THE FORMATION OF AU-RICH PORPHRY DEPOSITS: WHERE IS THE CU?

A global analysis of porphyry-type ore deposits indicates significant variation in the metal tenor and Cu/Au ratios among deposits within a single tectonic regime. For example, an analysis of porphyry deposits along the Andean arc reveals the presence of Cu-rich, Au-poor deposits within tens of kilometers of Cu-poor, Au-rich deposits. The variation in Cu/Au does not correlate strictly to bulk magma composition or the timing of deposit formation. If one assumes that the observed variation in metal tenor and Cu/Au ratios is not caused by significant variation in the initial metal abundances or Cu/Au ratios of the parental pre-differentiated silicate melts along the arc, then processes occurring during magma evolution, i.e., melt/crystal and melt/fluid partitioning, must be responsible for the variation in the observed tenor and metal ratio(s) among deposits that form quasi-contemporaneously from compositionally similar magmas in a relatively restricted volume of crust. In this study, we focus on the evolution of magmatically-derived formative ore fluids in the Pancho and Verde East and West deposits of the Refugio District of the Maricunga Belt, Northern Chile. Pre-,syn- and post-ore fluid inclusions were characterized by using CL and microthermometry, and Cu and Au concentrations were quantified by using LA-ICP-MS. The data constrain fluid chemistries at discrete space-time steps throughout deposit formation. The fluid inclusion data indicate that the Cu/Au ratios of pre-, syn- and post-ore fluids remains relatively constant and that the formative ore fluids possess a low Cu/Au ratio. These data are consistent with a magmatic control on the Cu/Au ratio of the mineralizing fluid. This implies that the fluid chemistry was controlled by processes in the magma plumbing system prior to the exsolution of the ore-forming fluid from the Cu- and Au-carrying silicate melt. The likely magmatic control involves the selective fractionation of a Cu-Fe-S phase(s) that preferentially sequestered Cu from the melt, resulting in a low Cu/Au ratio of the melt prior to volatile saturation. The data elucidate potentially critical controls on the formation of Au-rich porphyry deposits and the Cu/Au ratio of porphyry deposits in general that may be used as exploration criteria.