Ore-forming Mechanism Of The Xiyi Lead-zinc Deposit And Regional Ore Indications In Baoshan-Longling Area,yunnan Province

The Xiyi Pb-Zn deposit,located in the Baoshan Block of the Sanjiang region in western Yunnan,China,contains 1.2 Mt of Pb+Zn resources.In recent years,preliminary studies mainly focused on the geological characteristic of the deposit.The ore deposit geochemistry,nature and source of the ore-forming fluids,source of ore materials and metallogenic regularity are weak in this deposit.Some disputes still exist in ore genesis.Therefore,the prospecting work of the deposit and peripheral area has been restricted to some extent.In this paper,we systematically studied the deposit geology,geochemistry of host rocks and ore-forming fluid and stable isotope of the Xiyi deposit.Based on the detailed study and comparative studies on the regional Pb-Zn deposits,we proposed a definitive interpretation of ore genesis,established a genetic model,summarized regional ore-controlling factors and pointed out regional ore indications.The ore district is subdivided into three ore blocks,i.e.,the Zhaozhai,Dongjiazhai,and Lutu ore blocks.The orebodies are hosted by Devonian-Carboniferous limestone,structurally controlled by the NE-trending fractures,and occur in stratoid,vein and lenticular forms.The mineral assemblage of the ores includes sphalerite,galena,pyrite,arsenopyrite,chalcopyrite,and marcasite.Calcite,barite,and quartz occur as gangue minerals.Three stages of hydrothermal mineralization are recognized based on the ore textures,crosscutting relationship and mineral assemblages: an early stage of sphalerite + galena + calcite(stage I),a middle stage of galena + arsenopyrite + calcite ± sphalerite(stage II),and a late stage of calcite + pyrite± barite ± quartz(stage III).Fluid inclusion and isotopic studies indicated that the ore-forming fluids belonged to the H2O-NaCl system of low temperatures(100~200 ?),medium salinities(8~20wt%NaCleq)and medium densities(1.0~1.1g/cm3),and contained minor amounts of CH4.The H2 O in hydrothermal fluids was derived from basinal brine.The?13CPDB and ?18OSMOW values of calcite vary in the range of-5.2‰ to 3.2‰ and12.5‰ to 20.0‰,respectively,suggesting that the CO2 in the ore-forming fluids was derived from the dissolution of marine carbonate.The sulfides in mineralization stages have a narrow range of ?34S values(0-3‰),and the barite has a ?34S values of5~14‰.The sulfur was derived from the regional marine sulfates,reduced by the BSR in the sedimentation-diagenesis period.The lead isotopic compositions in sulfides are very similar to those in the diagenetic pyrite in the host rocks,plotting in the field of the upper crust Pb evolution curve.Thus,the lead source was likely the Carboniferous and Devonian strata.Comprehensive studies suggest that the deposit is an epigenetic deposit,occurring in the carbonate strata and controlling by the structures.The principal similarities between the Xiyi Pb-Zn deposit and typical MVT Zn-Pb deposits include the tectonic setting,deposit geology,geochemistry of deposit,ore-forming fluid characteristics and the ore material sources.The geologic,fluid inclusion and isotopic evidence in this study suggests that the Xiyi Pb-Zn deposit is a Mississippi Valley Type deposit.Based on the study,we established a genetic model,summarized regional ore-controlling factors and pointed out regional ore indications.