An unusual Ni-Sb-Ag-Au association of ullmannite, allargentum, Au-rich silver and Au-bearing dyscrasite from Oselsk\xc3\xa9 p\xc3\xa1smo \xe2\x80\x9csilver\xe2\x80\x9d Lode of Kutn\xc3\xa1 Hora Pb-Zn-Ag ore district (Czech Republic)

[摘要] A rare and unusual Ni-Sb-Ag-Au association of ullmannite, NiSbS, associated with allargentum, Ag6Sb, Au-rich silver, Au9.7Ag85.0Sb2.9, and Au-bearing dyscrasite was found in medieval mine dump material of the Oselsk\xc3\xa9 p\xc3\xa1smo \xe2\x80\x9dsilver\xe2\x80\x9d lode of the historic Kutn\xc3\xa1 Hora Pb-Zn-Ag ore district. Ullmannite is the first and only Ni sulphide found in the base-metal paragenesis of otherwise exclusively Ag-(Cu)-(Pb)-Sb sulphosalts. No other nickel mineral is known from this base-metal ore district. The mineral was identified in two polished sections of one sample as anhedral grains up to 50 \xc2\xb5m across enclosed in allargentum. The average chemical composition of ullmannite Ni 26.76-27.17, Sb 57.80-59.17, S 15.14-15.28 wt. % corresponds to the empirical formula Ni0.98Sb1.01As0.01S1.00 on the basis of Ni + Sb + S = 3 apfu, i.e. it is close to the ideal formula o with only trace contents of As. Au-rich silver with 15.52-16.34 wt. % of gold was found in association with freibergite and pyrargyrite as an anhedral inclusions up to c. 5 \xc2\xb5m across. The chemical composition corresponds to Au9.7Ag85.0Sb2.9 on the basis of 100 apfu. One more gold-containing phase was determined: Au-bearing dyscrasite with ~0.7 wt. % of Au. The sample also produced an example of Ag-rich \xe2\x80\x9cbonanza\xe2\x80\x9d with stephanite, acanthite, pyrargyrite and miargyrite. The likely source of nickel and gold were the serpentinized ultrabasic bodies, cut by the \xe2\x80\x9csilver\xe2\x80\x9d lodes in the South of the ore district. The serpentinites contain 2000 ppm of Ni and low but stable contents of Au. Penetration of hydrothermal fluids could have caused the mobilization of nickel and gold in serpentinites and in earlier ore mineralization (arsenopyrite). This process resulted in the breakdown of earlier silver minerals while allargentum, dyscrasite and stephanite crystallized. The discovery of Ni-Sb-Ag-Au association in Kutn\xc3\xa1 Hora ore district sheds new light on the elemental variability of this hydrothermal vein-type deposit of Variscan age.

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