Multiple Sources of Metals of Mineralization in Lower Cambrian Black Shales of South China: Evidence from Geochemical and Petrographic Study

摘要: Black shales of the Lower Cambrian Niutitang Formation in southern China (Huangjiawan mine, Zunyi region, northern part Guizhou Province) host regionally distributed stratiform polymetallic Ni-Mo-platinum group elements (PGE)-Au phosphate- and sulfide-rich ores. These are confined to a ≥0.2-m thick ore horizon composed mineralized bodies algal onkolites, phosphate nodules, sulfide shale clasts organic matter-rich matrix. Compared footwall hanging wall shales, bed is strongly enriched Ni (up 100-fold), As 97-fold), Mo 95-fold), Sb 67-fold), Rh 49-fold), Cu 37-fold), Pd 33-fold), Ru 24-fold), Zn 23-fold), Pt 21-fold), Ir 15-fold), Co 14-fold), Pb 13-fold). Even black significantly by (21-fold) (12-fold) but depleted Cr comparison average shale. Organic matter represented separate accumulations dispersed rock matrix or as biotic bitumen droplets veinlets clasts. Similar carbon (Corg) values an enclosing little mineralization indicate that metal accumulation was not controlled only biogenic productivity rate. Evaporitic conditions during sedimentation basal were documented occurrence preserved Ni-, V-, Cr-, Cu-enriched phosphate-rich hardground with halite anhydrite pseudomorphs on paleosurface underlying Neoproterozoic carbonates. Doushantuo characterized increased concentrations. Comparison abundances both could have become source metal-rich weathering. The Ni-Mo-PGE interpreted represent remnant shallow-water rich metals, which originated sediment-starved, semi-restricted, seawater environment. During Early transgression influx fresh intensive evaporation, together hydrothermal enrichment semi-restricted basin, resulted formation dense metalliferous brines; co-precipitation metals phosphates sulfides occurred at above oxic–anoxic sediment interface. Metal-enriched disintegrated action waves bottom currents deposited deeper anoxic basin. Contemporaneously Ni-Mo-PGE-Au bed, economic sedimentary exhalative (SEDEX)-type barite deposits forming stratigraphically geotectonically similar setting. results geochemical study Shang Gongtang SEDEX-type Ba deposit concentrations Ag, As, Cr, Cu, Fe, Mn, Ni, Pb, Sb, V, other decrease from top body toward also display element Formation, developed body. But composition layer different Ni-Mo showing elevated values. It suggested brines might leached sequences became, after mixing normal seawater, additional Zn, PGE, V metals.