Epithermal Cu mineralization in the Stary Lesieniec rhyodacite quarry, Lower Silesia: primary and secondary mineral paragenesis

Abstract

Primary epithermal and secondary Cu mineralization in the Stary Lesieniec rhyodacite quarry, located within the Intra-Sudetic Depression, was studied using reflected light microscopy, powder X-ray diffraction, and electron microprobe. Samples containing copper sulphides, baryte, and secondary weathering minerals were collected from mineralized veinlets in the Upper Carboniferous rhyodacite. Copper sulphides (chalcocite Cu2S, djurleite Cu31S16, anilite Cu7S4 / digenite Cu9S5, and covellite CuS) are the major ore minerals and are associated with quartz, hematite, and very minor uraninite. The samples studied indicate phase transformation from chalcocite to anilite, which indicates that Cu sulphides began to crystallize at ~100°C. Then, during the epithermal stage of precipitation, the temperature of the solutions dropped <72°C, based on the Cu-S ternary diagram and anilite stability. Admixtures of Ag, Fe, Bi, and Se in the sulphides are very minor. Supergene paragenesis is represented by chrysocolla with minor brochantite and very scarce malachite. These only bear trace impurities at the anionic sites. The supergene oxidation process began with the formation of abundant chrysocolla, at a relatively neutral pH. After dropping of the pH to ~4–6, brochantite was deposited.