LAUSR

Abstract Mississippi valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits, that are formed by fluid expulsion from sedimentary sulfide successions. The sulfides were generated by thermochemical sulfate reduction (TSR) of the evaporitic sulfates dissolved in fluids. The initiation, processes and principles for TSR occurring in MVT ore deposition and the respective influence of major minerogenetic metal ions such as Pb2+ and Zn2+, as well as Fe2+, Sr2+, Ba2+ has not been clearly resolved. To evaluate the TSR activity of metal cations in MVT minerogenetic systems, a series of 300 °C to 450 °C gold-tube hydrous-pyrolysis experiments were separately conducted with FeSO4, PbSO4, ZnSO4, BaSO4 or SrSO4 and with n-octadecane (n-C18) as the hydrocarbon substrate. Based on the yields (from gas chromatography (GC) analysis) and carbon isotopic compositions (determined by GC-irMS) of the gases produced in the hydrous-pyrolysis gold-tube experiments, the TSR reactivity of the minerogenetic metal sulfates was ranked FeSO4 > ZnSO4 > Sr/BaSO4 > PbSO4. TSR occurred easily in the FeSO4 experiments at 300 °C, but hardly at all in PbSO4 experiments at 450 °C. Hence, S2− for the formation of the gelenite (PbS), sphalerite (ZnS) and pyrite (FeS2) in the MVT ore deposits appears to be related to Fe2+, which could initiate the TSR easily to produce reduced sulfur. The following two potential routes would provide good support for the TSR of FeSO4: (1) Hydrolysis of Fe2+, or the formation of a ferrous hydroxide-sulfate-hydrate complex, that increases the H+ concentration, resulting in the formation of HSO4− that initiates TSR; and (2) The oxidation of Fe2+ to Fe3+, which on subsequent hydrolysis (or the formation of iron-hydroxide-sulfate-hydrate complex), would greatly increase the concentrations of H+ and HSO4−, reduce the pH of the brine fluids and maintain acidic conditions favorable to TSR. However, the precipitation of pyrite greatly consumes S2−, limiting the concentration of H2S and thereby affecting the rate of TSR. In short, TSR is difficult to simulate using Zn2+ and Pb2+ sulfates, but easy with sulfates containing Fe2+ and Mg2+. This suggests that the occurrence of the FeSO4 and MgSO4 is critical in the formation of the large-scale MVT ore deposits, where they react as acid buffering agents, decreasing and maintaining a lower pH in the brine fluids and accelerating and sustaining TSR with higher concentrations of HSO4−. H2S/S2− would then be produced continually and participate as FeS2, ZnS and PbS in the large-scale MVT ore deposits.