Platinum group elements: indicators of sulfide saturation in intermediate to felsic magmatic systems and implications for porphyry deposit formation

Abstract

Sulfide saturation during the magmatic evolution of porphyry systems is emerging as an important control on the fertility of magmas with respect to the chalcophile elements. Platinum group elements (PGE) have extreme sulfide melt-silicate melt partition coefficients that make them sensitive indicators of the timing of sulfide saturation in evolving magmatic systems. This study reports PGE and Re concentrations, measured using a NiS fire assay isotope dilution technique, of three igneous suites: 1) the El Abra porphyry Cu deposit, Chile; 2) the Grasberg-Ertsberg porphyry-skarn Cu-Au district, Papua, Indonesia: and 3) Mt. Taranaki, a young stratovolcano in New Zealand. The El Abra and Grasberg samples allow direct comparison between a Cu-only and a Cu-Au porphyry system, whereas, Mt. Taranaki samples were analysed to investigate sulfide saturation in an active magmatic system with no known associated porphyry deposit. The El Abra results show a rapid drop in Pt and Pd abundances, which is indicative of sulfide saturation, at ca. 2.5 wt. % MgO. However, at El Abra a porphyry Cu deposit was still able to form because the amount of sulfide melt that formed was small, stripping the magma of Au and PGE but not significant Cu. In contrast the Grasberg PGE results suggest that sulfide saturation did not occur during magmatic evolution of the intrusions, and so Cu, Au, and PGE were concentrated by fractional crystallisation and then partitioned into the mineralising fluid. Sulfide saturation has a first order influence on both the availability of the chalcophile elements to partition into the hydrothermal ore-fluid phase and the type of porphyry mineralisation that forms, i.e. Cu, Cu-Au, or Cu-Au-(Pd). The PGE concentrations of the Mt. Taranaki samples indicate that sulfide saturation occurred at ca. 4 wt. % MgO during the evolution of the Taranaki magmatic system. If sulfide saturation of the developing Taranaki magmas persists with each new magmatic episode then the oxidation state of the magmas, the amount of sulfide melt that forms, and the timing with respect to saturation of volatile phases will influence any future porphyry mineralisation. To compliment the PGE analyses of the Grasberg-Ertsberg samples, Th disequilibrium corrected 238U-206Pb ages of zircons from the intrusions were determined by laser ablation inductively coupled plasma mass spectrometry. The ages range from 2.8 to 3.6 Ma and indicate that the duration of magmatism of the Grasberg-Ertsberg intrusions was ca. 730 ± 50 k.y.