Early sulfide saturation in arc volcanic rocks of southeast China: Implications for the formation of co-magmatic porphyry–epithermal Cu–Au deposits

Abstract

It is widely considered that arc magmas are highly oxidized, and can thus dissolve a large amount of sulfur, thereby preventing significant sulfide saturation prior to fluid saturation. Therefore, chalcophile metals (e.g., Cu and Au) are retained in the magma and finally extracted by exsolved hydrothermal fluids. To test this hypothesis, we analyzed the abundances of platinum-group elements (PGEs) in a suite of arc volcanic rocks, which are the extrusive counterparts of magmas associated with the world-class Zijinshan porphyry–epithermal Cu–Au ore district in SE China. All of these volcanic rocks are depleted in PGEs (e.g., Pd = 0.02–0.40 ppb) in comparison with fertile arc magmas, and the PGE contents decrease with decreasing MgO from basalt to dacite. The results show that the parental magmas for these rocks reached sulfide saturation before they evolved to 7 wt.% MgO, which is earlier than the onset of magnetite crystallization. Sulfide liquid and monosulfide solid solution (MSS) were removed from the magma before it evolved into a dacitic composition. This process extracted most of the Au and Cu from the magma. Modeling results indicate that Au concentrations would not exceed 7–10 times the Pd concentrations in the residual silicate magma, irrespective of the partition coefficients between the sulfide and silicate melt. Given the low Pd contents in dacitic arc magmas worldwide, depletion of Au and Cu might be a common feature of highly evolved arc magmas. Thus, the hydrothermal ore fluids separated from such evolved arc magmas would not contain enough metals to form the giant Zijinshan porphyry–epithermal Cu–Au ore district. We propose a model in which the magmas parental to the arc volcanic rocks achieved S saturation, and thus large amounts of sulfides accumulated in the underlying magma chamber before the magmas were extruded from depth to the surface. During solidification of the magma chamber, these deposits formed from exsolved magmatic hydrothermal fluids that dissolved abundant pre-existing magmatic Cu–Au-rich sulfide assemblages. Release of large amounts of Au, Cu and S to the ore-forming aqueous fluids by dissolution of sulfides can account for the metal and S budgets of giant porphyry–epithermal Cu–Au deposits.