Mineralogical study and significance of the basalt-hosted Carlin-type Au deposits in southwestern Guizhou Province, China

Abstract

The Jiadi and Damaidi gold deposits in southwest Guizhou Province are the largest basalt-hosted Carlin-type gold deposits recently discovered in China. This study uses the Tescan Integrated Mineral Analyzer, supported by detailed field investigations, regional geological data, and extensive sample collections, including mineralized ore, altered wall rock, and unaltered basalt samples, for ore-bearing and geochemical analyses. Comparative analysis between altered and unaltered basalt samples revealed a mineral assemblage of sericite, quartz, and pyrite. This mineral composition forms through the hydrothermal alteration of unaltered basalt, originally containing feldspar, pyroxene, and ilmenite. The wall rock primarily features sericite, quartz, and hematite. During the alteration process, major, trace, and rare earth elements notably migrate. In the Jiadi deposit, K₂O, Rb, Au, and REE significantly increase, while Na₂O, CaO, MgO, and MnO decrease. SiO₂, Al₂O₃, and Fe₂O₃ levels remain relatively stable. In the Damaidi deposit, K₂O, Rb, and Au enrich, contrasting with the depletion of Na₂O, CaO, MgO, and MnO, while SiO₂, Fe₂O₃, Al₂O₃, TiO₂, and REE show no significant changes. In the wall rock, TiO₂, Al₂O₃, K₂O, and REE increase, while Na₂O, CaO, MgO, and MnO decrease; SiO₂ and Fe₂O₃ content remains unchanged. The mineralization process likely originated from mid- to low-temperature, reductive magmatic hydrothermal fluids rich in CO₂, CH₄, N₂, H⁺, S²⁻, HS⁻, H₃AsO₃, and [Au(HS)₂]⁻. These fluids migrated to tectonically weak zones in the Lianhuashan area, where Emeishan basalts are present. They reacted with Fe-bearing minerals in the basalt, such as ferro-hornblende and ilmenite, forming pyrite, arsenic-bearing pyrite, and arsenopyrite, thus enriching Au in these minerals. Additionally, K⁺ and H⁺ in the fluid reacted with plagioclase in the basalt, forming sericite and quartz. As the fluid entered the wall rock from structural weak zones, its oxidation increased, leading to the complete or partial reaction of Fe-bearing minerals in the wall rock, resulting in the formation of hematite or magnetite. This mineralization process is similar to that observed in carbonate-hosted Carlin-type gold deposits in southwest Guizhou, with the primary distinction being the iron source. In carbonate deposits, iron originates from ferridolomite within the wall rock, while in basalt-hosted deposits, it derives from ferripyroxene and ilmenite.