epitermales de oro

Journal of Geochemical Exploration, 36 ( 1990 ) 197-232 197 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

The geology of hydrothermal gold deposits in Chile

FRANCISCO CAMUS

Compania Minera el Bronce, Carmencita 240, Las Condes, Santiago, Chile

(Received January 10, 1989; revised and accepted July 25, 1989 )

ABSTRACT

Camus, F., 1990. The geology of hydrothermal gold deposits in Chile. In: J.W. Hedenquist, N.C. White and G. Siddeley (Editors), Epithermal Gold Mineralization of the Circum-Pacific: Ge- ology, Geochemistry, Origin and Exploration, I. J. Geochem. Explor., 36: 197-232.

A detailed geologic study has been undertaken of the characteristics of 20 of the best known hydrothermal gold deposits in Chile. Characteristics include production and reserve data, metal grades, geotectonic setting, morphology, mineralogy, zonation, alteration assemblages and fluid- inclusion studies. Based on this geologic data base, two broad categories of gold deposits have been recognized: volcanic-hosted epithermal deposits of the adularia-sericite and acid-sulphate types and porphyry-related deposits.

Volcanic-hosted epithermal deposits of the adularia-sericite type appear to be more abundant than the acid-sulphate type. Very few examples of porphyry-related deposits have been recognized. The majority of the acid-sulphate type deposits were formed during the Oligocene-Quaternary time-span and no adularia-sericite type examples are yet known from this period. This latter epithermal type was formed exclusively during the Mesozoic and Early Tertiary.

Presently accepted ore-deposit models have been applied to the Chilean epithermal gold deposits, and an empirical model for adularia-sericite epithermal system is presented.

INTRODUCTION

Background

The surge in precious-metals exploration worldwide in the past 15 years has resulted in a greater interest in the study of hydrothermal deposits, especially of gold. These studies are aimed at defining the geological characteristics, depositional environments, and origin of these deposits in order to develop better tools for exploration. The works of Buchanan (1981), Henley and Ellis (1983), Berger and Bethke (1985), and Heald et al. (1987), among others, are examples of general studies of this type.

The marked increase in gold-silver exploration in Chile during the past 10 years has resulted in the discovery of the E1 Indio ore deposit, east ofLa Serena

0375-6742/90/$03.50 © 1990 Elsevier Science Publishers B.V.

198 F. CAMUS

in the high Andes - presently the largest gold producer in Chile - and the recent discoveries at La Coipa and Marte.

Chile's total recorded gold production between 1545 and 1987 was 688,000 kg. Almost 50% of this gold (341,000 kg) has been mined since 1932. Since 1980 there has been a strong revival of exploration and mining. At present the official average annual Chilean gold production is 16,000 kg.

Geological studies of Chilean gold and silver deposits have not been devel- oping at the same rate as exploration. A review of the literature shows a scarc- ity of published detailed geological work even for the principal gold camps of the country. The only published compilations are papers by Flores (1942) and Ruiz et al. (1965). Recently, Camus (1985) and Cabello (1986) described the principal geological features of a number of these deposits using updated concepts.

This paper represents an updated version of Camus' (1985) review of new research results and describes the geological characteristics of the best known gold deposits in Chile. A major aim is the understanding of the genesis and evolution of the deposits within the geological context of the country. A pos- sible classification is established, based on conceptual models that may be use- ful in exploration.

Available data of the 20 best known gold districts or their most important

TABLE 1

List of 20 epithermal gold deposits/districts with selected references considered in this study

Deposit/district References

Choquelimpie Faride San Cristobal E1 Guanaco La Coipa Marte E1 Hueso Inca de Oro Cachiyuyo de Oro E1 Capote E1 Indio Andacollo Los Mantos de Punitaqui Las Vacas E1 Bronce Alhu~ Chanc6n El Tigre E! Chivato Minas del Prado

Thomas (1973) Camus (1987) Lowell and Aspillaga (1987), Rivera (1984) Llaumett {1979) Cabello (1986) Mining Journal (1988) Minerfa Chilena (1987) Flores and Ruiz (1946), Villarroel (1972) Salinas (1975) Flores (1943), Frank et al. {1985) Siddeley and Araneda (1986) Llaumett (1980) Lepeltier (1964), Galay (1974) Swayne (1949), Camus and Reichhard (1987) Camus (1982), Camus et al. (1986) Flores (1948) Camus and Duhalde ( 1981 ), Dfaz (1986) Camus (1981) Camus and Drummond (1979) Ambrus and Araya (1981)

GEOLOGY OF HYDROTHERMAL GOLD DEPOSITS IN CHILE 199

deposits have been compiled and summarized in Tables 1 to 14. This information was used to group the deposits into certain deposit types. In addition, an empirical model for vein deposits is presented in an attempt to establish a point of reference for Chilean vein gold deposits.

Geological and metallogenic evolution of Chile

The geological and metallogenic evolution of the country has been the sub- ject of several detail studies in recent years. Ruiz et al. (1965), Aguirre et al. (1974), Sillitoe (1976), Coira et al. (1982) and Frutos and Pincheira (1985) have covered several aspects of the geological evolution of Chile. According to these workers, the Chilean continental margin is the result of two tectonic cycles developed during the Paleozoic (Hercynian cycle) and the Mesozoic- Cenozoic (Andean cycle) time spans. Most metallogenic events in Chile are related to these two cycles. The Hercynian cycle is associated mainly with iron, and unimportant chrome and nickel deposits, while the Andean cycle produced the porphyry copper systems and the epithermal gold deposits. These deposits formed in a geotectonic regime and associated intrusive-extrusive magmatism which has been correlated with the tectonic activity on the western boundary of the South American plate (Davidson, 1987). As a result of this tectonic evolution an east-west zonation of several metallogenic belts developed on this margin (Sillitoe, 1976; Frutos and Pincheira, 1985). The westernmost belts contain iron-apatite contact metasomatic deposits as well as stratabound copper deposits. On the other hand, the Cu-Mo porphyries and the polymetallic Ag-Pb-Zn deposits in Chile form a second belt located to the east of the Fe-Cr- Ni belt. Finally, towards the altiplano of Peru and Bolivia the Sn-Bi-W belt is situated.

These belts are interpreted as being the reflection of changes in the tectonic setting and igneous activity during the evolution of the Andean orogen, with the development of a magmatic arc-back-arc basin setting in an early stage, towards more evolved stages with the presence of a magmatic arc accreted to the continental margin. Important north-south-trending structures, broad folding and numerous volcanic centers developed along this margin. Magma- tism also evolves from calc-alkaline and locally tholeiitic in composition during the early stages, to intermediate and finally K-rich calc-alkaline in composition in the later stages (Frutos and Pincheira, 1985).

Gold mineralization occurs throughout the whole Andean cycle. There is no known direct evidence of gold-bearing mineralization during the Paleozoic. The existence of gold placer deposits associated with metamorphic Paleozoic terrains provide indirect evidence for the presence of primary gold deposits at this time. This is particularly true along the Nahuelbuta cordillera in southern Chile.

200

TABLE 2

Time distribution of gold districts

F. CAMUS

Magmatic Arc Upper Magmatic Arc Upper Magmatic Arc Jurassic-Lower Cretaceous Cretaceous-Lower Tertiary Miocene-Recent

Inca de Oro Faride Choquelimpie El Capote San Cristobal El Indio Las Vacas E1 Guanaco Minas del Prado Los Mantos de Punitaqui E1 Bronce La Coipa E1 Chivato Alhud Marte Cachiyuyo de Oro Chancdn El Hueso Andacollo E1 Tigre

The 20 districts reported in this paper all formed during the Andean Cycle and can be grouped chronologically as follows (Table 2):

(1) deposits associated with the Upper Jurassic-Lower Cretaceous magmatic arc;

(2) deposits associated with subvolcanic systems developed during the Up- per Cretaceous-Lower Tertiary; and

(3) deposits associated with subvolcanic systems developed during the Mio- cene to Recent.

Most of the gold was introduced during this latter time span (Fig. 3). The geographical location of these groups are shown in Figure 1.

Gold deposits formed during the Late Jurassic-Early Tertiary time span are mainly volcanic-hosted epithermal veins of the adularia-sericite type, associated in most cases with volcanic centers. Towards the Late Oligocene-Early Miocene period, changes in the geological setting (and different erosion levels? ) are reflected in the predominance of the acid-sulphate epithermal deposits of the stockwork/disseminated type associated with large hydrothermal alteration zones. Finally, during the Miocene-Pleistocene, gold hydrothermal systems appear to be more related to an increased geothermal activity, so all the existing epithermal systems are of the acid-sulphate type.

From the above discussion we could presume an evolution of the Chilean gold deposits from more deep-seated deposits of the adularia-sericite bonanza type, with a relatively simple mineralogy, occurring in the early stages of the Andean cycle to morphologically, mineralogically and structurally more complex, shallow acid-sulphate epithermal deposits, associated with volcanic centers, during the later stages of this cycle.

Nevertheless, since the acid-sulphate deposits are generally associated with shallow hydrothermal systems, erosion might well account for the fact that no known deposits of this type has yet been found in the older Mesozoic rocks.


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THE GEOLOGY OF THE HYDROTHERMAL GOLD DEPOSITS

General

Most of the known Chilean hydrothermal gold deposits are epithermal in nature and belong, as was mentioned above, to one of two types : the acid- sulphate type and the adularia-sericite type, as defined by Heald et al. (1987). The mineralogy and textural characteristics indicate a shallow emplacement and low to moderate temperatures of formation (White, 1981 ). Of deeper emplacement, are plutonic or porphyry-related deposits including skarns, breccias and vein or stockwork style deposits. Over 80% of these deposits are emplaced within the Coastal Range morphological unit, a 1700-km-long belt extending from latitude 18 ° south to 37 ° south, bounded approximately by longitudes 70 ° and 72 ° west (Fig. 1 ). The remainder occur in the Precordillera and the High Cordillera of the Andes and include some important recent discoveries.

The majority of hydrothermal deposits found in Chile are veins associated with volcanic centers. Some of the new discoveries in the high Andes are large- tonnage low-grade deposits of the stockwork or disseminated type, usually hosted in permeable stratigraphic horizons.

The gold deposits discussed in this paper include those in which the principal metal is gold, as well as those containing significant concentrations of other metals. Within the Coastal Range numerous vein deposits exploited for copper contain minor gold values. These deposits are important in the metallogenic evolution of the country but will not be described in detail.

E s t i m a t e d reserves

The lack of data on grade and tonnage for most of the deposits allows only a rough estimate of their metal content. Figure 2 was constructed using the published information, for the better known deposits and tonnage and grade estimate based on personal evaluations of the author for deposits without published data. For the majority of the vein orebodies, reserve figures are based on the length, width and depth of the mineralized structure and assuming that only 25-30% of the vein volume carries economic grades. The grade-tonnage distributions for the 20 districts are shown graphically on log-log scales in Fig- ure 2. Forty-five percent of the deposits, by size, fall in the range of 200,000- 500,000 tonnes of contained ore; 25% in the 1,500,000-7,500,000 tonnes range and the remaining 30% fall in the 10,000,000-40,000,000 tonnes range. This last group represents the great majority of the stockwork/disseminated deposits, while the smaller groups are mainly vein-type deposits. The smaller and medium-sized deposit groups have grades between 2.8 and 14 g/ t Au. The larger deposits contain between 1.6 and 2.1 g / t Au. The richest deposit is El Indio

204 F. CAMUS

TONN S (Au~

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Fig. 3. Relative gold deposition during Mesozoic and Cenozoic time-span for the 20 selected deposits.

with 106 tonnes of contained gold; the smallest deposit considered is Minas del Prado with 0.5 tonnes of gold, according to current information (Fig. 2 ).

Most of the smaller deposits are located in the Coastal Range; their smaller size probably reflect the deeper erosion level of this morphological unit. The larger deposits occur at elevations greater than 3500 m in the High Andes, with the exception of the San Cristobal deposit.

Associated metals

Most of the gold-bearing vein deposits also contain metals such as Ag, Cu, Pb, Zn, As and Hg. However, with the exception of silver, these minor metals are generally not recovered.

Silver: Silver grades typically range from < 5 to 10 g/tonne. Silver is of economic significance in the Choquelimpie, Faride, La Coipa and E1 Indio deposits. Au/Ag ratios are generally less than one for most of these ore deposits (Table 3 ).

Copper: The copper content rarely exceeds 0.50% Cu, and as a result is rarely recovered. However, San Cristobal, Faride, E1 Guanaco, and E1 Indio have av-


TABLE3

Production data for 20 selected hydrothermal gold deposits

Deposit or district Ton ( × 10 e) Au Ag Au/Ag Base metals (Prod÷Res) (g/t) (g/t) (%Cu+Pb+Zn)

Choquelimpie 11.75 2.11 60 0.035 Traces Faride 1.50 2.80 240 0.012 2.50 San Cristobal:

(Diss) 15.90 1.58 NP > 1 NP (Vein) 0.15 4.00 50 0.080 2.0% Cu

El Guanaco 5.40 3.70 17 0.22 2.0% Cu La Coipa 33.80 1.70 88 0.019 Traces Marte 36.00 1.26 NP > 1 NP El Hueso 16.00 1.68 NP > 1 NP Inca de Oro 0.20 4.00 NP > 1 Traces Cachiyuyo de Oro 0.30 12.00 NP > 1 NP El Capote 0.50 9.00 NP > 1 NP El Indio:

(Bonanza) 0.18 218.00 109 2.0 2.87% Cu (Plant-Grade) 7.40 9.10 91 0.10 4.33% Cu

Andacollo 10.00 1.50 NP > 1 NP Los Mantos de Punitaqui 2.50 9.00 NP > 1 0.5% Cu Las Vacas 0.25 12.50 NP > 1 NP E1 Bronce 4.30 5.00 20 0.25 2.20 Alhu~ 0.50 5.00 20 0.25 0.10% Cu ChancSn 0.55 5.00 10 0.50 1.50 E1Tigre 0.25 8.00 50 0.16 1.50 E1 Chivato 0.20 7.50 NP > 1 NP Minas del Prado 0.20 2.30 NP > 1 NP

NP = not present; ? = unknown; diss-- disseminated.

erage grades over 2% Cu. In some of these deposi ts , coppe r va lues t e n d to in- c rease wi th d e p t h re f lec t ing a ve r t i ca l zona t ion , s imi la r to e p i t h e r m a l s y s t e m s e l sewhere ( B u c h a n a n , 1981 ).

Lead-Zinc: T h e s e two m e t a l s occur in 9 of t he depos i t s deposi ts . In on ly four of t hese depos i t s was lead a n d zinc ac tua l ly assayed , y ie ld ing va lues b e t w e e n 1.0 a n d 2.0% c o m b i n e d P b + Zn. In t he five n o n - a s s a y e d depos i t s the a m o u n t of ga l ena a n d spha le r i t e is minor . L e a d a n d zinc va lues t e n d to increase wi th dep th , showing a pos i t ive co r re l a t ion wi th copper .

Arsenic: I m p o r t a n t a m o u n t s of a r sen ic are p r e s e n t as ena rg i t e in E1 G u a n a c o a n d E1 Indio a n d as a r s e n o p y r i t e in E1 Capote . R e p o r t e d As grades for E1 Ind io r ange b e t w e e n 0.60 a n d 4.0% As (S idde ley a n d Araneda , 1986). A t E1 Capo t e F r a n k e t al. (1985) ind ica te g rades of 0 .70-5 .90% As. In all depos i t s a rsenic t ends to inc rease w i th dep th .

Mercury: M e r c u r y m i n e r a l s have on ly been repor ted , a s s ayed for a n d eco- n o m i c a l l y r ecove red a t t he Los M a n t o s de P u n i t a q u i deposi t . G r a d e s a re in the

206 F. CAMUS

order of 0.25-0.30% Hg and the mercury distribution within the orebody shows a horizontal zonation.

Geotectonic setting

In the better known deposits, such as El Indio and El Bronce, a close association of the veins with nearby caldera structures or volcanic centers is recognized (Camus et al., 1986 ). At least 11 deposits are closely related to margins of calderas or stratovolcanoes. These include deposits at Choquelimpie, E1 Guanaco, La Coipa, Marte, E1 Indio, E1 Bronce and Minas del Prado (Table 4). As noted by Heald et al. (1987), caldera settings represent excellent plumb- ing systems for the development of hydrothermal convection cells. Volcanic centers commonly are capable of generating the necessary zones of structural weakness along which the hydrothermal fluids were channeled.

Deposits like Los Mantos de Punitaqui, E1 Tigre, E1 Capote or Las Vacas, are associated with structurally complex fault zones.Other deposits like E1 Hueso or Andacollo, are marginal to porphyry copper systems, which may also be related to volcanic centers.

Present evidence, for all these deposits, suggests that hydrothermal mineralization occurred at the end of a phase of active volcanism, in what Smith and Bailey (1968) define as "the terminal stage of waning volcanic activity".

Geological characteristics

Host-rock lithology The main host rocks for Chilean gold deposits are andesites, volcanic brec-

cias, ignimbrites, tufts, rhyolites and dacites. As a rule these rocks are intruded by dioritic to granodioritic stocks. Only in two cases do sedimentary rocks act as host rocks. Andesite is the most common wall rock in the deposits assigned to the Upper Jurassic-Lower Tertiary time span. Some of these deposits in part occur in diorite to granodiorite intrusives. Rhyolites and dacites become more important in deposits associated with the Upper Cretaceous-Recent period. If we consider all reported gold deposits in Chile, including the 20 deposits listed in Table 1, the distribution of host rocks is as follows: andesites-dacites: 44%; granitic intrusives: 47%; and sedimentary rocks: 9% (Ruiz et al., 1965; Fuenzalida, 1974; Salinas, 1975; Sandoval, 1975; and Munoz, 1975).

In some deposits, especially in those hosted by andesites and granitic rocks, basic to intermediate dikes occur along the mineralized structures. Examples are E1 Capote, Inca de Oro and E1 Bronce districts. Barren diorite or diabase stocks are also spatially related to some of these districts.

GEOLOGY OF HYDROTHERMAL GOLD DEPOSITS IN CHILE

TABLE 4

Geotectonic setting, age and host rock

207

Deposit/district Geotectonic setting Age Host rock

Choquelimpie Stratovolcano Faride Marginal to porphyry system San Cristobal Volcanic complex El Guanaco Caldera complex La Coipa Volcanic complex (caldera?)

Miocene Upper Cretaceous Upper Cretaceous Lower Tertiary Late Miocene

Miocene Miocene Lower Cretaceous

Marte Marginal to stratovolcano El Hueso Marginal to porphyry Cu system Inca de Oro N-S to N45. E-trending structure

within magmatic island arc. Cachiyuyo de Oro Magrnatic arc. Mineralization along

N-S to NE-trending structures El Capote Magmatic arc. Mineralization along

N15-40W-trending structures El Indio Subvolcanic system associated with

magrnatic arc. Mineralization in structures marginal to volcanic center

Andacollo Radial fractures marginal to Upper Cretaceous porphyry Cu system

Los Mantos de N-S to NE-trending shear zone Lower Cretaceous Punitaqui Lower Cretaceous

Las Vacas

Upper Cretaceous

Upper Jurassic- Lower Cretaceous

Miocene-Pliocene

Magmatic arc. with mineralization along N80W-N85E-trending structures

El Bronce Structures marginal to caldera Upper Cretaceous- Lower Tertiary

Alhud Upper Cretaceous

Chancdn Upper Cretaceous- Lower Tertiary

Andesite Granodiorite Quartz porphyry Dacite, andesite, turfs Dacitic tuff, lutite, carbonaceous sediments Dacite Calcareous sediments Andesite, granodiorite

Subvolcanic system associated with magmatic arc. Mineralization along N25E-trending structures Subvolcanic system associated with magmatic arc. Mineralization hosted by N70W-NS-N75E shear zones. Subvolcanic system associated with Upper Cretaceous magmatic arc. Mineralization controlled by N45-50E-trending structures

Granodiorite

Granodiorite, tonalite

Andesite, dacite, rhyolite

Rhyolite, dacite

Meta-andesite Granodiorite

Andesite, breccia, tuff

Andesite, breccia, granodiorite, tuff

Andesite, breccia

El Tigre Andesite, breccia

E1 Chivato Magmatic arc. Mineralization along Lower Cretaceous Granodiorite, monzonite N-S-trending shear zones

Minas del Prado Volcanic center. Mineralization Mid-Tertiary Andesite, hydrothermal marginal and breccia hosted, breccia

Morphology As already mentioned, in all deposits the control of mineralization is pri-

marily structural and related to volcanic centers. Usually, the vein ore deposits in these volcanic related environments are contained in complex structures in which several generations of faults or fractures are present. Common types of

208 F. CAMUS

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structures in vein or stockwork type deposits include cymoid loops, tension fractures, horse tails, vein intersections, branches and stockworks (Fig. 4). These structures are the result of pre-mineral displacements along major faults, which created open spaces due to changes in dip and strike, or as a result of changes in pore fluid pressure. Mineralization occurs within the dilatant spaces of these structures, forming discrete lens-shaped ore-shoots of variable dimen- sions which alternate with barren zones. If movement continued during the intra and post-mineral stages, ore bodies may show breccia textures or can be surrounded by stockwork envelopes. This will result in considerable increase in the width of the vein.

The length of the veins ranges from 100-150 m to up to 8 km, as in the case of E1 Bronce (Camus et al., 1986). The ore shoots within these veins vary in length from 50-60 m to up to 600 m, have widths between 0.20 and 23 m and have a vertical extent of up to 400 m (Table 5).

Where fault or shear zone intersections occur, ore shoots are breccias and stockworks elongated in a vertical sense, thus forming pipe-shaped bodies, e.g., Inca de Oro, San Cristobal and E1 Chivato (Camus, 1985). Breccias are common morphological features in deposits of Neogene-Quaternary age. The E1 Tambo deposit in the E1 Indio district contains both breccia and vein orebodies (Siddeley and Araneda, 1986). The breccias are irregularly shaped, steeply dipping, either elliptical or circular in plan, with typical diameters of 100 m (Fig. 5a). The mineralization is irregularly distributed within the breccia. At depth these breccias pass into barite-alunite veins, interpreted by Siddeley and Araneda (1986) as the hydrothermal conduits.

At Minas del Prado, Ambrus and Araya (1981) described a hydrothermal breccia bound by concentric fractures carrying gold mineralization (Fig. 5b). Irregular stockwork bodies developed at the intersection of these fractures and the breccia. The Choquelimpie deposit in northern Chile (Fig. 1 ) also contains a gold-bearing breccia (Thomas, 1973).

Recent exploration activity in Chile has led to the recognition of stratabound stockwork-disseminated type of gold deposits, hosted by either volcanics or sediments (Fig. 5c). These deposits represent large tonnage low-grade re- sources suitable for bulk-mining (Table 2). Examples include La Coipa, Marte, Andacollo, San Cristobal and E1 Hueso. The orebodies in these deposits con- form strongly to bedding, defining stratabound deposits controlled in part by fracture density and rock lithology. Fault control is evident, with the faults possibly acting as hydrothermal fluid vents.

Mineralogy and primary zoning The primary mineralogy for both vein and stockwork/disseminated gold de-

posits is summarized in Tables 6 and 7. Two main mineralogic associations can be recognized: (a) quartz-pyrite-chalcopyrite-galena-sphalerite with minor carbonates,

barite, hematite, sulphosalts and,

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KIMBERLY BRECCIA ORE BODY

PLAN AND SECTION

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SOURCE :MINERIA CHILENA 1 9 8 7 . -

Fig. 5. (a) Plan and section of Kimberly breccia, E1 Tambo. {Taken from Siddeley and Araneda, 1986 ). (b) Plan and section of Minas del Prado gold deposits. (c). Longitudinal section of Pajon- ales orebody, El Hueso stockwork/disseminated deposit.

(b) quartz-barite-alunite-pyrite-enargite with minor amounts of chalcopyrite, galena, sphalerite, sulphosalts and silver sulphides.

Gold occurs as native gold, associated with silver (electrum) or copper (cu- proauride ) or with sulphides such as pyrite or arsenopyrite. In some cases gold is found in cracks or fractures in chalcopyrite, sphalerite or tetrahedrite. Silver

212

TABLE 6

Mineralogy of the hydrothermal vein districts

F. CAMUS

District/deposit Major Minor Rare

Faride Ag-Au-gal Ag sulfides Tet/tenn bar-qtz-hem sph-cpy-py Mn-oxides calc-sid-rho-Mn-silicates-bol

El Guanaco Au-Ag Ag sulphides qtz-bar sph-gal py-cpy-en alun

Inca de Oro qtz-py-Au Au tellurides-calc-cpy

Cachiyuyo de Oro qtz-py-Au hem-cpy

El Capote Au-qtz cpy-py bn-gal arsenopy calc-hem-bar cov-po

E1 Indio-Tambo Au-Ag tet/tenn Au tellurides en-py cpy stib-Ag-sulphides bar-qtz arsenopy sph-gal

anh-gyp cov-bn-po alun-nat S

Los Mantos de Punitaqui schw-cinn mgtt

qtz-spec py-cpy calc-Au

Las Vacas qtz-py-Au calc-hem -

E1 Bronce qtz-py-Au Ag-gal-tet/tenn po-arsenopy sph-cpy hem-bn calc-sid ank

Alhud qtz-py-Au cpy-sph -

Chanc6n qtz-py-Au cpy-sph bn-mgtt

E1 Tigre qtz-py-Au gal-sph - arsenopy cpy-stib

KEY: qtz=quartz; bar=barite; calc=calcite; sid--siderite; ank=ankerite; spec--specularite; mgtt--magnetite; po--pyrrhotite; hem=hematite; rho=rhodochrosite; alun=alunite; anh = anhydrite; gyp-- gypsum; gal=galena; sph =sphalerite; py--pyrite; cpy--chalcopyrite; arsenopy=arsenopyrite; en--enargite; tet/tenn=tetrahedrite/tennantite; cov=covellite; bn-- bornite; schw = schwazite; cinn = cinnabar; nat S = native sulphur; stib = stibnite; bol = boleite.

is c o m m o n l y associa ted wi th galena and sulphosal ts bu t also occurs as nat ive meta l or as silver sulphides.

The vein deposi ts m a y show vert ical and lateral meta l zona t ion pa t te rns . Table 8 shows a compos i te vert ical zona t ion pa t te rn . Th ree separa te zones can be defined: an upper or p rec ious -meta l -bea r ing zone; an in te rmedia te or base-


TABLE 7

Mineralogy of stockwork/disserninated deposits

213

District/deposit Major Minor Rare

Choquelimpie qtz-bar-py-gal orp-real-arg Au-Ag-acan alun-calc

San Cristobal Diss: qtz-py-Au - Veins: qtz-py-Au gal-sph

cpy-calc-bar hem-anh

La Coipa Ag Au-sph-gal qtz-py arg-sulphosalts Ag chlorides

El Hueso Au-py cpy

Andacollo Au-py -

El Chivato qtz-py-Au gal-sph

Minas del Prado qtz-py-Au -

Hg-Sn sulphosalts

cinn

po-bn-cc

KEY: same as Table 6 except for: orp = orpiment; real = realgar; arg = argentite; acan = acanthite.

TABLE 8

Vertical zonation of primary mineralization along vein deposits

Zone Mineral associations Gold (g/t)

2 4 6 8 10 15 20 I: Upper precious (a) quartz-pyrite with native gold-barite metals zone + carbonates, galena or sulphosalts

with associated silver (b) quartz-enargite-alunite-pyrite-barite

II: Intermediate base (a) quartz-pyrite-chalcopyrite-sphalerite- metals zone galena-native gold; carbonates-

hematite (b) quartz-enargite-alunite-sulphosalts

and copper sulphides locally galena- sphalerite

Ill: Lower root zone quartz-pyrite-arsenopyrite

metal-bearing zone; and a lower or root zone. Note that the location of both mineral associations distinguished above are shown in Table 8 together with the gold distribution. In contrast, stockwork/disseminated deposits do not show such well defined vertical zonation.

Lateral zonation has been described for some vein oreb0dies, but available

214 F. CAMUS

data are insufficient to define a generalized zonal pattern. Based on our present information, gold tends to be located in the central part of the system with lateral increase of, first Ag-Cu and then Pb-Zn minerals. At Los Mantos de Punitaqui, the lateral zonation is unusual with a gold zone grading into copper zone and finally into a peripheral mercury zone.

Most deposits show several mineralization stages, each of which probably represent structural reactivation accompanied by a respective pulse of mineralization. Each deposit is the result of a unique tectonic and depositional con- tinuum, involving changes in both precious- and base-metal-bearing mineral stabilities, thereby explaining the differences observed in the mineralogy of the 20 deposits. However, the basic assemblages are always present. Gold-silver deposits invariably are the result of more than one pulse of mineralization. Thus after a silver mineralization stage may follow a gold or base-metal pulse, provided permeability requirements were met. La Coipa and Faride are good examples of this style of mineralization.

Hydrothermal alteration The major alteration types recognized in the 20 districts or individual de-

posits are: { a) Quartz-sericitic, (b) Silicification or silicic, (c) Argillic of the intermediate and advanced types, (d) Chloritic and/or propylitic, and (e) K-feldspar in the form of adularia.

In addition tourmalinization (El Chivato, Faride), albitization (Andacollo) and carbonatization (El Bronce) may also be present. In all cases there is evidence of multiple alteration stages.

The determination of alteration types in some of the vein deposits is difficult because underground workings are usually driven along orebodies. The alteration descriptions and personal observations were interpreted on the basis of Meyer and Hemley's (1967) classification.

The alteration most commonly found includes the quartz-sericitic and argillic assemblages (Tables 9 and 10). These assemblages occur in all vein and stockwork/disseminated districts. However, at the El Guanaco and E1 Indio vein districts advanced argillic assemblages prevail. In the stockwork/disseminated deposits silicic and argillic alteration types predominate, with the advanced argillic assemblage prevailing at La Coipa and possibly Marte. Adularia has been recognized locally only at Faride in association with quartz veinlets. No adularia has yet been reported in other well-studied vein deposits.

The majority of the individual vein deposits are characterized by quartz- sericite halos or by argillization bordering the vein. Occasionally, argillic alteration predominates intermixed with quartz-sericite. The argiUic alteration consists mainly of kaolinite with minor proportions of montmorillonite. Chlor-


TABLE 9

Alteration assemblages in vein districts

District/deposit Type Description

Faride Quartz-sericite, argillic, propylitic, tourmaline

El Guanaco Silicic, quartz-sericite, advanced argillic. Propylitic halos

Pervasive destruction of plagioclase and K- feldspar by sericite/kaolinite adjacent to veins. Farther away increase in chlorite. Presence of adularia in association with quartz veins. Locally tourmaline.

Strongly silicified zones, surrounded by advanced argillic alteration zones. Propylitization is developed farther outward in the form of a weak chlorite halo.

Inca de Oro

Cachiyuyo de Oro

El Capote

El Indio

Argillic

Argillic

Quartz-sericite, argillic

Advanced argillic, silicic, weak chloritic

Weak development of kaolinite around veins. Locally sericitic alteration.

Weak development of kaolinite on the vein walls. Locally sericitic alteration.

Narrow quartz-sericite envelopes around veins. Kaolinite probably supergene.

Patchy silicification around veins, faults and volcanic vents. ArgiUization occurs widespread within the entire vein system. Presence of kaolinite, sericite, dickite and minor pyrophyllite, and montmorillonite. Earthy alunite, alunite-native sulphur, silica and gypsum, also occur especially at higher elevations.

Los Mantos de Punitaqui

Las Vacas

E1 Bronce

Alhud

Chancdn

Argillic

Quartz-sericite, chloritic

Quartz-sericite, argillic, propylitic

Quartz-sericite

Quartz-sericite, chloritic

Strong argillization around veins.

Weak quartz-sericitic alteration around veins but strong chloritic envelopes farther outward.

Late-stage carbonatization affecting the andesite dikes.

Strong envelopes around veins.

3-10 m envelopes around veins.

E1 Tigre Argillic, quartz-sericite Well developed alteration envelopes.

216 F. CAMUS

TABLE 10

Alteration assemblages in stockwork/disseminated breccia-hosted and shear zones-related deposits

District/deposit Type Description notes

Choquelimpie Argillic, silicic, propylitic

San Cristobal Quartz-sericite, argillic

La Coipa Advanced argillic, argillic, silicic, propylitic halo

Matte Advanced argillic, silicic

E1 Hueso Silicic, argillic

Andacollo Propylitic zone

E1 Chivato Quartz-sericite, argillic (supergene)

Minas del Prado Silicic, chloritic

Alteration associated with hydrothermal breccia. Propylitization surrounds the altered area on a regional basis. Locally, alunite coexists with silicification.

Alteration is iithologically controlled, showing weak to strong development.

Abundant native sulphur, silica and alunite. Pyrophyllite and traces of zunyite occur coexisting with the orebodies. Presence of sinter.

Well-developed silica cap. Supergene argillic alteration.

Alteration is clearly structurally controlled. Argillic alteration is composed of montmorillonite, halloysite.

Mineralization coexists within weakly albitized flows, marginal to the Andacollo porphyry copper system.

Extensive quartz-sericitic zone coexisting with tourmaline and surrounded by a propylitic halo.

Both alteration types coexist with a weakly developed sericite alteration.

itic or propylitic alteration represents the peripheral alteration assemblage. The width of these alteration halos varies according to lithology. In those deposits where volcanics are the prevailing host rocks, the alteration halo can be two meters wide. In deposits occurring in part in intrusive rocks, alteration halo often range from 0.20 to 0.30 m, suggesting a lower permeability or a deeper setting for these rocks. The pat tern described is particularly well developed at E1 Bronce, Chancdn, Las Vacas, E1 Tigre and Faride. Carbonitiza- tion is present in some of these deposits, e.g., El Bronce, where it represents a late-stage overprint on top of the earlier alteration assemblages.

In deposits such as E1 Indio and E1 Guanaco, where advanced argillic alteration predominates, the alteration halos around the veins are much wider. In both deposits the high vein density produced an alteration zone which extends for at least 500 m from the main structures. The advanced argillic assemblage consists of kaolinite, halloysite, dickite, pyrophyUite and very abundant alu-


nite. The presence of zunyite has also been reported locally. Propylitization occurs farther outward in the form of a weak chlorite envelope.

The alteration types associated with vein deposits also appear in stockwork/ disseminated orebodies. However, both types of ore deposits show significant differences in the extent and distribution of the alteration assemblages. Wide- spread silicification and argillization seem to be typical of the stockwork/disseminated deposits. Silicification occurs as quartz veinlets or silica flooding, in some cases accompanied by sericite. Argillization, characterized by the occurrence of kaolinite with lesser montmorillonite and halloysite, is the most frequent of the feldspar destructive type of alteration. At La Coipa, advanced argillic alteration occurs as a major phase of alteration together with silicification. The advanced argillic assemblages in these deposits include alunite, pyrophyllite, native sulphur and traces of zunyite. A propylitic outer zone has been reported for at least four of the deposits listed in Table 10. So far, no propylitization has been found in the other deposits. Vertical zoning patterns have not been well documented. At E1 Hueso and La Coipa silica caps with sinter development are preserved on the surface, grading into argillic (El Hueso) to advanced argillic (La Coipa, Marte? ) assemblages with depth.

The surface distribution and extent of the various alteration types reflect the intensity and extent of the hydrothermal activity. Vein deposits associated with quartz-sericite/argillic assemblages tend to have very narrow alteration envelopes, while vein deposits, with important development of advanced argillic alteration, are associated with altered areas covering several square kil- ometers. At E1 Indio, the alteration zone directly associated with the mineralization occupies a surface area of 3 by 10 km, which occurs within a 200-km- long and 1 to 10-km-wide belt of alteration (Siddeley and Araneda, 1986). In contrast, at E1 B ronce (Camus et al., 1986), the structural system that hosts the ore deposits extends for more than 8 km, with only weakly developed surface alteration.

In the stockwork/disseminated deposits the mineralization covers large areas and the alteration is equally widespread. This gives rise to strong colour anomalies. For example, at La Coipa the hydrothermal alteration zone covers an area of approximately 20 by 3 km.

Differences in level of erosion and in the intensity of the hydrothermal alteration-mineralization process may explain the variability of surface expression between deposits.

Oxida t ion zone

In most of the studied deposits a zone of oxidation developed in which the primary gold grades have been increased two to ten times. The depth of the oxidation zone varies from one to ten meters in those deposits located in southern Chile, to between 200 and 250 m in deposits situated in the northern arid part of the country. In some vein orebodies, the oxidation zone passes into a

218

T A B L E 11

Depth and grades of the oxidation and primary zones

F. CAMUS

Deposit/district Depth of Grade Primary zone grade oxidation zone ( m ) ( g / t Au) ( g / t A u )

Choquelimpie 80 -200 1- 2 0 .5- 1 Faride 200 2 - 3 1 - 2 San Cristobal

Veins: 100 50 4

Disseminated: 100 2 < 1.0 E 1 G u a n a c o 50- 70 50 -60 4 - 5 La Co ipa 100-150 1.80 < 1.0 E 1 H u e s o 100-150 1- 2 < 1.0

I n c a d e Oro 40 - 80 20 -40 5 -10 C a c h i y u y o de Oro 200 12-30 5 -10 E 1 C a p o t e 100-150 10-15 5 - 6 E1 Ind io 20 30 10 -15 Andacollo 10- 20 2 0 .5- 1.0 Los M a n t o s de Punitaqui 45 - 60 8 - 1 0 3 - 4 L a s V a c a s 20 - 30 13-30 8 - 1 0 E 1 B r o n c e 20 - 30 20 5 - 6 Alhud 15- 20 10-20 5 Chancdn 50 30 3 - 4 E1Tigre 30 - 40 10-15 2 - 3 E 1 C h i v a t o 20- 70 10-15 3 - 4 Minas del Prado - 2 - 3

thin blanket of secondary sulphides before entering the primary zone. The depth and gold grades of the oxidation and primary zones for each of the selected deposits are summarized in Table 11.

The oxidation zone is characterized by the presence of complex oxides, carbonates, sulphates and chlorides, products of oxidation and supergene leaching of pre-existing Cu, Fe, Pb, Zn, Ag sulphides and sulphosalts. Coexisting with those minerals, pervasive clayey minerals of secondary origin are also present. The principal minerals present in the oxidation zone are listed in Table 12.

The degree of oxidation and supergene enrichment in each of these deposits is controlled by the primary mineralogy and permeability of the host rocks. As previously mentioned, the majority of the gold deposits contain iron as well as copper sulphides which carry precious metals. Oxidation and leaching of these sulphides liberates the gold content, which is then transported and fixed within the oxidation zone. As a result, a zone of high gold or silver concentration is formed in the upper part of the oxidation zone of most of these deposits, which gradually diminishes in grade with depth. The importance of this zone will depend on the primary gold-bearing sulphide content and host-rock permeability and reactiveness. If the permeability of the rock is sufficient, gold may


TABLE 12

Principal minerals present in the oxidation zone of Chilean gold deposits

219

Major minerals Minor minerals

Goethite Scorodite Jarosite Osarizawaite Hematite Mackayite Alunite Boleite Anglesite Emmonsite Cerussite Beudantite Atacamite Rodalquilarite Brochantite Native gold Malachite Native silver Native sulphur Gypsum

percolate by gravity and accumulate in favorable locations such as fracture planes, joints and breccia zones, forming irregular platelets, dendrites or foils. As a rule, gold only tends to accumulate in the oxidation zone if the host rocks are unreactive. These factors may have contributed to local enrichments of two to four times the primary old grades in the oxidation zone at the E1 Indio deposit (Araneda, 1982).

Base metals and silver-bearing sulphides present in these deposits may also be locally enriched. Copper and silver are concentrated in the secondary enrichment zone, while lead and zinc deposit are immobilized as sulphates and carbonates in the oxidation zone.

The depth and enrichment characteristics of the oxidation and secondary enrichments zones described above, are a function of the geomorphological evolution and climatic changes that affected the Chilean gold belt. This process is well known between latitudes 29-26 ° south. According to Mortimer (1973) and Mortimer and Saric (1972), the secondary enrichment process in this area occurred in two periods: the phase I or "Cumbre Surface" period during the Eocene; and the phase II or "Sierra Checo del Cobre Surface" period during the Upper Miocene. Consequently, the oxidation profiles present in the gold deposits in this area would be the result of these two enrichment periods. Nevertheless, between latitudes 26 and 21 ° south, climatic conditions are the most arid in the Atacama desert, with less than 1-5 cm of rain per year since the Upper Eocene (Maksaev and Zentilli, 1988). In addition, sedimentation diminished to a minimum during the Upper Miocene, limiting erosion processes. Consequently, gold deposits in this part of Chile have encountered very limited erosion but very deep oxidation (Table 11 ) with only one enrichment process.

220 F. CAMUS

South of 29 ° south latitude, the conditions of geomorphological and climatic evolution in Chile were different and, as a result, only one enrichment stage of gold took place. The degree and importance of the enrichment process diminishes gradually as we go south, disappearing at approximately 36 ° south latitude. Thus at the Minas del Prado gold deposit (Fig. 1 ) only incipient and very surficial oxidation occurred and no evidence of any enrichment was found.

GEOCHEMISTRY

Surface geochemistry

Quantitative studies about the Chilean gold deposits with regards to primary metal dispersion are scarce. Of the 20 deposits considered in this study, only the detailed surface geochemical study undertaken at E1 Indio is available (Siddeley and Araneda, 1986). In this study 2000 rock samples derived from fine colluvium and weathered subcrop were analyzed for Au, Ag, As, Cu, Pb and Zn. With the exception of Cu and Zn, all elements were highly anomalous. Arsenic proved to be a very good pathfinder, showing a positive correlation with gold and silver (Siddeley and Araneda, 1986). Gold and silver anomalies defined the approximate location of the orebodies.

Studies elsewhere remain confidential at the time of writing this paper.

Fluid-inclusion studies

Preliminary or detailed studies on fluid inclusions, tied to mineral parage- netic sequences, were undertaken at Faride (Skewes, 1986), E1 Bronce (Skewes and Camus, 1988), E1 Indio (Jannas and Araneda, 1985; Jannas et al., 1990, this volume) and E1 Guanaco (Cuitino et al., 1988). According to these au- thors, precious metal deposition occurred between 190 and 330 ° C (Table 13 ).

The Faride, E1 Guanaco and E1 Indio deposits show relatively low temperatures and low salinities. In these deposits there exists evidence for boiling during the precious-metal deposition stage; Skewes (1986), Cuitino et al. (1988) and Jannas and Araneda (1985) considered boiling the main depositional mechanism. Salinities, determined from freezing point depression measurements, range in value between 0.1 and 3.5 eq. wt.% NaC1 at El Indio, E1 Guan- aco and Faride (Table 13). These values suggest a significant influx of dilute meteoric water into the hydrothermal system, which perhaps also contained a minor magmatic component.

The E1 Bronce deposit, on the other hand, shows relatively higher temperatures and salinities. A boiling event is also documented but in this case took place during barren late-stage mineral deposition. At E1 Bronce, the deposition of precious metals and gold-rich zones occurred below the boiling zone. A mix-


TABLE 13

Summary of available fluid-inclusion data

221

District Measured Selected °

T Salinity- T Salinity- ( ° C ) equiv. ( ° C ) equiv.

(wt.%) (wt.%)

Evidence Paleodepth of estimation boiling b ( m )

Reference

Faride 180-270 1.5- 7.6 190-250 3.5 Yes 150- 200 (90) (75)

ElIndio 190-280 0.1-40 220-280 0.1-1.4 Yes 250

E1Bronce 150-358 0.5-10 230-330 5.1 No 400-1200 (256) (205)

EI Guanaco 130-290 0.8-3.6 240 3.0 Yes 160 (60) (14)

Skewes (1986)

Jannas and Araneda (1985) Jannas et al. (1990) Skewes and Camus (1988) Cuitifio et al. (1988)

Number in parentheses denote number of measurements. aTemperature and salinities during precious-metal deposition. byes when boiling is associated with metal deposition. No when boiling is not related to metal deposition.

ing of fluids is suggested as the depositional mechanism for El Bronce, with ascending hot and saline metalliferous fluids mixing with cooler dilute meteoric waters.

The relatively higher salinities of E1 Bronce as well as some of the Faride data (Table 13), could explain their relatively high base-metal contents (Barnes, 1979).

Paleodepth estimates based on pressure-temperature-salinity relationships (Haas, 1971) determined for the Faride, E1 Guanaco and E1 Indio deposits, show a relatively shallow setting while at the E1 Bronce orebody, a deeper environment was documented (Table 13). No liquid CO2 phase was present in fluid inclusions of any of the deposits studied.

Isotopic studies

Of the 20 deposits considered in this work the only isotope data come from :El Bronce. Sulphur isotope compositions with ~4S ranging from 0.5 to 2.3 %o (Camus et al., 1986) suggest a magmatic source for sulphur in the sulphide :minerals of this deposit (Ohmoto and Rye, 1979).

G E N E T I C M O D E L S A N D T Y P E S OF H Y D R O T H E R M A L G O L D D E P O S I T S

Based on the reported morphological, structural and mineralogical characteristics, the Chilean hydrothermal gold deposits can be grouped into two broad genetic categories: volcanic-hosted epithermal deposits and porphyry-related deposits.

222

TABLE 14

Classification of the Chilean gold deposits

F. CAMUS

Epithermal deposits A dularia- se ricite

Faride San Cristobal Inca de Oro Cachiyuyo de Oro E1 Capote Los Mantos de Punitaqui Las Vacas E1 Bronce Alhu~ ChancSn El Tigre E1 Chivato Minas del Prado

Porphyry-related deposits Andacollo E1 Hueso

Acid-sulphate Choquelimpie E1 Guanaco La Coipa Marte E1 Indio

According to Heald et al. (1987), volcanic-hosted epithermal deposits can be grouped into two types: acid-sulphate and adularia-sericite, based on their mineralogy, lithotectonic setting and geochemistry. The great majority of the Chilean gold deposits fit into these two categories. Heald et al. (1987) have modeled the occurrence of these two types based on the Henley and Ellis (1983) models of geothermal systems. In Figure 6, these models are applied to the Chilean Andean geotectonic environment and emphasize the close relationship of both types of deposits with volcanic centers. Distinctive deposits for each setting are shown in Figure 6.

Porphyry-related gold deposits are not very common, and they have been recognized mainly within the Mesozoic metallogenic belts. Table 14 shows the 20 deposits classified according to these two broad categories: volcanic-hosted or porphyry-related.

From a morphological point of view, these two categories of hydrothermal gold deposits can be grouped into the five following types: (a) Vein deposits, (b) Ore shoots associated with shear zones, (c) Breccia-hosted deposits, (d) Deposits in fractures, marginal to porphyry systems, and (e) Large-tonnage stockwork/disseminated low-grade deposits, hosted by fa-

vorable rocks. Of these five groups of deposit types based on morphology, veins predomi-

nate in 12 deposits, low-grade orebodies in 4 cases, breccias in 2 cases, and


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224 F. CAMUS

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shear zones and fracture-related types have one example each. Since veins are the more common type of deposits they are referred to in more detail below.

Volcanic-hosted adularia-sericite epithermal deposits

These deposits are the most frequent among the Chilean gold epithermal deposits, occurring generally in the Upper Jurassic-Lower Tertiary time span.

Thirteen of the 20 deposits outlined in Table 14 belong to this type. Mor- phologically, 10 of these thirteen deposits are veins (group a), one deposit being a combination of vein and stockwork/disseminated (San Cristobal ), an-


other is a breccia-hosted deposit (Minas del Prado) (group c), and the last one being related to a shear zone (group b).

a. Vein deposits This group includes all districts where mineralization is hosted by pre-ex-

isting structures. The resultant orebodies are lense-shaped and they are more persistent laterally than vertically. Within some of these veins stockwork zones are developed.

Based on host-rock types, mineralogy and alteration, three subgroups of veins have been distinguished. These three subgroups, according to their geomorphological setting, are interpreted to represent different erosion levels of an original vein orebody. The three subgroups are the following (Fig. 7).

a. 1. Veins in intrusive. They generally correspond to the root zones of adularia- sericite type vein deposits, hosted by intrusives of dioritic to granodioritic composition. Veins are narrow, generally less than one meter in width, decreasing in width with depth. Mineralogy is simple, consisting of quartz, pyrite, arsenopyrite and associated gold. Locally, galena, sphalerite and chalcopyrite occur in the upper portions of the veins. Gold grade in the primary zone of these veins is frequently low (less than 4 g/t Au), so oxidation and supergene processes would be required in order to make these veins economically feasible. No other valuable metals are present. Alteration is weakly developed and is represented by a narrow silicified envelope 20-30 cm wide.

Generally, this subgroup of vein deposits is found to be emplaced in Upper Jurassic-Upper Cretaceous rocks. Examples are the Las Vacas, Cachiyuyo de Oro and E1 Capote districts (Fig. 1 ).

a.2. Veins associated with volcanic rocks of intermediate composition. These veins are morphologically similar to the veins described in (a.1). However, these veins tend to be wider ( > 1 m) and mineralogically more complex, containing quartz, pyrite, chalcopyrite, sphalerite, tetrahedrite-tennantite, calcite, barite and, locally, bornite and anhydrite. These minerals display lateral and vertical zonations with chalcopyrite-pyrite-sphalerite-tetrahedrite/tennantite assemblages located in the central part of the deposit, grading laterally and with depth into pyrite. Gold occurs associated with pyrite and to a lesser extent with chalcopyrite and sphalerite.

Alteration envelopes up to 50 cm wide around the veins consist of quartz, sericite and clay minerals, grading marginally into weak propylitization.

Primary gold grade in these group of vein deposits may exceed 10 g/t Au. Generally, recoverable quantities of silver and copper are also present. The majority of these deposits belong to the Upper Cretaceous-Early Tertiary time span. Examples of these subgroup are E1 Bronce, E1 Tigre, ChancSn and Alhud (Fig. 1 ).

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a.3. Veins in volcanic and intrusive rocks. They correspond to a combination of subgroups a.1 and a.2. The upper portion has the characteristics of the latter group and the lower part is typical of group a.1. Part of the Inca de Oro district is a good example of this subgroup.

Since the volcanic-hosted adularia-sericite epithermal vein deposits are the most common type of gold orebodies in Chile, an empirical model has been developed and is portrayed in Figure 8. Subgroups a.1, a.2 and a.3 as described and shown in Figure 7 are considered as portions of one complete epithermal system. Three zones recognized in this model are: an upper barren zone; a zone of precious and base metals; and a lower zone or root zone.

As shown in Figure 8, the upper barren zone of such an epithermal vein system consists of a quartz vein stockwork several meters in width. Such stockwork may be closed or open to the surface depending on the evolution of the epithermal system. In the latter case an active hot spring may be present within the upper portions of such a system, as suggested by Henley and Ellis (1983). However, we may not see such surface expression today because most vein deposits of this type are of Mesozoic age, and their original hot-spring surface expression is likely to have been eroded during post-Mesozoic times.

The zone of precious and base metals underlies the upper barren zone. Oc- casionally, it consists of an upper precious-metal subzone and a lower base- metal subzone. Generally, this zone is emplaced within volcanic rocks of intermediate composition. This zone shows a well-developed quartz-pyrite-chalcopyrite-galena-sphalerite-stibnite-tennantite/tetrahedrite-carbonates assemblage, with associated gold and silver.

The lower zone is characterized by the presence of narrow quartz-pyrite- arsenopyrite gold-bearing veins which tend to disappear with depth, or other- wise give way to a stockwork of quartz veins. The host rocks are generally dioritic to granodioritic intrusive stocks. Alteration is weakly developed and consists of very narrow silicified envelopes. They represent the roots of the epithermal vein system. The Pichidegua district (Fig. 1 ) is a good example of this situation.

b. Ore shoots associated with shear zones They consist of irregular-shaped ore shoots formed at the intersection of two

or more major shear zones. These bodies are generally emplaced in intrusive rocks of dioritic to granodioritic composition. Structurally, these ore shoots are stockwork zones filled with quartz, pyrite, calcite and, locally, sphalerite, galena and tourmaline. Gold is exclusively associated with pyrite. Primary gold grades are in the order of 4 g/ t Au, and no other metals are present in important amounts.

Hydrothermal alteration is related to the shear zones and covers areas of 500-1000 m 2. These areas are characterized by a central quartz-sericite zone

228 F. CAMUS

surrounded by a propylitic halo. The quartz-sericite alteration zone coincides with the orebodies.

Deposits of this group only occur in association with rocks of the Upper Jurassic-Lower Cretaceous period. A representative example is the E1 Chivato orebody.

c. Breccia-hosted deposits Such deposits are irregular or pipe-like concentrations of pyrite and free gold

together with quartz veining, which appear within as well as on the margins of hydrothermal breccias bodies.

The hydrothermal alteration coexisting with these ore bodies consists of silicification and chloritization, as well as abundant pyrite and minor sericite. Primary gold grades are between 1 and 2 g / t Au, but locally, along individual quartz veins, grades as high as 10-20 g / t Au are found. No other metals of economic interest exist.

Deposits of this type, as for example the deposits at Minas del Prado (Fig. 5b ), are apparently genetically related to volcanic activity of Tertiary age.

Volcanic-hosted acid-sulphate epithermal deposits

Five deposits of this type are recognized: E1 Guanaco, La Coipa, Choquelim- pie, E1 Indio-Tambo and possibly Marte. They are Early Tertiary to Recent in age. Morphologically, they form vein deposits (group a) or low-grade stockwork/disseminated orebodies (group e) showing a close association with volcanic centers of acid to intermediate affiliation.

a. Vein deposits These vein type deposits are hosted by dacitic to rhyolitic domes or stocks.

The orebodies are lense-shaped and occur along pre-existing faults or fractures, locally forming stockworks and/or breccia zones. Mineralogy consists of quartz, alunite, barite, rhodochrosite, enargite, pyrite, chalcopyrite and gold, free as native gold or associated with sulphides.

Gold grades in the primary sulphide zone of these deposits are high ( > 10 g/ t Au), with bonanza zones reaching grades of 50 to 100 g / t Au. Copper and silver are significant metals and are economically recoverable.

Alteration is widespread, covering areas of 200 m wide by 500-1000 m long, within which the veins are situated. A very distinct alteration zonation is generally recognized, with a central zone of sericite-quartz and clay minerals grading gradually into a marginal propylitic zone. Toward the top of the deposits, extensive advanced argillic alteration zones are present. This alteration pattern can be recognized both locally and district-wide.


b. Low-grade deposits The low-grade deposit type (group e) was recognized only very recently in

Chile. Consequently, published information regarding its geological characteristics is scarce.

These type of deposits contain low-grade stratabound mineralization hosted by favorable volcanic horizons, usually slightly or strongly silicified. These horizons tend to overlie acid leached argillized zones. Mineralization occurs as fine-grained disseminations and as quartz vein stockworks. Some of these deposits developed silica sinters at the surface. In others the presence of silicified breccias is conspicuous. Quartz is generally of the chalcedonic type.

Most of these deposits are hosted by Oligocene to Miocene volcanics, being spatially and possibly temporally associated with deeply eroded stratovolcanoes. Considering its geological setting, the mineralization may have occurred in very shallow epithermal systems associated with hot-spring environments of the High Andes. Erosion at these elevations has been minor, leaving the deposits of this type almost untouched. Normally, these orebodies contain gold and silver, and are large in size and bulk mineable.

Porphyry-related gold deposits

The gold mineralization is found either marginal or distal to porphyry copper systems. The best example of this type is Andacollo, where stratabound gold mineralization occupies a series of fractures radial to the Andacollo porphyry system. These fractures cut a sequence of rhyolite to dacite flows occurring in the propylitic halo of this porphyry copper system. Gold is associated with disseminated pyrite and the grades in the primary zone are on the order of 0.5 to 1.0 g/t Au. Due to oxidation and supergene enrichment processes, gold has reached locally, in zones of stockwork type fractures, grades up to 10-15 g/t Au, the resultant orebodies being irregular in shape. The mineralization is possibly of Upper Cretaceous-Early Tertiary age.

The E1 Hueso low-grade stockwork/disseminated deposit (Fig. 5c) is probably another example of this type. The gold mineralization, hosted by sedimentary rocks, is structurally and lithologically controlled by N70E to E-W faults. This deposit overlies the Potrerillos porphyry copper system, suggesting a close relationship between the two deposits.

A C K N O W L E D G M E N T S

This paper has benefited from the critical comments of R.H. Sillitoe, G. Westra, E. Reichhard and J. Davidson, to whom I am very grateful. I wish to thank also to M.A. Duhalde who helped me in the preparation and was my coauthor of an earlier version of this work. I am also grateful for the excellent drafting of G. Alarcon.

230 F. CAMUS

REFERENCES

Aguirre, L., Charrier, R., Davidson, J., Mpodozis, C., Rivano, S., Thiele, R., Tidy, E., Vergara, M. and Vicente, J.C., 1974. Andean magmatism: its paleogeographical and structural setting in the central part (30 °-35 ° ) of the southern Andes. Pac. Geol., 8: 1-38.

Ambrus, J. and Araya, M., 1981. Notes on the geology of Minas del Prado, Chile. Unpubl. Rep., 21 pp.

Anonymous., 1987a. Chile's gold expansion. Min. J., 310: 376-377. Anonymous, 1987b. Operacion aurifera E1 Hueso Codelco-Chile Division E1 Salvador. Mineria

Chilena, 72: 21-31. Araneda, R., 1982. E11ndio, yacimiento de oro, plata y cobre, Coquimbo, Chile. Minerales, 37: 5-

13. Barnes, H.L., 1979. Solubilities of minerals. In: H.L. Barnes (Editor), Geochemistry of Hydro-

thermal Ore Deposits, 2nd Edition. John Wiley and Sons, New York, NY, pp. 404-460. Berger, B.R. and Bethke, P.M. (Editors), 1985. Geology and Geochemistry of Epithermal Sys-

tems. Soc. Econ. Geol., Rev. Econ. Geol., 2,298 pp. Boyle, R.W., 1979. The geochemistry of gold deposits. Geol. Surv. of Canada, 280:584 pp. Buchanan, L.J., 1981. Precious metals deposits associated with volcanic environments in the

Southwest. Ariz. Geol. Soc. Dig., 14: 237-262. Cabello, J., 1986. Precious metals and Cenozoic volcanism in the Chilean Andes. In: C.E. Nichols

(Editor), Exploration for Ore Deposits of the North American Cordillera. J. Geochem. Explor., 25: 1-19.

Camus, F., 1981. E1 Tigre gold prospect, Yaquil district, IV Region. Unpubl. Rep., 8 pp. Camus, F., 1982. Evaluacion geologica-economica de los yacimientos de oro El Bronce y Pedro de

Valdivia. Distrito E1 Bronce de Petorca. Compania Minera E1 Bronce., Intern. Rep., 72 pp. Camus, F., 1985. Yacimientos epitermales de oro de Chile. In: J. Frutos, R. Oyarzun and M. Pinch-

eira (Editors), Geologia y recursos minerales de Chile. Univ. de Concepcion, Concepcion, pp. 654-689.

Camus, F., 1987. Evaluacion geologica y programa de exploracion para el proyecto Faride, II region. Compania Minera El Bronce., Intern. Rep., 73 pp.

Camus, F. and Drummond, A.D., 1979. The Chivato gold prospect. Unpubl. Rep., 18 pp. Camus, F. and Duhalde, A., 1981. Evaluacion geologica del distrito minero de Chancon, VI Region,

Chile. Unpubl. Rep., 20 pp. Camus, F. and Reichhard, E., 1987. Evaluacion geologica preliminar del yacimiento de oro Las

Vacas, IV region. Sociedad Minera Las Vacas., Intern. Rep., 24 pp. Camus, F., Boric, R. and Skewes, A., 1986. E1 distrito de oro El Bronce y su relacion con la caldera

Morro Hediondo region de Valparaiso, Chile. Rev. Geol. Chile, 28-29: 95-101. Coira, B., Davidson, J., Mpodozis, C. and Ramos, V., 1982. Tectonic and magmatic evolution of

the Andes of northern Argentina and Chile. Earth-Sci. Rev., 18: 303-332. Cuitifio, L., Diaz, S. and Puig, A., 1988. Aspectos de la mineralogia, geoquimica y geotermometria

de los yacimientos epitermales E1 Guanaco y Cachinal de la Sierra, Antofagasta, Chile. In: 5th Congreso Geologico Chileno, I: B273-B298.

Davidson, J., 1987. Tectonic and magmatic evolution of the southern central Andes. In: IUGS/ UNESCO, Deposit Modeling Workshop, Hydrothermal System in Volcanic Terrains {abstract), 3 pp.

Diaz, S., 1986. Geologia economica y prospeccion geoquimica de suelos del area de lamina La Leona, distrito minero Chancon, VI region. Geol. Thesis, Univ. de Chile, 175 pp. (unpubl.).

Flores, H., 1942. Geologia de los yacimientos de Cu y Au de Chile. In: 1st Congr. Panam. Ing. Geol., pp. 1145-1185.

Flores, H., 1943. Informe sobre el mineral de Capote aurifero de Freirina. Instituto de Investiga- ciones Geologicas., Intern. Rep., 8 pp.


Flores, H., 1948. Informe geologico sobre los yacimientos de la sociedad Aurifera de Alhue. Direc- cion de Minas y Petroleo., Intern. Rep., 8 pp.

Flores, H. and Ruiz, C., 1946. Estudio geologico del distrito de Inca de Oro, provincia de Atacama, Chile. In: 2nd Congr. Panam. Ing. Geol., 2: 243-273.

Frank, M., Inostroza, C. and Mattheus, F., 1985. Evaluacion geologica del distrito aurifero de E1 Capote de Freirina. Compania Minera E1 Bronce, Intern. Rep., 52 pp.

Fuenzalida, D., 1974. Prospeccion de recursos auriferos en el sector oriental de la cuenca del cajon del Maipo. Instituto de Investigaciones Geologicas. Unpubl. Rep., 29 pp.

Frutos, J. and Pincheira, M., 1985. Metalogenesis y yacimientos metaliferos chilenos. In: J. Frutos, R. Oyarzun and M. Pincheira (Editors), Geologia y recursos minerales de Chile. Univ. de Concepcion, Concepcion, pp. 470-487.

Galay, I., 1974. Inventario geologico minero de la Hoja Ovalle. Instituto de Investigaciones Geo- logicas. Unpubl. Rep., pp. 21-31.

Haas, J.L., 1971. The effect of salinity on the maximum thermal gradient of a hydrothermal system at hydrostatic pressure. Econ. Geol., 66: 940-946.

Heald, P., Foley, N. and Hayba, D.O., 1987. Comparative anatomy of volcanic-hosted epithermal deposits: acid-sulfate and adularia-sericite types. Econ. Geol., 82: 1-26.

Henley, R.W. and Ellis, A.J., 1983. Geothermal systems ancient and modern: A geochemical review. Earth-Sci. Rev., 19: 1-50.

Jannas, R.R. and Araneda, R., 1985. Geologia de la veta Indio Sur 3500: una estructura tipo bonanza del yacimiento E1 Indio. Rev. Geol. Chile, 24: 49-62.

Jannas, R.R., Beane R.E., Ahler, B.A. and Brosnahan, D.R., 1990. Gold and upper mineralization at the E1 Indio deposit, Chile. In: J.W. Hedenquist, N.C. White and G. Siddeley (Editors), Epithermal Gold Mineralization of the Circum-Pacific; Geology, Geochemistry, Origin and Exploration, II. J. Geochem. Explor., 36: 233-266.

Lepeltier, C., 1964. Geochemical prospecting for mercury at Los Mantos de Punitaqui. Instituto de Investigaciones Geologicas, Unpubl. Rep., pp. 56-83.

Llaumett, C., 1979. Evaluacion geologica del distrito minero E1 Guanaco, II region, Chile. Enami., Intern. Rep., 49 pp.

Llaumett, C., 1980. Antecedentes sobre produccion recursos y expectativas de la mineria del oro en Chile. Minerales, 35(150): 9-20.

Lowell, J.D. and Aspillaga, J., 1987. Deposito de mineral de oro diseminado San Cristobal. Mineria Chilena, 80: 5-9.

Maksaev, V. and Zentilli, M., 1988. Marco metalogenico regional de los megadepositos de tipo porfido cuprifero del norte grande de Chile. In: 5th Congreso Geologico Chileno, I: B181-B212.

Meyer, C. and Hemley, J.J., 1967. Wallrock alteration. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Holt, Rinehart and Winston, New York, NY, pp. 166-235.

Mortimer, C.E., 1973. The Cenozoic history of southern Atacama desert, Chile. J. Geol. Soc. London, 129: 505-526.

Mortimer, C.E. and Saric, N., 1972. Landform evolution in the coastal region of Tarapaca province, Chile. Rev. Geomorphol. Dyn., 4: 162-170.

Munoz, M.I., 1975. Recursos auriferos de la provincia de O'Higgins, VI region. Instituto de Inves- tigaciones Geologicas, Unpubl. Rep., 20-25.

Ohmoto, H. and Rye, R.O., 1979. Isotopes of sulfur and carbon. In: H.L. Barnes (Editor), Geo- chemistry of Hydrothermal Ore Deposits, 2nd Ed. John Wiley and Sons, New York, NY, pp. 509-567.

Rivera, S., 1984. Esquema de distribucion de vetas y tipos de clavos mineralizados en el distrito aurifero San Cristobal, II region, Chile. In: IV Congr. Ing. Minas, Copiapo, Chile, pp. 209-213.

Ruiz, C., Aguirre, L., Corvalan, J., Klohn, E. and Levi, B., 1965. Geologia y yacimientos metaliferos de Chile. Instituto de Investigaciones Geologicas. Publ. Espec., 385 pp.

232 F. CAMUS

Salinas, M., 1975. Yacimientos auriferos de la provincia de Atacama. Instituto de Investigaciones Geologicas, Unpubl. Rep., 216 pp.

Sandoval, R., 1975. Recursos auriferos de la IV region, provincia de Coquimbo. Instituto de In- vestigaciones Geologicas, Unpubl. Rep., 40 pp.

Siddeley, G. and Araneda, R., 1986. The El Indio-Tambo gold deposits, Chile. In: A.J. MacDonald (Editor), International Symposium on the Gold Deposits, Gold '86, Toronto, pp. 445-456.

Sillitoe, R.H., 1976. Andean mineralization: a model for the metallogeny of convergent plate margins. Geol. Soc. Can., Spec. Pap., 14: 59-100.

Skewes, A., 1986. Informe de inclusiones fluidas en proyecto Faride, sur de Sierra Gorda, Unpubl. Rep., 19 pp.

Skewes, A. and Camus, F., 1988. Inclusiones fluidas y mecanismo de precipitacion de metales preciosos en el yacimiento epithermal El Bronce de Petorca, Chile. Rev. Geol. Chile, 15 (I): 31-40.

Smith, R.L. and Bailey, R.A., 1968. Resurgent cauldrons. Geol. Soc. Am. Mere., 116: 613-662. Swayne, W.H., 1949. Reconnaissance report on the Las Vacas gold property near Los Vilos. De-

partament of Ulapel, Province of Coquimbo, Chile, Intern. Rep., 4 pp. Thomas, A., 1973. Geologia del yacimiento argentifero polimetalico de Choquelimpie. Enami-

Junta Ltda, Intern. Rep., 10 pp. Villaroel, P., 1972. Informe geologico de lamina Rhodesia. Instituto de Investigaciones Geologicas,

Unpubl. Rep., 12 pp. White, D.E., 1981. Active geothermal systems and hydrothermal ore deposits. In: B.J. Skinner

(Editor), Econ. Geol. 75th Anniv. Vol., pp. 392-423.

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