Role of magma differentiation depth in controlling the Au grade of giant porphyry deposits

Abstract

Porphyry deposits are the world's most important source of Cu and a major source of Au. It has been recognized that low Au grades generally characterize porphyry deposits in thick continental arcs, like the Andes, where the magmas are likely to differentiate at depth. In contrast, Au-rich porphyries are mainly found in thin island arcs, for example, those of the southwest Pacific, or associated with short extensional periods in continental arcs, where the magmas are likely to experience relatively shallow differentiation. However, the key factors that control this difference remain debated. This study shows that the Au grade of giant porphyry deposits and La/Yb ratios of the ore-associated suites are negatively correlated (r=∼0.7, p=10−7). We attribute the negative correlation to be mainly due to varying sulfide saturation histories, modulated by the depth of magma differentiation. Magmas differentiating in deep crustal reservoirs are likely to reach sulfide saturation early due to high pressure and early depletion of FeO (calc-alkaline trend). Early sulfide saturation causes most Au to be held in cumulus sulfides, making it unavailable to enter ore-forming fluid released in the upper crust, resulting in Au-poor porphyry systems. In contrast, magmas evolving in shallow reservoirs are likely to experience late sulfide saturation because of the high sulfur solubility induced by low pressure and the high FeO content of the melts (tholeiitic trend). Late sulfide saturation enhances the potential of a magmatic system to form Au-rich porphyry deposits due to the high Au content of the magma at voluminous fluid saturation and efficient Au transfer from melt to the ore-forming fluids via sulfide-fluid interaction. The link between average magma differentiation depth and Au content is supported by platinum-group element geochemistry of the porphyry ore-forming suites, which shows that the magmas differentiating at shallower depths (lower La/Yb) reach sulfide saturation later and therefore have higher Au concentrations than those that differentiate at deeper levels (higher La/Yb). Numerical models for ore-associated magmas with different sulfide saturation histories indicate that variations in the Au grades of giant porphyry deposits can be explained by the variations in the timing of sulfide saturation. Based on the results, we propose that Au concentration in the magma, modulated by average magma differentiation depth and sulfide saturation history, is one of the critical factors controlling the Au grade of giant porphyry Cu deposits.