Variable redox states of sulfur in porphyry copper-fertile arc magmas.

Porphyry copper systems formed in magmatic arcs represent the largest concentrations of copper and most massive anomalies of sulfur worldwide. Whether the ore-fluid-source magmas were dominated by oxidized or reduced sulfur remains the subject of intense debate. Here, we quantitatively measured in situ the relative abundances of S6+, S4+, and S2- in sulfur-bearing primary apatite grains hosted in zircon, amphibole, and biotite in igneous rocks related to porphyry and skarn copper-gold deposits in the Middle-Lower Yangtze River metallogenic belt, Eastern China. Calculated S6+/ΣS ratios of the sulfur-bearing apatite grains vary from 0.18 to 0.97 and are independent of the crystallization pressure and temperature of their host minerals. The data reveal two distinct groups of ore-fluid-source magmas: (i) highly oxidized, dominantly sulfate (anhydrite)-saturated and (ii) relatively reduced, dominantly sulfide-saturated. The results demonstrate that economic porphyry copper systems can form from intermediate-felsic magmas over a broad range of S6+/ΣS ratios and oxygen fugacities (fO2), possibly throughout Earth history including the Proterozoic and Archean.