The Late Devonian Dajiangping pyrite deposit in South China is a giant sediment-hosted stratiform deposit and contains >200 Mt of pyrite ores with an average grade of 30 wt % S. The orebodies are hosted in carbonaceous siltstone and black shale and consist mainly of laminated pyrite ore. Small euhedral pyrite crystals (pyrite 1) and larger subhedral pyrite crystals (pyrite 2) are the main ore mineral and occur as laminated aggregates of subhedral to euhedral grains. Anhedral pyrite crystals (pyrite 3) associated with pyrrhotite are present in the host rocks and are interbedded with the ore layers. Mass-balance calculations reveal that the pyrite ores have Al-normalized Ca, Mg, Mn, Si, Na, K, and organic carbon contents similar to the host sedimentary rocks, precluding substantial carbonate dissolution and a stratiform replacement origin for the ores.
Pyrite 1 and 2 crystals in the laminated ores have δ34S values ranging from −28.7‰ to 23.6‰, and mostly lower than −16‰. Such highly negative values indicate that the sulfur in these ores was sourced from bacterially reduced seawater sulfate in an open system. Pyrite 1 and 2 crystals within a single laminated ore sample have similar δ34S values. The mostly negative sulfur isotope composition implies that the pyrite crystallized above to just below the seawater-sediment interface by consuming aliquots of H2S supplied by bacterial sulfate reduction (BSR) in sulfate-dominated shallow muds. Pyrite 3 crystals have mostly positive δ34S values, ranging from −3.2 to 42.8‰ with a mean of 5.7‰. There are also large ranges of δ34S values within a single sample, for example, from 12.1 to 42.8‰ in sample YF0204. Such positive and highly heterogeneous δ34S values likely resulted from thermochemical sulfate reduction (TSR) and anaerobic oxidation of methane (AOM) coupled with sulfate reduction in a tightly closed system of deep sediments that experienced strong Rayleigh fractionation during diagenesis.
We propose that the laminated pyrite ores of the Dajiangping deposit formed by sedimentary exhalation in an anoxic seawater column. Hydrothermal fluids leached Fe from the metamorphic basement underlying the stratiform orebodies, as indicated by the similar Pb isotope compositions of the laminated ores and metamorphic basement, and vented onto the sea floor. Mixing of Fe2+ from the exhaled metalliferous fluids with H2S from the ambient anoxic seawater resulted in the precipitation of pyrite 1 and 2 crystals above and immediately below the sea floor, respectively, and the formation of laminated ores. This was followed by crystallization of a small proportion of pyrite 3 in the pores within the deep sediment pile.