Sulfide saturation during the magmatic evolution of porphyry Cu ± Au systems

Why some porphyry deposits are Cu-only whilst others are Cu-Au or even Cu-Au-Pd remains unresolved but sulfide saturation during the magmatic evolution of porphyry systems is emerging as an important control of magma fertility with respect to 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 an evolving magmatic system. We compare PGE and Re concentrations of intrusions from the El Abra porphyry Cu deposit, Chile, and the Grasberg-Ertsberg porphyry-skarn Cu-Au district, Papua, Indonesia. The El Abra results show a rapid drop in Pt and Pd abundances, which is indicative of sulfide saturation [1]. However, El Abra was still able to form a porphyry Cu deposit because the amount of sulfide melt that formed was small, stripping the magma of Au and PGE but not significant Cu [1]. 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 crystallization and partitioned into the mineralizing fluid. Sulfide saturation has a first order influence on both the availability of the chalcophile elements to the hydrothermal ore-fluid phase and the type of porphyry mineralization that forms, i.e. Cu, Cu-Au, or Cu-Au-(Pd).

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4th International Workshop on Highly Siderophile Element Geochemistry

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