Chemical and mineral composition of sulfide nodules from the Mikhaylovsky and Belousovsky ancient mines, Orenburg Gis-Urals

Abstract

Cu-sulfde and rare Fe-sulfde nodules up to 2–5 cm in size are found in gray marl siltstones, which occur in a sequence of Permian sandstones of Orenburg Cis-Urals. The Cu-sulfde nodules are characterized by high Cu content (>20 %) and low contents of trace elements (Fe <1.2 %, As, Ag, V, and Pb <130 ppm). They are composed of Cu sulfdes (minerals of the chalcocite group and covellite) with minor barite and mottramite. The Cu sulfdes are transformed to malachite and, to a lesser degree, to chrysocolla and brochantite. The decomposition of Ag-bearing Cu sulfdes leads to the formation of a Ag sulfde. The Fe-sulfde nodules of the Belousovsky mine are composed of pyrite. In the oxidation zone, pyrite is replaced by ferrous opal; single grains of plumbogummite are associated with Fe hydroxides.