Genetic Aspects of the Manto-type Copper Deposits Based on Geochemical Studies of North Chilean Deposits

摘要

Recent studies on mineralogy, geochronology, fluid inclusion and stable isotope (Pb, Os, S, C, O, Sr) characteristics were reviewed to determine constraints for genetic models of the Chilean manto-type copper deposits. The Chilean manto-type deposits are divided into the two geologic categories of the northern areas (Arica-Iquique, Tocopilla-Taltal) and the central areas (Copiapo, La Serena, Santiago). The former is distributed in the coastal range composed of Jurassic andesite-dominated volcano-sedimentary piles and younger plutonic intrusions, and yields chalcocite (-digenite) and bornite as the principal hypogene copper sulfides. The latter is hosted mostly in Lower Cretaceous volcano-sedimentary sequences, and has chalcopyrite-rich mineral associations. The fluid inclusion data indicate that the primary copper mineralization was commonly generated in the temperature range 150-360 deg C under low-pressure conditions near the boiling curve, mediated with relatively saline brines. Generally, homogeneous Pb and S isotope compositions for primary copper minerals imply direct magma source or leaching of igneous rocks. Pb and Os isotope data published for some deposits, however, suggest that ore-forming metals were derived mainly from the volcano-sedimentary host rocks. The noticeably negative isotope ratios of primary sulfide sulfur and hydrothermal calcite carbon of some central area deposits indicate influx of sedimentary rock components, and the high ~(87)Sr/~(86)Sr initial ratios of hydro-thermal calcite from the Tocopilla-Taltal area deposits imply contribution of the contemporaneous seawater or marine carbonates. These isotopic constraints imply a formation mechanism in which the Chilean manto-type copper deposits formed epigenetically in the process of hydrothermal interaction of non-magmatic surface-derived brine with the volcano-sedimentary host rocks, which is inferred to have been induced by a deep-seated plutonic complex as the possible heat source.