1990

EconomicGeology
Vol. 85, 1990, pp. 1604-1628
Geomorphologic
EnvironmentandAge of SupergeneEnrichment
of the Cuajone,Quellaveco,andToquepalaPorphyry
Copper Deposits, SoutheasternPeru
ALAN H. CLARK,
Departmentof GeologicalSciences,
Queen'sUniversity,Kingston,Ontario,CanadaK7L 3N6
RICHARD M. TOSDAL,
U.S. GeologicalSurvey,345 MiddlefieldRoad,Mail Stop901, MenloPark,California94025
EDWARD FARRAR,
Departmentof GeologicalSciences,
Queen'sUniversity,Kingston,Ontario,CanadaK7L 3N6
AND ARMANDO
PLAZOLLES
V.
SouthernPeru CopperCorporation,Casilla 303, Tacna,Peru*
Abstract
Copperoregradesof theCuajone,Quellaveco,
andToquepala
porphyryCu(-Mo)deposits,
situatedat 3,000 to 4,000 m a.s.1.on the Pacificslopeof the CordilleraOccidentalof south-
ernmostPeru (lats17ø02'-17ø15
' S), havebeenmarkedlyincreased
by supergene
sulfide
enrichment.
Followingthe emplacement
of the hypogene
mineralization
in the earlyEocene
(52-57 Ma) asthe terminalstagein the development
of the ToquepalaGroupcontinental
volcano-plutonic
terrane,thisAndeantransect
wasgradually
reducedby erosionin a semiarid
climateto a low altitudetopography;
unroofingof the deposits
hadtakenplaceby the midOligocene.
Theinitiationofcordilleran
upliftatca.25 to 26 Ma,accompanied
by theepisodic
eruptionof felsicash-flow
tuffsat theoceanward
frontof thevolcanic
arc,ledto morerapid
erosionandtheconversion
of themid-Tertiarylandscape
intothesubplanar
Altosde Camilaca
surface.
Thisregionallyextensive
pediplainconstitutes
the majorlandformcomponent
of the
presentprecordillerasurrounding
the porphyrycenters;its finalconfiguration
wasattained
at ca. 18 to 19 Ma. Ash-flowtuff eruptionwaswidespread
andfrequentat thistime.
Supergene
enrichmentbeganin the late Oligoceneduringthe progressive
loweringof
topography
andcontinuedthroughthe moreabruptwater-tabledepression
resultingfrom
the latestOligoceneto early Mioceneuplift. However,the distributionof chalcocite,sensu
lato, in the three porphyrydepositsand the local postmineralization
landformsandvolcanic
historiesdifferedsignificantly
duringthe latter interval.At Toquepalaand,to a lesserextent,
Quellaveco,the landformregimesweredominatedby openvalleysandignimbriteblanketing
was short-lived,whereasthe exposedCuajonedepositwasboth the site of aggressive
early
Miocenevalleyincisionandthe depositionof anunusuallythick,in part welded,ash-flowtuff
at 22.8 ñ 0.7 Ma. As a result, the enrichmentblanketat Cuajoneremainedrelativelythin
andwasevenpartiallyerodedin the early Miocene,whereasthe blanketsat Quellavecoand
Toquepalawere thickened.
Continuedstrongupliftin the mid-Miocene
(ca.8-15 Ma) generated
the arrayof apron
andterracepedimentsof the MultiplePedimentstagewhichdominates
the lowercordilleran
slopesand,in the mineralizedareas,led to the deepening
of existingvalleysandthe devel-
opmentof newfluvialchannels.
Ignimbrite
eruptionpersisted
throughout
theMiocene.Again,
localregimes
of erosionandvolcanism
provedinimicalto supergene
activityat Cuajone,but
enrichmentcontinuedat Quellavecoand, particularly,Toquepala,where there is no record
of laterMioceneash-flow
accumulation.
Upliftin thisperiodwasapparently
alsomostextensive
in thevicinityof theToquepala
deposit,whichexperienced
theformation
of a deepchalcocite
blanket,while the extantenrichmentzoneat Quellavecowasthickened.
The development
of enrichedassemblages
containing"massive"chalcocitehad clearly
terminatedby 13.1 ___
0.4 Ma at Cuajoneandby 9.5 ___
0.5 Ma at Quellaveco;the timingof
laterenrichment
atToquepala
islessconstrained,
but a mid-Miocene
ageisprobable.Supergene
* Presentaddress:MMagaGrenet316, Umacollo,Arequipa,Peru.
1604
CUAJONE,QUELLAVECO,AND TOQUEPALA
DEPOSITS,SEPERU
1605
upgradingof the three Peruviandepositswascontemporaneous
with that of coppermineralizationin northernmostand northernChile, asat Chuquicamata
andLa Escondida,and in
the Copiap6miningdistrict,in all of whichareasa late Oligoceneto mid-Mioceneagehas
been inferredfor supergenealteration.This major and regionallydevelopedmetallogenic
episodetookplaceduringandbetweentwo majorperiodsof cordilleranuplift, undersemiarid
climaticconditions.
The terminationof intenseenrichmentalongthis2,000-kmstretchof the
Cordillera Occidental in the late Miocene wasa direct result of marked climatic desiccation,
whichnot onlyreducedthe overallsupergene
activitybut focusedfluvialerosion,leadingto
the incisionof steep-walled
canyonslessfavorablefor supergene
alterationthanthe earlier
subplanarlandforms.
Introduction
work of Segerstrom(1963), definedempiricalcorrelationsbetweenepisodesof supergeneenrichment
of regionallyexment in the later Tertiary have contributedto the of copperandsilveranda sequence
developmentof high coppergradesin manyof the tensivesubplanarerosionallandforms,the agesof
Cretaceous
andPaleogene
porphyrycopperdeposits which were delimited by K-Ar geochronologyof
in the central Andes of Peru and Chile, as in other overlyingignimbrite flows.FollowingHollingworth
in thisdeserticarea,close
comparable mineralized regions. However, the (1964), the erosionsurfaces
weatheringof thesemajordeposits
hasreceivedlittle to the southern limit of the Atacama Desert, were
concertedstudyfromtheperspective
of regionaland interpretedaspedimentsor, whereregionallyextenlocal geomorphology,
despitethe widely accepted sive,pediplains,whichwere inferredto haveformed
to episodes
of abruptepeirogenic
importanceof landforms
in the development
of su- rapidlyin response
flankof the presentCordillera
pergeneprofiles.This paper addresses
the physio- upliftof the oceanward
graphicenvironmentandageof supergenealteration Principal.Two distinctperiodsof metal enrichment
in the Eocene porphyry Cu(-Mo) subprovinceof were recognized,eachascribedto a specificepisode
It wasassumed
thatsupersouthernmost
Peru(Fig. 1), with particularreference of landformdevelopment.
to the Cuajone(17ø02' S-70ø42' W) andToquepala genesulfideformationwasmostactiveduringthe pe(17ø15' S-70ø3T W) mines of the SouthernPeru
CopperCorporation(SPCC)andthe nearbyQuellavecoprospect(17ø06' S-70ø37' w) of Mineroper6.
Each depositis situatedat elevationsbetween 3,000
and 4,000 m a.s.1.on the Pacificslopesof the CorSUPERGENE oxidation and attendant sulfide enrich-
dilleraOccidental.The mineralizedareanowexperiencesan arid climate(Meigs, 1953; InstitutoNacionalde Planificaci6n,1965) andisborderedto the
BOLIVIA
southwest
by the hyperaridPeruvianCoastalDesert,
the northern extension of the Chilean Atacama Desert
(di Castri and Hajek, 1976). However, it has been
widelyinferredongeomorphologic
andothergrounds
(e.g.,Mortimer,1969, 1973) that semiaridconditions
prevailedin thisregionuntil the onsetof drasticcli-
CHUQUICAMATA©
• /
matedesiccation
in thelateMiocene(e.g.,Alpersand
Brimhall, 1988).
ßLA ES(•ONDIDA
ß25 ø
/
The time scaleof Berggrenet al. (1985) is used
here, although the term mid-Miocene rather than
middleMioceneis employedinformallyfor intervals
significantly
transgressing
the boundaries
of the latter
time-stratigraphicunit, and mid-Oligoceneis usedto
..
ß EL SALVADOR
COPlAP6
DISTRICT
•;.
/
./! ,
ARGENTINA
referbroadlyto the middlepartof thatepoch.
Previousresearchon supergeneenrichmentin the
Andean deposits
Detailedinvestigations
(Clarket al., 1967a,b; Sillitoe et al., 1968; Mortimer, 1969, 1973; Sillitoe,
1969) of Mesozoicto Paleogenevein systems
in the
broaderCopiap6miningdistrictofnorth-central
Chile
(ca.lat 26ø-29øS;Fig. 1),buildingonthepioneering
:1
/
:,.•i•
i
• 7øøw
o
200
500
k ' * -- km
85ø
FIG. 1. ' Map showingthe Cuajone,Quellaveco,andToquepala
porphyry copper depositsin southen•Peru, and other mineralized
areas cited in the text.
1606
CLARK,TOSDAL,FARRAR,
AND PLAZOLLES
riods of relative tectonicquiescenceand reduced and Toquepala deposits subsequentto hypogene
erosionalrate interveningbetween the incisionsof mineralization and to correlate those histories with
the associated
erosionsurfaces.
The earlierepisode the distributionof supergeneore mineralsat eachsite.
ofenrichment
wasfoundtobeasoldasearlyEocene, K-Ar dating of felsicash-flowtuff unitspermitsthe
whereas
thelaterenrichment
periodwaslessprecisely delimitationof the agesof landformswhich underlie
assigned
to the late Oligoceneandearly (or middle) or truncate them in the mine areas. The occurrence
Miocene interval. The prevalenceof an arid or semi~ andnatureof supergenemineralshavereceivedonly
arid climatein thisregionthroughoutthe Paleogene reconnaissance
study(Horlicket al., 1981; Satchwell,
is implied both by the nature of the dominantland- 1983; Clark, in prep.). Both "massive"and "sooty"
formsandbythewidespread
occurrence
ofevaporites supergenesulfideassemblages
occurin the three dein the continentalsupracrustal
cover.All significant posits.As Sillitoe and Clark (1969) have shown,the
sulfideenrichmentoccurredpriorto the middleand initial stages,or lower limits, of enrichmentzonesare
lateMiocene(•12.9-9.8 Ma; agesrecalculated
with characterizedby the developmentof powderyaggrethe decayconstants
of SteigerandJ•iger,1977) de- gatesof coppersulfides,whereasmore extensiverevelopmentof the very extensiveAtacamapediplain, placementof hypogenemineralsresultsin coarser
whichwidely truncatedpreexistingsupergenepro- grained, sectile massesof the supergenesulfides.
files.Renewedtectonicuplift, andinferredclimatic• Sooty assemblages
are, however, also developedin
desiccation
duringthe latestMioceneto Pliocenein- the early stagesof oxidativeconversionof chalcocite
terval(ca.9-2 Ma), wasaccompanied
by the termi- anddjurleiteto moresulfurrich coppersulfides(e.g.,
nationof pedimentation
andthe restrictionof major anilite; Clark and Sillitoe, 1971). At Cuajone and
erosion to the channels of the main exorheic rivers
Quellaveco,the singleenrichmentblankets,or chal(Mortimer,1973). Only limitedoxidationandsuper- cocitezones,aredefinedlargelyonthe basisofmegaficialsulfideenrichmentof theveinsystems
occurred scopicmineralogicalrelationshipsand copper ore
at thistime (Segerstrom,1963; Sillitoeet al., 1968). grades.At Toquepala,at leasttwo spatiallyseparated
Supergeneprocessesare all but inactive under the enrichmentzoneshavelongbeenrecognized(Richard
hyperaridclimaticconditionsnow prevailingin the and Courtright, 1958), but the irregulardistribution
Chileannortechicobelowca.3,000 m a.s.1.(di Castri of supergenechalcocite(sensulato) hasnot been inand Hajek, 1976; UNESCO, 1980).
vestigatedin detail.Regardless
of thesedeficiencies,
In the E1Salvadorporphyrycopperdeposit(Fig. we considerthat the enrichedzonesin eachdeposit
1), the intensesupergene
enrichmentof the upper are sufficientlydelimitedto permit their broadcorEocenestockworkmineralization
maynothavebeen relation with stagesin local landformevolution.
contemporaneouswith that which affected the vein
systemsof the nearbyCopiap6district.On the basis
of K-Ar datingof supergenealunite,Gustafson
and
Hunt (1975) concluded
that supergene
upgradingat
E1 Salvadorbeganat ca. 37 Ma, within 5 Ma of the
emplacementof the hypogeneores.Mortimer et al.
(1977), however, have shownthat oxidationand enrichmentof the ca. 29-Ma Chuquicamata
porphyry
deposit,located450 km to the north(lat 22o19'-20'
S; Fig. 1), and the concomitantformationof the contiguousEx6tica(MinaS6r)chrysocolla
mineralization,
occurredin the late Oligoceneto earlyMioceneand
wasthusessentiallycoevalwith the later enrichment
in theCopiap6district.Alperset al. (1984)andAlpers
andBrimhall(1988) morerecentlypresentedmiddle
MioceneK-Ar datesfor supergenealuniteassociated
with the intense sulfide enrichment at the La Escon-
didaporphyryprospectat 24o16' S (Fig. 1). It was
concludedthat supergenesulfideenrichmentat La
Escondidaoccurredin the mid-Tertiary, and prior to
the lateMiocene,asin the Coplap6andChuquicamata
districts.
Methodologyof the presentstudy
Our aimsin the presentresearchwere to establish
the geologic-geomorphologic
evolutionof the areas
immediatelysurroundingthe Cuajone,Quellaveco,
It shouldbe emphasizedthat our methodologydiffers radicallyfrom that employedby Gustafsonand
Hunt (1975) andAlperset al. (1984) andAlpersand
Brimhall (1988) in their analysesof the age and formationalconditionsof supergeneenrichmentzones
at E1SalvadorandLa Escondida,Chile, respectively.
This is in part becausecareful searchhassofar failed
to revealthe occurrenceof supergenealunitesuitable
for K-Ar dating. More critically, the undoubtedly
complexgeomorphologic
evolutionof the mineralized
areas during the Neogene would introduce unacceptableuncertaintiesto the massbalancecalculations
centralto the modeldevelopedby Alpersand Brimhall at La Escondida,where a muchsimplersequence
of landformeventsattendedthe supergeneprocesses.
The physiographicdevelopmentof the precordillera
of southernmostPeru has played a key role in promotingandrestrictingthe enrichmentof theporphyry
copper depositsand is thus critical to our analysis.
Our interpretationsof the interrelationsof supergenemineralzonesandgeomorphology
arebasedon
the standardsupergeneprofile first definedin detail
by Emmons(1917): a land surfacegeneticallyassociatedwith alterationis underlainsuccessively
by a
leached(lixiviated)zone, an oxidizedzone, a zone of
supergenesulfideenrichment,and the protore. The
transitionfromoxideto sulfideassemblages
isassumed
CUAJONE,
QUELLA
VECO,ANDTOQUEPALA
DEPOSITS,
SEPERU
1607
to correspondapproximatelyto the water tableextant
A simplifiedchronologyof mid- to late Tertiary
at the time of alteration.Becausepyrite-poorhypo- geologicandgeomorphologic
eventsin southernmost
gene mineralassemblages
are not developedin the Peru is providedin Table 1, basedlargely on Tosdal
upper partsof the three depositsunder discussion, et al. (1981, 1984). In outline, four major physiothe broad configurationsof the upper and lower graphicterrainsconstitutethe present-daytopograboundaries of the sulfide enrichment zones are in-
phy (Fig. 2a): (1) the Cordillerade la Costa,a disferred essentially
to reflectthoseof the superjacent continuousbelt of mountainsattainingelevationsof
landforms (cf. Ambrus, 1977; Gullbert and Park,
1986, p. 800-802).
ca. 1,000 to 1,800 m a.s.1.;(2) the LlanurasCostaneras,a slopingfacetedterrain risingnortheastward
from ca. 350 m a.s.1. at the inner foot of the coastal
Geologyand Geomorphology
mountainsto ca. 3,000 m a.s.1.;(3) the precordillera
(informalterm), a subplanarca. 4,000-m benchwith
Selectedfeaturesof the present-daytopographic a steepsouthwesterninterfacewith the Llanuras;and
relationships,
thepost-Eocene
geologic
units,andthe (4) the mainCordilleraOccidental,which at thislatin part glamajorNeogenelandformdomains
oftheareaof study itude is dominatedby stratovolcanoes,
areoutlinedin Figure2a-c. The regionincorporates ciated, and reachinga maximumelevationof 5,815
a transectof the PeruvianCoastalDesert,itselfpart m a.s.1.at VolcftnTutupaca(Fig. 2a).
of the AtacamaDesert,andconforms
physiographi- The porphyrycopperdepositsunderconsideration
callyto thebasinandrangeor mountain
andpiedmont were emplacedin the early Eocene (52-57 Ma, Esdesertsof, e.g., CookeandWarren (1973). The land- trada, 1975; McBride, 1977; Zweng, 1984; Beckinsale
scapeis dominatedby an assemblage
of steepmoun- et al., 1985; Clark et al., 1990) asthe terminalstage
tain slopes,rock-cutalluviatedpedimentsor pedi- in the developmentof an Upper Cretaceousto Paplains,andalluvialplains,traversed
by widelyspaced leogenesubaerialvolcano-plutonic
arccomprising
the
exorheicriver systems,the RiosMoqueguaand Lo- Toquepala Group and a segmentof the Peruvian
cumbaandtheir majortributaries,fedby snowmelt Coastalbatholith (Bellido, 1979; Beckinsaleet al.,
in the high cordillera.
1985).The geologyoftheToquepaladeposithasbeen
of Southernmost Peru
D Post- mid-Miocene
]
I
huntacala
Formation
Huaylillas
Formation
D Moquegua
Formation
!D Pre- Oligocene
rocks
Valley-and-Terrace
Terrain
Multiple-Pediment
Terrain
(PL= Pampa Lagunas Pediment)
Altos
deCamilaca
Surface
'•,•'D
-Basement-
FIG. 2. The study area on the Pacific slope of the Cordillera Occidental,southernPeru. a. The
majorphysiographic
unitsand locationsof the Cuajoneand Toquepalaminesand the Quellaveco
prospect.b. Simplifiedgeologicmapof area(afterBellidoandLanda,1965;modifiedby Tosdalet al.,
1981). c. Geomorphologic
map,showingdistributionof Tertiarylandformdomains(simplifiedafter
Tosdalet al., 1984). Abbreviations:
C -- Cuajone,Q -- Quellaveco,
T = Toquepala.
1608
CLARK,TOSDAL,FARRAR,AND PLAZOLLES
TAI•LE1. CenozoicGeologicandPhysiographic
Evolutionof the PacificSlopeof the CordilleraOccidentalof Southernmost
Peru
(after Bellido, 1979; Tosdalet al., 1981, 1984; Clark et al., 1990).
Geologicevents
Major geomorphologic
events
Pliocene
Developmentof large andesitic-daciticstratovolcanoes
of Barroso
Group (-•5.3 Ma)
Capillune Formation sedimentationand minor volcanism
Valley andTerrace stage:continued
episodicuplift, with incisionof
steep-walledvalleysin
precordilleraand upper Llanuras
Costaneras(ca. 8.5 Ma to present)
Miocene
"SenccaFormation" ignimbrite eruption (ca. 6.5 ___
0.3 Ma)
"Maure Formation" sedimentationin basinsof high cordillera
Multiple Pediment stage:renewed,
episodicuplift, causing
developmentof apronandterrace
pedimentsin LlanurasCostaneras
(9-15 Ma) andof broad,open
valleysin evolvingprecordillera
Final configurationof Altos de
Camilacasurface(ca. 18-19 Ma)
Chuntacala Formation
felsic volcanism and associated clastic
sedimentation(14.2 ___
0.4-8.9 ___
0.6 Ma)
Eruption of youngestignimbriteassignedto HuaylillasFormation
(18.6 _ 0.3-18.3 _ 0.3 Ma)
Emplacementof major early ignimbritesof Huaylillas Formation
(-•22.8 ___
0.7 Ma)
Oligocene
Eruptionof ignimbriteflows(-•29 - 0.2 Ma) anddepositionof
conglomeratichorizonsin uppermostMoqueguaFormation
Accumulationof MoqueguaFormationfine clasticsand evaporites
(? _•45-50 Ma)
Initiation of stronguplift andrapid
erosion,tilting and modifying
mid-Tertiary landscape
Development of subdued,lowaltitudelandscapethroughslow
erosion of Paleocene-Eocene
Eocene
arc
Hypogenemineralization:Toquepala,ca. 57 Ma; Quellaveco,ca.
56 Ma; Cuajone, ca. 52 Ma
PMeoeene
Final stagesof subaerialToquepalaGroupvolcanism(to ca. 59 Ma)
Intrusionof granitoidplutons(ca. 58.7-65.5 Ma)
describedby Richardand Courtright (1958), Zweng
(1984), and Zweng and Clark (1984, and in prep.),
the Cuajonedepositby Lacy (1958), Manrique and
Plazolles (1975), and Satchwell (1983), and the
Quellavecodepositby Estrada(1975), Kihien (1975),
Guerrero and Candiotti (1979), Torpoco(1979), and
Zimmermanand Kihien (1983).
Followinghypogenehydrothermalactivity,a prolonged mid-Tertiary period of erosionensued,with
the gradualreductionof the arc terrain to a topography of subduedrelief which, in the vicinityof the
deposits,maynot haveexceeded500 to 1,000 m a.s.1.
(Table1). The predominantlycontinentalclasticstrata
of the Oligoceneto earliestMioceneMoqueguaFormation (Barfa, 1961; Bellido and Landa, 1965; Bellido, 1979; andMaroccoand Noblet, 1990) constitute
the aggradational
faciesof this erosionalsystem(Fig.
2b). Beginningin the late Oligoceneand continuing
into the early Miocene,a succession
of moreabrupt
uplift episodesresultedin the developmentof a regionallyextensive,generallysubplanar,largely degradationallandscape,now preservedin the precordillera (Fig. 3a) and at the summitsof the Cordillera
de la Costa(Fig. 2a andc). Relationships
in the area
upstreamfrom the town of Moqueguaillustratethat
significantrapid degradationof the precordillerahad
takenplaceprior to 25 Ma, generatingconglomerate
horizonsin the uppermostMoqueguaFormationto
the southwest. The erosional surface attained its final
form immediatelyprior to 18 Ma and is referred to
asthe Altosde Camilacasurface(Tosdalet al., 1984;
Table 1). A clear backscarpis preservedonly locally
for this landform in the immediate study area (e.g.,
Fig. 3d), but an extensivetopographicstep defines
muchof the upper limit of the surfacein the Desierto
de Clemesl north of the Rio Moquegua (Fig. 2); in
the area of the ore deposits,the Altos de Camilaca
surfacehas, in part, the form of a pediment dome
(Mabbutt, 1955; Cooke and Warren, 1973, p. 196).
During the middle and earliestlate Miocene (ca.
15-8 Ma), the Altos de Camilacasurfacewas itself
uplifted, variablytilted and eroded,with the formation of a succession
of apronand terracepediments
(nomenclature
of CookeandWarren,1973),assigned
to the Multiple Pedimentstage(Figs.2c, 3b and c;
Table 1). Broadopen valleysdevelopedat this time
acrossthe precordillera to the northeast.The later
Miocene uplift of the southernPeruviantransectof
the Cordillera Occidentalwasprobablygreaterthan
that which occurred at this time in other transects of
the continentalmargin(Tosdalet al., 1984) andpresumablyrepresentsthe topographicresponseto unusuallyabrupt crustalthickening.Pedimentationin
the Cordillera Occidental, at this asat other latitudes,
ceasedin the late Miocene, contemporaneous
with
inferred climatic desiccationat lower topographic
levels.Sincethat time, steep-walledvalleysandcanyonshave been incisedinto the Pacificslopeof the
mountainsas a result of continued episodicuplift
CUAIONE,QUELLA
VECO,ANDTOQUEPALA
DEPOSITS,
SEPERU
1609
b
c
d
FIG. 3. Landformsof the Cuajone-Quellaveco-Toquepala
district. a. Characteristiclandscapeof
the precordillera:dissectedarea of the lower Miocene Altos de Camilacasurface,near Larampahuane
(10 km north-northeastof Toquepala).View looking east-southeast.
Accordantsummits(ca. 4,1004,200 m a.s.l.)representremnantsof the pediplain,whereasthe southwest-dipping
slopesare defined
by the surfacesof flow units of the Upper Cretaceousor PaleoceneSerie Alta Voltanits. b. Thinly
alluviatedPampaLagunasapronpediment (11-14 Ma), the dominantlocal landformof the middle to
late Miocene Multiple Pediment stagein the upper LlanurasCostaneras;Pampais some20 km westsouthwestof Toquepala.Steepslopesin backgroundrepresentthe back scarpof the pedimentand the
lower slopesof the precordillera.The valleys(QuebradaCuculein foreground)dissectingthe pediment
formedmainlyin the PlioceneValley andTerrace stage.c. View to the southwestfrom Cerro Botifiaca,
at the southwestmarginof the precordillera,acrossthe LlanurasCostanerasand toward the Cordillera
de la Costa.Cerro Bafil,the mesaat right, is undedainby sedimentsof the Upper MoqueguaFormation
and cappedby a 25.3 q-0.8-Ma tuff. The courseof the Rio Tumilaca(left) is flankedby upper Miocene
terrace pedimentsof the later Multiple Pedimentstage.Another tuff, correlativewith that overlying
the Cuajonedeposit(Tosdalet al., 1981), is interbeddedwith the MoquequaFormationand underlies
the apronpedimenton the distant(left) skyline.d. View to the northeastacrossthe upper valley of
Rio Asana,northeastof Quellaveco,in the inner precordillera.Major morainesare drapedacrossthe
dissected
Altosde Camilacasurface.CerroCondoriquifia
(5,085 m), onthe skyline,isprobablyundedain
by the Upper CretaceousToquepalaGroup Voltanits, andthusconstitutesthe localback scarp,albeit
subsequentlyglaciated,of the pediplain.
duringthe still activeValley and Terracestageof initial late Oligocenephasesof rapid uplift. Several
landformdevelopment(Table 1), causingconsider- tuffs are intercalated in the coarse elastic sediments
abledegradation
of the earlierplanateor gentlyin- of the uppermostMoqueguaFormation(Fig. 3c;Belcisedlandforms(Figs.2c and 3).
lido, 1979; Tosdalet al., 1981). Subsequently,
felsic
In the early Neogene, the mineralizedarea was pyroelasticeruptionsaccompaniedthe development
situated,asit is today,at the oceanwardfront of the of both the Altos de Camilaca surface and the landvolcanicare in the precordillera.The eruptionof formsof the MultiplePedimentstage,resultingin the
rhyodacitic
ash-flow
tuffsapparently
beganlocallyat mantlingof wide areasof the precordilleranslopeof
ca. 29 Ma. (R. J. Langridge,pers. commun.,1989), the CordilleraOccidental(Tosdalet al., 1981, 1984).
but becamemoreintenseat 25 to 26 Ma duringthe The widespreadtuffscontemporary
with incisionof
1610
CLARK,TOSDAL,FARRAR,
AND PLAZOLLES
the Altosde Camilacasurfaceare assigned
to the relief, with numerousinselbergsstandingup to 30 to
HuaylillasFormation.In contrast,tuffsthat are predominantly
restrictedto valleyswereeruptedduring
the Multiple Pedimentstageand are groupedasthe
ChuntacalaFormation(Tosdalet al., 1981, 1984).
Thisdistinction,firstrecognizedat Cuajoneby ManriqueandPlazolles(1975), is criticalto the interpre-
60 m abovethe plain, and interspersedwith gravelfilled depressions,the remnants of paleodrainage
channels.The altitudeof thisold surfacein the vicinity
of the mines rangesfrom 3,500 to 4,000 m a.s.1.To
the northeast, it constitutes the foundation for the
youngstratovolcanoes,
the flanksof whichextendto
within 10 km of the deposits.To the southwest,a
the accumulation
of even thin ignimbriteshasbeen shortdistancefrom the mines,an abrupt decreasein
shownelsewhere
to disruptground-water
systems
and altitudemarksthe erosionalbackscarpandthe upper
of the PampaLagunasapronpediment,the
terminatesupergeneactivitybeneathsubjacentsur- extensions
faces(e.g.,Sillitoeet al., 1968), definitionof the time- oldestof the middleMioceneerosionsurfaces(Figs.
spacerelationshipsof the pyroclasticunits and the 2c and 3b). Thus, the porphyrycopperdepositsare
associated
landforms
isclearlyofpotentialimportance intersectedby erosionallandformsof differentorigins
in the prediction of the extent of local enrichment whichextendbackin time to the Paleogene-Neogene
horizons.In the studyarea,the basaltuff overlying boundary(ca.23-24 Ma). Moreover,it is evidentthat
theerodedCuajonedepositisunusually
thick(_<180 the Cuajoneand Quellavecodeposits,and probably
m) andincludesa majorweldedzonecontaining
es- alsotheToquepalaarea,wereblanketedfor sometime
sentiallyundevitrifiedshards.Similarlythe upper by Mioceneignimbritesheets.
tation of the landform evolution of the area. Because
Miocenetuffoverlyingthe Quellaveco
depositin the
Local GeomorphologicSettings
Rio AsanaValley exhibitsrelicsof flattenedshards,
and Enrichment Zones
suggestiveof originalwelding.In general,mosttuffs
of the HuaylillasFormationshowonlyweakdevitri- Cuajonedeposit
ficationand hydrothermalalteration.Those of the
The Cuajonedepositoccursonthe south(left) flank
Chuntacala
Formationareextensively
devitrifiedbut of the 500-m-deepcanyonof the R•o Torata (Fig. 4)
are otherwiseunaltered.We conclude,therefore,that and more extensivelyin its tributary, the Quebrada
the ash-flowtuffsemplacedin the vicinityof the ore Chuntacala,the westerlycourseof which transected
depositspossessed
low permeabilitiesandwere thus the centralpart of the mineralizedzone (Manrique
likely to act asaquicludes.
andPlazolles,1975). Lacy(1958) firstdrewattention
Duringthe Pliocene,a localtransition
fromignim- to the inversionof fluvialrelief whichtookplacedurbriteeruptionto predominantly
andesitic-dacitic
vol- ing the recentgeologichistoryof the area;thus,the
canismculminatedin the developmentof the stra- presentinterfiuvebetween the steep-walledTorata
tovolcanoes
of the BarrosoGroup (Table 1). Radical and ChuntacalaValleysis essentiallycoaxialwith a
coolingof the climate in the Pleistoceneled to the paleovalleywith gently dipping margins,partially
formationof alpineglaciersandto the development preservedthroughinfillingby volcanicflows.
of extensivemoraines,which are well preserved The subplanar(Fig. 5a) northernand southerninnortheastof Cuajoneand Quellaveco(Fig. 3d) but terfiuvesof theTorata-Chuntacala
valleysystem,lying
not in the immediatevicinityof the deposits.
at ca. 3,800 to 4,200 m a.s.1.anddippingat ca. 4ø to
Around the three depositsunder discussion,
the the west-southwest,
are largelydefinedby the upper
precordilleranlandscape(Figs.2a and 3a) is domi- surfaces
of ash-flowtuffsof the HuaylillasFormation,
natedbya high,southwest-sloping
plain,representingwhichlocallycomprises
up to 280 m of predominantly
both the degradational
portionof the Altosde Cam- rhyodaciticash-flowtuffswith minorconglomerates
ilacasurface
and,overwideareas,thegentlydipping and agglomerates(Manrique and Plazolles,1975).
surfaces
of ignimbriteflowsbroadlycoevalwith that
In the immediate Cuajone mine area, the basal
landform.The plainhasbeenvariablydissected
by memberof the HuaylillasFormationisa thickwelded
west- to southwest-trending
valley systems.Some tuff (Fig. 5b; the "trachyte" of Manriqueand Plavalleysoriginallypossessed
broadshallowcrosssec- zolles,1975, p.), 22.8 +__
0.7 Ma in age (Tosdalet al.,
tions and representedthe uppermostextensionsof 1981). The tuff overliesan irregular,southwesterly
the middle and upper Miocene pediments,which inclinedtopographywith localrelief up to 120 m (ca.
constitute much of the Llanuras Costaneras at lower
3,580-3,700 m a.s.1.).This flow is also coeval with
altitudes.Theywere subsequently
extensively
mod- an unweldedtuff intercalatedin the uppermostMoified duringthe ValleyandTerracestage,whereas queguaFormation(Fig. 3c) 30 km to the southwest
othervalleyswere initiatedduringthe latter stage, (Tosdalet al., 1981). Ash-flowtuffs,therefore,probanddisplaysimpleV-shaped
profiles.Priorto mining, ablycoveredmuchor all of the Cuajoneareaat this
the porphyrycopperdeposits
were exposedin com- time. We considerthe undulatingerosionaltopogpositepolystagevalleys.
The Altos de Camilaca surface of the immediate
areaischaracterized
bya significant
degreeofprimary
raphybeneaththe HuaylillasFormationto represent
an early stage in the developmentof the Altos de
Camilaca surface.
CUAJONE,
QUELLA
VECO,ANDTOQUEPALA
DEPOSITS,
SEPERU
588,000
8e.ooo
1611
E
N
FIG.4. Simplifiedpreminingphysiographic
mapof the Cuajoneminearea.Contourintervalis33.3
m. Shadedarea,coveringmuchof the interfiuves
betweentheCocotea(southof maparea),Chuntacala,
and Torata valleys,indicatesapproximateextent of subplanarO!igoceneand Miocenelandscape,a
confiation
of Huayli!!as
Formationash-flowtopsandthe erosional
Altosde Camilacasurface.Areaof
Figure 6, centeredover the Cuajonemine, is outlined.
thattheseunits
In QuebradaCocotea,the activedrainagesystem well (pers.commun.,1986)concluded
Formation,
withtherhyolitic
immediatelyto the southof QuebradaChuntacala arepartoftheHuaylillas
probablyrepresenting
its localbasal
(Fig. 4), the basaltuff attainsa maximumthickness
of conglomerate
postdating
thebasaltuff.
180 m, displaysa completeweldedtuff profile (Tos- memberandtheagglomerate
isin partunderlainby thinmudflows
dal, 1978), andisunderlainby a ca. 100-m-thickcon- The agglomerate
glomerate(Manrique and Plazolles,1975). These that containclastsidenticalto partsof the tuff.
The conglomerate,
the lower contactof whichhas
featuresstronglysuggestthat a significantvalley existedin that area early in the evolutionof the Altos been intersected at elevations of ca. 3,565 to 3,670
de Camfiacasurface.Consolidation
of thismajorash- m (Fig. 6), probablyaccumulated
in a valleyof modflowtuffwouldprobablyhavedisplacedthe mainlocal erate depth in the pre-HuaylfilasFormationtopogdrainageto the southandnorth, in the latter caseto raphy, perhapscontinuouswith the palcovalleyunderlyingthe presentQuebradaCocotea.The grayagthe presentpositionof QuebradaChuntacala.
Detailedmappingof the Cuajoneopenpit in recent glomeratefilled in a relativelydeep and probably
years(Satchwell,1983; F. B. Stevenson,writ. com- moresteep-walledchannel,the floorof whichlay at
mun., 1983) hasoutlinedseveralpostorestratigraphic elevations of ca. 3,475 and 3,430 m in the east and
units (Figs.5c and 6) not clearlyidentifiedon the westquadrants,
respectively,of the presentopenpit
preminesurface.Theseincludea rhyoliteclastcon- (Fig. 6). Thus, significantvalley development,apglomerate,exposedat the easternextremity of the proximatelyparallelto the more recent Quebrada
mineralizedzone,anda younger"gray agglomerate" Chuntacala,is inferred to have occurred in the im(a local informal term), exposedin the western and mediatevicinityof the Cuajoneorebodyprior to and
of
eastern sectorsof the pit. Manrique and Plazolles alsoat somelater stageduringthe localdeposition
the youngest
local
(1975, fig. 2) mappedthe westernarea of the latter the HuaylillasFormation.Because
unit aspart of the ChuntacalaFormation.Satchwell ash-flow member of the formation is 18.6 _ 0.3 Ma
(1983) originallyinterpretedboth unitsto be older (recalculatedfrom date reported by Tosdalet al.,
episodes
probably
thanthe basalignimbriteof the HuaylillasFormation 1981), the laterof theseerosional
andassigned
thema lateOligoceneage.Subsequently, tookplaceat ca. 19 to 21 Ma. The grayagglomerate
by conglomerate,
thereby
L. J. France(pers.commun.,1984) andP. C. $atch- is overlainunconformably
1612
CLARK, TOSDAL, FARRAR, AND PLAZOLLES
a
d
c
FIG. 5. LandformsandNeogeneunitsin Cuajoneminearea.a. View to the northwestfrom vicinity
ofCerro Vifia Biancatowardthe Cuajoneminearea(seedumps).The west-southwest-sloping
subplanar
ridgesare largely definedby the tuffs of the lower MioceneHuayli!!asFormation,but are locally
degradational
andrepresentthe Altosde Camilacasurface.The thickestdevelopmentof earlyHuay!fllas
Formationtuffsis exposedon the far (north)slopesof QuebradaCocotea,the majorvalleyin the middle
distance.b. South wall of the Cuajone open pit, June 1980 (note selectedbench elevations).Basal,
22.8 + 0.7-Ma, ashflow of the Huayli!!asFormationoverliesan irregul.arunconformity(dashedline)
incisedinto the Que!!avecoquartz porphyry of the ToquepalaGroup. The leachedcappingextends
from ca. 3,800 to ca. 3,495 m, overlyingthe tapered southernlimit of the enrichmentblanket. Lower
part of the pit is in primary or "transitional,"i.e., weakly enriched,ore (seetext). c. Easternquadrant
of the Cuajone open pit, lookingeast-northeast,
June 1983. Premine courseof QuebradaChuntacala
is clearly shown.Ridge on the right is underlainby Huayli!!asFormationashflow (22.8 _+0.7 Ma) and
thatontheleftby ashflowsandagglomerates
ofthemiddleand!ateMiocene
Chuntacala
Formation
(--•13.1 + 0.4 Ma). Trough-shapedbody of gray agglomerate,herein assignedto the mid-Huayli!!as
Formation,fills in the steep-walledalluvialchannel(dashed),incisedinto enrichedores.The exposed
baseof the agglomerateis at ca. 3,475 m.d. View to the west-southwest
down QuebradaChuntacala
to upper levels of the Cuajone mine. Skyline is dominatedby slopingignimbritesof the Huayli!!as
Formation.The ridge separatingthe ChuntacalaandTorata(right)valleysis underlainby the Chuntacala
Formationandcorresponds
to the courseof an openmid-Miocenevalley.Axesof earlier lower Miocene
valleyslay along,or to the southof, the areaof the openpit.
suggestingthat east-westvalleyswere incisedhere (Tosdalet al., 1981), this valley was filled in by a
more than oncein the later HuaylillasFormationin- thick (max 485 m) succession
of ash-flowtuffs, agterval.
glomerates,and laharsof the ChuntacalaFormation.
With thetransitionto the MultiplePedimentstage
after about18 Ma, uplift andrenewederosionled to
the development
of a broadopenvalley,the axisof
which lay 600 to 800 m to the north of the earlier
channelor channels.Beginningat 13.1 + 0.4 Ma
The tuffs now underlie
the interfiuve
between
the
Torata and ChuntacalaValleys(Figs.5d and 6). This
palcovalleywasprobablycoevalwith the regionally
extensive middle Miocene (11.5-14.1 Ma) Pampa
Lagunasapronpediment(Tosdalet al., 1984), which
CUAJONE,
QUELLAVECO,AND TOQUEPALA
DEPOSITS,
SEPERU
1613
541,200 E
t
•
•
!
.......... I
.............
•
..............................
FIG.6. Simplified
mapdelineating
themajorpostmineralizati.on
geologic
unitsin theCuajonemine
area, includingthe HuaylillasFormation (<22.8 _ 0.7 Ma) and ChuntacalaFormation(<13.1 _ 0.4
Ma) volcanicson the southand north marginsof the open pit, respectively.The approximatelocations
of the rhyolitic conglomerateand gray agglomerateare shown(after Satchwell,1983). Line A-B correspondsto the sectionin Figure 7.
definesmuch of the upper parts of the Llanuras approximateboundariesof the supergenezones.
Costanerasto the southwestof the Cuajone mine Cuajonewasoriginallya 470-million-metric-tonde(Fig. 2c).
posit, with an averagegrade of 1 percent Cu, and
Intraformation unconformities within the Chuncontainingca. 75 millionmetrictonsof enrichedsultacalaFormation (Manrique and Plazolles,1975) in- fide ores(avggrade> 1.5% Cu). The sulfideenrichdicatethatthe mineareaexperienced
episodicmiddle ment,dominatedby chalcocitesensustricto,wasapandlate Mioceneuplift, presumablyduringthe suc- parently confinedto a single blanketlike horizon,
cessivedevelopments,to the southwest,of the Cerro some20 m in averagethickness,
whichdippedgently
de lasChulpas,PampaSitana,andCerroSagolloter- to the west, and in the north-south cross section,
race pediments(Tosdalet al., 1984), each of which showeda gentletroughingin itscentralportions(Fig.
recordsa discretestagein the middleand late Mio- 7). To the south,the enrichmentzonethinnedmarkcene uplift of the Cordillera Occidental.However, edly toward the limit of the mineralizedzone (Fig.
erosionat that time apparentlydid not unroof the 5b). Partiallyoxidizedoreswith remnantsupergene
orebody.Thereafter(probablypost-6.5Ma), renewed chalcocitewere locallyintersectedby the (premine)
uplift duringthe Valley and Terracestageinitiated floor of Quebrada Chuntacala,and beneath the
the development
of theextantvalleyof theQuebrada northerninterfiuve,wereclearlyexposedonthe floor
Chuntacalaandthe deepercanyonof the R;oTorata of the palcovalleyat the baseof the ChuntacalaFor(Fig. 4). This involvedthe erosionof muchof the local mation;oxidized remnantsof chalcocitepersistto
Huaylillasand ChuntacalaFormationsvolcaniccover within a few metersof this erosionsurface.Moreover,
and, eventually,renewedincisioninto the orebody
alongthe axisof the Chuntacala
Valley.
A crosssection(Fig. 7) illustratesthe preminetopographyin the immediateCuajonemine area,the
broaddistributionof postoregeologicunits,andthe
it is evidentthat in the easternarea of the pit ores
containingboth coherent (massive)and powdery
(sooty)chalcociteoccurredwithin a few metersof
the unconformity(palcovalleysidesandbottom)underlyingthe grayagglomerate.
It isnotknownif chal-
1614
CLARK,TOSDAL,FARRAR,AND PLAZOLLES
A
B
SW
NE
•
CHUNTAC 37oo
-..
[] Leached
zone
_•
- • Su•erge•
sulfide
zone
-
Ou,ro,
Oxide
zone
0
'
x
200
;
'
/
3•oo
m
FIG. 7. Schematicsouthwest-northeast
crosssectionthroughthe Cuajonedepositshowingpremine
topography,the approximatelimit of ore-gradeCu mineralization,the major supergenezones,and
the dispositionof Neogenevolcanic-sedimentary
units.The smalloutlier of volcanicrockscloseto the
pit axisis mappedby Manrique and Plazo!!es(1975) asthe ChuntacalaFormation,but we concurwith
Satchwe]!(1983) in assigning
it to the HuaylillasFormation(grayagglomerate).Note that there are
otherminordiscrepancies
betweenthe two descriptions
of the postoregeology.Simplifiedandmodified
from Manrique and P!azo!les(1975).
cocite occurred immediatelybeneath the rhyolitic
conglomerate.
On the southernbenchesof the open
pit, the taperedextremityof the enrichmentzonelay
approximately150 m belowan irregularunconformity, representingan earlystagein the evolutionof the
Altos de Camilacasurface(at ca. 3,500-3,640 m
a.s.l.), and here it is overlainby the major ash-flow
tuff of the lower HuaylillasFormation(Fig. 5b). By
the early 1980s,muchof the mainenrichmentzone
had been mined out, and the lower part of the open
pit lay in "transitional"ore (Satchwell,1983), characterizedby largelysuperficialcoatings
of sootychalcooitcon hypogenesulfides.The supergeneassemblagecomprises(Clark, in prep.) varyingproportions
ofdjurleite,roxbyite((Cu, Fe)l.SaS,Clark, 1972; (Cu,
Fe)].74-1.82
S, Mumme et el., 1988), and anilite. The
grade of the underlying primary ore averages0.8
percentCu (Satchwell,1983).
the lower part of the leachedzone formedthrough
oxidationof sulfide-enriched
(chalcocitic)ores,once
continuous with the extant enrichment zone. It is,
therefore,inferredthat sulfideenrichmentmayhave
originallyoccurredashigh asthe 3,550-m level, i.e.,
within 20 to 100 m of the latestOligoceneor earliest
Mioceneerosionaltopography,and that subsequent
lowering of the water table causeda significant
downwardpenetrationof oxidationandleaching.The
transitionalore (seeabove)showsfeaturescharacteristicof interfacesbetweensupergenesulfideandprimary ore zones(seeAlpereand Brimhall, 1989) but
probably representsa moderate redistributionof
copperin responseto the fall in the water table.
The observedrelationshipsat Cuajoneare in permissiveagreementwith a modelin whichsulfideenrichmenttookplacebeneatha subplanartopography
ancestral to the Altos de Camilaca surface. If the
The sulfide-enriched
zonepasses
upwardintothe leachedcappingdevelopedthroughdegradationof
oxidezone(Fig. 7), whichhasan averagethickness previously
enrichedsulfideores,thenthe upperzones
of 15 m andanaveragegradeof 1.3 percentCu. The of the depositmay bear a recordof a relativelyprooxidezone is considerably
thicker (to 40 m) in the longedperiodof supergene
alterationbeneatha subnorthernpart of the deposit,i.e., in the area which duedlandscape
undergoing
gradualdissection;
hence,
liesbeneaththe gentlyinclinedsouthernsideof the supergeneactivitymay havebegunsignificantly
bevalley which formsthe depositionalsurfacefor the forethe endof the Oligocene.In anycase,enrichment
Chuntacala Formation. The oxide ores are dominated
probablyceasedat 22.8 _ 0.7 Ma wheneruptionof
by malachiteandchrysocolla,
butremnants
of super- the basal(or near-basal)tuff of the HuaylillasForgenesulfidesare widespread.The oxidezoneitselfis mationmantiedthe deposit.The thick leachedzone
overlainby a hematite-bearing
leachedzone (0.1- underlyingthat unit, and exposedon the southwall
0.2% Cu), with a maximum(preserved)thicknessof of the openpit (Fig. 5b), probablydevelopedlargely
120 m. The mineralogicand textural relationships prior to that volcanicepisode.
(Stevenson,1972; Horlick et el., 1981; Satchwell,
Subsequently,but before 18.6 _ 0.3 Ma (see
1983) suggestthat the entire oxidezone and at least above),further oxidationand leachingof the super-
CUAJONE,
QUELLA
VECO,AND TOQUEPALA
DEPOSITS,
SEPERU
%
%
%
ASANA
\
i
outerlimit
-78,000
N
D
- Holocene
alluvium
Neogene
volcanlcs
andsediments
D
pre- Neogene
FIG. 8. Geologicsketchmapof the Quellavecoprospectarea
(simplifiedandmodifiedafterEstrada,1975) showingdistribution
ofpostmineralization
Neogenevolcanicandsedimentary
unitsof
the Huayli!las,Chuntacala,and "Seneca"Formations.The outline
of thephyllic-argillic
alterationapproximately
corresponds
to the
limit of ore-grademineralization.The northwest-southeast
line
(C-D) indicateslocationof the crosssectionin Figure12 andthe
southeast
part of the crosssectionin Figure 11.
1615
overliesthe orebody,andis now in contactwith ores
originallymoderatelyrich in chalcocite.Satchwell
(1983) ascribesthe "troughing"of the leachedzone
andthelocaldevelopment
of extensive
supergene
kaolinitc to weatheringbeneaththisvalley.However,
the steepconfiguration
of the valley,andthe probability that it wasrapidlyfilled in by the agglomerate,
suggeststhat supergeneprocesseswould not have
beenveryeffectiveat thisstageand,moreover,would
haveagainbeen terminatedon the onsetof eruption
of the Chuntacala
Formation
at 13.1 _ 0.4 Ma. It
remainspossiblethatthe troughingof the enrichment
blanket(Fig. 7), and/orthe developmentof the transitionalenrichedores,may reflect valley incisionin
mid-HuaylillasFormationtimes,but it is moreprobablethat they were controlledby the undulatingnature of the earliest Miocene landforms ancestral to
the Altos de Camilaca surface.
Supergeneoxidationwas probablyrenewedfollowingthe erosionof the ChutacalaFormationstrata
duringthe Valley andTerracestage,but it is consideredunlikelythatsignificant
enrichmenthasoccurred
since the middle Miocene.
Quellavecodeposit
Copperorereserves
at the Quellaveco
prospect
(Fig. 1) are variouslyreportedas200 millionmetric
tonsat 0.95 percentCu (Hollister,1974) or 405 million metrictonsat 0.8 percentCu (Estrada,1975).
The Quellavecoorebodyisexposedonbothflanksof
the deep westerlytrendingvalley of the Rio Asana
gene chalcociteores may have resultedfrom their andiselongatedin a northwesterly
direction(Fig.8).
exposure
duringtheerosionof the deepvalleysystem The broadphysiographic
relationships'
(Fig. 9) are
occupiedby the gray agglomerate,which directly- similarto thoseat Cuajone.The AsanaValley is in-
,•.oo..s4,..o.
ooE
4•,o,
,•,...o,
FIG.9. Simplified
physiographic
mapof theQuellaveco
prospect
area(of.Fig. 4). Notethecourses
of the RiosAsanaand CharaqueValleysand the interveningunnamedquebrada.The shadedarea
indicatesthe approximate
extentof the subplanar
Oligo-Miocene
landscape,
but the smoothlower
(southwesterly)
part of the interfiuvebetweenthe Asanaandunnamed
valleysis definedby gently
dippingflowsof the "SenecaFormation"(seeFig. 10b).The areaof Figure8 is outlined.
1616
CLARK,TOSDAL,FARRAR,
AND PLAZOLLES
b
cisedinto a modifiedAltosde Camilacasurface,representedhere (Fig. 10a)by a gentlyrollinghighplain
ca. 3,800 to 4,100 m a.s.1.;the valley floor at the
Quellavecocampis 3,500 to 3,600 m. Discontinuous
remnantsof Huaylillas and ChuntacalaFormation
ignimbritesoccuron higherinterfiuves.Onesuchashflow tuff, exposedca. 7 km to the southwest
of the
deposit,has yielded a K-Ar age of 18.4 ñ 0.6 Ma
(Tosdalet al., 1981), similarto that of the younger
of the datedHuaylillasFormationflowsat Cuajone.
On the southeastedge of the AsanaValley, a small
erosionalremnantof a 13.1 ñ 0.7-Ma ignimbriteassignedto the ChuntacalaFormation (Tosdalet al.,
1981) ispreservedin a swaleonthe AltosdeCamilaca
surface.To the northwestof the deposit,a similar
middleMioceneageof 12.5 ñ 0.6 Ma is reportedfor
a thick sequenceof alternatingash-flowtuffsandlaharswhich is exposedin the valley of the Rio Charaque, a right bank tributary of the Rio Asana(Fig.
8). The tuff sequenceis lithologicallysimilarto the
Chuntacala Formation-type section at Cuajone.
However,the uppermostpreservedtuff in the CharaqueValley hasan ageof 6.5 ñ 0.6 Ma anddisplays
violetquartzphenocrysts
consideredcharacteristic
of
the Pliocene ash flows of the Seneca Formation in the
region (Tosdal, 1978; Tosdal et al., 1981). This assignmentis, however,probablyincorrect(R. J. Langridge,pers.commun.,1989).
The interfiuvesbetween the valleys of the Rio
Charaqueand R.ioAsanahave quasiplanartopsand
dip gently to the west-southwest
(Figs.9 and 10b).
The lower slopesin bothvalleysare underlainby volcanicrocksof the Upper Cretaceous-Paleocene
ToquepalaGroup and intrudedby broadlycoevalplutons(Bellido, 1979). The smoothinterfiuvebetween
the AsanaandCharaqueValleysisthe slightlyeroded
depositional
surfaceof an ash-flowtuff that is tentatively correlatedeither with the SenecaFormationin
the CharaqueValley, or with an underlyingflow of
the upper part of the ChuntacalaFormation.Other
tuffs assignedto the ChuntacalaFormationare exposedon the northernslopesof the upperAsanaValley at an approximateelevationof 3,750 to 3,775 m
a.s.1.Thus,the Quellavecodepositwasburiedat differenttimesby volcanicrocksof the middleandupper
FIG. 10. Landformsand Neogene units in the Quellaveco Miocene Chuntacala and uppermost Miocene and
prospectarea. a. View to east-southeast
from the slopesof Cerro PlioceneSeneca
Formations.Undoubtedlythe deposit
Charaquetowardthe areaof the Quellavecoprospect(notedrill
was
also
at
one
time coveredby the HuaylillasForaccessroadson S slopeof the AsanaValley). Unnamedquebrada
visiblein middledistance.The skylineis modifiedAltosde Camilacasurfacecappedlocallyby flowsof the HuaylillasandChun-
mation.
A conspicuous
featureof therightbankoftheAsana
Valleyfor a considerable
distanceupstreamfromthe
ValleyandQuellavecomineralizedarea.West-southwest-dippingQuellaveco
deposit(Fig. 10c)isa thick,white-weathinterfiuve is cappedby a tuff tentatively assignedto the upper
Miocene-PlioceneSenecaFormation, underlain by flows of the ering, unweldedash-flowtuff underlainby a disconmiddle MioceneChuntacalaFormation.c. View northeastup the tinuous fluvial conglomerate,together constituting
Rio AsanaValley from the southernexposuresof Quellavecode- the Quellaveco
brecciasandconglomerates
of Estrada
posit. White tuff (9.5 ___
0.5 Ma) of the uppermostChuntacala (1975). The flowis 9.5 ñ 0.5 Ma (Tosdalet al., 1981)
Formationis evidentalongthe north bankof the valley,where it
andlacksthe violetquartzeyesof the upperunit in
lies on a rock cut terrace incised into the Chuntacala Formation
and underlyingToquepalaGroup. Smoothsurfacesin distance the CharaqueValley.The tuff cannotbe tracedalong
are morainesoverlyingthe Altosde Camilacasurface.
the northern slopesof the AsanaValley to the west
tacala Formations. b. View to the northwest across the Rio Asana
CUAJONE,
QUELLA
VECO,AND TOQUEPALA
DEPOSITS,
SEPERU
1617
erosionin the area of the presentCharaqueValley
and in the approximate
positionof the presentRio
Asana,where these valleysare narrower and with
steeperslopes
thantheearliersystems;
(6) deposition
of fluvialconglomerates
andthe 9.5 _+0.5-Ma ignimvalley incisedinto the older rocksto a level closeto brite, whichwasassigned
to the uppermostChuntathe Paleogene-Neogene
unconformity.Similarstrati- ealaFormation(Tosdalet al., 1981); (7) eruptionof
graphicand geomorphologic
relationships
probably the upper Miocene and PlioeeneSenecaFormation
occuron the southslopeof the presentAsanaValley, ignimbrites,now preservedonlyaboveca. 3,800 m
but the tuff andthe underlyingbenchare lessexten- a.s.l.;and (8) incisionof the presentdeepcanyonof
sivelypreserved.Moreover,the erosionalsurfaceun- the Rio Asana,the valley of the Rio Charaque,and
derlyingthe conglomerate
andyoungtuff liesca. 75 the interveningquebradas.
The age of the faulting
m higher on the southside of the valley, probably episodealongthe axisof the Rio Asanacannotbe
resultingfrompost-9.5-Mafaultingalongthe present preciselydefined,but it probablycoincidedwith eiAsanaValley (Estrada,1974).
ther stages(6) or (7) above.The approximaterelaOn the basisof the aboverelationships,
the follow- tionshipsof the abovefeaturesare illustratedin the
ing sequenceof Neogeneeventsin the area is pro- sketchcrosssectionof Figure 11, alongthe northposed:(1) finalstagesin the developmentof the Altos westerlytrendingaxisof the Quellaveeoorebody(el.
de Camilacasurfaceat ca. 19 to 25 Ma; (2) eruption Figs.8, 9, and 12). Stage(3) is correlatedwith the
of the youngerHuaylillasFormationignimbritesat developmentof the PampaLagunaspedimentin the
andstage(5) with the Cerro Saabout 18.4 Ma; (3) incisionof a broadopenvalley, LlanurasCostaneras
the axisof whichprobablycoincidedapproximately gollo pediment,respectivelythe earliestand latest
with the presentinterfluvebetween the Asanaand episodesin the Multiple Pedimentstage(Tosdalet
deepening
of the Asana
CharaqueValleys;(4) depositionof a thicksuccession al., 1984).The post-9.5-Ma
of Chuntacala
Formationignimbritesandlahars,be- and other valleysoccurredduring the subsequent
ginningat ca. 13.1 _+0.7 Ma andfillingin the palco- Valley andTerrace stage.
Estrada(1974, 1975) hasshownthat a singlesuvalley andlocallyspillingover its southernrim onto
the Altosde Camilacasurface;(5) focusingof fluvial pergenesulfideenrichmentblanketis developedat
of the deposit.Upstream,a topographic
benchincised
into volcanicflowsand intrusionsof the Toquepala
Group and the overlyingChuntacalaFormationvolcanics(above)locallytakesthe placeof the whitetuff,
and we concludethat the latter was depositedin a
NW
SE
4100
Baseof Chuntacala
Fro.
Aolxoximate
0o 's•tio•
ofAltos
deCam•aca
surface
I
Se•cca
Fm.'(?6.5Ma)
12.5 Ma'"
_....•e
..........
..__,.
• •, •, •, •,
I
•'ø••
t./'/' /
13.1*Ma
,,?
3900
,
o.c,
'
'
,oo
•3500
fo
ma.s.t
R•
LEGEND
•
• ........
AolxoxJ•ate Limitof Quelaveco Deposit
To•)ogr.•y
of
volcanic
flow
•,•_• Oosiltons
Exlx)•ed
(R)
and
projected
(L)
0.5
---- ? -' Tentativeextrapolation
of flow
-""'"f.,. /
•ferredOosltion
of I:)aleosurface
Fault
!
0
I km
FIG. 11. Schematicnorthwest-southeast
crosssectionalongaxisof the Quellavecoorebody,illustrating interrelationships
of presenttopography,Neogeneignimbriteflows(someundated),inferred
paleolandforms,
andthe Que!lavecoenrichmentzone.The sectionis approximately
orthogonalto the
extantriver valleys.The lastepisodeof faultingalongthe axisof the Asanavalley is assumedto have
occurredafter eruptionof the 9.5 ___
0.5-Ma ignimbrite.Verticalscaleis exaggerated.
1618
CLARK,TOSDAL,FARRAR,AND PLAZOLLES
-3coo
c
D
(SE)
--3300
SUPERGENE
[] Alluvium
--3300
--
I
[] Neogene
volcanice
[] 'Quellaveco
con91omerale'
[] pre-Neogene
rocks
ZONES
[] Leached
(and
oxidized)
zone
.r•]moderately-to-strongly
enriched
zone
0
2oo
4oo
[] weakly-enriched
zone
m
•m....I.
FIG. 12. Crosssection(lookingnortheast)of the Quellavecosupergenezones,after Estrada(1974;
sect. 13). Note the different distributionof postoresedimentaryand volcanicunits on the southand
north flanksof the AsanaValley. The volcanicsare undifferentiated,but in this sectionthey probably
comprisemainly the 9.5 _+0.5-Ma white tuff (seeFig. 10c) of the upper ChuntacalaFormation.Note
the intersectionof the enrichmentzone by the erosionsurface(valleybench)underlyingthe fluvial
conglomerateand the displacementof the supergeneprofileby the easterlytrendingfault.
Quellaveco,as at Cuajone (Fig. 12). Two zonesare humedAltosde Camilacasurface(Fig. 11), which is
greaterrelativedepththanat Cuadistinguished:
an upper zone of moderateto strong at a significantly
enrichmentoverlyinga lowergradezone,presumably jone (seeabove).A verticalseparationof thisextent
representinga downwardtransitionto unalteredhypogeneores.The chalcocite-bearing
zone is significantlythicker (50-60 m) andmoreirregularthan at
Cuajone.It attainsdepthsof œ50to 300 m belowthe
presentsurfaceand hasa gentletroughlikeform in
verticalcrosssection,crudelyreflectingthe configuration of the AsanaValley slopes(Estrada,1975).
The enrichedzoneis generallyoverlainby a leached
and/oroxidizedzone,extendingfromsurfaceto maximum depthsof 80 m. An east-west-trending
fault
closeto the presentaxisof the AsanaValley offsets
the northern part of the enrichmentblanket downwardabout75 m (Fig. 12; Estrada,1975). Restoration
of this offsetwould enhancethe subparallelrelationshipof the presentvalley and the troughin the enrichmentblanket(Fig. 12).
The enrichedzone at Quellavecois truncatedby
the lower slopesof the presentAsanaValley. On the
north flank of the valley, the unconformitybeneath
the 9.5 + 0.5-Ma ignimbriteandthe underlyingconglomerateintersectsthe sulfideenrichmentzone as
well asthe overlyingleached-oxidized
zone(Figs.19.
and 13); oxidizedremnantsof chalcociteoccurwithin
50 cm of this old valley floor. Similarrelationships
areapparentbetweenthe enrichmentblanketandthe
vestigiallypreservedoutliersof tuff on the southern
slopesof the valley(Fig. 12; seeEstrada,1974, 1975).
The relationships
betweensupergeneenrichment FIG. 13. View to the north-northeastacrossthe AsanaValley
and the older topographicallyhigher landformsare incisedinto oxidized and enriched Que!!avecomineralization.
lessclear.The highestexposures
of the enrichedzone, White cliffson the far slopeof tributary quebradaare outcrops
of 9.5 ___
0.5-Ma tuff, which immediatelyoverliesa ca. 8-m-thick
at its southernextremity,lie at altitudesof ca. 3,750 conglomerate.Lower slopesare in the ore zone, with remnants
m a.s.l.approximatelyœ50m below the horizontally of supergenechalcocitepersistingto within 50 cm of the unconextrapolatedposition(ca. 4,000 m a.s.l.) of the ex- formity.
CUAJONE,
QUELLA
VECO,
ANDTOQUEPALA
DEPOSITS,
SEPERU
between land surface and water table would not be
unusual in a mountainous semiarid environment. For
example,in physiographically
similarsettingsin the
Copiap0districtof Chile (Fig. 1), enrichmentassemblagesare intenselydevelopedat depthsof ca. 200
to 300 m belowthe presentsurfacein the Chafiarcillo
silverveins(Whitehead,1919), andsupergenechalcociteis abundantat depthsin excessof 275 m in the
Dulcineade Llamposcoppermine, at a level approximatelycoincidingwith the preminewater table (Sillitoe et al., 1968; Sillitoe, 1969). Similarly, the
leachedzoneoverlyingthe enrichmentblanketat the
La Escondida
porphyrydeposit(Fig. 1) locallyattains
depthsof 400 m belowsurface(Brimhallet al., 1985;
Ojeda, 1986; AlpersandBrimhall,1989). It is therefore possiblethat enrichmentat Quellavecocould
1619
Neogenetuffsin the immediateminearea.However,
anearlyMioceneK-Ar ageof 18.3 Ma hasbeendetermined(Tosdalet al., 1981)for a tuffwhichcapsa
40-m-thicksuccession
of tuffsandconglomerates
of
theUpperMoquegua
Formation
some20 kmsouthsoutheast
ofToquepala.
Thistuffislithologically
and
chronologically
similarto the 18.4 +_0.6-Ma ignimbrite in the Quellavecoarea,some15 km northof
Toquepala
(seeabove).Othertuffsof the Huaylillas
FormationnearCuajoneandalsosome50 km to the
south near the Peru-Chile border have similar K-Ar
agesof 18.6 ___
0.6 and18.4 ___
0.5 Ma, respectively
(seeabove;Bel16nand Lef&vre,1976). The widespreadoccurrence
of tuffsof similaragethroughout
theregionrendersit likelythattheToquepala
deposit
wascoveredby tuffsat thattimein the earlyMiocene.
TheToquepala
porphyrycopperdepositoriginally
croppedoutin a southerly
trendingvalley,Quebrada
prior to its mantlingby the 18.4-Ma flow.
The openvalleyinferredto havesubsequently
de- Toquepala,andits shorttributary,QuebradaHuanvelopedat Quellavecowould (Figs.11 and 12) have canani,which were eroded to depthsof ca. 300 m
been incisedto within 75 to 100 m of the preserved into an erosion surface with a local mean altitude of
enrichmentzone. If hydrologicconditionshad been ca. 3,600 m a.s.1.andwith a gentlesouth-southwestsuitablefor supergene
alteration,it ishighlyprobable erly slope(Figs.14 and 15a). As at Cuajoneand
thislandscape
nearToquepalahadsigthat enrichmentwouldhavetakenplaceat that time, Quellaveco,
ocperhapsinvolving upgradingand deepeningof an nificantprimaryreliefbut, fromthe widespread
earlier formed blanket.The troughlikeform of the currenceof accordant,in part planatesummits(Fig.
to the Altosde Camilaca
surenrichedzone may broadly reflect this valley. En- 15a),it maybe assigned
richment would again have been interrupted and face(Tosdalet al., 1984). Immediatelyto the southto ca.
probablyterminatedby depositionof the thickChun- westof Toquepala,the surfaceslopesteepens
have occurred beneath the lower Miocene
surface
tacalaFormationsuccession,
beginningat ca. 13.1 Ma.
Our model, therefore, involves two or more superimposedenrichmentphasesresulting,not in the
development
of verticallyseparateblanketsbut in the
thickeningand upgradingof a singlezone (see for
example,Locke, 1926; Brimhall et al., 1985; Alpers
and Brimhall,1989). The apparentlygreaterdevelopment(and/orpreservation)
of enrichedsulfideores
at Quellavecothan at Cuajone may be in part the
result of a lessintenseregimeof localvalley incision
into the Altosde Camilacasurfaceduringthe early
Mioceneover the intervalrepresentedby the Huaylillas Formation.Althoughwe infer that enrichment
at Quellavecowasmoreprotractedthanat Cuajone-possiblyextendingfrom the later Oligoceneto the
middle Miocene--it had similarlyterminatedbefore
the closeof the Multiple Pedimentstageprior to 9.5
7o (Fig.15b)beforefallingabruptlyto analtitudeof
ca. 2,600 m a.s.1.at the northeasternmarginof the
PampaLagunaspediment(Fig. 2). This landform
configuration
postdates
all significant
localmovement
on the major west-northwest-trending
Incapuquio
fault (WilsonandGarcia,1962), whichtraverses
the
lowerprecordilleran
slope(BellidoandLanda,1965;
Tosdal,1978). A comparably
steepinclinationto the
Altosde Camilacasurfaceis observedelsewhereonly
at the marginof an apparentMiocenecalderasome
30 km southeastof Toquepala.A similar volcanic
eventmayhaveoccurrednorthor northeastof the
mine, althoughno circular structureis visible on
Landsatimagesin thatarea,perhapsdueto burialby
youngervolcanics.
Prior to mining,QuebradaToquepalapossessed
a
composite,goblet-shaped
profile which, by analogy
Ma. Oxidation, but little sulfide enrichment, accom- with othervalleyprofilesin thisregion,suggests
that
paniedthe more recent stagesin the deepeningof the gentlydippingupperslopesprobablydeveloped
the AsanaValley.The majorfaultalongthe valleyhas during the Multiple Pedimentstage,and the more
notfocusedsignificant
sulfideenrichmentor oxidation steeplyincisedlowerslopesduringthe ensuingValley
of the ores.
Toquepaladeposit
Of the three porphyrycenters,the Toquepaladepositdisplays
the mostextensive
recordof sulfideenrichment(Richardand Courtright,1958; Anderson,
1982), but paradoxically,this cannotbe precisely
constrained in time due to the absence of datable
and Terrace stage.
Supergenesulfide enrichmentin the Toquepala
depositextendsovera muchgreaterverticalinterval
thanat Cuajoneor Quellaveco;secondary
chalcocite
after chalcopyriteand bornite occurred between
3,525 and at least ca. 3,100 m a.s.1.(Richardand
Courtright, 1958; Southern Peru Copper Corp.,
unpub.repts.).Much of the enrichedore hasbeen
1620
CLARK,TOSDAL,FARRAR,AND PLAZOLLES
1982). The mostextensiveenrichmentzone,andthe
only one systematically
delimitedon the basisof ore
grade by mine personnel(Fig. 17), has a broadly
planar,but locallyirregular, upper surfacewhich is
subhorizontalin east-westprojectionbut whichfalls
from an altitude of ca. 3,350 m at the northern limit
of the depositto ca. 3,250 m at the southernlimit (a
horizontal distanceof ca. 700 m). In contrast,the
lower boundaryof the enrichmentzone is very indented, with deep roots resultingin enrichedore
thicknesses
in excessof 150 m. Althoughlocallysuch
downwardextensionsof chalcocitedevelopmentlie
beneaththe valleyaxis,in generalthere is no correlationbetweenenrichmentblanketconfiguration
and/
or thicknessand the preminevalley form. A local
b
FIG. 14. Preminephysiography
of the Toquepalaminearea.
QuebradaToquepalaand its tributariesare incisedinto a south-
southwest-dipping
planatelandformassignedto the Altos de
Camilaca surface.
mined,but its originaldistributionmay be defined
approximately.Remnantsof zonesaffectedby enrichmentarevisible(Fig.16a)in theopenpit because
of the associated
supergene
argillicalteration,which
givesriseto a pale,bluish-graycolorationon broken
rock surfaces
and,in the upperlevels,throughthe
FIG. 15. Landformsin the Toquepalamine area. a. View to
the northeastfrom Cerro Toquepala,acrossthe easternhalf of
the Toquepalaopen pit (June1982). In the distance,Cerro Condoriquifiastandsabovethe southwest-sloping,
planate,rock-cut,
landform--the Altosde Camilacasurface.Upper coursesof the
QuebradasToquepala(center)and Huacanani(right) incisethe
pediplain.b. View to the east-southeast
from BarrioAzul (5 km
southwestof the Toquepalaopen pit), along the southwestern
margin of the precoralii!era.The southwest-dipping
landform
representsa tilted andstronglydissectedremnantof the Altosde
Camilacasurface,developedhere in the Upper CretaceousToquepalaGroupvolcanics
andPaleocenegranitoidintrusions.
The
developmentof reddish-brownhematite,an oxidation flat-toppedmesarisingabovethe surfacein the middlegroundis
productof chalcocite-pyrite
associations
(Anderson,
a remnantof the Huayli!!asFormation(Be!!idoandLanda,1969).
CUA]OHE,QUELLA
VECO,AND TOQUEPALA
DEPOSITS,
SEPERU
1621
a
3400
SECTION
34
32O0
3
FIG. 17. Schematicwest to east crosssectionsthrough the
lower main enrichmentblanket of the Toquepalamine, showing
itsrelationsto the premineconfiguration
of QuebradaToquepala.
The sectionstraversethe southern(J), central (P), and northern
(U) parts of the deposit.The central break in the enrichmentin
the northernpart of the depositdelimitsthe postoreandessentially
barren daciteagglomeratebrecciapipe. The enrichmentblanket
shownis asdelimited by premine drilling on the basisof copper
grades(+0.45%: after unpub.data,SouthernPeru CopperCorp.).
controlof the depth of enrichmentby postorefaults
is apparentin someparts of the deposit.This does
not, however,explainwhy the mainenrichmentzone
wasboth thicker (Fig. 17) and of higher meangrade
in the western quadrantof the depositthan in the
easternpart in manyeast-westsections.In the absence
of evidencefor major north-strikingfaults through
the deposit,this asymmetricaldistributionof enrichment maybe inferredto reflect primary-gradevariations,or the preservationof thick impermeablerhyolites(SerieAlta) of the premineralization
Toquepala
Group over the easternpart of the orebody(Richard
and Courtright, 1958), which could have inhibited
copperleachingandenrichment.
At the northern and southernlimits of the open
pit, the main enrichmentzonewaslocallyseparated
from the lower slopesof QuebradaToquepalaby less
FIG. 16. Distribution of enriched zones on west and northwest
than 20 m (Fig. 17), andits formationthereforecerwallsof the Toquepalaopenpit. a. Stronglyforeshortened
view
tainly predatedat leastthe later stagesof valley inlookingwestfromthe 3,145-mlevel (June1975). Dark (in part
shadowed)benchesin foregroundare in transitionalore, with cision.The overallsoutherlydip (ca. 8o-9 ø) of its
erraticdistributionof powdery(sooty)chalcociteon hypogene
assemblages.
In higherlevels,the sulfide-enrichedzonesare whitish,due to the developmentof supergenekaolinRe,andthe oxidizedandleachedzonesare darker.The lower (main)enriched
zone immediatelyoverliesthe transitionalore and is essentially
continuous
aroundthe westwall of the pit. The overlyingolder
enrichedzonesare lessregularin form.Selectedbenchelevations
strQnglyoxidized,extendsto the marginof the pit andto within
50 m of the dissectedAltosde Camilacasurface,here partially
mantiedby dumps.The lower three visiblebenchesare mainly
in transitionalore. Selectedbenchelevationsare shown.c. Oblique
view of the westwall of the openpit, lookingsouthwestfrom the
areshown(m.a.s.1.).
b. Northwest
quadrant
oftheopenpit (August 3,280-mbench(June1982),ComparewithFigure17a--enriched
1975), showing(areasdelimitedby white lines)the main lower ores(pale)are nowexposedquasicontinuously
fromthe shallowest
enrichedzone and the irregular,discontinuous,
distributionof levels (top-right) to the upper boundaryof the hypogeneore
theupperenrichedzones(viewisessentially
contiguous
withthat (lower 4-5 benches).The oxidizedzone on the upper benches
in 17a).The highestdevelopment
of chalcocite
(attop-right),now correspondsto dark areas.Bottom level at 3,085 m a.s.i.
1622
CLARK,TOSDAL,FARRAR,AND PLAZOLLES
upper surface,broadlyparallelingthat of the upper
precordilleranslopein the southernpart of the mine
area(Fig.15b),impliesa controlby a subplanar
southdippingtopography.Whereasthe upper and central
parts of this enrichmentzone were dominatedby
cocite zone to the Altos de Camilaca surface indicates
that enrichmentbeganprior to the final stagesin the
development of that landform, i.e., in the earliest
Mioceneor before.Althoughno tuffscropout in the
mine area,the widespreadpreservationof ca. 18-Ma
massivecoherent chalcocite,its lower sectionand lat-
ash-flowtuffsthroughoutthe regionrendersit proberal peripheriesdisplayextensivedevelopmentof able that the depositwas then buried, resultingin
sootysupergenesulfides,givingrise to transitional interruptionof the supergeneprocess.Moreover,an
ore, as at Cuajone.The lowermostenrichedoresare earlier episodeof coveringby tuff maybe recorded
mineralogically
similarto thoseat Cuajone,compris- by the occurrenceof a 23.3 ___
0.8-Ma tuff (Tosdalet
ing blackto midnight-bluepowderycoatingsof djur- al., 1981) in the MoqueguaFormationsome25 km
to the westof, andbroadlydownslopefrom,Toqueleite,roxbyite,andanilite(Clark,in prep.).
The originallarge-scaledistributionof chalcocite- pala.It cannotbe confirmedwhetheranytuffsof the
emplaced
bearingoresin the shallower
levelsof the openpit is ChuntacalaFormationwere subsequently
now difficultto reconstruct,largelybecauseroutine in the area.
mineralogicobservations
have not been carried out
The higher altitude early enrichment,at least in
duringmining,while, to the authors'knowledge,no part, clearlypredatedthe initialincisionof Quebrada
suchstudies
weremadein theinitialdrillingprogram. Toquepala.Indeed, the subplanarupper surfaceof
It is evidentthat suchhigheraltitudeandpresumably the lower (main)supergenesulfidezone,anditsclose
olderenrichmentwasalsofocusedin the western(or approachto the preminevalleybottom,impliesthat
northwestern)quadrantof the deposit.Anderson it was similarlygenerated,largely or entirely, prior
(1982,fig.12.15)identified
suchasinglezonedipping to local valley development.However, the available
fairly steeplyto the south.However,observationof datado not permit usreliably to assignan ageto the
thedistribution
ofrelictchalcocite
andsupergene
ka- importantuplift episodewhichwe infer immediately
olinite on the western and northwestern benches of precededthe mainenrichmentperiod.Three possible
the pit hasrevealedextremelyirregularandvarying modelsmayexplainthe interrelationsof enrichment
patternsasthe pit wallswere displacedto the west andlandformdevelopmentat Toquepala.In the first
from1975 to 1982. Whereasin 1975 to 1978 (Fig. model, all enrichment would be ascribed to weath16a,b), two, apparentlyseparate,supergene
sulfide eringbeneaththe evolvingAltosde Camilacasurface
in the late Oligoceneand early Miocene,the supergeneactivitybeingeventuallyterminatedby eruption
visible(Fig. 16c) on mostbenchesbetweenthe 3,280- of the 18.3- to 18.4-Ma HuaylillasFormationignimand 3,490-m benchesand were locallyapparently brite. Accordingto thismodel,the majorupliftwhich
continuous with the lower main enrichment zone.
precededthe main enrichmentwouldbe assignedto
However, as pointed out by Anderson(1982), ores the latest stagesin the generationof the surface.In
immediatelyoverlyingthe lowerenrichmentzoneare the secondmodel, the early upper chalcocitezones
characterized
by extensivedevelopmentof jarosite are inferredto haveformedduringthe finalstagesin
afterhypogenechalcopyrite.
The highestsurviving the developmentof the Altos de Camilacasurface,
enrichmentobservedduringthe periodof study,at whereasthe deep zone may have developedin rethe 3,535-m level, lies only 40 to 75 m below the sponseto rapid uplift, tilting, andwater tabledescent
accordantsummitsassignedto the Altosde Camilaca causedby localcalderaformation,presumablyin the
surfaceadjacentto the northwestmarginof the pit earlyMiocene(at 18.3 Ma ?).Thismodelmustremain
hypotheticalin the absenceof evidencefor a caldera
(Figs.15a and 16b).
We concurwithRichardandCourtright(1958) and to the north of Toquepala.However, becauseashAnderson(1982) thatthe unusuallythickenrichment floweruptionwouldnormallybe intimatelyassociated
in theToquepala
deposit
resulted
fromperiodicuplift with crustalflexure in a calderasetting (Smith and
andthatthe spatialisolation
of thelower(main)zone Bailey,1968), the periodimmediatelyfollowingsuch
probablyrecordsan episodeof uplift of a sufficient an event would probablybe unpropitiousfor supermagnitudeto have depressedthe local water table gene activity. The third model would correlatethe
belowthelevelpreviously
affected
bysupergene
sul- early enrichmentwith the Altosde Camilacasurface
fideformation.
Theirregularenrichment
zonesabove and the later with the earlier part of the Multiple
ca. 3,330 m mayreflecteither two or moreperiods Pedimentstageandprobablyspecificallywith the inof alteration
separated
by significant
uplifteventsor termediatestagesin the developmentof the Pampa
a singleprotractedperiodof enrichmentaccompa- Lagunasapronpediment,for whichthe tilted surface
zoneswere evident,centeredat ca. 3,330- and 3,440m elevations, in 1982 remnants of chalcocite were
nyinga relativelyslowdepression
of the watertable.
As notedpreviously,no firm temporalconstraints
maybe placedon the supergene
sulfideenrichment
at Toquepala.
The proximityof the uppermost
chal-
southof the mineconstitutes
the originalbackscarp.
The lower main enrichment would thus have devel-
opedfollowingerosionof the 18,3 ___
0.6-Ma ignimbrite and probably at ca. 13 Ma (see Tosdalet al.,
CUAJONE,
QUELLAVECO,AND TOQUEPALA
DEPOSITS,
SEPERU
1623
the early Miocene;and(2) the localfrequencyofemplacementandpersistenceof Miocenetuffs.Bothparameterswere themselvesstronglyinfluencedby the
local regime of fluvial channeldevelopment.
In passingfrom Cuajone to Toquepala,there was
evidently a progressivediminution in the intensity
(i.e., extent and frequencyand/orrapidity) of valley
developmentacrossthe evolvingsubplanarprecorcene.
dilleran landscapeduring the earliestMiocene and a
Whereas we consider it clear that enrichment of
concomitantdecreasein blanketingby tuff flows.At
the Toquepaladeposithadbegunpriorto the Multiple Cuajone,we infer that the supergenesulfideenrichPedimentstage,very probablyin the latestOligocene mentblanketis largelyrelict fromthe late Oligocene
1984, fig. 5). In this model,the considerableextent
of the lower chalcocitezone would reflect (1) the
stronguplift andresultingdownwarddisplacement
of
the water table which characterizedthe Multiple
Pedimentstage(Tosdalet al., 1984); (2) the absence,
or rapid removalby erosion,of the ChuntacalaFormationin the immediatearea;and(3) the lack of deep
incisionof QuebradaToquepalauntil the later Mio-
to earliest Miocene, we cannot decide between the
and was formed in association with the evolution of
abovemodelsfor the age of the later, more intense
enrichmentstage.Nevertheless,we prefer the third
modelin whichchalcocitedevelopment
persisted
into
the middle Miocene (ca. 11-18 Ma; see above).
the subduedlandscapeancestralto the Altosde Camilaca surface. At that time, gradual erosion and
depression
of the water tablepermittedthe progressivedownwardextensionof chalcocitedevelopment
Synthesis
enriched horizons. Since then, the local conditions
The abovediscussion
of the Tertiary geomorphologic contextof supergeneprocesses
in the area surroundingthe Cuajone,Quellaveco,and Toquepala
mines demonstratesboth the potential of this approachin the elucidationof the controlson the economicallycritical enrichmentand the uncertainties
inherent in the unravelingof complexcordilleran
landformdevelopment.
The ore deposits were emplaced in the early
Eocene(Clark et al., 1990) at shallowdepths(equivalent to confiningpressuresof ca. 100-150 bars;
Zweng, 1984) but, given the considerablethickness
of subaerialvolcanicstrataconstitutingthe Toquepala
Group, at a significantelevation(>2,000 m a.s.1.).
Followingtheir unroofingin the Eoceneand Oligocene during a prolongedperiod of comparativetectonic quiescence,all three porphyry centersexperienceda historyof episodicuplift andmantlingby tuffs
during the initial stagesin the developmentof the
present-dayCordillerraOccidentalat thislatitude.It
is evident that supergeneactivity was most intense
duringconversionof a regionallyextensiveerosional
surfaceof low altitudeandrelief into the faceted,polyphase,and mountainouslandscapeof the present
precordillera.
Despite the lacunae in our knowledge of local
Neogene geologicevents, a comparisonof the enrichmenthistoriesof thesethree depositsprovidesa
basisfor an understandingof the factorswhich influence supergeneprocesses.In particular, the progressiveincreasein the extent of chalcocitemineralization,fromthe modestblanketat Cuajonethrough
the thickerenrichedzone at Quellavecoto the multiple blanketsat Toquepala,can be linked with differencesin erosionalhistoryandthe extent of burial
beneathNeogenepyroclasticrocks.It is apparentthat
the criticalparametersaffectingthe formationand/
or preservationof enrichmentzonesin thesedeposits
were (1) the locationandrate of valleyincisionduring
havebeenlargelyunfavorablefor supergeneactivity,
due almostentirely to the presenceof extensivetuffs
on top of the deposit.Valley incisionacrossthe tuffs
and subjacentorebodyand potentiallyfavorablewater table lowering were of insufficientdurationfor
anysignificantenrichment,becausethe valleyswere
quicklyfilledby successive
tuffsandotherpyroclastic
rocks of the Huaylillas and ChuntacalaFormations.
It is clear in any casethat all significantsupergene
activitypredatedconsolidation
of the basal(13.1-Ma)
ashflowof the ChuntacalaFormation.Thus,the major
episodicuplift associated
with the MultiplePediment
stage failed to generate important enrichment at
Cuajone,althoughit is possiblethat somedevelopment of the sootychalcociteof the transitionalores
may have persistedinto this period. Only minor oxidationmaybe assigned
to the later Mioceneandsub-
and the encroachment of the leached zone over earlier
sequentinterval.
The 80-m-thick enrichmentblanket in the Quellavecodepositformedprior to the 9.5 ___
0.5-Ma tuff
of the ChuntacalaFormationwhich directly overlies
oxidized chalcociteores. The markedly troughlike
form of the enrichedzonesuggests
that it developed,
at least in part, beneath an extensiveopen valley
which we infer to have been in existence in the mid-
Miocene, between 18.4 and 13.1 Ma. However, we
stronglyfavora modelin whichsuchenrichmentduring the Multiple Pedimentstagewas superimposed
upon an earlier supergeneprofile which wasgenerated beneath
the Altos de Camilaca
surface in the
earliestMioceneor latestOligocene,probablycoeval
with the enrichment at Cuajone. Thus, despitethe
complexsuccession
of Mioceneerosionalandvolcanic
eventsrepresentedin the Quellavecodistrict,the episodesof valleyincisionwere, in aggregate,favorable
for the deepeningof the main chalcocitezone, and
the blanketingby the HuaylillasandChuntacalaFormationtuffs was insufficientlyprotractedto impede
seriouslythe supergeneactivity.
1624
CLARK, TOSDAL,FARRAR,AND PLAZOLLES
In our opinion,the considerable
verticalextentand
apparentlypolystagenatureof the Toquepalaenrichment is a direct reflectionof the ephemeralnatureof
Miocene ignimbritecover in the mine area and of
unusuallystronguplift and tilting, probably in the
mid-Miocene.The dispositionof the highestaltitude
residual chalcocitezones showsclearly that they
formed beneath the Altos de Camilaca surface or its
late Oligoceneantecedents.A brief interruptionof
supergeneactivityat ca. 18.3 Ma probablyoccurred
in responseto burialbeneathtuffs.The deepestmain
enrichmentzonewasgeneratedfollowinga significant
episodeof uplift and oceanwardtilting of the precordillerain the Toquepalatransect.Thiseventcannot
be dated,but we favor an origin duringthe earlier
part of the Multiple Pediment stage,i.e., at ca. 13
Ma, rather than prior to the probableemplacement
of the 18.3-Ma flow. Thus, developmentof massive
chalcociteores may have persistedsomewhatlater
thanat Quellavecoandit isimplicitthatnosignificant
burial by the ChuntacalaFormationignimbritestook
place.
It is evident that both physiographicand climatic
conditionswere broadlyfavorablefor supergeneactivityin the late Oligoceneto middleMioceneinterval
in southernPeru.Economicallyimportantenrichment
certainlyoccurredin the early Miocene (ca. 23-18
Ma) but had ceasedby the earliestlate Miocene.The
correspondence
between the uppermostpreserved
zonesof supergenechalcociteandthe Altosde Camilacasurfacestronglyimpliesthat enrichmentbegan
as early as approximately25 to 30 Ma, probably
shortlyfollowingexhumationof the deposits.Subsequently,whereasclimaticconditionsfavorablefor
enrichment clearly persisted into, and perhaps
throughout,the middleMioceneMultiple Pediment
stage,the extentof new enrichmentandpreservation
of extant supergenezoneswas entirely dependent
upon the local configurationof valley development
and the presenceor absenceof Miocene tuffs over
the deposits.Thus, the aggressivedevelopmentof
cordilleranreliefin the Miocenecausedtheupgrading
of an earlierformedchalcociteblanketat Quellaveco
and generatednew enrichmentzonesat Toquepala,
but led to oxidationanderosionof supergeneprofiles
at Cuajone.
RegionalCorrelations
Supergene enrichment has upgraded numerous
smallcoppervein systems
exposedon the Pacificslope
of the Cordillera
Occidental
elsewhere in southern-
most Peru. The Norviii mine, 13 km southeastof To-
quepala(V•trgas,1975), workedoresin which chalcocite,after chalcopyrite,hadbeen oxidizedto malachiteandchrysocolla(Clark, unpub.data).The depositcropsout on the southwesterly
tilted segment
of the Altosde Camilacasurface(seeFig. 15b), and
the supergene
enrichmentwaspresumably
relatedto
an earlystagein the developmentof that landform.
In the smallLluta (Cercana)Cu(-Ag, Pb) district
(V•trgas,1975), situated120 km southof Toquepala,
veins were exploited in which massivesupergene
chalcocitewas extensivelyconvertedto malachite.
The enrichedveinscrop out on a rock-cutbenchof
the QuebradaLluta. This bench is interpreted as a
restrictedterracepediment,anditselevationstrongly
suggeststhat it is equivalentin age to the Multiple
Pedimentstageof the Toquepala-Cuajone
area.Thus,
enrichmenthere is at least middle Miocenein age
andmay havebegundevelopmentin the early Miocene.
Elsewhere in southern Peru, 110 km west-north-
west of Cuajone,the large Cerro Verde-SantaRosa
porphyry copper center (16ø32' S), broadly coeval
with the depositsunder discussion(Estrada,1975),
displays
anextensive
supergene
profile(Kihien,1975;
Mineroper6,unpub.data).Miningoperationsby Mineroper• in the Cerro Verde sectorhavebeenbased
on high-gradeoxidizedores,dominatedby brochantite, which overlie a zoneof sulfideenrichmentup to
300 m in thickness,attaininga depthof 465 m below
the 2,903-m-a.s.1. summit of Cerro Verde. In the ad-
jacent SantaRosasector,1.5 km to the southeast,a
muchthinner (30-50 m), but laterallyextensive,subhorizontal enrichment zone lies ca. 50 m beneath an
interconnected series of small intermontane basins.
The surroundingsummitsare accordantand,in many
cases,areflattopped.They aretentativelyinterpreted
as the remnants of the landform which controlled the
enrichmentprocess,the "Caldera surface"of ]enks
(1948). Both the accordantsummitsand the intermontanebasinsareconsideredto representpediments
thatareasyet undated.However,onthebasisof longdistance correlationsof landforms, we tentatively
suggest
that the oldersummitsurfaceis equivalentin
age to the Altos de Camilacasurface,whereasthe
basinfloors,representingthe upperextensions
of the
very extensivealluviatedpediplainwhichconstitutes
the PampasCostaneras(Pampade La ]oya) ca. 15 to
20 km southwest
of the minearea,areprobablycoeval
with the Multiple Pediment stage.If this model is
correct, the enrichment of the Cerro Verde-Santa
Rosadepositwould be inferred to be, at leastin its
earlier stages,of late Oligoceneor earliestMiocene
age. The great depthsattainedby the enrichment
processbeneath Cerro Verde itself may reflect the
permeabilityof the brecciabodieswhich host the
mineralization in this area and may record a protractedhistoryof supergeneactivityextendinginto
the Multiple Pedimentstage.
The age and physiographicsettingof supergene
enrichmentin northernChile are remarkablysimilar
to thoseestablished
in southernPeru. Supergeneenrichment of the Chuquicamataporphyry systemin
CUAJONE,QUELLAVECO,AND TOQUEPALADEPOSITS,SEPERU
1625
northern Chile (Fig. 1) occurredin the interval 16.5 mentsandopenvalleys.That both supergeneactivity
terminatedin this
to 26 Ma (Mortimeret al., 1977)beneatha regionally andpedimentationhaveessentially
developedplanatelandscape
broadlycorrelativewith regionhaslongbeen ascribedto the onsetin the later
the Altos de Camilaca surface. To the south, the Mioceneof anaridandevenhyperaridclimaticregime
youngerof the two enrichmentepisodesrecognized (e.g., Mortimer, 1969; Mortimer and Sari•, 1975;
in the Copiap6 district (e.g., Sillitoe et al., 1968; Mortimer et al., 1977), itself a productof the northMortimer, 1973), thoughnot preciselydelimited,had ward propagationof a coldlittoral current.Thisaspect
terminatedprior to the developmentof the Atacama of the later historyof the Andeshasbeen analyzed
pediplain,before 12.9 Ma (Clark et al., 1967a; Sillitoe in detailby AlpersandBrimhall(1988), who identify
et al., 1968). Thisepisodefolloweda lateEocene(39- the essential causative factors to be the establishment
to 43-Ma) episodeof copper mineralization(Farrar
et al., 1970; Clark et al., 1976). A late Oligoceneand
early Miocene age for enrichment in that area of
northernChile maythereforebe inferred. The attendant regional erosionallandform, the Sierra Checo
of the circum-Antarcticcurrent and the subsequent
formation of the Antarctic ice cap at 13 to 15 Ma.
They alsoemphasizethe key importanceof the cli-
del Cobre surface(Mortimer, 1973), thusprobably
zones in what is now the Atacama Desert. In this con-
matic desiccation
and resultant
decrease
in erosion
ratesin the preservationof earlier formed supergene
formed at the same time as the Altos de Camilaca
text, it shouldbe emphasizedthat economicallysigsurfacein the studyarea.Finally,AlpersandBrimhall nificantenrichmenthasalsoaffectedthe very young
(1988) report early and middleMioceneK-Ar ages Rio Blanco-Disputada
(_•7.4 Ma; Warnaarset al.,
for supergene
alunitewithinthe unusually
extensive 1985) andE1Teniente(HowellandMolloy,1960;
and high-grade chalcocite blanket and associated _•4.7Ma; Clarket al., 1983) depositsof centralChile,
leachedcappingoftheLaEscondida
porphyrydeposit whichhaveexperienced
a pluvialcold-temperate,
or
(Fig. 1).
evenperiglacial,climatethroughoutmuchof their
It is thereforeapparentthat the late Oligoceneto brief exposurehistory.
early Miocene interval proved favorablefor intense
Althoughthere is someevidencethat major ensupergeneupgradingof copperdepositsalonga ca. richmentmayhavelocallypersistedto a slightlylater
2,000-kmstretchof the Pacificslopeof the evolving period in southernPeru than in northern Chile, the
CordilleraOccidental(-Principal),fromsouthernPeru availableage constraintsare insufficientto define a
to north-central Chile. The essential simultaneous
significant
northerlychangein the time of enrichment
enrichmentdefinesa discretemetallogenicepisode, cessation.Instead,in comparingthe two broadtranor perhapstwo closelyspacedepisodes,of great im- sects,we would emphasizethe greater extent, both
portance, directly correlatedwith major pulsesof absoluteandrelative,of the late Oligoceneto middle
uplift of the cordillerain the latestOligocene(ca. Mioceneuplift eventsin southernPeru.Thus,whereas
18-25 Ma) andearlymiddleMiocene(ca.12-16 Ma). muchof northernChile hadalreadyachievedconsidIn thiscontext,the ca.37-Ma ageof supergenealunite erablealtitudesby theEocene(e.g.,Mortimer,1973),
in the E1Salvadordepositrecordedby Gustafsonand southernPeru lay closeto sealevel until the late OliHunt (1975) is anomalous.
For thisdepositthe geo- gocene(Tosdalet al., 1984). Thereafter, uplift and
morphologyonlyconstrains
enrichmentto havepre- crustalthickeningwere far more abrupt in this area
datedthe local formationof the Atacamapediplain, thanfartherto the south.This wouldfavordeep enprior to 12.9 Ma. Lackingany evidenceto the con- richmentof the ore deposits.However, the region
trary, we canonlyconcludethat climaticandlandform surrounding
the Cuajone,Quellaveco,andToquepala
conditionsat E1Salvadorwere suitablefor majoren- porphyry centersexperiencedmore intensevolcanic
richmentin the earlyOligocene,althoughno regional activityduringthe late OligoceneandearlyMiocene
erosionalsurfaceor enrichmentepisodeof this age interval than did geomorphologicallycomparable
hasbeenrecognizedin the region(Mortimer,1973). areasin northern Chile. The closelyspacederuption
of thicktuffsat thiscriticaljuncturein southernPeru
Conclusions
musthaveplayeda key role in limiting the extentof
Supergene enrichment processesin the central enrichmentin the depositsunder studyand particuAndescontextachievedtheirmaximum
development larly in the caseof Cuajone.In contrast,there is no
in the late Oligoceneand early to middleMiocene, evidencefor localvolcanism
in the areasurrounding
stimulated
by anoptimalconjunction
ofepisodic
uplift La Escondida,Chile, during the early and middle
oftheCordillera
Occidental
anda prevailing
semiarid Miocene, thus permitting essentiallyuninterrupted
climate.The moderateprecipitation(perhapsex- developmentof an enrichmentblanket both considceeding10 cm/annum)inferredfor this periodnot erably thicker and of higher grade than thosepreonlypermittedsignificantcirculationof meteoricwa- servedin southernPeru. Volcanismof late Oligocene
ter throughthe upper partsof the exposedore de- and early Miocene age (ca. 25-15 Ma) affectedwide
positsbut favoredthe formationof subplanarpedi- areasof northernChile, extendingfromthe Peruvian
16 2 6
CLARK,TOSDAL,FARRAR,AND PLAZOLLES
borderto at leastlatitude26ø S (e.g.,Zentilli, 1974; Andersondisplayedconsiderable
patiencein the typCoiraet al., 1982). However,the scatteredgeochro- ing of the penultimateandfinalmanuscripts.
nologicdata suggestthat major mid-Tertiarytuffs
Revisionof the manuscript
wasconsiderably
aided
were erupted lessfrequentlysouthof ca. 21ø S in by the detailed,sympathetic,
andconstructive
reviews
northern and north-central Chile where the Paleocene of two Economic
Geologyreviewers.
to mid-Oligoceneporphyrycopperdepositsoccur,
This paper is a contributionto the Queen'sUnithan in northernmost Chile and southernmost Peru versity Central Andean Metallogenetic Project
(Mortimer et al., 1974; Tosdalet al., 1981; Lahsen, (CAMP).
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Acknowledgments
Thisprojectwasundertakenwith the full logistical
cooperation
of the SouthernPeruCopperCorporation
(SPCC),who alsogavepermissionfor the publication
of this paper. Our studieswere initiatedthroughthe
goodofficesand encouragementof the late Frank W.
Archibald, former SPCC president. Field studies
benefitedgreatlyfrom the supportof G. ChuckPreble, then general managerof the SPCC Toquepala
area operations,and from the adviceand unstinting
assistance
of chief geologistFrank B. Stevenson,and
of chiefmine geologists
JorgeManrique(Toquepala)
andNobertoSocolitchandPaulSatchwell(Cuajone).
Studiesin the Quellaveco area were carried out in
part throughthe kind cooperationof Tom•sGuerrero,
then project head for Mineroper6, Alfredo Kihien,
and GermanG•rate. We greatlyappreciatedthe opportunityto read a preprint of the paperby Charles
AlpersandGeorgeBrimhallon the enrichmentof the
La Escondidadeposit.The seniorauthorprofitedimmeasurably
from the insightsandhardwork of Cedric
Mortimer, Richard Sillitoe, and Ronald Cooke at an
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