EXTREME METAL ENRICHMENT IN EARLY CAMBRIAN SULFIDE-RICH BLACK SHALE IN SOUTH CHINA: A SEAWATER MODEL

Black shales of the Early Cambrian Niutitang Formation mark the outer passive paleomargin of the Yangtze craton in SE China. The lowermost part of the black shale sequence hosts a regionally distributed concordant, several cm-thick, sulfide- and carbon-rich polymetallic layer which displays extreme metal enrichment. Molybdenum, Ni, Se, Re, Os, As, Hg and Sb are >1000 times, and Ag-Au-Pt-Pd >100 times enriched over bulk continental crust. Large phosphorite, barite and coal (sapropelic alginite) deposits occur in the same general stratigraphic position. The polymetallic sulfide layer has a Re-Os isochron age of 541 ±16 Ma (2s) with an initial 187Os/188Os ratio of 0.78 ±0.19. This initial osmium isotope ratio corresponds to that of the black shale host, and is similar to present-day seawater. The sulfide layer displays a positive Y anomaly (Y/Ho >50) which suggests a mostly hydrogenous origin of REE, as opposed to the black shale host with Y/Ho ~30. Metal ratios such as Au:Pd:Pt ~1, and Mo:Pb and Mo:Cu >>1 are similar to seawater, but very different from hydrothermal sulfide precipitates. The sulfide layer/average seawater metal ratio is on the order of 106-108 for a broad redox-sensitive element spectrum, which is about 10-100 times higher than for the black shale host and for average metalliferous black shale. Synsedimentary metal enrichment from seawater under anoxic (sulfate-reducing) conditions appears likely, provided a very low terrigenous sedimentation rate and replenishment of a stagnant basin by oxic seawater on a time scale of hundreds of years or less. Present-day analogs for such metal enrichment are the Cariaco Trench and the Black Sea which, however, have much larger dilution by terrigenous input. The model suggests that Mo and other redox-sensitive bioessential metals were as abundant in the Early Cambrian oceans as today, as probably opposed to the largely sulfidic mid-Proterozoic oceans where Mo scarcity would have limited the bioavailability of nitrogen (MoFe-nitrogenase), and possibly hindered animal diversification.