Evidence for low-temperature alteration of sulfides in the Kupferschiefer copper deposits of southwestern Poland

Abstract

Minerals of the Cu-Fe-S system from the Lubin and Rudna mines in the Polish Kupferschiefer display textures and compositional trends consistent with copper depletion. The compositions, optical properties, textures, and distribution of Cu-S minerals indicate progressive alteration from chalcocite to covellite in the sequence chalcocite-djurleite-anilite-geerite-spionkopite-yarrowite-covellite. Bornite compositions plot on the Cu-Fe-S phase diagram along a line between ideal bornite (Cu 5 FeS 4 ) and Idaite (Cu 3 FeS 4 ). The range in bornite compositions extends from ideal bornite to Cu (sub 3.5) FeS 4 . Copper-poor bornites within the composition range Cu (sub 4.5) FeS 4 to Cu (sub 3.5) FeS 4 texturally appear to have partially decomposed to covellite and chalcopyrite. Both of these mineralogical trends have been previously reported from zones of supergene alteration in other copper deposits, where they result from copper leaching. Grains observed to be of uniform anilite composition indicate that the alteration occurred at low temperatures, probably less than 40 degrees C. In the Lubin mine samples, the most copper-depleted minerals, yarrowite, covellite, copper-poor bornite, and chalcopyrite occur in association with veins of gypsum, anhydrite, and atacamite. This association probably results from the interaction of saline mine waters with the sulfides. In the Rudna mine samples, copper leaching is confined to distinct impermeable horizons, most notably at the top of the Weissliegendes and in the Kupferschiefer at the contact between the Cu and Pb zones. On both horizons, the mineralogical change passing upward corresponds to increasing copper depletion. Considering the impermeability of the horizons and the low temperature of anilite formation, the horizon-specific copper depletion probably occurred during early diagenesis...