Metallogenesis Of The Jiadi Golden Deposit In Southwestern Guizhou Province

The Jiadi golden deposit,located in the southwestern Guizhou Province in southwest China,one of wold famous ore-concentration areas as named the "Golden Triangle" of Yunnan,Guizhou,and Guangxi,is is a newly discoverd large-scale fine disseminated deposit in southwestern Guizhou,and is also the largest primary golden deposit found in basalt rocks in China.However,few research has been carried out on the nature and evolution of ore-forming fluids,and the source of the ore-foming materials,resctricitng our knowledge of the ore-forming mechanics,the genesis of the deposit.This paper mainly focuses on the characteristics of the mineralized-related alteration,ore-forming fluid and ore-forming materials,based on the detailed field investigation together with laboratory microscope observations by the method of the measurement of the temperature in fluid inclusion assemblage and the isotope geochemical analysis.The goal of this paper is to discuss the ore-foming mechanics and to establish a metallogenic model of the Jiadi golden deposit,and futhur to provide guidance to the ore-prospecting exploration.The main research results are as follows:(1)The Jiadi golden deposit is outcropped in the strongly-deformed area of the southeast wing of the Lianhuashan anticline in the northeast of the southwestern Guizhou ore concentration area.There are two layers of orebodies.The lower one developed inside the structural breccias between the limestone of the Permian Maokou Formation and the Emeishan basalt and the upper one developed in the basalt breccia of the second member of the Emeishan basalt.Baed on the field investigation and microscope observation in the laboratory,the mineralization period can be divided into the sedimentary diagenesis pre-enrichment stage and the hydrothermal mineralization stage.The later can also be divided into three stages,incluing pyrite mineralization,smoky quartz fomation and complex sulfide stage.(2)Filed structural analyses suggested that structural superimposition and structural transition deformation area together with the regional unconformity surface should be favorable for the migration of ore-forming fluid and precipitation of ore-forming materials.The analyses of fluid inclusions are carried out for the hydrothermal transparent minerals formed at different stages.The content of the fluid inclusions are mainly NaCl-H2O type and CO2-NaCl-H2O type,including single-phase,two-phase and three-phase inclusion types.Our analyses showed that the early stage of mineralization were dominated by CO2-rich inclusions,with the temperature of 195?231?,and the value of salinity ranging of 2.1%?7.6%(NaCleq).The inclusions in the main mineralization stage are well developed and the homogenization temperature ragnes from 175? to 210?,with the salinity of 11.4%?5.9%(NaCleq).The homogenization temperature in the late stage is generally less than 180?,and the salinity is 10.9%?5.3%(NaCleq).Laser Raman analysis revealed that the fluid mainly contained CH4,N2,SO2 and other gas phases,and the composition of the gas phase in the inclusions became more simple as the time evolution of the ore-forming fluid.Our analyese suggested that the ore-forming fluids should be a type of medium and low temperature,low salinity,and low density fluid as a whole,similar with other Carlin-types of golden deposits in the region.(3)The analyese of the carbon isotope composition in hydrothermal quartz showed that there probably were attributes of magma source in the initial ore-forming fluid,and obvious multi-source mixing of the deep-cycle atmospheric precipitation and organic fluids in the hydrogen and oxygen isotope composition map.There are two groups of pyrite,including the sedimentary period and the hydrothermal mineralization period.The value of pyrite ?34S in the sedimentary diagenetic group is less than-0.5%?,and that in the later group is between 0.5 and 4.5‰,suggesting that there shouble be a source of magma.(4)A new ore-forming model of the Jiadi golded deposit is established basen on the comprehensive studies of the ore-geological features,hydrothermal alternations and the ore-foeming fluid,suggesting that the ore-forming fluids and materials of the Jiadi golden deposit should probably be of magma source,and the mixed action of the deep-circulating atmospheric water with the organic fluids in the shallow crust of the basin should be the main mechanics for the unloading and enrichment of ore-forming materials.