Characteristics and Formation of the Jerónimo Carbonate-Replacement Gold Deposit, Potrerillos District, Chile
John F. H. Thompson, Vanessa G. Gale, Richard M. Tosdal, William A. Wright, 2005. "Characteristics and Formation of the Jerónimo Carbonate-Replacement Gold Deposit, Potrerillos District, Chile", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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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.
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Andean Metallogeny: New Discoveries, Concepts, and Updates
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