Hydrothermal reduction of sulfates by ferrous carbonate: Implications for the origin of reduced sulfur in carbonate-hosted Pb-Zn deposits associated spatially with basalts

Abstract

Reduced sulfur derived from thermochemical sulfate reduction (TSR) in Pb-Zn deposits is one of the long-debated issues in economic geology. The formation of - 2 valent sulfur (S2-) in Pb-Zn deposits associated spatially with basalts, may be related to the thermal reduction of SO42- by ferrous ions (Fe2+). In the present study, hydrothermal experiments on the reduction of magnesium sulfate (MgSO4) by ferrous carbonate (FeCO3) were carried out to investigate the thermodynamics feasibility, kinetic characteristics and reaction mechanisms for the inorganic reduction of sulfates. The results show that the reduction of MgSO4 by FeCO3 is thermodynamically stronger than TSR of oil, gaseous hydrocarbons and solid bitumen. Moreover, this reduction reaction occurs at threshold temperatures below 300 °C under hydrothermal conditions, yielding the main products of magnetite (Fe3O4), pyrite (FeS2) and pyrrhotite (Fe7S8). The reaction mechanism for the studied multiphase system at 400 − 525 °C involves at least nine elementary steps. The chemical kinetics for the FeCO3 −MgSO4 − H2O system is in accordance with the phase boundary control reaction model with apparent activation energy of 70 kJ/mol. The formation of S2- in carbonate-hosted Pb-Zn deposits associated spatially with basalts is most likely a two-stage sulfate reduction process involving Fe2+ (90 −140 °C) and organic matters (>140 °C) in series. The new findings from the inorganic reduction of sulfates would contribute to clarification of the genesis of Pb-Zn deposits and help to reconceptualize some of the long-standing controversial issues in this field.