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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
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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
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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-
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1320 CAMUS ET AL.
A,XA A A
AI A
J v
o o o o o o
/ oo o o
O o
A A A
A A A
V
V
V
V
V
AAAAA _
A A
A A A
A
A A
A A A A A A A A A
A A A A A A A A A A A
X X A A A A
X X X X
X X X
o
o o
o
o
o
o
o o Do o
o
o o
o
o
o o
o oooo
o
\ o
A A A ^ ^ ^ ^ ^ ^
o
o
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.
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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).
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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
/
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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-
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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-
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EL BRONCE EPITHERMAL VEIN SYSTEM, CENTRAL CHILE 1325
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13 2 6 CAMUS ET AL.
I
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EL BRONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1327
o
o g o
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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-
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- 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
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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.,
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EL BRONCE EPITHERMAL VEIN SYSTEM, CENTRAL CHILE 1331
,, .
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.,
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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
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EL BRONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1333
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.
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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.
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EL BRONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1335
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
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1336 CAMUS ET AL.
STAGE I I II III IV V
TEMPERATURE 235-344C 200C ?
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EL BRONCE EPITHERMAL VEIN SYSTEM, CENTRAL CHILE 1337
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
(
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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
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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
(
7/24/2019 Geologic, Structura and FI Studies of Eoithermal Vein Sustem, Chile
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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.
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EL BRONCE EPITHERMAL VEIN SYSTEM, CENTRAL CHILE 1341
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.
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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
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EL BRONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1343
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
7/24/2019 Geologic, Structura and FI Studies of Eoithermal Vein Sustem, Chile
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1344 CAMUS ET AL.
I-
z
uJ
CONDUCTIVEOOLIN
A B C
0
SALINITY
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EL BRONCE EPITHERMAL VEIN SYSTEM,CENTRAL CHILE 1345
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.
Recommended