Rhenium-osmium isotopes of base metal porphyry deposits

Abstract

This study uses Re-Os isotopes from sulfide minerals (molybdenite, pyrite, chalcopyrite, bornite, sphalerite, arsenopyrite, and galena) and magnetite from high (400-500°C) and low and low ( < 300°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 ¹⁸⁷Os/¹⁸⁸Os 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 ¹⁸⁷Os/¹⁸⁸Os ratios from sulfides of the ore deposit and amount of copper present in porphyry copper deposits. Eleven deposits sampled from different crustal blocks and different mineralization ages form a trend in which deposits that contain large amounts of copper have less radiogenic ¹⁸⁷Os/¹⁸⁸Os 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 ¹⁸⁷Os/¹⁸⁸Os ratios are greater then the chondritic mantle. This indicates that there must be a crustal source (recycled in a metasomatized mantle, lower/upper continental crust, or subducting slab) that contributes Os and by inference other metals present in porphyry coppers. Another feature of the data set is that gold-bearing systems in arc environments associated with porphyry intrusions have crustal signatures (0.5-2.5) regardless of the style of the related ore body. For instance, the low/high sulfidation epithermal gold systems in Bucaramanga Columbia (¹⁸⁷Os/¹⁸⁸Os initial 1.2), the massive sulfide ores of the Grasberg, Irian Jaya (¹⁸⁷Os/¹⁸⁸Os initial 0.7--1.2), and Cu-Au skarns in the Hedley District, British Columbia (¹⁸⁷Os/¹⁸⁸Os initial 1.7-2.5) all have large crustal components.