Magmatic sulfides from the Rincón-Portezuelo de las Ánimas Volcanic complex, northwest Arentina: insights on magma fertility and comparison with mineralized volcanic systems

Abstract

The composition and fate of magmatic sulfies are some of the most critical factors invoked to play a role in thechalcophile metal fertility of arc magmas. Examination of magmatic sulfie accessory minerals in nonmineralized volcanic systems may help to understand the behavior of chalcophile metals at sulfie saturation. This studypresents compositional data on magmatic sulfies in lavas of the late Miocene Rincón-Portezuelo de las ÁnimasVolcanic Complex, northwest Argentina. This is the easternmost magmatic occurrence in the back arc of theSouthern Central Andes, at 27°S, about 75 km northeast from the world-class Bajo de la Alumbrera porphyryCu-Au deposit. At this latitude the late Miocene volcanic activity migrated eastward as a consequence of theshallowing slab subduction. Both copper-rich and pyrrhotite magmatic sulfie inclusions have been identifidin the Rincón-Portezuelo de las Ánimas volcanic suite, straddling the high K calc-alkaline?shoshonite boundary.We discuss the sulfie composition in the framework of magmatic evolution and in comparison to the metalcontent of magmatic sulfies of the coeval Farallón Negro Volcanic Complex, associated with the Bajo de laAlumbrera porphyry Cu-Au and other mineralized systems. The results show that sulfie liquid, exsolved fromsilicate melts of intermediate composition, stores Cu, Pb, Ag, and Bi in crystal mushes, reducing the mineralizing potential of residual melts while fertilizing the middle-upper crust. Gold behavior seems to be controlledby additional mechanisms, linked to the magma source or to an early partitioning into an S-bearing flid phase.The high Au/Cu ratio of sulfies formed as monosulfie solid solution may be associated with the potassic character of the magmas in this sector of the Central Andes.