Re-os-isotope systematics of sulfides from base-metal porphyry and manto-type mineralization in chile

Chile is a major world producer of copper, most of which occurs in base-metal porphyry and in manto deposits. A fundamental difference between these two types of deposits is the relative importance of intrusions spatially associated with the mineralization. The porphyry deposits are set within Mesozoic to Tertiary intrusive complexes. The manto-type deposits are restricted to volcanic and volcano-sedimentary sequences of Middle-Late Jurassic, Early Cretaceous, Late Cretaceous, and Early Tertiary times. Large intrusive centers are not spatially associated with these deposits, although minor intrusions are common. A central question in the metallogenesis of these deposits is the source of ore-forming components, in particular the ore metals. Initial 187Os/188Os isotopic data from sulfides from the El Teniente, La Disputada, and Andacollo base-metal porphyry deposits range from 0.19 to 1. These data indicate that the Os (and, by inference, Cu) is mostly crustally derived, since it is more radiogenic than that of the mantle, which has a ¹⁸⁷Os/¹⁸⁶Os ratio of ∼0.12. The isotopie data for El Soldado, which is an important example of manto-type mineralization, are significantly more radiogenic, with ¹⁸⁷Os/¹⁸⁸Os ratios greater than 3. These radiogenic values require that the Os come from a crustal reservoir with a high Re/Os ratio, such as black shales. The Os-isotopic data indicate that the source for these two types of base-metal deposits is different, but that both Os reservoirs reside in the crust.