Re–Os isotopes of base metal porphyry deposits (original) (raw)

This study uses Re-Os isotopes from sulfide minerals (molybdenite, pyrite, chalcopyrite, bomite, sphalerite, arsenopyrite, and galena) and magnetite from high (400-500°C) and low (OOO^C) temperature mineralization assemblages associated with the intrusion of igneous rocks to trace the source of Os and by inference the other metals. The source of Os is used as a proxy for the source of copper and gold in these systems. Concentrations of Re-Os for all sulfides except molybdenite in various deposit types vary from 0.005-2 ppb Os and 0.120-500 ppb Re. Initial '*^Os/***Os ratios derived from isochron plots ranges from 0.15-50. These data show interesting relationships among deposits of different styles within the district, region, and worldwide scales. On the district-deposit scale, Re-Os isotopes illustrate a complex behavior for the source of Os and by inference other transition and noble metals. The source of Os can be linked to one intrusive event, multiple intrusive events, and the sedimentary rocks into which the magmas intersect. On a regional scale, in the Andean Cordillera, Re-Os isotopes reveal a correspondence between the initial '*^Os/***Os ratio from sulfides of the ore deposit and amount of copper present in porphyry copper deposits. Eleven deposits sampled from difTerent crustal blocks and different mineralization ages form a trend in which deposits that contain large amounts of copper have less radiogenic "^Os/'^'Os initial ratios, suggesting a greater mantle component. Smaller deposits have more radiogenic Os suggesting greater crustal components. On a global scale, the significant observation for the deposits that form in arc environments is that all of the calculated initial '*^Os/'**Os ratios are greater then the Chapter 2-Re-Os molybdenite ages for mineralization and source information from low concentration sulfides for the iron oxide-rich Cu-Au ore at Candelaria, Chile