Silver, cobalt and nickel mineralogy and geochemistry of shale ore in the sediment-hosted stratiform Nowa Sól Cu-Ag deposit, SW Poland

Abstract

The Nowa Sól deposit is a part of the newly discovered Northern Copper Belt and is situated some 30 km north-west of the Lubin-Sieroszowice Mining District (so called New Copper District) in SW Poland. The ore horizon spans across the upper part of the Lower Permian (Rotliegend) terrestrial redbeds and the lower part of the Upper Permian (Zechstein) marine rocks and comprises three lithotypes: sandstone, shale and carbonate. The high-grade shale ore has polymetallic characteristics and is a crucial host for by-product metals such as silver, cobalt, and nickel (studied in this paper), but also molybdenum, vanadium, and rhenium. The results of bulk-rock and electron microprobe as well as mineralogical (optical and scanning electron microscope) data of the mineralized, organic-rich shale ore from the Nowa Sól deposit are presented. This thin stratigraphic horizon, ranging from 0.06 m to 0.59 m, shows notable concentrations of critical metals, including on average 15.9 wt. % copper, 715 g/t silver, 318 g/t cobalt, and 345 g/t nickel. It constitutes less than 10% of the total ore mass, but accounts for 36% of the silver, 40% of the nickel, and 42% of the cobalt found within the deposit. The ore sulfides in the mineralized shale in the Nowa Sól deposit include chalcocite, djurleite, bornite, accompanied by digenite, covellite, tennantite, galena, sphalerite, and pyrite. The silver content within the copper sulfides exhibits a linear decrease: chalcocite > djurleite > bornite. Three primary silver minerals are identified within the shale ore, namely native silver, silver amalgam, and stromeyerite. Two types of silver amalgam are observed: Hg-rich and Hg-poor. Cobaltite and gersdorffite represent the primary cobalt and nickel minerals, occurring as micrometer-sized inclusions within chalcocite and djurleite. Textural observations suggest that the silver, cobalt, and nickel mineralization postdates the major phase of copper sulfide precipitation. It is shown that in the Nowa Sól deposit, the Kupferschiefer horizon has acted as a geochemical barrier for abovementioned metals during protracted time – from early, syndepositional to late, epigenetic stage of basin evolution. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6873631