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Mineral chemistry of the Qitianling granitoid and the Furong tin oredeposit in Hunan Province, South China

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, China

^* e-mail: shyjiang{at}public1.ptt.js.cn

The Qitianling granitic stock is a stanniferous granite located in Hunan Province, China. Recently, the Furong tin ore deposit was found in this stock, with a tin reserve of approximately 600,000 tons. The main rock-forming minerals including amphibole, biotite, plagioclase and ore minerals including cassiterite and rutile were analysed chemical compositions by electron microprobe. The biotite is Fe-rich annite, and has high Ti and Cl concentration. The biotite has high Fe³⁺/(Fe²⁺+Fe³⁺) ratios and the oxygen fugacity calculated by biotite compositions is above Ni-NiO (NNO), and near the Fe₂O_3-Fe₃O₄ (MH). The amphiboles are ferropargasite and ferro-edenite hornblende. The pressure of the granite estimated by Al-in-hornblende barometer is 3.6 ± 0.9 kbar. An amphibole-plagioclase thermometry and a semiquantitative hornblende thermometer yield a forming temperature of 750820°C. The chlorite from the orebody has negligible K₂O, Ti₂O, F, and Cl, but shows similar Fe/(Fe+Mg) ratios with amphibole and biotite. Cassiterite is observed closely associated with rutile disseminated in chlorite alteration veins and envelopes. The ore-forming temperature is estimated to be 290405°C from chlorite geothermometry.

The Qitianling granite contains Sn-rich hornblende and biotite, and shows a high oxygen fugacity for the magma, which is slightly different from the common S-type stanniferous granites worldwide. Fractional crystallization of the magma and tin deposition directly from exsolved magmatic-hydrothermal fluids may not be the major mechanism for the tin mineralization in this deposit. Instead, we suggest that post-magmatic hydrothermal alteration of the granite may have released tin and other metals (e.g., Ti) from the Sn-bearing biotite and hornblende in the granite. Then cassiterite and rutile precipitated together with chlorite when the physical and chemical condition of the Sn- and Ti-rich fluids changed.

Key-words: granite, tin deposit, oxygen fugacity, chloritization, South China.