Reduction-Induced Sulfide Saturation in the Huangshandong and Huangshanxi Magmatic Ni-Cu Deposits in the Central Asian Orogenic Belt of China

Abstract Reduction of fO2 in magma by organic carbon or addition of external S to magma can play a major role in the formation of magmatic Ni-Cu sulfide deposits in convergent tectonic settings. This study evaluates two competing genetic models for two coeval, spatially related magmatic Ni-Cu sulfide deposits (Huangshandong and Huangshanxi) in the Central Asian orogenic belt using whole-rock C isotopes and S/Se ratios, sulfide mineral S isotopes, and calculated magmatic fO2. The whole-rock S/Se ratios range from 3,085 to 7,372, overlapping to slightly higher than the mantle ratios (3,250 ± 400). The δ34S values of pyrrhotite, pentlandite, and chalcopyrite range from −0.25 to 2.1‰. The calculated bulk sulfide δ34S values are slightly more restricted, from 0.8 to 1.2‰ for Huangshandong and from 0.8 to 1.3‰ for Huangshanxi, within the range of initial arc basalts from the Central American volcanic arc (−0.7 to 3.2‰). The δ13C values of these deposits range from –28.7 to –21.1‰, significantly lower than the values of the Xiadong sulfide-barren ultramafic intrusion (–9.6 to –7.1‰) and the mantle (–6 ± 1‰) but are within the range of marine organic matter preserved in Paleozoic sedimentary rocks worldwide (–30 to –20.7‰). The magmatic fO2 values, calculated using a reliable spinel-olivine-orthopyroxene oxybarometer, range from ΔFMQ + 0.21 to ΔFMQ + 0.76 for Huangshandong and from ΔFMQ + 0.73 to ΔFMQ + 0.96 for Huangshanxi (where FMQ is the fayalite-magnetite-quartz buffer), more than one order of magnitude lower than those of the older Xiadong ultramafic intrusion in the region as well as the initial arc basalts of the Central American volcanic arc. Using reported S contents in initial arc basalts (3,000 ± 400 ppm) and the redox control on S content at S saturation in hydrous mafic magma, we demonstrate that external S addition alone cannot trigger sulfide saturation in the parental magmas for the Huangshandong and Huangshanxi deposits. Significant reduction of fO2 in the magmas by organic matter-rich sedimentary rocks is required to induce sulfide saturation. This study implies that exploration of magmatic Ni-Cu sulfide deposits in convergent tectonic setting should focus on the mafic-ultramafic intrusions that intruded organic carbon-rich sedimentary rocks.