Molybdenum isotope systematics of subduction-related magmas from the Zhongdian region: Assessing the Mo fractionation behavior in magmatic-hydrothermal processes

Abstract

The behavior of molybdenum isotopes in magmatic–hydrothermal systems is poorly known. Late Triassic andesitic rocks from the Disuga and Lannitang areas, and contemporaneous porphyry–skarn Cu polymetallic deposits (e.g., Pulang, Xuejiping, Yaza, Hongshan, and Langdu), from the Zhongdian region, southeastern Tibetan Plateau, China, present an ideal opportunity to study Mo isotope fractionation in magmatic–hydrothermal systems. In this study, we report Mo isotopic compositions, analyzed by multi-collector–inductively coupled plasma–mass spectrometry, of ore-forming porphyries and hydrothermal chalcopyrite, pyrite, and molybdenite from the Pulang deposit, and coeval barren andesites from the Disuga and Lannitang areas. The δ98Mo values of the Pulang porphyries (−0.59‰ to −0.05‰) are lower than those of the Disuga and Lannitang andesites (−0.29‰ to −0.03‰). In the Pulang deposit, whole-rock δ98Mo values are lower (−0.59‰ to −0.05‰) than those of pyrite (+0.73‰ to +2.09‰). Chalcopyrite has lower δ98Mo values (−0.16‰ to +1.06‰), than pyrite, but similar δ98Mo values to molybdenite (−0.18‰ to +0.06‰). We infer, tentatively, that fractional crystallization was not a major control on the Mo isotopic composition of the Pulang magmatic–hydrothermal system. Heavy Mo isotopes partitioned into pyrite relative to molybdenite, and 98Mo is thought to partition into Mo6+-bearing species relative to Mo4+-bearing species. Therefore, if MoO42− was the predominant Mo species in exsolved hydrothermal fluids, then formation of pyrite from these fluids explains the high δ98Mo values of pyrite. We infer that the chalcopyrite from the Pulang deposit contains micron-scale molybdenite inclusions, which partially explains the comparable δ98Mo values of chalcopyrite and molybdenite. Pyrite has a strong influence on Mo isotope fractionation, at least in the magmatic–hydrothermal system described here.