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Recently, a new type of skarn tin ore formed by B-F-metasomatism has been found inthe central part of the Malage ore field at Gejiu, Yunnan province, which contains 1-7%Sn. The principal tin minerals are nordenskiol dine, schoenfliesite, varlamoffite, etc., accountingfor about 4%. Cassiterite is rarely seen(less than 0.2%). Outward from endoskarns are dis-tinguished four alteration zones. ore fluids were rich in Al and Si in the early stage ofskarnization, and became enriched in Fe, Mg, Sn, B, F, H_2O, etc. in the later stages. Two epi-sodes of hydrothermal metasomatism of B and F took place during post-skarnization, and the dif-ferent alteration zones may be due to reaction between B-and F-bearing hydrothermal solutionsand country rocks(calcareous carbonates). The stability of tin minerals is controlled by μB_2O_3,μCO_2, μF_2O_(-1) and μF(OH)_(-1) . It has been concluded from the discussion in terms of the systemCaO-SnO_2-SiO_2-H_2O-B_2O_3-CO_2-F_2O_(-1) proposed by Burt(1978)and the topological diagrams const-ructed by the author that nordenskioldine was formed in the depletion of Si at about 400℃ inthe skarn environment, where metallogenetic conditions are characterized by rich oxygen butpoor sulfur, and high boron but low fluorine, while schoenfliesite and varlamoffite were for-med at the lower temperature (about 150℃)in response to hydrothermal metasomatism offluorine. In comparison with the metallogenetic conditions for adjacent endoskarn tin ores andother tin ore deposits at Gejiu, the role of the characteristic elemental association Sn-B-F isemphasized in this paper.
Recently, a new type of skarn tin ore formed by BF-metasomatism has been found inthe central part of the Malage ore field at Gejiu, Yunnan province, which contains 1-7% Sn. The principal tin minerals are nordenskiol dine, schoenfliesite, varlamoffite , ore, accounting for about 4%. Cassiterite is rarely seen (less than 0.2%). Outward from endoskarns are dis-tinguished four alteration zones. ore fluids were rich in Al and Si in the early stage ofskarnization, and became enriched in Fe , Mg, Sn, B, F, H_2O, etc. in the later stages. Two epi-sodes of hydrothermal metasomatism of B and F took place during post-skarnization, and the dif-ferent alteration zones may be due to reaction between B -and F-bearing hydrothermal solutions and country rocks (calcareous carbonates). The stability of tin minerals is controlled by μB_2O_3, μCO_2, μF_2O_ (- 1) and μF (OH) _ (- 1) terms of the system proposed by Burt (1978) and CaO-SnO_2-SiO_2-H_2O-B_2O_3-CO_2-F_2O_ (-1) the topological diagrams const-ructed by the author that nordenskioldine was formed in the depletion of Si at about 400 ° C inthe skarn environment, where metallogenetic conditions are characterized by rich oxygen butpoor sulfur, and high boron but low fluorine, while schoenfliesite and varlamoffite were for -med at the lower temperature (about 150 ° C) in response to hydrothermal metasomatism offluorine. In comparison with the metallogenetic conditions for adjacent endoskarn tin ores andother tin ore deposits at Gejiu, the role of the characteristic elemental association Sn-BF isemphasized in this paper .