What follows is the text of an article that I co-ordinated for publication in the prestigious American professional journal 'Economic Geology' in 1991 (Volume 86, pages 1206 to 1221).
The purpose was to place on record in a technical journal a scientific account of the Choquelimpie gold and silver deposits - which was the highest precious metal mine of any commercial significance in the world.
Although the article came out under the aegis of Sociedad Contratual Minera Vilacollo, it was, in reality, sponsored by persons within Shell Chile and Billiton International Metals in The Hague, supported by Peter McAleer and Andy Meldrum who were then of Westfield Minerals. The task of liasing with Dick Sillitoe, the co-ordinator of the Special Edition of Economic Geology of which this article was a part, was mine and it was completed only in 1991 after I moved back to live in England.
There is an acount of the more human side of the exploration and implementation of the project on this site here.
I include a number of photos from the published article, but those wishing to consult the figures will need to access the article through library services.
The Epithermal Gold-Silver Deposit of Choquelimpie, Northern Chile by H. Gröpper, M. Calvo, H. Crespo, C.R. Bisso, W. A. Cuadra, P. M. Dunkerley and E. Aguirre.
The Choquelimpie gold-silver deposit is located at an elevation of 4,860 m above sea level, 115 km due east of Arica in northern Chile. A 5,500-metric-ton/day open-pit mine-heap leach operation was brought on stream in 1988 by the Sociedad Contratual Minera Vilacollo, whose shareholders are Shell Chile, Westfield Minera, and Citibank.
The deposit is located near the Arica bend of the South American continent, where the earth’s crust has a thickness of more than 70 km. It lies in a belt of calc-alkalic to alkali-calcic Miocene to Recent volcanism which runs parallel to the Pacific coast and contains a series of volcanic-hosted precious metal deposits. These have been worked in Peru, Chile, and Bolivia since the time of the Spanish conquest, usually for Ag but also with locally important Au values. These deposits are characterized by Ag-Au-Pb-Zn.
The volcano hosting the mineralization is a subalkalic stratovolcano of late Miocene age; a late magmatic event has been dated at 6.60 ± 0.20 Ma. The volcano is located at the intersection of a major N 60° E-trending lineament with a N 35° E fault, active from the late Miocene to the Recent. The volcano consists of dacitic and andesitic volcanic flows and breccias and is intruded by dacite domes. Erosion of its central part has exposed a zone of hydrothermal alteration and mineralization centered upon an andesitic feldspar porphyry, which is probably a dome.
Most of the mineralization worked until recently came from narrow argentiferous veins, but the present operation is centered on a series of hydrothermal breccia bodies emplaced in a N 600 E-trending zone 2 km long.
Peripheral to the mineralized zone, propylitic alteration exists, grading inward to a sericite-pyrite assemblage. Toward the breccia bodies this assemblage is overprinted by silicification associated with base metals, pyrite, kaolinite, and barite. The breccias themselves and some adjacent lithologies contain precious metal species including native Au and Ag, electrum, and argentite. A final stage of mineralization is represented by base and precious metals in veins which cut the breccias. Oxidation has produced supergene enrichment of Ag, but probably not of Au.
Limited fluid inclusion data from structures peripheral to the breccia bodies indicate homogenization temperatures ranging from 213° to 305°C. Salinities are below 5 wt percent NaCl equiv. Samples from one location show evidence of boiling at a minimum depth estimated as 300 m below the paleowater table.
Based on the available evidence, Choquelimpie is interpreted as a volcanic-hosted, acid sulfate, epithermal deposit. It is associated with an Ag-Au-Pb-Zn metallogenic province which contrasts with the Maricunga and El Indio Au-Ag-Cu-As provinces of central Chile.
The Choquelimpie gold mine is owned and operated by Sociedad Contratual Minera Vilacollo. It is located in Parinacota province, in Region I of Chile, some115 km due east of the regional capital, Arica, 35km east of the provincial capital, Putre, and 20 km west of the border with Bolivia (Fig. 1). The mine and neighboring zones of hydrothermal alteration are situated within the eroded Choquelimpie stratovolcano at 4,860 m above sea level at lat 18˚ 18΄ 19˝ S and long 69˚ 16΄42˝ W.
Silver veins were intermittently mined by the Spaniards and there is a reference to them dated about 1643 (E. Veliz, pers. commun.). Documentation of mining activities is available for the period 1883 to 1930. Production peaked around 1920, when the Arica Mining Company mechanized vein extraction. Between 1960 and 1976 various projects were planned to treat the old sulfidic dumps by flotation for recovery of their considerable silver values, but none came to fruition. With the rise in silver and gold prices around 1980, however, a local company began shipping material from the dumps to a flotation plant in Arica and later expanded its activities to include gold-silver ores which it extracted from small open pits and treated in a heap leach plant located some 60 km from the mine at a lower elevation. Total production by all previous operators has been estimated at 1.5 metric tons of gold and 200 tons of silver.
At the end of 1985, Shell Chile signed an option to buy Choquelimpie. Exploration in 1986 and the first half of 1987 demonstrated the presence of a reserve totaling 6.7 million metric tons of oxide ore at grades estimated as 2.23 g/metric ton Au and 87 g/ metric ton Ag within a mineral resource totaling over 11 million metric tons (Mining Journal, 1987). As a result the company exercised its option in October 1987, Citibank and Westfield Minerals farmed into the property, and a 3,000 metric ton/day heap leach plant was built alongside the mine during 1988. Production has since been expanded to about 5,500 metric tons/day.
The only known description of the old workings is by Venables (1926). Mario Pinto (unpub. rept., 1968) first recognized the bulk mining potential of the area and Mario Sanchez (1970) described silicification, pyritization, and kaolinization of breccias. Geologic studies carried out in connection with evaluations made by the present owners form the basis for this paper. Among these studies, Sillitoe (1985) first described the mineralization in terms of modern epithermal models and Cuadra et al. (1986) carried out the first detailed mapping.
Choquelimpie is one of a series of epithermal mineral deposits lying in a belt located at an altitude of 3,600 to 4,600 m above sea level and running parallel to the Pacific coast. These include Orcopampa, Arcata, and Cailloma in Peru and Todos Santos and Carangas in Bolivia (Fig. 1), all of which appear to be adularia-sericite-type deposits (W. Cuadra, unpub. data). All are hosted by apparently Miocene volcanic complexes developed on the exceptionally thick crust, >70 km (James, 1971a, b), in this part of South America. The epithermal deposits lie east of, but parallel to, a belt of older porphyry copper deposits (Fig. 1).
With the exception of a narrow strip of Precambrian schists (Pacci et al., 1980), which are caught up in the fault zone which forms the western edge of the Altiplano, all rocks in the area surrounding Choquelimpie are of Tertiary age or younger (Fig. 2) (Aguirre, 1988). These make up sequences of volcanic rocks intercalated with a variety of continental and lacustrine sediments. The volcanic rocks either occur as rhyolitic to andesitic ignimbrite sheets or form isolated or grouped stratovolcanoes of calc-alkalic to alkali-calcic composition (Beeson, 1989). The sediments fill basins developed between the stratovolcanoes and comprise volcaniclastic detritus of coarse to fine grain size and local chemical or biochemical sediments.
The oldest group of stratovolcanoes appears to be associated with ignimbrites dated at 19 ± 0.6 Ma (E. Aguirre, unpub. data). They crop out principally west of Choquelimpie and are composed of andesitic and dacitic lavas; strong glacial erosion has often exposed zones of intense hydrothermal alteration in their centers. Larancagua is an example of this group (Fig. 2).
A second group of paleovolcanoes is also of Neogene age and displays evidence of glacial erosion. They form prominent landmarks on the Altiplano and include Choquelimpie; they too are of andesitic to dacitic composition. Among them are some large-diameter craters (e.g., the Lauca caldera; Fig. 2) of probable collapse type. The most important representatives of the group and their K/Ar ages, as indicated by Wörner et al. (1988), are Lauca (10.5 ± 0.3 Ma), Choquelimpie (6.60 ± 0.20 Ma), and Ajoya (7.06 ± 0.21 Ma).
A third group of stratovolcanoes probably formed during the late Pliocene but was reactivated during the Pleistocene. They are well preserved, and although glacial landforms are absent, their flanks preserve thin moraine covers. The most prominent examples are Taapaca and Guallatire (Fig. 2), each > 6,000 m high and with surface native sulfur deposits. The solfatara at Guallatire is still active.
The most recent volcanoes are of upper Pleistocene to Holocene age (Wörner et al., 1988) and form the most imposing landforms in the area; their ice-capped summits attain 6,500 m above sea level. Parinacota is a stratovolcano with five recognized eruptive events ranging in age from 0.112 to 0.015 Ma. One of these events was an eruption similar to that of Mount St. Helens and gave rise to a widespread volcanic avalanche deposit.
The most prominent structural features are faults, lineaments, and flexures, which in some instances have exerted controls on either the emplacement of volcanoes or the present morphology of the area.
The Tignamar reverse fault places rocks of Tertiary age over the Precambrian strip located along the western margin of the Altiplano (Pacci et al., 1980).
Numerous normal faults and lineaments have been detected, trending north-northwest, north-northeast, northeast, and north-south; the last mentioned are high-angle, west-dipping features that form large blocks which apparently postdate the north-northwest structures. Folding is of minor importance.
District Geology of the Choquelimpie Volcanic Complex
Choquelimpie is a stratovolcano of roughly circular shape which coalesces to the northeast with the Ajoya twin stratovolcano of similar age (Fig. 3). The Choquelimpie volcano measures 4.5 km northeast by 3.8 km northwest. Its central area is deeply eroded, exposing the mineralized core, with well-preserved, outward-dipping walls to the north, south, and east; to the southwest the wall has been breached by the Milluni Creek (Fig. 3). Dips on the flanks range from 30° to 70°.
The volcanic complex rests on the Altiplano and has a maximum elevation of 5,300 m above sea level and a lowest point, within the eroded central area, of about 4,550 m above sea level. Within the central depression three hills are related to porphyritic domes; one of these, which reaches 4,860 m above sea level, is Cerro Choquelimpie, the principal host to mineralization.
A reconstruction of the stratovolcano suggests an original maximum elevation of the volcanic edifice of 5,600 to 6,000 m above sea level, similar to the elevations of the Pleistocene to Recent volcanoes in the region.
The Choquelimpie volcanic complex rests unconformably on the Condoriri Ignimbrites which are probably of middle Miocene age (19 Ma) (E. Aguirre, unpub. data). The volcanic stratigraphy suggests a rather homogeneous magma production of intermediate composition during the life of the volcano (Bisso, 1989). The five principal rock units, from oldest to youngest, are given here.
Dacitic volcanic breccia (unit CH-1): This is up to 200 m thick, trends N 30° to N 60° and dips 20° to 40° N (Fig. 3). It covers an area of approximately 4 km2 in the central-western part of the stratovolcano, which is centered on Cerro Antena. The rock is made up of 40 to 50 percent rounded to subangular, centimetric to decimetric dacitic fragments in a tuffaceous matrix. The fragments are predominantly of porphyritic texture. The unit is strongly affected by sub- vertical faulting and fracturing of generally northeast orientation.
A possible variety of this rock is the dacitic autobreccia which occurs at the southwest breach of the stratovolcano.
Andesites and andesitic breccia (unit CH-2): These rocks form a homogeneous sequence approximately 150 m thick which overlies the dacitic volcanic breccia and occupies the peripheral zones of the stratovolcano, forming most of the crest of the cone. Andesitic volcanic breccias predominate over flow rocks. A dark gray aphanitic matrix supports angular fragments of andesitic composition with a porphyritic texture. Hornblende, biotite, and plagioclase are the principal phenocrysts. K feldspar and quartz occur sporadically.
Dacitic porphyry and dacitic breccia (unit CH-3): These rocks form the uppermost unit of the volcanic sequence and occupy the highest peaks of the volcanic rim. The rock is typically gray red, porphyritic, and locally exhibits flow textures. Quartz eyes, plagioclase, biotite, and oxyhornblende are the principal phenocrysts.
Dacite domes (unit CH-4): Four small intrusive domes are associated with a circular structure of 1.6- km diam in the southern center of the stratovolcano. These rocks appear to cut the previously mentioned lithologies and are dacitic porphyries containing phenocrysts of plagioclase, quartz, biotite, and oxyhornblende in an aphanitic to glassy groundmass.
Of similar composition, but exhibiting a more distinct flow texture, is the dacite sheet at Cerro Chivaque. This rock overlies altered and mineralized porphyries in the center of the stratovolcano. The rock is tentatively interpreted as a lava dome and may be related to a major fault.
Minor intrusions: Andesitic dikes cut units CH-1 to CH-3 and appear to postdate the alteration and mineralization events of the Choquelimpie deposit. The dikes are up to 2 km in length, the principal directions being N 65°, N 75° to 80°, and N 340°.
They are made up of a dark gray porphyritic rock containing plagioclase (labradorite), hornblende, and biotite phenocrysts in an aphanitic groundmass. Apatite, sericite, and magnetite occur in trace amounts.
A porphyritic dacitic pipe has been mapped in the mine area. It is subvertical, measures 5 x 5 m, and intrudes a N 35°-trending, high-angle fault zone, causing local bleaching of the fault gouge. The pipe is distinctly melanocratic and contains 20 percent phenocrysts of plagioclase, hornblende, quartz, and biotite in an aphanitic matrix. Magnetite, calcite, and apatite occur in traces.
A major N 60°-trending lineament - the Guahacallalla-Chungará - has been traced for more than 50 km from the Choquelimpie stratovolcano to the northeast (Fig. 2). This structure appears to have controlled magma emplacement, mineralization, and hydrothermal activity, suggesting that it has been in-termittently active during the entire volcanic history of the stratovolcano.
A second fault zone of regional importance crosses the volcano with a N 35° trend (Fig. 3). Four Chilean volcanoes of late Miocene to Recent age (Choquelimpie, Ajoya, Parinacota, and Pomerape) and a number of Bolivian volcanoes are distributed along this trend. The Choquelimpie segment of the structure forms the contact between relatively unaltered andesitic breccias in the eastern sector of the volcano and the hydrothermally altered and mineralized rocks of the center.
There are additional fault directions of local importance, i.e., N-S, N 75°, and N 330°.
All structures, together with subsidiary and parallel faults, intersect within or close to the center of the volcano causing intensive fracturing and ground preparation in this area.
A prominent circular feature is related to the crater walls. Sillitoe (1985) suggested it is a small summit caldera, and Aguirre (1988) thought it might mark the rim of a collapsed caldera, but no associated pyroclastic rocks have been identified.
Lithologies and alteration
Lithologies at Cerro Choquelimpie, the location of the present open pit, have been hydrothermally altered to such an extent that their original composition and texture are often difficult to determine. On the basis of contact relationships and analysis of the nature of unaltered breccia fragments, it is tentatively concluded that the bulk of the rock was originally an andesitic feldspar porphyry, perhaps an intrusive dome (Sillitoe, 1985), which probably postdates the volcanostratigraphic units (CH-1-CH-3).
The principal host rocks to mineralization are brecciated feldspar porphyries, polymictic breccias, and late hydrothermal breccias, which are described in detail below. These rocks have evolved from the porphyry by successive superimposed phases of brecciation, argillic and siliceous alteration, and supergene leaching. Figures 4 and 5 show geologic relationships and Figures 6 and 7 show the distribution of alteration in the main Choquelimpie orebody, as known in May 1989.
This is a white to yellow rock exposed at the margins of the present open pit. It also occurs as metric to decametric internal waste blocks within the central siliceous breccia bodies. The rock is characterized by completely kaolinized feldspar phenocrysts, up to 5 mm in size, and ghosts of biotite booklets, in an argillic and silicified groundmass. Strongly silicified domains of light gray color are impregnated by hyaline quartz. Feldspar phenocrysts in these zones are leached rather than silicified, forming a spongy texture with voids stained by iron oxides.
Microscopic examination of this rock shows that the first stage of hydrothermal alteration produced a sericite-pyrite-quartz assemblage. Sericite replaced the phenocrysts and much of the groundmass. Pyrite comprises 1 to 10 vol percent of the rock, occurring as disseminations and in fine fractures. The quartz is almost entirely developed in the groundmass and is fine, dirty, and anhedral. Later supergene processes oxidized the pyrite and resulted in the formation of considerable amounts of kaolinite, jarosite, goethite, and minor hematite. Locally, mineralization is indicated by the development of barite in vugs and fractures and then the rock typically assays 0.5 g/metric ton gold.
Along the northwest contact zone of the porphyry, a white, vitric tuff of very porous, pumice-like texture crops out. It is composed of a devitrified kaolinitic matrix with lithic clasts, quartz crystals, and crystal fragments of less than 0.5 mm of kaolinite after feldspar. The rock may represent an extrusive equivalent of the feldspar porphyry.
Brecciated feldspar porphyry
This rock occurs at the contact of feldspar porphyry and siliceous hydrothermal breccias and is characterized by a stockwork of light and dark gray silica (Fig.
8). The fragments are angular, more than 5 cm in size, and consist exclusively of feldspar porphyry. Fine disseminations of pyrite occur within the dark gray silica veins, within the matrix, and in the phenocrysts of the fragments. In the oxide zone, iron oxides fill vugs and accompany silica veining. The typical gold content of the porphyry breccia is between 0.5 and 1.0 g/metric ton.
This rock is intensely silicified and has lost its original texture, except for the preservation of ghosts of some of the phenocrysts. The contact relationships with feldspar porphyry and late hydrothermal breccias suggest that it evolved from feldspar porphyry. Silicified porphyry exposures in the Choquelimpie main orebody zone form decametric, elongated bodies of greater vertical than lateral extent. The rock consists of light and dark gray silica, which occasionally displays banded and schlieren textures, sometimes cut by a late stockwork within a generally massive host rock. In the oxidized zone, locally abundant fractures are filled by kaolinitic clays and iron oxides. The massive silica rock is in contact with a vuggy siliceous variety with more than 10 percent pore space (Fig. 9). The vugs are filled or coated by jarosite, goethite, and minor barite.
Microscopically the silicified porphyry consists of secondary anhedral quartz, which almost completely replaces the original groundmass. Relics of phenocrysts (10 vol %) are totally replaced by silica and clay minerals. The late stockwork consists of microcrystalline quartz or gray silica (60-80 vol %) containing finely disseminated pyrite, clear anhedral quartz (20-40 vol %), and barite, alunite, anhydrite, and jarosite (0-5 vol %).
Typical gold grades in the silicified porphyry are 2.0 g/metric ton.
This rock forms decametric (Intermedio sector) to hectometric bodies (Vizcachas sector) and is found in contact with the late hydrothermal breccias. The light gray-colored, matrix-supported breccia is of hydrothermal origin. The fragments are subrounded to angular, and their size varies from 1 mm to 1 m (Fig. 10). Most of the clasts are made up of feldspar porphyry, porphyry breccia, cream-colored or dark and light gray silica fragments. Feldspar porphyry and porphyry breccia fragments are argillized, silicified, and pyritized to various extents and locally exhibit concentric alteration rims. Some argillized feldspar phenocrysts contain traces of alunite.
The matrix is composed of white rock flour, a white clay aggregate, and light and dark gray silica containing finely disseminated pyrite. The siliceous sectors sometimes exhibit a fine banding. Millimetric to centimetric vugs in the fragments and matrix give the rock a high porosity. In the oxide zone, vugs are often filled or encrusted by jarosite, goethite, silica, and minor hematite.
In the Vizcachas sector, northeast of Cerro Choquelimpie (Fig. 11), the polymictic breccia is the principal host rock to mineralization. Here, in addition to kaolinite or sericite and silica, alunite is also a common alteration mineral, occupying vugs together with silica, orpiment, and realgar, or replacing feldspar phenocrysts together with silica or kaolinite.
Gold grades of the polymictic breccia vary typically between 0.75 and 2.0 g/metric ton.
Late hydrothermal breccia
These breccias form elongated bodies, with a vertical extent exceeding 100 m, and are related to deeply oxidized troughs in the main Choquelimpie orebody. Due to the abundance of iron oxides in fractures and vugs, the rock is typically stained red, brown, and yellow on the surface. The late hydrothermal breccia is similar to the polymictic breccia and is often flanked by it, but it has suffered one or more additional hydrothermal events. The rock is characterized by intense fracturing and brecciation, with much silicification of both the clasts and matrix and, typically, the presence of large open spaces. These reach several decimeters in size, are lined by pyrite, and filled by thin masses of white kaolinite, crystalline barite plates up to 6 cm long, and rare gray sulfide crystals, probably enargite. In the oxidized zone, kaolinization is much more intense and the vugs contain millimetric clasts of silica, goethite, and jarosite, sometimes forming crystals up to 0.4 mm across. The barite crystals may be encrusted by several, locally colloform, millimetric layers of jarosite, goethite, and hematite some of which have suffered a final, probably supergene, leaching leaving a delicate boxwork of kaolinite and hematite with traces of silica.
Typical gold values in the late hydrothermal breccia are 2.0 to >5.0 g/metric ton.
Two types of mineralization have been worked at Choquelimpie, siliceous veins and hydrothermal breccia bodies. Until 1926, production came both from veins and a sulfide breccia stockwork; all present production comes from oxidized breccia bodies.
The vein mineralization was of interest exclusively for its silver content. Over a 12-month period in 1925 to 1926, ore with a head grade of 240 g/metric ton was mined from the veins and hand cobbed to produce a shipping product grading 1.28 kg/metric ton silver and 31 percent lead. Typical richer ore assayed 6.7 percent lead, 0.9 percent copper, 6.6 percent zinc, 490 g/metric ton silver, and traces of gold (Venables,
Mineralized veins are arranged in a crudely radial pattern around the southwest, south, southeast, and eastern sides of the central sector of the stratovolcano. The principal strike directions are N 10°, N 70°, and N 300°. The veins were worked to 80 m below adit level (at an elevation of about 4,470 m above sea level), almost 400 m below the highest outcrop of the principal mineralized breccia body. The veins are generally subvertical and vary in width from a few millimeters to 1 m, but locally widen to form siliceous breccia bodies several meters thick. Most of the production appears to have come from the southwestern sector of the deposit, where several drifts and cross- cuts were developed on at least three levels.
The vein mineralogy consists of quartz, kaolinite, barite, pyrite, sphalerite, galena, copper minerals, and silver-bearing minerals including tetrahedrite, pyrargyrite, polybasite, minor argentite, and native silver (Venables, 1926).
Predominantly argillic alteration generally extends from the veins for distances of a few meters into the wall rocks and may have been caused by supergene oxidation of wall-rock pyrite.
The main gold-silver mineralization occurs in a fault-controlled belt 2 km long, trending N 60°, and with a maximum width of 200 m. The principal mineralized bodies are, from west to east, the San Miguel breccia, the Choquelimpie Main orebody comprising five major breccias (Intermedio, Fortuna, Abundancia, Inglesa, and Zorro), the Cerro Hundimiento breccia, the Vizcachas orebody, and the Rajo Hundimiento orebody (Fig. 11). Parallel to the main trend and 250 m to the southeast occur the mineralized bodies of Animita, Chivaque, and Santana and 600 m farther southeast is the mineralized Española trend.
The orebodies comprise brecciated porphyries, silicified porphyry, polymictic breccia, and late hydrothermal breccia. Individual bodies along the N 60° trend form topographic highs up to 30 m above the general base. They are hosted by argillic feldspar porphyry which was first thought to be a fault zone breccia. However, Sillitoe (1990) suggests that the argillized, brecciated porphyry may be a postmineral hydrothermal breccia. The breccia consists of angular to rounded clasts, many of andesitic porphyry, in an extremely clayey rock flour matrix. Argillic alteration is dominated by supergene kaolinite in the oxidized zone but by hypogene illite-smectite in the underlying primary zone.
Faults, marked by slickensides and gouge, are common in the argillized breccia, which appears to have acted as a stress guide because of its enhanced ductility as compared with that of adjoining lithologies.
The mineralized breccia bodies typically measure several tens of meters on surface, extend to 150m depth, and are of irregular shape, often elongated along N 60° and N 330° directions. At Cerro Hundimiento and Rajo Hundimiento, funnel-shaped bodies have been drilled which measure close to 100 m on surface, but reduce to decametric and metric sizes over a vertical distance of 30 m. Some of the breccias, however, narrow upward. The present pit has exposed the Interrnedio breccia over a vertical distance of 80 m, from level 4840 (meters above sea level) to 4760. On the 4760 level the mineralized body measures 120 m across, on 4800 the body is split in two, exposed over 20 and 40 m, respectively. On the 4,825 m bench only a silica stockwork is present and at 4,840 m only an intense fracture pattern indicates the presence of a silicified and mineralized body at depth.
The oxide-sulfide contact generally parallels the present erosion surface and varies between a 10 and 40 m depth in feldspar porphyry host rock. However, deep oxide troughs, reaching down 130 m in the Intermedio zone, have developed in fracture-controlled breccia bodies.
Hypogene mineralization at Choquelimpie took place in several stages, four of which have been documented (Bisso, 1989). Stage I caused widespread propylitic and sericitic alteration and produced a calcite-chlorite-pyrite-hematite assemblage; this is most evident in volcanic units CH-1 to CH-3.
Stage II produced impregnation and fine veining of the central feldspar porphyries with pyrite, sphalerite, galena, and chalcopyrite, and perhaps a little pyrrhotite, some magnetite and arsenopyrite. Pyrite is the earliest species and is often overgrown or replaced by the others. Some of the galena is argentiferous. Sulfides occupy up to 10 vol percent of the rock, occurring as fine disseminations or millimetric veins. Kaolinization overprinted the earlier sericite, and barite is an additional gangue mineral.
Gold and silver appear to be almost entirely associated with late stages of mineralization. Stage III includes native gold and silver, electrum, argentite, and minerals identified as aramayoite (Ag2S.(Sb,Bi) 2S3) and schapbachite (PbS,Ag2S.Bi2S3), associated with plentiful pyrite and lesser enargite, tennantite, stibnite, and traces of lead sulfosalts. The accompanying gangue minerals are barite, quartz, chalcedony, kaolinite, dickite, and a small amount of alunite. Stages II and III are associated with episodes of hydrothermal brecciation and silicification. Native gold occurs in interstices, or as inclusions within the sulfides, particularly enargite, and sulfosalts, such as liveingite (5PbS.4As2S3) and guanajuatite (Bi2Se3), but also in stibnite, forming particles of generally less than 10 µm but which reach a maximum of 70 µm. Late tabular barite, a few millimeters to 6 cm in size, formed in open spaces contemporaneously with stage III.
Stage IV appears to be the latest event associated with the hypogene mineralization process, forming rare, narrow, subvertical veins along fractures trending N 750 E which clearly cut the hydrothermal breccias. The veins contain near-massive sphalerite and minor galena but no pyrite and carry both gold and silver values. It is not known whether these postbreccia veins are of the same age as the veins formerly exploited around the periphery of the breccia bodies.
Oxidation of the deposits is thought to have been caused by weathering although some observations have kept open the possibility that part of the abundant jarosite associated with the stage III gold-silver mineralization may be of hypogene origin. However, breccias below the zone of surface oxidation encountered only sulfides. The depth of oxidation is extremely variable, ranging from just a few meters to 130 m in some of the more permeable mineralized breccias (Figs. 5 and 7). The oxide-sulfide contact zone is usually less than 1 m wide and fairly regular, but along structural discontinuities roots of oxidation penetrate into the sulfides. Although there is a clear color change at the interface, the oxide zone normally contains traces of unoxidized pyrite encapsulated in silicified breccia fragments.
The hypogene mineralization contains abundant pyrite and wide pyritic halos are present around the ore zone. Oxidation of pyrite produces acid conditions, causing pHs of between 2 and 3 and precipitation of sulfates in the local streams. Hypogene kaolinite and minor alunite are found in the sulfides underlying the economic mineralization but much of the kaolinite and alunite in the deposit was produced by weathering. The oxidized parts of the orebodies and pyrite halos are typically jarosite rich.
Native gold and silver are more common in the oxide than in the sulfide zone. Native gold occurs as inclusions of up to 20 µm in goethite and jarosite, which is concentrated in fractures and open spaces in siliceous breccias. Traces of gold selenides have been observed in a goethite matrix. Chlorargyrite is
found exclusively in the oxide zone and there are also arsenic-, lead-, and copper-bearing silver sulfosalts.
In general, silver grades are significantly higher in the oxide zone than in the sulfide zone. The average silver content for 1,605 oxide ore samples was 78 g/ metric ton compared with 9 g/metric ton for 1,014 underlying sulfide samples, and it is usual to find abrupt changes in silver grades across the oxide-sulfide interface. Despite clear evidence of supergene silver concentration by downward-percolating solutions, mass balance considerations suggest that this may well have been superimposed on a primary system in which silver values increased markedly upward as the paleosurface was approached.
Irregular zones of silver enrichment exist within the oxidized part of the main deposit and it is frequently possible to identify horizons which contain unusually high values (averaging >300 g/metric ton). These occur irregularly in the deposit, apparently controlled by changes in lithologies or structural discontinuities. Leached rocks above zones of enrichment always contain some silver values (typically 30 g/metric ton). No satisfactory explanation has been proposed to account for the silver distribution of the deposit. There is no evidence of supergene enrichment of copper, zinc, or other metals at Choquelimpie.
Although gold values are above average in the deeply oxidized breccia bodies, this is considered to be a hypogene phenomenon. The present gold distribution appears to reflect the original distribution, a conclusion supported by the fact that above-average gold values in the oxidized facies overlie similar values in the underlying sulfide facies. No consistent correlations have been established between gold and silver distributions in the oxidized zone.
Geochemistry and Fluid Inclusions
Major and trace element analyses and fluid inclusion studies have been carried out on a small number of samples. The results provide only a preliminary characterization of the geochemical environment at Choquelimpie.
Samples containing measurable fluid inclusions could not be found in the main breccia bodies, but three samples of hyaline quartz with suitable inclusions were obtained from peripheral zones (Skewes, 1989). One of the samples is from a siliceous body, one from a vein, and one from a breccia matrix at Animita (Fig. 11). A total of 72 thermometric measurements of primary inclusions were carried out (Fig. 12). The salinities were determined using the depression point of ice and the homogenization temperatures were not corrected for pressure. The conclusions drawn are preliminary and require confirmation by a more detailed study.
Two types of primary inclusions were identified, one rich in vapor (<25% liquid) and one rich in liquid (<20% vapor). The sample from Animita showed coexisting primary inclusions of both types, suggesting that they were formed under boiling conditions. Homogenization temperatures range from 213° to 305°C and salinities from 0.5 to 4.5 wt percent NaC1 equiv. No carbon dioxide phases were observed in the inclusions. The depth of the boiling sample is estimated to be 305 m below the paleowater table.
Major and trace elements
The host rocks of the gold-silver mineralization are hydrothermally altered lithologies, which have been subsequently modified by weathering. It has not been possible to characterize the primary andesite feldspar porphyry because no unaltered samples have been found. However, 23 unaltered samples of lithologies of volcanic units CH1 to CH4 were collected from outcrops adjacent to the deposit, and 27 hydrothermally altered rocks were analyzed for major and trace elements (Bisso, 1989). Three kilograms of material was crushed and pulverized and a 100-g split was used for analysis. Major elements were determined by the low-dilution fusion method and plasma emission spectrometry. Trace elements were analyzed by neutron activation or by plasma spectrometry on aqua regia extracts.
The unaltered samples range petrographically from normal andesites and dacites to potassium-rich varieties (Fig. 13) and show a uniform content of major elements. According to Kuno’s classification they are subalkalic in composition (Beeson, 1989), range between 60 and 67 percent silica by weight, and have low normative Ti and P values. The altered samples are depleted in oxides of titanium, calcium, and sodium and to a minor extent also in ferrous iron, magnesium, aluminum, manganese, and probably phosphorus. However, they are enriched in silica (65—80 wt %), potassium oxide, and volatiles. The late hydrothermal breccia is the most altered rock (Table 1).
Among the altered lithologies the more argillic varieties contain relatively high values of Rb, Cs, Li, Ce, Tb, Sc, and Sm, and the more siliceous rocks are characterized by high contents of Au, Ag, B, volatiles (loss on ignition values of up to 15 wt %), and probably Ba, Pb, Sb, Ni, Cr, and W.
According to the available evidence, the mineralization at Choquelimpie was formed by hydrothermal activity in the central part of a stratovolcano of subalkalic composition and Neogene age. Reconstruction of the land form of the volcano suggests that it originally had a height of 5,600 to 6,000 m above sea level, which is similar to the Pleistocene to Recent volcanoes in the region. Mineralization in the hydrothermal breccias is known from 4,860 to below 4,760 m above sea level, about 1,000 m below the original summit of the volcano. The extent to which erosion had reduced the height of the volcano at the time when mineralization occurred is unknown.
Mass balance considerations of silver leaching and enrichment suggest that silver grades may have increased markedly upward as the surface was approached by the mineralizing fluids, whose flow was focused in and around breccias of hydrothermal origin. Surrounding the breccias there developed a vein field, whose temporal relation to the breccias is unknown. Limited fluid inclusion data from one vein suggest that mineralization took place at a minimum depth of 300 m below the paleosurface.
The close spatial relationship between mineralization, the hydrothermal breccias, and feldspar porphyry suggests that the latter played a role in the control of the mineralization. Two possibilities which seem worth considering are either that the feldspar porphyry could represent a late magmatic phase of the volcano with which the mineralization is genetically associated, or that the mineralization made use
of the same fracture system which controlled the ascent of the feldspar porphyry. In either case, the mineralization in the hydrothermal breccias is later than emplacement of the feldspar porphyry, since it cuts and alters the porphyry.
The breccia-hosted mineralization shows the characteristics of an acid sulfate system (Heald etal., 1987) and is surrounded by a wide pyritic halo, formed largely by replacement of mafic minerals by sulfides. This sulfur-rich system is probably typical of those which are still forming native sulfur deposits in the area by sublimation around solfataras at elevations of around 6,000 m near volcanic summits such as Guallatire. It is therefore likely that some of these Recent volcanoes are forming epithermal deposits in their interiors.
There is clear evidence of variations in hypogene mineralogy among the different hydrothermal breccia bodies at Choquelimpie. One group is pyritic with economically important Au and Ag values, another contains arsenates and important Au but insignificant Ag, and a third group carries important trace values in Pb, Zn, Cu, As, Sb, Ag, and sporadic Au. This suggests that mineralization may have occurred in pulses at different times along the fracture system which provided the dominant control on the localization of hydrothermal activity.
We wish to thank the management of Sociedad Contratual Minera Vilacollo for permission to publish this article. The undergraduate theses of E. Aguirre (Aguirre, 1988) and C. Bisso (Bisso, 1989) were supported by Shell Chile scholarships and their work was supervised by H. Moreno of Universidad de Chile and A. Skewes, who also carried out the fluid inclusion study (Skewes, 1989). Many others have contributed, among whom we would like to mention E. Veliz who studied the history of Choquelimpie, E. Coronado, H. Luengo, S. Araya, and P. Alarcon who produced the line drawings, and P. Jamett who did the typing. Finally, our particular thanks are due to J. Ambrus, R. Freraut, 0. Ponce, and A. Faunes who participated in the initial evaluation of the deposit. The manuscript was patiently reviewed by R. H. Sillitoe and two Economic Geology reviewers.
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