Bismuth Sulfosalts from the Nistru Metallogenetic Field, Baia Mare Zone, NW Romania

An association of bismuth sulfosalts coexisting with native gold is described from a sulfide-rich copper vein in the Nistru area, Baia Mare metallogenetic district, NW Romania. This mineralization is hosted within a Neogene calc-alkaline subvolcanic porphyry quartz-micromonzodiorite stock on the southern border of the Gutâi Mountains. Cu+Au+Bi mineralization represents the inner part of a zoned vein of a type specific to the SE part of the Nistru orefield. The Pb-Zn and Au-Ag veins are located towards the external zone at the boundary of the stock with the surrounding rocks. Bismuth sulfosalts are mainly represented by bismuthinite derivatives and members of the lillianite homologous series. Cosalite, matildite and wittichenite are also present in subordinate amounts. Bi-rich members of the bismuthinite-aikinite series (from krupkaite to bismuthinite) are predominant. A phase with the empirical formula CuPbBi7S12 was also identified and could potentially be a new bismuthinite derivative. The lillianite homologous series is represented by phases with composition between Gus73 and Gus59, with a dominance of members closer to gustavite (Gus97–79) and less abundant members closer to lillianite (Gus49). Native gold and Bi-sulfosalts are closely associated with the main sulfides (pyrrhotite, pyrite, chalcopyrite) and quartz, indicating simultaneous crystallization. Fluid inclusion data for quartz indicate a temperature interval between 205 and 247 °C. The assemblage within this vein was deposited from a low-salinity fluid (0.4–2.6 wt.% NaCl equiv.) and density from 0.80 to 0.87 g/cm3.