geologic, structura and fi studies of eoithermal vein sustem, chile

7/24/2019 Geologic, Structura and FI Studies of Eoithermal Vein Sustem, Chile

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EconomicGeology

Vol. 86, 1991, pp. 1317-1345

Geologic,Structural, ndFluid InclusionStudies f E1Bronce

EpithermalVein System,Petorca,CentralChile

FRANCISCO CAMUS,

Camino del Valle Alto 1327, Los Dominicos,Las Condes,Santiago,Chile

RICARDO BORIC,

CompaniaMineraDisputada e Las Condes, edrode Valdivia291, Santiago, hile

MILKA ALEXANDRA SKEWES,

Casilla 970, Correo Central, Santiago,Chile

JUANCARLOSCASTELLI,

Empresa acional e Petroleo-Magallanes,oseNogueira 101,PuntaArenas,Chile

ENRIQUE REICHHARD, AND ANDRESMESTRE

CompaniaMineraEl Bronce,Carmencita 40, Santiago, hile

Abstract

The E1Bronce pithermal ein system,ocated n the western oothills f the AndeanCor-

dillera of centralChile, contains 5 metric tonsof gold, 105 metric tonsof silver,and 16,000

metric tonsof copper.The veinsare hostedby volcanic ocksof the Cerro Morado Early

Cretaceous) ndLas Chilcas Early o Late Cretaceous) ormations hichconsist hieflyof

breccias, uffs,and avasof andesitic omposition. he subcircularMorro Hediondocaldera,

with a diameterof 14 to 16 km and of Late Cretaceous ge, s located mmediatelynorth of

the district. Dacitic tuffs and andesitic flows and breccias associated with the caldera are

assignedo the Lo Valle Formationon the basisof K-Ar agesof 83 to 80 Ma.

Two groupsof Early to Late Cretaceousntrusive ocks,occupying orth-trending elts,

are recognizedn the area.The older,a quartzmonzodiorite odywestof Petorca, ntruded

the Cerro MoradoFormation.The youngercomprises ioritic o granodioritic tocks, ikes,

and sillsand ncludes he PetorcaPorphyry 86 _ 3 Ma) and he dioritic-tonaliticing dike.

The latter defines he marginof the Morro Hediondocaldera 80-79 Ma). Large zonesof

hydrothermalilicificationndargillic lteration reassociatedith the two groups f ntrusive

rocks.

The Morro Hediondo caldera s the mostprominent geologic eature in the area. Faults

and ractures elated o the caldera rovided tructural ontrolsor the mineralization. everal

northeast- o northwest-trendingaults are either radial or concentricwith respect o the

caldera.The mostoutstandingtructures re the Quebrada e CastroandE] Bronee tructural

systems.

The E] Bronee tructural ystem, ontainingmostof the minera]izationn the district,consists

of extensionalaults,dikes,andveinsexposed ithina 3-km-wideanda 17-km-long, ortheast-

trendingzone delimitedby the north-northwest-south-southeastuebradade Castroand

Petorca-E1Durazno dextral strike-slip aults.

The relative movementof this fault pair was responsibleor the structuralpattern at E1

Bronce,which nvolveddevelopment f a first-orderdilational og. Within this og, two major

groupsof veinsare recognized:he El Bronce-Guanaco-Lalla-SanLorenzogroup ocated

north of the E1 BronceCreek fault and the Pedro de Valdivia-E1EspinoNorte group ocated

southof this structure.Both groupsof veinsare vertical o subvertical nd strikenorthwest

to northeast.

Detailed studiesof the orebodieshave shown hat each ore shoot s composedof several

lenseswhich containas many as four ore types and a dike: (1) hydrothermalbrecciaore

cemented, 2) massive re, (3) stockwork one, and (4) disseminatedone.Orebodiesmay

alsocontainbarren andesitcdikes.The contactsbetween the four ore types are generally

abruptbut are ocallygradational. he andesitc ikesgenerallydisplay harp, ocallysheared

contacts.

0361-0128/91/1272/1317-1953.00 1317


2/29

CAMUS ET AL.

Ore minerals re dominated y coarse rystalline yrite, sphalerite, halcopyrite, alena,

tetrahedrite-tennantite,nd minor bornitc.Gangueminerals onsist f quartz, carbonates,

barite,andchlorite.Fiveparagenetictagesre ecognized:1) quartz-pyrite-gold,2) quartz-

pyrite-sphalerite-chalcopyrite-gold,3) tetrahedrite-tennantite-galena-silver,4) barite, and

(5) carbonates-chlorite-sphalerite.ydrothermalalteration,with the formationof sericite,

kaolinitc, chlorite, and carbonates,mainly affected he host rocksof the stockwork nd dis-

seminated re types.Carbonates re the principalalterationminerals n the andesitc ikes.

Fluid inclusionsassociatedemporally with the preciousmetal mineralizationshow ho-

mogenization emperatures hat range from 235 to 344C and salinities rom 4 to 10 wt

percentNaC1 quiv.Goldmineralization nderlies shallow oneof boilingof the hydrothermal

fluids.The fluid inclusiondata suggesthat the preciousmetal zone was generated400 to

1,200 m beneath he palcosurface.

Temperatureand salinity end to decreasen the shallow evelsof the veins.The calculated

verticalvariationsn enthalpyof the hydrothermalluidssuggesthat a fluidmixingmechanism

was esponsibleor ore deposition. ot, relativelysaline,metal-rich luidsascendedndmixed

with coolermeteoric luids o causeprecipitationof the iron, copper,and zinc sulfides, nd

subsequentlyhegold.Preliminaryulfursotope tudies/4S;0.5-2.3%0) uggestmagmatic

source for the sulfur in sulfides at E1 Bronce.

Introduction

Scopeof work

THIS paper presents he resultsand interpretations

of geologic,mineralogic,etrographic,tructural, nd

fluid inclusionstudiesundertakensince 1985 by

CompaniaMinera E1Bronce n the E1Broncemining

districtandsurroundings.hesestudieswere oriented

towardunderstandinghe geologic nvironment nd

processesnvolved n the depositgenesisn order to

generategeologicand geneticmodels or use n ex-

ploration or additionalore in the district.

Location

The E1 Broncemining district s located 150 km

northof SantiagoFig. 1) and 8 km northof the small

townofPetorca. he approximateeographicocation

is 32011 Slat and 70056 W long. Altitude varies

between 600 and 2,700 m above sea evel.

The vein orebodies in the district are located on

the steep lanksof the E1BronceValley and extend

eastward oward the high peaksof the area.

Previous studies

There are no detailed,publishedgeologic eports

covering he E1Broncedistrict.Recently,Camus t

al. (1986) and Skewes nd Camus 1988) described

specific spects f the districtgeology.Unpublished

but readily availabledocumentationncludesRuiz

(1945) on the geology f the E1Broncedistrictwith

emphasisn he RosarioI ore shoot,Baranovskynd

Fresno 1940) on the geologyof the Pedrode Valdivia

mine,andCamus 1982) summarizinghe then-avail-

able geologic nowledge f the E1BronceandPedro

de Valdiviadeposits.

History and production

According o Vicuna-Mackena1881), veins n the

E1Broncedistrictwere workedsurficiallyor goldby

local ndians n precolonialimesand then by the

Spaniardsduring the 16th to early 19th century.

During the late 19th century, the E1 Bronce,E1 Es-

pino, andPedrode Valdiviaminesof the districtwere

in production. n 1939, A. Callejas ounded he Com-

paniaMineraE1Bronce e Petorca, ndapproximately

500,000 metric onsof ore averaging 1 g/metric on

Au were mined rom hese hree mines hrough1955,

when mining was discontinued ecauseof low gold

prices. The company recommencedoperations n

1980, and up to 1989 about 2.5 million metric tons

of ore averaging .8 g/metric on Au, 20 g/metric on

Ag, and0.30 percentCu hadbeenextracted rom he

new Rosario II ore shoot.The total goldproduction

from the district s estimated o be approximately .3

million metric tons 630,000 oz). Presentmine pro-

ductionandplant capacityat E1Bronce s 1,200 met-

ric tonsper day.

RegionalGeology

The E1 Bronce district is located in the western

foothills of the Andean Cordillera in an area where

outcroppingrocks are volcanicsand intrusionsof

Cretaceousage (Fig. 2). The numerouspolymetallic

(Au, Ag, Cu, Pb, andZn), epithermal eins n the dis-

trict were emplacedwithin a north- to N 20 o E-strik-

ing structuralsystem utting volcanic ocksof Early

Cretaceous ge. The structuralsystemextends rom

Petorca in the south to Morro Hediondo hill, 17 km

farthernorth Fig. 2). The epithermal eins rerelated

genetically o a Late Cretaceous86-79 Ma) magmatic

event characterized y emplacement f subvolcanic

intrusions, evelopment f a collapse aldera,andex-

tensivehydrothermalalteration.

Stratigraphy

Dacitic to andesitic volcanic and volcaniclastic

rocksof Early to Late Cretaceous ge,whichassigned

to the Cerro Morado, Las Chilcas, and Lo Valle For-

mations,are exposed n the E1 Bronce district and


3/29

EL BRONCEEPITHERMALVEIN SYSTEM, ENTRALCHILE 1319

FIG. l. Location of the E1 Bronce district, central Chile.

surrounding reas Fig. 2). The lithology, thickness,

attitude, and age relationsof these hree formations

are summarized in Table 1. Their distinction can be

rather difficult because of the absence of marker ho-

rizons and tectonic disruption.

The El Broncedistrict s hostedby the Cerro Mo-

radoFormation,which strikesnortherlyandunderlies

conformably, nd in part interfingerswith, the Las

ChilcasFormation mmediatelynorth of the district.

The Las ChilcasFormation, which crops out east as

well as north of the district, underlies the Lo Valle

Formation.Units of this latter formationcrop out as

two compositionally ifferent north-trendingbelts

northeast of the district. The rocks of the western

belt constitutepart of the Morro Hediondo caldera

complex Boric, 1986): a sequenceof interfingered

lava flows, andesitic volcanic breccias, and dacitic la-

pilli tuffs characterizedby weak chlorite-clayalter-

ationand the presence f calcite.The eastern elt

consists f a sequence f andesitic ocks hat uncon-

formablyoverlie the northwestern ortionof the

Morro Hediondo caldera.

Intrusive rocks

Several plutons of intermediate composition n-

truded he Cretaceous olcanic equencesf the area

(Fig.2). The plutons reeitherpartof thebatholithic

Illapelsuperunit f Early o LateCretaceousge 86-

134 Ma; K-Ar) or stocks nddikesof the SanLorenzo

superunit f Late Cretaceousge 86-79 Ma;K-Ar).

Both ntrusive hases ere definedand datednorth

of the districtby Rivanoet al. (1985).

Quartzmonzodioritentrusions f the Illapel su-

perunitcropoutwest,south, ndup to 100 kmnorth

of the district and cut the Cerro Morado Formation

(Fig. 2). The intrusive ocksare light colored, ha-


4/29

1320 CAMUS ET AL.

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EXPLANATION

Unconsolidated debris

A-- Andesiticlows/br ec cia

Andeste

tuffs

_ Oaciteorphyry

(T,ndesire

orphyry

Hydrothermal

lferoion zone

Contact

Vein

Fault

Strike and dip of

bedding

] Cu mine

( Au-Ag-Cu-Pb-Znine

"' ,2' K-Ar age

o 5km

i i

FIG. 2. Regionalgeologicmapof the E1Bronceminingdistrictshowing tructure, ithology, lteration

zones,and mines.Also shownare the locationsof samples ated by the K-Ar method.


5/29

TABLE 1.

EL BRONCE EPITHERMAL VEIN SYSTEM, CENTRAL CHILE

GeologicCharacteristics f the Cerro Morado,Las Chilcas,andLo Valle Formations

1321

Formation Lithology Attitude Thickness Age

Cerro Morado Lavas and andesitic breccias, N-S 10-30 E >1,000 m

intercalations of tuffs and

tuffaceous sandstones; N-S 10-30 E

gray to purple and

greenishcolored rocks

Las Chilcas Conglomerate, uffs, lavas, N-S 10-30 E >12,000 m

andesitic breccias; brown

to reddish colored

Lo Valle

West belt: Lavas, andesitic ? >1,000 m

volcanic breccias; dacitic

lapilli tuffs (Morro

Hediondo volcanic

center); light brown to

gray colored

East belt: Porphyritic N-S 10-30 E >700 m

andesRes and andesitic

breccias;gray to greenish

colored

Early Cretaceous;ntruded by Petorca

Porphyry (K-Ar: 86 +_3 Ma) and by

quartz monzodiorite 109-96 Ma);

overlies post-NeocomianVeta

Negra Formation

Early to Late Cretaceous; ntruded by

dikes of 82 to 79 Ma; underlies

lavasof Lo Valle Formation (82-80

Ma)

Late Cretaceous;K-Ar dating of tuffs

and andesites; 89 to 79 Ma

Late Cretaceous;67 to 60 Ma plutons

intrude this formation to N and S

neritic, and medium grainedand are affectedby in-

cipient alteration o chlorite, epidote, clays,and/or

hematite. Where these intrusive rocks are in contact

with volcanic rocks of the Cerro Morado Formation,

the latter displays he effectsof contact metamor-

phism.

The dioritic (andesitic) o granodioritic dacitic)

stocks nd dikesof the SanLorenzosuperunitcrop

out in the E1 Bronce district and also to the east and

up to 100 km farther north. These rocks ntrude the

volcanicsof the Cerro Morado, Las Chilcas, and Lo

Valle Formations nd are light colored,porphyritic,

andalteredweakly o chlorite,epidote,clays, ericite,

prehnite, and/or calcite. The outcrop areas of the

stocks are less than 7 km 2.

The goldveinsof the E1Broncedistrictand other

gold, silver, and/or copperveinswithin the general

area shown n Figure 2 were emplacedaroundand

locallywithin ntrusions f the SanLorenzosuperunit.

Vein mineralization an hereforebe related geneti-

cally o emplacement f specificntrusions f the San

Lorenzounit, including he PetorcaPorphyryand he

ring dike.

The PetorcaPorphyry 86-79 ___ Ma) cropsout

on the easternmargins f the E1Broncedistrict Fig.

2), where it intrudes the Cerro Morado Formation,

producinga narrow metamorphic alo. The south-

ernmost utcrops f the porphyryappear o constitute

a stockwhereas he northernmost utcrops re sill-

like. Both ntrusivebodiesare likely to be connected

at depth. The porphyry is porphyritic, onalitic to

granodioritic n composition, nd affectedby weak

alteration and numerous carbonate veins and veinlets.

The ring dike cropsout in the north and east of

the area (Fig. 2) and formsa discontinuousartial

annuluswith a diameterof 14 to 16 km. The ring dike

defines he externalmarginof the Morro Hediondo

caldera,dips vertically, and averages10 to 20 m in

width. However, widths of over 100 m are observed

locally Fig. 2). The ring dike hasa dacitic o quartz

monzodioritic omposition,porphyritic exture,and

is very weakly altered. The ring dike yielded K-Ar

agesof 80 ___ and 79 ___ Ma (Camus t al., 1986).

The Dulcineacopperbrecciaorebody,emplaced d-

jacent o the ring dike near ts southeasternnd (Fig.

2), is thought o be genetically elated o it.

Structure

Numerous structures occur in the E1 Bronce area,

the most outstandingof which are the northwest-

strikingQuebrada e Castro ystem,he north-striking

E1 Broncesystem,and the Morro Hediondocaldera

(Camuset al., 1986; Fig. 3).

The Quebradade Castro structuralsystemcom-

prisesa regional ineament,which strikesN 22 W,

and extends between latitudes 31 and 33 S. It is

located3 km eastof E1Bronceasa series f subparallel

dextralwrench aults hat canbe followedalongstrike

for about 27 km. The faults intersect the Cerro Mor-

adoFormationandplace his formation n contact o-

callywith the LasChilcasF9rmation Fig. 2), thereby

suggestingrelativedownthrow f the eastern lock.

A few faults in this systemhost Au, Ag, and/or Cu

vein-type mineralization.

The E1 Bronce structuralsystemhosts he main

veins in the district and extends for almost 18 km

(Fig. 2). The system omprises seriesof tensional

fractures,dikes,andveinsdelimitedby the Quebrada

de Castro and the Petorca-E1 Durazno dextral wrench

faults Fig. 3).


6/29

1322 CAMUS ET AL.

i

E=$15.000

- N=6.446.000

-43

I

E=$19.000

ixMorroediondo

cala _

/

i /'. Ring ike

'/

,,'Sa Lorenzo

'

El Espino

Ii

/


7/29

EL BItONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1323

TABLE 2.

K-Ar Data for Volcanic and Intrusive Rocks and Alteration Zones in the E1 Bronce District

Sampleno. Coordinates Sampledescription K (%) Age (Ma)

RB-200 3202'2" S Basaltic andesitc from the Lo Valle Formation; 2.501 80 +__ 3

7053'3"W marginof the Morro Hediondocaldera;

whole rock

RB-203 32003'2" S Andesitc rom the Lo Valle Formation;margin 1.728 82 +__3

7053'3" W of the Morro Hediondo caldera; whole rock

EB-600 3205'6" S Dacitic lapilli tuff; intracalderasequence;biotite 6.052 86 +__3

7052'0" W separate

RB-6 3214'2" S Granodiorite porphyry; PetorcaPorphyry; 2 km 0.984 86 +__3

7054'2"W from Petorca Fig. 2); plagioclase eparate

RB-149 3204'3"S Dacite porphyry; ing dike; whole rock 3.144 80 +__3

70055'6" W

RB-157 3206'4"S Monzodioriteporphyry;ring dike; plagioclase 2.352 79 +__3

7055'6" W separate

RB-502 3205'6"S Argillized rock; Morro Hediondoalterationzone 0.183 81 +__4

7054'2" W (Fig. 2); whole rock

RB-126 3205'1" S Argillized tuff; Morro Hediondo alteration zone 0.416 82 +__9

70054'3" W (Fig. 2); whole rock

RB-91 32011'2" S Argillized and silicified ava flow; El Durazno 1.753 109 __+4

7059'2"W alterationzone (Fig. 2); whole rock

SD-316 3211'9" S Sericite; alteration envelope of the Rosario II 6.577 79 +__3

7056'17" W ore shoot;drill hole sericite separate

The polymetallic deposits predominate in the

westernpart of the districtand the copperdeposits

are concentratedn the easternpart and within the

Morro Hediondo caldera.

The hydrothermalalterationzonesare predomi-

nantlyof the argillic ype and are present n volcanic

rocks,particularlynear intrusions,n the vicinity of

major faults,and/or alongmore permeablehorizons.

The economicallymost mportantmetallicdeposits

in the area are the E1 Bronce-Guanaco,Pedro de Val-

divia, and E1 Espino-Boton e Oro epithermalvein

systemsFig. 3).

Geologyof the E1 BronceEpithermalSystem

Distributionand structuralcontrolof veins

The E1 Broncestructuralsystem rendsN 10 E

and is exposedover an area 3 km wide and 17 km

long (Fig. 3). In this structural ystem,he E1Bronce

epithermal ein system ontainsmajorconcentrations

of preciousmetals.The principalcomponentsf the

E1Broncesystem re the E1Bronce-Guanaco-Lalla-

San Lorenzo and the Pedro de Valdivia-E1Espino

Norte structures. Both structures are subvertical and

north to N 10 E striking.The E1 Bronceveinsare

separated rom the Pedro de Valdivia veins to the

southby the N 45 E strikingQuebradaE1 Bronce

fault. The main veins of the Pedro de Valdivia area

are controlledby north- to N 10 W- and N 45o-60

E-striking premineralizationaults.There are alsoa

series f minorsubparalleltructurestriking etween

N 20 W and N 30 E and dipping45 or more to

either eastor west.Thesedifferentstructuresmay

converge r diverge o form multiplecymoid oops

(e.g., Guanaco rea) or crossone other and conse-

quently undergodisplacement r truncation.Some

structuresrend east-northeastEspinomine area;Fig.

3), and someminor west-northwest iagonalexten-

sionalstructuresmay alsobe recognized.

Many of the faultsare normaland showvertical

displacementsf up to 250 m. The displacementsave

producedhorsts nd grabens. here are alsodextral

and sinistralstrike-slip aultsshowingdisplacements

of up to 50 m as demonstrated y horizontalslick-

ensides and dike offsets.

The faults elated o the Quebradade Castrosystem

displaced he SanLorenzovein of the E1Broncesys-

tem (Fig. 3). On the other hand, he northernend of

the SanLorenzovein intersectshe ring dike, giving

rise to a series of veins within the caldera in the Morro

Hediondo area (Figs. 2 and 3).

On the basis of the distribution of ore shoots, the

E1Broncesystem s subdividednto four sectors: l

Bronce, Guanaco,La Olla, and SanLorenzo. The best

known sectorsare the first two, whereas the last two

are under explorationand consequentlyess well

known Fig. 3). The four sectors trikenorthward or

a total of 7 km and consist f a complexset of tension

fractures, dilational jogs, or cymoid loops, and

brancheswhich havecontrolled he emplacement f

a series of lenticular ore shoots and barren andesitc

dikesof variablesizesand morphologiesTable 3).

The E1 Bronce sector consists f a single, major

complex tructure, xtendingor 2 km, whichcontains

three major ore shoots:Rosario , II, and III. At its

northernend, the structuresplits nto a seriesof par-


8/29

1324 CAMUS ET AL.

TABLE 3. Ore Shoot Dimensions

Length Depth Thickness

Sector Ore shoot (m) (m) (m)

E1Bronce

Guanaco

Rosario I 250 150 1-3

Rosario II 250 280 1-5

Rosario III 550 400 1-20

Guanaco N 450 200 0.9-6.8

Guanaco S 250 450 0.7-8.2

Paulino 90 220 0.4-4.8

Central 300 350 0.7-4.7

Del Medio 330 240 0.5-6.9

Maria Isabel 500 500 0.9-8.6

Range:maximum ndminimum

allel to subparallel einsor extensionalractures hat

extend500 to 600 m alongstrikeand encompassn

area 200 to 250 m wide (Figs. 4 and 5); this is the

Guanacosector and includes six veins, the samenum-

ber of ore shoots,and several barren dikes. The six

veins are Guanaco Norte, Guanaco Sur, Paulino, Del

Medio, Central,andMaria Isabel Figs.4 and5). To-

ward the northern limit of the Guanaco sector, the

veinsmerge o form hreemainstructuresecognized

onlyon the basis f exploration rill holes.These hree

main structuresconstitute the La Olla sector which,

in turn, toward the northern end of the systemhas

developed nto what s knownas he SanLorenzosec-

tor, where here s a singlemainstructure xtending

for 2.5 km and endingat the Morro Hediondo ing

dike (Fig. 3).

The strikeof the structural ystemn the E1Bronce

sectorvariesbetween N 10 W and N 20 E, pre-

dominantlyhe latter, anddips ange rom 70 o 85

E at Guanaco to 60 to 85 E elsewhere. At San Lo-

renzo, the strike s northerlyand the dipsvary from

vertical to 75 W at the northern end to vertical to

60 W at the southern nd, and approximately 5

W in the central part of the sector.

In addition o the premineralization r even ntra-

mineralization displacements, postmineralization

faultingalsooccurred.This wasvery important,es-

pecially n the E1Broncesector,where postmineral-

ization movementcaused ranscurrentand gravity

faultingwhich duplicatedpartsof the orebodies.Net

verticaldisplacements f 50 to 100 m havebeen mea-

sured n partsof the RosarioII ore shoot.The post-

mineralization aults are characterizedby clayey

gouge,1 m wide, which ocallyencloses ein andhost-

rock fragments.The barren andesitcdikes ntruded

along he veinswere utilizedasdisplacementurfaces

by thepostmineralizationaults ndconsequentlyre

shearedor brecciated n places.The principalpost-

mineralization ault affecting he Rosarioore shoots

extends northward and divides the Guanaco sector

into severalblocks.The easternblocks,containing

the Central and Maria Isabelveins Fig. 5), were dis-

placed downward.

Ore shoots nd their morphology

The E1 Bronce and Guanaco sectors contain well-

differentiated ore shoots: three at E1 Bronce and six

at Guanaco Table 3).

In the E1 Bronce sector, in the three individual ore

shoots,he width hasbeen nfluenced y vein dupli-

cationdue to postmineralizationaulting; he original

averagewidth is estimated o be 6 to 8 m. At the

elevationof the Carmen evel (1,090 m; Fig. 6), the

three ore shoots are interconnected, with Rosario I

the smallest ndRosarioII the largest Table 3). The

latter ore shoot is the best known in this sector, and

all three are shown n the longitudinalsopach ection

of Figure 7a. Note that only original hicknessesave

been used n the construction f the isopachsection.

The Rosario I and II ore shoots are not as well known

due to intense exploitation n the past, and partial

reconstructions basedon mapsand sections aken

from Ruiz (1945) and company ecords.

In the Guanacosector (Table 3; Figs. 4 and 5),

their morphologies re similar o thoseof the Rosario

ore shoots,but they are smaller n size and extend

farther downdip han alongstrike. Figures8a and 9a

are longitudinal sopachsectionsof the Central and

Maria Isabel ore shoots. Note that the roots of both

ore shoots re poorly knownbelow the Rosario evel

(1,232 m) due to lack of data and hat both ore shoots

plunge southwardat 50 to 55

In the La Olla sector, no ore shoot has been found

to date, whereas the San Lorenzo sector shows the

presenceof at least one ore shootwhich has been

intersectedby explorationdrill holes.

The constructionof the longitudinal sopachsec-

tions Figs.7a, 8a, and 9a) is basedalmost ntirely on

diamond drill hole data. Ore shoot widths based on

channelsamplingwere not used because rue vein

widthsare rarely depicted n undergroundworkings.

In contrast,drill core data give more reliable infor-

mation on the true widths of ore shoots.Moreover,

the regulardrilling pattern at E1Bronce s conducive

to accurate hickness ontouring.Basedon the 1-m

contour, he upper limit of the Rosario II ore shoot,

just above he Capote evel (1,450-1,500 m), is reg-

ular with few inflectionsor digitations Fig. 7a). In

contrast, ts ower imit is very irregularandpossesses

severaldigitations. he southern imit is also egular,

whereas the northern limit is rather irregular and

forms obesand wedges oward the upper part of the

shoot Fig. 7a). The Rosario and II ore shoots re

best known near their lower limits where they show

the samekind of digitationsas does the Rosario II

ore shoot.The upper imitsare poorlyknownbecause

of a lack of information rom the old stopes.

A consistent elationshipexistsbetween the con-


9/29

EL BRONCE EPITHERMAL VEIN SYSTEM, CENTRAL CHILE 1325


10/29

13 2 6 CAMUS ET AL.

I


11/29

EL BRONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1327

o

o g o


12/29

1328 CAMUS ET AL.

E= IZ4OO

I

//

d SECTION

38800--N

w

E=lZ500

E=7.700

/

/

SECTION

3

KEY

]Vein

... DisseminatedAndesre Fault [[ Tuff 0 lOOm

one dike I I

FIG. 5. Vertical sectionsmarked n Fig. 4) showing istribution f veinsand andesitedikes n the

E Bronce and Guanaco sectors, ] Bronce district.

toured vein widths, he sinuosity f the structuresn

which the orebodieswere emplaced,and the con-

toured analyticaldata (Figs. 7b and c, 8b, and 9b).

Each ore shoot s composed f a seriesof small, en-

ticular bodies or lenses of various sizes. In the Rosario

III ore shoot,a total of 17 such ensesmay be rec-


13/29

- 2.000 m.

I I I I I

GUANACO ORE SHOOTS

S GUANACOOR

EL BRONCRE HOOTS UNACOSU

- MARISAB__

% %.ooo ": /

FIG. 6. Longitudinalsectionshowing ocationand morphologyof the main ore shoots n the El

Broncesystem.Note that the three ore shoots re in different vertical planes, he Maria Isabel s east

of the Guanaco.

i i i i [ i i i

N= 7.200 N=37.600 N=38.OO0 N= 9.400

-1.500m

-1.0Om

-900m

i

N= 39.200

KEY

Om

- 1.500m

-1.00m

-1:lOOm

- 900m

KEY

[--1 < I g/t Au

'l--Sg/t Au

5-30g/tAu

IT[1Tm O- 2Og/t Au

> 20g/t Au

-1.50Om

- 900m

I

c

I I I I I I I I I I

KEY

[] EOa/t Aa

FIC. 7. Longitudinalection f El Bronce ector howinga) originalhicknesssopach,b) gold

distribution,and (c) silver distribution.

1329


14/29

1330 CAMUS ET AL.

KEY

.o---- (m)

~=s:ooo ~=s.'oo ~=s:oo ~=se.'soo

N=1000 N=Se'.00 N=Se'.=00 N=Se;00

1.700 m-

1.600 m -

Drill hole

i) 'KOm K E Y

l.O(g/t Au) .Drill hole

FIG. 8. Longitudinalsectionof the Central vein, Guanacosector,showing a) thicknesssopachs,

and (b) gold distribution.

ognized. igures b andc, 8b, and9b and ieldob-

servationsuggesthateach ndividualens epresents

a zone of extension hat arose rom strike and/or dip

changes orming second-order ilational ault jogs

(i.e.,


15/29


,, .

N = 38.900 N= 39.100

I I I I

N=39.300 N=39.500

i I i

N:38.900 N=39.100

I

N:39.300

.800

.700m -

KEY

2.o j

Drill hole

0 lOOm

I I

KEY

.-- to (e/t A,)

Orill hole

FIG. 9. Longitudinal ectionof the Maria sabelvein, Guanaco ector, howing a) thicknesssopachs,

and (b) gold distribution.

tionships.Relative chronologyndicates hat the hy-

drothermalbrecciaand massive re typeswere the

first to develop, ollowedby the stockwork nd dis-

seminated ones Fig. 11). The dikesare generally

pre- or intramineralizationn age, althoughone dike

in the RosarioII ore shoot ontainsmassive re frag-

mentsand s thereforepostmineralizationn age.

Hydrothermalbrecciaore:This ore type generally

occursn dilationalogsassociated ith sinuousault

planes. n plan view, brecciaore shootwidthsvary

from 0.2 to 2.0 m and lengths luctuate rom 3.0 to

20 m. The hydrothermalbrecciaore may be subdi-

vided into that cementedby sulfidesand that ce-

mentedby gangueminerals.

Brecciascemented by sulfide minerals are com-

posed of rounded to subangular ragmentsof tuff,

ocoitas andesitic avaswith large plagioclase he-

nocrysts), ndesite ike, and ocallyquartz.Fragments

range n size rom I to 10 cm andare generally ltered

to sericite,quartz,hematite,chlorite,epidote,and/

or kaolinite.The cement s commonly phalerite nd

chalcopyritewith minor pyrite. The breccias re cut

by multidirectional uartzand/orcarbonate einlets.

This ype of brecciacontainshe highestgoldgrades

(> 10 g/metric on Au) in the ore shoots f all sectors.

The hydrothermal recciaore cemented y gangue

minerals onsistsf rounded o subangularragments,

I to 10 cm in size, of the same ithologiesas the sul-

fide-cemented reccias lus ocal quartz and sulfide

fragments.The cement generally consists f silica,

barite, and minor carbonates.

In some areas of the Guanaco sector, the breccias

containaltered clastssurroundedby sulfide(sphal-

erite or pyrite) rims but also cementedby silica.

Therefore brecciationwas followedby periodic de-

positionof sulfideand gangueminerals. n general,

the gangue-cementedreccias ontain essgold han

do the sulfidebreccias i.e.,


16/29

1332 CAMUS ET AL.

GUANACO SOUTH

ORE SHOOT

2'0 3'0 4'0 5'0 6'0 7'0 8'0 9b

Distancebetween lenses (in)

I 0 7or-8o , -

Distance between lenses (in)

50.

45.

40'

35'

-30'

.20-

, 15.

.10'

...a 5'

GUANACO SOUTH

ORE SHOOT

'D-305)'0b '0'0

ength of lens (in)

MARIA ISABEL

ORE SHOOT

4;0 ,Sb 6'0 7b 8b' 9b--

Length of lens (in)

FIG. 10. Frequency istogramshowingensdistributionn ore shoots,ncluding he numberof

lenses and the distances between them.

zontal fractures. The massive ore is subdivided into

sulfide- ndgangue-bearingones.

The sulfide-bearingassive recontains ore han

50 percentsulfides y volume,whichconsistmainly

of pyrite,sphalerite, ndminorchalcopyritendga-

lena.The zones re always ounded y faultplanes

betweenwhich he massivere ocally isplaysym-

metric depositional eatures, with the latest mineral

precipitated n the central part of the vein. Com-

monly, quartz or ankerite veins are intercalatedwith

or transectmassive ulfideore and provideevidence

for ate-stageydraulic nd/or ectonicracturing. s

in the caseof the sulfide-cemented re breccias, he

goldcontent f the massivere shigh 10 g/metric

ton Au) and ocallyattains kg/metric on.

The gangue-bearing assive re contains t least

50 percent by volume of gangueminerals,mainly

subhedraluartz, arite,andcarbonatesankerite nd

minorcalciteandsiderite). ocally, hese ones ccur

with thin bands 1-5 cm) of sulfides.Generally,

gangue-rich assive re acks ontinuity ndgrades

into gangue-cementedhydrothermal breccia. The

goldcontent f gangue-rich assivere sextremely

low,exceptwheremicrocrystallineuartz spresent.

Stockwork one:This mineralizationype is ocated

adjacent o hydrothermalbrecciaore and/or massive

ore. It consists of stockworks of multidirectional vein-

letsof shear,hybrid,or extension rigin.The veinlets

are filled with one or more of quartz,barite, carbon-

ates,pyrite, and subordinate phalerite, halcopyrite,

galena, and tetrahedrite-tennantite.Veinlet thick-

nessesange from 1 to 15 mm, although ocally they

reach 40 cm. The densityof veinlets ncreasesn the

vicinityof the brecciaandmassive resbut diminishes

graduallyaway from them. The widths of the stock-

work zonescan attain 5 m, and the adjoininghost

rock is altered to quartz, sericite, and kaolinite. The

stockworkzonesare gold bearing, but gold grades

are much lower than those of the breccia or massive

ores as is the sulfide content.

Disseminated one:These zonescomprisehydro-

thermally altered rocks around the hydrothermal

breccia, massiveore, and stockwork zone. Dissemi-

natedzonesare 2- to 1O-m-wide nvelopes f quartz,

sericite, kaolinite, and carbonates with abundant

coarsegrainsof disseminated yrite. Gold contentof

thesezones s essentially il.

Andesitedikes:The dike rocks are light to dark


17/29


N

183.9

KEY

Massivere

,r -] Hydrothermalrecciaore)

*.; Hydrothermalreccia(gancJue)

Stockworkone

Andesireike

Host ock

?y Fracture

Fault

F F Fracture fre;uency

(froCtures/m)

820 156o 0

.^/ ^ /

STRUCTURALOMAIN

STRUCTURALOMAIN

o / FF:2 73

i I

FIG. 11. Exampleof a typical enspresent n an El Bronceore shootshowingore typeswithin a

second-order ilationai og. Alsoshown s a gold distributionhistogram hat reflects he metal content

of the different re ypes. he histogramsbased nchannel ampling.he ensbelongso the Guanaco

Southore shoot ndhasbeensubdividednto two structural omainsor which racture requency ata

are shown.Note that the highest oldgrades re concentratedn the hydrothermal reeeiaandmassive

ores.

green in color, aphanitic o porphyritic n texture,

andcontainabundant lagioclaserystals ltered o

calciteand siderite.The widthsof the dikes ange

from0.5 to 20 m. n general,he dikes resubvertical,

tabular odies mplacedlong ension ndshearaults

and ractures. hey canalsooccuraseast-trending,

50 - to 70-dippingbodieswhich cut the ore shoots,

or assills.Fracturingwithin he dikes s generally f

extensionalrigin,with carbonateillingsn all cases.

Host-rock ithology

The rocks ostingheEl Bronce einsystem onsist

of andesiticlows,andesitic reccias, gglomerates,

porphyritic ndesitesocoitas),apilli tuffs,and uff-

aceous andstones. ll belong o the Cerro Morado

Formation f Early Cretaceousge Boric,1986).

Fragmentalocks: heseare he predominantock

units n the district and average>1,000 m in thick-

ness.

The breccias onsist f andesitic, enerallyangular

fragmentsn an aphanitic r clearly uffaceousmatrix.

Tuffsand tuffaceous andstonesccuraselongate,

lenticular horizons of reddish- to violet-colored rocks

composed f pyroelasticragmentsn an ash-bearing

matrix stainedby iron and manganese xides.The

tuffaceous ocks exerciseda strong structuraland

lithologic ontrolon the alterationand mineralization

and causedpinchingof the ore shoots.

Flow rocks: The andesitic flows are characterized

by plagioclase nd ferromagnesianhenocrystsn an

aphaniticgroundmass.heir color s dark gray,red-

dish,or dark green.They usuallydisplayamygdules

containing hlorite andcalcite.The thickness f each

andesitc flow is about 40 m. Weak alteration to ohio-

rite, epidote, and hematite s widespread.


18/29

1334 CAMUS ET AL.

Ocoita, as noted above, is a local name for an an-

desite with a markedly porphyritic texture charac-

terizedby large (1-3 cm) plagioclasehenocrystsn

anaphanitic roundmass.enerally, coita onstitutes

flowsbut locally may be intrusivesillsor dikes.

Hydrothermal alteration

Hydrothermal alteration at El Bronce affected

mainly he rockshosting tockwork nddisseminated

zones as well as the andesite dikes. The wall rocks of

the veins and brecciasare only weakly altered for

distances f 1 or 2 m from vein margins.

Alteration of andesite and ocoita was not texture

destructive.Plagioclase as totally or partially re-

placedby quartz,carbonates, nd/orsericite,whereas

the marlcswere altered to chlorite and, to a lesser

degree, epidote.

In the brecciasand tuffs, fragmentswere altered

to sericite whereas the matrix was transformed to an

aggregateof clay and carbonates.

The andesite ikeswere alteredpervasivelyo car-

bonates up to 70 vol %), with lesseramounts f ser-

icite, kaolinire, chlorite, and montmorillonite. These

mineralsoccur ogetherwith late, coarse yrite. The

original extureof the dikeswascompletely estroyed

near ore.

In the orebodies, he most ntensehydrothermal

alteration occurs in the stockwork and disseminated

zonesn whichoriginal ock extureswere obliterated.

The principalalteration ype s quartz-sericite, hich

is followedwith decreasingntensityby argillization

and carbonization. oward the margins f the veins,

propylitization predominates to form well-defined

envelopes.Modal analysis f alterationproducts n-

dicate80 to 85 percentsericite,6 to 10 percentcar-

bonates,and 2 to 4 percent chlorite. Kaolinite and

montmorillonitewere recognizedon the basisof X-

ray diffraction nalysis, ut their modalproportions

are still unknown.

Figure 12 shows he distribution of sulfidesand

alteration minerals in the Rosario II ore shoot, El

Bronce sector.Quartz-sericite,clays,and carbonates

occur ubiquitously hroughout he deposit and are

notshownn the figure.Chloriteoccursmainly n the

lower part of the ore shoots, elow the 1,200-m ele-

vation.The chlorite ripidoliteaccordingo X-raydif-

fractionanalysis) asa radialhabit and occursilling

cavitiesasa late phasenot only in the stockwork nd

disseminated zones but also in druses in the breccia

and massive res.Propylitization s not shown n the

figure but extendsaround the orebodiesas narrow

envelopescontainingchlorite, epidote, and calcite

suite.

Mineralogyand paragenesis

The primarymineralogy f the entire epithermal

systemhas been studied on more than 250 conven-

tionalpolished nddoublypolishedhin sections, ith

more detailed work concentrated on the E1 Bronce,

Guanaco,and SanLorenzo sectors. hirty X-ray dif-

fraction and ten electron microprobeanalysiswere

alsodone.Thesemineralogic tudieswere performed

in the laboratories of the Servicio Nacional de Geo-

1ogiay Mineria, Centro de InvestigacionesMinero

Metalurgicas, nd the Departamentode Geologiaof

the University of Chile.

The ore mineralogyof the systemconsists f sul-

fidesand sulfosalts. he oxidationzone is weakly de-

veloped and extendsno deeper than 40 m (Fig. 12).

The bestsurface xposures f the oxidation oneoccur

in the San Lorenzo sector, where limonitic boxworks

after pyrite, chalcopyrite, phalerite,and galenacan

be recognized.The presenceof cerussite nd smith-

soniteplus a few oxidized copper mineralsare the

evidence or underlyingore. n Figure 13, a summary

of the hypogenemineralogyof the E1Broncesystem

is presented,which is relatively simple n terms of

both the number of speciesand their spatialdistri-

bution.The principalminerals re, n decreasing rder

of importance,quartz, pyrite, sphalerite,chalcopy-

rite, and carbonates.Minor barite, galena, tetrahe-

drite-tennantite,hematite, and bornite are alsopres-

ent. The relative proportionsof these mineralsvary

within the ore shoots Fig. 14).

Quartz is the most abundantmineral in all ore

shootsand is present in all the parageneticstagesas

three varieties: chalcedonicquartz, coarse-grained

euhedral quartz, and anhedral granular quartz. In

general, n the upper portionsof the veins, chalce-

donicquartz predominates nd occurs n crustified

form with coarse-grained uhedralquartz developed

aswell-defined nterbanding.n the deeperpartsof

shoots, anding ends o decrease nd the quartz oc-

cursassmalleranhedralgrainswith granular exture.

Pyrite s he most bundant ulfide long he entire

systemand amounts o about 70 to 75 percent by

weight of total sulfides.t is one of the earliestmin-

eralsdeposited nd is replacedand/or cut by other

basemetal sulfides. yrite is the main host or gold,

whichoccursillingcracks r fracturesn pyrite grains

or rimming hem.The earlypyrite s fine grained nd

locallypulverulent, enerally ssociatedith granular

quartz, and locally with euhedral coarse-grained

quartz.A late stageof coarse-grainedyrite, ascubic

crystals p to 1 cm in size, is completelybarren of

preciousmetals. t is concentrated n the vein sel-

vages.

Sphalerites he secondmostabundant ulfide 10-

15 wt %) and occurs smassive, oarse-grainedrys-

tals. Color varies rom black through reddishbrown

to almost ranslucent reen.The black o darkbrown

sphaleritebelongs o the earlier parageneticstages

and occurscloselyassociated ith chalcopyrite. ts

distribution s shown n Figure 12. Locally, gold oc-

curs n fractures n the black to dark brown sphalerite.


19/29


N= 8.00 .= 8.S00 N= .00

KEY

Bose of oxidation

.kl.z.,.. Chlorite:outer limit

-- 1.800m-

_...--.....----fi.__arte: ower imit

Sphalerite:uterimit N

.....%..-""**.nnduurfldes:owerimit

CG Galena:uterimit

Hematiteouterimit __

- ... k .

I I / .....

->,. ................-...-.? - '

................ .. - '...y '..., .....

............ -.

I I

FIG. 12. Distribution and zonation of ore, gangue, and alteration minerals n the Rosario II ore

shoot. Quartz, pyrite, ehaleopyrite, serieite, and kaolinitc distribution are not shown because hey

occur hroughout he deposit.Alsoshown or reference s the presenteconomic imit of the orebody.

Chalcopyrite 8 wt %) occurs sa replacement f

pyrite but more commonly s rregularlydistributed

blebs n dark-colored phalerite.This texture resem-

blesvery much he chalcopyrite iseasen sphalerite

recentlydescribed y BartonandBethke 1987) and

explained s a possibleeplacement f chalcopyrite

in sphalerite.

Of the remaining ulfides, alena ccursocally n

associationwith tetrahedrite-tennantite r pyrite.

Locally,goldwasobserved ssmallblebs n galena.

In places,galenawasreplacedby late chalcopyrite,

tetrahedrite-tennantite, nd sphalerite.The assem-

blagegalena, etrahedrite-tennantite, ndbornitc ac-

counts or 4 wt percentof the total sulfides nd ends

to be more mportant n the Guanacoore shoots.Of

these hree minerals, ornitc s especiallymportant

becausets abundance auses n ncreasen the cop-

per content of the Guanaco ore shoots.

The sulfosalts,etrahedrite-tennantitend ocally,

schwarzitc mercurian etrahedrite), the latter im-

portant in the shallow parts of the Guanacosector,

occur as coarse tetrahedral crystals ogether with

crystalline uartz.All the sulfosaltsre silverbearing.

Schwarzitc replaced chalcopyrite and is cogenetic

with the other sulfosalts.

Gold occurs as the native metal and as electrum.

Gold grainsvary n size rom 5 to 28 #m, but locally,

1-mmgrainswere observed.t occursmainly n frac-

tures and as nclusionsn pyrite, galena,chalcopyrite,

sphalerite,and uncommonly n tetrahedrite-tennan-

tite.

Barite occursonly in the upper levels of the de-

positsas ypical elongate,prismaticcrystals.

Carbonates of the siderite, ankerite, and calcite va-

rieties were depositedduring the late stages f min-

eral depositionand after the barite. They occur as

fine- to coarse-grainedrystals, oating he lastquartz

depositedand filling open spaces,especially n the

upper partsof the veins,and ining geodesor druses.

Carbonates re closelyassociated ith the late-stage,

low iron, green sphalerites Fig. 13).

Hematiteoccurs sacicular rystals ssociatedith


20/29

1336 CAMUS ET AL.

STAGE I I II III IV V

TEMPERATURE 235-344C 200C ?


21/29


Hydrothermalrecciore

'] .,

x

Hostock - - - X

X

x

x

Andesitc o X

dike X

X

X

I OOOm X

Veintructure

METAL VALUE

ZONE DESCRIPTION g/t Au

Upper barren zone

A with cholcedonic quartz

_+arbonate

Zoneithuartzt-

arbonates +barite,

a subeconomicoldontent

+Ag +_. ose metals

pyrite

High-gradeone

ith economic values

ofAuAg+aseetals.-

Quartz +- pyrite + sphole-

ritechalcopyrite/

arbonates

/

/

/

Loweroneithnhedrol

ranular quartz +pyrite

D + hlorite.ubeconomic

old-silver volues. No

bose metols present

FIG. 14. Vertical oningmodelof El Bronce pithermal ystem howing eincomponents, ineral

distribution, and relative metal content of each zone. The metal value curve indicates that zone C is

the ore nterval. = the approximateimitsof the oreshoot, nd///= subeconomicrades.

Sphalerite ccurs nlysporadicallyn the lower evels

of zone C. The galena-sulfosaltssociation,ogether

with the late chalcedonic uartz, tends o be located

in the uppermost100 m of the ore shoot,near the

top of zone C and close o the lower limit of barite.

Locally,however,some solatedpatchesof galena-

sulfosaltsre found n the centraland deeperparts

of the ore shoot Fig. 12). Near the bottom of zone

C, there is a gradual ncrease n copper contentas

chalcopyrite. yrite becomes elativelymore abun-

dant n the deeperpartsof zoneC due o the decrease

in sphalerite ndchalcopyrite ndextends venbelow

the baseof the economicmineralization. ipidolite

increasesn abundance longwith pyrite. The com-

plete absence f basemetalsulfides elow the bottom

of zoneC marks he topof zoneD. Hematite spatch-

ily developedn the basalpart of zoneC (Fig. 12).

ZoneD is definedby the disappearancef hydro-

thermalbrecciaore and massive re and by an in-

crease in the stockwork and disseminated zones car-

rying coarse-grained uartz, pyrite, chlorite, and car-

bonates. n the deeper levels of the system, he

stockwork ones end to give way to a well-defined

structure ndicated only by chloritizationand traces

of pyrite.

Gold and silver distributionswithin the four depth

zones Fig. 14) are illustratedby a preciousmetal

value curve hat indicates he tenor of gold and silver

expressedas gold equivalent. As shown, the gold

equivalent content of zonesA and D is subeconomic

(


22/29

1338 CAMUS ET AL.

Castro and Petorca-E1 Durazno en echelon, dextral

wrench faults. The relative movement of this fault

pair is responsibleor the structuralpattern in the E1

Bronce vein system,which developed a first-order

( 1-km-wide)dilational ault og (Sibson, 987) with

a length of more than 20 km. The tensional ractures

are the loci for the ore shoots in the various miner-

alized sectors.

Based on structural studies of the Guanaco ore

shootsCastelli,1989), major aults,minor oints,and

veinsmay be distinguishedn a typical dilational og

as he two main typesof structures ontaining ither

massive re, brecciaore, or faultbreccia-gouge. hese

two fracture types at E1 Bronce are interpreted as

being he resultof an ncreasen either he differential

tectonicstress nd/or n the fluid pressure o exceed

the minimumhorizontalprincipal stress S3).A local

increase n fluid pressureat E1 Bronce could have

arisen rom the heatingof hydrothermal luidsby the

PetorcaPorphyry.The minor fracturesand faultsat

E1Broncecouldbe the productsof brittle rupture n

the near-surface environment when such a differential

stresswaspresent. n thisregard, luid nclusion tud-

ies (Skewesand Camus, 1988) suggest hat the min-

eralization at El Bronce took place 400 to 1,200 m

beneath he palcosurfacesee below), levels where

inhomogeneouseformation ndbrittle failure are to

be expected.

Faultsand associatedillings

The faults of the Guanaco sector have strikes of N

5 W to N 26 E with subvertical o 60 dips. The

faultsextend long trike or 0.2 to 5 km andvertically

for 80 to 600 m. They can be divided on the basisof

morphology nto three types with characteristic ea-

tures, which can be extrapolated o the entire epi-

thermal system: ectilinear faults, ault swarms, nd

second-orderault jogs.

Rectilinear aultshavestrikeextensions o greater

than 5 to 30 m and strike and dip variationsof only

+__ . The fillings of these faults are narrow (5-50

cm) which indicates hat the physicalconditions fluid

pressure,uniaxial strength,and porosity)prevailing

within the hydrothermalconduitswere such hat fur-

ther dilation became mpossible.

Fault swarmsoccurover strike engthsof 5 to 20

m and consist of multidirectional faults and fractures

associatedwith major structures and generating

stockworkpatterns.

Second-order ault jogs occur along main struc-

tures, showing he former strike changesover dis-

tances of 5 to 15 m.

The four ore types,brecciaore, massive re, stock-

work zones,and disseminated onescan occur n any

oneof these hree typesof structuresn the E1Bronce

hydrothermalsystem.

Massive ore is restricted to the second-order di-

lational ault jogs. These ogs formed through he

reactivation of sinuous faults, whereas breccia ore

within these ogs is interpreted to have been gener-

ated when contemporaneoushydrothermal fluids

converged oward these ogs. Under favorablephys-

ico-chemicalconditions, hese fluids may have then

precipitatedgangueand/or sulfideminerals hereby

cementing he fault brecciafragments.

Commonly, he wider and economicallymore im-

portant hydrothermalbreccia bodies at El Bronce

showevidenceof havingbeen hydrothermallybrec-

ciatedand cementedby gangueand/orsulfidesmore

than once. This suggests hat there were several

phasesof shearingand/or extensional eactivation

during the period of hydrothermal luid circulation

and deposition f gangueand sulfideminerals.

Massiveore associated ith rectilinear faults may

be present sbanded,syntaxialillings, uggestinghat

fracture filling extended rom the center toward the

margins.Bandedveins are interpreted at E1 Bronce

asbeing syntectonic Castelli, 1989). Generally, he

less mportantore shoots t El Bronce,asat Guanaco

South,are related to slightly sinuous aults with no

significant isplacementsollowingvein generation.

Zonesof stockwork einletsat E1Bronce Fig. 11)

occurboth adjacent o dilational ault ogsand/orhy-

drothermalbreccias, ndadjacent o the massive ul-

fide zone of the ore shoot.Both types of stockwork

are a network of multidirectionalveinlets that may

havebeen generated hrough ectonic eactivation r

hydraulic racturing seeabove)either n preexisting

andsyntectonicxtensional,ybrid,or shear ractures

which were filled later by variousminerals.

Fracturesand their fillings

Fractures veinsand oints)differ rom faults n that

they have not been affectedby tectonic eactivation

and subsequent isplacement nd shearing long he

structuraldiscontinuity. hree typesof fracturesare

identified in the Guanacoore shoots: xtensional, y-

brid, and shear.These hree typesof fracturescanbe

explainedwithin the context of the effective stress

componentso'1 0'2 0'3)anddihedralangle 20)

under which these fracture types may develop. Han-

cock (1985) proposedan interrelationship etween

the magnitude f the differential ffective tressesnd

the tensilestrengthof eachrock to predict the gen-

eration of these hree fracture types.

Veins and joints show rectilinear patterns at E1

Bronce. Their trace architecturalstylesare of V, X,

or I shapeswhich indicate, for the first two cases,

conjugateractureshybridor shear) nd, or the third

case,extensionalractures.This last casegenerally s

associated with the andesitc dikes.

The veins are of various compositions, lthough

quartz, pyrite, and carbonatespredominate see

above).

Thin section studies of veins demonstrate that the

effective stresses related to these structures are of


23/29

EL BRONCEEPITHERMAL EIN SYSTEM, ENTRALCHILE 1339

extension and shear. Extension veins in andesite dikes

show ndeformedalcite iberswithsyntaxialrowth

textures,whereas hear einsshowquartzandcalcite

crystals rowing t obliqueangleswith respect o the

vein margins,denoting hat the externalstresses ar-

ied during crystal precipitationand syntectonic

growth.

Ore shootemplacementmechanism

The first-order ilationalault og system,with its

multiplestrikeanddip orientationsnd epeated if-

ferentialmovements,roducedhestructuralatterns

that provided he necessaryermeabilityor the cir-

culationof hydrothermalluids.Thesesamedilational

openings ermitted the emplacement f barren dikes

as well.

Figure 15 is a schematic llustration, based on Sib-

son 1987), to show he four stageshat canbe in-

ferred or the development f an individual re lens

alonga sinuous trike-slipault; t incorporateshe

variousstructuraland morphologic omponentso-

getherwith the associatedre andganguemineral-

ization.

The structural emplacementmechanismcom-

menceswith a dextral senseof the fault shearwhich,

along he sinuous egment f the plane, orms he

jog. Fluid pressure radientswill favor ocalbreccia-

tion within the jog and the precipitationof ore and

gangueminerals o form a hydrothermal reccia. n

the more inearsegmentsf the faultplane,massive

andbandedore s precipitated. owardboth endsof

the jog, there s stockwork evelopment ith multi-

directional hear ndhybrid ractures roduced y

hydraulicracturedue o circulation f hydrothermal

fluids Phillips, 972).Thehigherprecious etalval-

ues are concentrated n the hydrothermalbreccias

andmassive artsof the lens Fig. 11).

As described bove, he thickestpartsof the ore

shoots orrelatewell with the highest oldor silver

concentrationsFig. 7). Thiscorrelation howswhere

the rupture-inducedxtensionalracturing ccurred

and, consequently,he favoredconduitsor the cir-

culation f hydrothermalluids.n Figure16, a fluid

circulationmodel s presented, n which the rootsof

the ore shoots re interpreted s he pointsof fluid

inflow nduced y extensionalracturing.

Hydraulic racturing ppearedo haveplayedan

important ole in the development f the E1Bronce

epithermal ystem ndprobably ccurred henhy-

drothermalolutionsoseo shallowevels. ccording

to Phillips 1971 , partof the energy f hydrothermal

fluids nd/ormagmas usedn displacingr hydrau-

lically racturinghe rocks ncountered.ydraulic

fracturing, s alreadyexplained,s considerede-

sponsibleor the generationof stockwork onesor

hydrothermalreccias here luids ose long truc-

tural discontinuitiesFig. 15).

Fluid Inclusions

Primary,pseudosecondary,ndsecondaryluid n-

clusionsn quartz,calcite,sphalerite, ndbarite from

the E1Bronceveinswere analyzed hermometrically

using a Linkham 600 heating and freezing stage.

Where possible, luid nclusionsn minerals rom dif-

ferentparagenetic tages--theearlypyrite-quartz o

the late carbonates--were studied. More than 400

inclusions ere studied n 12 samples ver a vertical

intervalof 400 m in the Guanaco ector Figs.3 and

17; Skewes, 1986a, b, and c, 1988; Skewesand Ca-

mus, 1988). Other samplesrom the Rosario II ore

shoot n the E1Broncesectorand from the E1Espino

(south of the QuebradaE1 Bronce fault, Skewes,

1987), La Olla (Skewes, 1988), and San Lorenzo

(Skewes, 986a) sectorswere alsoanalyzed.

Fluid inclusionsrom the E1Broncesystem onsist

of two phases: aporand iquid.The eutecticminima

for the fluid inclusionss close o -20.5C, indicative

of solutionsn the H20-NaC1 system Potter et al.,

1977). Neither liquid CO2 nor clathrateswere ob-

servedupon cooling.Most inclusions re liquid rich

(


24/29

1340 CAMUS ET AL.

E4

E3

A A

A A --'-'-- A

---.__A A. - .'-'.__..-.e...:.....:.___- Outerimitf lterationnd

'" ,--........_..

E2

E1

A

o

I

KEY

Hosf ock

Hydrofhermalreccia

ore)

Hydrothermalreccia

gangue)

5m

_____Stockworkmainlyhear

ractures )

Fault

Con a c t (defined,inferred)

Principal extension

orientation :

Direction f hydrothermal

fluid inflow

FIG. 15. Schematic epresentationof the four rupture stages uring the developmentof a dilational

jog at E1 Bronce. E1 is the prerupture stage,E2 and E3 are intermediate stages,and E4 is the final

(presentlyobserved) tage.E4 s a real case aken rom the GuanacoSouthore shoot.Note the presence

of hydrothermalbreccia n the jog andbanded,massive re in the more inear sectors f the fault. The

small arrows ndicate inferred fluid inflow directionsand the large arrows n E4 indicate the direction

of the leasteffectivehorizontalprincipal stress S3)direction.


25/29


i i i i i i i i i i i

N=$7.200 N=7.600 N=$8.000 N=38,40,800 _ N=3.200

'Ira thickness contour

Fluid inflow channelwaits

I I I I I I I I II I I I

FIG. 16. Hypothetical model of fluid inflow channelways n the Rosario II ore shoot, El Bronce

district.

idenceof neckingwasobserved. hese nclusions ith

highlyvariable iquid/vapor atioshomogenize ithin

the same angeof temperaturesuggestingoilingof

the hydrothermalluids Roedder ndBodnar,1980).

Primary nclusionst this evelhomogenizedetween

235 and 270C and had salinities between 4 and 7

wt percentNaC1equiv (Figs. 18 and 19). Secondary

inclusions omogenize t about 200C and have sa-

linitiesbetween and5 wt percentNaC1equiv Fig.

18). Late-stage arbonatest this evel have nclusions

that alsohomogenize t about150C (Fig. 18). Thus,

from the intermediate to the shallow levels of the

Guanacosector,a decrease s observedonly in the

homogenizationemperatureswhile the ranges or

salinitiesoverlap (Fig. 19). For fluid inclusionsn

sampleshatshow vidence f boilingandare ocated

at the presentsurface ver the Guanaco ector,min-

imum homogenizationemperaturesof 235C and

salinities f 5 wt percentNaC1equiv ndicatea depth

of formation,under hydrostatic onditions, lose o

GUANACO SECTOR

ELEVATION L OLL SECTOR

meters

"'"'-

Guanaco oreshoot

Fluidnclusionampleite

Fluidnclusionornogenizafion

temperature h{stogroms

1000 ? i,m __250.__sotherm

( Sornpleshowingoiling

FIG. 17. Distribution of fluid inclusionsamples n the Guanacoand La alia sectors t E1Bronce.

The paleosurface, efinedon the basisof the fluid inclusionstudy, s shown.


26/29

1342 CAMUSET AL.

TIME

F,I. IN CARBONAT'

.I. IN QUARTZ

PRIMARY SECONDARY

Si 02

PRIMARilY

235-270C

4 -7%

u.I

/

p

FIG. 18. Generalizedparageneticdiagram or the E1 Bronce

epithermalsystem.The horizontalaxis epresents ime and the

vertical xis epresentshedepthduring hevarious volutionary

stages f the system. he verticalbars epresenthe most mpor-

tant minerals n the different paragenesesPY = pyrite, CPY

-- chalcopyrite,SPH -- sphalerite, ET/TEN = tetrahedrite-ten-

nantite,BAR -- barite, CAR = carbonates). ilicawasdeposited

throughout he sequence s quartz. The homogenizationem-

peraturesof primary and secondaryluid inclusionsFI) are in-

dicated t different levationsn the deposit. he segmentedine

showshe evelabovewhichboiling ookplaceduring he different

paragenetic tages. he rectanglewith diagonal inesrepresents

the area of gold mineralizationn both time and space.

300 m beneath he paleosurfaceSkewes ndCamus,

1988; Fig. 18).

Stable sotopeStudies

Preliminary ulfur ndoxygensotope tudies ere

undertaken n mineralsamplesrom the RosarioII

ore shoot at the Geochron Laboratories Division of

KruegerEnterprises.Six 34Sdeterminations ere

made romchalcopyrite,yrite,sphalerite, ndbarite

separates. nalyses f two of the sulfide airs,sphal-

erite-pyriteand chalcopyrite-pyrite, ere evaluated

for use n geothermometry.he two mineralpairs

were from massive re and were hoped to have

formedunder equilibriumconditions.However, sul-

fur isotopeequilibration emperaturesor the two

pairscalculated sing he fractionation quations f

Ohmotoand Rye (1979) were much ower than fluid

inclusion omogenizationemperatures,uggesting

thatneitherpair formed n sulfur sotope quilibrium.

Stable sotopeanalyses nd temperaturedetermina-

tions are summarized in Table 4.

The three sulfides tudied howa relativelynarrow

rangeof 34S alues:0.5 to -0.6 per mil for chal-

copyrite, -2.3 to +0.7 per mil for sphalerite,and

-3.8 to +2.1 per mil for pyrite. The narrow angeof

a4S aluesor thesulfidesnd heproximityo 0 per

mil suggestsmagmatic ource or the sulfur nvolved

in the E1 Bronceepithermalsystem.Spiroand Puig

(1988) arrived at similarconclusions singsamples

of galena, phalerite, ndchalcopyriterom he upper

portionsof the RosarioII ore shoot.The enrichment

of 34S aluesor the ate-stagearitesample9.1%0)

indicates near-surfacencrease n the sulfide/sulfate

ratios n the ore fluid (OhmotoandRye, 1979).

Discussion

Depth o formation

Based on the thermometric information of fluid in-

clusions,t is estimated hat the top of the zone of

gold mineralization evelopedno more than 400 m

beneath he paleosurface.he depthcalculationsre

basedon the dataof Haas 1971), assuminghat hy-

drostatic ressure onditions revailed n the upper

levelsof the deposit ssuggestedy open-spaceex-

tures.

The fluid nclusion videnee uggestshat he gold

mineralization tagewasnot related o boilingpro-

eesses shascommonlybeen reported elsewhere or

epithermal epositse.g.,Buchanan, 981). Evidenee

for boilingwas eeognized t E1Bronee nlyat levels

above he zoneof eeonomiemineralizationFigs.17

and 18). This boilingzone, n the early paragenetie

stages, eaehed a maximum depth of 350 m below

the paleosurfaee.n the Guanaeoseetor,where evi-

denee or boilingeanbe reeognizedeadily, t iseon-

eluded hatonlyabout350 m of erosion aveoeeurred

during he last 80 m.y.

Enthalpies f the luids and mechanism f gold

precipitation

Enthalpyversussalinityplotsare useful or deter-

mining the eoolingmechanisms f aseendant ydro-

thermal luids Fig. 20a; Fournier,1979). Coolingof

the fluidscanoeeurby mixingwith eooler,moredilute

fluidwhiehdeereasesothsalinity ndenthalpy path

OA; Fig. 20a),boilingwhiehdeereasesnthalpy ut

inereasessalinity (path OC), eonduetionof heat to

thehost oekswhiehdecreasesnly heenthalpypath

OB), or by a eombination f theseproeesses.

For the Guanaeo ectorat E1Bronee,a plot of en-

thalpyversus alinityshowshat the highestenthalpy

andhighestsalinityoccur n the deepest evelsof the

depositand that both deerease oward ntermediate

levelsof the veins segment ; Fig. 20b). Comparing


27/29


16.

12,

4.

12

26o

200 240

ELEVATION (above sea level )

1 738 - 1810m

12

8

ELEVATION (above sea level)

1 738 -1 810m

24Q0

30 30

1563- 628m 1

nn 2 4 6 8 10 12

I

280 320 30 Z

2 4 6 8 10 12

I 563-1 628m

2{)0 ' 2,0 ' 280

I 500m

HOMOGENIZATION TEMPERATURE (C)

16-

12.

8.

4-

320 360 2 4 6 8 10 12

SALINITY (weight NaCI equivalent)

FG. 19. a. Homogenization emperatures C). b. Salinity wt % NaCI equiv). Fluid inclusiondata

for primary nclusionsn quartz rom stages and3 at different evelsof the Guanaco ector,E1Bronce

district.

I 500m

TABLE . SulfurandOxygen sotopeData, RosarioII Ore

Shoot, E1 Bronce District

Sample Mineral Stage 34S%0) 180 %0)

CH-1 Chalcopyrite II -0.5 to -0.6

CH-1 Sphalerite II +0.7

CH- 1 Pyrite II +2.1

R2-1 Pyrite II -3.8

CH~2 Sphalerite V -2.3

CP-1 Quartz IV-V

CH-2 Barite IV +9.1

+11.9

Calculated emperatures sampleCH- 1)

Mineral pair

Sphalerite-pyrite

Chalcopyrite-pyrite

T (C)

192 30

143 _+ 25

this o the paths or differentcoolingmechanismsFig.

20a) suggestshat coolingwas causedby mixingof

hot, saline fluids with cooler, more dilute fluids.

From the intermediate to the shallowest levels of

the Guanaco ector,enthalpycontinueso decrease

but the salinityremainsconstant segment ; Fig.

20b). In the shallowestevels, he coexistencef liq-

uid-rich and liquid-poor fluid inclusions nd their

same angeof homogenizationemperatures,uggest

that boiling occurred.However, t is not possibleo

explain he observed hangesn enthalpy ndsalinity

by boilingalone,becausehere is no increasen the

salinity f the fluids.A combinationf boiling ollowed

by mixingof fluidswouldproduce he observed om-

bination f decreasingnthalpy ndconstant alinity.

Becausemixingof hot, salineandcooler,dilute fluids


28/29

1344 CAMUS ET AL.

I-

z

uJ

CONDUCTIVEOOLIN

A B C

0

SALINITY


29/29


minationsrom fluid inclusion tudies: arth-Sci.Rev., v. 8, p.

263-301.

Ruiz, C., 1945, Informe geologico-minero obre el mineral E1

Bronce de Petorca:Santiago,Chile, Inst. Inv. Geol., unpub.

rept., 16 p.

Seward,T. M., 1984, The transport nd deposition f gold in

hydrothermal ystems,n Foster,R. P., ed.,Gold 82: Rotterdam,

A. A. Balkema,p. 165-181.

Sibson, . H., 1987,Earthquakeupturing sa mineralizinggent

in hydrothermal ystems: eology,v. 15, p. 701-704.

Skewes,M. A., 1986a, nformepreliminar obre nclusionesluidas

de a vetaGuanaco, inaE1Bronce e Petorca: antiago, hile,

CompaniaMinera E1 Bronce,unpub. rept., 27 p.

-- 1986b,Consideracionesetrologicasde nclusionesluidas

en la vetaGuanaco SanLorenzo:Santiago, hile, Compania

Minera E1Bronce,unpub.rept., 13 p.

-- 1986c, Inclusionesluidasen el sistema 1Bronce:Santiago,

Chile, CompaniaMinera El Bronce,unpub.rept., 20 p.

-- 1987, Inclusiones luidasen E1Espino,E1Bronce Sur: San-

tiago, Chile, CompaniaMinera El Bronce,unpub. rept., 16 p.

-- 1988, Estudiopreliminarde inclusionesluidasen el sector

La Olla del yacimientoE1 Broncede Petorca:Santiago,Chile,

CompaniaMinera E1 Bronce, unpub. rept., 23 p.

Skewes,M. A., and Camus,F., 1988, Inclusionesluidasy me-

canismose precipitacion e metales reciososn el yacimiento

epitermal E1Broncede Petorca:Rev. Geol. Chile, v. 15, p. 31-

39.

Spiro,B., andPuig,A., 1988, The source f sulphurn polymetallic

deposits n the Cretaceous island arc of the Chilean Andes,

initial assessment: outh American Earth Sci. Jour., v. 1, p. 1-

6.

Vicuna-Mackena, B., 1881, La edad del oro en Chile, 2nd ed.

1968: Santiago,Chile, Franciscode Aguirre, 417 p.

geologic, structura and fi studies of eoithermal vein sustem, chile

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