Genetic association between carbonates and gold precipitation mechanisms in the Jinshan deposit, eastern Jiangnan orogen

Abstract

Carbonates have been traditionally regarded as post-ore minerals in hydrothermal deposits, but they can also occur in pre- and syn-ore stages. However, the genetic relationships between multistage carbonates and gold precipitation mechanisms remain unclear. The Jinshan gold deposit, the largest gold deposit in the eastern Jiangnan orogen, is characterized by the occurrence of abundant hydrothermal carbonates. The nature of the carbonates, especially their association with gold precipitation, was examined in this study through petrographic and geochemical analyses as well as geochemical modeling. Field and petrographic work recognized four paragenesis stages in the Jinshan deposit: (1) quartz (Q1)-carbonate, (2) pyrite-quartz (Q2), (3) gold-polysulfide-quartz (Q3)-carbonate, and (4) chlorite-quartz (Q4)-carbonate. Carbonates associated with gold mineralization are mainly present in the first and third stages. Among them, the first-stage carbonates are pre-ore ankerites (Ank1), which are generally crosscut by syn-ore sulfides. Such carbonates mostly occur in altered host rocks that are characterized by bleaching. Mineralogical and geochemical analyses revealed that chlorite was consumed while ankerite was produced, and Fe contents of the host rocks remained generally unchanged during alteration. Consequently, ankerite (Ank1) is interpreted to have been generated by the interaction of CO2-bearing fluids and chlorite in the host rocks. Geochemical modeling revealed that both Fe-bearing chlorite and ankerite (Ank1) can lead to gold precipitation by triggering sulfidation, but ankerite has higher chemical reactivity and therefore is kinetically favorable for more efficient gold deposition. Syn-ore carbonates were mainly formed in the third stage, primarily consisting of ankerite (Ank2) with minor calcite, and they were coprecipitated with native gold, galena, sphalerite, and chalcopyrite. Two stages of carbonates shared similar δ13CVPDB values and Pb, Zn, and Cu contents but different Ca, Mg, Mn, and Sr contents, indicating they formed at different stages of the same hydrothermal event. Geochemical modeling demonstrated that pH increases, as indicated by the formation of syn-ore carbonates, together with the consumption of S due to the precipitation of pyrite and arsenopyrite in the second stage, resulted in the decrease of Au, Pb, Zn, and Cu solubility in aqueous fluids. This would have led to the coprecipitation of native gold with galena, sphalerite, and chalcopyrite, forming the general small-scale but commonly occurring native gold−polysulfide veins in the hydrothermal gold deposits. Consequently, both pre- and syn-ore carbonates in the Jinshan deposit were genetically associated with efficient Au-polymetallic deposition through distinct mechanisms. Pre-ore carbonates caused the large-scale bleaching of the host rocks, while syn-ore carbonates were closely related to native gold−polysulfides, and they can both be used as exploration indicators for the Jinshan deposit and other similar hydrothermal gold deposits worldwide.