Journal
EUROPEAN JOURNAL OF MINERALOGY
Volume 17, Issue 4, Pages 635-648Publisher
COPERNICUS GESELLSCHAFT MBH
DOI: 10.1127/0935-1221/2005/0017-0635
Keywords
granite; tin deposit; oxygen fugacity; chloritization; South China
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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 Fe2O3-Fe3O4 (MH). The amphiboles are ferropargasite and ferro-edenite homblende. 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 750 similar to 820 degrees 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 290 similar to 405 degrees 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 Sri- and Ti-rich fluids changed.
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