Economic
Vol, 83, 1988, pp. 1368-1378
SedimentaryRock-HostedDisseminated
PreciousMetal Mineralizationat
PurlsimaConcepci6n,YauricochaDistrict, Central Peru
ANGEL ALVAREZ
A.
Departamento
de Geologfa,
CENTROMIN-PERU
S.A.,La Oroya,Perd
AND DONALD
C. NOBLE
MackaySchoolof Mines,Universityof Nevacla-Reno,
Reno,Nevada89557
Abstract
Sedimentary
rock-hosted
disseminated
goldoreslithologicallyand chemicallysimilarto
thoseof Carlin-type
deposits
ofthewestern
UnitedStates
arepresentin theYauricocha
district,
centralPeru. The PurlsimaConcepci6ndepositis locatedin the core of a steeplyplunging
anticlineseveralhundredmetersbeyondlarge pipe-shapedCu-Zn-Pb-Ag-Aureplacement
orebodies
in limestoneborderinga late Miocenegranodiorite
stock.The centralpart of the
stockispotassium-silicate
alteredandcontains
high-salinity
fluidinclusions.
Mineralizedimpurelimestoneis generallydecalcifiedand silicifiedand appearsto have
beencarbonaceous.
Originallaminatedbeddingandothersedimentary
structuresare locally
preserved.
Characteristic
minerals
includequartz,rhodochrosite,
pyrite,calcite,sericite,
and
barite.Pyrrhotite,
arsenopyrite,
plagionite
(PbsSbsS17),
andaltaitc(PbTe)havebeenrecognized
locally.Although
thechemical
composition
oftheoresisgenerally
similartothatoftheCarlin
deposit,rocksof PurlsimaConcepci6n
havehigherFe, Mn, Te, Ag, andT1 andlowerHg
contents.Mostof the Au-bearingrockscontainbetween3 and6 wt percentFe, andTe (•4
ppm)andMn (•4 wt %) are abouttwo ordersof magnitude
higherthanat Carlin.These
elevatedelementalconcentrations
are all epigenetic;thereis no regionalor localgeologicor
texturalevidencefor a syngenetic
and/ordiageneticorigin.The Ag/Au ratio of the oresis
about2.5. Gold correlatespositivelywith Ag, As, Hg, Mo, Sb,Te, andT1 andAg strongly
with Mn. Porous,leached,but not silicified,bedstendto carryhighergoldvaluesthansilicified
rocks.
Thepresence
ofenargite,
tetrahedrite,
andhighScoppersulfideminerals
in thereplacement
oresandof pyrophyllite,
diaspore,
alunite,anhydrite,andhigh-temperature
(•400øC), cornpositionally
complex,
hypersaline
fluidinclusions
in alteredrocksbordering
enargiteveinsin
the stock,andthe intensedissolution
of limestoneduringdepositionof the polymetallicore-
bodiessuggest
thatthe fluidsinitiallyhadlow pH andhighf o, andj•, values.The magmatic
hydrothermal
system
mayhavebeensimilarto thatofJulcani
andotherenargite-tetrahedrite
districtsin Peru, with the differencesin ore typesat leastin part reflectingthe carbonate
composition
of the hostrockat Yauricocha.
The elementalchemistry
of the PurlsimaConcepci6n
orescanbe qualitatively
explained
by a process
of hydrothermal
differentiation
in whichselectiveprecipitation
of basemetal
andAgsulfides
andsulfosalts
depletedthefluidsin thesecomponents
andproduced
a decrease
in Cu/Zn,Cu/Pb,As/Sb,basemetal/precious
metal,andAg/Auratios.Themoderate
pH and
lowfo•andj• valuesimpliedbythemineralassemblage
ofPurisima
Concepci6n
presumably
reflectin part the presenceof carbonaceous
material.
Similardeposits
canbeexpected
elsewhere
in theWesternCordillera
ofcentralandnorthern
Peruwherehydrothermal
fluidsrelatedto Neogene
stocks
encountered
impurecarbonaceous
limestonebedsof thePariatambo
andJumasha
Formations
andotherunits.Certainsedimentary
rock-hosted,disseminated
Au-Agdepositselsewheremay havea similargeneticrelationto
porphyry,skarn,and/orlimestone-replacement
systems.
Introduction
1980; BagbyandBerger,1985; Bonham,1985; Tin-
gleyandBonham,1986;Percivalet al., 1988),similar
deposits
arepresentin otherpartsof the world,for
example,China(Geng,1985; Liu andGeng, 1985;
Cunningham
et al., 1987).
CentralPeru, a majorregionof baseandprecious
DURINGthe past two decadessedimentaryrockhosted,disseminated
preciousmetal depositshave
becomean importantsourceof gold, and to a lesser
extent, silver. Although most known sedimentary
rock-hosted
deposits
arelocatedin thewesternUnited
impurecarStates,andparticularlyin Nevada(e.g.,Radtkeet al., metalmining(Petersen,1965), possesses
0361-0128/88/856/1368-11
$2.50
1368
PUR•$IMACONCEPCIC•N
DEPOSIT,PERU
13 6 9
bonate rocks and calcareous siltstones and shales,
somecarbonaceous
(M•gard, 1978b). These rocks,
largelyof Mesozoicage,occurin a transitional
environmentreminiscentof that of lithologicallysimilar
stratain the western United States(Stewart, 1980)
andare cut by manystocksof Tertiaryage(M•gard,
1978aandb). Unitsof relativelypurecarbonatestrata,
sandstone,and shalemlocally coal-bearing--are
presentin the section,but volcanicrocksare absent.
The impurecarbonatebedsshouldbe goodexplorationtargetsfor disseminated
preciousmetaldeposits.
The purposeof this paperis to providea preliminary geologicand geochemicaldescriptionof the
PuHsimaConcepci6ngolddepositandof the intimate
spatialandtemporalassociation
of the depositwith a
porphyrysystemand importantpolymetalliclimestonereplacementorebodies,featuresthat are critical
in providinginsightsinto the origin of the deposit.
PurlsimaConcepci6n,locatedin the Yauricochadis-
FIG. 1. Map showingthe location of the Yauricochadistrict
and certainother major miningdistrictsin centralPeru.
trict at an elevation of about 4,700 m in the Western
Cordilleraof centralPeru (Fig. 1), is one of the first
sedimentary rock-hosted, disseminatedprecious
metaldepositsto be recognizedin the centralAndes
(NobleandAlvarez, 1988). This firstreport is based
mostlyon informationobtainedduring mining,development,and evaluationactivities,supplemented
by smallamountsof chemicalandpetrographicdata
obtainedfor this study.
The principalorebodies
of the Yauricocha
district,
locatednear the westernmarginof the Yauricocha
stock,arerelatedspatiallyandgenetically
to themain
bodyofthestockandtonarrowfingers
ofgranodiorite
thatpresumably
connectat depthto a westernextensionof thestock(Figs.2 and3). The orebodies
consist
ofverticallyelongate
pipescomposed
largelyofpyrite
GeologicSettingand Associated
Mineral Deposits andothersulfidemineralsthat replacelimestoneof
the Jumasha
Formation.Skarnis developed
adjacent
The geologyandmineraldeposits
of the Yauricocha to the stockbut doesnot hostappreciableamounts
districthavebeendescribed
by Lacy(1949), Thomp- of ore.The pipestypicallyexhibitbothverticaland
son(1960), Petersen(1965), andthe Departamento radialzoningandthereis a pronounced
districtzonde Geologlade Cerro de PascoCorporation(1970). ing,withaninnerzoneofenargite(theprincipalcopThe ore depositsof the districtare spatiallyand ge- permineral)givingwayoutwardto anenargite-chalneticallyrelatedto the Yauricochastock,a composite copyrite-bornite
zone,whichin turnis succeeded
to
by sphalerite,
galena,
intrusivebody of granodioriticto quartzmonzonitic thewestby zonescharacterized
composition(Fig. 2) that hasbeen radiometrically lead, andsilver(Lacy, 1949; Thompson,1970; Figs.
datedat about7.5 m.y. (Giletti andDay, 1968). The 2 and 3).
stockintrudestightly foldedbedsof the Late CretaEnargiteveinsthatcutthe Yauricocha
stock(Fig.
ceousJumasha
(Coniacian)andCelendln(Santonian) 2) aresurrounded
by granodiorite
alteredto a mixture
Formationsand the overlyingCasapalcaFormation of pyrophyllite,diaspore,alunite,andsmallamounts
(latestCretaceousandPaleocene?)
(M6gard,1978a; of anhydrite.Largequartzgrainsin theserocksalso
containabundantsecondaryfluid inclusions,some
Noble et al., 1979).
The centralpart of the Yauricochastockisaffected whichcontainhaliteandasmanyas14, andperhaps
includingtwo phaseswith exby K silicatealteration.Abundanthigh-salinityand 16, other daughters,
vapor-dominated
inclusions
arepresent,with the hy- treme birefringence,probablycarbonate,and a vapersalineinclusions
commonly
containing
sylviteand riety of othermoderatelybirefringentminerals.Vacarbonatedaughtersand in somecasesone or more por-dominantinclusionsaccompanythe daughterandsmallamounts
of liquidCO2
moderately
birefringentmineralsandchalcopyrite(?) bearinginclusions,
in additionto halite.The K silicate-altered
granodi- havebeententativelyidentifiedin a few inclusions.
orite contains0.1 to 0.2 wt percentCu andlocallyas Preliminary heating- and freezing-stagemeasuremuch as 12 ppm Mo (Wright, 1966); veinlets in ments(P. C. Gibson,pers.commun.,1988) reveala
quartz-sericite-altered
granodioriteexposedat the stronglytelescoped
fluidinclusionassemblage,
with
surfacecontainas muchas 0.02 to 0.03 oz/ton Au onetype of hypersaline
inclusionhavingvaporhomogenization
temperatures
in excessof 400øC and
and0.5 to 1 oz/tonAg.
1370
A. ALVAREZ A. AND D.C. NOBLE
i
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EXPLANATION
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GRANDDIORITE
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i-I- ENARGITE VEIN$.•-•,
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r;--I
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SLATE
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,7o ,7o ,•
,7o,.
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FIC.2. Mapshowing
theprincipalorebodies
andzoningpatternof theYauricocha
district.Modified,
with additions,frommappingby K. E. Sicgrist(unpub.)andThompson(1960). Line A-A' showsapproximatezoneacross
whichthe generalized
crosssection(Fig. 3) wasdrawn.
,•3oo level
++
+
FIC. 3. Generalizedcrosssectionshowingthe geologyand mineraldepositsof the Yauricocha
district.Modified,with additions,
fromThompson
(1960). SeeFigure2 for symbols
andabbreviations,
Kp?possible
Pariatambo
Formation.
PURlfSIMA
CONCEPCI(•N
DEPOSIT,
PERU
13 71
salinitiesof greater than 50 wt percent. These are Au andAg contents,high Au/Ag ratios,andlow base
accompanied
by a varietyof other inclusions
having metal contents.
On the basisof Iten's (1952) assays,
CENTROMIN
a very wide range of temperaturesand salinities.
ofthePurlsima
Concepci6n
zone
Basedon the alterationassemblage
andthe very com- beganareevaluation
plexcationandanionchemistry
of the fluidsindicated in 1986. It wasrecognizedthat, in additionto the
lithologicalcontrol,the plunginganticlinein conjunctionwitha sillofbasaltandandesitc
andtheshear
zonesprovidedstructural
controlforthe mineralizing
solutions.
The Departamentode Geologlarealized
simiis anexcellentexampleof advancedargillicalteration that the deposithad geologicandgeochemical
rock-hosteddepositsof the
producedby hot, salineascendant
fluidswhichprob- laritiesto sedimentary
ablyincludedanimportantmagmatic
component
(cf. western United States.
by the manydaughters
(cf.Metzgeret al., 1977),the
hydrothermalfluidsresponsible
for the sulfideorebodiesappearto havebeenrelativelyacid,oxidized,
andrich in sulfurandprobablyCO•. The alteration
Bruha and Noble, 1983; Drexler and Noble, 1983).
The Yauricochamagmatic-hydrothermal
systemis
interpretedasa highO• andS• fugacitysystemthat
wasemplacedinto carbonatestratarather than into
dominantlyquartzofeldspathic
rock. The solutions
were highlyreactiveandintenselyattackedthe carbonatewallrock,producing
the pipe-shaped
channels
within whichthe rich sulfideorebodieswere rapidly
deposited(Lacy, 1949; Thompson,1960). Baseand
preciousmetalswere largelyprecipitatedwithin several hundredmetersof the stockrather than being
carriedupwardalongstructuresinto anoverlyinggeneticallyrelatedvolcanicpile as,for example,at Julcani (Petersenet al., 1977; Noble and Silberman,
1984).
PurlsimaConcepci6nGold Deposit
The Purlsima Concepci6ndisseminateddeposit
(alsoknown as PurlsimaConcepci6nOeste), one of
severalbulk-mineablepreciousmetal depositsin the
Yauricochadistrict(Alvarezet al., 1989), is located
outsideof the sphalerite-galena
andsilverzonesabout
300 m from the outermostpart of the enargitezone
(Figs.2 and3). Althoughthe orebodyis in the early
stagesof exploration,about200,000 metrictonsat a
grade of about 0.1 oz Au/metric ton have been
proven.Columntestshaveyieldedabout90 percent
extractionof Au and 45 percentextractionof silver.
Preliminaryresultsof a pilot heap-leachingoperation
have been favorable.
Previous work
The first recordedstudyof the Pur•simaConcep-
ci6nareawasthatofIten (1952),whowasprincipally
Lithologyand mineralogy
Carlin-typeores:The oresof the PurlsimaConcepci6ndepositare sedimentary
rock-hosted,
disseminated
oreshavingphysicalcharacteristics
typical
oftheCarlin-typesubset
ofsedimentary
rock-hosted,
precious
metaldeposits
ofBagbyandBerger(1985).
Thegeologic
settingandthephysical
featuresof the
depositare inconsistent
with a syngenetic
or diagenetic originsuchas that proposedfor sedimentary
rock-hosted, stratiform base metal deposits (e.g.,
Gustafsonand Williams, 1981) or gold-bearingbut
basemetal-poormassive
sulfidedeposits(e.g.,Barnett
et al., 1982).
Theoresarelargelyunoxidized
andconsist
mostly
of decalcifiedand silicified impure carbonaceous
limestone
(cf.Armstrong
et al., 1987),whichin places
is hydrothermally brecciated and/or sheared. Althoughsomebedspossess
well-preservedthin beddingwith certainlaminaepreferentiallyreplacedby
rhodochrosite(Fig. 4A), the depositdoesnot exhibit
the texturalandstructuralfeatures,for example,sulfidebanding,of sedimentary
exhalativedeposits.Oxidized zonescontainingabundantlimonite also are
locallypresent,but quartzveinletsandbodiesofjasperold are uncommon.It is not presently clear
whetherthe oxidizedoresare hypogene,supergene,
or of mixedorigin.The averagegradefor the deposit
appearsto be somewhathigherthanthe median(3.3
ppm)for the sedimentary
rock-hosted
depositsof the
westernUnited States(Bagbyet al., 1986). About 5
percentof the ore consists
of bedscomposed
largely
of soft,porous,fine-grained,decalcifiedbut not silicified, and in part carbonaceous,
materialthat commonlycontainshighpyrite contents(Fig. 4B). These
rockstypically have higher gold contentsthan the
interestedin evaluatingthe potentialfor lead ores.
Iten recognizedthat metal valueswere presentin a silicified rocks.
Most mineralizedrock is composedlargely of
subunitof impure limestone(the "slate bed") that
formsthe coreof an anticlineplungingabout50ø to quartz,withlesseramounts
of veryfinegrainedrhothe southeast
andwere localized,particularlyin crush dochrosite,
pyrite,calcite,andsericite_+bariteand
and shear zones within the subunit. The slate bed is _+ carbonaceousmaterial and traces of detrital(?)
sphene,
andzircon.Preliminary
X-raydifprobablya subunitof the JumashaFormation,but it tourmaline,
maybelongto the underlyingPariatamboFormation. fractionstudysuggests
thatsomeor all of the sericite
Iten (1952) alsoobtainedassaysfor Cu, Zn, Pb, Ag, maybe muscovite
ratherthanilliticmaterial.Quartz
andAu on20 specimens
that clearlyshowedelevated istypicallyveryfinegrainedandapparently
largely
13 7 2
A. AL VAREZ A. AND D.C. NOBLE
arsenopyrite,
andboulangerite,2.5 percenteachof
pyrrhotiteandgalena,1 percentchalcopyrite
andaltaitc, and tracesof sphaleriteand nativegold in a
ganguecomposed
largelyof microfractured
quartz.
Plagionitealsohasbeenreported(Iten, 1952). The
nativegold,whichoccursassociated
variouslywith
the majorminerals,in somecasesfills fracturesin
quartz,pyrite,andboulangerite,
suggesting
late depositionand/orremobilization.
It is unclearff these
veins and veinlets,which do not comprisean eco-
nomically
important
partofthedeposit,
wereformed
before,after, or contemporaneous
with the disseminated ores.
Geochemistry
The followingsectionsummarizes
the resultsof a
largenumberof assays
for the preciousmetalsanda
lessernumberof analyses
forthe commonbasemetals
madeduringevaluation
of PurisimaConcepei•nplus
a muchsmallernumberof analysesfor a wider range
of minor elementsobtainedfor geochemicalcharacterizationof the deposit.Theseresultsare com-
paredwith dataavailablefor the Carlinsediment
.a.•
rock-hosteddepositof the westernUnited States
(Radtke,1985), for which the mostcomprehensive
geochemical
coveragein the publicdomainis available. It shouldbe emphasizedthat the data base
availablefor sedimentary
rock-hosted,
disseminated
precious
metaldeposits
suggests
anappreciable
range
in traceelementcomposition
(e.g.,BagbyandBerger,
1985).A moredetaileddiscussion
of the geochemistry
of the Purlsimadepositwill be presentedelsewhere.
FIG.4. Handspecimens
of ore fromthe PurlsimaConcepci6n
Figure 5 summarizesanalysesfor Ag, Au, and a
deposit.A. Finelybeddedsilicifiedrockcontaining
light layers
composed
largelyof rhodochrosite;
specimen
is 12 cm long.B. wide rangeof otherminorelementsobtainedon 12
Porousalecalcified
rock;specimen
is 10 cmlong.
specimens
of mostlyunoxidized,alecalcified,
and silicified ore from diamond drill hole PC-1-86
in the
replacescarbonateremovedby hydrothermalsolu- northernpart of Pur{simaConcepci•nandcompares
tions. It is unclear to what extent carbonaceous mathesedatawith averagevaluesreportedfor the Carlin
terial hasbeenaddedto and/orredistributedwithin deposit.(Althoughthe Purlsimadataare notnormally
the deposit.Pyrite is the principalopaquemineral, distributed,the arithmethicmeanisusedsoasto prooccurringassmallirregulargrains,in partframboidal, vide a moredirectcomparison
with thosefromCardisseminated
withinthe rock.Inclusions
of pyrrhotite lin.) Meanvaluesfor Bi andCd at PurlsimaConcephavebeenidentifiedin pyrite in onespecimen.
He-' ci•n are 1.7 andabout0.5 ppm,respectively.
Analyses
matitc,limonite,and/or Mn oxideshavebeen iden- for Cu, Pb, and Zn availablefor 33 specimens
from
tifiedin severaloxidizedsamples,
andgalenahasbeen otherpartsof the depositconfirmthe verylow con-
identifiedin severalspecimens.
Grainsof nativegold tents of these elements.Concentrationsof As, Cu, K,
4 to 6 t*min diameter,associated
preferentiallywith Mo, Pb, Sb, Se, and Zn are similarto thoseof the ores
quartzandoxideminerals,havebeen observedin two from the mainpit at Carlin. Fluorinecontentsof a
of ten specimens
of ore-gradedrill corestudied.A smallsuiteof samplesrangefrom 0.06 to 0.46 wt
portionof the goldappearsto occuralongfractures percent,with a medianof 1.10 wt percent.Kfilb ratios
andin otherreadilyaccessible
sites.
of about500 are muchhigherthanthoseof the metaSulfide-quartz
veinlets:In the southernpart of somatizeddaciticlavasthathostenargite-tetrabedrite
Purlsima
Concepci6n
theslatebediscutby veinsand veinsat Julcani(Scherkenbach
andNoble, 1984).
veinletscomposedlargely of sulfideand sulfosalt Analyses
for morethan150 specimens
containing
minerals.A vein cut by drill hole PC-4-$6 contains •0.01 oz/ton Aushow that mostof the mineralized
about40 percentpyrite,10percenteachofmarcasite, rockscontainbetweenabout3 and6 wt percentFe,
PURfSIMACONCEPCIdNDEPOSIT,PERU
•10
relationwith Ag, with samples
with morethan0.1
oz/tonAu havingan averageAg/Auratioof about
2.5. Silverexhibitsa strongpositivecorrelation
with
Mn (Fig.6). Manganese,
mostcommonly
asrhodonite,
rhodochrosite,
and/oralabandite,is typicalof Agbearingveinsin Peru,andthecloserelationship
betweenAg andMn at Purlsimasuggests
a closeasso-
•
_
bJ
¸
z
1373
10 •
o
ciationof theseelementsduringhydrothermaltransport and deposition.
•, 10•
--
/
ct'
,0ø6'/
•
Onebulksampleof soft,porous,andfine-grained
black,barite-bearing
materialselectedfor chemical
andmetallurigical
studycontained
about2.7 oz/metric ton Au, 1.0 oz/metricton Ag, 730 ppm As, 5.0 wt
percent Ba, 10.4 wt percent Fe, 78 ppm Hg, 240
10 -=
10 -•
1
10
10 =
10 •
10 *
10 •
MEAN CONCENTRATIONS,
CARLIN MAIN PIT (ppm)
•C. 5. •1ot comparingmean concentrationsof selectedelementsfor the •urisima Coneepei6nore zone with meanvMues
for the Oarlinmainpit (Badtke,1985). The 1• PurisimaConeepei6n specimens
consistof siliei•ed impurelimestonefrom drill
hole •O-1-86. The Te and TI vMuesfor •ur•sima Coneepei6nare
the mean of duplicate anMysesby OeoehemieMServices,Inc.,
andBondarClegg,Inc., and6SI •d •ringer Laboratories,Inc.,
respectively.Other values•e basedon analysesby GSI, Hunter
Minin½Laboratory, Inc., and CEN•OMIN-PEBU.
severaltimesthe averageof about1.5 wt percentfor
normaloresfrom Carlin. The 12 core specimensof
ore from PC-1-86 average3.5 wt percent Fe. Manganesecontentsare morethan 100 timeshigherthan
at Carlin(Fig.5). The averageforthe 12 coresamples,
5.3 wt percent,is supportedby fouranalyses
of other
rocksrangingfrom2.3 and3.6 wt percentMn. Based
on XRD study of representativesamples,the manganeseis presentmainlyas fine-grainedrhodochrosite. The high silicacontentsof the rocksare supportedby routinelyobtainedinsolubleresiduevalues
mostlybetween60 and 80 wt percent.
The averageHg contentof the core specimens
is
morethananorderof magnitudelowerthanat Carlin
and Ag, and to a lesserdegree Ba, Se, and T1, are
higher.Tellurium (•3-4 ppm) averagesabouttwo
ordersof magnitudegreaterthanthe meanfor Carlin,
which is consistentwith the presenceof altaite. All
the elevatedconcentrations,
includingthoseof Fe and
Mn, clearlyrepresentepigeneticadditionof the ele-
ppmMn, 26 ppmMo, 460 ppmSb,10 ppmTe, 0.23
wt percentnoncarbonate
carbon,and lessthan 10
ppmeachofCuandPb.Thethalliumcontentisabout
0.6 wt percent(basedon replicateanalyses
by GeochemicalServices,Inc., and BarringerLaboratories,
Inc.);twosimilarrocksfromotherpartsofthe deposit
have thalliumcontentsof 2,600 and 3,800 ppm. Al-
thoughthesiteofthethalliumin notclear,thesesamples demonstrate
that very high concentrations
of
thalliumcandevelopin a disseminated
formin finegrained,homogeneous,
and unfracturedrock.The
carbonaceous
materialis apparentlyactive,because
only2 percentof the goldpresentwasextractedduring a standardcyanideroll test.
Discussion and Conclusions
Comparison
with deposits
in thewestern
United States
The ores of the PurlsimaConcepci6ndepositare
similarin generalappearance,
lithology,mineralogy
andchemicalcomposition
to thoseof the sedimentary
rock-hosteddepositsof the westernUnited States
(e.g.,BagbyandBerger,1985;Radtke,1985;Bakken
-
i
I
i
i liii
I
I
t
•
I IIII
I
I
I
,
f • I
ments.
The elementsAg, As,Hg, Mo, Sb,Te, andT1show
a positivecorrelation
withAuin the smallandspatially
very restricteddata set availablefrom PC-1-86. The
covarianceis similarto that reported by Joralemon
(1951), Wrucke and Armbrustmacher(1975) and
Harris and Radtke (1976). Basedon more than 200
fire assays,Au showsa general,but scattered,cor-
+
1
10
10 •
Mn (wt. •)
FIG. 6. Plot of Ag versusMn for specimens
from drill hole
PC-1-86.
1374
A. AL VAREZ A. AND D.C. NOBLE
and Einaudi, 1986; Birak and Hawkins, 1985; Rom-
berger, 1986; JohnsonandAbbott, 1987; Percivalet
al., 1988). With an averageAg/Auratio of about2.5,
the Purlsima ores are similar to those of the Bald
MountainandStandarddeposits,
whichprovidea link
betweenthe typicalsedimentary
rock-hosted
deposits
of the westernUnitedStates,in whichgoldgreatly
exceedssilver and silver-dominantsystemssuchas
Taylor(BagbyandBerger,1985). Notabledifferences
betweenPurlsimaConcepci6nandthe U.S. deposits
includethe intimateassociation
with pluton-related
hydrothermal activity and large-scalebase metal
mineralization,and basedlargely on the data from
Carlin, the higher concentrationsof introducedFe,
and particularly,Mn and Te.
Model for the origin of the Purfsima
Concepci(;n
deposit
Althoughigneousrocksare exposed
at or nearmost
sedimentaryrock-hosteddeposits,the connection
between igneousactivity and mineralizationis in
manycasestenuous(e.g.,Romberger,1986; Percival
et al., 1988). In contrast,at Yauricochathere existsa
clearandcompellingconnectionbetweenmagmatic
activity.andmineralization.
The low Ag to Au andbasemetalto preciousmetal
ratiosof the Pur•simaConcepci0noresrelativeto the
sulfide-richorebodiesof the districtcan reasonably
be interpretedin termsof progressive
differentiation
of hydrothermalsolutions.The enargiteoresof the
TABLE1.
inner part of the zoning pattern contain Cu, with
lesseramountsof Au, Ag, Zn, and Pb, and have an
Ag/Au ratio of about 100, althoughenargite-rich
specimens
commonlyhaveAg/Au ratiosof lessthan
20 (Table 1). Progressiveremovalof Cu, Zn, and Pb
in the enargite, enargite-chalcopyrite-bornite,
and
sphalerite-galena
zonescouldhave largely depleted
the solutionsin basemetals.Silver to gold ratiosof
100 to 500 in the sphalerite-galenazone, and the
presenceof orescontaining10 or moreoz/tonAg and
lessthan 0.02 oz/ton Au in the silverzone, suggest
that the Ag/Au ratio of the solutionswere progressivelyreducedasthe oreswere deposited.The relativeabundanceof silverin the PurlsimaConcepci0n
depositis consistentwith the generallysilver-dominant natureof mineraldepositsin centralPeru (e.g.,
C. E. Vidal and D.C. Noble, in prep.) and with the
closeness
of the depositto the probablesourceof the
hydrothermalsolutions,which appearto havebeen
salineandthusgoodcarriersof Ag.
The elevatedTe contentsappearto reflecttheclose
proximityto the Yauricochastock(cf. Afifi et al.,
1988). The stocksand associated
volcanicrocksof
the main part of the late Neogenemagmaticarc are
not alkalic,althoughalkalicrockswere eruptedeast
of the volcanicfront (Thompson,1960; Noble et al.,
1975; Noble and Bowman, 1976; Beckinsaleet al.,
1985). AlthoughelevatedTe contentsare conventionallyconsidered
to be associated
with alkalicrocks
(e.g.,Wattersonet al., 1977;Bonham,1988;Hastings,
AverageCompositionsof Ores from the Enargite,Enargite-Chalcopyrite-Bornite,
Sphalerite-Galena,
and Silver Zonesand the PurlsimaConcepci6nOrebody
I
2
3
Au
0.35
0.038
0.049
0.061
0.030
Ag
5.9
3.4
4.3
6.8
4.6
16.5
2.4
3.0
4.3
1.7
3.8
6.5
1.2
3.6
7.5
0.43
0.68
4.4
7.9
0.4
8.5
8.1
0.16
0.0025
0.0035
0.009
0.046
0.05
0.017
Cu
Pb
Zn
As
16.4
Sb
Ag/Au
Ag/Cu*
Ag/Pb*
Au/Cu*
Pb/Cu
Zn/Cu
Pb/Zn
Sb/As
4
5
0.05
17
1.2
0.07
(very low)
89
4.9
3.9
0.06
1.3
1.8
0.7
(low)
88
8.7
3.9
0.08
2.2
3.8
0.6
(low)
111
19.4
6.5
0.18
3.0
6.3
0.5
0.12
153
23
3.6
0.15
6.5
12
0.6
6
0.020
808
141
6.7
0.18
16
20
1.0
0.33
7
0.12
0.30
2.5
411
294
165
1.4
3.6
0.39
0.38
Specimens:
1, meanof 12 bulk specimens
of rich enargiteore, Main Catasorebody;2, meancomposition,Cuye orebody;3, mean
composition,Main Catasorebody;4, meancomposition,EasternContactorebody;5, meancomposition,Antacacaorebody;6, mean
composition,
WesternContactorebody;7, meancompositionof 12 samplesof PudsimaConcepci6norescut by drill hole PC-1-86
Au and Ag in oz/ton, other elementsin wt percent;data compiledfrom varioussources;exceptasindicatedvaluesfor columns2
through6 are calculatedore-blockaverages;Au, As, and Sb valuesfor EasternContactand Western Contactorebodiesare analyses
of speciallypreparedcompositesamples;ratiosare weight ratios,thosemarkedwith an asteriskhavebeen multipliedby 1,000; the
qualitativeSb/Asratiosin columns1, 2, and 3 are basedon concentrateassaydata
PUR[SIMA
CONCEPCI6N
DEPOSIT,PERU
13 7 5
1988), it appearsthat a significantTe signaturecan Pointerdeposit(Smithet al., 1988),whichis closely
developin nonalkalicterranes(cf. Sillitoe,1988).
relatedto the Ruth(Nevada)porphyrysystem.
Althoughwe lackstableisotopedata,the geologic
for the originof sedimentary
relations,mineralogy,and chemistryof the deposit Generalimplications
suggest
that the initialfluidswere largelyof deep- rock-hosteddeposits
seatedoriginwhichpresumably
mixedto greateror
Becauseof its clear association
with a porphyrylesserdegreeswith meteoricfluidsprior to and/or skarnsystemandmajorpolymetallicmineraldeposits,
duringdifferentiation.
Theseevolved
basemetal-poor Pur•simaConcepci6nhasimplicationsfor the origin
residualfluidswere then structurallyfocusedon a and evolution of the fluids from which certain of the
zoneofrelativelypermeable
andlithologically
favor- sedimentaryrock-hosted,preciousmetal depositsof
ablestrata.The localpresenceof pyrrhotiteandar- the westernUnited Statesandotherregionswere desenopyrite,
the scarcityof arsenicsulfides(andthe posited.Ore solutionscompositionally
appropriateto
presumedpresenceof arsenian
pyrite,arsenopyrite, formAu-, As-,Sb-,T1-andHg-richandAg- andbase
and/ornativearsenic;
Rytuba,1985), andthe pres- metal-poor sedimentaryrock-hosteddepositscan
enceof carbonaceous
materialsuggest
that Au-Ag formfrommagmatically
derivedand/orcloselyrelated
deposition
tookplaceunderconditions
of low S2and hydrothermalsolutions
by a processof hydrothermal
O2 fugacitiesmarkedlydifferentfrom thoseof the differentiation(see,for example,Petersenet al., 1977;
initial solutions.
Hackbarthand Petersen,1984). Derivation of metals
The depthof formationwasat least400 m, because and other componentsfrom metal-bearingcarbonastronglyglaciallyeroded remnantsof Cretaceous ceousor other sedimentaryrocks(e.g., Dicksonet
stratareachelevations
of greaterthan5,000m several al., 1979; Romberger,1986) wouldnot appearto be
kilometers
northandwestof the deposit.The coex- requiredfor the formationof all sedimentaryrockistinghypersalineand vapor-dominated
fluid inclu- hosted deposits.Admixture with meteoric waters
sionsin the Yauricocha
stocksuggest
anevengreater couldmaskanyhigh•i•80 fluidsinvolved.The high
depth,whichwouldbe consistent
with depthsof Au/Ag ratiosof many sedimentaryrock-hostedde1,000 m or moreinferredby somegeologists
from positsmay largelyreflect a combinationof changes
geologicand fluid inclusiondata for certain of the in the Au/Ag ratio producedduringearlybasemetal
sedimentaryrock-hosteddepositsin the western depositionand subsequentselectiveprecipitationof
United States(Kuehnand Bodnar,1984; Bonham, low concentrations
of silverin limestone(seebelow)
1985; Schnorret al., 1986).
and perhapsin other rock typesthat were not lithologicallyfavorablefor the depositionof gold. The
Relationto contactmetasomatic
distaldeposits
presenceof hypersalinefluid inclusionssuggestsa
Sillitoe(1983, 1988) and Bonham(1985, p. 72) similar mechanismof formation for the Candelaria,
Nevada,silver deposit(Fosteret al., 1988). Boiling
have discusseda classof bulk mineable contact metasomaticdistal preciousmetal deposits"believed to (e.g., Cole and Drummond, 1986) may not be rebe related to graniticintrusiverocksbut which lie quiredto explainthe highAu/Ag ratiosandvery low
rock-hosted
beyondthe zoneof well-developed
skarnformation," basemetalcontentsof manysedimentary
as,forexample,
thegolddeposits
at CopperCanyon, deposits.The relative abundanceof carbonate
Nevada (Blake et al., 1984; Theodore et al., 1986; daughtermineralsin the fluid inclusionsobservedin
Wotruba et al., 1986, 1988; see also Orris et al., the potassiumsilicate-alteredportion of the Yauri1987).Purlsima
Concepci6n
wouldappearto blurthe cochastock,andthe localpresenceof pyrrhotite(cf.
with the reldistinction between the "contact metasomatic distal Rytuba,1985), wouldappearconsistent
atively
high
CO2
contents
of
the
solutions
fromwhich
deposit"andsediment-hosted
disseminated
preciousthe
Carlin
and
other
sedimentary
rock-hosted
deposits
metal depositcategories(a possibilitypresciently
recognized
by Sillitoe,1983),with the termapplied in the westernUnited Stateswere deposited(Kuehn
reflecting
thedegreeofassociation
ofthedeposit
with andBodnar,1984; BagbyandBerger,1985; Schnorr
intrusiverocks,the nature of the host material, and et al., 1986).
the initialcomposition
anddegreeof evolutionof the
hydrothermal
solutions
fromwhicha givendeposit Explorationfor similar depositsin centraland
wasformed.Otherexamples
of the closespatialand northern Peru
genetic relationshipbetween sedimentaryrockPrimeexplorationtargetsin theWesternCordillera
hosted,disseminated
ores,gold-bearing
calc-silicate include localitieswhere thin-bedded,silty to sandy
rocks,andhigh-levelsilicicintrusions
are provided carbonaceous siltstone or carbonate rock of the Parby the Bau district, Indonesia(Wolfenden,1965;
T. J. Percivalet al., in prep.), and the Te-rich Star
iatamboand JumashaFormationsis cut or underlain
by stocksor other Cenozoicintrusivebodies.How-
1376
A. AL VAREZ A. AND D. C. NOBLE
ever, the rich distalsilverorespresentlybeingpro- Directorio of CENTROMIN, Guillermo Guanilo,
duced at Colquijirca from the fresh-water Calera Gerente de Minas, Concentradoresy Geologla,ArLimestoneof Paleogeneage (Vidal et al., 1984) and naldoLe6n, Jefe de la Offelnade PlanemientoCorthe rangeof lithologiesknownto hostdisseminated porativo,and JulioPastor,formerDirector del Deoresin the westernUnited Statestogethersuggest partamentode Geologla,withoutwhoseinitiativeand
that explorationshouldnot be restrictedto the Par- supportthis studywouldnot havebeen possible.
iatamboandJumasha
Formations.Telluriummaybe
a usefuladditionto the standardgeochemical
explo- January11, July 7, 1988
rationsuiteof Au, Ag, As,Sb,andHg. Alterationand
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