MAGMA MIXING AND MINERALIZATION: EMPIRICAL EVIDENCE FROM QUESTA(MO), N.M., USA; NUKAY(AU-CU), GUERRERO, MEXICO; LAS BAMBAS(CU-AU), PERU

The comingling and mixing of mafic and felsic magmas has been, and remains, a largely overlooked phenomenon in the empirical study of mineralized porphyry systems. Extensive work at three ore deposits reveals not only that magma mixing occurred, but suggests that it was essential to the linked magmatic-hydrothermal processes that ultimately led to ore formation. At Questa, N.M., a compositionally zoned dike swarm provides evidence that residual magmas of a silicic caldera mixed with basaltic andesites to produce porphyries that yielded world-class porphyry molybdenum deposits. At Nukay, Guerrero, Mexico, the coexistence of andesite and granodiorite porphyry magmas produced the proper physical conditions for the development of the Los Filos bulk-tonnage Au deposit. At Las Bambas, Peru, world-class Cu(-Au) skarn mineralization is spatially and temporally associated with odd hybrid porphyry phases, the development of which is in ample evidence in field exposures. In each deposit the 'role' of magma mixing was somewhat different. At Questa, the mineralizing ‘source’ porphyries appear to be the fractionated byproducts of hybridization. At Los Filos, the andesite portion of the mafic-felsic magma 'pairing' ultimately served as a passive, brittle host rock to mineralizing fluids. At Las Bambas, hybridization appears to have occurred nearly contemporaneously with peak Cu mineralization of exoskarn bodies. The empirical evidence suggests that the presence of mafic magmas was integral to ore development in all three systems. Importantly, none of these phenomena had been noted by prior workers in any of these mineral districts. This suggests that magma mixing in productive porphyry systems is likely a far more common and widespread phenomenon than is currently appreciated.