Geology, Geochemistry And Genesis Of The Jinchuantang Tin-bismuth Deposit, Hunan Province

There are a series of large-and medium-sized W-Sn-Mo-Bi polymetallic depositsdistributed around the Qianlishan granite in the Dongpo ore field. Previous studieshave focused on the Qianlishan granite and Shizhuyuan deposit, and there is a littleresearch on other deposits in Dongpo ore field. The Jinchuantang deposit is a large-sized skarn-type tin-bismuth deposit in the Dongpo ore field, and its elements aredifferent from the Shizhuyuan deposit. Based on detailed field investigation of theJinchuantang tin-bismuth depsosit, this study is focused on systematic microscopeobservation, electron microprobe analyses and situ LA-ICP-MS analyses of the skarnminerals, detailed micro-thermometric measurement of the fluid inclusion, and LaserRaman spectroscopic analysis, direct Re-Os dating on molybdenites collected fromthe skarn ore. Then, we preliminarily discussed its geological characteristics, fluidevolution and the age of mineralization:(1) The mineralizations of the Jinchuantang deposit can be divided into skarnstage, retrograde stage, greisen stage, cassiterite-sulfide stage, and barren quartz-carbonate stage. The analyses of electron microprobe show that the end member ofgarnet is mainly grossularite and andradite, followed by spessortite. The compositionof pyroxene is dominated by diopside, with minor hedenbergite. The amphibole ismainly ferrotschermakite, followed by tschermakite, actinolite, tremolite. Based onthe above, it is suggested that skarn in the Jinchuantang tin-bismuth deposit iscalcareous skarn, with minor manganoan skarn. Accodring to the studies of thecharacteristics of skarn minerals, we have futher discussed the mechanism ofcassiterite precipitation, and considered that tin maybe replace Fe3+with Sn4+in theoctahedron of crystal structure of andradite in the early skarn stage, because of thehigher the oxygen fugacity. While, in the late skarn stage, tin dominantly as Sn(II)chloro-complex species were transported in fluid with the decrease of the oxygenfugacity. During the process of the fluid evolution the changes of the temperature,salinity, pH and oxygen fugacity are responsible for cassiterite precipitation due tothat the Sn2+was oxidized to Sn4+. (2) The study of fluid inclusions indicate that the composition of the ore-formingfluid is mainly H2O, and there is a little CO2, CH4, SO2in the gas phase, and the solidis mainly NaCl, containing a little KCl, and therefore we consider that the ore-formingfluid is a system of H2O-NaCl(±KCl)-CO2(±CH4). The homogenizationtemperatures of the fluid inclusions show that the ore-forming fluid has a tendency tobecome low temperature and low salinity from the early to late stage in general, thedensity of the fluid is about0.65~1.0g/cm3, the pressure is120~200MPa, and thecorresponding metallogenic depth is about4~7km. The ore-forming fluid weredominantly magmatic origin, with the progressive mixing with cooler meteoric waterin the process of fluid evolution, and the fluid boiling induced by pressure decreaseand the mixing of high and low temperature fluids maybe the main mechanism of thecassiterite precipitation.(3) The Re-Os dating on molybdenites of the Jinchuangtang deposit show thatthe isochron age is158.8±6.6Ma. Combined with the Re-Os isochron age (151.0±3.5Ma) of Shizhuyuan deposit and zircon SHRIMP U-Pb ages (152Ma±2Ma) ofthe Qianlishan granite, it is suggested that the mineralization of the Jinchuantang Sn-Bi deposit is temporally and genetically related to the granite in this area. Based onprevious studies, petrologenesis and metallogenesis of the Dongpo ore field, includingthe Jinchuantang deposit, mainly occurred149-161Ma, which is similar to the large-scale tungsten-tin polymetallic mineralization (150-160Ma) in the Nanling region.Coupled with previous studies in this region, it is suggested that mantle componentswere extensively involved in the petrologenic and metallogenic processes of theMesozioc W-Sn polymetallic ore district, southern Hunan Province, indicating that themiddle-later Jurassic explosive mineralization of this region is probably as results ofcrust-mantle interactions under a geodynamic setting of the lithospheric thinning andextension.