Oxygen fugacity of Yanshanian granites in South China and implications for metallogeny

Abstract

Oxygen fugacity (fo2) of magmas exerts an important control on the speciation of multivalent elements in magmatic-hydrothermal processes, which has a great significance for mineralization. This paper examines the relationships between oxygen fugacity and mineralization associated with Yanshanian (Jurassic to Cretaceous) granites in South China, which hosts many world-class W, Sn, Cu, Mo, Au, Pb and Zn deposits. The oxygen fugacities of Yanshanian granites were evaluated using published chemical compositions of biotite, and the redox states were further constrained with whole-rock Fe3+/Fe2+ as well as zircon Ce4+/Ce3+ ratios from the literature. Systematic differences in oxygen fugacity between granites related to Cu–(Au)–Mo, Cu–Pb–Zn, W, Sn, and W–Sn mineralization, and between ore-bearing and barren granites, were found. Logfo2 values (relative to NNO buffer) are +1.08 to +13.07 (average +4.22) for Cu–(Au)–Mo-related granites, +1.01 to +5.36 (average +3.51) for Cu–Pb–Zn-related granites, +1.51 to +5.68 (average +3.78) for W-related granites, −1.70 to +3.22 (average +1.71) for W–Sn-related granites, and −5.32 to +2.43 (average −0.83) for Sn-related granites. In the biotite Fe3+–Fe2+–Mg2+ ternary and logfo2–T diagrams, the Cu–(Au)–Mo-related granites are located between the MH and NNO buffers and above the MH buffer, and Sn and W–Sn-related granites are centered around the NNO buffer and spread toward the FMQ and MH buffers, whereas the Cu–Pb–Zn- and W-related granites are plotted between the NNO and MH buffers. Similar trends were also observed for whole-rock Fe2O3/FeO ratios and zircon Ce4+/Ce3+ ratios. In areas of Cu–(Au)–Mo mineralization, ore-bearing granites have higher oxygen fugacities than barren granites. Regional-scale variations of oxygen fugacities, coupled with changes in La/Sm and Ba/Th ratios which are considered as proxies of subducted sediments and slab dehydration fluids, support the distinction of two metallogenic systems in the Yanshanian period in South China, a Jurassic system related to NE to SW subduction, and a Cretaceous one related to SE to NW subduction, of the Paleo-Pacific plate, each with a trend of decreasing oxygen fugacities in the direction of subduction.