Disseminated gold mineralization under varied redox conditions: Constraints from microscopic observation, geochemistry, and thermodynamic modeling on fluid-rock interactions in lamprophyres, Zhenyuan gold deposit

Abstract

The controls of fluid redox states on disseminated gold mineralization remain ambiguous. To address this, this study compared the hydrothermal alteration and auriferous pyrite geochemistry in lamprophyres, Zhenyuan gold deposit, which were infiltrated by fluids with varied redox states. Vein-A, vein-B, and vein-C were distinguished and they contain different sulfur-bearing minerals, i.e. pyrite, pyrite and barite, and barite, respectively. This suggests the gradual oxidzing formation conditions from vein-A to vein-C. Alteration halos enveloping vein-A and vein-B develop disseminated pyrite. In the vein-A halos, alteration minerals transit from pyrite to Fe‑carbonates outwards, which was well replicated by the thermodynamic simulation of the successive reaction of H2O-CO2-H2S-Au-bearing fluid with lamprophyres. The precipitation of pyrite grains in vein-B halos is farther from vein-B, suggesting progressive fluid-rock interaction (FRI) first induced the reduction of oxidized S followed by the pyrite precipitation. Additionally, δ34S values of pyrite are more scattered in vein-B halos (−8.0 to 4.7‰) than in vein-B, vein-A, and vein-A halos, implying the sulfate-sulfide disequilibrium. Pyrite in vein-B halos displays higher As concentrations than that in vein-A halos due to the strong redox dependence of As partition from fluids to pyrite. Au and Au/As of halo-hosted pyrite grains decrease outwards, and pyrite in vein-B halos has low Au and Au/As (below 93.93 ppm and 0.005, respectively) compared to that in vein-A halos (below 1597.08 ppm and 0.031, respectively). It is thus suggested that Au concentrations in ore fluids decreased during outwards FRI, and the lack of pyrite grains in proximal alteration zones results in lower gold concentration in pyrite under relatively oxidized conditions. This study reveals that the fluid redox conditions can affect As concentrations and spatial distribution of pyrite in alteration halos, which directly controls invisible gold precipitation.