New find of silver tellurosulphides from the Funan gold deposit, East Shandong, China

Two unnamed silver tellurosulphides were discovered in the Funan quartz vein-type gold deposit, East Shandong, China. One is optically isotropic and has the composition Ag_8.98Fe_1.01Te_1.98S_4.04 (stoichiometric Ag₉FeTe₂S₄) according to the microprobe analyses of 13 grains. It occurs in close association with galena, chalcopyrite, hessite, cervelleite (?), acanthite and native silver. The second one has the composition Ag_16.08Fe_1.01Bi_0.94Te_3.03S_7.95 (stoichiometric Ag₁₆FeBiTe₃S₈) from 3 microprobe analyses of one grain about 5×10 μm in size and is associated with unnamed Ag₉FeTe₂S₄, acanthite and native silver. Fifteen electron-microprobe analyses of the compounds in the (Ag, Cu)₆TeS₂- (Ag, Cu)₄TeS series, with the generalized formula (Ag_12-x, Cu_x)₁₂(S_6-y, Te_y)₆, 0 ≤ x ≤ 3,2 ≤ y ≤ 3, reveal a wide range of substitution between Ag and Cu as well as Te and S both in cervelleite-like mineral and a possibly-separate phase Ag₆TeS₂. An unnamed secondary Ag-Au-Cu sulphide, with the composition Ag_6.10AU_1.06Cu_1.86S_4.98 (stoichiometric Ag₆AuCu₂S₅) from the microprobe analyses of 6 grains, was observed in association with secondary gold-silver alloy and cerussite replacing primary gold-silver alloy. Microthermometry of fluid inclusions in quartz indicates that the silver tellurosulphides were possibly formed at temperatures around 250°C within the range of Logf_Te2 from -17.4 to -13.4 and Logf_S2 from -13.6 to -9.0.