Re-Os geochronology, fluid inclusions and genesis of the 0.85Ga Tumen molybdenite-fluorite deposit in Eastern Qinling, China: Implications for pre-Mesozoic Mo enrichment and tectonic setting

摘要

The East Qinling Molybdenum Belt (EQMB), central China, hosting tens of Mesozoic magmatic hydrothermal Mo deposits, is one of the largest molybdenum belts in the world. Recently, a new type of Mo mineralization characterized by molybdenite-fluorite veins was discovered with ongoing prospecting at the Tumen area in the Huaxiong block, representing the southernmost tectonic unit of the North China Craton. The molybdenite-fluorite veins occur in faults or ductile shear zones cross-cutting the Neoproterozoic Luanchuan Group. At the Tumen deposit, CO2-rich, aqueous and daughter mineral-bearing fluid inclusions are observed in fluorite. These fluid inclusions resemble those reported from magmatic hydrothermal Mo systems formed in intracontinental tectonic settings and yield homogenization temperatures up to 450°C, with salinities up to 39.8wt.% NaCl equiv, suggesting that the mineralization resulted from a magmatic fluid system. Seven molybdenite analyses from the molybdenite-fluorite veins yield Re-Os isotope ages ranging from 845.8±7.3 to 965.3±7.2Ma, with an isochron age of 847.4±7.3Ma (2σ, MSWD=23), marking the timing of mineralization as Neoproterozoic. These ages are broadly comparable with the 844.3±1.6Ma age reported from the Shuangshan syenite located near the mining area, and the ca. 830Ma gabbros occurring within the same tectonic unit, indicating that the deposit was possibly related to Neoproterozoic rifting at the southern margin of the North China Craton. The age data correspond to the transition from the culmination of the assembly of the Neoproterozoic supercontinent Rodinia to the beginning of its break-up. Our results provide new insights into the regional tectonics and show that the Mo mineralization is not related to the much younger Mesozoic Yanshanian magmatism as considered in previous studies.