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Corresponding author: e-mail, jperello@aminerals.cl

Abstract

The Esperanza porphyry copper-gold deposit is located approximately 60 km south of Calama, in the porphyry copper province of northern Chile. Although partly exposed, historically mined from small-scale pits, and intermittently explored over many years, its true size and potential were appreciated only in 1999. Discovery was the direct result of detailed geologic mapping of key rock types and hydrothermal alteration assemblages and zoning and was partly underpinned by a property-wide ground-magnetic survey.

The geology of the region is typical of the Cordillera de Domeyko and includes several fault-controlled basement blocks of late Paleozoic age and a number of sedimentary and volcano-sedimentary sequences of Mesozoic and Cenozoic age. Of these, the Late Cretaceous Quebrada Mala Formation and the middle Eocene domes of the Estratos de Cerro Casado are widely distributed in the area. Much of the region is mantled by moderately consolidated gravels of middle Eocene to middle Miocene age, collectively grouped as the Calama and Tambores Formations. The regional structure is dominated by several north-northeast-trending splays of the Domeyko fault system, which display evidence for both strike-slip and reverse movements and exert a strong control on the location of Esperanza. The deposit is part of a northeast-trending corridor of middle Eocene porphyry deposits that includes Telégrafo, Centinela, and Polo Sur. At Esperanza, a series of structurally controlled, medium-grained granodiorite porphyry dikes intrude a sequence of massive andesite flows and interbedded pyroclastic and calcareous volcano-sedimentary horizons of the Quebrada Mala Formation. Hydrothermal alteration consists of a core of potassic alteration partly overprinted, but mainly surrounded by, intermediate argillic, quartz-sericitic, and propylitic assemblages. Early biotite-bearing alteration from the central potassic zone yields a 40Ar-39Ar age of 41.3 ± 0.3 Ma.

Hypogene copper-gold mineralization occurs dominantly as chalcopyrite and bornite in multiple stockworks of pyrite-poor, A- and B-type veinlets with quartz, K-feldspar, biotite, magnetite, apatite, and anhydrite, which are spatially and genetically associated with the potassic assemblages. Primary fluid inclusions in these veinlets possess homogenization temperatures (Th) of between 435° and 592°C and salinities in the 41 to 60 wt percent NaCl equiv range. Minor molybdenite accompanying these veinlets yields an Re-Os age of 41.80 ± 0.13 Ma. Overprinted intermediate argillic alteration is characterized by chlorite, illite, smectite, and greenish sericite, with chalcopyrite and pyrite, whereas quartz-sericitic assemblages are barren of copper and dominated by disseminated and veinlet pyrite in classic D-type veinlets. Primary fluid inclusions in quartz veinlets from these assemblages show lower Th (217°–330°C), although still retaining a magmatic component to generate salinities of 40 to 53 wt percent NaCl equiv. Within the potassic core, anhydrite becomes increasingly abundant with depth and, locally, forms a large structurally controlled massive body with interbedded proximal skarn rich in garnet and diopside. Supergene copper mineralization is developed in the upper 150 m of the deposit where it is characterized by atacamite and chrysocolla with subordinate brochantite, copper wad, and copper-rich clays. Minor amounts of chalcocite, covellite, native copper, and cuprite occur near the redox interface.

From a regional standpoint, Esperanza confirms that copper-gold and copper-molybdenum deposits coexist in continental arcs within the same metallogenic belt, and porphyry copper and copper-gold mineralization in the northern Chile porphyry copper province was, at least in part, intimately associated with contractional deformation during the middle to late Eocene Incaic orogeny.

Introduction

The Esperanza copper-gold deposit (22°58′00″lat S; 69°03′30″long W) is located approximately 30 km southeast of the mining town of Sierra Gorda and 60 km south of the city of Calama, in the Antofagasta región of northern Chile (Fig. 1). The project area, at an average elevation of 2,300 m, is characterized by the rolling topography of the western foothills of the Cordillera de Domeyko, a major morphotectonic unit of the Atacama región of northern Chile. The deposit is the property of Antofagasta Minerals S.A., a wholly owned Chilean subsidiary of Antofagasta plc of London.

Fig. 1.

Location map of the Esperanza deposit in relationship to the El Tesoro mine and other districts and prospects in the region. For reference, note the location of the recently discovered Spence porphyry copper deposit near Sierra Gorda and the Chuquicamata mine north of Calama. Porphyry prospects of the Esperanza-Polo Sur corridor also shown.

Fig. 1.

Location map of the Esperanza deposit in relationship to the El Tesoro mine and other districts and prospects in the region. For reference, note the location of the recently discovered Spence porphyry copper deposit near Sierra Gorda and the Chuquicamata mine north of Calama. Porphyry prospects of the Esperanza-Polo Sur corridor also shown.

Esperanza occurs at the northeastern end of a 40-km-long corridor of copper deposits that includes porphyry-type mineralization at Polo Sur, Centinela, Telégrafo, and other occurrences, and exotic copper mineralization at El Tesoro and Tesoro NE (Fig. 1). At present, mining takes place only at El Tesoro, which produces 85,000 metric tons (t) of copper cathodes per year (Mora et al., 2003, 2004). The corridor constitutes a segment of the larger late Eocene to early Oligocene porphyry copper belt of northern Chile and is located between the giant Chuquicamata and Escondida mines and approximately 30 km southeast of the Paleocene Spence porphyry copper deposit.

The present contribution on Esperanza summarizes the exploration history that led to discovery, reviews the geology of the region, and describes the salient geologic, hydrothermal alteration, and mineralization features of the deposit. The paper builds mainly on the geologic work by the authors during the various exploration phases (Perelló and Brockway, 2000; Perelló et al., 2000, 2003a) but also incorporates in-house work (Mora, 2002) and published descriptions of the regional geology (Boric et al., 1990; Mpodozis et al., 1993a; Marinovic et al., 1996; Marinovic and García, 1999), as well as unpublished work on certain aspects of the geology and alteration-mineralization of the deposit (Sillitoe, 2001).

Exploration and Discovery

Although exposed, historically mined from small-scale pits, and intermittently explored by several major companies between 1983 and 1992, the true size of Esperanza was appreciated by Anaconda Chile (now Antofagasta Minerals S.A.) only in 1999. This is surprising given the easy access of the area and the proximity to known mineralization at El Tesoro, not to mention the fact that most of the geologic elements of the system are readily visible at surface. The following account summarizes, in chronologic order, the main events of the exploration history and explains the key decisions that led to final discovery of the deposit.

Prediscovery exploration

The earliest modern exploration efforts recorded in the area were by Minera Utah de Chile Inc. in 1983 as part of the evaluation of their Loreto property around the El Tesoro exotic copper mineralization. Minera Utah conducted geologic mapping and 629 m of conventional rotary drilling in five holes (ET23-ET27). One of these holes (ET23: 185 m; Fig. 2a) intersected andesite with intense sericitic alteration and erratic copper values (50–1,200 ppm). Owing to the lack of significant results, Minera Utah terminated exploration in the area and let their property rights expire.

Fig. 2.

Location of discovery drill holes in the Esperanza deposit and selected follow-up holes. a. Plan showing the discovery drill holes with respect to previous holes by other companies and main current and historical mining claims in the area. See text for discussion b. Schematic geologic section constructed before first-stage drilling in 1999 and discovery drill holes S525 and S526 with main intersections.

Fig. 2.

Location of discovery drill holes in the Esperanza deposit and selected follow-up holes. a. Plan showing the discovery drill holes with respect to previous holes by other companies and main current and historical mining claims in the area. See text for discussion b. Schematic geologic section constructed before first-stage drilling in 1999 and discovery drill holes S525 and S526 with main intersections.

In 1984, Kennecott Chile S.A. conducted a district-wide IP survey and drilled six rotary holes, totaling approximately 953 m, between Esperanza and Telégrafo. The exact locations of these holes remain unknown, although it is inferred that two of them were collared at Esperanza. Apparently, one of these holes was later twinned by Anglo American in 1991 (see below) and corresponds to their hole DTHTEL5 (Fig. 2a). In 1985, Geoestudios carried out additional exploration in the area on behalf of Northern Mines, a subsidiary company of Antofagasta Holdings. The main objective of the program was to define the source of the El Tesoro exotic copper mineralization (Ambrus and Romo, 1986) for which 18 rotary holes totaling 2,008 m were drilled between Esperanza and El Tesoro. Two of them (TES1: 168 m and TES17: 78 m) were collared in the Esperanza area (Fig. 2a). Hole TES1 intersected andesite and porphyry with chloritic and sericitic alteration, with average grades between 0.2 and 0.3 percent Cu. Hole TES17 intersected intensely sericitized andesite, with copper values of <0.1 percent. Gold was not assayed during this program.

In 1991, Empresa Minera de Mantos Blancos (EMABLOS), currently a wholly owned subsidiary of Anglo American, commenced an exploration program over their Telégrafo and Pimienta (today Esperanza) concessions (Fig. 2a). EMABLOS geologists identified the general alteration pattern of the Esperanza system, obtained highly anomalous copper and gold values in rock chip samples from Esperanza, and recommended a series of drill holes at both properties (Toro and Muehlebach, 1991). These holes, totaling 5,878 m, were drilled in both main targets during 1992 with a conventional rotary rig, with holes DTHTEL1 to DTHTEL5 (1,474 m) at Esperanza (Fig. 2a). Holes DTHTEL1, DTHTEL 2, and DTHTEL 5 intersected appreciable intervals of copper-gold mineralization in potassic alteration, whereas holes DTH-TEL3 and DTHTEL4 penetrated peripheral chlorite and sericite assemblages with low copper and gold contents. EMABLOS concluded that Esperanza contained a geologic resource of 20 million metric tons (Mt) at 0.45 percent Cu and 0.3 ppm Au in oxides plus an additional 19 Mt at 0.55 percent Cu and 0.3 ppm Au in sulfides (Lyall, 1992). Simultaneously with the EMABLOS exploration, Anaconda Chile S.A. drilled two reverse-circulation holes (S110 and S111) in their Santa Carmen concession on the boundary with the EMABLOS Pimienta property (Fig. 2a). Hole S110 (250 m) intersected andesite with intense sericitic alteration, with <400 ppm Cu. Hole S111 (300 m) intersected andesite with moderate potassic alteration, with >0.2 percent Cu and >0.2 ppm Au.

Exploration in the area was discontinued by both companies in 1992. Anaconda concentrated efforts on its El Tesoro exotic copper target (Mora et al., 2004), whereas EMABLOS halted exploration because of the small size of the resource at Pimienta. In 1993, Anaconda acquired these two concessions from EMABLOS, thereby annexing them to the Santa Carmen property.

Discovery

Exploration by Anaconda resumed in 1999 with a program to test several conceptual targets throughout the entire concession. This new effort included ground geophysics (magnetics and gravity) and geochemistry over covered areas and detailed geologic mapping of selected outcropping areas. A detailed geologic map (1:2,000 scale) prepared for the Esperanza area in September 1999 defined the salient geologic features, including rock types, hydrothermal alteration zoning, distribution and intensity of quartz-magnetite stockworks, and structure, and assisted in construction of a conceptual model for drill testing. One of the main conclusions of this work, together with a reinterpretation of available drilling data, postulated that Esperanza was a tilted system (Fig. 2b), implying that previous exploration efforts had not necessarily determined the full potential. This geologic conclusion was also supported by the results of the recently completed ground-magnetic survey, which showed Esperanza to be associated with a southeast-dipping magnetic signature. Similarly, the intimate relationship between the magnetite content of the quartz veinlets mapped at surface and vein intensity suggested that zones with a higher magnetic response had the potential to be associated with higher grade mineralization than that intersected by previous drilling.

A first-phase drilling program was proposed to test these concepts as well as the shallow oxide mineralization exposed in the area. The idea of testing the oxide mineralization was a strategic element of the program, since any oxide resource at Esperanza was viewed as potential feed for the El Tesoro operation. Drilling commenced in September 1999 with testing of shallow oxide copper mineralization and, by October, the first drill hole (S525) programmed to test the sulfide mineralization had intersected 190 m of 0.56 percent Cu and 0.28 ppm Au (Figs. 23a). This hole was immediately followed by hole S526, located to the south, which intersected 200 m of 0.66 percent Cu and 0.38 ppm Au. These discovery holes at Esperanza not only satisfactorily confirmed the geologic model but also helped to significantly increase the size potential of the target and opened new ground for exploration. Holes S525 and S526 were the thirteenth and fourteenth holes drilled at Esperanza if previous drilling (2,744 m) by Minera Utah, Kennecott, Geoestudios, Mantos Blancos, and Anaconda are included. By February 2000, a total of 9,491 m in 27 holes had been drilled, and by April a first geologic resource estimated 42 Mt at 0.46 percent Cu (oxide) and 210 Mt at 0.51 percent Cu and 0.29 ppm Au (sulfide), using a 0.3 percent Cu cutoff (Perelló et al., 2000). Although these figures were an order of magnitude larger than previous estimates (see above), they also posed the question as to whether Esperanza could host additional high-grade coppergold mineralization that would make the deposit economics more attractive to management.

Fig. 3.

Evolution in size of the high-grade core of the Esperanza deposit between April 2000 and May 2001. Note how a conventional grid drilling program on 200-m centers could have left this core undiscovered. See text for further discussion.

Fig. 3.

Evolution in size of the high-grade core of the Esperanza deposit between April 2000 and May 2001. Note how a conventional grid drilling program on 200-m centers could have left this core undiscovered. See text for further discussion.

Detailed modeling of the geologic resource indicated that a smaller tonnage of higher grade copper-gold mineralization (>0.8% Cu, >0.5 ppm Au) was present (Fig. 3a) in association with potassic-altered porphyry and the contiguous andesite wall rock containing abundant quartz-magnetite-chalcopyrite veinlets and subordinate bornite. It was suggested that the system is vertically zoned, with a shallower, chalcopyrite-rich, bornite-poor assemblage underlain by chalcopyrite-bornite mineralization, with corresponding increases in copper and gold contents (Perelló et al., 2000). Five reverse-circulation drill holes recommended to test this core zone were soon endorsed by management and, by July 2000, an additional 2,500 m of drilling in holes S603 through S607 were completed (Fig. 3b). This campaign successfully tested the zoning concept and prompted a third, definition drilling phase between November 2000 and May 2001, which comprised 13,923 m in 24 reverse-circulation and combined reverse-circulation and diamond holes. By June 2001, a revised resource estimate, using both geologic and geostatistic methods, concluded that Esperanza contained 71 Mt at 0.42 percent Cu in the oxide zone and 443 Mt of 0.63 percent Cu and 0.26 ppm gold in the underlying sulfide zone, using a 0.3 percent Cu cutoff. A higher grade, open-pittable core of 128 Mt of 1.0 percent Cu, 0.48 ppm Au, and 3.5 ppm Ag (Perelló et al., 2001, 2003a; Fig. 3c) significantly improved the economics of the Esperanza deposit and made it one of the largest goldrich porphyry copper centers of the middle Eocene to early Oligocene belt of northern Chile (e.g., Perelló et al., 1996; Paleczeck and Cáceres, 2003; Rivera et al., 2003).

Regional Geology

The geology of the región is typical of the Cordillera de Domeyko as previously described by Mpodozis et al. (1993a, b), Marinovic et al. (1996), and Marinovic and García (1999). It is characterized by several blocks of pre-Mesozoic basement and by a number of sedimentary and volcanic sequences of Mesozoic and Cenozoic age. Much of the región is covered by several piedmont gravel units of Cenozoic age (Fig. 4).

Fig. 4.

Sketch map of the geology of the Esperanza-Polo Sur region. Compilation based on Mpodozis et al. (1993a), Marinovic and García (1999), Mora (2001), and mapping by the authors.

Fig. 4.

Sketch map of the geology of the Esperanza-Polo Sur region. Compilation based on Mpodozis et al. (1993a), Marinovic and García (1999), Mora (2001), and mapping by the authors.

Pre-Mesozoic basement is exposed in several large, structurally controlled blocks at Sierra Limon Verde in the north and elsewhere east of Esperanza. Basement rocks include a series of volcanic and plutonic assemblages of Late Carboniferous to Permian age, dominant among which are the Pampa Elvira and Limon Verde intrusive complexes and the felsic volcanic and subvolcanic Cas Formation and Peine Group. The plutonic rocks comprise granite, granodiorite, monzogranite, diorite, monzodiorite, and subordinate gabbro, whereas the Cas Formation is composed of rhyolite porphyry, dacite domes, and ignimbrite, and the Peine Group of dominantly andesitic and minor sedimentary rocks. Scarce outcrops of metamorphic rocks in Sierra Limon Verde consist of mica schist, amphibolite, and metasedimentary rocks, which are correlated with the metamorphic basement in the Chuquicamata area (Boric et al., 1990; Mpodozis et al., 1993a).

The Mesozoic and Cenozoic rocks comprise three main associations, including Late Triassic to Early Cretaceous volcanic and sedimentary sequences, Late Cretaceous to early Eocene volcanic, subvolcanic, and intrusive units, and late Eocene to Pliocene cover sequences (Fig. 4). The oldest group includes the subaerial andesitic rocks of the Late Triassic to Early Jurassic Estratos Las Lomas and the Middle Jurassic limestone, shale, and evaporites of the Caracoles Group. The second group is dominated by the andesitic, pyroclastic, and volcano-sedimentary rocks of the Late Cretaceous Quebrada Mala Formation, the partly bimodal, andesitic-rhyodacitic tuffs and flows of the widespread Paleocene to early Eocene Cinchado Formation, and the andesitic, dacitic, and rhyodacitic flow-dome complexes of the middle to late Eocene Estratos de Cerro Casado. The youngest group includes moderately to poorly consolidated gravel sequences correlatable with parts of the Eocene Calama Formation and the more widespread Miocene Tambores Formation. The latter is associated with pediment surfaces of late Miocene age and includes a 10 Ma ignimbrite of regional extent (Artola ignimbrite; Fig. 4).

The Mesozoic and Cenozoic intrusive rocks comprise several units of which the Late Cretaceous quartz monzodiorite and granodiorite stocks of the composite Sierra del Buitre batholith and the gabbro and diorite plutons of the Caracoles area are the most widespread. In addition, numerous stocks of Paleocene and Eocene age, scattered throughout the area shown in Figure 4, constitute the partly unroofed tops of inferred batholiths (e.g., Behn et al., 2001).

The regional structure is rather complex and dominated by the Domeyko fault system (Maksaev, 1990; Boric et al., 1990; Mpodozis et al., 1993a, b). Within the area shown in Figure 4, the fault system has a width of approximately 15 km, between the bounding Centinela and Los Toros faults, and is characterized by a series of submeridianally striking faults with continuous exposed lengths of up to 80 km. Both bounding faults are part of the larger “Caracoles Structural Fan” of Mpodozis et al. (1993a), which constitutes the regional contact between Paleozoic basement to the east and Mesozoic to Cenozoic cover sequences to the west. The Domeyko fault system continues south along the Sierra de Varas master fault to the Escondida area, thus comprising a >300-km-long system (Mpodozis et al., 1993a). Recent studies indicate that these faults continue north of the Limon Verde basement block to Chuquicamata and El Abra and played key pre-, syn-, and postmineral roles in the localization, evolution, and displacement of the giant Chuquicamata porphyry deposit and its satellite systems (Dilles et al., 1997; Tomlinson and Blanco, 1997).

Deformation of Mesozoic cover sequences in the región includes north- and northeast-trending folds as well as high-and low-angle reverse faults developed during a regionally extensive, Late Cretaceous contractional event (Marinovic and García, 1999). Younger Paleocene to Eocene volcanic sequences are irregularly deformed by large open folds mapped beyond the study area, with more intense deformation being intimately associated with the faults of the Domeyko fault system. This deformation, as well as dextral displacement on the Domeyko fault system, is interpreted as products of shortening during Eocene Incaic compression (Mpodozis et al., 1993a, b). Recent work, including evidence from the study area (see below), however, suggests that reverse faulting (e.g., Centinela fault) was an additional component of the Incaic deformation (Marinovic and García, 1999; Perelló et al., 2001).

Geology of the Esperanza Deposit

The geology of the Esperanza area comprises two main lithologic units, including andesite flows and volcano-sedimentary horizons of the Quebrada Mala Formation and several flow-dome centers and related pyroclastic rocks of the Estratos de Cerro Casado (Fig. 5). Quebrada Mala volcanic rocks unconformably overlie marine carbonate horizons of the Caracoles Group east of Esperanza. Postmineralization cover consists of a thin, irregular blanket of unconsolidated piedmont gravel and talus deposits underlain by a thick sequence of moderately consolidated Calama and/or Tambores Formation gravel that hosts the El Tesoro deposit approximately 3 km northwest (Fig. 4).

Fig. 5.

Simplified geologic map of the Esperanza-El Tesoro area.

Fig. 5.

Simplified geologic map of the Esperanza-El Tesoro area.

Premineral rocks

The Quebrada Mala Formation is characterized by greenish, massive to thickly bedded lava flows, flow breccia, block and ash deposits, and tuff of dominantly andesitic composition. These rocks, monoclinally tilted west at ~60°, are at least 600 m thick, and comprise two main members. The lower member includes basal conglomerate, with abundant limestone and volcanic fragments in a calcareous cement, followed by 50 to 100 m of fine-grained andesitic tuff, calcareous volcanogenic sedimentary rocks, thinly bedded to laminated limestone, and andesitic block and ash flows. A progressive increase in volcanic rocks upward in the sequence culminates in an upper member of massive, porphyritic to aphanitic andesite flows and flow breccia, with minor interbedded tuff of similar composition.

The Estratos de Cerro Casado occurs immediately west and south of Esperanza as a 4-km-long, northeast-trending corridor of structurally controlled endogenous domes emplaced in Quebrada Mala rocks (Fig. 5). The domes are intimately associated with a series of marginal block and ash flows. Silty lacustrine sediments with limestone horizons and replacement onyx and travertine deposits near similar dome complexes west of Telégrafo suggest that a large, multicenter, dome-related geothermal field may have existed.

Dome-related rocks include a variety of texturally zoned stocks of andesitic to dacitic composition characterized by massive, porphyritic cores surrounded by flow-banded breccias of similar texture and composition. In general, these rocks contain abundant plagioclase (30–35 vol %), pyroxene (25–30 vol %), and amphibole (1–2 vol %) in a flow-banded, pilotaxitic groundmass. Bedded rocks consist of massive and chaotic block and ash flows and lapilli tuff, although andesite and even basaltic andesite compositions containing olivine and pyroxene phenocrysts occur locally. Pyroclastic rocks contain broken crystals and lithic fragments and ash-rich matrices, whereas glass is common in the andesite flows.

The age of this volcanism is middle Eocene as constrained by a K-Ar age of 37.6 ± 2.5 Ma and a 40Ar-39Ar plateau age of 44.8 ± 0.4 Ma for dacitic and andesitic domes, respectively, west of Esperanza (Tables 12). These ages imply that the magmatism in the area evolved from early andesitic to late dacitic compositions.

Table 1.

K-Ar Ages from the Esperanza Area (see Fig. 5 for locations)

Sample no.UnitMaterial datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (±2σ)
TELEGKAR02Cerro CasadoAmphibole0.4340.6417037.6 ± 2.5
TES12Cerro CasadoAmphibole0.3960.5795237.2 ± 1.6
S497Artola IgnimbriteBiotite6.8412.8188410.6 ± 1.2
Sample no.UnitMaterial datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (±2σ)
TELEGKAR02Cerro CasadoAmphibole0.4340.6417037.6 ± 2.5
TES12Cerro CasadoAmphibole0.3960.5795237.2 ± 1.6
S497Artola IgnimbriteBiotite6.8412.8188410.6 ± 1.2

Constants: λβ = 4.962 × 10−10y−1; λε = 0.581 × 10−10y−1; 40Ar/36Ar = 295.5; 40K = 0.01167 atom %

Table 2.

40Ar-39Ar Ages from the Esperanza Area (see Fig. 5 for locations)

Sample no.UnitMaterial datedPlateaux age (±2σ)
TELEGKAR05Cerro CasadoAmphibole44.8 ±0.4 (99.3% of gas)
S625EsperanzaBiotite41.3 ±0.3 (100% of gas)
Sample no.UnitMaterial datedPlateaux age (±2σ)
TELEGKAR05Cerro CasadoAmphibole44.8 ±0.4 (99.3% of gas)
S625EsperanzaBiotite41.3 ±0.3 (100% of gas)

Postmineral units

The postmineral cover is dominated by massive, moderately consolidated, pebbly to blocky gravels widely distributed east, north, and west of Esperanza (Figs. 45). The >500-m-thick sequence is made up of two main members (see also Mora et al., 2004). The lower member includes coarsegrained, poorly to moderately sorted, dominantly matrix supported gravel with interbedded sandy, silty, and calcareous horizons. Clast composition is varied, with appreciable amounts of Paleozoic granitoids and andesitic volcanic rocks, although andesite and porphyry fragments with intense quartz-sericitic and advanced argillic alteration dominate. The upper member is composed of massive, chaotic, polymictic, matrix-supported gravel with limestone, andesite, and granitoid fragments. Uppermost levels contain abundant gypsum cement, locally constituting gypcrete. Copper mineralization at El Tesoro occurs in several mantos in the lower member (Mora et al., 2004), which grade laterally to, and interfinger with, hematite-cemented gravel.

The age of the sequence is constrained by stratigraphic relationships and K-Ar data. Basal gravel horizons unconformably overlie middle Eocene (44.8 ± 0.4 Ma) andesite dome and related facies of the Estratos de Cerro Casado immediately north of Esperanza but are interbedded with a 37.2 ± 1.6 Ma dacitic tuff to the south (Table 1). In addition, a biotite-rich ignimbrite near the top of the upper member yields an age of 10.6 ± 1.2 Ma (Table 1) and is correlated with the Artola and Sifón ignimbrites of the Salar de Atacama and Calama regions (Blanco et al., 2000; Mpodozis et al., 2000). Thus, the age of the sequence in the Esperanza-Tesoro-Telégrafo area is latest middle Eocene to middle Miocene. Regionally, these gravel sequences are assigned to the Calama (Blanco et al., 2003), Tambores (Ramírez and Gardeweg, 1982; Mpodozis et al., 2000), and Sichal (Maksaev, 1978) Formations and the informally termed Atacama Gravels (Mortimer, 1973), whereas the regional pediment surfaces are traditionally correlated with the Atacama Pediplain (Mortimer, 1973) of the Copiapo región (e.g., Marinovic and García, 1999).

Porphyry-related rocks

Porphyry copper-gold mineralization at Esperanza is spatially associated with a series of dikes and minor stocks of quartz-feldspar porphyry that intrude massive andesite and thickly bedded pyroclastic horizons of the Quebrada Mala Formation. The dikes and stocks are small, with surface dimensions of 8 to 60 m in width and lengths up to 400 m in a dominantly northeasterly direction (Fig. 6). In general, the porphyries are irregular in form, with abundant smaller dikes connecting larger masses. At depth, drilling shows that most of the dikes merge into a larger, ~600- × 300-m, ~50° southeast-plunging body. Locally, contact zones between the porphyries and their wall rocks are marked by intrusion (igneous) breccia containing variable amounts of andesitic clasts in a porphyritic matrix (Figs. 5, 7).

Fig. 6.

Simplified geologic and hydrothermal alteration map of the Esperanza deposit. Note the classic alteration pattern centered on the granodiorite porphyry intrusions and the highest quartz bearing vein intensity associated with a central potassic alteration zone.

Fig. 6.

Simplified geologic and hydrothermal alteration map of the Esperanza deposit. Note the classic alteration pattern centered on the granodiorite porphyry intrusions and the highest quartz bearing vein intensity associated with a central potassic alteration zone.

Fig. 7.

Simplified sections through the Esperanza deposit. See Figure 6 for locations. a. Salient geologic units of the system and main structures, including the reverse fault associated with the anhydrite horizon. b. Geometry of hydrothermal alteration assemblages for the same section and the distribution of copper mineralization. Note the intimate association of the >1.0 percent Cu core to the system with main-stage granodiorite porphyry and the bulk of the mineralization hosted by potassic alteration. The low-grade (<0.2% Cu) mineralization hosted by a centrally located late mineral porphyry is also shown.

Fig. 7.

Simplified sections through the Esperanza deposit. See Figure 6 for locations. a. Salient geologic units of the system and main structures, including the reverse fault associated with the anhydrite horizon. b. Geometry of hydrothermal alteration assemblages for the same section and the distribution of copper mineralization. Note the intimate association of the >1.0 percent Cu core to the system with main-stage granodiorite porphyry and the bulk of the mineralization hosted by potassic alteration. The low-grade (<0.2% Cu) mineralization hosted by a centrally located late mineral porphyry is also shown.

Quartz-feldspar porphyry typically displays a medium- to coarse-grained (0.2–0.5 cm) porphyritic texture marked by phenocrysts of plagioclase (25 vol %), biotite (5 vol %), quartz (3–4 vol %), and hornblende (3–4 vol %), in a microcrystalline groundmass of quartz, K-feldspar, plagioclase, and biotite. The overall composition is granodioritic, based on the phaneritic texture of the groundmass and the presence of abundant, interstitial, and poikilitic phenocrysts of magmatic K-feldspar in the larger intrusions. Smaller intrusions tend to develop finer grained texture and an aphanitic groundmass and are thus classified as dacite porphyry. Quartz-feldspar porphyry at Esperanza is made up of numerous phases, of which two are separated and shown in Figures 6 and 7. The main and late mineral phases display similar textural and compositional features, in accord with the petrographic descriptions above, although younger intrusions generally tend to develop finer grained textures. The two main phases may also be distinguished on the basis of intensity of hydrothermal alteration and quartz veining: early porphyry intrusions are characterized by more intense potassic alteration (see below) and quartz-magnetite veining, both of which are characteristically lacking in the late mineral phase. Late mineral porphyry also develops intrusion breccia bodies along wall-rock contacts, with the clasts typically including earlier phase porphyry and andesite.

Deep drilling (>600 m) detected the presence of a large, massive anhydrite horizon, approximately 70 m thick, composed of upper crystalline anhydrite and lower anhydrite and calc-silicate minerals (Fig. 7). The upper part is medium- to coarse-grained anhydrite containing up to 5 vol percent of sulfides, magnetite, and calc-silicate minerals, typically concentrated in unidirectional, streaky bands of massive to disseminated, millimetric aggregates spaced 1 to 2 cm apart. The bands tend to be dominated by either magnetite and sulfides or magnetite and calc-silicate assemblages. The lower zone comprises anhydrite with garnet, diopside, epidote, and hedenbergite intergrown with copper sulfides.

Structure

Esperanza lies between two main northeast-trending fault zones (Fig. 6), which are subsidiary splays of the regional Domeyko fault system. At the deposit scale, a number of smaller faults define a fanlike array, with north-northeast- to east-northeast-trending structures that control the orientation and shape of the porphyry dikes (see below and Fig. 6). Individual fault zones are typified by fault breccia and gouge, locally with abundant supergene gypsum fill, and mortar texture in quartz veins as a result of crushing. Most of the faults dip southeastward (Fig. 7a), roughly subparallel to the main porphyry intrusions.

The upper and lower contacts of the anhydrite horizon (Fig. 7a) are marked by brecciation and gouge, whereas the sulfide and calc-silicate bands comprise discontinuous schlieren, locally displaying folding (Sillitoe, 2001). These observations are consistent with a syntectonic formation of the anhydrite horizon during porphyry emplacement and potassic alteration (see below).

Hydrothermal Alteration

Hydrothermal alteration zoning at Esperanza conforms with the classic Lowell and Guilbert (1970) model and includes, in plan and section, a core of potasssic alteration surrounded by zones of quartz-sericitic and propylitic alteration. A zone of intermediate argillic alteration is located between the potassic core and the quartz-sericitic halo, whereas calc-silicate assemblages are present at depth in calcareous sedimentary and volcano-sedimentary rocks (Fig. 7).

Potassic alteration

The potassic core is centered on the porphyry intrusions and immediate andesitic wall rocks and comprises various associations of quartz, K-feldspar, and biotite, with abundant hydrothermal magnetite, anhydrite, and subordinate apatite. Copper sulfides are common and pyrite is scarce (see below). Where potassic alteration is more intense, within main-phase porphyry and adjacent andesite, there is complete to partial transformation of original constituents by coarse-grained assemblages of quartz and K-feldspar (orthoclase) as well as abundant biotite and anhydrite, the last increasing rapidly with depth. Graphic intergrowths between quartz and K-feldspar are characteristic and, locally, give way to breccialike fabrics in which pegmatoidal biotite is common. Original biotite grains undergo replacement by K-feldspar, rutile needles, and secondary biotite, whereas hornblende crystals are typically replaced by intergrowths of fine-grained, reddish biotite, K-feldspar, and apatite. Potassic alteration in andesite is characterized by fine-grained, locally granoblastic, mosaics of quartz, plagioclase, K-feldspar, and biotite with subordinate chlorite, apatite, zircon, and anhydrite.

Several generations of veinlets characterize the potassic alteration and include the classic A and B types (Gustafson and Hunt, 1975). A-type veinlets in porphyry from Esperanza are millimeters to centimeters in width, irregular and discontinuous, and typically made up of quartz and K-feldspar, anhydrite, magnetite, chalcopyrite, and bornite. Biotite is conspicuously absent from these veins, and the anhydrite can be partly to totally hydrated to gypsum. A second generation of B type includes planar and continuous veinlets with quartz, K-feldspar, fine-grained biotite rosettes, and interstitial anhydrite, magnetite, chalcopyrite, and bornite. They also carry trace amounts of molybdenite and pyrite, and possess biotite and chlorite borders and incipient external halos of K-feldspar and biotite. In andesite, these veinlets tend to be straight and planar and contain fine-grained biotite, perthitic feldspar, and anhydrite, plus additional chlorite, calcite, magnetite, and trace chalcopyrite. These veinlets are truncated and offset by a second generation of stockwork veinlets with quartz mosaics, K-feldspar, magnetite, and fine-grained chalcopyrite and bornite, with incipiently to well- developed K-feldspar halos. Magnetite intergrowths with biotite and sulfides, titanite around magnetite grains, appreciable amounts of apatite, and trace epidote are additional features.

In general, a broad spectrum of A- and B-type veinlets associated with much of the potassic alteration and mineralization is observed at Esperanza (see below) and includes early, intermediate, and late generations of each type. As a general rule, the earliest veinlets tend to be irregular in shape and lack alteration halos, with sulfides occurring as disseminated grains. Later generations tend to be more planar and have only incipient halos rich in K-feldspar, which become better developed in younger generations. Sulfide grains in these veinlets are typically dispersed but constitute semicontinuous center lines in the younger sets. As shown in Figure 6, an empirical correlation between veinlet intensity and distribution and the potassic core is apparent, all features being centered on the porphyry stocks and dikes.

Calc-silicate assemblages

Lower volcano-sedimentary and calcareous horizons of the Quebrada Mala Formation, intersected locally by drilling at depths of >600 m, are transformed to calc-silicate assemblages as an irregular aureole to the main mass of quartz-feldspar porphyry. The assemblages are the same as those present in the anhydrite horizon described above: typically prograde, fine-grained, greenish pyroxene and coarser grained, pale brown andraditic garnet. Pétrographic work shows that both diopside and hedenbergite are present and, like garnet, are characteristically intergrown with magnetite and copper sulfides to constitute a classic anhydrous copper skarn.

Intermediate argilllic alteration

The potassic core is overprinted by, and grades laterally to, intermediate argillic assemblages dominated by quartz and chlorite but also with abundant greenish soapy sericite, illite, and smectite. Where the assemblages are incipiently to moderately developed, a weak background of preexisting potassic alteration is present. Intermediate argillic alteration assemblages are texture destructive where alteration is intense, with biotite and other ferromagnesian minerals being replaced by chlorite and smectite, and plagioclase and K-feldspar grains being selectively replaced by sericite (fine-grained muscovite) and varied amounts of illite.

The dominant veinlet type includes planar, millimeter- to centimeter-thick veinlets of quartz, chlorite, sericite, and sulfides, with halos of chlorite, smectite, illite, and sericite. Chalcopyrite and pyrite characteristically occupy a well-defined center line, and the alteration halos are up to several times wider than the veinlets themselves. The halos are typically zoned from an inner, white zone with sericite > chlorite to an outer, greener zone, in which chlorite and smectite predominate over sericite. Magnetite is absent from these veinlets, but martite and remnant martitized magnetite may be present within and beyond the alteration halos. These veinlets are considered to be transitional to the D type of Gustafson and Hunt (1975) and may be an early generation of the same. They are invariably cut by classic D-type veinlets accompanied by well-developed quartz-sericitic alteration.

Quartz-sericitic alteration

Intermediate argillic alteration is overprinted by, and grades laterally outward to, a zone with quartz-sericitic assemblages dominated by quartz, white sericite, and illite. These assemblages are mainly hosted in andesitic rocks, whose textures are completely destroyed and replaced by fine-grained mosaics of these minerals. Chlorite in this zone is present in proximity to the inner intermediate argillic and the outer propylitic zones. Dominant veinlets in this zone are of D type and characterized by planar, centimeter- to decimeter-scale veins of quartz and interstitial sericite, with thick, white alteration halos of similar composition. Copper sulfides are absent from these veins, but pyrite is characteristic and occurs as coarse-grained crystals that define a continuous, locally massive center line.

Propylitic alteration

The propylitic halo at Esperanza is defined by the first appearance of epidote beyond the quartz-sericitic zone. In addition, the assemblage includes chlorite, calcite, and pyrite. Epidote occurs as replacements of ferromagnesian minerals and plagioclase and as fillings of amygdules and fractures. As shown in Figure 6, this zone is free of quartz veinlets, but calcite- and/or gypsum-filled fractures are common.

Mineralization

Copper

The bulk of the copper mineralization at Esperanza is spatially and temporally associated with the potassic core, which contains >80 percent of the sulfide and oxide resources (Fig. 7b). Hypogene copper mineralization comprises several associations of chalcopyrite and bornite, accompanied by abundant hydrothermal magnetite and subordinate amounts of steely chalcocite and pyrite. In general, chalcopyrite-bornite mineralization occurs as micron-sized to millimeter-scale grain disseminations and in multiple generations of millimeter- to centimeter-scale veinlets of the A and B types described above. The average sulfide content of the deposit varies between 2 and 4 vol percent, with sulfides being most abundant in zones of intense stockwork veining at the contact between main-phase porphyry and andesite. Typical veinlet associations include chalcopyrite-bornite, chalcopyrite-bornite-magnetite, chalcopyrite-magnetite, and bornite-steely chalcocite, all of which are accompanied by various proportions of quartz, K-feldspar, anhydrite, and biotite, either within the veinlets themselves or in the alteration halos. Hydrothermal magnetite is a common accompaniment to these veins and contributes to the average 5 to 7 vol percent estimated for the deposit. The elevated magnetite contents explain the excellent correlation between copper mineralization and the magnetic response. Disseminated mineralization occurs as individual chalcopyrite or bornite grains or as groups of three to four grains with additional magnetite and subordinate chalcocite. Simultaneous deposition of chalcopyrite, bornite, and magnetite, evidenced by intergrowths and exsolution textures, is common in both the disseminated and vein-hosted mineralization. Chalcopyrite and bornite inclusions in magnetite are characteristic. Deep-seated mineralization includes meter-scale intersections of massive to semimassive magnetite-chalcopyrite of skarn type, hosted by proximal calc-silicate assemblages rich in garnet and pyroxene (see above). The overall volume and significance of this style of mineralization remains to be fully assessed, although it seems to be confined to an irregular aureole around mainphase porphyry.

Intermediate argillic alteration assemblages are characterized by chalcopyrite contained in veinlets and hairline fractures with pyrite and as fine- to medium-grained disseminations. The dominant veinlet type consists of quartz-sericite-chlorite with center lines filled by chalcopyrite and pyrite and greenish alteration halos with disseminated chalcopyrite and pyrite. Copper grades in intermediate argillic alteration are lower than those in potassic alteration, with some of the highest grades either partly inherited from earlier formed mineralization in potassic alteration or associated with monomineralic chalcopyrite veinlets. Bornite is absent from these assemblages, magnetite is typically transformed to martite, and the pyrite content is higher than in potassic alteration.

Quartz-sericitic alteration assemblages are essentially barren of copper mineralization. Pyrite is the dominant sulfide and trace amounts of molybdenite and sphalerite occur locally. Much of the pyrite is present in classic D-type veinlets as coarse-grained fillings in quartz-sericite veinlets with well-developed alteration halos. Overall veinlet intensity decreases with distance from the core of the system, and pyrite in the external parts of the quartz-sericitic zone occurs mainly as hairline fractures and disseminations.

Gold, silver, and molybdenum

Empirical relationships show that gold and copper grades vary sympathetically (Fig. 8a). This suggests that the bulk of the gold is associated with the copper sulfides, an observation that is partly confirmed by the presence of micron-sized native gold grains in chalcopyrite and bornite. SEM studies indicate that, in addition to native gold, electrum and cupriferous gold also occur in subordinate amounts. In general, gold is observed as fine inclusions (up to 50 μm) in chalcopyrite and bornite, whereas coarser grained native gold of ~250 μm in size is typically attached to either chalcopyrite or bornite. Cupriferous gold occurs in rounded grains between 200 and 400 μn containing up to 23 wt percent Cu.

Fig. 8.

Sections through the Esperanza deposit displaying the relationship between various elemental associations. See Figure 6 for locations. a. Copper and gold. b. Copper and silver. c. Copper and molybdenum. Note the intimate association between copper, gold, and silver and potassic alteration, as well as the inverse association between them and molybdenum, the latter mostly contained in intermediate argillic alteration halos. See text for discussion.

Fig. 8.

Sections through the Esperanza deposit displaying the relationship between various elemental associations. See Figure 6 for locations. a. Copper and gold. b. Copper and silver. c. Copper and molybdenum. Note the intimate association between copper, gold, and silver and potassic alteration, as well as the inverse association between them and molybdenum, the latter mostly contained in intermediate argillic alteration halos. See text for discussion.

Silver mineralization is poorly understood. No silver minerals have been detected by either optical or SEM methods. Analytical data confirm that much of the silver occurs within the copper-gold orebody, with silver grades displaying a sympathetic relationship to both copper and gold (Fig. 8b). No appreciable silver enrichment or depletion is present in the supergene oxide zone of the deposit. Molybdenum contents average <100 ppm for the entire deposit. Molybdenite is observed as a minor constituent of B-type veinlets, as well as in veinlets of the intermediate argillic alteration stage. Indeed, at the deposit scale, molybdenum grades display a spatial association with intermediate argillic alteration. An overall antipathetic relationship of molybdenum with copper, gold, and silver, and potassic alteration is apparent (Fig. 8c).

Summary of hypogene copper-gold mineralization

Field relationships indicate that hypogene copper-gold mineralization at Esperanza evolved from an early, high-grade, potassic alteration stage rich in chalcopyrite and bornite, through an intermediate stage associated with intermediate argillic alteration and lower grade chalcopyrite plus pyrite, to a late quartz-sericitic stage with abundant pyrite. The analytical data coupled with detailed drill core observations suggest that the original copper sulfide mineralogy of the early potassic alteration stage was reconstituted to more oxidized, higher sulfidation state assemblages during overprinting by the intermediate argillic and quartz-sericitic events, with consequent depletion of copper and gold. Figure 9 shows that this process took place at all scales within the system and was also responsible for the vertical and lateral zoning of the copper sulfide mineralization observed at Esperanza (Fig. 10), with a deeper core rich in bornite and a shallower zone dominated by chalcopyrite.

Fig. 9.

Schematic representation of the various alteration-mineralization events at Esperanza based on detailed observation of a ~10-cm core sample. Note how successive intermediate argillic and quartz-sericitic alteration events modified the original sulfide mineralogy of the potassic stage, with consequent decrease in copper grades. Also note the size of the sample, which implies that these obliteration processes took place at all scales in the system. See text and Figure 11 (below) for further discussion.

Fig. 9.

Schematic representation of the various alteration-mineralization events at Esperanza based on detailed observation of a ~10-cm core sample. Note how successive intermediate argillic and quartz-sericitic alteration events modified the original sulfide mineralogy of the potassic stage, with consequent decrease in copper grades. Also note the size of the sample, which implies that these obliteration processes took place at all scales in the system. See text and Figure 11 (below) for further discussion.

Fig. 10.

Schematic cross section through Esperanza displaying the intimate relationship between the high-grade copper values and bornite distribution. Truncation of this core zone by the anhydrite horizon and a late mineral porphyry dike is also apparent. See text for discussion and Figure 6 for section location.

Fig. 10.

Schematic cross section through Esperanza displaying the intimate relationship between the high-grade copper values and bornite distribution. Truncation of this core zone by the anhydrite horizon and a late mineral porphyry dike is also apparent. See text for discussion and Figure 6 for section location.

Supergene mineralization

Supergene copper mineralization is dominated by atacamite followed by variable amounts of paratacamite, chrysocolla, brochanthite, pseudomalachite, and chalcanthite, plus additional copper clays, copper wad, and copper pitch. Trace relict chalcopyrite and pyrite are common. These minerals and mineraloids occur in the upper 100 to 200 m of the deposit, with mineralization typically occurring as fracture fillings, replacements of plagioclase phenocrysts, and in quartz stockworks showing in situ oxidation of chalcopyrite and chalcocite. The phosphate, sampleite (NaCaCu5(PO4)4Cl·5H2O), is also an important supergene constituent, as determined by SEM studies.

At the redox front (Fig. 7b), between the surfaces defined by the top of sulfides and the top of dominant sulfides, an irregular zone 10 to 20 m in thickness contains mixed oxide and sulfide mineralization comprising supergene chalcocite and covellite and subordinate amounts of cuprite and native copper.

Age

The age of Esperanza is constrained by field relationships and radiometric dating. Premineral rocks include the domes and associated pyroclastic flows of the middle to late Eocene Estratos de Cerro Casado which, to the west and southwest of Esperanza, are dated at 44.8 ± 0.4 and 37.6 ± 2.5 Ma, respectively (Tables 12). Similarly, hydrothermal biotite from potassic-altered main-phase porphyry yields a 40Ar-39Ar plateau age of 41.4 ± 0.3 Ma (Table 2), which places the potassic alteration and related mineralization in the middle Eocene. These ages are further confirmed by an Re-Os age of 41.80 ± 0.13 Ma for molybdenite from a B-type veinlet from the potassic core (H. Stein, pers. commun., 2003). These ages are similar to those of other systems in the region, including Centinela (44.0 ± 1.5 and 43.8 ± 1.5 Ma; Boric et al., 1990) and certain intrusions in the vicinity of Polo Sur (Marinovic and García, 1999), and define a 40-km-long, northeast-trending corridor of structurally controlled porphyry systems of middle to late Eocene age.

Although not dated directly, supergene alteration and mineralization at Esperanza are inferred to be early Miocene in age, based on a K-Ar age of 20.0 ± 1.0 Ma for supergene alunite from the leached capping at the nearby Telégrafo prospect (Sillitoe and McKee, 1996).

Fluid Inclusion Data

A preliminary fluid inclusion study was undertaken on seven core samples in order to characterize the inclusions, determine the temperature and salinity of the fluids, and obtain rough estimates for pressure and paleodepth conditions (Collao and Campos, 2003). Five samples correspond to A- and B-type veinlets from the core of the system and two samples are from D-type veinlets in intermediate argillic and quartz-sericitic alteration. The following three main inclusion types are observed:

  1. Type I: multiphase inclusions with a vapor bubble, liquid, and up to five daughters (halite, metallic opaques, hematite, sylvite, anhydrite). The vapor bubble and/or halite crystal constitute up to 50 vol percent of the inclusions. These inclusions possess the highest homogenization temperatures, between 435° and 592°C and have salinities of 41 to 60 wt percent NaCl equiv.

  2. Type II: two-phase inclusions, in which the vapor bubble occupies between 60 and 90 vol percent of the inclusions. They typically coexist with type I and III inclusions, homogenize to vapor at temperatures between 312° and 512°C, and possess low salinities, between 0.5 and 5 wt percent NaCl equiv.

  3. Type III: two-phase liquid-rich inclusions, in which the vapor bubble occupies between 5 and 20 vol percent of the inclusions. They homogenize to liquid at temperatures between 276° and 406°C and possess low salinities, between 1 and 11 wt percent NaCl equiv.

Coexistence of type I and III inclusions and the dominant presence of type I inclusions in quartz stockworks associated with intense potassic alteration in main-phase porphyry, along with their elevated temperatures (up to 592°C) and salinities (up to 60 wt % NaCl), suggest that they were trapped from undiluted, hot, boiling fluid compatible with a magmatic derivation. Coexistence of several subpopulations of type I inclusions further suggests a multiepisodic evolution of the main-stage potassic alteration and associated mineralization.

Type I inclusions in quartz veinlets from intermediate argillic and quartz-sericitic alteration zones show lower ho-mogenization temperatures (217°–330°C), although a magmatic signature is retained with high salinities of 40 to 53 wt percent NaCl equiv. Type II and III inclusions coexisting with type I inclusions in these assemblages are characterized by lower temperatures (200°–300°C) and salinities (6–7 wt % NaCl equiv) and are interpreted to have been trapped from different boiling fluid with varied meteoric and magmatic components. Pressure-corrected depths of formation for type I fluid inclusions under lithostatic conditions (Shepherd et al., 1985) are between 1 and 2 km below the paleowater table. This suggests that much of the mineralization at Esperanza was emplaced in a shallow sub volcanic environment, consistent with the regional and district geologic evidence that relates Esperanza to the intermediate and terminal stages of dome-type volcanism represented by the Estratos de Cerro Casado.

Discussion

Porphyry evolution

The evolution of the Esperanza system is inferred to have involved several alteration-mineralization stages associated with the porphyry intrusions. Early, potassic-stable mineralization was associated with a magnetite-rich assemblage of quartz, K-feldspar, biotite, bornite, and chalcopyrite. This event was generated by juvenile fluids of magmatic derivation, judging by the high-temperature, hypersaline fluid inclusions found in quartz from these assemblages (cf. Bodnar, 1995; Hedenquist and Richards, 1998).

Potassic alteration at Esperanza is overprinted by intermediate argillic assemblages. These assemblages are, in turn, overprinted by quartz-sericitic alteration. Overprinting may have been caused either by the influx of heated meteoric water in response to cooling of the hydrothermal system or by direct contribution from magmatic fluid, judging by the high salinities and lower temperatures measured in these fluid inclusions (e.g., Ulrich et al., 2001). Field evidence at several scales indicates that the sulfide mineralogy of the early-stage, bornite- and chalcopyrite-rich mineralization was reconstituted to more oxidized, higher sulfidation-state sulfide assemblages richer in pyrite during overprinting by the intermediate argillic and quartz-sericitic alteration, with consequent lowering of the copper, gold, and silver contents (Fig. 11). Heat to sustain fluid circulation and additional magmatic fluid input for sulfide reconstitution and remobilization was provided by emplacement of intermineral porphyry phases. Similarly, hydrothermal magnetite from the early potassic stage underwent transformation to martite during the overprinting phases. Molybdenite, on the contrary, accompanied the overprint, judging by the increased molybdenum contents in the intermediate argillic alteration assemblages (Fig. 11).

Fig. 11.

Graph showing the relationships between copper, gold, silver, and molybdenum and the various hydrothermal alteration associations at Esperanza. Average grades and ranges represent geostatistical averages for > 15,000 samples. Note the positive correlation between copper, gold, and silver and potassic alteration, and association of intermediate argillic and quartz-sericitic stages with lower grades. Also note the positive correlation between molybdenum contents and intermediate argillic ateration and background copper, gold, silver, and molybdenum values in peripheral propylitic alteration. See text for discussion and Figures 8 and 9 for reference.

Fig. 11.

Graph showing the relationships between copper, gold, silver, and molybdenum and the various hydrothermal alteration associations at Esperanza. Average grades and ranges represent geostatistical averages for > 15,000 samples. Note the positive correlation between copper, gold, and silver and potassic alteration, and association of intermediate argillic and quartz-sericitic stages with lower grades. Also note the positive correlation between molybdenum contents and intermediate argillic ateration and background copper, gold, silver, and molybdenum values in peripheral propylitic alteration. See text for discussion and Figures 8 and 9 for reference.

Metallogenesis

Esperanza is part of a northeast-trending corridor of structurally controlled porphyry systems and associated remnants of porphyry-related lithocaps that is ~40 km long and 5 km wide and includes the Polo Sur, Centinela, and Telégrafo prospects. The age of this corridor is middle Eocene, as constrained by K-Ar, 40Ar-39Ar, and Re-Os ages (J. Perelló unpub. data, 2002), temporally and spatially linked to the dome-centered, intermediate composition, calc-alkaline magmatism of the Estratos de Cerro Casado (Perelló et al., 2003a). Between Esperanza and Telégrafo, and perhaps elsewhere in the region, this volcanism and associated lacustrine sediments form part of a large paleogeothermal field that also deposited onyx and travertine in a distal, structurally controlled hot-spring environment. Subvolcanic emplacement of Esperanza is further corroborated by the depths of entrapment determined for some of the highest temperature fluid inclusions from the core of the system, between 1 and 2 km below the paleowater table.

Porphyry-type mineralization along the belt comprises a complete spectrum of types, from copper-gold at Esperanza, through copper-gold-molybdenum at Telégrafo, to copper (-molybdenum) at Centinela and Polo Sur (Perelló et al., 2003a; J. Perelló unpub. data, 2002), thereby confirming that these end members coexist under similar geologic conditions and that formation of any particular type is independent of host-rock composition, intrusion petrochemistry, geotectonic setting, and crustal thickness. However, in conformity with many gold-rich porphyry copper deposits elsewhere (e.g., Sillitoe, 1979, 2000), abundant hydrothermal magnetite accompanies copper-gold mineralization at Esperanza and Telégrafo, but it is lacking from the gold-poor examples at Centinela and Polo Sur.

Structural control on porphyry emplacement by the Domeyko fault system is apparent in all the systems in the corridor. At Esperanza, structure played an important role in determining the shape, orientation, and attitude of the porphyry intrusions and associated mineralization, with the field evidence showing that at least early-stage potassic and anhydrite-bearing calc-silicate alteration took place simultaneously with contractional deformation. Reverse faulting and thrusting in the area correlate with the widespread Incaic orogeny, an Andean-scale contractional event that caused significant regional deformation, surface uplift, and exhumation in both Peru and Chile (Noble et al., 1979; Sébrier et al., 1988), including formation of the Cordillera de Domeyko in northern Chile (Maksaev and Zentilli, 1988; Maksaev, 1990; Mpodozis et al., 1993a, b; Maksaev and Zentilli, 1999).

Evidence from the corridor and elsewhere (e.g., Mpodozis et al., 2000; Perelló et al., 2003b) suggests that synorogenic clastic deposits, located both east and west of the Cordillera de Domeyko, accumulated in structurally controlled basins during Incaic deformation and uplift. In the area of Esperanza-Telégrafo, these deposits include thick gravel accumulations, which acted as the recipient for the copper-charged solutions that gave rise to the exotic copper mineralization at El Tesoro and Tesoro NE (Mora et al., 2003, 2004). This exotic mineralization constitutes an integral part of the metallogeny of the corridor.

Conclusions

Exploration

Although historically known and having been formally explored by different companies over a period of at least 16 yr since 1983, full appreciation of the potential of Esperanza dates from 1999, when detailed mapping defined the salient geologic elements of the system. Geologic modeling of surface data was complemented by reinterpretation of available drilling data from previous campaigns and all underpinned by the results of a property-wide ground-magnetic survey. Discovery was by the thirteenth and fourteenth holes drilled in the area of the deposit. A key decision point was the end of the first drilling program, when the potential of the system to host a high-grade copper-gold core was first appreciated. Modeling and anticipation of this high-grade zone was based on simple geologic observations, which resulted in the siting of additional key drill holes. Discovery was made possible by the strong commitment of the exploration team and the backing of company management.

Geology, alteration, and mineralization

Esperanza is a classic porphyry copper-gold deposit. It is associated with a series of structurally controlled, porphyry dikes of granodioritic composition that intrude a sequence of Late Cretaceous massive andesite flows and volcano-sedimentary rocks. The age of the alteration and mineralization is middle Eocene, based on a 40Ar-39Ar age of 41.4 ± 0.3 Ma for hydrothermal biotite and an Re-Os age of 41.80 ± 0.13 Ma for molybdenite. Porphyry copper formation was coeval with emplacement and development of a large, district-wide dome field, which also included distal, low-temperature geothermal activity in the form of carbonate hot springs that formed onyx and travertine in restricted, structurally controlled basins.

The system developed hydrothermal alteration zoning that complies with the classic model of Lowell and Guilbert (1970), including a core of potassic alteration surrounded by quartz-sericitic and propylitic zones. It differs from this model, however, in having intermediate argillic alteration between potassic and quartz-sericitic zones, in the abundance of hydrothermal magnetite in the potassic core, in the location of the copper mineralization in the potassic core, and in its elevated gold content. Potassic alteration in volcanic rocks from the upper parts of the system gives way, at depth, to proximal calc-silicate assemblages rich in garnet and pyroxene where country rocks are dominated by calcareous pelite and sandstone.

The system displays profund structural control. Current evidence supports the inference that the productive porphyries were emplaced in a contractional environment, contemporaneously with reverse faults that controlled the shape and attitude of the intrusive bodies and the alteration-mineralization geometry. Under such tectonic conditions, in response to regional uplift during Incaic contraction, Esperanza evolved from early-stage potassic alteration with copper-gold mineralization as chalcopyrite and bornite, through a transgressive stage of intermediate argillic alteration with chalcopyrite and pyrite, to a late overprint of quartz-sericitic alteration dominated by pyrite. A wealth of observational and analytical data support the concept that copper sulfides from the early stage were reconstituted to more oxidized, pyrite-rich, copper-poor assemblages during subsequent alteration-mineralization events, with a consequent decrease in copper, gold, and silver contents. Molybdenum, instead, was mainly introduced during the intermediate argillic alteration. Inter- and late mineral mobilization of copper by oxidized fluids has been documented at Butte, Montana (Brimhall, 1980), Sungun, Iran (Hezarkhani and William-Jones, 1998), and Bajo de la Alumbrera, Argentina (Proffett et al., 1998; Ulrich and Heinrich, 2001), and has also been empirically inferred at Oyu Tolgoi, Mongolia (Perelló et al., 2001), Cotabambas, Peru (Perelló et al., 2002, 2004), and in numerous gold-rich porphyry deposits elsewhere (Sillitoe, 2000).

Comparison with other deposits

Esperanza displays similarities in scale, alteration zoning, and copper-gold mineralization with the Bajo de la Alumbrera, Argentina (Sillitoe, 1979; Ulrich and Heinrich, 2001), and in the geometry of the tilted dikes with Island Copper, Canada (Perelló et al., 1995). Esperanza, however, lacks the centrally located, copper- and gold-barren zone of intense quartz-magnetite stockwork veining with K-feldspar at Bajo de la Alumbrera and amphibole at Island Copper. On the contrary, the most intense quartz-magnetite-K-feldspar stockwork veining and flooding, either hosted in the main porphyry intrusion or in immediately adjacent andesite, contain the highest copper, gold, and silver values in the entire deposit. Thus, the temporal and spatial association of iron metasomatism, potassic alteration, and copper-gold deposition at Esperanza contrasts with the observations at Island Copper by Arancibia and Clark (1996), who interpreted the iron metasomatism to be an early, discrete event. Additional evidence from Cotabambas (Perelló et al., 2002, 2004) indicates that iron, potassium, and calcium metasomatism and copper-gold mineralization can occur in multiple overlapping events during the lifespan of potassic alteration in any magnetite-rich, porphyry copper-gold system.

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Acknowledgments

Héctor Poblete and Claudio Montecinos of Antofagasta Minerals are thanked for their drafting work. Rodrigo Mora, Edmundo Martínez, Rodrigo Arcos, and Oscar Rodriguez assisted with fieldwork at various stages of the exploration program. Javier Chacana, Fredis Poblete, and many others provided excellent field support. Holly Stein obtained a precise Re-Os age on a minute sample of molybdenite and Carlos Pérez de Arce from the Chilean Geological Survey carried out the K-Ar and 40Ar-39Ar geochronologic work. Lucía Cuitino conducted detailed petrographic descriptions of selected samples from the deposit. Santiago Collao and Eduardo Campos carried out preliminary fluid inclusion studies. Field discussions with Dick Sillitoe improved our understanding of the geology of the system, while Constantino (Cocho) Mpodozis contributed his knowledge of the regional geologic setting. Special thanks are due to Ricardo Muhr and Nicolás Fuster for their constant support, particularly at key decision points, during the exploration program at Esperanza. The manuscript benefited from reviews by Peter Pollard, John Proffett, and Dick Sillitoe, although the interpretations and conclusions are the sole responsibility of the authors.

Figures & Tables

Fig. 1.

Location map of the Esperanza deposit in relationship to the El Tesoro mine and other districts and prospects in the region. For reference, note the location of the recently discovered Spence porphyry copper deposit near Sierra Gorda and the Chuquicamata mine north of Calama. Porphyry prospects of the Esperanza-Polo Sur corridor also shown.

Fig. 1.

Location map of the Esperanza deposit in relationship to the El Tesoro mine and other districts and prospects in the region. For reference, note the location of the recently discovered Spence porphyry copper deposit near Sierra Gorda and the Chuquicamata mine north of Calama. Porphyry prospects of the Esperanza-Polo Sur corridor also shown.

Fig. 2.

Location of discovery drill holes in the Esperanza deposit and selected follow-up holes. a. Plan showing the discovery drill holes with respect to previous holes by other companies and main current and historical mining claims in the area. See text for discussion b. Schematic geologic section constructed before first-stage drilling in 1999 and discovery drill holes S525 and S526 with main intersections.

Fig. 2.

Location of discovery drill holes in the Esperanza deposit and selected follow-up holes. a. Plan showing the discovery drill holes with respect to previous holes by other companies and main current and historical mining claims in the area. See text for discussion b. Schematic geologic section constructed before first-stage drilling in 1999 and discovery drill holes S525 and S526 with main intersections.

Fig. 3.

Evolution in size of the high-grade core of the Esperanza deposit between April 2000 and May 2001. Note how a conventional grid drilling program on 200-m centers could have left this core undiscovered. See text for further discussion.

Fig. 3.

Evolution in size of the high-grade core of the Esperanza deposit between April 2000 and May 2001. Note how a conventional grid drilling program on 200-m centers could have left this core undiscovered. See text for further discussion.

Fig. 4.

Sketch map of the geology of the Esperanza-Polo Sur region. Compilation based on Mpodozis et al. (1993a), Marinovic and García (1999), Mora (2001), and mapping by the authors.

Fig. 4.

Sketch map of the geology of the Esperanza-Polo Sur region. Compilation based on Mpodozis et al. (1993a), Marinovic and García (1999), Mora (2001), and mapping by the authors.

Fig. 5.

Simplified geologic map of the Esperanza-El Tesoro area.

Fig. 5.

Simplified geologic map of the Esperanza-El Tesoro area.

Fig. 6.

Simplified geologic and hydrothermal alteration map of the Esperanza deposit. Note the classic alteration pattern centered on the granodiorite porphyry intrusions and the highest quartz bearing vein intensity associated with a central potassic alteration zone.

Fig. 6.

Simplified geologic and hydrothermal alteration map of the Esperanza deposit. Note the classic alteration pattern centered on the granodiorite porphyry intrusions and the highest quartz bearing vein intensity associated with a central potassic alteration zone.

Fig. 7.

Simplified sections through the Esperanza deposit. See Figure 6 for locations. a. Salient geologic units of the system and main structures, including the reverse fault associated with the anhydrite horizon. b. Geometry of hydrothermal alteration assemblages for the same section and the distribution of copper mineralization. Note the intimate association of the >1.0 percent Cu core to the system with main-stage granodiorite porphyry and the bulk of the mineralization hosted by potassic alteration. The low-grade (<0.2% Cu) mineralization hosted by a centrally located late mineral porphyry is also shown.

Fig. 7.

Simplified sections through the Esperanza deposit. See Figure 6 for locations. a. Salient geologic units of the system and main structures, including the reverse fault associated with the anhydrite horizon. b. Geometry of hydrothermal alteration assemblages for the same section and the distribution of copper mineralization. Note the intimate association of the >1.0 percent Cu core to the system with main-stage granodiorite porphyry and the bulk of the mineralization hosted by potassic alteration. The low-grade (<0.2% Cu) mineralization hosted by a centrally located late mineral porphyry is also shown.

Fig. 8.

Sections through the Esperanza deposit displaying the relationship between various elemental associations. See Figure 6 for locations. a. Copper and gold. b. Copper and silver. c. Copper and molybdenum. Note the intimate association between copper, gold, and silver and potassic alteration, as well as the inverse association between them and molybdenum, the latter mostly contained in intermediate argillic alteration halos. See text for discussion.

Fig. 8.

Sections through the Esperanza deposit displaying the relationship between various elemental associations. See Figure 6 for locations. a. Copper and gold. b. Copper and silver. c. Copper and molybdenum. Note the intimate association between copper, gold, and silver and potassic alteration, as well as the inverse association between them and molybdenum, the latter mostly contained in intermediate argillic alteration halos. See text for discussion.

Fig. 9.

Schematic representation of the various alteration-mineralization events at Esperanza based on detailed observation of a ~10-cm core sample. Note how successive intermediate argillic and quartz-sericitic alteration events modified the original sulfide mineralogy of the potassic stage, with consequent decrease in copper grades. Also note the size of the sample, which implies that these obliteration processes took place at all scales in the system. See text and Figure 11 (below) for further discussion.

Fig. 9.

Schematic representation of the various alteration-mineralization events at Esperanza based on detailed observation of a ~10-cm core sample. Note how successive intermediate argillic and quartz-sericitic alteration events modified the original sulfide mineralogy of the potassic stage, with consequent decrease in copper grades. Also note the size of the sample, which implies that these obliteration processes took place at all scales in the system. See text and Figure 11 (below) for further discussion.

Fig. 10.

Schematic cross section through Esperanza displaying the intimate relationship between the high-grade copper values and bornite distribution. Truncation of this core zone by the anhydrite horizon and a late mineral porphyry dike is also apparent. See text for discussion and Figure 6 for section location.

Fig. 10.

Schematic cross section through Esperanza displaying the intimate relationship between the high-grade copper values and bornite distribution. Truncation of this core zone by the anhydrite horizon and a late mineral porphyry dike is also apparent. See text for discussion and Figure 6 for section location.

Fig. 11.

Graph showing the relationships between copper, gold, silver, and molybdenum and the various hydrothermal alteration associations at Esperanza. Average grades and ranges represent geostatistical averages for > 15,000 samples. Note the positive correlation between copper, gold, and silver and potassic alteration, and association of intermediate argillic and quartz-sericitic stages with lower grades. Also note the positive correlation between molybdenum contents and intermediate argillic ateration and background copper, gold, silver, and molybdenum values in peripheral propylitic alteration. See text for discussion and Figures 8 and 9 for reference.

Fig. 11.

Graph showing the relationships between copper, gold, silver, and molybdenum and the various hydrothermal alteration associations at Esperanza. Average grades and ranges represent geostatistical averages for > 15,000 samples. Note the positive correlation between copper, gold, and silver and potassic alteration, and association of intermediate argillic and quartz-sericitic stages with lower grades. Also note the positive correlation between molybdenum contents and intermediate argillic ateration and background copper, gold, silver, and molybdenum values in peripheral propylitic alteration. See text for discussion and Figures 8 and 9 for reference.

Table 1.

K-Ar Ages from the Esperanza Area (see Fig. 5 for locations)

Sample no.UnitMaterial datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (±2σ)
TELEGKAR02Cerro CasadoAmphibole0.4340.6417037.6 ± 2.5
TES12Cerro CasadoAmphibole0.3960.5795237.2 ± 1.6
S497Artola IgnimbriteBiotite6.8412.8188410.6 ± 1.2
Sample no.UnitMaterial datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (±2σ)
TELEGKAR02Cerro CasadoAmphibole0.4340.6417037.6 ± 2.5
TES12Cerro CasadoAmphibole0.3960.5795237.2 ± 1.6
S497Artola IgnimbriteBiotite6.8412.8188410.6 ± 1.2

Constants: λβ = 4.962 × 10−10y−1; λε = 0.581 × 10−10y−1; 40Ar/36Ar = 295.5; 40K = 0.01167 atom %

Table 2.

40Ar-39Ar Ages from the Esperanza Area (see Fig. 5 for locations)

Sample no.UnitMaterial datedPlateaux age (±2σ)
TELEGKAR05Cerro CasadoAmphibole44.8 ±0.4 (99.3% of gas)
S625EsperanzaBiotite41.3 ±0.3 (100% of gas)
Sample no.UnitMaterial datedPlateaux age (±2σ)
TELEGKAR05Cerro CasadoAmphibole44.8 ±0.4 (99.3% of gas)
S625EsperanzaBiotite41.3 ±0.3 (100% of gas)

Contents

GeoRef

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