Bismuth and bismuth–antimony sulphosalts from Neogene vein mineralisation, Baia Borşa area, Maramureş, Romania

Abstract

AbstractSeveral complex Cu-Pb-Bi, Cu-Pb-Bi-Sb and Ag-Pb-Bi sulphosalt minerals have been identified in samples from hydrothermal vein mineralisation associated with the Toroiaga sub-volcanic body in the Baia Borşa area of Maramureş County, northwest Romania. This is the first chemically-documented report of Bi-sulphosalts in the Neogene metallogenic province around Baia Mare. The investigated samples contain abundant amounts of matildite solid solution within galena, the Cu-Pb/Bi sulphosalts aikinite, friedrichite, krupkaite, hammarite, lindströmite and gladite as well as nuffieldite and berryite. Within the Ag-Pb/Bi group, the majority of analysed grains can be regarded as members of the lillianite homologous series. Three distinct lillianite homologues were identified, which correspond to (i) phases along the lillianite-gustavite solid solution join (Pb3Bi2S6-AgPbBi3S6), (ii) phases within solid solution field of heyrovskyite, and (iii) compositions which best correspond to ‘schirmerite’,sensuMakovicky and Karup-Møller (1977b), but may represent disordered gustavite, vikingite or eskimoite. Some of the analysed lillianite homologues contain excess Cu, which may occupy interstitial sites. Furthermore, a large proportion of the lillianite homologues display significant substitution of Sb for Bi within the limits predicted by experimental investigations. Cosalite, also showing a range of compositions including Sb-rich varieties is recognised. Izoklakeite, Cu2Pb22(Sb,Bi)22S57, is an abundant phase throughout the analysed samples, its composition is in good agreement with previously published analyses, except for excess Cu and Fe beyond the limits previously reported. The description of several minerals from this new occurrence and compositional data on them, including the Sb-bearing varieties, provides valuable additional information on compositional limits in natural samples.