The origin of the late paleozoic Dajiangping sedex-type pyrite deposit in Southeastern China

Abstract

The Dajiangping pyrite deposit in Southeastern China is a giant sediment-hosted stratiform deposit and contains >200 Mt of pyrite ores with an average grade of 30 wt% S. Stratiform orebodies are hosted in black shale, and consist mainly of laminated pyrite ores. Stratiform orebodies contain the youngest detrital zircon grains in ages of ca.429 Ma and the pyrite ores have pyrite Re-Os isochronal age of 389 ± 62 Ma. As the South China Block was in extensional passive margin setting and created numerous fault bounded basins during Devonian after it was broken up and migrated northward from Gandwana, such a mineralization age implies that the Dajiangping deposit near the Wuchuan-Sihui fault zone formed in a fault bounded basin in such rifting setting. The enrichment of TOC (total organic carbon), Mo, As, Pb, Zn and Cd and the high values of redox proxies (V/(V+Ni)>0.6, V/Cr>1) of black shales indicate an anoxic-euxinic deep basin. Lead isotope ratios of the Dajiangping deposit distribute in a linear array, with predominantly unradiogenic Pb similar to the Paleozoic metamorphic basement and the radiogenic Pb similar to the sedimentary covers, indicating metals of this deposit were mostly sourced from the metamorphic basement. Lead isotope ratios of other SEDEX deposits in the Cathaysia Block are in similar linear array, indicating lead isotopic compositions of basement beneath this Block are relatively homogenous. The SEDEX deposits in the Yangtze Block, which is another block in South China, have very different lead isotopic compositions, indicating the basement beneath these two blocks are different. Predominantly syngenetic pyrite grains of the laminated ores in the Dajiangping deposit have mostly negative δ34S values (<-16 ‰) which are homogenous within a single sample, indicating that these ores formed in a sulfate-dominated water column in which H2S were produced by bacterial reduction of seawater sulfate (BSR). Diagenetic pyrite grains in black shales have positive δ34S values (-3.2 ‰ to 42.8 ‰) which are heterogeneous within a single sample, indicating that these grains formed in sediment pores where sulfate was reduced to H2S by thermochemical sulfate reduction (TSR) and anaerobic oxidation of methane (AOM). These features suggest 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, during or after which minor diagenetic pyrite grains also crystallized in sediments. The δ34S values of pyrite ores 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 opening of a fault-bounded basin.