GeoScienceWorld
Volume

Andean Metallogeny:

New Discoveries, Concepts, and Updates

Edited by Richard H. Sillitoe, José Perelló and César E. Vidal

Abstract

A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.

Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo

  1. Page 1
    Abstract
    E-mail, aucu@compuserve.com

    A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.

  2. Page 15
    Abstract
    Corresponding author: e-mail, vmaksaev@cec.uchile.cl

    Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40Ar/39Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40Ar/39Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40Ar/39Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district.

    Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40Ar/39Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40Ar/39Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y.

    The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartzsericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40Ar/39Ar).

    The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105° ± 20°C, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40Ar/39Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody.

  3. Page 55
    Abstract
    Corresponding author: e-mail, cdeyell@utas.edu.au

    Epithermal mineralization in the El Indio-Pascua belt, a world-class Au-Ag-Cu district located along the Chile-Argentina frontier, formed in the late Miocene within an actively uplifting tectonic block of the Cordillera Principal. Previous studies have demonstrated a spatial and temporal relationship between epithermal processes and a series of regional erosional surfaces (pediplains) for all the major deposits in the belt. Ore deposition occurred beneath the 12.5 to 14 Ma Azufreras-Torta pediplain but near the head of actively incising 6 to 10 Ma Los Ríos pediment valleys. New isotopic and paragenetic data for several major deposits (Pascua-Lama, Tambo, El Indio) suggest that this geomorphologic setting had a significant impact on the processes that produced preore alteration, ore deposition, and postore processes.

    Barren, preore alteration of late Oligocene to early Miocene age occurs throughout the belt and is well-developed in the vicinity of the major ore deposits. This alteration is of a hypogene advanced argillic type, and isotopic data for alunite and associated sulfide minerals are characteristic of a magmatic-hydrothermal origin. Alunite-pyrite pairs, where available, suggest depositional temperatures of 190° to 350°C. All economic mineralization in the belt is of an epithermal type and is dominated by high-sulfidation—state mineral assemblages. At both Pascua-Lama and Tambo, alunite is intergrown with, or host to, much of the precious metal mineralization. Isotopic data are consistent with a magmatic-hydrothermal origin for much of this alunite, and ore deposition resulted from the boiling of magmatic-dominated fluid at temperatures of ca. 200° to 300°C. A transition from magmatic-hydrothermal to magmatic steam-dominated processes is evident in both systems, and at Tambo this transition is associated with significant Au deposition. Large steam-heated alteration blankets developed above these mineralized centers during this time, although the isotopic signature of this alteration is not typical of most steam-heated zones and is characterized by magmatic-dominated fluid. In contrast to Pascua-Lama and Tambo, alunite at El Indio is restricted to banded alunite sulfide veins that postdate most other mineralized veins in the district. Constraints on mineralization are more difficult to determine for this deposit due to its wide age range (<6.2–7.8 Ma) and different ore styles. A lithologic contact between the Tilito Formation volcanic rocks and overlying andesites of the Escabroso Group most likely enhanced fluid mixing and suppressed widespread boiling of magmatic fluid, although evidence for local boiling in the late, banded alunite sulfide veins is recognized.

    Overall, isotopic studies and other features of these deposits suggest that magmatic-hydrothermal processes and epithermal ore deposition were strongly influenced by pediment erosion and semiarid climatic conditions. The dominance of magmatic condensates for the duration of the major mineralizing systems, even in near-surface alteration zones, is consistent with an arid climate and limited availability of meteoric water. Telescoping of alteration assemblages and the occurrence of supergene alteration to appreciable depths below the surface suggest drops in the paleowater tables during late stages of the hydrothermal systems. At both Tambo and Pascua-Lama, fluid inclusion and mineralogic evidence for fluid boiling during the main stages of mineralization is recognized. Similarly, in both these systems, a transition to magmatic steam-dominated processes also argues for rapid changes in hydrostatic pressures. In the absence of large volcanic edifices and the potential for topographic collapse, erosion at the head of the Los Ríos pediment valleys, where most deposits are located, is thought to have had a major influence on high-level fluid-flow regimes. Rapid draining of the outflow zones by local catastrophic erosion may have promoted boiling in the upflow zones and allowed lateral outflow at lower topographic levels, thereby significantly modifying the nature of these richly mineralized systems.

  4. Page 75
    Abstract
    Corresponding author: e-mail, john.thompson@teckcominco.com

    The Jerónimo sedimentary rock-hosted disseminated Au deposit is located within the Potrerillos district of the Atacama region of northern Chile, east of the Potrerillos porphyry Cu-Mo and El Hueso high-sulfidation Au deposits. Prior to development, the Jerónimo deposit contained a resource of approximately 16.5 million metric tons (Mt) at 6.0 g/t Au. Production began in the oxidized, nonrefractory portion of the deposit in 1997 and terminated in 2002. During that time, approximately 1.5 Mt at 6.8 g/t Au was mined by underground room-and-pillar methods, from which a total of approximately 220,000 oz of Au was recovered by heap-leach cyanidation.

    Jerónimo mineralization occurs as irregular strata-bound lenses within particular bioclastic limestone units of the Jurassic Asientos Formation. The manto-shaped mineralized zone extends over an area of approximately 2.0 by 1.3 km and averages 6 m in thickness. Mineralization and alteration are focused along subvertical fractures and joints within the bioclastic units. Alteration involved decarbonatization followed by the formation of the following assemblages: (1) intense, pervasive, replacement-style silicification; (2) carbonate, mainly restricted to vugs, consisting of Mn carbonate (rhodochrosite and kutnohorite) in the center of the orebody and calcite-dolomite on the margins; and (3) argillization, consisting of illite as widespread disseminations and veinlets and kaolinite as vug fillings in the center of the deposit. Other common alteration minerals include apatite, rutile, monazite, and barite. The ore mineral suite consists of pyrite, arsenopyrite, sphalerite, lead sulfosalts, orpiment, and realgar, with minor coloradoite, altaite, cinnabar, and cassiterite. Gold is present dominantly as submicron-sized grains, ranging from 140 nm to 1.13 μm, that are encapsulated in pyrite, arsenopyrite, quartz, and realgar and also occur within vugs in the silicified matrix.

    Lead isotope results of the main-stage sulfide and sulfosalt minerals (206Pb/204Pb: 18.564–18.644; 207Pb/204Pb: 15.592–15.662; 208Pb/204Pb: 38.536–38.638) indicate that lead in the ore fluids was dominantly from a Tertiary magmatic source, with input from a more radiogenic source—igneous Carboniferous to Triassic basement rocks and/or the overlying Jurassic limestone and sandstone. Carbon and oxygen isotope compositions of ore zone rhodochrosite and kutnohorite, ranging from δ18O of 16.65 to 22.52 per mil (VSMOW) and δ13C of −2.84 to −1.3 per mil (PDB), suggest contributions from both magmatic and Jurassic limestone wall-rock sources.

    The critical features that define the style of mineralization at Jerónimo include lithological and structural control, enrichment in Au-As-Mn-Zn-Pb-Ag-Hg, silicification and carbonate alteration, the presence of native Au grains, the close spatial association with porphyry and related styles of mineralization, and isotopic evidence for a magmatic contribution to metals and hydrothermal fluids. These characteristics are more similar to carbonate-replacement deposits than to typical Carlin-type sediment-hosted Au deposits. Structural and isotopic data suggest that Jerónimo is late Eocene-early Oligocene in age, but the precise temporal and genetic relationships of Jerónimo to other magmatic-hydrothermal systems in the district are unknown.

  5. Page 97
    Abstract
    Corresponding author: e-mail, srive003@codelco.cl

    The porphyry copper systems of the El Salvador region, represented mainly by El Salvador, Potrerillos, Exploradora, Sierra Jardín, and Coya, are located in the southern part of the middle Eocene to early Oligocene porphyry copper belt of northern Chile. They have distinctively higher gold grades (between 0.1 and 0.5 g/t) and slightly lower molybdenum grades (<100 ppm) than the systems located in the northern parts of the belt. The El Salvador and Potrerillos deposits each contains resources of >600 million metric tons (Mt) at >0.6 percent Cu and 0.1 to 0.2 g/t Au. The Exploradora porphyry copper-gold prospect contains geologic resources of ~100 Mt at 0.3 percent Cu and 0.2 g/t Au, with an overlying leached capping enriched in gold, averaging ~0.5 g/t. The Sierra Jardín and Coya prospects show the lowest Cu contents (0.1–0.2 %), but the Au tenor is in the 0.1- to 0.5-g/t range.

    The deposits are related to discrete magmatic pulses emplaced in different lithotectonic environments, including the borders of Paleocene volcanic structures (El Salvador, Sierra Jardín; 45–40 Ma) and dilational jogs and reverse and transfer faults (Exploradora, Potrerillos, and Coya; 37–31 Ma). Host rocks are mainly volcanic and marine sedimentary rocks of Jurassic to Cretaceous age. The most remarkable feature of the deposits is their relationship to multiphase, syntectonic tonalite to granodiorite intrusions. There is strong superimposition (telescoping) of intrusion and alteration-mineralization phases, including early, magnetite-rich, potassic alteration-mineralization, moderately to weakly developed sericitic alteration, and late-stage overprinted advanced argillic alteration. Postmineral phreatomagmatic activity is characterized by pebble dikes and/or diatreme breccias.

    The appreciable number of gold-rich porphyries (as opposed to their gold-poor counterparts) discovered during the past ten years in the Chilean Andes (e.g., La Fortuna, Cerro Casale, Esperanza) implies that geologic conditions that favor their development are more widespread than previously considered. Several key characteristics of gold-rich porphyry copper deposits in the El Salvador region can be used for exploration purposes and open new ground to prospecting.

  6. Page 113
    Abstract
    Corresponding author: e-mail, epithermalau@hotmail.com

    The El Peñón gold-silver deposit comprises six epithermal veins, which contain a geologic resource (measured + indicated + inferred) of 3.8 million oz (Moz) Au and 63 Moz Ag. Three of the veins are currently being mined by underground and open-pit methods. El Peñón is located in the Central Depression of northern Chile, where the geology is dominated by Paleocene and Eocene mafic to felsic volcanic rocks and minor intermediate to felsic subvolcanic rocks and intrusions. The deposit is located in the central portion of the Paleocene metallogenic belt, 165 km southeast of Antofagasta. The property covers an area of ~440 km2, and the ore deposit occurs within an area of approximately 15 km2.

    Late Cretaceous, Paleocene, and Eocene andesitic to rhyolitic flows, domes, tuffs, and minor intrusive rocks characterize the geology of the district. The deposit occurs within flat-lying to gently dipping, andesitic to rhyolitic pyroclastic and flow units, and volcaniclastic breccias of Paleocene and early Eocene age and is partly assignable to the Augusta Victoria Formation. El Peñón veins are partly hosted by, and spatially associated with, a 54 to 55 Ma (40Ar-39Ar, U-Pb) rhyolite dome complex that occurs over an area at least 18 km2, and similar rhyolite lavas occur over an area of tens of kilometers2.

    Rocks in the district display two distinct types of hydrothermal alteration: widespread alteration associated with near-neutral pH, reduced fluid and localized alteration associated with acidic pH, oxidized fluid. Near-neutral pH, reduced fluid produced widespread replacement of phenocrysts and groundmass by quartz, adularia, albite, illite, chlorite, smectite, calcite, and pyrite; quartz-adularia flooding and cement to hydrothermal breccia intensify in the vicinity of veins. Where upflow of these fluids was focused along dominantly north- and northeast-trending structures, Au-Ag ± base metal-bearing crustiform quartz ± adularia ± carbonate veins formed, including the six veins that comprise the El Peñón deposit and several outlying prospects. Adularia from the two largest veins has been dated at 52 to 53 Ma (40Ar-39Ar), indicating formation 1 to 3 m.y. later than the host rhyolite domes.

    Acidic pH, oxidized fluid produced lithocaps of massive quartz-alunite alteration, quartz-alunite cemented breccia, and, locally, weak Cu mineralization above inferred Late Cretaceous and Eocene intrusions. Isolated occurrences of quartz-alunite alteration covering hundreds of meters2 are located at the periphery of the property, in addition to several larger areas beyond it. Locally, quartz-barite veins occur peripheral to quartz-alunite alteration and contain variable amounts of base metals and Ag with little or no Au.

    The veins that comprise the El Peñón deposit range from <50 cm to 22 m wide. Pervasive supergene oxidation extends to 400 m below surface. Limited drill intercepts at deeper levels consist of banded and brecciated quartz, adularia, and massive, bladed, and acicular, Ca-, Fe-, Mn-, and Mg-bearing carbonate minerals, with minor amounts of pyrite, chalcopyrite, sphalerite, and galena. Veins exhibit a wide range of crustiform textures, including comb, colloform, and lattice quartz, rhombic adularia, and abundant massive and bladed Fe and Mn oxide minerals. Recrystallization textures suggest amorphous silica and chalcedony precursors for some quartz. Coexisting liquid- and vapor-rich inclusions, lattice textures, and vein adularia are evidence for boiling conditions that were likely responsible for Au-Ag deposition. Ore minerals observed in oxidized veins consist of electrum (mostly 40–60 wt % Au), acanthite, gold, silver, silver sulfosalts, silver halides, and rarely pyrite, chalcopyrite, and galena. High ore grades are generally associated with massive bands of fine-grained quartz and adularia, breccias composed of vein material in a matrix of fine-grained quartz and adularia, and, less commonly, colloform quartz bands. Supergene processes resulted in local remobilization of Au and Ag, leaving nearly pure gold (up to 98 wt % Au) along fractures and associated with oxide masses.

    Fluid inclusion data from the El Peñón deposit indicate vein formation from low-salinity (<2 wt % NaCl equiv), boiling hydrothermal fluid at temperatures mostly from 230° to 260°C. Fluid inclusion data from other mineralized quartz veins in the district indicate formation from commonly boiling, dilute fluid (<3 wt % NaCl equiv) at temperatures between 180° and 330°C. Quartz-barite veins peripheral to quartz-alunite alteration formed from boiling fluid between 175° and 225°C with apparent salinities of 1 to 6 wt percent NaCl equiv.

    Geochemical gradients in altered rocks surrounding veins in the El Peñón district indicate that Au, Ag, As, and Sb concentrations increase toward quartz veins, and Au, Ag, As, Sb, and base metal concentrations increase toward quartz-barite veins. Geochemical analyses of altered rocks from drill holes surrounding the Quebrada Colorada vein, the highest grade vein of the El Peñón deposit, show enrichment of Au and Ag and depletion of Ca, Na, and Sr toward the vein. Comparing the geochemical data to elevation, the highest mean values for Au (205 ppb) and Ag (5.9 ppm) occur at mid levels, the highest mean values for Pb (35 ppm) and Zn (183 ppm) occur at lower levels, and the mean values for As (139 ppm) and Sb (34 ppm) increase with elevation. Arsenic (100s of ppm) and Sb (10s of ppm) anomalies occur in rocks above the Quebrada Colorada vein that are barren or contain only low levels of Au and Ag; such anomalies may be useful indicators of blind mineralization.

    Epithermal deposits in the Paleocene belt of northern Chile are preserved in rocks located at a long-lived convergent plate boundary with a complex history of compression and extension that has formed linear morphotectonic and metallogenic belts parallel to the plate boundary. In northern Chile, Paleocene and early Eocene epithermal deposits occur west of the uplifted Cordillera Domeyko, which contains younger porphyry Cu deposits and intrusions characteristic of deeper environments. This paradoxical situation is partly explained by a protracted history of structurally controlled basins in the Paleocene belt of northern Chile. From Late Cretaceous to Eocene time, the Paleocene belt was characterized by fault-bounded basins that experienced both subsidence and inversion; however, cumulative postmineral uplift was minimal because fluid inclusion data from El Peñón indicate erosion of only several hundreds of meters. Since the Miocene, erosion has largely ceased due to the onset of hyperaridity.

  7. Page 141
    Abstract
    Corresponding author: e-mail, padillaruben@hotmail.com

    At Escondida, the propylitic, potassic, and quartz-sericitic hydrothermal mineral assemblages are centered on a 38 Ma granodioritic stock composed of at least four phases. These early intrusive and related hydrothermal events were followed by rhyolitic intrusive and extrusive rocks and sulfide-rich veins of two advanced argillic alteration events at ~ 36 and ~34 Ma. The final hypogene copper grade of the deposit varies between 0.2 and 1.0 percent, reflecting varying degrees of overprinting by the successive copper mineralization stages.

    The homogenization temperatures of primary fluid inclusions from quartz-orthoclase- (stage A), quartzsericite- (stage B), and quartz-alunite-bearing veins (stage C) vary between 500° and 560°, 280° and 380°, and 200° and 340°C, respectively, with estimated depths of trapping ranging from 1.5 to 3.0 km.

    The δ34S values of sulfide minerals from the potassic assemblage range from −3.2 to −2.0 per mil, whereas values from the quartz-sericite assemblage vary between −1.1 and +0.6 per mil. These values suggest a common source of magmatic sulfur and indicate that sulfide minerals of stage B were not formed by leaching sulfide from stage A. The δ34S values of stage C sulfide minerals range from −2.5 to +2.7 per mil, which suggests that sulfide for the advanced argillic event may have been derived, in part, by leaching sulfide from the two earlier hydrothermal stages. Calculated values of δD and δ18O for water coexisting with igneous and hydrothermal minerals indicate a dominantly magmatic component in stages A and B and a mixture with meteoric water at the lower temperatures of stage B and during the advanced argillic event (stage C). Stage C may have formed by circulatión of surficial meteoric water heated by the 34 to 36 Ma subvolcanic rhyolitic rocks. It is also possible that some 2 m.y. after the emplacement, uplift, and denudatión of the main Escondida porphyry system, a new porphyry copper stock was emplaced in the same structural weakness zone, and that the subvolcanic rhyolite and advanced argillic alteration and associated mineralization represent the upper levels of this younger porphyry system.

  8. Page 167
    Abstract
    Corresponding author: e-mail, jperello@aminerals.cl

    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.

  9. Page 187
    Abstract
    Corresponding author: e-mail, rmora@aminerals.cl

    The El Tesoro exotic copper deposit is located approximately 70 km south of Chuquicamata, northern Chile. Prior to the involvement of Anaconda Chile S.A. (Anaconda) in 1990, the area was known historically as a group of small copper occurrences, where small-scale mining had taken place intermittently since 1886. Between 1980 and 1990, exploration programs undertaken by several companies, including Anaconda, targeted the source of the exotic copper mineralization. In 1990, a renewed exploration effort by Anaconda, originally designed to outline a small oxide copper resource around the old workings at El Tesoro, was quickly expanded after intersecting significant ore-grade mineralization in the first two drill holes. Follow-up exploration and delineation, carried out in several stages between 1990 and 1995, included 54,500 m of reverse-circulation drilling at El Tesoro. Discovery of 206 million metric tons (Mt) of 0.86 percent Cu was the direct result of geologic mapping and modeling, strongly supported by company management. Mining of the El Tesoro deposit by Compaúía Minera El Tesoro (61% Antofagasta Minerals S.A., 39% Compañía Contractual Minera Leonor) commenced in November 2001, with current production amounting to ~85,000 t of copper cathode per year. With the present mining schedule of 25,000 t/d, mine life is estimated to be 19 yrs.

    Regional geology is characterized by a late Paleozoic intrusive and volcanic basement and several sedimentary and volcanic sequences of Mesozoic age, which are locally intruded by Cretaceous granitoids. An early Tertiary volcanic sequence unconformably overlies the Mesozoic units. A series of flow-dome complexes of middle to late Eocene age is the youngest bedrock component. Local cover comprises a thick sequence of moderately consolidated gravel of middle to late Tertiary age, which is blanketed by a 10-Ma ignimbrite. A locally important north-northeast-trending fault zone, part of the regionally extensive Domeyko fault system, transects the district and offsets all units, including a post-10-Ma pediment surface. These faults display both strike-slip and reverse motion and were accompanied by minor folding of the gravel sequence.

    The El Tesoro deposit is completely contained in the Tertiary gravel sequence. It is composed of two main, northeast-trending, northwest-dipping mantos with a maximum thickness of 150 m and an areal extension of 2 × 2.5 km. The host gravel is dominated by crudely stratified, pebbly to blocky horizons interbedded with coarse-grained sand and silt, some of which are calcareous. Clast composition is varied and allows definition of two main units, namely Gravas I and Gravas IL The basal Gravas I sequence includes several subunits of which the Gravas de Pórfido is the main host to the exotic copper mineralization. The Gravas II is essentially postmineralization in timing. Mineralization is dominated by paratacamite and atacamite, accompanied by subordinate chrysocolla and copper wad. These mineral(oid)s occur as cement to the gravel, the clasts of which were not mineralized, although very locally they contain encapsulated chalcocite and copper oxide remnants. Mineralization is clearly controlled by original host porosity and permeability, with silty horizons generally being barren. A copper-depleted, hematitic ferricrete horizon that is locally interbedded with gravels in stratigraphically higher parts of the El Tesoro deposit correlates laterally with nearby exposures of Gravas Rojas. Broad ore mineral zoning comprises a central zone of paratacamite and atacamite concentrated in upper horizons and a zone of chrysocolla that dominates the lower parts of the deposit. Copper wad is common and is irregularly distributed, although it generally tends to occur along the fringes of individual mineralized lenses.

    El Tesoro is a classic exotic copper deposit formed by lateral migratión of copper-charged solutions and subsequent deposition of copper species in lithologically favorable alluvial gravel horizons. Its dominant mineralogy and mineralogic zoning, as well as the nature of the host gravel, suggest that copper precipitation took place in a distal environment, located perhaps several kilometers from a porphyry-related chalcocite enrichment source area.

  10. Page 199
    Abstract
    Corresponding author: e-mail, srive003@codelco.cl

    The Toki porphyry copper deposit is located in the Chuquicamata district, northern Chile, approximately 15 km southwest of the Chuquicamata mine. It is the newest giant copper deposit discovered in Chile and one of the most significant base metal exploration discoveries worldwide in the last 10 years. Inferred resources at Toki are >2,400 million metric tons (Mt) at ~0.5 percent Cu, one-third of it oxide and mixed oxide-sulfide mineralization.

    The discovery was made by Codelco-Chile in 2000, as a result of a systematic exploration program begun in 1998 in the Chuquicamata district. During that year, all available geologic information was analyzed, a simple district geologic model was assembled, and exploration targets were conceptually evaluated and prioritized. The program was guided by the belief that remaining potential in the district was restricted to covered zones, and the West Fissure paradigm needed to be changed. One of the highest ranked targets was the Pampa Genoveva covered area, where inspection of core from holes drilled by the Chuquicamata division at the beginning of the 1990s, some 3 km north of the eventual Toki discovery, showed subtle but encouraging copper geochemistry and alteration features. An exploration model was generated and quickly tested by drilling reverse- circulation scout holes on a 1-km grid. These holes revealed evidence of porphyry-style alteration-mineralization and high copper contents in two sectors. They were followed-up by more closely spaced holes, resulting in the discovery of the Genoveva deposit by the end of 1999 and the Toki deposit at the end of 2000. Together with Opache, discovered in 1996, the deposits comprise the Toki cluster.

    The geologic setting of the Toki deposit consists of Paleozoic schist and Permian to Triassic volcanic rocks covered by Jurassic to Early Cretaceous marine and terrestrial sedimentary rocks, intruded by Eocene stocks. The older stratified units are folded and faulted, following a dominant north-northeast structural trend. The deposit is completely concealed by 120 to 200 m of Miocene gravel and lacustrine sediment. Plan dimensions of the deposit are 2.5 × 1 km. The tested mineralization has a vertical extent of ~800 m, one-half of which is oxide, mixed oxide-sulfide, and supergene enriched sulfide mineralization, with the other half being a hypogene copper-bearing sulfide zone. The alteration and mineralization events are related to emplacement of a multiphase tonalite porphyry complex hosted by a Permian to Triassic bimodal volcanic sequence and late Eocene equigranular intrusive rocks. Hypogene mineralization is zoned laterally from a core of bornite (-digenite) and chalcopyrite to a zone of chalcopyrite and pyrite. Alteration displays a centrally located, pervasive potassic assemblage related to moderate to intense quartz-K-feldspar veins, grading outward to propylitization. Biotite from the potassic zone yields a K-Ar age of 37.3 ±1.3 Ma. A series of conspicuous northwest-trending D-type veins associated with sericite alteration crosscuts the earlier hypogene sulfide zoning pattern and represents a late hydrothermal event. Sericite from a D vein halo yields a 40Ar-39Ar age of 34.52 ± 0.20 Ma. The supergene profile consists of an upper 100- to 160-m-thick oxide zone (malachite, chrysocolla, atacamite, wad), locally interrupted by leached D veins and parallel faults. This is followed below by irregular, structurally controlled, mixed oxide-sulfide (atacamite-chalcocite), and weakly to moderately enriched chalcocite-covellite zones, between 50 and 200 m thick. Postmineral, northwest-trending faults interrupt the continuity of the mineralized body, which is tilted to the south. At the district scale, the mineralizing events that generated the Toki deposit at ~37 Ma preceded emplacement of the giant Chuquicamata porphyry copper complex, dated between 35 and 31 Ma. Evidence from Toki confirms that this world-class copper district is characterized by several discrete pulses of intrusion and mineralization.

  11. Page 213
    Abstract
    Corresponding author: e-mail, jperello@aminerals.cl

    The Cotabambas porphyry copper-gold cluster includes at least four porphyry centers in an area of 5 × 3 km, namely Ccalla, Azulccacca, Huaclle, and Ccarayoc, of which the first two are the best known. The geology of the area is dominated by two large granodiorite and diorite plutons and smaller stocks and dikes of microdiorite and andesite, all of which form part of the middle Eocene to early Oligocene Andahuaylas-Yauri batholith. At Cotabambas, granodiorite (K-Ar age of 39.8 ± 1.5 Ma) intrudes diorite (K-Ar age of 43.2 ± 1.1 Ma) and both, in turn, are intruded by a series of composite, structurally controlled porphyry stocks and dikes of granodioritic to quartz monzodioritic composition associated with porphyry copper-gold mineralization. The main structural features include several intersecting north-northeast- and west-northwest-trending faults, which are interpreted to have controlled the emplacement and architecture of the porphyry centers.

    Most of the copper-gold mineralization at Cotabambas is associated with early-stage potassic alteration, including multiphase, magnetite-rich stockworks with quartz, K-feldspar, biotite, chalcopyrite, and bornite. Apatite and anhydrite are common constituents. An early, copper-bearing potassic-calcic assemblage made up of quartz, K-feldspar, biotite, actinolite, hornblende, diopside-hedenbergite, and magnetite is also preserved locally. Early, biotite-rich alteration from Ccalla yielded a K-Ar age of 35.7 ± 0.9 Ma. These mineral associations are overprinted by greenish-colored, intermediate argillic assemblages dominated by quartz, chlorite, illite, smectite, halloysite, and greenish soapy sericite, which have partially to completely destroyed earlier formed chalcopyrite-bornite associations but have contributed pyrite as disseminated grains and veinlets. These associations are spatially related to a series of inter- to late mineral porphyries.

    All the previously described rocks and alteration assemblages are cut by a large, late mineral, domelike body of dacitic composition and an associated dike swarm. Both dome and dikes developed incipient alteration to calcite, illite, and chlorite and host a set of centimeter-wide veins with open spaces filled by quartz, calcite, sphalerite, and galena.

    Supergene mineralization is present as patchily distributed copper oxides near surface underlain by an irregular supergene enrichment blanket dominated by sooty chalcocite. Supergene kaolinite and alunite are common. A K-Ar age of 3.3 ± 0.2 Ma on alunite shows a late Pliocene age for supergene enrichment and leaching processes in the region.

    The mineralization at Cotabambas is assigned to the porphyry copper-gold class because (1) gold grades are typically >0.3 ppm; (2) copper-gold mineralization is accompanied by abundant (>5 vol %) hydrothermal magnetite in potassic alteration; (3) hydrothermal amphibole and pyroxene are present in potassic-calcic alteration; (4) copper and gold display a sympathetic relationship, and all observed Au occurs as micron-sized inclusions in chalcopyrite; (5) intense pyrite-rich intermediate argillic alteration overprints earlier formed potassic and calcic-potassic alteration and associated copper-gold mineralization; and (6) molybdenum contents are low (<0.01%).

    The coincident K-Ar and fission-track (apatite) ages (38.6 ± 3.4 and 33.3 ±1.4 Ma) for plutons from the Cotabambas area, together with the late Eocene age of Cotabambas (see above) and regional data, confirm that porphyry copper emplacement in the Andahuaylas-Yauri belt of southeastern Peru took place simultaneously with intense shortening, surface uplift, and rapid exhumation during the middle to late Eocene Incaic orogeny.

  12. Page 231
    Abstract
    Corresponding author: e-mail, cesarvid@buenaventura.com.pe

    The Marcapunta enargite-Au deposits are located in the center of the Colquijirca mining district, 310 km northeast of Lima and 10 km south of the Cerro de Pasco mine. The regional geology comprises folded Permo-Triassic red-bed deposits of the Mitu Group succeeded by Pucará Group limestone and dolomite of Triassic to Jurassic age, which are overlain by carbonate breccia, conglomerate, and fresh-water limestone of the Eocene Calera Formation. These units are intruded and overlain by dacitic domes and pyroclastic rocks of the Marcapunta volcanic center. The north-trending Longitudinal fault controlled basin morphology during both the Pucará and Calera Formation sedimentation as well as emplacement of the Cerro de Pasco and Marcapunta Miocene volcanoes.

    The Marcapunta Cu-As-Au deposits are zoned symmetrically northward into the Zn-Pb-Ag ores of Colquijirca and southwestward into the Zn-Pb-Ag San Gregorio deposit. The Miocene volcanic center at Marcapunta is intensely altered to advanced argillic alteration assemblages in the form of quartz-alunite ledges with argillic halos. Alunite mineral separates have been dated at 11.6 ± 0.1 Ma by K-Ar and 10.6 ± 0.1 Ma by 40Ar/39Ar methods. The main silicification and quartz-alunite alteration are controlled by several prominent east-west fractures and attain thicknesses from a few centimeters to ~50 m. Mineral assemblages are zoned outward from a central zone of vuggy quartz to quartz-alunite ± dickite, illite-kaolinite ± montmorillonite and external chlorite-calcite envelopes. Copper mineralization surrounds an interpreted subsurface diatreme vent and flares outward along the base of the dacitic domes comprising the Marcapunta volcanic center. Semimassive to massive quartz-pyrite bodies preferentially replaced limestone breccia and conglomerate of the Calera Formation and are sandwiched between the underlying Mitu Group sandstone and the overlying lava domes.

    The northern, western, and southwestern flanks of the Marcapunta volcanic center are characterized by a recently determined and drill-tested, crescent-shaped gravimetric high. Ore zones attain thicknesses as much as 100 m adjacent to the steep diatreme walls and thin laterally into discrete strata-bound manto and breccia horizons. Ore mineralogy is dominated by enargite, covellite, native gold, and several precious metal-bearing telluride phases. Hypogene chalcocite and digenite occur in a discrete lower manto beneath the enargite zone of western Marcapunta. Gold appears to be concentrated in the southwestern parts of the replacement bodies. Cu/As ratios increase from a homogeneous value of 3/1 in the Smelter area, immediately north of the volcano, to variable values of 4 to 40/1 in the western Marcapunta enargite and digenite-chalcocite mantos.

  13. Page 243
    Abstract
    Corresponding author: e-mail, ulrichp@aol.com

    Although known since at least 1897, Uchucchacua was first explored on a major scale by Compaúía de Minas Buenaventura since 1960. Narrow vein mining started in 1975, but orebodies discovered at depth enabled expansion to today’s 2,000-t/d operation, transforming “Chacua” into the largest primary silver producer in South America.

    The ores occur in fractures and faults, as well as in pipes, irregular replacement bodies, and mantos hosted by Late Cretaceous limestone. Porphyritic dacite bodies are probably pre-, syn-, and postore. Most of the ore occurs in distal manganiferous exoskarn and limestone and is mineralogically diverse, consisting mostly of the following.

    View this table:

    The grade of the ore mined varies between 16 and 20 oz/t Ag combined with about 10 percent Mn, 1.5 percent Zn, and 0.9 percent Pb. Between 75 and 80 percent of the reserves are high in silver and manganese, whereas about 7 percent contain high zinc and lead grades with only moderate silver and low manganese.

    Logarithmic-grade graphs show very good positive linear correlations for zinc versus lead, moderate correlations for silver versus manganese, and arcuate correlation bands for silver or manganese versus zinc or lead. These relationships indicate that the outward zoning sequence is from lead-zinc to silver-manganese or vice versa. The corresponding longitudinal vein sections can generally be contoured unambiguously, showing that the bands of highest grades of lead and zinc coincide very well. The highest silver grades can be contoured convincingly as a band that is zoned outward and/or at a higher elevation than the lead and zinc bands. However, the manganese grades often require two high-grade bands: a main band that mostly coincides with the highest silver grades and a thinner upper band that may represent near-surface manganese enrichment.

    Ore intervals in individual veins, pipes, and replacement bodies are up to 200 m in vertical extent. However, the elevations of these intervals change progressively, reflecting the overall geometry of the hydrothermal cell (or cells) responsible for the mineralization. In addition, postore faulting has displaced the ore intervals. As a result, ore has been found to date over a vertical interval of 600 m, between 4,730 and 4,040 m.

    At surface, manganese oxide stains in the host limestone and limonite in fractures and faults indicate proximity to ore. Underground, multiple calcite veinlets constitute a guide to nearby orebodies. Geochemical anomalies of 60 to 80 ppm Ag have been documented up to 15 m from an orebody. By extrapolation, 10 ppm Ag anomalies may extend 25 m from ore, and 1 ppm Ag anomalies may attain 40 to 45 m. Ore continues to be found at depth as well as laterally and between known ore zones.

  14. Page 259
    Abstract
    E-mail: stewart@goldquestcorp.com

    Antamina, the world’s largest copper-zinc skarn deposit, entered production in 2001. This paper describes the development of the geologic model for the feasibility study (1996–1998). Antamina is located in the eastern part of the Western Cordillera of northern Peru at latitude 9° 32′ S and longitude 77° 03′ W and 4,200 to 4,800 m in elevation.

    Antamina has a long history of exploration and is a case study of successful creation of an orebody from a mineral resource. While small-scale mining is recorded intermittently since 1860, the first serious exploration was not begun until a century later by Cerro de Pasco Corporation (1952–1970), followed by a Minero Peru-Geomin (Romania) partnership, which conducted a feasibility study (1970–1976) with a reserve of 128.6 million metric tons (Mt) at 1.6 percent Cu and 1.3 percent Zn.

    Privatization of the project was won by Compañía Minera Antamina in 1996. This consortium undertook a major exploration program and completed a full feasibility study in 1998 that defined a minable, open-pittable resource of 500 Mt at 1.2 percent Cu, 1.0 percent Zn, 0.03 percent Mo, and 12 g/t Ag within a global resource of 1,500 Mt. Production is by open pit and flotation at 70,000 t/d, producing 270,000 t of copper and 162,000 t of zinc in concentrates per year. This makes Antamina the seventh largest copper and the third largest zinc mine in the world.

    Antamina is located in the polymetallic belt of central Peru, which comprises copper, zinc, silver, lead and gold deposits related to mid to late Miocene calc-alkaline stocks. The regional geologic setting comprises Late Jurassic to Late Cretaceous siliciclastic to carbonate sequences in a northwest-trending foreland fold-thrust belt of mid-Eocene age, the Incaic II deformation phase. Antamina is hosted by calcareous siltstone and mudstone of the Late Cretaceous Upper Celendin Formation. Skarn mineralization forms a shell over and around a quartz monzonite porphyry stock of late Miocene age, which itself hosts subeconomic porphyry copper-molybdenum mineralization. The skarn body is approximately 2,500 m long in a northeasterly direction and up to 1,000 m wide, with a known vertical extent of 1,000 m. The skarn consists mainly of andraditic garnet. It is symmetrically zoned around the intrusion from proximal brown garnet endoskarn and exoskarn outward to green garnet exoskarn, with peripheral wollastonite-diopside exoskarn. Significant copper mineralization is hosted by endoskarn. Retrograde chlorite skarn and hydrothermal breccia are minor.

    Metals are zoned laterally from a central copper-only zone to a peripheral copper-zinc zone. Chalcopyrite is distributed throughout all skarn zones. Appearance of sphalerite approximately coincides with the transition from brown to green garnet. The copper-zinc zone thins at depth and originally appears to have closed over the top of the intrusion, although most of it has been eroded. The main copper mineral in the wollastonite-diop-side skarn is bornite, and this zone also has elevated gold values. Silver, lead, and bismuth values are highest in the outer part of the copper-zinc zone and adjacent marble. Molybdenite occurs in the intrusion and adjacent skarn, as well as being abundant in the wollastonite-diopside skarn. Sulfides were deposited during the late prograde and retrograde phases and occur disseminated interstitial to garnet; as irregular massive sulfide zones; and as veinlets. The deposit was unroofed by glaciation and is exposed in a glacial valley; hence there is no significant oxidation or enrichment.

    Antamina is an oxidized calcic copper skarn related to a calc-alkaline quartz monzonite porphyry stock containing subeconomic porphyry copper-molybdenum mineralization. The outer zinc zone is unusually well developed. Features that appear to have contributed to Antamina’s world-class status include a possible mantle origin of the intrusions, the basin-margin setting of the host sedimentary rocks, favorable structural preparation, limited retrograde alteration, and partial preservation of the intrusion roof zone.

  15. Page 279
    Abstract
    Corresponding author: e-mail, lgustaf685@aol.com

    At least 14 porphyry copper-gold deposits and 19 epithermal gold deposits are known within 60 km of Cajamarca. The partly explored porphyry deposits vary in grade, Cu-Au-Mo proportions, and depth of erosion. Associated epithermal mineralization occurs at Perol, Peña de las Águilas, Kupfertal, Yanacocha Norte, Maqui Maqui, and Pampa Verde but not at Michiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sorpresa, Colpayoc, and Chamis. These deposits are associated with Miocene magmatic activity, northwest-trending folds and thrusts, and northeast-trending faults.

    In the porphyry deposits, granular A quartz veins, associated with K-feldspar-biotite alteration and disseminated chalcopyrite-magnetite with bornite or pyrite, are typically present within and about multiple coeval porphyry intrusions. Banded quartz veins occur near the tops of some shallowly eroded systems, and late sericite-pyrite ± chalcopyrite is superimposed on most. Epithermal mineralization is mostly of high-sulfidation character, with pyrite-enargite-covellite typically underlying oxide Au zones leached of Cu. Epithermal Au-Cu is associated with multiple stages of brecciation and intense silicification, zoned outward and downward with decreasing SiO2 and Au through quartz-pyrophyllite-diaspore-alunite-dickite to quartz-alunite and kaolinite. Structurally controlled, high-grade Au is apparently late and associated locally with intermediate-sulfidation assemblages, barite, and chalcedony.

    The transition between porphyry and epithermal environments is exposed at Perol and Huaylamachay, La Zanja, and especially Tantahuatay and Yanacocha. At Perol and Huaylamachay, porphyry gold-copper deposits are adjacent to generally contemporaneous volcanic vents altered to quartz-alunite with minor structures containing quartz-pyrophyllite-alunite-Au. At Perol, the dacitic vent is intruded by a later mineralized porphyry, whereas at Huaylamachay the vent breccia contains clasts with quartz-molybdenite veins and is cut by banded quartz veins, which we interpret as indicating a second, deeper porphyry Au system.

    At Tantahuatay, an andesitic dome complex is pervasively brecciated and altered to quartz-alunite-pyrophyllite-diaspore ± dickite, with extensive pyrite-enargite-covellite-(bornite) veins and disseminations beneath Aurich oxide mineralization. A gusano texture of soft, round patches of pyrophyllite-diaspore and/or alunite in a silicified matrix is widespread and associated with anomalous concentrations of Mo. Only one of several drill holes to 600-m depth encountered A quartz veins and minor porphyry intrusions. This hole provides evidence for prograde advance of quartz veining associated with one or more porphyry intrusions into the epithermal environment and subsequent retrograde collapse.

    At Yanacocha, the most abundant evidence of direct, albeit complex, spatial and temporal relationships between multiple centers of epithermal mineralization and porphyry intrusion and mineralization has been partially deciphered. At Kupfertal, the matrix of gusano alteration above the top of the porphyry becomes increasingly silicified and patchy downward, developing very contorted wormy quartz veins that overlap the top of A quartz veins. Intense pyritic quartz-pyrophyllite-diaspore-alunite and underlying sericite alteration is superimposed on K-feldspar-biotite alteration of the early stage. Fluid inclusions in quartz are vapor dominant, with downward-increasing proportions of high-salinity inclusions and amounts of minute relict chalcopyrite ± bornite grains “locked” in A vein quartz. A-veined and advanced argillic-altered xenoliths in pyroclastic rocks intruded by porphyries and hosting gold mineralization demonstrate multiple generations of porphyry and epithermal mineralization. Early Cu and Au of the porphyry event appear to have been remobilized and incorporated into the overlying epithermal system.

  16. Page 301
    Abstract
    Corresponding author: e-mail, Patrick.Mallette@Newmont.com

    With premining reserves plus resources of 10.2 million oz (Moz) of gold, 424 million metric tons (Mt) at 0.75 g/t, La Quinua is one of several world-class gold deposits that comprise the Yanacocha mining district in the Cajamarca province, northern Peru. Unlike volcanic-hosted high-sulfidation gold deposits that characterize much of the Yanacocha district, La Quinua is hosted by unconsolidated gravel. In 1996, Newmont and Minera Yanacocha geologists discovered gold-bearing gravel while drilling an alluvial basin to test “blind” basement targets. Exploration and development drilling proceeded rapidly; 1 yr after initial discovery the reserve plus resource stood at 7 Moz. Production began in mid-2001.

    At La Quinua, there is infilling of coarse clastic sediments in two structural basins situated along the western flank of the Yanacocha Sur and Yanacocha Oeste gold deposits. Gravel fans reach a maximum thickness of 350 m on the downthrown side of the basin-bounding La Quinua fault. Sediments fine down gradient from chaotic boulder gravel in proximal facies to gravelly silt and sand in distal facies. Bedding becomes more pronounced down gradient with a decrease in bed thickness. Five deposit-scale stratigraphic units are recognized. These include, from bottom to top, (1) regolith directly overlying basement rocks, (2) high-to low-energy deposits of clay-and alunite-bearing sand and gravel, (3) low-energy deposits of organic-rich mud, peat, and bog iron, (4) ferruginous gravel, and (5) high-energy deposits of pebble-cobble-boulder gravel grading distally to fine-grained sandey silt.

    La Quinua gold was derived from erosion, transportation, and deposition of gold particles and mineralized clasts from the Yanacocha Sur and Yanacocha Oeste deposits. However, a portion of the gold may have resulted from chemical mobilization and reprecipitation. Gold particles are mostly micron sized, liberated within mud matrix, and disseminated within mineralized clasts, although liberated particles to 0.2 mm have been observed. Gold is disseminated throughout the deposit with only gradual lateral and vertical grade transitions. Placer “paystreaks” have not been encountered. The La Quinua gold-trapping system was efficient; grade dilution from source deposits is <25 percent. The Ag/Au ratio is 6/1, compared to >10/1 for the Yanacocha Sur and Yanacocha Oeste deposits. Copper and iron are locally enriched in specific stratigraphic horizons. Copper values up to a few percent are associated with detrital and authigenic minerals. Copper mineralization is erratic and not considered economic and may present future ore-processing challenges. Authigenic iron is present as bog-iron lenses, ferricrete cement, and gravel matrix impregnation.

    La Quinua formed in response to dynamic interaction of climate and tectonics. A subsiding tectonic basin preserved gold-bearing sediments during periods of intense mechanical weathering in the adjacent highlands. Cold alpine climatic conditions resulted in two pulses of rapid sedimentation and basin infill. Temperate conditions resulted in diastems marked by organic accumulation and surficial iron deposits. La Quinua is characterized by a paucity of channel deposits, lack of coarse-grained placer gold, and preservation of fine-grained gold. Structural offset and warping of sedimentary units indicates that basinal tectonism continued after deposition of the gravel sequence.

    Gold production from La Quinua will exceed 1 Moz/yr during 5 yrs of an 8-yr mine life, with a peak of nearly 2.5 Moz predicted in 2006. Light blasting improves mining efficiency. Oxide ore is belt agglomerated prior to placement on the leach pad. The majority of gold is recovered in carbon columns with a smaller portion recovered in a Merrill-Crowe facility. Mine equipment includes Hitachi 5500EX shovels, Cat 992 loaders, and Cat 785 and 793 haul trucks.

  17. Page 313
    Abstract
    Corresponding author: e-mail, dcn@kori.reno.nv.us

    A 40Ar/39Ar age of 16.94 ± 0.34 Ma on hypogene alunite from the La Virgen sedimentary and volcanic rockhosted Au deposit and new ages of 16.06 ± 0.11 to 15.58 ± 0.12 Ma on porphyry Cu-Au deposits at Minas Conga confirm the existence of early Miocene and earliest middle Miocene mineralization in the eastern part of the Miocene metallogenic belt of northern Peru. A new K-Ar age of 15.3 ± 0.3 Ma for the Magistral porphyry-skarn Cu-Mo deposit extends the belt of known early and middle Miocene deposits southeast of La Virgen. A 40Ar/39Ar age of 20.02 ± 0.15 Ma is reported for a late intrusive phase at the Michiquillay porphyry Cu deposit.

    A 40Ar/39Ar plateau age of 15.61 ± 0.12 Ma on alunite from the San Pedro Sur zone at the La Zanja epithermal Au district is consistent with the location of the district within the middle Miocene Quiruvilca-Pierina subbelt of the Miocene metallogenic belt. 40Ar/39Ar mineralization ages of 20.79 ± 0.10 and 21.78 ± 0.11 Ma have been obtained on mineral deposits of the Malvas and Huinac districts in the Cordillera Negra to the south. Together with a published age of 18.7 ± 0.6 Ma on the Churropampa Au prospect in the eastern part of the Cordillera Negra and a new Re-Os age of 18.15 ± 0.06 Ma on the Pachagón porphyry Cu-Ag prospect northeast of Trujillo, an early Miocene mineral activity in the western part of the metallogenic belt is indicated. The new ages show that clearly magmatically related mineralization was formed at the same latitudes in both the eastern and western parts of the Miocene metallogenic belt during early Miocene and earliest middle Miocene times.

    The deposits of the giant Yanacocha Au district, which were formed at about 11 Ma, are younger than deposits to the west and to the east. Deposits of the Hualgayoc district to the north, which have yielded mineralization ages from 12.4 ± 0.4 to 14.4 ± 0.2 Ma, may overlap in time with those of Yanacocha. These districts, which are located within large coeval volcanic fields, could reflect a younger episode of magmatic activity of late middle Miocene age.

  18. Page 319
    Abstract
    Corresonding author: e-mail, lwinter@eos.ubc.ca

    The Cretaceous Tambogrande volcanogenic massive sulfide (VMS) deposits of northwestern Peru represent some of the largest Cu-Zn-Au-Ag bimodal-mafic VMS deposits in the world. There are currently three known deposits each with approximately 100 million metric tons (Mt) of massive pyrite-rich sulfide. The deposits are intimately associated with dacite lava dome complexes and were deposited within steep-sided basins on the sea floor. Reconstructed sea-floor paleogeomorphic models suggest that sulfide deposition was concentrated in the deepest parts of the basins. Sulfide deposition accompanied synvolcanic faulting and episodic dacitic and basaltic eruptions. A series of time-stratigraphic horizons are defined at the TG1 and TG3 deposits and mark stages in the development of the volcanic complex and massive sulfide bodies. There is only limited evidence for replacement of host rocks during formation of the Tambogrande deposits, in contrast to many other large massive sulfide deposits. The deposits at Tambogrande resulted from focused hydrothermal fluid flow along synvolcanic faults with deposition of sulfide minerals within deep and restricted basins. These depressions, the results of the structural and volcanologic setting, acted as efficient traps for sulfide deposition and were also important for the preservation of the sulfide masses as they acted to shield them from submarine oxidation and weathering. Steep basins and episodic bimodal lava eruptions are key geologic attributes of the depositional setting at Tambogrande and may be necessary for the formation of anomalously large VMS deposits in a volcanic-rock–dominated environment.

  19. Page 341
    Abstract
    Corrresponding author: e-mail, lluis.fontbote@terre.unige.ch

    The Nambija gold district, southeastern Ecuador, consists of oxidized skarns developed mainly in volcaniclastic rocks of the Triassic Piuntza unit, which occurs as a 20-km-long, north-trending, contact-metamorphosed lens within the Jurassic Zamora batholith. High gold grades (10–30 g/t) are accompanied in most mines by very low Fe, Cu, Zn, and Pb sulfide contents. The skarn is constituted dominantly by massive brown garnet (mean Ad38). Subordinate pyroxene-epidote skarn developed mainly at the margins of brown garnet skarn bodies. Mostly idiomorphic and more andraditic garnet (mean Ad45) occurs in blue-green skarn formed as a later phase, in places with high porosity, at the transition with vugs and discontinuous dilational type I veins. The last garnet generations are mainly andraditic and occur largely as honey-yellow to red-brown clusters and cross-cutting bands (mean Ad84). As typical for other skarns developed in volcaniclastic rocks, mineral zoning is poorly defined.

    The retrograde overprint is weakly developed, commonly fails to alter the prograde minerals, and is mainly recognized in mineral infilling of structurally controlled (N10°–60°E) vugs and up to several-centimeter-wide type I veins, as well as interstices in blue-green skarn. Retrograde minerals are milky quartz, K-feldspar, calcite, chlorite, and hematite, ±plagioclase, ±muscovite, plus minor amounts of pyrite, chalcopyrite, hematite, sphalerite, and gold. Vugs and type I veins are cut by thin (1–2-mm) throughgoing type II veins that show similar orientations and mineralogy. Native gold is associated with retrograde alteration, mainly in the irregular vugs and type I veins, and subordinately in interstitial spaces and throughgoing type II veins. It is not observed in sulfide-rich type III veins, which cut the previous vein generations.

    High-temperature (up to 500°C) and high-salinity (up to 60 wt % NaCl equiv) inclusions in pyroxene represent the best approximation of the fluid responsible for a significant part of the prograde skarn stage. Such a highly saline fluid is interpreted as the result of boiling of a moderately saline (~8–10 wt % NaCl equiv) magmatic fluid at temperatures of ~500°C. Moderate-to low-salinity fluid inclusions (20−2 wt % NaCl equiv) in paragenetically later garnet as well as in epidote and quartz from vugs and type I veins may represent later, slightly lower temperature (420° −350°C) trapping of similar moderately saline fluids with or without some degree of boiling and mixing. The similarity of salinities and homogenization temperatures in late garnet, epidote, and quartz fluid inclusions is consistent with the apparent continuum between the prograde and retrograde skarn stages, as illustrated by the general lack of prograde mineral alteration, even at the contacts with retrograde fillings.

    Gold deposition, together with that of small amounts of hematite, chalcopyrite, and pyrite, took place during fluid cooling in the retrograde skarn stages but not during the last retrograde alteration, as indicated by the absence of gold in the sulfide-rich type III veins. The abundance of gold-bearing samples with high hematite/sulfide ratios and generally low total sulfide contents suggests high oxygen fugacities during gold deposition. The northeast structural control of vugs and type I veins, compatible with regional northeast-striking structures, in part with a dilational character, suggests that skarn formation, including gold deposition in the retrograde stage, took place under conditions of tectonic stress.

    Minimum Re-Os ages of 145.92 ± 0.46 and 145.58 ± 0.45 Ma for molybdenite from type III veins are compatible with skarn formation and gold mineralization during Late Jurassic magmatism. A genetic relationship with felsic porphyry intrusions that cut the Jurassic Zamora batholith and crop out near several gold skarns is suggested by a published hornblende K-Ar age of 141 ± 5 Ma for a felsic porphyry in the northern part of the Nambija district. Furthermore, the minimum Re-Os ages of ~146 Ma are just slightly younger than the published K-Ar ages (154 ± 5, 157 ± 5 Ma) for the Pangui porphyry copper belt about 70 km north of Nambija.

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