A revised oxygen barometry in sulfide-saturated magmas and application to the Permian magmatic Ni–Cu deposits in the southern Central Asian Orogenic Belt

Abstract

Oxygen fugacity is a key parameter in controlling the petrogenesis of mafic-ultramafic rocks and their associated sulfide mineralization, especially in convergent settings. This study uses new and previously published experimental data on olivine-sulfide pairs to reparameterize an expression for oxygen barometry using the distribution coefficient K D FeNi for Fe-Ni exchange between olivine and sulfide. We derive a new expression, ΔQFM = (9.775 + 0.416 ∙ C Ni − K D FeNi)/4.308, where ΔQFM denotes divergence from the fayalite–magnetite–quartz buffer. The revised oxygen barometry has been applied to the Permian magmatic Ni–Cu deposits in the Central Asian Orogenic Belt, NW China. The Ni–Cu deposits in the East Tianshan—North Tianshan, Central Tianshan, and Beishan—are considered as a single mineral system, whereas the spatially separated deposits in the East Junggar are considered as a separate system. The deposit of the East Tianshan group exhibits a large range of oxygen fugacity (QFM − 2 to ~QFM + 1) and Ni tenor (metal concentration in pure sulfide, ~ 5 to 16 wt%). The Poyi and Huangshannan deposits contain high-Ni tenor sulfides, varying from 12 to 16 wt%. The relatively high Fo values (> 85 mol%) and Ni contents (> 2000 ppm) in olivine of these deposits indicate that the high-Ni tenor sulfides were segregated from less differentiated high-Ni magmas that also had relatively high oxygen fugacity (~QFM + 1). The remaining Ni–Cu deposits in the East Tianshan—the Huangshandong, Huangshanxi, Hulu, Tulaergen, Tudun, and Xiangshanzhong deposits—have intermediate Ni tenors (5–8 wt%). These sulfides correspond to intermediate Fo values (80–84 mol%) and Ni contents (700–1400 ppm) in the coexisting olivine, illustrating that they were segregated from magmas with lower Ni contents thought to be the result of a large amount (15–20%) of olivine fractionation at depth. These magmas are more reduced (− 2 < ΔQFM < + 0.3) than the less evolved magmas (~QFM + 1). It is shown that the ΔQFM value calculated for the deposits in East Tianshan decreases with decreasing Fo value, indicating that the host magmas became gradually reduced during evolution, which can be explained by primarily oxidizing magma progressively assimilating crustal material containing reducing agents, such as carbon. The Kalatongke deposit in the East Junggar belt, with the lowest Ni tenors in sulfides (3–5 wt%) and low Fo values in olivine (< 78 mol%), was derived from relatively oxidizing magmas (~QFM + 1) that had experienced significant olivine plus clinopyroxene and plagioclase fractionation at depth. We propose that the variation in oxygen fugacity and Ni tenor in the Permian Ni–Cu deposits in the Central Asian Orogenic Belt is the result of gradual contamination and a variable degree of fractional crystallization.