the porphyry copper deposit at el salvador chile l.gustafson,j.hunt

8/9/2019 The Porphyry Copper Deposit at El Salvador Chile L.gustAFSON,J.hunt

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ECONOMIC GEOLOGY

AID TIE

BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

Vo.. 70 AucusT, 1975 No. 5

The PorphyryCopperDepositat E1 Salvador, hile

LEWIS B. GUSTAFSONND JOHN P. HUNT

Abstract

The formation of the porphyry copper deposit at E1 Salvador culminated volcanic

activity in the Indio Muerto district. Host rocks for the ore are Cretaceous andesitic

flows and sedimentary ocks overlain unconformablyby lower Tertiary volcanics. Early

rhyolite domes, ormed about 50 m.y. ago and roughly contemporaneous ith voluminous

rhyolitic and andesitic volcanics, were followed by irregularly shaped subvolcanic in-

trusions of quartz rhyolite and quartz porphyry about 46 m.y. ago. Minor copper-

molybdenum mineralization accompanied this event. A steep-walled granodioritic

porphyry complexand the closelyassociatedmain center of mineralizationand alteration,

were emplaced 1 m.y. ago.

The oldestof theseporphyries, X" Porphyry, is fine grained, equigranular o weakly

porphyritic. Porphyritic textures are seen n deep exposures,whereas strong K-silicate

alteration at higher elevationshas developed he equigranular exture. Next, a complex

seriesof feldsparporphyrieswas intruded. These includean early group, "K" Porphyry,

and a late group, "L" Porphyry, definedby mappedage relations at intrusive contacts.

Strong alteration and mineralization of most "K" Porphyry bodies have partially

obliterated the porphyry texture. The larger "L" Porphyry complex is relatively un-

altered and unmineralized. A wide range of textural variation in "L" Porphyry is

spatially elated o its conta.ts and evidenceseactionwith intrudedandesitc. Rela-

tively minor porphyrydikes and igneousbrecciacut the composite orphyry stock and

are followed by postmineral atite dikes and clastic pebble dikes. Below the present

surface,pebbledikes exhibit a striking decreasen abundance nd a change rom a

radial-concentric o a nearly orthogonalpattern.

Petrologic trends are obscuredbecausemost intrusive rock types are not exposed

away from the area affectedby alterationand mineralizationand because hemicaland

mineralogicvariation within a single fresh major intrusive unit, "L" Porphyry, is

apparentlygreater than it is across he entire porphyry series. However, rhyolitic

volcanism n the district was clearly more felsic than younger granodioritic porphyries

and producedhigher K20/Na20 ratios. Compared o average granodiorite, he E1

Salvador orphyries re low in total ron and havea smallerK20/NagO ratio. Composi-

tional trends n "L" Porphyry correlate with textural variations. The initial 87Sr/86Sr

ratio of early siliceousextrusive rocks and domes,as well as of the main porphyry

series ndall alterationproducts,s a consistent.704.

Early alteration-mineralizationas mostly accomplishedefore the intrusionof the

last major feldsparporphyry ("L" Porphyry) and contributed robably hree-quarters

of the 5 million tons of copper n the orebody. Early mineralization s characterized y

distinctive quartz veins and largely disseminatedK-silicate assemblages f alkali

eldspar-biotite-anhydrite-chalcopyrite-borniter chalcopyrite-pyrite.Early quartz

veinsare typically ranularquartz-K-feldspar-anhydrite-sulfide,enerallyack internal

symmetry, ndare irregularanddiscontinuous.-silicatealteration f someporphyries

appearso haveoccurred uring inal consolidationf the meltsas well as later. Bioti-

zation of andesiticvolcanicsand an apparently contemporaneousuter fringe of propy-

litic alteration were producedduring this Early period. Except at deepest. xposed

elevationsn the youngerporphyries,ncipientK-silicatealterationconverted ornblende

phenocrystso biotite-anhydrite-rutile,lmeniteo hematite-rutile,ndspheneo rutile-

857

Cortesia de

_Geolibros_


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The PorphyryCopperDepositat E1 Salvador,Chile

Fig. 1. Aerial view of Cerro Indio Muerto, lookingwest, during construction f the E1 Salvadormine. Volcanic peaks

of the high Cordillera, he Salar de Pedernales, nd low hills underlain by folded Mesozoicsediments nd Paleozoicgranite

are in the background. A major north-south fault separates his structural block from the volcanic rocks of the Indio Muerto

district. The main orebodyunderliesTurquoise Gulch, the northwest-facing mphitheater eneath he peak of Indio Muerto.

Limonite-stained iliceous hyolite,quartz porphyry,and Tertiary ignimbrites orm high ridges around Turquoise Gulch.

Dark Cretaceous andesitic rocks are bleached on the lower flanks of the mountain.


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858 L. B. GUSTAFSON AND ]. P. HUNT

anhydrite.Anhydritedepositionccurredhrough he entirehistoryof primaryminer-

alization, nd probablymoresulfur'was ixed as sulfate n anhydrite han in sulfides.

Outward within a central zone of K-silicate alteration with chalcopyrite-bornite,he

proportion f bornitedecreasesntil pyrite appears nd increases s chalcopyrite

diminishes.Pyrite abundancencreases,hen decreasesn an outer propyliticzone

with epidote-chlorite-calcite.n the outermost ropylitic one,minor chalcopyrite-

magnetite einsgive way outward o specular ematite. Pyrite is very closely sso-

ciatedwith sericiteor sericite-chlorite,nd pyrite-sericite-chloriteeining is clearly

youngerhanbothK-silicate nd propylitic ssemblages.he major fringe zoneof

pyrite-sericiteppearso bea relativelyate eature uperimposedcrosshe ransitional

boundaryf the Early-formedones.Patterns f alteration-mineralizationre strongly

influencedy the intrusion f "L" Porphyry,which emoved art of the previously

formedEarly pattern nd argelycontrolledubsequentate events.

A Transitionalypeof quartzveinwas ormed fter consolidationf all major ntru-

sions ndprior to the developmentf Late pyriticandK-feldspar-destructivelteration

assemblages.ransitionaluartz eins ccupyontinuouslanar ractures, hich end

to be flat. They are characterizedy a lack of K-feldspar nd associatedlteration

halosand by the presencef molybdenite. he assemblage-feldspar-andalusiten

deepevelss probably Transitionallteration ssemblage.ourmalinen veinlets

and breccias s closelyassociatedn time with Transitionalquartz veins. The

abundancef tourmalinencreasespward oward hepresent urface.

Late mineralization, haracterized y abundantpyrite and K-feldspar- destruc-

tive alteration,ends o be more fracturecontrolledhan Early and more dis-

seminated mineralization. Late sulfide veins and veinlets cut all rock types, except

latite,andall EarlyandTransitionalgeveins.Theycontain yriteand esser ut

upward-increasingmountsf bornite, halcopyrite,nargite,ennantite,phalerite,r

galena.Quartzand anhydritere the mostcommonangueminerals.Alteration

halossurroundinghesepyrite veinlets re principally ericiteor sericite-chlorite.

These eins ccupy radial-concentricracture etat all evels f exposure.

Vertical oning f Latealterationndsulfidessemblagess welldeveloped.eripheral

sericite-chloriteivesway upward o sericite, hichencroachesnwardon central

zones.Upper evelassemblagesre dominatedy sericite nd andalusitend are

superimposedn EarlyK-silicatessemblages.ericite-andalusitessemblagesre

gradationalith underlyingndalusite-K-feldsparones.Deep-levelarly sulfide

zones, ithantitheticyrite ndbornite,re abruptlyruncatedy laterdisseminated

sulfideones ontainingontactssemblagesf pyriteandbornite ndvariable mounts

of chalcopyritend chalcocite."videnceor sulfideoning igher ithin he eached

cappings based n study f relictsulfide rains.Pyrite-borniteulfideones re

generallyound ith ericiteradvancedrgilliclterationssemblages,ut he roots"

of these ones xtenddownwardnto K-feldspar-bearingower level alteration ones.

Advancedrgillic lterationssemblagesontaining'bundantyrophyllite,iaspore,

alunite,morphorousaterial,ndocal orundumrestronglyevelopedt high le-

vations.These ssemblages,resentn postoreebble ikes,were ormed ery ate in

theevolutionf mineralization. herepreserved,heassociatedulfides pyrite.

Two ypesf fluidnclusionsre oundn Early ndTransitionaluartzeins ut

nevern Latepyritic eins.Theycontainigh-salinityluidcoexistingith ow-

densityluid.Bothxhibitomogenizationemperaturesn he angef360 o>600C.

A thirdype f nclusions foundn veins f all ages,ontainsow-salinityluid, nd

homogenizest lesshan350C.

Supergenenrichmentormedhecommercialrebody,oughly00millionons f

1.6%Cu. Secondaryu-Smineralsxtensivelyeplacedhalcopyritend ornireut

coatedyrite ith ittleor no eplacement.aolinitend lunitere heprincipal

supergenelterationroducts.aoliniteeplaceseldspar,iotite,nd hloriteutnot

sericite. hezonesf supergeneaoliniteredevelopedeneathheupperevel ones

ofstrongericiticlterationndwithinheupperreservedortionsf heunderlying

K-silicatendsericite-chloriteones.Magnetites oxidizedo hematitey supergene

alteration. nhydrites hydratedo gypsumnd hen issolvedy supergeneater o

depthssgreat s900m beneathhepresenturface.

Sulfidesriginallyresentn .theeachedappingave een xidizedo limonite,

composedostlyf arosite,oethite,nd ematite. dominantlyarositicapping

overlies ost f theorebodynd he nner yriticringe.This s surroundedy a


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860 L. B. GUSTAFSONAND .1.P. HUNT

? 8

" MOCHA

' 2Ca' '' C, COLORADO

IQUIQUI=) I

Q.ANCA

TALTA

PELAM.ES 0 50 100 150 200 250

KM.

VALPARAISO

/ SANTIAGO

EL TENIENTE

FiG.2. Locationmapof northernChile.

using the Mulchay and Stephensmap as a guide

examined the leached outcrops within Turquoise

Gulch. During this visit particular emphasiswas

placed on the distributionof quartz mineralization

and other featuresof limonite and alteration, ndicat-

ing a favorable exploration target. Subsequently,

Anaconda'smanagement pprovedPerry's vigorous

recommendationor a major explorationeffort to

test the possibilityof a secondary nriched arget

beneathTurquoiseGulch. Swayne,assisted y John

Bain and Hans Langerfeldt, was then assigned o

map in detail the ruggedslopes f Indio Muerto and

the surrounding istrict and to plan a drilling cam-

paign. The mappingproject was a major under-

taking as no adequatebase maps, roads, or water

existed within the district. These difficulties were

overcome nd an accuratemap was preparedupon

whichPerry and Swayne ogetheraid out four initial

test holes. Swayne'smapping and interpretations

were supplementedy mineralogical tudiesof rock

specimens y Charles Meyer in Anaconda'sButte

laboratories. Meyer was then in charge of Ana-

conda'sgeological esearchand also inspected he

prospect ccompaniedy Salesand Perry during the

subsequent rilling campaign.

In 1951, approval or drilling was given by Ana-

condamanagement. A singledrill rig was allocated

for the initial exploration program. The prime

target in Turquoise Gulch was inaccessiblet the

start and the first two holes were drilled in more,

easily eachableocations. They intersectednterest-

ing but low-gradesecondary oppersulfides n what

subsequently roved to be the outer pyritic fringe

of the orebody. The third hole was drilled to ex-

plore the readily accessible amp Area target and

intercepted1,000 feet of plus one percent primary

coppermineralization. Such an encouraging how-

ing threatened o divert management'snterest rom

the secondarily nriched arget in TurquoiseGulch,

and a fourth hole was drilled near Hole 3. In spite

of the tempting distractionof.the Camp prospect,

Swayne moved the rig back to the relatively nac-

cessible rime targetarea n TurquoiseGulch,where,

in the meantime,a drill road and site had been com-

pleted,and startedHole 5. Completion f this hole

was delayed due to management'snterest in the

interceptof primary mineralization n the Camp

Area, where two additional holes were finished.

Finally, in 1954, Swayne, supportedby Perry,

managedo completeHole 5, intercepting igh-grade

secondarilyenriched ore beneath the barren out-

cropsof TurquoiseGulch, and it ,wasevident hat a

major discoveryhad been achieved.

The development f the E1 Salvador Mine fol-

lowing the discoveryof the TurquoiseGulch ore-

body hrough1959, he first year of production, as


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THE PORPHYRY COPPERDEPOSIT AT EL SALVADOR, CHILE 861

alsobeendescribed y Perry (1960). 'Swayne nd

Trask (1960) described anyof the generaleatures

of the mine and district as well as the geologic

mappingand officeproceduresoutinelyused at E1

Salvador. Several mportantaspects f the geology

were reportedduring he course f the work (Hunt,

1964, 1969; Hemley, 1969; and Gustafsonand

Hunt, 1971).

During the period of Anacondamanagement f

E1 Salvador rior to July 1971,more than 80 man-

years of detailedgeologicmappingand study were

investedn the property. The presentauthorshave

the privilegeof summarizing omeof the resultsof

this effort. A significantpart of this commitment

of manpowerand money was deliberately imed at

the broad objective of developingnew exploration

concepts nd tools hrougha "case-history" nalysis

of a major porphyry copper deposit. E1 Salvador

was selectedor studybecause f excellent nd com-

plete geologic ecordsand because f the unusually

good rock exposure,consistingof surface outcrops

overlyingmore than 200 km of tunnelsand diamond

drill holes which extend over a vertical range of

900 meters.

The main thrust of geologic esearch t E1 Salva-

dor was directed at understanding he detailed

anatomyand evolutionof the Turqu'oiseGulch ore-

body. Broader studies,such as t.he relation of the

Turquoise ulch rebodyo othersmallermineralized

centers n the district and the geologyof the district

itself in relation to the Mesozoic and Cenozoic his-

tory of the AndeanCordillera,were begunbut never

completed. We also regret tha. critical petrological

and chemicalstudiesof both the regional ocksand

alteration-mineralization suites within the mine were

never completed.

The presentpaper attempts o focuson what we

consider to be the main scient. fic result of Ana-

conda'.s eologiceffort at E1 Salvador,namely, de-

scriptionand interpretation f the space-timeela-

tions of volcanism nd porphyry intrusionwith the

concurrentlyevolving mineralizationand alteration

in the main orebody eneath urquoiseGulch.

Geologic Setting

The E1 Salvador mine is located in the Indio

Muerto district in the Atacama Desert of northern

Chile, some 00 km north of Santiago Figs. 1 and

2). During 12 yearsof operation nder Anaconda,

the mineproduced 0 million short onsof sulfide re

averaging1.5.% Cu. The orebody s a "chalcocite"

enrichmentblanket oughly1.5 km in diameterand

up to 200 m thick, underlying he TurquoiseGulch

area. .Surface indications of alteration and mineral-

ization can be observed in the Indio Muerto district

in a north-northeastlongate oneof some5 by 10

km. Actual ore reserves prior to production

(January 1, 1957) were about 300 million short

tons averaging1.6 percent total copper, approxi-

mately 5 million tons of coppermetal. This repre-

sents roughly one-third to one-half of the total

amountof copperdepositedn the district.

The Indio Muerto district and the Potrerillos

porphyry copper deposit, 25 km southeastof E1

Salvador,both lie along the northernedgeof a dis-

sected and eroded lower Tertiary volcanic field,

roughly50 x 200 km in extent,whichcontains hyo-

lite and andesiteextrusives nd numerousgranodio-

rite and quartzmonzonite tocks.These ower Terti-

ary Volcanicswere laid down unconformablyover

foldedand erodedUpper Cretaceous ndesitic ol-

canicand related sedimentaryocks. The Quater-

nary volcanic belt, lying some 60 km east of E1

Salvador n the High Andes,appears o be a recent

analogueof the lower Tertiary field. Erosion and

dissection f the lower Tertiary rockswere aidedby

major northerly rending aults,mostshowing own-

to-the-west relative .displacements nd unknown

strike-slip components. Both E1 Salvador and

Potrerillos have been exposedby erosion, which

progressedo the point of largelystripping he lower

Tertiary volcanics ut not deeplyeroding he under-

lying Mesozoicocks.

Upper Cretaceous(?) ocks,approximately to

5 km thick, are exposedn the northernhalf of the

Indio Muerto district and at lower elevations within

Cerro ndio Muerto itself (Fig. 3). The lower part

of this Cretaceousections dominantly edimentary

and composed f andesitic onglomeratesnd sand-

stone, uffaceousn part, with subordinate ndesitc

flows. The upper part of the Cretaceous ection

containsnumerousandesite lows, subordinate nde-

sitic conglomeratesnd sandstones,nd at least one

silicicpyroclastic nit. Theserocksare very similar

to and probablycorrelatewith the lower and upper

members f the Cerrillos ormation n the Copiapo

area (summarized y ,Segerstrom, 967). In the

Indio Muerto district, he Upper Cretaceousocks

are folded into a faulted antif'ormal structure trend-

ing northerlyand havinga steepwestern imb. In

the vicinity of the orebodies,distinctionbetween

igneous and clastic units within this formation is

impossibleecausef strong lteration nd heyhave

beenmappedsimplyas "andesite."

A series f lower Tertiary andesitic nd rhyolitic

extrusives,ncludingabundantgnimbr,ites, verlies

the Cretaceous ocks and comprisesntertongued

volcanicpiles whose hicknesshas not been deter-

mined. In the vicinityof TurquoiseGulch,at 'least

400 m of siliceous ignimbrites verlie the uncon-

formity and dip gently o the south. The fact that

the steepporphyrycontacts nd sulfideveins n the


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mine dip northerly,perpendicularo thesevolcanics,

suggestsminor southerly ilting or warping of the

districtafter mineral.ization. hesevolcanics rob-

ably correlate with the I-Iornitos formation in the

Copiapo rea (Segerstrom, 967), and therefore he

un.conformityas beennamed he "I-Iornitosuncon-

formity".

On the southeast lank of Cerro Indio Muerto, a

secondunconformitywith sharp ocal relief is seen

cutting hrough he Horn.itosvolcanicsnto an under-

lying windowof Cerrillos ock.. This unconformity

and the thick seriesof overlyingandesitic nd rhyo-

liti'.c volcanics and sediments have been called the

Indio Muerto unconformity nd series, espectively.

Mapping o define he extension f this unconformity

and he detailwithin he volcanicson he southslope

of the mountainwas never completed, o these ea-

tures on Figure 3 are somewhatspeculative. The

Indio Muerto series ocksprobably orrelatewith the

Cerro La Peinetavolcanicsn the Copiapo egion

(Clark et al., 1967).

Intrusive activity centered n the Indio Muerto

districtbeganduringmid-Eocene ith the emplace-

ment of a groupof rhyolite domes, hichapparently

formed one of the volcanic centers for the Indio

Muerto series xtrusives.A second roupof quartz

rhyoliteandquartzporphyry ntrusions as ollowed

by the granodioritic orphyrycomplexaroundTur-

quoise Gulch at the end of the Eocene. It is not

clearhow muchof an edificewas built by either of

thesevolcanicepisodes r how much erosionpre-

ceded he intrusion f the main porphyrysequence.

Only minor copperand molybdenummineralizat.ion

was related o the quartz rhyolite and quartz por-

phyry volcanic vent., but the bulk of mineralization

and alterationaccompaniedhe emplacementf the

final porphyry omplex. Subsequentupergenexi-

dationand sulfide nrichment f the primarymin-

eralization formed the commerc.ial orebodies at E1

Salvador. Supergene nrichment as accomplished

long before he presenterosionsurfacewas formed,

as notedelsewheren the Atacamadesert (Sillitoe

et al., 1968). Oxidizedportionsof the originalen-

richment lanket re exposed n the lower slopes f

Indio Muerto and are overlainby Miocenegravels.

The presentpaper will concentrate n thoseevents

that took place n the TurquoiseGulch area at the

culmination f volcanic ctivity and produced he

main orebodyof the E1 Salvadormine.

Principal Intrusive Rock Types

The Turquoise Gulch center of mineralization

contains complexof siliceouso intermediatentru-

sive rock types.

Indio Muerto Rhyolitedomes

The main peak of Cerro Indio Muerto and the

high ridge to the southwest Figs. 1 and 4) are

formed by two rhyolite domes. Undergroundpene-

trationshavepartially defined he geometryof each

as flaring outwardabove he elevationof the I-Iornitos

unconformity Fig. 5). A third rhyolite body,

locatedon the east flank of Indio Muerto, is petro-

logicallyvery similar to thesedomesand is prob-

ably a more deeplyerodedand steep-walled olcanic

neck. The smaller irregular masseson the north-

east flank of the mountain are dikes and sills of

similar rock intruding Cerrillos "andesites." The

rhyolite domesclearly intrude the rhyolitic pyro-

clastics bove he I-Iornitos unconformitynear Tur-

quoiseGulch. A flow brecc.a of identical rock on

the southeast lopeof the mountain gradesdownhill

into water-worked debris derived from the domes.

These rocks directly overlie the Indio Muerto un-

conformity,which thereforemarks the surfaceat the

time of emplacement f theseearly rhyolites. These

rhyolites re clearlyolder than quartzporphyryand

granodiorite orphyry,being cut by dikes of these

rocks.

Theserhyolites re readily dentifiable s a single

rocktype, alled ndio Muerto Rhyolite (Fig. 6A).

They containpracticallyno quartz phenocrysts, ut

all containmore or lessabundant, to 3 mm pheno-

crystsof alkali feldspar, ecognizableven n strongly

altered areas. A variety of matrix textures are

seen, all suggestingdevitrificationof glass. Flow

banding s commonand widespread.

A singlecomplete hemical nalysis Table 1) and

a few partial analysesndicatea silicacontentrang-

ing from about74% to 77% S.iO2,with K20 ranging

from 4.0% to 6.5% and Na20 from 1.6% to 3.6%.

Quartz rhyoIite

Close to the northeast flank of the mountain lie

two hills of quartz rhyolite, known as Cerro Pelado

and Rhyolite Hill (Fig. 3). This rock type is

characterized y abundant nd usuallysmall quartz

phenocrystsnd relativelyabundant eldsparpheno-

crysts (Fig. 6B). Small biotite booksand K-feld-

spar phenocrysts re commonlypresentbut sparse,

and opaques re pract.ically bsent. Age relations

with the Indio Muerto Rhyolite domes and with

quartz porphyry, describedbelow, are inconclusive.

Cerro Pelado s a steep-walled,omplex ntrusive

center. Quartz rhyolite forms an arcuate massive

plug with arcuateand tangentialdikes. The mar-

gins of the plug are stronglybrecciated.Enclosed

within the circularoutline s what is probably col-

lapsed breccia, containing fragments of andesitic

sediment artiallyengulfed y quartzrhyolite. Cerro

Peladohas many of the characteristicsf a shallow


8/57

SIMPLIFIED GEOLOGIC MAPof the INDIO MUERTO DISTRICT

o z =

MIXED HYITIC &

FAULT

POLD

__ TERTiJ,IY OLeAN,CS--UNJFFEIIENTI&TE0.NCLUDESOMENTRUYEHYOLI?E15 OF UNDETERMINI[:D AGE D(JE TO INCOMPLETE M;INQ

Fro. 3. Simplified geologicmap of the Indio Muerto district. Cortesia de_Geolibros_


9/57

Cortesia de

_Geolibros_


10/57

19500 hi

ROCK TYPES ,TURQUOISE GULCH AREA

o

RECENTETRITUS

LATITE

PEBBLEIKE

CLASTICR CCIA

TOURMALINERECClA

IGNEOUS BRECCIA

"A"PORPHYRY

I'Ll'pORPHYRy

ioo .oo ::5oo

"" PORPHYRy

QUARTZ GRAIN I:ORPHYRy

*[-J"X" ORPHYRY

QUARTZ PORPHYRY

' RHYOLITE INTRUSIVE

RHYOLITEXTRUSIVE

Hornifol

% UNCONFORMITY

ANDESITICEDIMENTS

CerrlllOI fn )

. i

'e.o


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Cortesia de

_Geolibros_


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FIG. 5. Rock types in the E1 Salvador mine, isometric projection.


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Cortesia de

_Geolibros_


14/57

THE PORPHYRY COPPER DEPOSIT AT EL SALVADOR, CHILE 869

volcanic eck It is not certainwhether he nearly

fiat baseof the quartz rhyolite n Rhyolite Hill was

the surfaceon wh.ch it was extrudedor represents

merely he baseof an intrusivesill.

Quartx porphyry

Quartz porphyry s a major intrusive ock type in

the TurquoiseGulchand Old Camp centersof min-

eralization. It is characterizedy usuallyabundant

and large quartz and plagioclase henocrystsn a

siliceous ine-grained roundmass.The texture is

similar o the coarsest uartzrhyolite above), ex-

cept that plagioclase henocrystsre larger (some

> 1 cm) and more abundantand biotite booksmore

prominent Fig. 6C).

Clearly more than one intrusive unit has been

included s quartz porphyry,but only in the Old

Camp area have contacts etweenwo quartz por-

phyries eenmapped..The rregularnorth-trend.ag

dike belweenurquoiseGulchand the Old Camp

area (Fig. 3) containsbundant roken henocrysts,

suggestinghat it was a feeder or pyroclastic x-

trusives. Quartz porphyryat the Old Camparea

formsan arcuate ike,presumablyring dike,wh.ich

occupiesearly170 degrees f a circlearoundCerro

Peladoquartz rhyolite. The large areas of quartz

porphyry n and surrounding urquoiseGulch are

exposures f rather extensiveand thick sills of quartz

porphyry which were .intrudedat the base of and

within he Hornitosvolcan.c pile. In mineexposures

and drillingbeneathheseoutcrops, nly a few small

dikes are seen below the Hornitos unconformity

(Figs. 4 and 5).

There is a striking difference n shape between

the quartz porphyry intrusionsand both the earlier

Ind,o Muerto R'hyolitedomesand later steep-walled

granodioritic orphyries. This suggestshat quartz

porphyrywas intrudedat a differentdepth or at a

different rate than these other intrusions. Quartz

rhyolitehascloser ffinitieso quartzporphyry han

to Indio Muerto Rhyolite in texture and shape.

Quartz rhyoliteand quartzporphyryare interpreted

as beingclosely elated ntrusions.

All of the quartzporphyry n the mainTurquoise

Gulcharea is moderatelyo stronglyaltered. The

singlechemical nalysis f quartz porphyry (Table

1) is of a sericite-chlorite altered dike rather than

of fresh rock. The alterationmay account or the

relativelyhigh FeOa/FeO and KO/NaO ratios

reportedn the analysis.

Fit;. 6. Texturesof intrusive ocks elated o early rhyolitic olcanic vents.

A, IndioMuertoRhyolite.Flowbanding,evitrificationexturesquartz ndalkali eldspar), ndsparse mallpheno-

crysts f alkali eldsparharacterizehecluster f rhyolite omes n andaroundCerro ndio Muerto. Quartzor biotite

phenocrysts re not seen. (Nonpolarized ight)

B, Quartz hyolite.Abundant henocrystsf quartzandalkali eldspar recommonlyragmental,ndbiotite books"re

smallandsparse.Groundmasss a very finegranularntergrowth f quartz,alkali feldspar, nd sericite,which shows

neitherlowbandingor heusual evitrificationextures.Coarse arieties ith some lagioclasehenocrystspproach

quartz porphyry n texture. (Cross-polarizedight)

C.,.Quartzorphyry. argephenocrystsf plagioclasend quartz re set n a fine-grainedroundmassf quartz nd

sencte.Biotite books"reprominent,ut n thisspecimenre alteredo sericite,s s theplagioclase.Cross-polarized

light)

Note hat ike thephotographsn Figures , 8, and9 these re negativerintsmade y usinghinsectionsirectly s

negativesn the enlarger,with or withoutpolarizing heets.


15/57

870 L. B. GUSTAFSON AND .L P. HUNT

TABLE1. ChemicalAnalysesof Intrusive Rocks. The samples re from the freshest nt most weakly mineralizedexposures

of each ype in the minearea,but mosthave beenaffectedby significantmineralization nd alteration. Analyseswere

madeby the Japan Analytical Research nstitute, except for (1), which was made by The AnacondaCo. In

sampleswith significant mountsof sulfides, he ratio of Fe2Oa o FeO is erroneously igh.

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

SiO2 75.86 60.11 57.75 62.93 56.58 64.31 64.53 65.09 62.46 53.85 59.23

AlcOa 12.87 15.01 .16.44 14.66 17.41 16.29 16.10 15.03 17.39 16.66 15.59

FeOa 0.44 1.40 0.59 1.00 3.44 2.63 1.23 2.05 2.42 2.03 3.10

FeO 0.75 0.47 2.22 1.00 2.72 1.77 1.37 1.27 1.64 1.05 1.71

MnO 0.00 trace 0.06 0.01 0.03 0.03 0.01 0.02 trace 0.02 0.05

MgO 0.05 1.36 2.58 1.33 2.15 1.60 1.34 1.31 1.48 2.43 2.13

CaO 0.23 5.44 6.39 4.66 6.14 4.34 4.55 3.87 4.40 6.64 5.41

NaO 3.44 1.99 4.04 6.73 4.65 4.79 3.99 3.56 4.29 5.59 4.31

KO 5.13 3.77 2.28 1.45 1.57 1.79 2.30 2.68 3.58 1.89 2.73

H20(+) 0.39 2.55 1.04 0.62 1.18 0.98 1.24 2.47 0.93 1.25 1.32

HO(--) -- 1.09 0.28 0.95 0.53 0.41 0.53 0.77 0.29 0.51 2.30

P20, 0.00 0.28 0.85 0.55 0.48 0.32 0.26 0.22 0.20 0.22 0.29

TiO 0.24 0.14 0.99 0.62 0.73 0.71 0.43 0.48 0.45 0.66 0.96

SOs 0.00 6.30 3.79 3.43 2.24 0.55 2.27 1.83 0.45 6.23 trace

S 0.02 0.63 0.20 0.38 0.22 0.08 0.28 0.57 0.25 0.07 trace

CO2 0.17 0.27 0.08 0.26 0.33 0.24 0.40 0.35 0.04 0.53 1.55

F 0.013 0.02 0.04 0.03 0.06 trace 0.04 0.04 0.04 0.04 0.03

Cu 0.00 0.26 0.53 0.50 0.06 0.03 0.15 0.13 0.03 0.27 0.01

Subtotal 99.60 101.09 100.15 101.11 100.52 100.87 101.02 101.74 100.34 99.94 100.72

Less O

equivalent

for S --0.01 -0.32 -0.10 -0.19 -0.11 -0.11 --0.14 --0.29 -0.12 -0.04 --

Total 99.59 100.77 100.05 100.92 100.41 100.83 100.88 101.45 100.22 99.90 100.72

Sp. gr. 2.52 2.68 2.69 2.66 2.73 2.67 2.66 2.66 2.66 2.70 2.56

(1) Indio Muerto Rhyotite, ES 1693; practicallyunmineralized nd unaltered;surface.

(2) Qua,,rtzorphyry,S2702; ericite-chlorite-anhydrite-chalcopyrite-bornite;400evel.

(3) "X Porphyry, ES 2699; K-feldspar-biotite-anhydrite-chalcopyrite-bornite;400 level.

(4) "K" Porphyry, DDH 547-180 m; K-feldspar-biotite-anhydrite-chalcopyrite-bornite; ,460-m elevation.

(5) "L" Porphyry, ES 2691; no aplitic groundmass,iotizedhornblende nd anhydriteveinlets;2400 evel.

(6) "L" Porphyry, ES 2689; (-) aplitic groundmass, ractically reshand unmineralized; 400 level.

(7) "L" Porphyry, ES 2688; (4-) aplitic groundmass, eak chloritization,sparse halcopyrite n "alkali seams";2400 level.

(8) "L" Porphyry, ES 2687; (4-) aplitic groundmass, eak sericite-Na-plagioclase-chloriteith sparse halcopyrite-pyrite;

2400 level.

(9) "L" Porphyry,ES 2703; (4-) coarse pliticgroundmass,ractically reshand unmineralized;400 evel.

(10) "A" Porphyry, ES 2701; "mineralized" exture, biotitie-alkali feldspar-anhydrite-chalcopyrite-bornite;400 level.

(11) Latite, ES 2695; moderatemontmorillonite-calcite lteration; 2400 level.

"X" Porphyry

The oldestof the main seriesof granodioritic or-

phyries n TurquoiseGulch s knownas "X" Por-

phyry. (The main intrusive ock types n the E1

Salvadormine were arbitrar.ily iven etter designa-

tions,X, 14,L, etc., eferring o crosscutsn original

explorationworkingswhere hese ock typeswere

well exposed.) This porphyrywas referred o by

Swayneand Trask (1960) as "fine-grained rario-

diorite." As shown n Figure 4, there are three

mainbodies f "X" Porphyry yingalong he north-

northeastxial rendof theporphyry omplex.The

central body forms a discontinuousringe about a

younger eldspar orphyry ntrusion.

"X" Porphyrycharacteristicallyendsmany r-

regular .dikes nto andesite. Recrystallizationf

andesitentoa relatively oarse, quigranulariotized

rock n the immediate icinityof the contactocally

makesrecognition f the intrusivecontactdifficult,

especially here urther complicatedbysuperimposed

hydrothermal lteration. Definitive age relationsat

contacts etween X" Porphyryand quartzporphyry

have not been found, but gross geometry strongly

implies hat the steep"X" Porphyry stockscut the

quartz porphyry sills. Younger feldsparporphyries

("K" and "L") clearly ntrude"X" Porphyry. "X"

Porphyrycontactsocally runcateearly quartzveins

with sulfides n andesire,but most quartz veins cut

across these contacts.

The weakly porphyritic exture of "X" Porphyry

is best observed n deep undergroundexposures,

where the rock is least altered (F. g. 7A). In ex-

posuresat higher elevations, he rock is strongly

altered and appearsequigranular,with only sparse

evidence f a porphyritic exture (Fig. 8C). Plagio-

clasephenocrystsre commonly bliterated y alkali

feldspars nd hornblende henocrystsy biotiteand

alkali feldspar. Over broad areas, there is n'o evi-

dence f an originallymore orphyritic exture.Small


16/57

THE PORPHYRY COPPERDEPOSIT AT EL SALVADOR,CHILE 871

dikes of "X" Porphyrygrade into aplitesat their

extremities, as do some small dikes of other por-

phyries. It is not fully clear whether the equi-

granular texture of "X" Porphyry at upper levels

was developed hroughpost-consolidationl.teration

of the rock or whether it was developedprimarily

during final stagesof consolidation f the melt.

"K" Porpl,yry

Followingeraplacementf "X" Porphyry, a com-

plex series of feldspar porphyries was intruded.

"Feldspar porphyry" is a textural term meaning

porphyry characterized primarily by plagioclase

phenocrysts, ith an abundance f mariephenocrysts

but lackingprominentquartz and K-feldsparpheno-

crysts. The main massof feldsparporphyry in the

TurquoiseGulch area is separated nto an early "K"

Porphyryand a later "L" Porphyry.

"K" Porphyry occupieshe southeasternobe of

the main mass of feldspar porphyry in Turquoise

Gulch. It is older than the main mass of "L" Por-

phyry to the northwestbut intrudesandesiteand the

fringingmassof "X" Porphyry. This is established

not only by dike shapes ut by truncationof quartz

veins and alterationassemblages.While rock tex-

ture, degree of alteration and nfineralization, and

locationare useful or field recognition f these ock

types, t is the age relationships t the intrusivecon-

tacts that were used to define each porphyry rock

type.

"K" Porphyry is best described s an intrusive

complex,as within its main body many local con-

tacts between intrusive surges of "K" Porphyry

have been mapped. There is a fairly wide range

of textural variation within "K" Porphyry. As

with "X" Porphyry, it is not entirely clear how

much of this is due to post-consolidationlteration

and how much o reacti'on etween rystals nd late-

stage melt and fluids during final crystallization

(Figs. 7B, 8A, 8B). Most "K" Porphyryexposed

in the mine is at least moderately ltered to po-

tassium ilicate ssemblages.

"L" Porphyry

The largestmassof feldsparporphyry n Tur-

quoiseGulch is "L" Porphyry. It is a complex

steep-walled tock with a crudely arcuate outline,

nearly 1 km across. "L" Porphyry cuts quartz

porphyry, K" Porphyry,and "X" Porphyry. It is

also younger han much,but not all, of the altera-

tion and mineralizationn the deepcentralpart of

the ore zone. Although intrusive contactswithin

the massare difficult o recognize, nough avebeen

seen ocally to indicate hat this stock s also made

of a numberof separatentrusiveunits. The south-

eastern obe shows he clearestevidenceof multiple

intrusionof feldsparporphyrymagma. Here dikes

of both marie feldsparporphyry ("A" Porphyry)

and igneous receiawhichclearlycut "L" Porphyry

are in turn cut by dikesof porp'hyrywhich are in-

distinguishablerom the host "L" Porphyry. So

close s the similarityof early and late surgesof "L"

Porphyry hat contacts etween hem can be traced

for only short distances.

"L" Porphyry s the only one of the major intru-

sive rockswith exposure resh enough o determine

the original composition nd petrography. The tex-

ture and composition ary markedly. However, all

textural variants are characterized by abundant

phenocrysts f plagioclase, iotite, hornblende, nd

locally quartz. These are enclosedn a matrix of

quartz, alkali feldspar, and biotite and(or) horn-

blende,with accessory ircon, apatite, sphene,mag-

netite,and ilmenite (Figs. 9 and 10).

The major texture variation is in the abundance

and grain size of the groundmass. Where micro-

scopicextureof the groundmassisa "sugary"equi-

granular mixture of relatively fine grained quartz

and alkali feldspar, with marlcsand other accessory

minerals, it has been called "aplitic" groundmass.

This is characteristic f most "L" Porphyry as well

as the least altered exposuresof "K" Porphyry.

Along with variation in the abundanceof aplitic

groundmassre seen ather systematic ariations n

the abundance f quartz phenocrysts, olor index,

and ratio of identifiable iotite and amphibole heno-

crysts o total biotite plus amphibole. The sizesof

plagioclase henocrysts nd the ratio of hornblende

to biotitephenocrystshowno systematic ariations.

Systematic extural patterns have been mapped

within "L" Porphyry (.Fig. 11). Areas of abund-

ant aplitic groundmass,quartz phenocrysts, ow

marie content, and a high proportion of marlcs as

phenocrystsgrade into relatively nonporphyritic,

more marie rock with no quartz phenocrysts ear

contacts with biotized andesite. This transition is

accomplishedy both truly gradationaland abrupt

changesn one or a combination f the textural fea-

tures. Such marie contacteffectsare absentor only

weakly developedwhere "L" Porphyry intrudes

early porphyriesor previouslymineralizedand bio-

tized andesite.A miniature (5 cm), nonporphyritic,

mafic porphyry rim has .been ound surrounding

small (10 cm) inclusionof biotized andesitewithin

one of the high groundmass orphyry centers. On

both scales,Na20 risesand K20 dropsapproaching

the "andesite"rom high-groundmassorphyry.Evi-

dently, reaction with the intruded "andesite" s the

chief cause of the textural variations.


17/57

872 L. B. GUSTAFSON AND I. P. HUNT

Fro. 7. Textures of intrusive porphyriesof the main Turquoise Gulch porphyry series (except "L" Porphyry, Fig. 9).

A, "X" Porphyry. Euhedralplagioclase nd biotite clustersafter subhedral ornblende re seenonly in deepexposures.

The anhedralgranular exture o trongK-silicatealteration (Fig. 8C) is more tpical. The anhedral nterstitialmaterial s

quartz, K-feldspar,biotite,and anhydrite. Biotite "books"are rare and confined o a few contactzones. (Cross-polarized

light)

B, "K" Porphyry. Euhedralplagioclase henocrysts ith biotite "books" nd local quartz "eyes" n an "aplitic" ground-

masscharacterize elatively unaltered"K" Porphyry. This is petrologically ery similar to "L" Porphyry (,Fig. 9), but

low-groundmassariantsare not seen n "K" Porphyry. Argillic alteration gives mottled appearanceo the plagioclase.

(Cross-polarized light)

C, "A" Porphyry. Plagioclasend hornblendearrow) phenocrystsre surroundedy an abundanteldspathic"round-

mass Fig. 10C), which s characterizedy tiny plagioclaseaths and abundantmarlcs, suallyhornblende.A "wormy" n-

tergrowthof alkali feldspars seen n outergrowthzonesof someplagioclase.A wide rangeof normal extural variation,

involvingmostly he abundance,exture,and mineralogy he groundmass,s seen n essentiallynmineralizedndunaltered

"A" Porphyry. (Cross-polarized light)

D, Latite. Euhedralplagioclase henocrystsre commonly ltered o a "wormy" intergrowthcontainingmostly alkali

feldspar,montmorillonite, nd calcite. Quartz, amphibole, nd biotite phenocrystsre relatively sparse. Groundmass on-

tains iny sodic lagioclaseathswith interstitialK-feldspar, uartz,andabundantmphibole, agnetite,nd lmenite. Non-

polarized light)

"A" Porphyry

"A" Porphyry is the name given to a group of

relatively minor intrusive bodiescharacterized y

rather sparse plagioclasephenocrysts n a fine-

grained,dark groundmassontaining bundant, mall,

growth-zonedplagioclase rystals (Figs. 7C, 8D,

10C). "A" Porphyryoccurs n dikesranging rom

a few centimeters to more than 10 m in thickness.

Although the largest dikes are continuous or more

than 100 m, most are quite irregular and cannotbe

traced for more than a few tens of meters. Some

seem o have been raplaced s a seriesof discontinu-

ous pods. Most "A" Porphyry dikes are younger

than most of the "L" Porphyry, perhapsemplaced

during he ate stages f "L" Porphyry ntrusion. In

a few exposureshere appears o be a closespace-

time association between the intrusion of "A" Por-

phyry and the formation f tourmaline reccias.


18/57

THE PORPHYRYCOPPERDEPOSIT AT EL SALVADOR,CHILE 873

FxG. 8. Textures of strong K-silicate alteration in "X", "K", and "A" Porphyry.

A, "K" Porphyry with porphyritic texture (Fig. 7B) stronglyobliteratedby replacement f phenocrysts nd recrystalliza-

tion of groundmass. Plagioclase s rimmed and veined by perthite, with oligoclase ypically separating any unreplaced an-

desine rom the perthire. The relatively coarse, ragged "perthitic" groundmass Fig. 10B) assemblage eplacesbiotite pheno-

crysts as edgesof plagioclase. Diagonal "A" quartz vein. (Cross-polarized ight)

B, "K" Porphyry with poorly defined area of fairly clean, residual porphyry texture within an area of texture obliterated

by intenseK-silicate alteration. Within "K" Porphyry there is a general correlation between ntensity of texture obliteration

and abundanceof "A" quartz veining. (Macrophotograph)

C, "X" Porphyry with much of the plagioclase eplaced by alkali feldspar and relatively coarsequartz and perthite in the

matrix (Fig. 7A). Irregular clots of "shreddy" biotite do not suggest hornblende pseudomorphs. This texture is wide-

spread n "X" .Porphyry with no evidenceof any structural control. (Cross-polarized ight)

D, "A" Porphyry "mineralized' 'texture (cf. Fig. 7C). Some plagioclasephenocrystsare replaced by alkali feldspar-

biotite-anhydrite (B), and a miarolitic cavity(?) filled with anhydrite-biotite-quartz-bornite s marked C. The trachytic

groundmasscontainsvery fine grained plagiodase laths and biotite. This texture characterizesdikes (or extensionsof dikes

with normal texture, as in Fig. 7C) which intrude previouslywell mineralizedrock in the central portions of the deposit.

(,Nonpolarized ight)

Igneousbreccias

Intrusive rockscontainingmore or lessabundant

heterogeneousock fragments n an igneous (i.e.,

originally magmatic,not alastic) matrix are here

called"igneousbreccias." Four of the largestbrec-

cia masses re shown n Figure 4B, as they are ex-

posedon the 2600 level.

The Main Breccia,which is an arcuate eature near

he contact between "L" Porphyry and "K" Por-

phyry, is the bestexposed. Near the 2,600-m eleva-

tion, where he Main Brecciacutsseveral ock types

(Fig. 5), it contains bundantheterogeneousock

fragmentsn a groundmass f alkali feldspar,quartz,

and biotite with chalcopyrite, ornite, and futile.

Sixty metersbelow, the breccia s .smaller,contains

practicallyno fragments, nd is confinedwithin "L"

Porphyry. The rock, which has a sharp intrusive

contactwith the "L" Porphyry, ooks ike little more

than a foliatedor "stretched ut" surgeof "L" Por-

phyry magma. At higherelevations,he Main Brec-

cia crossesnto the "K" Porphyrywith little change

other han an increasen "K" Porphyry ragments.

Above the 2,710-m elevation, an arcuate mass of

intenselyrecciatedK" Porphyry ontainingbund-


19/57

874 L. B. GUSTAFSON AND J.P. HUNT

ant,quartz ragments verlies he upward projection

of the Main Breccia. The deepexposures ppear o

represent he roots of the breccia. It is not clear

whether he arcuatebrecciatedoneat highereleva-

tions epresentsrecciationelated o the intrusion f

igneous reccia r to a prior structural ventwhich

Fro. 9. Textural variations within E1 Salvador "L" Porphyry.

.A_, aximum-groundmassexture.Most plagioclasehenocrystsre isolatedn (+) "aplitic"groundmassseeFig. 10.A_).

Plagioclasere oscillatory oned An.so,suallyhavenormally oned ims Am, havemoderately ell ordered tructures,

and range rom 1 to 5 mm in size. Phenocrystslsoof quartz, biotite,and hornblendebiotized); accessoriesre zircon,

apatite, phene,magnetite, nd lmenite. (Cross-polarizedight)

B, Intermediate-groundmassexture.Mostplagioclasehenoc.rystsn point ontactn () "aplitic" roundmass.lagio-

claseare slightlyaltered,but there s no systematicariation n size of plagioclase. (Cross-polarizedight)

C, Low-groundmassexture. Most plagioclasehenocrystsn edge ontact. --) "aplitic"groundmasss relatively oarse

andragged ndhasa relativelyow alkali feldspar-quartzatio. (Cross-polariz.edight)

D, No-groundmassexture.Quartz ndbiotite utalmost o K-feldspar re interstitialo plagioclase.his texture s

developedearcontactsithbiotized ndesiteFig. 11) and s a reactionim about n nclusionf andesite ithinporphyry

with (+) "aplitic"groundmass. Cross-polarizedight)

E, Porphyriticabitof marlcs,iotite, iotizedornblendeithinporphyry ithmaximumaplitic" roundmass.A_). ine

disseminatedpaquesre magnetite nd hematite-rutile fter ilmenite. (Nonpolarizedight)

F, Irregular shreddy"abitof biotitewithinno-groundmassorphyryD). Degree f anhedral abitof marlcsanges

between andF, correlates ell with abundancef "aplitic"groundmass,nd s easier o map. (Nonpolarizedight)


20/57

THE PORPHYRY COPPERDEPOSIT AT EL SALVADOR,CHILE 875

O.mrn

FiG. 10. Microscopicexturesof groundmassn porphyritic ocks.

A, "Aplitic" groundmass,ypicalof unaltered eldsparporphyries.Sugarygranularmixture

of subrounduartzandalkali feldsparwith moreor less inegrainedbiotiteand accessorye-

Ti oxides. The alkali feldspar s not perthitic, but its composition nd structureare not known.

Minor amounts f sodicplagioclase ay be present. In hand specimen,specially f rock with

no sericiticor argillic alterationwhere the groundmasss relatively fine grained, this kind of

groundmass ommonly ppearsaphanitic. (Cross-polarizedight)

B, Perthitic groundmass,ypical of strong K-silicate altered feldsparporphyries. lelatively

coarse, ragged mixture of quartz and perthitic alkali feldspar with more or less fine grained

biotite. This texture is developed oth by alteration of "aplitic" groundmass nd by original

crystallization_ Becauseof its coarseness,his groundmass arely appears to be aphanitic,

even n hand specimensacking sericiteor argillic alteration. (Cross-polarized ight)

C, 'eldspathic" groundmass, ypical of "A" Porphyry. It is composed argely of plagio-

clase laths, usually growth zoned with more calcic cores, and abundant marlcs with minor

quartz and rare K-feldspar. Marlcs are most commonly hornblende, usually biotized, and

commonly with a fine acicular habit. Groundmass intermediate between this and "aplitic"

groundmass ccurs n some "L" Porphyry. (Cross-polarized ight)

was merely ollowedby the intrusionof the igneous

breccia. Dikes of "A" Porphyry and of still later

"L"-type porphyrycut this brecciaon the 2600 level.

Latite

A seriesof northwest-trendingatite dikes s ex-

posedacross he district, as well as in the mine area

(Figs. 3 and 5). These are the only truly postore

intrusive ocksat E1 Salvador. The dikescut practi-

cally all mineralizationand alteration features n the

mine. The typical texture and petrography f the

latite are illustrated n Figure 7D.

Pebble Dikes

Pebbledikesare a conspicuouseatureat E1 Sal-

vador, especially t the surfaceand on upper evels.

Like latite dikes,with which they showvery close

spatialand temporalrelationships,he pebbledikes

postdate earlyall primary mineral.ization.Much of

our understandingf the pebbledikesat E1 Salvador

is derived rom the work of Langerfeldt 1964a).

Pebbles dikes at El Salvador are dikelike features

filled with alastic material, generally containing

abundant oundedpebbles Fig. 12). The width of

thesedikesranges rom less han cm to 2 m, with

rare bulges o 6 m. Their continuity long strike

ranges rom a few meters o more than 1 km. Few

pebbledikeshave a verticalcontinuityof more than

600 m below the present surface. There is one

circular outcrop of pebble breccia on the surface

which s presumably "pebblepipe." The abundance

of pebbles elative to matrix varies widely. The

matrix consists f pulverized ock and vein material,

ranging in size from silt to coarsesand size.

The degreeof roundingof a pebblecorrelates n a

rough way with the distanceof travel of the pebble.

Angular pebblesalmost nvariablyare of the same

rock types as the immediate endosing wall rock.

Well-rounded ebblesmay haveoriginallycome rom

lower or higher elevations han where exposed,al-

though this is usually indeterminate. The Cre-

taceous ndesRes re readily converted o sand ma-

trix and angular slabs,while porphyry rock types

tend to round readily and can be found relatively

far from their source. There is a general ack of

evidence f long-distanceransport f pebblesn these

pebble ikes. However, n the two largest nddeep-

est known pebbledikes, pebblesof barren, coarse,

subporphyriticock, presumablyrom significantly

deeper evels,are found. T,hesepebbles ould be

samplesof a subjacentcupola of a granodiorifie

batholithying below he porphyrycomplex.

Cortesia de

_Geolibros_


21/57

876 L. B. GUSTAFSON AND 1. P. HUNT

Fro. 11. Abundance f apliticgroundmassn "L" feldsparporphyry.

Flow bandingof the matrix of pebble dikes is

commonlyobserved. Many pebble dikes, particu-

larly the small ones,are irregular in both thickness

and attitude. Thesecommonly ollow sharpchanges

in direction between ntersectingstructures.

Peb.ble dikes occupy preexisting throughgoing

structures,especially ate 'hydrothermal ein struc-

tures. Late hydrothermal ein materialand ground-

up alteration halo material are very abundant n

pebbledikes. T.he surfacepattern of pebble dikes

(Fig. 4A), as mappedby Hans Langerfeldt,shows

a distinct adial pattern with a few circumferential

structures. There is a strongcorrespondencef this

structural pattern with the pattern of "D" veins

described elow (see Fig. 22). In strikingcontrast

to this pattern is the nearly orthogonalconjugate

pattern of pebble dikes at the lower levels in the

mine (Fig. 4B), even at levels where radial vein

fractures do exist. On these lower levels, pebble

dikes have the northwest and northeast trends of

la-te egional aults n the district and do not occupy

the radial vein set except n areaswhere this trend

is parallel to the northwestor northeastdirections.

Evidently he adial set of fractureswas opennear

the surface t the time of pebble-dikeormationbut

was not openat depth.

There is a striking decreasen the abundance f

pebbledikes rom the surfacedownward, specially

below he Hornitosunconformity t roughly2,800-

m elevation. Many pebble dikes seem simply to

terminate downward. In other areas, especially

whereparallelswarmsof pebbledikeson the surface

overliesinglemajor pebbledikesa.tdepth,a splitting

of the major dikesupward s implied.

Very close elationships etween atite dikes and

pebblebrecciashave been noted in a number of ex-

posuresFig. 12). The margins f latite are usually

faulted ndoccupiedy pebble ikes.Round,polished

pebbles lucked rom the pebbledikesare occasion-

ally included n latite, and in at least one instancea

pebbledike is clearly truncatedby latite. On the

other hand, pebbledikes ocallycontaincompletely

isolatedbut unrounded ragmentsof latite. Latite

dikesalso avor northwest-trendingaults,whichare

the principalociof the deeppebble ikes.

On deep evels,pebbledikesare relatively resh,

with weak calcite and chlorite alteration of their

matrix material. Near the surface,many pebble


22/57

THE PORPHYRYCOPPERDEPOSITAT EL SALVADOR, HILE 877

5 mm

I Real16sticatrixboutyroelasticebblesn

pods;oesowell efinedebbleikecrossack.

Alteration banding n latite

J ..... ( Incipientosticroundmossevelopedn

tmcreamngl ltered toward margin

I plocen rregularones boutragmenled

I )/andesJte(?)witholiati... dislurbed.

I-'i(-:'/,/ ./[/(_.'..'*.'?"_(///.yroclastics

-/- f/:' .; / ,: - I '..'- // ,Oh ood

l-,tJ .:.: : ,/ .: .: J' -- ' ///. -- /_-/ ' soft lat frag-

[ , .-'-' ' .' .-' / / - . d4.'f ..... ,,.

(Roundebblesncluded

in Iotite "squirt"

D

TUNNEL No. 5 LATITE DIKE-SKETCH EAST WALL (e?e to e9om

1:.1.00 L.B. GUST,&FSON

I

Fro. 12. Pebble breccias and latite dikes.

A, Surfaceexposure, howing oundedpebblesn sandyclasticmatrix. At high elevations,mostpeb-

ble dikes are altered to advancedargillic assemblages.

B, Sawed specimen f pebbledike from the deepest evel. Subangular o round porphyrypebbles re

not altered.

C, Photomicrograph f the clastic matrix of a pebbledike showing low banding.

D, Sketchof a drift wall, showingclosespatialassociationnd contradictory ge relationships etween

pebbledikes and latite dikes.

dikesobviously uide very intensesericiticand ad-

vancedargill.ic lteration. A few younger, elatively

nonsiliceousebbledikes lacking advanced rgillic

alteration cut siliceousand highly altered pebble

dikes. This and the fact that the youngerand older

pebbledikescan be interpretedas belonging o two

different radial sets about two different centers,

roughly 600 m apart, suggests t least two distinct

episodes f pebble-dikeormation.

Geochemistry] the ntrusive ocks

Somechemical nalyses f the leastalteredavail-

able exposures f intrusiverocksa.t E1 Salvadorare

presented n Table 1. In most cases,only single

samples f eachrock were analyzed. The one in-

trusiveunit, "L" Porphyry,which was sampled o

represent he range of textural variants, shows a

wide compositionalariation in most elements, l-

lustrating he problemof adequately ampling hese

rocks. An even more serioussamplingproblem s

the fact that truly fresh samples f the mineralized

rocks are not exposed. Unaltered samples an be

obtainedonly of postmineralntrusive ocksor so

far away from the centerof mineralizationhat cor-

relation with the mineralized rocks is uncertain.

With these qualificationsn mind, we tentatively

concluderom thesedata that the early rhyolitesare

more siliceousand have higher KaO/NaaO ratios

than the granodioriteporphyriesassociated ith the

main period of mineralization. Later dike.s "A"

Porphyryand atite) appear o havestill lower silica,

higher iron, and possiblyhigher alumina contents

than earlier intrusive rocks. Comparedwith Daly's

and Nockolds'granodiorites Poldervaart, 1955),


23/57

878 L. B. GUSTAFSON AND J. P. HUNT

I

I

Rb-SrAgeNumbernparenthesessnumber

, of somples n isochron )

,rTTTT'?Tq'rl',Ar Age

Indio Muerto Series Volcanics (4)

> Indio Muerto Rhyolite Domes 6)

461+_.s > QuortzhyohtesmclercHe)$)

SermHe,Cerroelodo

(1111111111111 erlcHeldComp

I Fspor.orph.-SericHe-LotHeBohte,hornblendeserralie)T)

(llll> Bmhzed ndesHeBiotite)

o'5os uHllspor.orphBinroe)I)

iillll]11111 Porph.Hornblende)

Gronitaulchspor. (Biohte)

SericifeK"Porphyry

9 LotHeBlotlie)

39.t 11

Prlmory lumte

Supergene lumte

o2 5

III Ill I III I1111111il I I Supervenelunile

I I I I I I I I [ I I MILLION YEARS B.RI I I I I I I I I I I

55 50 45 ,0 5 30

Fro. 13. Selected radiometric

the freshestE1 Salvador granodiorites i.e., "L"

Porphyry) are on the low side but within "normal"

limits for silica, ower in total iron, and have ower

K20/Na20 ratios. Relativelyconsistentomposi-

tional rendswithin"L" Porphyrycorrelatewith tex-

tural variation. Approaching ndesite ontactsrom

high-groundmass reas, there is a decrease n SiO2

and KO and an increase n AlcOa, CaO, Na20,

total Fe, MgO, and TiO2. This correspondso the

increase n plagioclase nd biotite (and/or horn-

blende)and decreasen quartzand alkali feldspar.

Reactionwith the andesitic ost rocks s indicated,

but insufficient work has been done to define the

processesnvolved.

Radiometric Age Dating

The "absolute" gesof events ttending ormation

of the El Salvador re deposit avebeen ather well

documentedy extensiveadiometric ating. In all,

37 independentge determinationsave beenmade

by K-Ar and Rb-Sr methods n whole ocks,biotite,

hornblende, ericite,alunite,and jarosite. Several

of these eterminationsereduplicationsy differ-

age dates, ndio Muerto district.

ent methods and different laboratories on the same

specimen.Most of the dateswere determined y

ChristopherBrooks at the Carnegie nstitution's

Department f TerrestrialMagnetism nd at Mon-

trealUniversity. The results resentedn Figure 13

are considered to be the most reliable. Determina-

tions consideredo be geologicallympossible r

which have been supersededy more geologically

consistent determinations have been discarded and

are not shown.

Rubidium-strontium echniqueswere required to

read hrough ater thermalevents o define he time

gap betweenhe two seriesof rhyolitedomes nd

the mainporphyry eries. An ageof about46 m.y.

is well establishedor the quartz rhyolite on Cerro

Pelado nd RhyoliteHill and for the sericite ltera-

tion in the Cerro Pelado center. Six whole-rock

specimensf quartz rhyoliteyield an isochron f

45.4--+1.4 m.y., with an initial strontium atio of

0.7040. Included n this isochron re two specimens

altered o sericite, ndicated y geologicmapping o

be closely elated n space nd apparently lso n

ime to the intrusive event. The most Rb-enriched

Cortesia de

_Geolibros_


24/57


of these-sericitepecimensieldsa mineralage of

46.1+--0.5m.y. whenan initial strontium atio of

0.7040 is used. A singleK-Ar age of 45.6---+ .3

m.y. on sericite rom the Old Camp supportshe

geologic rguments reviously resentedhat the

intrusion f quartzporphyry s closely elated o the

quartz hyolite olcanicvent nd hatmineralization

in these ntrusivecenters s closely elated to the

volcanicevents. To avoid obtainingan anomalously

low age or this specimen,ll but the coarsestq-100

mesh) fractionof the sericite ad to be separated ut.

The 50.4 +--- .8 m.y. ageon the early ndio Muerto

Rhyolitedomess lesswell established.Six speci-

mensof petrologicallyimilarbut separatedmasses

are includedn a single sochron.Geologicelations

suggesthat the quartz hyoliteswereemplacedfter

significant rosionof the Indio Muerto Rhyolite

domesbut do not conclusively rove even the rela-

tive age of the different hyolites. The inclusion f

all rhyolites with and withoutquartz eyes) in a

singlesochronidds45.1+ 1.1 m.y. The selection

of the 50 m.y. age as most probables a matter of

geologicudgment, nd the indicated pproximately

5 m.y. time gap between he two rhyolite events

cannot e consideredirmlyestablished. .he50.3+---

3.2 m.y. isochron n Indio Muerto seriesvolcanics

includes our whole-rock amples f rhyolitic flows

and ignimbrite rom the thick volcanicsequence n

the hills southeast of Indio Muerto. The indicated

initial strontium atio of 0.7041 is very close o all

other initial strontium ratios in the district.

K-Ar ages n the TurquoiseGulchcenter or bio-

tites from early-stage lteration o the postmineral

latite dikes, for hornblende rom three feldsparpor-

phyries, nd for alteration ericite ll fall close o 41

m.y. Rb-Sr analyses f the same specimensefine

an isochron t 41.3 --+1.1 m.y., with an initial stron-

tium ratio of 0.7042. The singlebiotite sufficiently

Fro. 14. Relations at intrusive contacts between feldspar porphyries.

A, Contactbetween L" Porphyry (below) and "K" Porphyry (above). Younger "L" has a clean

porphyry exture, s weaklyaltered,and containsmuch less quartz veining and sulfides han the older

"K". Bleachings due to supergeneaolinization, xtendingonly a shortdistancento "L" Porphyry.

B, Intrusivecontactwithin the "K" Porphyrymass. Older rock (right) containsmany quartz veins

which are truncatedat the contact,although many other quartz veinsof this sameEarly type are younger

than the intrudingporphyry. Both rocks are strongly altered to K-silicate assemblagesnd contain

abundant halcopyrite-bornite,lthoughalterationof the older rock is more ntense.

C, Thin section f "L" Porphyry above)"K" Porphyrycontact. Truncated arly quartzvein with

disseminatedhalcopyrite-bornitextends nto chilledmarginof "L" Porphyry. Feldspars re altered o

supergene aolinitc. (Cross-polarized ight)

D, Thin section f early quartzveins n "K" Porphyry (above), truncated nd included y younger

"K" Porphyry. Degreeof K-silicate lteration, s indicated y degree f obliterationf porphyryexture,

is muchstronger n the older rock. Supergene aolinizationof plagioclase. (Cross-polarizedight)


25/57

880 L. B. GUSTAFSON AND J. P. HUNT

A, ContinuousverticaI "B" vein, with relatively coarse quartz and sparse sulfide, cuts less continuous

lacing "A" veins, which are dark becauseof abundant disseminated ulfidesand fine granular habit. Rock

is "X" Porphyry bleachedby supergenekaolinization.

B, Two steep"D" pyrite-"chalcocite" eins with sericite halos cut a 10-cm "L" Porphyry dike within

"K" Porphyry. The veins have characteristicallyittle quartz, and one occupies small fault. Rock is

bleached y supergeneaolinization.

enriched n Rb to providean independent ineral

age s calculatedo be 41.5+-- .4 m.y. old. It is ap-

parent hat the agesof mineralizationventsof the

main Turquoise Gulch area are indistinguishable

within the analytical ccuracy f the combined at-

ing techniques.All appear o havebeencompressed

within a periodof less han onemillionyears. The

initial8*SrffSr atios or all isochrons,ncludinghe

groupof IndioMuertovolcanics,re remarkablyon-

sistentat 0.7041 -- 0.0003 m.y.

Alunite from primaryadvanced rgillicalteration

was formed essentially ontemporaneousith the

intrusion of latite. The 39.1 -- 1.1 K-Ar age on this

alunite s therefore lightlyanomalousut does ndi-

cate he general menability f alunites o K-Ar dat-

ing. The roughly36 m.y. agesof supergenelunites

could probably herefore be consideredminimum

ages. The main period of supergene xidationand

enrichmentrobablyollowed o more han5 million

years after the ,hypogenevent. Attempts to date

jarosite n leached apping ielded ges hat are much

too young. Five samplesndicateages ess . an 21

m.y., with two indicating gesyounger han the 10

to 13 m.y.-old gravelscapping he erosionsurface

which truncates the enrichmentblanket. It is ap-

parent hat evencoarse rystalline arositedoesnot

retain argon well enough to be useful for K-Ar

dating.

Relative Age Relations

Most of our understanding f the evolutionof min-

eralization nd porphyry ntrusionhas stemmedrom

surface nd undergroundmappingon a 1: 500 scale,

especiallyn the areas of intrusivecontacts. T,he

undergroundexposures n closely spacedhaulage

and grizzly drifts were particularlyvaluable n a-

curately working out the detailed hree-dimensional

geometry. Many kilometersof the-back and walls

of undergroundworkingswere scrubbedwith deter-

gent and wire brushes o reveal details. In many

places,1: 100-scalenoteswere also taken to supple-

ment the regular l:500-scale observations. Min-

eralogicdetail .wasmappedusing a color code.

It hasproved o be very important o differentiate

primary 'background"eatures rom those eatures

clearly related o later throughgoing eins and other

structures. Background-featuresnclude mineraliza-

tion which is disseminated or occurs on small dis-

continuouseinsand seams, nd associatedervasive

alteration. The distinctions generallyunambignous

in deep central zones where hydrothermal veins

with K-feldspar-destructive lteration halos are

clearly superimposedn .backgroundmineralization

characterizedy K-silicatealterationassemblagesnd

contrasting ulfideassemblages.However, the dis-

tinctions far from straightforwardn peripheral nd

in high-elevation mineralization ones w,here back-

groundmineralization nd alterationassemblagesre

commonly ndistinguishablerom the structurally

controlledassemblages.

The superpositionf supergenelteration ndmin-

eralization atterns n primaryassemblagesresents

anotherobstacle o correct nterpretation. At E1

Sal-vador, e weregreatlyaidedby the exposure f

a deep entral ulfate one,completelyree of super-

geneeffects,n which o studydeeperprimarypat-

terns. The sulfatezone, which will be discussedn

more detailsubsequently,s a zone n which he rock

is thoroughlympregnated ith anhydrite nd into

whichsupergeneolutions avenot penetrated e-

cause f extremelyow porosity ndpermeability.


26/57


Detailed mapping of undergroundexposuresof

the contactsbetween he porphyrieshas provided

strongevidence f an extremelyclose ime and space

relationship etween he processes f intrusionand

mineralization.Figures 14A and 14C showa major

contact etween L" and"K" Porphyries, nd Figure

15B shows a dike of "L" Porphyry cutting "K"

Porphyry. The intrusivenatureand relativeagesof

the porphyriesare clearly demonstrated y the trun-

cation of many early quartz veins at such contacts

(Figs. 14C and 14D). At this contact, here is a

strongcontrastbetween he nearly fresh,very weakly

mineralized L" Porphyry and the older "K" Por-

phyry, whichhas been ntensely ltered o K-silicate

assemblagesharacterized y alkali feldspar,biotite,

chalcopyrite, nd bornitc. The change n mineral

assemblages abrupt at the intrusivecontact. It is

clear hat mostof the primary alteration,mineraliza-

tion, and emplacementf quartz veins at this point

was accomplished efore the intrusion of the "L"

Porphyry. Some quartz veins and all later sulfide

veinswith hydrolyric lterationhalos (Fig. 15B and

Table 2) as well as supergene lterationand enrich-

ment, cut across uchcontacts.Other contacts, speci-

ally wthin the "K" Porphyry complex (Fig. 14B),

separateithologically imilarporphyrieswith a wide

range of intensity of alteration, mineralization,and

quartzveining.Mapping of relativeage relationshas

demonstratedhat the early processesf mineraliza-

tion were imposedupon each successiveurge of

porphyry magma and its wall rocks before and after

the emplacementf the next surge.

Early Alteration and Mineralization

The Early alteration and mineralization,which

were largely accomplished efore the intrusion of

the last major feldsparporphyry ("L" Porphyry),

are characterizedy distinctiveypesof quartzveins

and mineral assemblages.Alteration assemblages

with stablealkali feldsparand biotite and chalco-

pyrite-borniteor chalcopyrite-pyrite ith antithetic

pyriteandbornitc recharacteristicf both he quartz

veinsand backgroundmineralization. At E1 Salva-

dorperhaps smuchas 75 percent f the copperwas

emplaced uring his Early time of K-silicatealtera-

ation and low-sulfur sulfide mineralization.

"4" quartzveins

Quartz veins at E1 Salvadorwere originallyde-

scribed nd classified y Langerfeldt 1960). The

family of Early quartzveins,often runcated t in-

trusivecontacts y "X", "K", and "L" Porphyries,

.have een alled A" veins.As illustratedn Figures

15A, 16,andTable2, "A" quartzveinsare granular

assemblagesf quartz,perthitic eldspar, nhydrite,

chalcopyrite,nd bornitc. Pyrite, with chalcopyrite

'PLITI'GROUNDMASS[OUARTZ-ALKALI

"ALKALISEAM"

SULRDE,WITH

GROWTH O ING IS OBLITERATED BUT

TWINNING MAY CONTINUE INTO SODIC

'" QUARTZEIN

H Langerfeldt

Fro. 16. Composite dealized sketch of an "A" quartz

veinlet in feldspar porphyry, showing gradational relation-

ships between"A" veinlets and "alkali seams." No single

actual occurrenceshows this complete range of variation.

After H. Langerfeldt.

but never with bornitc, occurs n "A" veins only

near he edges f the deposit. Alterationhalosabout

theseveinsare practically ndistinguishablerom the

strongbackground -silicatealterationwith which

theseveinsare typicallyassociated.Where they cut

less pervasively ltered rock, perthitic K-feldspar,

anhydrite,chalcopyrite, nd bornitc form in halos

alongwith recrystallizeduartz,biotite,andaccessory

apatiteand rutile. With the rare exception f K-

feldspar-andalusite lteration halos (see below),

there is no hydrogen-ion etasomatismbout these

Early quartzveins. They are cut by all otherveins.

The oldest A" quartzveinsare typicallyvery ir-

regular, discontinuous,nd segmented.This is not

only becausehey have been subjectedo multiple

shearing, egmentation,nd recrystallization ut be-

causemany apparentlynever formed with parallel

walls. The fracturesoccupied y theseveinsappear

to have been formed before the rock was able to sus-

tain continuousbrittle fracture. The K-feldspar,

sulfides, nd anhydrite n "A" veins occur as dis-

seminated rains with the samesizesand shapes s

the associated uartz. Successively ounger "A"

veins end to have more parallel walls and to occupy

more continuous nd systematically rientedbreaks.

A few of these end to have some nternal symmetry,

which s lacking n earlier types,with the K-feldspar


27/57


Cortesia de

_Geolibros_


28/57


e ":"'"':":':':':':':"":": i:::::'":':':'""'"": i:::: :i i i iiiii:i : :": RELATIVEBUNDANCEFA'ndB'

" -'/''i:i?i::.;.i '. '-i: QUARTZEINS

IGNEOUSRECCIA

" SECTIONA" ELDSPARORPHYRY

......... :.:,............. , ... ............, .............

:.: . - .....-.-.*.-...-....- , -:+:.::ccc.*:.:. .......... ...-.....-..-..-.. *..c.,.:.:. . ..

-. :./ ;-.-. ... ::::::??:::::::-........:. ,, 'f ?g?:[??: , -':::-.-'::..:.::.:::.?. :: :

'/ * ':....................::-'"-...-:::'::: -;::::::::::"*':::-::::::-::::.:-.::::::: :t: : ,, "x"

,- . :.... ..-:.:.:::.:::::a ? :.>:?::::


29/57

884: L. B. GUSTAFSON AND .I'. P. HUNT

0.1 mm

' 0 ,0.1m

anhy

Fro. 18. Textures of Fe-Ti oxides.

A, Intergrowthof hematile-ruffle fter ilmenite. Incipient oxidationof ilmenite produces

hematite-rutile ntergrowths rientedalongbasalor rhombohedral lanesof the parent mineral,

but more completeseparationof phases nto this anhedral granular intergrowth apparently

represents common endency oward textural equilibrium. (Reflected ight)

B, Magnetite completelyreplaced by hematite (white). The octahedral orientation of this

marmatitie texture is characteristic f supergene xidation. The ruffle (light gray) "sponges"

are formedon removalof hematite rom hematite-ruffle ntergrowths. This hypogene lteration

of hematite-ruffle ntergrowths s accompanied y destructionof magnetite,except in a transi-

tional zone rom which his specimen as taken. (Reflected ight)

C, Granular intergrowth of rntile, anhydrite, and quartz, presumablyderived by alteration

of sphene. The rock is "X" Porphyry, which contains hemaffte-rutile after ilmenite and

biotite after hornblende (Transmitted light)

D, Ruffle cluster (medium grain) with pyrite (black), presumablyderived by sulfidationof

hematite-ruffle after ilmenite. In other specimenswith incomplete replacementof hematite,

there are no hematite-pyritecontacts, ematiteapparentlybeing dissolved head of the front

of pyrite precipitation. (Transmitted light)

of iron from the rock rather than a simpleaccom-

modation of the iron in biotite and sulfides. The

TiO: remainsas granular rutile (Fig. 18).

Evidence that these .same alteration effects are also

operative efore inal consolidationf the porphyry

'melt has been seen ocally at intrusivecontacts e-

tween surgesof "K" Porphyry. Within a zone a

few centimeters ide, the followingchanges re ob-

served within the intruding rock, going from the

main mass into a zone of reaction with the older

rock. The feldspar porphyry texture becomes

obliteratedhrough eplacementf plagioclaseheno-

crystsby perthireand oligoclase.Mafic phenocrysts,

iotite books,and biotized hornblendes re resorbed

or replaced y perthire-quartz ontainingnclusions

of rutile and oriented residuals of biotite. The

"aplitic" groundmass Fig. 10A) becomes oatset

and more raggedand perthitic. As t. e groundmass

getscoatsetand more perthitic,clear K-feldsparand

sodicplagiodase isappear.MagnetRe nd hema-

tite-rutile disappear,eaving only futile with no

evidencef anythingeplacinghem,except robably

groundmassilicates. he abundancef Early quartz

veins n the rock ncreases. Theseveinsare typically

segmented,nd .some ppear o be the last undi-

gested emnantsof the intruded ock.

Biotizationo] andesite

K-silicate alteration in andesite takes the form of

a broadhalo of biotization bout -heporp'hyryntru-

sions (Fig. 19B). The basic mineral assemblage

is biotite-sodic lagioclase-anhydrite-quartz.cces-

sory minerals are Fe-Ti oxides, sulfides,minor

apatite,and zircon.

At the outer edge of the biotized zone, roughly

500 to 1,000 m from the main intrusive contacts,

biotization s not megascopicallyecognizable, ut

biotite s present svery finegrainedlakes estricted


30/57


to the matrix of the rock. The original rock texture

is well preserved.

Closer o major intrusivecontacts,he increasing

intensityof biotizations markedby the appearance

of megascopicallyecognizable iotite as an altera-

tion productof intermediate lagioclase. n areasof

intense biotization close to intrusive contacts, the

rock is usuallyentirelyrecrystallizedo a fine equi-

granularassemblagef biotite,Na-plagioclase,nhy-

drite, and quartz. A few residualplagioclaseheno-

crysts may remain, but these are usually altered

with biotite, anhydrite, and occasionally ericite,

chlorite,and calcite. K-feldspar s generally bsent

frombiotized ndesite,xceptn stronglymineralized

zones,and is generallyrestricted o the immediate

vicinityof "A" quartzveins.

Propylitic alteration

Weak propyliticalteration orms a green fringe

about the mineralizedzonesat E1 Salvador, as was

originally noted by Swayne and later described n

more detail by Eckstrand 1967). The propylitized

rocks are mostly andesitic lows and sedimentary

rocks of the Cerrillos formation. Characteristic con-

stituents f the propyliticassemblagesre epidote,

chlorite,calcite,quartz,and plagioclase.They are

present as pervasivealteration and are controlled

by structures. Calcite is abundantas an alteration

product disseminatedn the rocksand in veins and is

alsoan abundant ndpossibly riginal ementing a-

terial in andesitic ediments ell beyond ny hydro-

thermal alteration.

Beyond he outer imits of biotization nd pyritic

sulfidemineralization, ron and titanium oxides are

magnetite, more or less altered to hematite, inter-

growths of magnetite-futile, hematite-rutile, and

locally lmenite. Veinletsof epidote-calcite-specu-

lar hematite re presentwith epidote lterationhalos.

Near the outer imits of the zone of biotization,dis-

seminatedhlorites presentwith fine-grainedpi-

dote after plagioclaserains and in tiny veinlets.

Chloritedoesnot appear o replacebiotite, n con-

trast to this characteristiceplacement equencen

mostof the biotized one. Veins of epidote-magne-

tite-chalcopyrite re associatedwith the chloritic

alteration. Pyritic veinswith sericite-chlorite ltera-

tion halosare later than epidote-magnetite-chalco-

pyriteveinlets. e-Ti oxides reconvertedo pyrite-

rutile, and a smallamountof chalcopyrites dis-

seminated within the sericitic alteration halos.

Alteration of hornblende nd Fe-Ti oxides

Hornblende,lmenite,and sphene re preserved

only in deep evelswithin "L" Porphyry. Horn-

blendephenocrystsre present n all stagesof re-

placemento assemblagesf biotite-anhydrite-rutile.

Clusters of "shreddy" biotite witl rutile and an-

hydrite occur throughouthigher exposuresof "L"

Porphyryand muchof the "X" and "K" Porphyries

and suggest riginalsitesof hornblende henocrysts.

In "L" Porphyry in which hornblendes partially

altered, lmenite s seen o be partially replacedby

an intergrowthof hematite nd rutile (Fig. 18). This

reactionseems o be a simple oxidation reaction: 2

FeTiOa + 1/2 0, --> Fe,Oa+ 2 TiO,. Sphene is

pseudomorphicallyeplaced by an intergrowth of

rutile and anhydrite, apparently as a result of the

reaction, CaTiSiO5 + SOa Son-'>TiO, + CaSO4 +

SiOn,Soln. Silica released in this reaction does not

seem to be fixed in place as quartz. Ilmenite

partially replacing phenen a few specimenss ap-

parentlypart of an unknownearlier reaction. Of 20

specimensf "L" Porphyrywith hornblende, nly 2

do not also contain lmenite. Only two specimens

with ilmenite but no hornblende have been seen.

Minor sphenes presentn several f thesespecimens

but in practicallyno others. Apparently there are

three concomitant reactions: biotization of horn-

blende, oxidation of ilmenite, and destruction of

sphene. Pseudomorphiceplacementproducts of

ilmeniteand sphene s well as hornblende re seen

at higher elevations n the well-mineralized or-

phyries nd"L" Porphyry. These eplacementsp-

pear to be the earliest (at any given point) and

deepestmanifestationsof K-silicate alteration.

Alkali seams

In weakly mineralized "L" Porphyry, a large

proportionof the sulfides re present n alkali seams,

small veinletsmarkedprimarily by alterationhalos

of alkali feldsparwhere they cut plagioclase heno-

crysts (Fig. 16). Only where alkali seamscontain

appreciable iotite, anhydrite,and (or) sulfideare

they usually traceable through the groundmass.

Sericite,either within the seamor as a halo about t,

is presentn those lkaliseamshat contain yritebut

is usuallyabsentwhere here s no pyrite. Tracesof

apatiteare occasionallyeen n alkali seamsn areas

of somewhatstronger mineralization,as in "K"

Porphyry. A gradationbetweenalkali seamsand

"A" veins s suggested,lthough o singlespecimen

displaysa complete ange of gradationbetween he

two. Extensionsof small "A" quartz veins across

plagioclase henocrysts ommonlyshow zoned re-

action halos, with K-feldspar separated rom the

plagioclasey a rim of moresodic lagioclase.

Anhydrite mineralixation

Anhydrite s among he earliest nd atestproducts

of mineralizationnd, n fact,spanshe entirehistory

of mineralization at E1 Salvador. The bulk of the

early anhydrite s disseminatednd is a character-


31/57

886 L. B. GUSTAFSON AND 1. P. HUNT

istic component f "A" quartz veins and K-silicate

alteration assemblages.Later anhydrite is domi-

nantly fracture controlled and is a characteristic

productof all youngerveins. Thus, depending n

timing,anhydrites an associatef a wide varietyof

mineral assemblages. hese ncludeearly feldspar

and biotite-stable, ow-sulfur, chalcopyrite-bornite

and chalcopyrite-pyrite uites and later feldspar-

destructive,sericite-bearing and even andalusite-

bearing)alteration ssemblagesith abundant yrite.

Within the sulfate zone, disseminated nhydrite

accounts for more total sulfur than all sulfides com-

bined. The abundance f anhydrite s greatest n

andesitic ost rocks (5 to 10 percentby volume)

and declinesn successivelyounger ntrusive ocks

(1 to 5 percentby volume). The anhydritecontent

of wall rocksapparently eflectsboth the original

available alcium ontentand the intensity nd dura-

tion of the mineralizingprocesseso which the rocks

were exposed. There is a rough inverse correlation

between he abundance f anhydriteand the abund-

ance of residualcalcicplagioclase nd hornblende.

Replacementf plagioclasey alkali eldspar, ericite,

and andalusite nd replacement f hornblende y

biotiteare believedo havebeen he principal nhy-

drite-fixing reactions.

Anhydrite-impregnatedock, the sulfate one,has

a porosityand permeability f nearly zero and thus

forms an effectiveunderground arrier for the move-

ment of ground water (and mine water). With

time, however, he upper and outer surfaces f the

sulfatezone are attackedby supergene nd other

groundwaterscausingirst a hydration f primary

an'hydrite o gypsumand then the dissolution f

gypsum, leaching both calcium and sulfate. The

supergeneemovalof anhydritecauses he wall rocks

to undergo significantncreasen porosity nd de-

crease n specificgravity, as well as a marked de-

crease n competency.The presentpositionof the

top of the sulfatezone at E1 Salvador ies below the

supergenenrichment lanketand s shownby cross-

hatching n Figures20 and 21.

Disseminatedulfide ssembla#esnd zones

Patternsof sulfide oningon deepmine levelsare

illustratedn Figure19A. Thesepatterns re based

on megascopic apping upported y quantitative s

well as qualitativemicroscopicxamination f hun-

dredsof samples. Background ssemblages,ccur-

ring as disseminationsnd in small discontinuous

fractures,have been carefullyseparatedrom as-

semblagesn large hroughgoingeinsand associated

alterationhalos. The distinctions important n

that the background ssemblagesefine well-devel-

oped zonal patterns. The assemblagesn younger

veinsand halos,althoughcon

the porphyry copper deposit at el salvador chile l.gustafson,j.hunt

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