Origin and geochemical variability of massive sulfide veins in the Permian Kupferschiefer Formation: insights from the Fore-Sudetic Monocline, Poland 

Abstract

The Fore-Sudetic Monocline, stretching from southwestern Poland to central Germany, hosts significant sediment-hosted copper deposits associated with the Kupferschiefer formation. While the Kupferschiefer generally displays dispersed mineralization in sandstone, black shale, and dolomite layers, massive mineralization is rare but noteworthy. Our investigation focuses on the Lubin district and aims to enhance understanding of the formation of massive sulfide veins and their setting in the complex stages of Kupferschiefer development. Four mineralogically distinct types: chalcopyrite-pyrite, chalcopyrite-galena, galena, and chalcocite veins are identified. Each forms complex structures with mineralogical overgrowths, replacements and minor occurrences of bornite, pyrite, sphalerite, tennantite, and tetrahedrite. The diversity extends to geochemistry, with observed variations in bulk trace element concentrations, where strong Hg, Co, and Mo enrichment in the chalcopyrite-galena zone (3500-7000 ppm, >2000 ppm, 25-50 ppm, respectively), Zn, Hg, and Cd in the galena zone (3-5 wt%, 0.5-1.5 wt%, 90-210 ppm, respectively), and Re in the chalcocite zone (200-1000 ppm) is observed. Chalcocite and galena veins show significantly stronger REE depletion, compared to the chalcopyrite-dominated ones. Finally, differences in Cu, As (pyrite), Pb, Ag, Co, Ga (pyrite and chalcopyrite), Zn (chalcopyrite), and Co (galena) concentrations are noted within the minerals from different mineralogical zones. The complex mineralogy of the veins exhibits isotope variations based on in-situ Cu isotope analyses. The δ65Cu in chalcopyrite, chalcocite, and bornite (chalcopyrite-pyrite and chalcocite veins) spans from approximately -1‰ to +~0.5‰, with lower δ65Cu in chalcopyrite compared to other minerals within a sample. This data suggests complex chalcopyrite formation and its subsequent alteration, mostly to secondary bornite. Relatively small differences between minerals within the sample result from the isotopic equilibration with the ambient fluid responsible for the vein’s origin. However, the disparity between various samples in their Cu isotope composition indicates differences in the ambient fluid origin, i.e. possible time-related separation in veins formation or the isotope fractionation process that took place after sulfides precipitation process. The study was funded by the National Science Centre grant to KD (2019/35/N/ST10/04524).