Re-Os isotopes and PGE geochemistry of black shales and intercalated Ni-Mo polymetallic sulfide bed from the Lower Cambrian Niutitang Formation, South China*

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Abstract
The Lower Cambrian Niutitang Formation consists of a thick black shale sequence with a regionally distributed conformable Ni-Mo polymetallic sulfide horizon and a chert bed at its basalstrata on the Yangtze Platform. South China. In this paper, we discuss all available data on Re-Os isotopes and Platinum Group Element (PGE) distribution pattern of the Ni-Mo polymetallic sulfide ore and its host rocks (black shales, cherts, and phosphorites) from Guizhou and Hunan provinces. Our results show that the black shales and the Ni-Mo sulfide ore have a high initial 187Os/188Os ratio of 0.78–0.86. indicating that the Early Cambrian ocean across the Yangtze Platform had a highly radiogenic Os value. This ratio is slightly lower than but still similar to present-day seawater. possibly as a result of high continental weathering at that time. The Ni-Mo sulfide ore yields a Re-Os isochron of 537 ± 10 Ma (MSWD =11.9). possibly representing the depositional age of the Niutitang Formation. The chondrite-normalized PGE pattern. Pt anomaly (Pt/Pt* ). Pt/Pd, Ir/Pd, Au/Ir and Re/Mo ratios of the Ni-Mo sulfide ore and its host rocks from South China indicate a varying source contribution of the PGE and other metals for different rocks. It is suggested that the cherts and Ni-Mo sulfide ore may have a significant proportion of PGE and probably other metals deriving from submarine-hydrothermal fluids with a mantle signature.