The platinum-group elements (PGE) are important metals in many technological applications, but some of the rarest elements in the Earth. Recent studies have shown that the PGE can be used to constrain the timing of sulfide saturation in evolving felsic systems. In this study, we report PGE and Re data for the barren and ore-associated suites of intermediate to felsic rocks from the Northparkes Cu-Au porphyry region and Escondida Cu porphyry district, emphasizing the timing of sulfide saturation and its influence on the tenor of the associated hydrothermal mineralization.
The concentrations of PGE in the barren suites decrease during fractional crystallization, indicating early sulfide saturation, which locked most of the Cu and Au in a sulfide phase in the cumulus pile of a deep parental magma chamber, well before volatile saturation, so that when the magma reached volatile saturation, it did not have access to the Cu and Au. Palladium in the ore-associated suites, in contrast, decreases abruptly at the final stage of magmatic evolution, representing late sulfide saturation, which was followed shortly afterwards by volatile saturation, as a result, the full complement of chalcophile elements will be available to enter the ore-forming fluid. This study also indicates that both the late sulfide saturation and water content are important in determining the magma fertility. The Pd/MgO and Y plot can be used to distinguish the ore-bearing from the barren suite, and porphyry Cu from the porphyry Cu-Au systems.