Abstract
The Late Cretaceous (ca. 92 Ma) Larong porphyry W-Mo deposit (39.44 Mt WO3 @ 0.1541%), a newly discovered deposit in the easternmost Bangong-Nujiang metallogenic belt (BNMB), is located in eastern Tibet. The W-Mo mineralization occurs as veins and stock-works hosted in monzogranite porphyry, granodiorite porphyry and surrounding quartz schist, and can be divided into two main paragenetic stages: the W-mineralization stage (Stage I) consists of various K-feldspar-quartz-scheelite veins and the quartz-sulfide stage (Stage II) is dominated by molybdenite and pyrite, of which the latter can be further subdivided into three sub-stages. Here, we present LA-ICPMS in-situ trace elements and S-Pb isotopes of sulfides from the Larong deposit, in order to constrain the sources of ore-forming elements and the processes of mineralization. Our study shows that pyrite is rich in Co and As, molybdenite is rich in W, Pb and Bi, and chalcopyrite has high Sn contents. These results are consistent with the mineralization characteristics of the Larong deposit, where the main metallogenic elements are W and Mo, accompanied by Bi, Sn and Cu. Overall, sulfides from the Larong deposit have calculated δ34SVCDT values ranging from 0.25‰ to 6.37‰, suggesting a predominantly magmatic origin for sulfur. From Stage II-1, to Stage II-2, to Stage II-3, the average δ34SVCDT values of pyrite change from 0.33‰, to 1.88‰, to 5.34‰, showing a progressive and gradual change in redox and temperature conditions. Molybdenite has high Pb isotopic ratios (208Pb/204Pb = 39.356–39.908, 207Pb/204Pb = 15.747–15.928, 206Pb/204Pb = 18.843–20.120) but low Re contents, and pyrite has low Ni contents, implying that the ore-forming materials were mainly derived from the crust. We propose that the W-Mo mineralization is genetically related to the Late Cretaceous monzogranite porphyry, with the mineralization of W and Mo occurring as a result of decreases in temperature and oxygen fugacity of the ore-forming fluid exsolved from the magma. In combination with previous studies, our study led to the identification of three W-dominated polymetallic metallogenic events and mineralization potential of igneous rocks in the BNMB.
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