Mineralogical chemistry of the Jinchuantang tin-bismuth deposit in Hunan Province and its geological significance

Abstract：The Jinchuantang deposit is a large-size skarn-type tin-bismuth deposit in the Dongpo ore field. Little is known about the mineralogical characteristics of the skarn in the Jinchuantang deposit. Based on microscopic observations and electron microprobe analyses, the authors investigated in detail compositional characteristics of skarn minerals in the Jinchuantang tin-bismuth deposit. The results show that the end member of the garnet is composed mainly of grossularite and andradite, followed by spessartite. The composition of pyroxene is dominated by diopside, with minor hedenbergite. The amphibole comprises mainly ferrotschermakite, followed by tschermakite, actinolite and tremolite. Based on the above data, the authors hold that skarn in the Jinchuantang tin-bismuth deposit is mainly calcareous skarn, with minor manganoan skarn. According to the characteristics of skarn minerals, this paper has further discussed the mechanism of cassiterite precipitation, and considered that tin probably replaced Fe3+ in the form of Sn4+ in the octahedron of crystal structure of andradite at the early skarn stage due to the relatively high oxygen fugacity. At the late skarn stage, however, tin dominantly existent as Sn (II) chloro-complex species was transported in fluid with the decrease of the oxygen fugacity. In the process of fluid evolution, the changes of the temperature, salinity, pH and oxygen fugacity were responsible for cassiterite precipitation because the Sn2+ was oxidized to Sn4+.