Platinum-group elements as a tracer of sulphide saturation in evolving magmas: application to the Rio Blanco Cu-porphyry deposit, central Chile

Recent studies show that the platinum-group elements can provide new insights into felsic magma fertility by determining the onset of sulphide saturation. This study aims to test the hypothesis that timing of sulphide saturation, relative to volatile exsolution, controls magma fertility in the Andes of central Chile. To address this aim, samples from the Rio Blanco porphyry Cu deposit are being studied. This deposit was formed in the Miocene-early Pliocene magmatic arc of central Chile and it represents the world's biggest copper reservoir. The samples include different units from the San Francisco Batholith and porphyritic intrusions associated with the deposit. They were analysed for major elements by XRF, trace elements by LA-ICP-MS, and Re, Au and PGE by a Ni-sulphide fire assay, isotope dilution method. Preliminary results suggest that sulphide saturation in the fractionated series occurred at ca. 2.2-1.8 wt.% MgO, slightly before volatile saturation at similar to 1.2 wt% MgO. This is similar to the El Abra porphyry Cu deposit (Cocker et al. 2015). It is suggested that the amount of sulphide that separated from the melt was small, enough to lower the PGE and Au contents, but not enough to have a significant effect on the ultimate Cu fertility of the magma due to its lower partition coefficient.