Seawater contribution to polymetallic Ni-Mo-PGE-Au mineralization in Early Cambrian black shales of South China: Evidence from Mo isotope, PGE, trace element, and REE geochemistry

The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni-Mo-PGE-Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy delta Mo-98/95 values of 0.94 to 138 parts per thousand. (avg. 1.13 +/- 0.14 parts per thousand., 1 sigma, n = 11), and the host black shale and phosphorite have slightly more variable delta Mo-98/95 values of 0.81 parts per thousand. to 1.70 parts per thousand. (n = 14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 10(7) compared to present-day seawater. PAAS-normalized REE + Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni-Mo-PGE-Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts. (C) 2012 Elsevier B.V. All rights reserved.