Mr. Wenhong Qiu Department of Earth Sciences, HKU
The late Devonian Dajiangping deposit in South China is a giant stratiform pyrite deposit (>200 Mt ores @ 32 wt.% S) and consists of laminated pyrite ores hosted in black shales. Syngenetic pyrite grains of the laminated ores have mostly negative δ34S values (<-16 ‰), indicating that sulfur of ores was reduced from seawater sulfate by bacterial sulfate reduction (BSR) in an open system. Diagenetic pyrite grains in black shales have heterogeneous δ34S values (-3.2 ‰ to 42.8 ‰), indicating that sulfur of these grains was reduced from infiltrated seawater sulfate by thermochemical sulfate reduction (TSR) in sediment pores. These features suggest that this deposit formed by precipitation of syngenetic pyrite grains on the sea floor when the exhaled metalliferous (Fe2+ rich) fluids encountered sulfuric (H2S rich) seawater. Minor diagenetic pyrite grains also crystallized in sediments under sediment-water interface.
The δ34S values of syngenetic pyrite vary systematically within each of four ore units. A sharp decrease from -1.8 ‰ to -24.8 ‰ in the bottom unit records the onset of BSR and basin subsidence. A gradual increase from -24.8 ‰ to -18.4 ‰ in the lower unit indicates a partially restricted basinal condition. The constant values of the middle unit (-20.6 ‰ to -17.6 ‰) and a further decrease from -25.3 ‰ to -28.7 ‰ of the upper unit suggest replenishment of seawater. These variations indicate that the mineralization occurred in an increasingly ventilated basin, which was likely related to the syn-rifting of a fault-bounded basin. Such syn-rifting basin was formed in a carbonated platform of South China Block, due to the break-up of South China Block from northern Gondwana during the late Devonian.