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

The Jiadi and Damaidi gold deposits in southwest Guizhou Province are the largest basalt-hosted Carlintype 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 orebearing 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,K_(2)O,Rb,Au,and REE significantly increase,while Na_(2)O,CaO,MgO,and MnO decrease.SiO_(2),Al_(2)O_(3),and Fe_(2)O_(3)levels remain relatively stable.In the Damaidi deposit,K_(2)O,Rb,and Au enrich,contrasting with the depletion of Na_(2)O,CaO,MgO,and MnO,while SiO_(2),Fe_(2)O_(3),Al_(2)O_(3),TiO_(2),and REE show no significant changes.In the wall rock,TiO_(2),Al_(2)O_(3),K_(2)O,and REE increase,while Na_(2)O,CaO,MgO,and MnO decrease;SiO_(2)and Fe_(2)O_(3)content remains unchanged.The mineralization process likely originated from mid-to low-temperature,reductive magmatic hydrothermal fluids rich in CO_(2),CH_(4),N_(2),H^(+),S^(2-),HS^(-),H_(3)AsO_(3),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.

Source and Evolution of the Ore-Forming Fluids of Carlin-Type Gold Deposit in the Youjiang Basin,South China:Evidences from Solute Data of Fluid Inclusion Extracts

The source and evolution of ore-forming fluids is important to understand the genesis of Carlin-type gold deposit.Constraints on the source and evolution of ore fluid components by the con-ventional geochemical methods have long been a challenge due to the very fine-grained nature and complex textures of hydrothermal minerals in these deposits.In this study,we present the crush-leach analyzed solute data of fluid inclusion extracts within quartz,calcite,realgar,and fluorite from the Shuiyindong,Nibao,and Yata gold deposits in the Youjiang Basin,providing new insights into the source and evolution of ore-forming fluids.The results show that the high molar Cl/Br ratios up to 2508 in fluid inclusion extracts are indicative of a contribution of magmatic hydrothermal fluids.Flu-ids mixing between basinal and magmatic-hydrothermal fluids are evident on the plots of Cl/Br versus Na/K ratios,showing that ore-stage milky quartz near the magmatic-hydrothermal fluids reflects magma origin of the ore-forming fluids,whereas late ore-stage drusy quartz and realgar near the de-fined basinal fluids suggest the later input of basinal fluids in late-ore stage.Although the predominate-ly host rocks in Shuiyindong,Nibao and Yata gold deposit are bioclastic limestone,sedimentary tuff,and calcareous siltstone,respectively,the solute data of fluid inclusion extracts records they underwent the similar fluid-rocks reactions between the Na-rich magmatic hydrothermal fluids and the Ca-and Mg-rich host rocks.This study highlights the solute data of fluid inclusion extracts obtained by crush-leach analyses have the potential to fingerprint the source and evolution of ore-forming fluids of the Carlin-type gold deposit.