Neoproterozoic mineralization in a hydrothermal cassiterite-sulfide deposit at Jiumao, northern Guangxi, South China: Mineral-scale constraints on metal origins and ore-forming processes

摘要

Most tin deposits worldwide are inferred to have formed from fluids derived from evolved (granitic) magma systems. Neoproterozoic silicate- and sulfide-stage Sn deposits at Jiumao of South China, in contrast, have been thought to be derived from metasedimentary and mafic-ultramafic host rocks. The Sn deposit may thus have formed by components and processes distinct from those that commonly contribute to Sn deposits, although spatially associated greisen-hosted Sn deposits also occur. To characterize in detail the formation of the Jiumao ore deposits, which contain similar to 28,000 t of Sn, we present new field and petrographic observations, major- and trace-element compositions for silicate and ore minerals, and U-Pb age data of cassiterites and zircons for the deposits and granites. Our research shows that (1) the granite-greisen system formed at similar to 830 Ma, which is the earliest Sn mineralization event in South China; (2) the silicate- and sulfide-stage ores in the country rocks formed at an fO(2) of similar to NNO with temperatures of 570 to 350 degrees C and 350 to 170 degrees C, respectively; (3) all vein-type Sn ores at Jiumao were formed by Sn-F-B-rich fluids that were derived from the Yuanbaoshan granite magma system rather than ultramafic rocks; (4) the Fe-rich character of the ores hosted by the metasedimentary rocks and the Mg-rich character of the ores hosted by the ultramafic rocks largely controlled the reactions and efficacy of cassiterite precipitation and thus ore grade. We highlight that differences in mineral assemblages largely relate to differences of the protolith compositions and fluid-rock reactions (e.g., B- and tourmaline deposition in the ores hosted by the metasedimentary rocks, but not in the ores hosted by the ultramafic rocks). Cassiterite and titanite trace-element compositions reflect fluid composition, protolith composition, and intensive parameters of crystallization.