Development of weakly euxinic basin environments in the Phanerozoic oxic ocean: Insights from Fe speciation and Mo isotopes of Devonian pyrite-bearing black shales, Dajiangping pyrite mine, South China

Abstract

Although the Phanerozoic ocean was generally oxygenated, local euxinic events occurred frequently, as witnessed by the widespread Paleozoic sediment-hosted stratiform sulfide (SHSS) deposits. Here, we use redox-sensitive Fe speciation and Mo isotopes of black shales and pyrite ores of the Devonian Dajiangping stratiform pyrite deposit in the South China Block (SCB) to decipher the linkage between redox conditions, basin restrictions and SHSS mineralization. From bottom to top, the host sedimentary succession on the deposit margin is a transgressive sequence of siltstones, carbonaceous limestones and black shales. The ratios of highly reactive Fe to total Fe (Fe_HR/Fe_T: 0.29 to 1.0), Fe in pyrite to highly reactive Fe (Fe_py/Fe_HR: 0.09 to 0.91), and δ^98/95Mo (0.50 ‰ to 1.30 ‰), show consistently increasing trends upward throughout the stratigraphic sequence, indicative of changes in the redox of bottom seawater from oxic, through anoxic, to weakly euxinic, in response to the subsidence of the basin. Black shales and interlayered laminated pyrite ores on the deposit center have δ^98/95Mo varying from +0.90 to +1.50 ‰ and higher than the host succession, clearly demonstrating the redox condition of bottom seawater in the depocenter is more euxinic than the margin of the basin. These δ^98/95Mo values lower than those of seawater were resulted from the occurrence of thimolybdates with variable Mo isotopic fractionations in the weakly euxinc conditions. In addition, the positive correlation (R² = 0.84) between Mo concentrations and total organic carbon (TOC) of the black shales suggests that Mo was authigenic in origin. These black shales have Mo/TOC of ∼1.7 ppm/wt%, which are much lower than those in poorly or unconfined euxinic basins (Mo/TOC > 11 ppm/wt%) and indicative of a restricted basin environment. The redox conditions of seawater changed from oxic to anoxic on the basin margin but maintained weakly euxinic in the basin center, consistent with a basinal environment showing subsidence and restriction. Such a restricted basin was due to the rifting of a fault-bounded trough on the passive margin of the SCB during the Devonian. Our study highlights the importance of locally restricted basins with euxinic and stratified seawater environments for the extensive development of SHSS deposits in an oxygenated Phanerozoic ocean.