Magmatic Sulfides from the Rincón-Portezuelo de las Ánimas Volcanic Complex, Northwest Argentina: Insights on Magma Fertility and Comparison with Mineralized Volcanic Systems

The composition and fate of magmatic sulfides are some of the most critical factors invoked to play a role in the chalcophile metal fertility of arc magmas. Examination of magmatic sulfide accessory minerals in non-mineralized volcanic systems may help to understand the behavior of chalcophile metals at sulfide saturation. This study presents compositional data on magmatic sulfides in lavas of the Late Miocene Rincón-Portezuelo de las Ánimas volcanic complex, NW Argentina. This is the easternmost magmatic occurrence in the back-arc of Southern Central Andes, at 27°S, about 75 km NE from the world-class Bajo de la Alumbrera porphyry Cu-Au deposit. At this latitude the Late Miocene volcanic activity migrated eastward as a consequence of the shallowing slab subduction. Both copper-rich and pyrrhotite magmatic sulfide inclusions have been identified in the Rincón-Portezuelo de las Ánimas volcanic suite, straddling the high-K calc-alkaline–shoshonite boundary. We discuss the sulfide composition in the framework of magmatic evolution and in comparison with the metal content of magmatic sulfides of the coeval Farallón Negro volcanic complex, associated with the Bajo de la Alumbrera porphyry Cu-Au, and other mineralized systems. The results show that sulfide liquid, exsolved from silicate melts of intermediate composition, stores Cu, Pb, Ag, Bi in crystal mushes, reducing the mineralizing potential of residual melts while fertilizing the middle-upper crust. Gold behavior seems to be controlled by additional mechanisms, linked to the magma source or to an early partitioning into a S-bearing fluid phase. The high Au/Cu ratio of sulfides formed as monosulfide solid solution may be associated with the potassic character of the magmas in this sector of the Central Andes.