The link between reduced porphyry copper deposits and oxidized magmas

Porphyry copper deposits account for more than 80% of the world's total Cu resources. However, the formation mechanism and controlling factors of porphyry copper deposits remain obscure. Previous studies have revealed that porphyry copper deposits are usually associated with oxidized, calc-alkalic, adakitic shallow intrusive rocks. Here we show that hematite-magnetite intergrowths are commonly found in porphyry copper deposits, suggesting high and fluctuating oxygen fugacity (fO₂). Oxidation promotes the destruction of sulfides in the magma source, and thereby increases initial chalcophile element concentrations. Sulfide remains undersaturated during the evolution of oxidized sulfur-enriched magmas where sulfate is the dominant sulfur species, leading to high chalcophile element concentrations in evolved magmas. The final porphyry copper mineralization is controlled by sulfate reduction, which starts with magnetite crystallization, accompanied by decreasing pH and correspondingly increasing fO₂. Hematite forms once sulfate reduction lowers the pH sufficiently and the fO₂ reaches the hematite-magnetite oxygen fugacity buffer, which in turn increases the pH for a given fO₂. The oxidation of ferrous iron during the crystallization of magnetite and hematite is the causal process of sulfate reduction and consequent mineralization. Therefore, the initial pH and fO₂ ranges of porphyries favorable for porphyry copper mineralization are defined by the hematite-magnetite oxygen fugacity buffer and SO₄^2--HS^--S₃^- reaction lines. Adakitic rocks have higher initial contents of copper, sulfur and iron than normal arc rocks, and thus are the best candidates for porphyry copper deposits. These provide a plausible explanation for the formation of copper porphyry deposits. The hematite-magnetite intergrowth marks the upper limits of fO₂ favorable for the mineralization, and thus may be a powerful tool for future prospecting of large porphyry copper deposits.