The formation mechanism of reduced porphyry Mo deposits in the West Junggar region, Xinjiang: The Suyunhe example

Abstract

Some porphyry Mo deposits with relatively reduced ore assemblages, characterized by high reduced gases (e.g. CH4) and a lack of magnetite and hematite, are generally considered as the reduced porphyry Mo deposits. Nevertheless, the role of oxygen fugacity (fO2) in controlling Mo enrichment and precipitation of molybdenite in such reduced porphyry Mo deposits is poorly understood. The Suyunhe reduced porphyry Mo deposit with a Mo metal reserve of 0.57Mt at an average grade of 0.05–0.09%, located in the West Junggar terrane in Xinjiang, is associated with Early Permian granitic rocks, which emplaced into the volcano-sedimentary sequences of the Middle Devonian Barluk Formation. In this study, we report that the variation of magmatic fO2, the possible mechanism of Mo enrichment, and the contributions of the reduced gases or matter to molybdenite precipitation. Results indicate that the primitive magma is still oxidized with fO2 > NNO (Ni–NiO oxygen buffer), but it is then reduced due to contamination by the reduced sequences. Furthermore, the decreasing magmatic fO2 does not significantly prevent the enrichment of Mo metals during magmatic evolution. By contrast, the evolved parental magma, characterized by highly differentiated and fluorite-rich, is responsible for improving Mo concentration of melts. Most of reduced gases are derived from decomposition of organic matter occurring in the reduced volcanic sedimentary rocks of Barluk Formation. More importantly, these reduced gases (e.g. CH4) play a role in molybdenite precipitation, which may effectively improve the efficiency of metal precipitation.