Variable redox states of sulfur in porphyry copper–fertile arc magmas
2026-09-04
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 S 6+ , S 4+ , and S 2− 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 S 6+ /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 S 6+ /ΣS ratios and oxygen fugacities ( f O 2 ), possibly throughout Earth history including the Proterozoic and Archean.