper(.ater Celend0n ormation 8-enaides, 19>=:,lar7ely Coniacian to Santonian in a7e 8Oaillard, 196": 5)t attainin7 the mid(Campanian innorthern Per) 8o)rier et al*, 1966:* The )pper part o% the 7ro)p, comprisin7 the O)mashaand Celend0n ormations 8i7* 4:, th)s represents a second maRor trans7ressie seH)ence*

The lo.er to middle Al5ian car5onate strata 8the 'rst trans7ressie seH)ence: are similarin litholo7y and thickness in 5oth northern and central Per) 8-enaides, 19>= Oaillard,196":* Jo.eer, north o% approximately 9 S, the oerlyin7 )pper Al5ian to mid(Campanian car5onate rocks are m)ch thicker, more %ossili%ero)s, and litholo7ically morearia5le than in central Per), and the O)masha ormation interal is diided into 'e%ormations 8-enaides, 19>=:* Oaillard 8196": proides correlations 5et.een thesestrati7raphic sections in northern and central Per)*

+Q* 4* +n%erred strati7raphic col)mn in the mine area compared .ith that %or the O)masha and Celend0n ormations,compiled %rom o5serations 8-enaides, 19>=: on the meas)red sections in the /0o P)chca alley, approximately 2#

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km north o% Antamina* The contact 5et.een the O)masha and Celend0n ormations coincides .ith the 5o)ndary5et.een the T)ronian and Coniacian sta7es and is indicated .ith a solid line )nkno.n sta7e 5o)ndary locations hae5een approximated and indicated .ith dashed lines and H)estion marks* The strati7raphic interal interpreted to hostthe skarn is sho.n* +nset sho.s an o)tline o% the re7ional strati7raphic section*

Tectonic relationshi&s

P)5lished descriptions o% the str)ct)ral settin7 o% the Antamina deposit 8-odenlosand ricksen, 19>> Terrones, 19>6 Petersen, 19=> /ed.ood, 1999: hae %oc)sed on thearan thr)st and %old 5elt, .hich deeloped circa 3# m*y* 5e%ore minerali;ation, .ithscant consideration o% the re7ional tectonic enironment that existed in the mid(iocene*$e ar7)e, ho.eer, that Antamina lies ath.art a lar7e(scale,cross(strike 8northeast(so)th.est: str)ct)ral discontin)ity 8$heeler, 19"6: that .as tectonically actie at the timeo% skarn %ormation and hence has metallo7enic si7ni'cance* &oe et al* 82##1: termed thestr)ct)re the E)erococha arch 5eca)se its so)th.estern limit at the mar7in o% the CalleRonde J)aylas lies close to &a7)na E)erococha 8i7s* > and =:* +ts in)ence on the a5)ndanceo% Beo7ene intr)sions, the re7ional strikes in the aran thr)st and %old 5elt, and the

strati7raphic relationships o% )nderlyin7 ississippian to &o.er O)rassic strata 8i7* 4: aredescri5ed 5elo.* The oerall strike o% the aran thr)st and %old 5elt chan7es, and thecommon pl)n7e directions reerse, across the proposed northeast(trendin7 cross(strikestr)ct)ral discontin)ity 8i7* >:* These thin(skinned ocene str)ct)res, attri5)ted to the+ncaic oro7eny, are the dominant tectonic elements in the re7ion, altho)7h t.o re7ional)ncon%ormities in the Cretaceo)s s)ccession in central Per), one at the 5ase o% the

O)masha ormation and the other 5elo. the Casapalca Qro)p and its eH)ialents,represent earlier emer7ence d)rin7, respectiely, the ochica and Per)ian tectonicphases 8Bo5le et al*, 19"9 7ard, 1964:* The aran thr)st and %old 5elt extends %rom> S to 12 3#! S, 7enerally strikin7 north(north.estYso)th(so)theast, parallel to thepresent plate 5o)ndary, and comprises str)ct)res that predominantly er7e northeast87ard, 1964 i7* >:*

Jo.eer, so)theast o% the cross(strike str)ct)ral discontin)ity, %olds and thr)st%a)lts strike north(north.est, .hereas to the north.est o% it, they strike northerly 8i7s* 1,>, and =:* oreoer, %old pl)n7es are reersed across this ;oneZ to the so)theast, mostmaRor anticlines and synclines pl)n7e to the so)th(so)theast, .hereas to the north.est,they pl)n7e north 8i7* >:* The loc)s o% chan7es in strike and pl)n7e extends northeast%rom &a7)na E)erococha, 5)t a5o)t > km so)th.est o% Antamina it steps 6 km to thenorth 5e%ore contin)in7 northeast.ard 8i7* >:* a)lts .ith the same oerall northeaststrike as the cross(strike str)ct)ral discontin)ity control some present(day draina7es, s)chas the northeast trendin7 /0o P)chca alley, 2# km north o% Antamina, .hich is discordantto the oerall north to north(north.est 7rain o% the terrain 8i7* >:*

The deection in the Antamina area is one o% seeral that artic)late the aranthr)st and %old 5elt* Bortherly strikes contin)e to &lamall0n, ># km to the north o%

Antamina in the eastern part o% the 5elt 8i7* 1:* Still %arther north, the oerall north(north.est re7ional strike o% the aran thr)st and %olds 5elt res)mes* A compara5lesharp deection to northso)th strikes occ)rs at the northern end o% the Cordillera -lanca8i7* 1:, 1"> km to the north(north.est o% Antamina, .here the Casma(Pasto -)eno %a)lt;one intersects the re7ional north(north.est strikes 8/iera, 199=:* -enaides 81999:identi'ed many se7ments in the %a5ric o% the aran thr)st and %old 5elt, incl)din7 thet.o descri5ed a5oe, altho)7h he proIered a diIerent mechanism %or their %ormation, asdisc)ssed 5elo.*

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Stratigra&hic ariations across the cross'stri(e structural !iscontinuity:

The contact relationships o% the )pper Paleo;oic and eso;oic strata 8i7* 4: to thelo.er Paleo;oic aran metamorphic complex ary in accordance .ith the se7mentationo% the thr)st and %old 5elt* i7)re = sho.s the relationships alon7 the .estern mar7in o% the aran complex thro)7ho)t an area more extensie than that sho.n in i7)re >*ississippian to &o.er O)rassic strata that normally separate the pre(rdoicianmetamorphic rocks %rom the Cretaceo)s sedimentary rocks are a5sent near the proposedcross(strike str)ct)ral discontin)ity* +n the north(strikin7 se7ment o% the %old 5eltimmediately north o% the Antamina area, the eastern limit o% the ississippian toCretaceo)s s)ccession is a s)5hori;ontal )ncon%ormity that strikes north oerall 5)t hasan irre7)lar s)r%ace trace o.in7 to the incised topo7raphy 8i7s* > and =:*

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+Q* >* Str)ct)ral 7eolo7y o% the aran thr)st and %old 5elt in the icinity o% Antamina, ill)stratin7 the markedchan7e in the orientations o% thr)st %a)lts and %old axes across a northeast(trendin7 ;one thro)7h Antamina 8a%ter7eler and de -ooy, 19>= $ilson et al*, 19=", 199> Co55in7 et al*, 199= Oacay, 199= and Str)sieic; et al*, 2###:* The northeast(so)th.estY trendin7 loci o% these chan7es in str)ct)ral attit)de is indicated 5y the heay dashed line,.hich is oIset in a le%t(steppin7 sense in the icinity o% Antamina* This area is the location o% the proposed cross(strike str)ct)ral discontin)ity disc)ssed in the text* +nset sho.s the location o% the map area relatie to the J)ari 819(i:, Sin7a 819(R:, /eH)ay 82#(i: and &a @nin 82#(R: H)adran7les* Bo attempt has 5een made to esta5lish contin)ity5et.een the map )nits o% $ilson et al* 819=", 199>: north o% 9 3#! S and those o% Co55in7 et al* 8199=: to the so)th* The )ndated olcanic rocks o% Pampa O)n0n hae 5een assi7ned to the Calip)y S)per7ro)p 8see text %or disc)ssion:*

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+Q* =* Simpli'ed 7eolo7y o% approximately 6,### km2 o% the aran thr)st and %old 5elt in the re7ion aro)ndAntamina the dominantly )pper iocene Cordillera -lanca 5atholith is remoed 8c%* i7s* 1 and >:, reectin7 the7eolo7y at the time o% intr)sion and minerali;ation in the late iocene* The northeast(so)th.est trendin7 cross(strikestr)ct)ral discontin)ity that passes thro)7h Antamina is delimited 5y heay dashed lines* ississippian to &o.er O)rassic sedimentary rocks are a5sent 5eneath the Cretaceo)s QoyllarisH)is7a Qro)p northeast o% Antamina alon7the cross(strike str)ct)ral discontin)ity 5)t are present north and so)theast o% the cross(strike str)ct)raldiscontin)ity, except .here c)t o)t 5y %a)ltin7* An )n)s)al a5)ndance o% i7neo)s 5odies intr)des the aran -eltalon7 the cross(strike str)ct)ral discontin)ity, compared to transects to the north and so)th* A%ter 7eler and de-ooy 819>=:, $ilson et al* 819=", 199>:, Co55in7 et al* 8199=:, Oacay 8199=:, and Str)sieic; et al* 82###:*

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19/95

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20/95

+n the north(north.estYstrikin7 se7ments o% the 5elt, ho.eer, the eastern extent o% theeso;oic rocks is 7enerally delimited 5y north(north.estYstrikin7 %a)lts 8e*7*, so)theast o% Antamina, and north.est o% &lamall0n i7* =:* So)theast o% Antamina, the so)th.estmar7in o% the aran metamorphic complex is lar7ely de'ned 5y a series o% maRornortheast(er7in7 reerse %a)lts that inoled 5asement, 5)t the ississippian to &o.er

O)rassic strata are presered 5et.een the pre(rdoician metamorphic rocks and the

Cretaceo)s sedimentary rocks 8i7* =:* Jo.eer, in the north(north.estYstrikin7 se7mentnorth.est o% &lamall0n, the aran complex is 5ackthr)st oer the Cretaceo)s rocks andthe thickness o% the ississippian to &o.er O)rassic strata is only locally apparent 8i7* =:*

Bortheast o% Antamina, ississippian to &o.er O)rassic strata 8i7* 4: are a5sentalon7 the proposed cross(strike str)ct)ral discontin)ity, and the clastic rocks o% the &o.erCretaceo)s QoyllarisH)is7a Qro)p lie )ncon%orma5ly on the pre(rdoician aranmetamorphic complex 8i7s* > and =:* +n contrast, north and so)theast o% the cross(strikestr)ct)ral discontin)ity, a relatiely thick seH)ence o% ississippian to &o.er O)rassicstrata separates these )nits 8i7* =:* Sandstones and shales o% the ississippian Am5oQro)p locally )ncon%orma5ly oerlie the aran complex near its .estern limit* Similarly,continental sedimentary rocks and alkaline to s)5alkaline olcanic rocks o% the &o.erPermian it) Qro)p commonly oerlie the Am5o Qro)p and also locally lie )ncon%orma5ly

on the aran complex alon7 its .estern ed7e 5)t are a5sent northeast o% Antamina*-oth north and so)th(so)theast o% the cross(strike str)ct)ral discontin)ity, thesesiliciclastic sedimentary rocks are oerlain 5y @pper Triassic to &o.er O)rassic car5onaterocks and shales 8P)carU Qro)p:* The a5sence o% these three 7ro)ps directly northeast o% Antamina records either a s)5(Cretaceo)s erosional )ncon%ormity or nondeposition %romississippian to &ate O)rassic times* The cross(strike str)ct)ral discontin)itythere%ore also coincided .ith a topo7raphic hi7h or arch, at least in the iddle to &ate

O)rassic, 5)t possi5ly persistin7 thro)7ho)t the ississippian to O)rassic interal* The cross(strike str)ct)ral discontin)ities pro5a5ly also in)enced the distri5)tion o%

the )ppermost Cretaceo)s to Paleocene red 5eds that )ncon%orma5ly oerlie theCretaceo)s strata 8i7* 4:* These are a5sent near the proposed cross(strike str)ct)raldiscontin)ity thro)7h the Antamina area, and they also thin si7ni'cantly near the morenortherly Casma(Pasto -)eno deection, 5)t they attain a considera5le thickness 5et.eenthese t.o transerse ;ones as .ell as to the so)th o% the arch 8i7* 1:*

Igneous actiity along the cross'stri(e structural !iscontinuity:

-et.een the Cordillera -lanca and the aran metamorphic complex, the 1Z1##,###H)adran7le maps o% Co55in7 et al* 8199=: and $ilson et al* 819=", 199>: record a 7reatera5)ndance o% Tertiary hypa5yssal and extr)sie rocks alon7 the proposed cross(strikestr)ct)ral discontin)ity than to the north.est or so)theast 8i7* =:* +n addition, .est o% thethr)st and %old 5elt, olcanic rocks o% the Calip)y S)per7ro)p 8Str)sieic; et al*, 2###: area5)ndant north.est and so)theast o% this cross(strike str)ct)ral discontin)ity 8e*7*, in theBeado J)antsUn area and the Cordillera J)ayh)ash i7*=:* These rocks are interpretedto 5e lo.er(middle iocene 5eca)se hypa5yssal intr)sions associated .ith similar olcanicrocks else.here in the re7ion 8J)ara; Qro)p o% the Calip)y S)per7ro)p i7* >: hae 5eensho.n 5y 4#Ar?39Ar step(heatin7 7eochronolo7y to hae persisted to 14*2 a 8Str)sieic;et al*, 2### &oe et al*, 2##1:* Jo.eer, the 11> km2 middle iocene 7ranodioriticCarh)ish pl)ton crops o)t on the axis o% the proposed cross(strike str)ct)ral discontin)ity8i7* >:, representin7 deeper(seated rocks o% 5roadly eH)ialent a7e 813*" a @?P5 ;ircondate on the main phase, )kasa, 1964 1=*> a K?Ar date on the mar7inal phase, Co55in7

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21/95

et al*, 1961:* The olcanic rocks o% Pampa O)n0n 87eler and de -ooy, 19>=: northeast o% the Carh)ish pl)ton 8i7* >: hae not 5een dated, so their si7ni'cance .ith re7ard to thetranserse str)ct)re is )ncertain* Jo.eer, .e propose that the cross(strike str)ct)raldiscontin)ity %oc)sed the distri5)tion o% i7neo)s actiity as it diminished thro)7h the mid(iocene, prior to intr)sion and minerali;ation at Antamina* +n i7)re = the arch is depicted.ith a .idth o% approximately 2# km to incorporate the le%t(steppin7 loc)s o% chan7es in

+ncaic strikes and pl)n7es 8i7* >:, the a5sence o% ississippian to &o.er O)rassic rocks5eneath the s)5(Cretaceo)s )ncon%ormity at the northeast end o% the arch, the diametero% the Carh)ish pl)ton at its so)th.estern mar7in, and the array o% iocene intr)sions*

o!a* +eo*o)i! Settin)

Se!imentary host roc(s

The contact o% the O)masha and Celend0n ormations .ithin the achay Qro)p has notpreio)sly 5een de'ned in the Antamina mine area, o.in7 to the intense skarn andhorn%els deelopment and to the pa)city o% 5iostrati7raphic markers* The cliI(%ormin7

strata s)rro)ndin7 the Antamina deposit .ere mapped initially 5y -odenlos and ricksen819>>: as marine limestones o% the O)masha ormation* O*O* $ilson 8)np)5lished report toCerro de Pasco Corp*, &ima, 19>9, in Petersen, 19=>: o5sered that the 5eds on thenorth.est slopes o% E)e5rada Antamina .ere strati7raphically hi7her than those at similareleations on the so)theast side, 5)t he did not locate the inter%ormational contact*Petersen 819=>: descri5ed the car5onate )nits as the achay ormation, and they .eres)5seH)ently reassi7ned to the O)masha ormation 5y Co55in7 et al* 8199=:* The skarn(hostin7 limestone strata at Antamina are herein s)5diided into t.o seH)ences, the )ppermarkedly more shaly than the lo.er, and assi7ned to the Celend0n and O)mashaormations, respectiely* These seH)ences are descri5ed and compared .ith a section o% the releant strati7raphic interal constr)cted %rom t.o sections meas)red 5y -enaides819>=: at @ch)pata in the /0o P)chca alley, 2# km north o% Antamina 8i7* 4:* A meas)red

section has not 5een esta5lished %or the Antamina minesite 5eca)se o% the str)ct)ralcomplexity o% the area 8descri5ed 5elo.: and the a5sence o% marker 5eds in the exposedhost rocks* The in%erred strati7raphic interal occ)pied 5y the skarn is also indicated ini7)re 4* The )pper seH)ence o% thin(5edded rocks that prior to minin7 )nderlay the rid7ecrests ankin7 E)e5rada Antamina 8i7* 2: is .idely altered to horn%els adRacent to theAntamina intr)sie center and 7rades into shaly limestone .ith increasin7 distance %rom it8i7* ":* The lo.er seH)ence

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22/95

+Q* "* The t.o distinct host rock types o% the Antamina skarn systemZ an )pper seH)ence o% thin(5edded, siltylimestones, here lar7ely conerted to calc(horn%elses, assi7ned to the Celend0n ormation 8Ce:, and a lo.er seH)enceo% thick(5edded, relatiely p)re limestones that %orm mar5les, interpreted as the O)masha ormation 8O:* &ocations%rom .hich the photo7raphs .ere taken are indicated in i7* 2* Photo7raphs taken in 199" thro)7h 1999 this rid7ehas no. 5een lar7ely remoed 5y mine deelopment* 8a: The so)theast side o% E)e5rada Antamina, lookin7so)theast* The contact 8lon7 5lack dashes: 5et.een the O)masha and Celend0n ormations is placed at the top o% the)ppermost massie thick(5edded limestone 8pale 7ray 5and: de%ormed 5y the Antamina anticline* Bote the irre7)lar)pper, so)theastern contact o% the skarn 8lon7 .hite dashes:, here lar7ely con'ned to the O)masha ormation* 85:&ookin7 northeast at the head o% E)e5rada Antamina sho.in7 the distinct control o% 5eddin7 in the Celend0n

ormation on skarn deelopment*o% calcitic mar5le contains only s)5ordinate intercalated diopside(rich )nits and 7radeso)t.ard into predominantly thick5edded, relatiely p)re limestone 8i7* ":* The lo.erseH)ence 8i7s* 4, "a, and 6a: 7enerally contains more than "> percent calcite andcomprises calcitic, aria5ly 5ioclastic limestones that ran7e in 7rain si;e %rom m)dstone to.ackestone* This seH)ence is 7enerally thick(5edded and massie and displays karstic.eatherin7 8i7* 65:* To.ard the top o% this seH)ence, medi)m to thick 5eds 82#Y># cm: o% imp)re, silty limestone are inter5edded .ith the p)re limestone

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23/95

+Q* 6* The O)masha and Celend0n ormations* 8a: CliI(%ormin7 thick 5eds o% massie limestone on the so)theast anko% E)e5rada Callapo 8see i7* 2 %or location:* 85: assie limestone o% the O)masha ormation, sho.in7 )ted.eatherin7 8so)th o% Manacancha area, i7* 2:* 8c: @pperZ massie 8i*e*, )nlaminated and .ith no pre%erred orientationo% %ossils: pelecypod car5onate .ackestone o% the O)masha ormation 8DDJ CA(#39, 1>> m:* &o.erZ sheared%ossili%ero)s car5onate .ackestone, eH)ialent to )pper piece, not marmori;ed, 5)t .ith a stron7 pre%erredorientation 8DDJ CA(#39, 1=# m:* 8d: ore recessie .eatherin7, e7etated, thin(5edded limestones and marls o% the Celend0n ormation croppin7 o)t on the so)theast slopes o% E)e5rada Ayash near its intersection .ith E)e5rada T)c)sh 8see i7* 2 %or location:* Photo7raph taken in 199" the lo.er hal% o% this area is no. o5sc)red 5y constr)ctiono% the tailin7s dam* 8e: Bod)lar limestone o% the Celend0n ormation, consistin7 o% li7ht 7ray, coalescin7, car5onate(

rich nod)les separated 5y .isps o% dark 7ray calcareo)s siltstone 8ort)na ine area, see i7* 2 hammer %or scale:*8%: @pperZ Celend0n ormation nod)lar limestone 8DDJ CAC3, 221 m:* &o.erZ lentic)lar 5eddin7 interpreted assheared nod)lar limestone o% the Celend0n ormation 8DDJ CA(C>, 6"*> m:*

8i7s* 4 and "a:* This seH)ence hosts ore at the s)r%ace and at depth in the so)th.est parto% the deposit, 5)t only in the s)5s)r%ace in the northeast* e. .hole %ossils are presered8i7* 6c: and no ammonites .ere o5sered in the mine area, precl)din7 5iostrati7raphiccorrelations* Jo.eer, .e assi7n this seH)ence to the O)masha ormation on litholo7ic7ro)nds* Sheared %ossili%ero)s car5onate .ackestone occ)rs locally in this seH)ence 8i7*

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24/95

6c:* +n north(central Per), -enaides 819>=: descri5ed the O)masha ormation asdominated 5y massie, thick(5edded, li7ht oran7e(5ro.n to yello.ish(5ro.n and 7ray,%ossil(poor dolostones and limestones that .eather dark yello.ish(5ro.n to 5ro.nish(7ray*

The %ormation has 5een %)rther descri5ed as comprisin7 topo7raphically prominent, cliI(%ormin7, li7ht(7ray limestones and yello.ish dolostones that are characteristically5ioclastic 8$ilson, 19=3:*

+n the @ch)pata section 8i7* 4:, the )pper 43" m is limestone, and the lo.er 3>3 mis dolostone* @nlikethe oerlyin7 Celend0n ormation, this %ormation only rarely contains calcareo)s siltstones,altho)7h it incorporates marly limestone 5eds near its top 8Oaillard, 196":* The )pper limito% the O)masha ormation .as de'ned lithostrati7raphically .here medi)m( or thick(5edded limestones pass )p.ard into thin(5edded marls and limestones 8$ilson, 19=3:*

The )pper seH)ence in the mine area 8i7s* 4, ", and 6d: comprises thin( to thick(5eddedimp)re limestone(marl that aries in car5onate?silicate ratio %rom relatiely calcite(richm)ddy limestone 87enerally >#Y">G car5onate: to calcareo)s siltstone 8less than >#Gcar5onate:* any )nits contain li7ht7ray, calcitic nod)les, composin7 1# to 9# percent o% the rock, enclosed 5y dark, silty calcareo)s m)dstone 8i7* 6e, %:* Bo limestone 5eds .ithsiliceo)s nod)les hae 5een o5sered* The nod)lar text)re is interpreted to 5e dia7enetic*

Sheared limestone .ith lentic)lar 5eddin7 exposed at the northeast head o% E)e5radaAntamina represents de%ormed nod)lar limestone in .hich the nod)les hae 5eenattened into lenses d)rin7 %oldin7 and %a)ltin7 8i7* 6%:* The )pper seH)ence lacksidenti'a5le %ossils 5)t is assi7ned to the Celend0n ormation on litholo7ic 7ro)nds* +n the@ch)pata section, 2# km north o% the mine, the Celend0n ormation comprises ery so%t,%ria5le, %ossil(poor, li7ht 7reenish(7ray, nod)lar, moderately silty marls and calcareo)sshales 8i7* 4 -enaides, 19>=:* This %ormation is 7enerally medi)m(5edded 8#*3Y#*6 m:and aria5ly dolomitic, and it ran7es %rom 'ne(7rained to pelletal 8$ilson, 19=3:*

The contact 5et.een the O)masha and Celend0n ormations in the mine area iscon%orma5le and 7enerally 7radational thro)7ho)t seeral meters and is marked 5y)p.ard(increasin7 siltiness and decreasin7 5ed thickness* This contact is interpreted to 5ethe top o% the )ppermost thick(5edded limestone or mar5le )nit on the 5asis o% litholo7yand 5eddin7 characteristics* This strati7raphic position is ill)strated in i7)re "a %oro)tcrops on the so)theast .all o% the Antamina alley* The contact is %olded 5y theanticlines exposed on the alley .alls adRacent to the so)th.estern part o% the deposit,and it dips northeast in the s)5s)r%ace in the northeast part o% the deposit, as do theoerlyin7 strata exposed in that area* The thin(5edded calc(horn%elses that predominateon the rid7e crests ankin7 E)e5rada Antamina 7rade laterally a.ay %rom skarn into themarly, aria5ly nod)lar seH)ence assi7ned to the Celend0n ormation, 5)t they do notcontain the shales characteristic o% its )pper part* The relatiely p)re calcitic mar5le o% the

O)masha ormation that hosts ore at s)r%ace in the so)th.est part o% the deposit and atdepth in the northeast sector does not contain the thick dolostones characteristic o% thelo.er part o% that %ormation, and no ma7nesian skarn has 5een %o)nd* n this 5asis, .econcl)de that the skarn deeloped in the )pper part o% the O)masha ormation and thelo.er part o% the Celend0n ormation 8i7* 4:* Jo.eer, .e estimate that approximatelythree(H)arters o% the minerali;ed exoskarn, and all o% the .ollastonite exoskarn, deelopedin the O)masha ormation, and that a similar proportion o% the conti7)o)s and 7eneticallyrelated endoskarn .as %ormed in the stock adRacent to that %ormation* +t is not kno.n.hether ma7nesian skarn deeloped in dolostone 5elo. the limit o% exploration anddeelopment drillin7*

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25/95

$lteration of se!imentary roc(s a!)acent to the s(arn

The o)ter contact o% coarse(7rained minerali;ed exoskarn is a5r)pt, conol)ted, and)nin)enced 5y 5eddin7 in the mar5les %ormed %rom the limestones o% the O)mashaormation 8i7* "a:, 5)t it is more 7radational and strati7raphically controlled in the 'ne(7rained calc(silicate rocks deeloped in the inter5edded limestones and marls o% theoerlyin7 Celend0n ormation 8i7* "5:* +n the O)masha ormation, the minerali;ed skarn is

R)xtaposed .ith mar5les containin7 local hori;ons rich in calc(silicate minerals that may 5eascri5ed to thermal metamorphism* +n contrast, the calc(silicate(5earin7 rocks deelopedin the marly Celend0n ormation incl)de rock types that are interpreted as the prod)cts o% either metamorphism or metasomatism* The latter do not host s)l'de minerals and diIerradically in text)re and mineralo7y %rom the exoskarn ore* Their .ide distri5)tion aro)ndthe )pper part o% the ore5ody constit)tes a si7ni'cant exploration tar7et*

Jumasha Formation:

n approachin7 skarn, dark 7ray O)masha limestone is conerted to coarselycrystalline, 7ray to .hite calcitic mar5le 8Ta5le 1, i7* 9:, %ormin7 an a)reole ran7in7 %romtens to h)ndreds o% meters .ide* $ithin the a)reole, local deelopment o% sparse diopside,.ollastonite, or scapolite porphyro5lasts 8i7* 9a: or sli7ht diIerential erosion and colorariation 8i7* 95: in calcite mar5le record minor compositional ariations in the limestone*/are, 'ne(7rained, li7ht 7reen, diopsidic layers in calcite mar5le 8i7* 9c:, the porcellaniteo% Terrones 819>6:, are interpreted as calc(horn%els, representin7 thermallymetamorphosed, medi)m to thick 5eds o% dolomitic, m)ddy, 'ne(7rained limestone nearthe top o% the %ormation* $ithin the mar5le a)reole, medi)m(7rained, mottled, or 5anded7ray mar5le predominates, locally 7radin7 in.ard to medi)m( to coarse(7rained, p)re.hite mar5le, reectin7 either de7raphiti;ation or metamorphism o% or7anic matter toclear itrinite* /are, medi)m( to coarse(7rained, thin, 5)I 7arnet layers in 5oth .hite andmottled %acies o% mar5le mimic the %orms o% %olded silicate(rich laminae 8i7* 9d: andappear to hae replaced them* This type o% 7arnet deelopment is interpreted as a5imetasomatic reaction skarn* +n 7ray mar5le, scapolite is locally deeloped in somedarker(7ray 5ands 8i7* 9e: 5)t does not persist into .hite mar5le or skarn 8Ta5le 1:*Scapolite there%ore %orms a discontin)o)s halo )p to tens o% meters .ide and commonlyseparated %rom

2>


26/95

the skarn %ront 5y tens o% meters* $hite mar5le deelops in patches 8i7* 9d and %: and isconcentrated alon7 stylolites and %ract)res 8i7* 9%:* The latter pro5a5ly represent )idpath.ays, s)77estin7 that deelopment o% .hite mar5le inoled the local channelin7 o% )ids* S)ch path.ays .ere not consistently )sed 5y later skarn(related )ids and arelocally c)t 5y %ract)res .ith .ollastonite(rich sela7es 8i7* 9%:* +mmediately so)th.est o%

the skarn, the O)masha ormation has a distinct planar 5anded %a5ric, locally a spacedcleaa7e, that dips northeast at a hi7h an7le to 5eddin7 8i7* 9a:* To the northeast, this%a5ric steepens and 5ecomes ertical adRacent to the skarn and alon7 the strike o% a maRor%old axis* The consistent strike, systematically chan7in7 dip, and relationship to %oldsisi5le in the adRacent limestones indicate that this is an )p.ard(%annin7 cleaa7eassociated .ith local %oldin7* +n the adRacent skarn, alternatin7 andradite(rich andsphalerite(rich 5ands, 7enerally > to ># cm .ide, are s)5parallel to this %a5ric, s)77estin7that at least some preexistin7 str)ct)res in)enced minerali;ation* This 5andin7 maycorrespond .ith the coarse(7rained 7arnet(de'ned %a5ric o5sered 5y Terrones 819>6:*

Celen!*n Formation:

+n the Celend0n ormation, on the rid7es aro)nd &a7o Antamina, the skarn has ahalo o% mineralo7ically dierse 'ne(7rained calc(silicate rocks .ith s)5conchoidal toconchoidal %ract)res* $ith increasin7 distance %rom the skarn, these rocks 7rade intointer5edded limestone and marl that reacted diIerentially to thermal metamorphismaro)nd the porphyry intr)sion and ore5ody, th)s hi7hli7htin7 the 5eddin7 8i7* "5:*

Dark 7ray massie )nits chan7e in color, mineralo7y, and 7rain si;e proximal tomon;o7ranitic dikes* Altho)7h .e hae not mapped the ario)s %acies o% these rocks indetail, .e reco7ni;e the deelopment o% three distinct ;onesZ a distal ery 'ne 7rained,li7ht(5ro.n phlo7opitic %acies an intermediate 'ne(7rained, li7ht(7ray, tremolitic %acies

2=


27/95

and a proximal medi)m(7rained, li7ht(7reen diopsidic %acies 8Ta5le 1, i7* 1#a, 5:* The5o)ndaries 5et.een these %acies are commonly sharp and smooth 5)t are locally irre7)lar.here controlled 5y stock.ork %ract)res 8i7* 1#a, 5:* The o)ter limit o% the distal 5ro.n%acies is typically h)ndreds o% meters %rom the 5o)ndary o% s)l'de(5earin7 skarn, and thepro7ression %rom 5ro.n thro)7h 7ray to 7reen %acies occ)rs oer distances ran7in7 %romtens o% meters adRacent to the main intr)sion to tens o% centimeters 8i7 1#5: adRacent to

dikes extendin7 5eyond the main intr)sion* Altho)7h local )id channelin7 and pro5a5lymetasomatism occ)rred at the 5o)ndaries o% the %acies, the systematic mineralo7ic;onation 8Ta5le 1: most likely reects increasin7 temperat)re* The 'rst appearance o% li7ht5ro.n calc(horn%els corresponds .ith the phlo7opite(in reaction, Dol N K%s V Phl N Cal,.hich occ)rs in the ran7e 3># to 4=>C )nder 7eolo7ically reasona5le conditions 8i*e*, 4 V1,### 5ars and \C2 V #*1 to #*9Z Tracy and rost, 1991:* The 7ray calc(horn%els containstremolite .ith or .itho)t phlo7opite, .hereas 5ro.n calc(horn%els is tremolite %ree 8Ta5le1:* The transition %rom 5ro.n to 7ray calc(horn%els there%ore is interpreted to represent aphlo7opite(o)t reaction that coincides .ith the %ormation o% tremolite, pro5a5ly thro)7hthe reaction Phl N Cal N Et; V Tr N K%s 8Tracy and rost, 1991 Ta5le 1:* The li7ht7reencalc(horn%els records the 'rst appearance o% massie diopside 8Ta5le 1:, altho)7h einletsand small 5le5s o% this mineral locally occ)r %arther %rom the intr)sie contacts* This ;one

pro5a5ly %ormed thro)7h the reaction Tr N Et; N Cal V Di, .hich )nder similar press)reand \C2 occ)rs at temperat)res o% 5et.een 4#> and 49>C 8Tracy and rost, 1991:* Thelocal narro.ness o% the 7ray calc(horn%els %acies and the a5sence o% tremolite in thephlo7opite %acies s)77est that the metamorphism took place at hi7h \C2* @nder s)chconditionsthe diopside(in and phlo7opite(in reactions are separated 5y only circa 1#C* Also, thedeelopment o% tremolite thro)7h the elimination o% phlo7opite is promoted )nder theseconditions, 5)t its %ormation .ithin the 'eld o% phlo7opite sta5ility thro)7h the reactionDol N Et; V Tr N Cal is inhi5ited*

+n contrast to these calc(horn%elses that %ormed )nder essentially thermalmetamorphic conditions, calc(silicate deelopment thro)7h metasomatism is reealed 5ydeelopment o% skarnoid in some 5eds o% the Celend0n ormation, extendin7 tens o% meters 5eyond the minerali;ed skarn %ront* Skarnoid is a descriptie term %or calc(silicaterocks that are relatiely 'ne 7rained, calci)m rich, and iron poor, and that reect, at leastin part, an al)minosilicate component in the protolith 8Fhariko, 19"#:* +t is 7eneticallyintermediate 5et.een a p)rely metamorphic horn%els and a p)rely metasomatic, )id(controlled skarn, .hich is typically coarser 7rained and does not as closely reect thecomposition or text)re o% the immediately s)rro)ndin7 rocks 8ina)di, 2###:* +n some 5edsin the Celend0n ormation, diopsidic nod)les stand o)t in relie% a7ainst the calcite(5earin7calc(silicate matrix 8i7* 1#c, d:* The nod)les, locally concentrically 5anded, ran7e %rom]1# cm to ^1 cm in diameter and %rom sparse to a5)ndant, independent o% their si;e 8i7*1#c, d:* They are interpreted as prod)cts o% the pre%erential metasomatism o% calciticnod)les 8c%* i7* 6e, %:*

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+Q* 9* armori;ed O)masha ormation* 8a: S)5tle 5eddin7, de'ned 5y porphyro5last a5)ndance 8o)tlined: in calciticmar5le, dips moderately so)th.est and is oerprinted 5y a northeast(dippin7 planar %a5ric 8lookin7 north.est, portalo% 5)lk sample adit, 4># m so)th.est o% .est shore o% &a7o Antamina hammer %or scale:* 85: S)5tle 5eddin7, de'ned5y sli7ht color diIerence and diIerential erosion 8o)tlined:, reectin7 cryptic diIerence in porphyro5last a5)ndancein calcitic mar5le 8lookin7 north.est, 3># m so)th.est o% .est shore o% &a7o Antamina 2# cm note5ook in%ore7ro)nd %or scale:* 8c: edi)m 5edded layer o% li7ht 7reen, 'ne(7rained, diopsidic calc(horn%els de'nes 5eddin7

.ithin coarse(7rained calcitic mar5le near the top o% the %ormation 8lookin7 northeast, 3## m so)th.est o% the

.estern shore o% &a7o Antamina 1> cm r)ler %or scale:* 8d: @pperZ mottled 7ray calcitic O)masha ormation mar5le

.ith ti7htly %olded, 5o)dina7ed, dark(7ray silty laminae* &o.erZ mottled .hite and 7ray mar5le .ith ti7htly %olded5)I 7arnet(rich layer, similar in shape to the silty layer in the )pper core 8DDJ CA(#6=, 216 m:* 8e: -anded 7ray O)masha ormation mar5le sho.in7 deelopment o% approximately 1# percent 5lack scapolite crystals .ithin darkerlayers 8DDJ CA(13=, 232 m:* 8%: @pperZ .hite mar5le in 7ray O)masha ormation mar5le 5oth as patches thro)7ho)tand locally aro)nd einlets and stylolites 8DDJ CA(13=, 226 m:* &o.erZ einlet(controlled .hite mar5le in 7ray O)masha ormation mar5le c)t 5y irre7)lar einlets .ith chalky(.hite .ollastonitic sela7es8DDJ CA(23=, 441 m:* Scale 5ars in centimeters*

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+Q* 1#* Jorn%els, skarnoid, and .ri77lite deeloped in the Celend0n ormation* 8a: +rre7)lar stock.ork o% sealed%ract)res controllin7 deelopment o% li7ht(7ray tremolitic calc(horn%els in li7ht(5ro.n phlo7opitic calc(horn%els 8on theso)theast rid7e crest, center o% i7* 12c 1> cm pencil %or scale:* 85: Calc(horn%els %acies .ith distal, li7ht(5ro.n, ery'ne 7rained phlo7opitic calc(horn%els at )pper le%t, li7ht(7ray tremolitic in the center, and proximal, li7ht(7reenmedi)m(7rained diopsidic calc(horn%els at lo.er ri7ht* These %acies are )n)s)ally closely spaced 5eca)se they are onthe mar7in o% a narro. dike, approximately 3># m so)theast o% the main intr)sion 8same location as _a` >> mm lenscap %or scale on the %ar le%t:* 8c: Sparse, lar7e, medi)m(7reen diopsidic nod)les in pale 7ray calc(horn%els 85ase o% cliIs, northeast o% &a7o Antamina 1# cm kni%e %or scale:* 8d: [ery a5)ndant coalescin7 diopsidic nod)les .ith calcite(5earin7 calc(horn%els matrix 85ase o% cliIs, east o% &a7o Antamina 1> cm pencil %or scale:* 8e: +rre7)lar concentric

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5andin7, interpreted 5y -odenlos and ricksen 819>>: to 5e o% al7al ori7in, 5)t reinterpreted as .ri77lite text)re o% metasomatic replacement ori7in 85ase o% cliIs, east o% &a7o Antamina 1> cm pencil %or scale:* 8%: -andin7 deelopedin calc(horn%els and apparently controlled 5y northeast strikin7 %ract)res 8on the rid7e crest so)theast o% Antaminaalley, R)st east o% the east end o% i7* 12c >> mm lens cap %or scale:*

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-oth close to the skarn %ront and tens to h)ndreds o% meters %rom it, s)l'de(%reeconcentrically layered str)ct)res occ)r in some massie 8nonnod)lar: 5eds o% alteredCelend0n limestone 8i7* 1#e:* These .ere interpreted as al7al str)ct)res 5y -odenlos andrickson 819>>:* Jo.eer, they do not hae the reH)isite three(dimensional 7eometry, andno )nam5i7)o)s al7al str)ct)res hae 5een seen in )naltered limestones in the area* Also,at the transition %rom 5ro.n to 7ray %acies in calc(horn%els, rhythmic 5andin7 is deeloped

adRacent to thro)7h(7oin7, northeast(strikin7 %ract)res 8i7* 1#%:* Similar laminated%eat)res occ)r in ore(5earin7 .ollastonite skarn* -eca)se o% their similarity to 5anded%eat)res descri5ed in other skarns 8e*7*, Knop%, 19#6 skola, 19>1:, .e interpret thelayerin7 as metasomatic .ri77lite 8K.ak and Askins, 1961:* rom these relationships, .econcl)de that minor metasomatism occ)rred 5eyond the extent o% metallic minerali;ationin the Celend0n ormation*

Structural geology of the mine area

The @pper Cretaceo)s strata o% the achay Qro)p that host the skarn .ereintensely de%ormed in the aran thr)st and %old 5elt* So)th.est o% the mine, the

O)masha ormation is thr)st oer the yo)n7er Celend0n ormation, and the &o.erCretaceo)s %ormations o% the QoyllarisH)is7a Qro)p are thr)st 5oth oer the Celend0normation and oer each other in reerse strati7raphic order 8i7* 11:*

+ocal fol!s an! faults:

The Antamina deposit is hosted 5y a relatiely at(lyin7 section o% the O)masha andCelend0n ormations, is de%ormed 5y open %olds, and is c)t 5y n)mero)s small thr)st(%a)ltramps and 5eddin7(parallel thr)st %a)lts 8i7* 11:, all interpreted to hae deeloped d)rin7the late ocene +ncaic oro7eny* Prior to open(pit deelopment, the steep slopes aro)nd the

Antamina alley proided clear, three(dimensional ima7es o% the local str)ct)ralrelationships 8i7* 12:, s)mmari;ed in an isometric 5lock dia7ram in i7)re 13* Theer7ence o% the thr)st %a)lts are interpreted %rom c)toI an7les, and it is ass)med that thestrata are all )pri7ht, )nless o5io)sly oert)rned* The local thr)stin7 and related %oldin7are descri5ed 5elo. in seH)ence %rom so)th.est to northeast* So)th.est o% Antamina, anortheast(er7in7 thr)st, herein named the MaH)irsh(-)H)e P)nta thr)st 8M-PT in i7s* 11,12, and 13:, has ca)sed str)ct)ral repetition o% the O)masha ormation* A %e. h)ndredmeters to the east o% this %a)lt, the O)masha ormation is thr)st oer the yo)n7er Celend0normation on the northeast(er7in7 Antamina thr)st %a)lt 8mine terminolo7y AT in i7s*11, 12, and 13:* The im5ricate thr)st tack is deeloped in massie limestones o% the

O)masha ormation 8i7* 6a: in the next transc)rrent alley to the north.est o% Antamina8E)e5rada Callapo in i7* 2:* This is part o% a d)plex in .hich the Antamina thr)st is the

sole and one o% the )nnamed thr)sts .est o% the MaH)irsh(-)H)e P)nta thr)st is the roo%,altho)7h the latter has 5een extensiely eroded* These im5ricate slices do not all contin)eso)th %rom E)e5rada Callapo into E)e5rada Antamina 5eca)se some %a)lt s)r%acesmer7e*

n the north.est slope o% the Antamina alley, only the O)masha ormation occ)rs5et.een the Antamina thr)st and the MaH)irsh(-)H)e P)nta thr)st, .hereas the O)mashaand Celend0n ormations in con%orma5le strati7raphic seH)ence separate these t.othr)sts on the opposin7 so)theast slope 8i7s* 11, 12, and 13:* The immediately

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)nderlyin7 thr)st slice, carried on the northeast(er7in7 ort)na thr)st 8mine terminolo7yZT:, occ)rs only on the north.est slope o% the alley, east o% and 5elo. the Antaminathr)st* +n this area, the ort)na thr)st repeats the Celend0n ormation and has a smallrec)m5ent anticline in its han7in7 .all 8i7s* 11, 12a, and 13:* Jo.eer, neither this thr)stnor its han7in7(.all anticline can 5e traced across the alley to its so)theast side 8i7*125, c:, either 5eca)se the thr)st s)r%aces mer7ed or 5eca)se the 5ranch line o% the t.o

thr)sts pl)n7es north and there%ore clim5s a5oe the erosion leel to the so)th* TheAntamina thr)st c)ts the shallo.ly dippin7 ort)na thr)st 8i7s* 11, 12a, 13: as .ell asother minor, 5eddin7(parallel thr)sts in the Celend0n ormation*

ost o% the %olds and thr)st %a)lts at Antamina are northeast er7in7, as ischaracteristic o% the aran thr)st and %old 5elt 87ard, 1964:, 5)t t.o so)th.est(er7in7 thr)stsocc)r so)theast o% &a7o Antamina 8i7s* 11, 12c, and 13:* These thr)sts areinterpreted as 5ackthr)sts related to the )nderlyin7 northeast(er7in7 scarina thr)st8mine terminolo7yZ T:, .hich is the lo.est in the immediate mine area* This thr)st can 5etraced alon7 the crest o% the so)th.est slope o% the T)c)sh alley 8i7* 2:, east o% Antamina* +t also repeats the Celend0n ormation and de%orms it into a han7in7( .allanticline 8i7s* 11 12a, 5, d and 13:* any other minor thr)st %a)lts are recorded 5yramps .ith 5eddin7 c)toIs %or distances o% tens o% meters, especially in the Celend0n

ormation, 5)t are not ill)strated here* Prominent north.est(strikin7, open, )pri7htanticlines are exposed a5oe the deposit on the north.est and so)theast .alls o% thealley 8i7s* "a 12a, c and 13:* They 5ecome 5roader and more open )p.ard 5)t theinterenin7 synclines are ti7ht* S)ch concentric %olds are common else.here in thearan thr)st and %old 5elt 8e*7*, Coney, 19"1:*

The lar7est o% the parallel %olds isi5le on the anks o% the Antamina alley has5een .idely re%erred to as the Antamina anticline, the axial plane o% .hich has anapparent dextral oIset o% approximately >## m in a northeast(so)th.est direction acrossthe alley* This oIset has 5een ario)sly ascri5ed to local 5ends in the strike o% the %olds8-odenlos and ricksen, 19>>:, dextral oIset on a post)lated northeast(strikin7 [alley%a)lt 8Terrones, 19>6:, transerse normal %a)ltin7 8Petersen, 19=>,: and cross(%oldin78cKee et al*, 19"9:* Jo.eer, in this st)dy .e sho. that the oIset is 5est explained 5y alateral ramp model* +n this model 8i7* 14:, the anticlines represent %a)lt(5end %olds relatedto han7in7(.all c)toIs %olded oer the top o% north.est(strikin7 %rontal ramps that areoIset 5eca)se a northeast(strikin7, >##(m(lon7 trans%er %a)lt or lateral ramp separates the%rontal ramps in an apparent dextral sense* Thispost)lated str)ct)re, the [alley lateralramp, .o)ld acco)nt %or the a5sence o% correspondin7 oIsets o% other str)ct)res to theso)th.est and northeast 8i7* 11:*

inor dikin7 and skarn alteration extend 5eyond the main intr)sion and ore5ody tothe so)th.est do.n E)e5rada Antamina and to the northeast and east at the head o% thealley* The ertical dikes that extend east %rom the main porphyry mass to the /osita dero area 8i7s* 11 and 12c: oIset t.o minor %orethr)sts and 5ackthr)sts, indicatin7 thatthere .as minor, apparently ertical, %a)ltin7 5et.een the time o% thr)st %a)ltin7 and%oldin7 and that o% intr)sion and minerali;ation* The orientation o% these dikes chan7esconsidera5ly alon7 strike near the main stock they are nearly ertical, strike east, and c)tstrata 8i7 12c:, .hereas %arther east 5eyond the rid7e crest they 5ecome sills and %ollo.either strata or 5eddin7(parallel thr)st ats, strike so)th, and dip moderately to the .est8i7* 125:* The dikes 5ecome pro7ressiely more altered to endoskarn to.ard the mainintr)sion, and altho)7h they transect ;oned exoskarn they do not appear to hae intr)dedit* A similarly aria5le orientation is sho.n 5y a dike that extends northeast %rom thenorthern shore o% &a7o Antamina it locally crosses strata at a steep an7le 5)t also %ollo.sat and moderately dippin7 strata 8i7* 12a:* The parallel %eat)res at opposite ends o% the

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main intr)sion and ore5ody s)77est an oIset, le%t(steppin7, northeast(so)th.est %ract)re;one that is lon7er than the >## m lateral ramp, the post)lated [alley %a)lt 8i7s* 35 and11:* A .ell deeloped northeast(strikin7, .idely spaced, nearly ertical %ract)re set occ)rsin the sedimentary host rocks thro)7ho)t the area 8e*7*, on the so)th.est ank o% Cerro/acpe i7* 1>a:, and .e interpret it as re7ional a(c Rointin7 related to %oldin7* A similarlyoriented %ract)re set is intensely deeloped near the deposit, especially alon7 the

so)theast side o% the Antamina alley 8i7* 1>5, c:*+n many places, calc(horn%els is more intensely deeloped and .idespread .here

these %ract)res are closely spaced* All o% the northeast(strikin7 %ract)res may hae 5eenrelated to ocene %oldin7 and thr)st %a)ltin7, 5)t they .ere 5etter deeloped in the minearea 5eca)se o% ex)re and tearin7 o% strata a5oe a trans%er %a)lt or lateral ramp8disc)ssed 5elo.:* ollo.in7 intr)sion, they proided access %or )ids to thehorn%els5earin7 strata* +n the Antamina district, there%ore, the O)masha and Celend0normations hae 5een thr)st(%a)lted, %olded, and R)xtaposed into a thick, complex thr)ststack* The total thicknesso% rock that oerlay the site o% minerali;ation in the late iocene cannot 5e estimated5eca)se the local thickness o% oerlyin7 )ppermost Cretaceo)s and Paleocene red 5eds is)nkno.n and 5eca)se it is )nclear i% the s)5aerial olcanic rocks and associated

sedimentary rocks o% the ocene to middle iocene Calip)y S)per7ro)p 8Str)sieic; et al*,2###: extended into this area*

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+Q* 11* &ocal 7eolo7y o% the Antamina area 8modi'ed a%ter Co55in7 et al*,199= Qloer, 199", )np)5lished report toCompan0a inera Antamina S*A*, &ima Palomino, 199", )np)5lished report to Compan0a inera Antamina S*A*, &imaand &oe and Clark, 1996, )np)5lished report to Compan0a inera Antamina S*A*, &ima:* The physio7raphy o% thisarea is sho.n in i7* 2* The areas depicted in i7* 13 are indicated 5y the three dia7onal rectan7les* AT V Antaminathr)st, T V ort)na thr)st, [ V le%t(steppin7 [alley %a)lt, M-PT V MaH)irsh(-)H)e P)nta thr)st*

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+Q* 12* Photomosaics ill)stratin7 the str)ct)re and strati7raphy o% the Antamina mine area* &ocations %rom .hich thephoto7raphs .ere taken are indicated in i7* 2* The ie.s in 8a: and 85: look north.est, .hereas those in 8c: and 8d:look so)th* Photo7raphs taken in 199" thro)7h 1999 m)ch o% 8a:, 85:, and 8c: has no. 5een remoed 5y minedeelopment, and the skyline o% 8d: has 5een modi'ed* 8a: The north.est side o% the Antamina alley the irre7)lar)pper or north.estern contact o% the skarn 8lon7 .hite dashes: c)ts oI the strati7raphic contact 8lon7 5lack dashes:o% the O)masha 8O: and Celend0n 8Ce: ormations* 85: The so)theast ank o% the rid7e extendin7 northeast o% Cerro-)H)e P)nta 8i*e*, the opposite side o% the rid7e ill)strated in _c`:, ie.ed %rom the Manacancha area* 8c: The so)theast side o% the Antamina alley theirre7)lar )pper, so)theastern contact o% the skarn 8lon7 .hite dashes: c)ts oI the strati7raphic contact 8lon7 5lackdashes: o% the O)masha 8O: and Celend0n 8Ce: ormations* 8d: The north.est ank o% the rid7e extendin7 northeast o%

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Cerro MaH)irsh 8i*e*, the opposite side o% the rid7e ill)strated in _a`:* a)lts and %old axes are sho.n 5y solid 5lacklines, the strati7raphic contact 5et.een the O)masha 8O: and Celend0n 8Ce: ormations 5y lon7 5lack dashes, thecontact o% the skarn 5y lon7 .hite dashes, intr)sie contacts 5y solid .hite lines, intr)sions 5y crosses* AT VAntamina thr)st, Ce V Celend0n ormation, T V ort)na thr)st, O V O)masha ormation, T V scarina thr)st, [ Vs)r%ace expression o% the northeastern se7ment o% the [alley %a)lt, M-PT V MaH)irsh(-)H)e P)nta thr)st*

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+Q* 14* Schematic dia7ram ill)stratin7 the lateral ramp model %or the str)ct)ral settin7 o% the Antamina deposit*&ookin7 so)th so that the %ace o% the lateral ramp is exposed, sho.in7 oIset anticlines prod)ced in the han7in7 .allo% a thr)st %a)lt 5y t.o %rontal ramps separated 5y a lateral ramp* 8a: Qeometry o% the %a)lt prior to moement* 85:Qeometry o% the %oot.all 8han7in7(.all 5lock remoed:, sho.in7 the so)th.est(dippin7 %rontal ramps linked 5y anortheast(strikin7 lateral ramp or trans%er %a)lt* 8c: A%ter minor thr)st moement, oIset ramp(c)toI anticlines

prod)ced in the han7in7 .all o% the thr)st %a)lt, .ith more intense northeast(strikin7 %ract)rin7 a5oe the lateralramp, represented 5y dashed lines* 8d: Jan7in7( .all c)toI anticlines separated %rom %a)lt(5end anticlines 5yadditional thr)st moement* xtensie %ract)rin7 deeloped in the thr)st sheet .here it exed oer the lateral ramp*

Structural control of mineraliation:

As in most intr)sion related skarn deposits, the most o5io)s %eat)re controllin7minerali;ation at Antamina is the contact 5et.een the main stock and the host limestones8Terrones, 19>6 Petersen, 19=> /ed.ood, 1999:* Jo.eer, the so)theast and north.estintr)sie contacts are themseles parallel to, and alon7 strike %rom, the t.o se7ments o%

the proposed northeast(strikin7 [alley %a)lt* The hydrothermal 5reccia sheets that arecommon at or near the endoskarn(exoskarn contact also hae this orientation* Theirre7)lar ;ones o% 5reccia and endoskarn .ithin the intr)sion are interpreted to strikepredominantly north(so)th 8i7* 35: and may hae 5een controlled 5y crossc)ttin7str)ct)res* The .estern end o% the main 5ody o% the intr)sion also 7enerally strikes north(so)th, as does the skarn %ront in that area 8i7* 35:* The parallelo7ram shape, in plan, o% the main 5ody o% the intr)sion and the s)rro)ndin7 skarn is complicated 5y the net.ork o% anastomosin7 dikes .ith enelopes o% 'ne(7rained 7arnet skarn extendin7 to hi7hereleations to the east o% &a7o Antamina 8i7* 12c:* At least one o% these dikes intr)des anormal %a)lt on .hich moement occ)rred 5et.een the time o% thr)st %a)ltin7 and %oldin7and that o% intr)sion and minerali;ation 8i7* 12c:* Seeral other dikes .ith skarnenelopes also extend 5eyond the main mass o% porphyry and most are controlled 5y

+ncaic str)ct)res* Alon7 the so)thern ed7e o% the porphyry and skarn, minor peripheralrhyodacitic dikes and sills and their skarn enelopes locally %ollo. 5eddin7 and thr)st%a)lts* Seeral minor P5(Fn( A7 eins 8i7* 2:, some o% .hich hae 5een mined on a smallscale, occ)r at or near the contacts 5et.een these dikes and the host limestone,descri5in7 a circa 3 3 km area o% dispersed hydrothermal actiity centered on theAntamina skarn 8-odenlos and ricksen, 19>>:*

Dis!ussion

-egional stratigra&hic an! tectonic relationshi&s

The post)lated [alley lateral ramp at Antamina may hae 5een controlled 5y )nderlyin7,northeast(strikin7 5asement %a)lts* &ateral ramps are tho)7ht to 5e lar7ely the res)lt o% the interaction o% thr)st sheets and old 5asement %ract)re systems 8Pohn, 2###:* -oth7ard 8196": and -enaides 81999: proposed that the se7mentation and artic)lation o% the aran thr)st and %old 5elt pro5a5ly reect control 5y 5asement str)ct)res, altho)7hthey diIer in their interpretations o% the mechanism* -enaides 81999: attri5)ted these7mentation

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o% the aran thr)st and %old 5elt to maRornortheaststrikin7, dextral 5asement %a)lts,.itho)t speci%yin7 .hen the %a)lts .ere actie*)rther, -)ssell and Pitcher 8196>: s)77estedthat a .ell deeloped set o% northeast(strikin7,en echelon, dextral %a)lts in the Cretaceo)s(

Paleo7ene Coastal 5atholith may haecontrolled some o% the contacts o% earlyPaleocene intr)sie rin7 complexes 8i7* 1=:*Jo.eer, no si7ni'cant tear %a)ltin7 parallel tothe northeast(trendin7 deections is apparentin the de%ormed eso;oic coer strata,s)77estin7 that the 5asement %a)lts hae not5een actie since the O)rassic, and that anydextral oIsets are apparent, not real* 7ard8196":, in contrast, proposed that en echelonsinistral 7ro.th %a)lts in the 5asement .erethe common

5o)ndary o% the mio7eosyncline and Ma)li shel% in the eastern part o% the $est Per)iantro)7h, and controlled the attit)de o% thepresent thr)st and %old 5elt* The eIects o% thecross(strike str)ct)ral discontin)ities onsedimentation hae aried thro)7h time and.ith rock type* A compilation o% doc)mentedstrati7raphic col)mns 8i7* 1=5: sho.s that%rom J)ancayo in the so)th to ># km northo% Antamina the Cretaceo)s strata ary ina77re7ate thickness %rom approximately >##to 1,>## m and thicken oerall to the north*Qenerally, only the easternmost sections hae5een )sed in this compilation, minimi;in7 theeIect o% thickness ariations perpendic)lar tothe mar7in, and th)s emphasi;in7 alon7(strikeariations that may 5e related to se7mentationo% the mar7in* +t is eident that the Cretaceo)sstrata exhi5it no clear relationship 5et.een

thickness and proximity to a crossstrike str)ct)ral discontin)ity*

+Q* 1>* Bortheast(strikin7, nearly ertical %ract)rin7 in the eastern and so)theastern part o% the Antamina mine area*&ocations %rom .hich the photo7raphs .ere taken are indicated in i7* 2* Photo7raphs taken in 1996 m)ch o% 85: and8c: has no. 5een remoed 5y mine deelopment* 8a: &ookin7 eastnortheast at the so)th.est ank o% Cerro /acpe.here all 5eddin7 dips so)th.est, to.ard the point o% ie.* $idely spaced northeast(strikin7 %ract)res are expressedas lineaments that coner7e in the distance and are perpendic)lar to the strike o% the Celend0n 8Ce: and )pper O)masha 8O: ormations on the lo.er slope, 5)t are more o5io)s as ertical %ract)res in the lo.er O)masha ormationon the steep )pper slope* &on7 5lack dashes indicate the strati7raphic contact 5et.een the Celend0n and O)mashaormations* 85: &ookin7 north(northeast alon7 the rid7e crest on the so)theast side o% the Antamina alley in theimmediate icinity o% the mine* The more intensely deeloped northeast(strikin7 %ract)re set in the Celend0normation can 5e seen as closely spaced ertical %ract)res in the cliI %ace* 8c: &ookin7 northeast alon7 the rid7e5o)ndin7 the so)theast side o% the Antamina alley, near its head, in the immediate mine area* The closely spacednortheaststrikin7 %ract)res in the Celend0n ormation strike a.ay %rom the point o% ie. across the steep slope in thelo.er(ri7ht %ore7ro)nd* The %ract)res are less stron7ly deeloped alon7 strike to the northeast on Cerro Aparina8)pper ri7ht:*

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$ mo!el for the geologic history of the $ntamina region

The 7eolo7ic history o% the Antamina re7ion %rom the %ormation o% the $est Per)iantro)7h in the O)rassic to the

+Q* 1=* Se7mentation and artic)lation o% the aran thr)st and %old 5elt and its eIect on Cretaceo)ssedimentation* 8a: Simpli'ed 7eolo7y o% the .estern Andes o% central and north(central Per), sho.in7 the maRoranticlinal axes, %a)lts and se7ment 5o)ndaries in the aran thr)st and %old 5elt 8-enaides, 1999:, the maRorpl)tonic centers o% the Coastal 5atholith and Cordillera -lanca 5atholith 8Pitcher et al*, 196>:, and the n)m5eredlocations o% the sections )sed in 85:* The maRor prod)cin7 mines, Manacocha, Pierina, Antamina and Cerro de Pascoare also sho.n* 85: &on7it)dinal %ence dia7ram o% the Cretaceo)s strati7raphic section o% central and north(centralPer) alon7 the eastern mar7in o% the aran 5elt* Sections -(14, -(1>, -(1=, and -(19 %rom -enaides 819>=:,sections =, 9, 11, 14, 1", 2#, and 22 %rom $ilson 819=3:, and section D %rom anriH)e 81996:* Antamina is located5et.een sections = and -(19*

iocene intr)sion and %ormation o% the ore5ody is s)mmari;ed schematically in i7)re 1"*+n this model, the mar7inparallel $est Per)ian tro)7h %ormed d)rin7 the iddle or &ate

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O)rassic 5y extension on en echelon, north.est(strikin7 normal %a)lts separated 5ynortheast(strikin7 trans%orm %a)lts 8i7* 1"a, 5 7ard, 196":* The normal %a)lts on the.estern mar7in o% the 5asement are th)s ri7ht(steppin7, 5)t the se7ments experienced norelatie displacement and each northeast(strikin7 trans%orm %a)lt experienced onlysinistral moement* This distri5)tion o% 7ro.th %a)lts in the O)rassic .o)ld res)lt in

promontories and reentrants in

the mar7in o% the$est Per)ian tro)7h, similar tothose o% the lar7er(scale earlyPaleo;oic eastern mar7in o% Borth America 8Thomas, 19"":*

The E)erococha archcoincided .ith an intermittenttopo7raphic hi7h that deelopedat least in the iddle O)rassic, oreen thro)7ho)t theississippian to iddle O)rassicinteral, and .hich also

in)enced the distri5)tion o% Cretaceo)s car5onate rocks,Paleocene red 5eds, and iocenei7neo)s rocks* +t is apparent thatthe deelopment o% thenortheaststrikin7 5asementstr)ct)res predated O)rassic(Cretaceo)s sedimentation* TheE)erococha arch apparentlyin)enced the distri5)tion o% ississippian to &o.er O)rassicrocks northeast o% Antamina,res)ltin7 in either localnondeposition in theississippian to arly O)rassic ora iddle to &ate O)rassicerosional )ncon%ormity 8i7* 1"5:*Additional minor extension atany an7le discordant to the

northeast(trendin7 %a)lts co)ld hae res)lted in reactiation o% the ori7inal ly sinistral %a)ltsas north(side(do.n normal %a)lts

+Q* 1"* /e7ional(scale schematic dia7rams, lookin7 east, ill)stratin7 the proposed str)ct)ral eol)tion o% theAntamina area* 8a: +n the iddle or early &ate O)rassic, the $est Per)ian tro)7h started to deelop thro)7h %ormationo% an en echelon pattern o% le%t(lateral 7ro.th %a)lts on the .estern ed7e o% the aran metamorphic complex* Theississippian to &o.er O)rassic sedimentary rocks that oerlay the metamorphic complex are remoed %rom thedia7ram %or clarity 8a%ter 7ard, 196":* 85: Also in the iddle or &ate O)rassic, minor extension at an an7le to thetrans%orm %a)lts se7mented the continental 5asement, prod)cin7 on the promontories str)ct)ral hi7hs parallel to theori7inal oIsets, s)ch as the E)erococha arch, alon7 .hich the ississippian to &o.er O)rassic sedimentary rocks .ereeroded* +n the &ate O)rassic the Chicama Qro)p .as deposited in the .estern, deeper(.ater part o% the 5asin, notill)strated* 8c: Cretaceo)s clastic and car5onate 8QoyllarisH)is7a and achay Qro)ps: sedimentation on the Ma)lishel% .as not stron7ly controlled 5y the se7mentation altho)7h car5onate %acies may hae extended %arther sea.ard

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adRacent to promontories* 8d: &atest Cretaceo)s to Paleocene deposition o% red 5eds 8checker5oard patterned: .ascontrolled 5y the se7mentation, .ith depocenters in the reentrants* 8e: +n the ocene +ncaic oro7eny, maRor strikedeections and en echelon %old patterns that con%orm to the se7mentation o% the 5asement deeloped in thearan thr)st and %old 5elt* /eentrants 5ecame str)ct)ral salients, and promontories 5ecame str)ct)ral recesses*8%: +n the iocene, additional )pli%t o% the E)erococha arch reactiated 5asement str)ct)res, .hich allo.ed intr)sionsto extend %arther inland alon7 the arch and to reach shallo. leels* A V Antamina mine, CP- V Casma( Pasto -)eno;one, E V E)erococha arch*

sinistral %a)lts as north(side(do.n normal %a)lts and hence erosion o% preexistin7sedimentary rocks %rom the transerse hi7hs 8i7* 1"5:* The eIect o% the E)erococha archin the &ate O)rassic cannot 5e ded)ced 5eca)se the @pper O)rassic Chicama Qro)p is notexposed in the eastern part o% the aran thr)st and %old 5elt*

+n the Cretaceo)s, the E)erococha arch and other transerse str)ct)res thatse7ment the aran thr)st and %old 5elt had little apparent eIect on the thickness o% clastic 8QoyllarisH)is7a Qro)p: and car5onate 8achay Qro)p: sedimentary rocks on the

Ma)li shel% 8i7s* 1=5 and 1"c:, 5)t it appears to hae aIected the distri5)tion o% thecar5onate rocks* The achay Qro)p does not extend as %ar .est to the north o% the archas it does on the arch and to the so)th o% it* rom the arch north the .esternmost limit o% o)tcrop o% the achay Qro)p there%ore crosses the 5elt in a northeastso)th.est direction8i7* 1:* Thomas 819"": noted a similar relationship 5et.een the extent o% car5onate %aciesand the locations o% str)ct)ral salients and recesses in the Appalachians* The distri5)tiono% latest Cretaceo)s and Paleocene red 5eds has also 5een in)enced 5y the transersestr)ct)res8i7* 1"d:* The red 5eds pinch o)t to.ard 5oth the E)erococha arch and the Casma(Pasto-)eno ;one, and they attain their 7reatest thickness 5et.een these arches in a str)ct)ralsalient, .hich .o)ld hae 5een a reentrant anddepocenter in the mar7in o% the $est Per)ian tro)7h at the time o% sedimentation*

Shortenin7 and ariations in the re7ional attit)des o% %olds and thr)st %a)lts7enerated in the ocene +ncaic oro7eny are represented in i7)re 1"e* The %olds and

thr)sts in the Antamina re7ion constit)te an artic)lated str)ct)ral recess in the mar7in o% the craton, .hich pro5a5ly %ormed thro)7h de%ormation aro)nd a 5asement promontory,the north.estern ed7e o% .hich is no. delineated 5y the E)erococha arch* -eca)se theorientation o% %olds and thr)st %a)lts in thinskinned tectonic 5elts 7enerally reects the)nderlyin7 ramps rather than the translation direction o% de%ormation 8Pohn, 2###:, thestrike and artic)lation o% the aran thr)st and %old 5elt mimic the 7eometry o% the5asement, despite ariations in the Ceno;oic plate coner7ence direction and rate 8Pardo(Casas and olnar, 196" Somo;a, 1996 Bora5)ena et al*, 1999:* ld trans%orm %a)lts inthe mar7in o% the $est Per)ian tro)7h did not experience extensie later strike(slipmoement 5eca)se the maxim)m shortenin7 direction .as not parallel to the orientationo% moement d)rin7 ri%tin7*

$e propose that the sin)o)s con'7)ration o% the mo)ntain 5elt 7enerally

reprod)ces the ori7inal ;i7(;a7 mar7in, altho)7h the artic)lation is prono)nced on theeastern side o% the 5elt, and shortenin7 in the interior o% the 5elt had a smoothin7 eIecton the se7mentation* Th)s, the re7ional strike at the .estern extent o% the Cretaceo)scar5onate strata 7rad)ally chan7es thro)7ho)t a distance o% approximately 1"> km alon7strike, %rom northerly near Antamina to north(north.esterly at the north.est end o% theCordillera -lanca 8i7* 1:* As Thomas 819"": concl)ded in the context o% eastern BorthAmerica, the aran thr)st and %old 5elt has %ormed a 5est('t c)re aro)nd oldpromontories and reentrants*

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The persistence o% iocene i7neo)s rocks %arther %rom the main axis o% thema7matic arc into the %oreland thr)st and %old 5elt alon7 a proposed 5asement transersestr)ct)re 8i7* 1"%: is consistent .ith o5serations in other 5elts* +7neo)s intr)sions hae5een doc)mented directly oer, and elon7ated parallel to, three o% the %o)r lateral rampsin the Appalachians analy;ed in detail 5y Pohn 82###:* )rthermore, the diIerence inexpos)re leel o% the iocene i7neo)s rocks on and adRacent to the so)th.est end o% the

arch implies that post(ocene )pli%t enhanced the pl)n7e reersals o% the ocene %oldsacross the arch 8&oe et al*, 2##1:* Th)s, %a)lts parallel to the arch may haeaccommodated its )pli%t and %)rnished the str)ct)ral anisotropies that proidedcond)its%or ma7ma ascent in the late iocene*

-egional eects on local structure an! mineraliation

At Antamina, the northeast(trendin7 %ract)re set, the [alley %a)lt and the [alley lateralramp are parallel to the re7ional cross(strike str)ct)ral discontin)ity, the E)erococha arch,and may hae 5een controlled 5y similarly oriented )nderlyin7 5asement str)ct)res* Atthe local scale, the [alley lateral ramp and the Antamina intr)sion hae 5een locali;ed 5ythe le%t(steppin7 Ro7 in the [alley %a)lt* /e7ionally, a5o)t > km so)th.est o% Antamina, thenortheast(trendin7 loc)s o% chan7es in strike and pl)n7e in the aran thr)st and %old5elt steps le%t 5y approximately 6 km 8i7* >:* Th)s the step in the [alley %a)lt deeloped.ithin, and mimics, a similar, lar7er(scale, le%t(steppin7 Ro7 .ithin the E)erococha arch*

The local(scale str)ct)ral eol)tion o% the Antamina area, incl)din7 intr)sion and%ormation o% the ore5ody in the late iocene, is s)mmari;ed schematically in i7)re 16*$hereas other models, s)ch as en echelon %oldin7, co)ld explain the apparent dextraloIset o% the Antamina anticline, the lateral ramp model is pre%erred here 5eca)se itproides a loc)s %or later, northeast(elon7ated intr)sion and hydrothermal actiity* Thenortheast(strikin7 %ract)re set in the host rocks peripheral to the ore at Antamina 8i7s* 1>and 16a: .as, .e contend, %ormed 5y de%ormation associated .ith thr)st translation alon7the )nderlyin7 and similarly oriented trans%er %a)lt or lateral ramp 8i7* 14:* The oerall%orm o% a thr)st sheet does not record its passa7e oer a lateral ramp 5eyond the limit o% that ramp 5)t, in oerlyin7 thr)st sheets, lon7it)dinal %ract)res .o)ld %orm in the lateralanticline oer the ramp* These %ract)res .o)ld hae sheared ertically in a north(sideYdo.n sense or opened o.in7 to ex)re a5oe the lateral ramp or trans%er %a)lt and .o)ldpersist 5eyond it 8i7* 1>d:, proidin7 eidence that a thr)st sheet had traersed s)ch aramp* At Antamina, the in%erred [alley lateral ramp extends only >## m %rom one oIsetanticlinal axis to the other, 5)t stron7, northeast(strikin7, nearly ertical %ract)rin7,interpreted herein as eidence o% tearin7 in the oerridin7 thr)st sheet, extends %arthernortheast 8i7s* 1> and 16a: and is interpreted as trace eidence o% the [alley lateralramp* The )p.ard(%annin7 axial planar cleaa7e related to the Antamina anticline also%ormed at this time 8i7* 16a:* +n this model, the le%t(steppin7 Ro7 in the trans c)rrent[alley %a)lt locali;ed the ocene deelopment o% the lateral ramp that res)lted in%ormation o% the oIset anticlines and also %oc)sed the iocene i7neo)s actiityresponsi5le %or the Antamina stock and skarn* Prior to intr)sion o% the main porphyrymass, the early east(strikin7 dikes east o% the main intr)sion %ormed in association .itheast(.est shortenin7 locali;ed at the northeast end o% the so)th.estern se7ment o% the

[alley %a)lt 8i7* 165:* any o% the peripheral dikes and minerali;ed eins 5eyondthe main porphyry 5ody intr)de, or 5ranch %rom, the same transc)rrent str)ct)res* Thedike 5ranches .ere controlled 5y other preexistin7 str)ct)res, predominantly 5eddin7 and+ncaic thr)sts, and they may hae intr)ded at this time* +n the O)masha ormation the

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deelopment o% .hite mar5le .as locali;ed 5y preexistin7 axial cleaa7es, stylolites, and%ract)res, proidin7 early )id path.ays* Also, in the Celend0n ormation calc(horn%els%ormation .as locali;ed in many places .here the northeast(strikin7, nearly ertical%ract)re set .as stron7ly deeloped* The primary control on the skarn and some o% the5reccia ;ones .as the mar7in o% the main intr)sion, .hich .e ar7)e .as itsel% locali;ed 5ythe post)lated step in the [alley %a)lt* The north.est and so)theast mar7ins o% the main

intr)sion appear to 5e de'ned 5y the northeast(trendin7 [alley %a)lt, and the main masso% the intr)sion occ)pies the le%t(steppin7 7ap 5et.een the t.o oIset se7ments o% thisstr)ct)re 8i7s* 35 and 11:* +n s)ch a system, any relaxation in east(.est compressionco)ld allo. sinistral moement on o5liH)e northeast( trendin7 str)ct)res s)ch as the[alley %a)lt and %ormation o% extensional north(so)th(oriented str)ct)res .ithin the le%t(steppin7 Ro7 8i7* 16c:, there5y proidin7 a loc)s %or intr)sion 8i7* 16d:* Deelopment o% northeast(strikin7 5reccia ;ones .ithin skarn on the north.est and so)theast mar7ins o% the intr)sion and north(strikin7 5reccia and endoskarn ;ones .ithin the intr)sionrepresent, in this model, str)ct)ral reactiation o% the maRor transc)rrent %a)lts and theminor north(so)th extensional %a)lts, respectiely*

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+Q* 16* Schematic dia7rams, lookin7 east, ill)stratin7 the proposed local(scale str)ct)ral eol)tion o% the Antaminaarea* 8a: Str)ct)ral elements in the Antamina area prior to intr)sion and minerali;ationZ %rontal thr)st ramps oIset 5ythe [alley lateral ramp, .hich .as locali;ed 5y a Ro7 in the transc)rrent [alley %a)lt %a)lt(5end(%old anticlinesoerlyin7 the tops o% the oIset %rontal ramps stron7ly deeloped northeast(strikin7 %ract)rin7 parallel to andoerlyin7 the lateral ramp and )p.ard%annin7 axial cleaa7e in the oerlyin7 anticlines* 85: ast(.est compressiono5liH)e to the [alley %a)lt ind)ced north(so)th extension and allo.ed intr)sion o% dikes in .in7 cracks locali;ed at theend o% one se7ment o% the [alley %a)lt* 8c: /elaxation o% east(.est compression allo.ed east(.est extensionmani%ested as north(so)thYstrikin7 normal %a)ltin7 in the Ro7 5et.een the t.o se7ments o% the [alley %a)lt* 8d:on;o7ranitic ma7ma intr)ded the parallelo7ram(shaped extensional re7ime 5et.een the t.o se7ments o% the[alley %a)lt, and )ltimately 7enerated skarn ore* 8e: /ene.ed east(.est compression %ormed east(.estYstrikin7 einsin skarn*

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Altho)7h the recrystalli;ation o% the host rocks d)rin7 skarn deelopment o5sc)redeidence o% controls 5y preexistin7 %ract)res or 5eddin7, .e ded)ce that the intersectin7str)ct)res that locally control ore 7rades in skarn may hae ori7inated at s)5stantiallydiIerent times* Terrones 819>6: reported hi7her exoskarn ore 7rades .here a set o% 1##minerali;ed sheeted eins intersects str)ct)res extendin7 %rom a north.est(strikin7anticlinal axis in mar5le, and .hich .e interpret as )p.ard(%annin7 axial planar cleaa7es*

The intersectin7 sheeted ein set diIers in orientation %rom the northeast(strikin7 %ract)resystem related to the lateral ramp descri5ed a5oe* $e propose that these eins, .hichc)t skarn and .ere there%ore late, %ormed in tension %ract)res associated .ith rene.edeast(.est shortenin7 8i7* 16e: a%ter the 5rie% episode o% relaxation* Th)s an +ncaic axialplanar cleaa7e .as intersected 5y iocene tension eins and deeloped a permea5lepath %or hydrothermal )ids*

Im&lications for local e/&loration an! ore genesis

The s)rro)ndin7 ;ones o% mar5le in the O)masha ormation, and partic)larly o% horn%elsand skarnoid in the Celend0n ormation, proide lar7er exploration tar7ets than the skarn

itsel%* An isometric 5lock dia7ram o% the simpli'ed 7eolo7y o% the deposit 8i7* 19: sho.sthe str)ct)re and alteration in the adRacent host rocks* Qray mar5le in the O)mashaormation 7enerally extends tens o% meters 5eyond skarn

copper skarn deposits

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