Sulfide saturation in evolving porphyry systems: El Abra porphyry Cu deposit, northern Chile, and the Grasberg-Ertsberg porphyry-skarn Cu-Au district, Papua, Indonesia

Sulfide saturation during the magmatic evolution of porphyry systems is emerging as an important control on chalcophile element fertility. Platinum group elements (PGE) have extreme sulfide melt-silicate melt partition coefficients that make them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. We report PGE and Re concentrations of intrusions from the Grasberg-Ertsberg porphyry-skarn Cu-Au district, Papua, Indonesia. Unaltered to weakly altered samples contain up to 0.023 ppb Rh, 5.5 ppb Pt, 11.6 ppb Pd and 162 ppb Re. The most altered and/or mineralized samples typically contain greater concentrations; up to 0.065 ppb Rh, 17.6 ppb Pt, 95 ppb Pd and 218 ppb Re. The results suggest that sulfide saturation did not occur during magmatic evolution of the intrusions, and so Cu, Au, and PGE were concentrated by fractional crystallization and partitioned into the mineralizing fluid. These findings contrast with the intrusions of the El Abra-Pajonal suite and porphyry Cu deposit, Chile, where a rapid drop in Pt and Pd abundance indicates that sulfide saturation started before ore-fluid saturation. 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 most of its Au and PGE but little Cu. Sulfide saturation therefore has a governing control over both the availability of the chalcophile elements to partition into the hydrothermal ore-fluid phase and the type of porphyry mineralization that can form, i.e. Cu, Cu-Au, or Cu-Au-(Pd)

Using platinum group elements to identify sulfide saturation in a porphyry Cu system: the El Abra porphyry Cu deposit, northern Chile

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra–Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt–silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to ore-fluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu–Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U–Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra–Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41–40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra–Pajonal intrusive complex drop rapidly in samples with MgO values below 2·5 wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu–Au deposit