Discovery and Geology of the Esperanza Porphyry Copper-Gold Deposit, Antofagasta Region, Northern Chile

Abstract

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.