Pyrite Concretions in the Lower Cambrian Niutitang Formation, South China: Response to Hydrothermal Activity

Abstract

The Early Cambrian Niutitang Formation is characterized by wide distribution of black shales on Yangtze Block, South China. Here we have reported the pyrite concretions in the bottom of the Niutitang Formation deposited in the slope-basin environment of Yangtze Block. The pyrite concretion was mainly composed of pyrite associated with hydrothermal minerals (barite, hyalophane, tetrahedrite), followed by quartz and organic matter. Trace elements Mo and U displayed significant enrichment (enrichment factors > 10), indicating the euxinic bottom water condition. Cu, Ni, and excess Ba concentrations were relatively high, denoting high primary productivity. In-situ sulfur isotope compositions of pyrite concretions δ³⁴S_py) showed little variations (13.2‰–19.4‰) and small fractionations compared to coeval seawater δ³⁴Sso₄. Petrological and geochemical analyses indicated the pyrite concretions were formed in the sediment-water interface during the early diagenesis, with H₂S diffusing from the euxinic water, and influenced by hydrothermal activity leading to the coexistence of barite, hyalophane, and tetrahedrite. These results imply euxinic bottom water featured by high primary productivity and increasing riverine flux of sulfate from chemical weathering during the Early Cambrian.

Keywords

shale / pyrites / black shale / pyrite concretion / depositional environment

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