Evolution Of Ore-forming Fluids And Metallogenesis Of The Jinchang Au Deposit In Mojiang County,Yunnan Province

The Ailaoshan metallogenic belt is an economically important Himalayan orogenic gold belt in Yunnan province,western China.The Jinchang Au deposit in Mojiang County represents for a typical Au deposit in the middle of the Ailaoshan belt.And the Au reserves are 29.7t and the average grade is 3.07g/t.On the basis of field observation,laboratory studies,including in-situ S isotope,H-O isotope,and electron microprobe analysis,were carried out to investige evolution history of ore-forming fluids and metallogenetic processes of the Mojiang Au deposit.Some significant conclusions have been reached as follows:（1）According to the field and microscopic observation and electron microprobe analysis（EMPA）,the metallogenic episodes of the Jinchang Au deposit in Mojiang County could be divided into three periods:sedimentary period,magmatic hydrothermal period,and epigenetic period.Among them,the hydrothermal period could be further divided into three phases:,The initial Au-enrichment stage are mostly dominanted by quartz,pyrite,serpentine,talc-chalcony,sulfur,nickel,and gold.The following Au-rich stage mostly consists of quartz,pyrite,gersdorffite,chromite,mica,polybasite,gold.Gold mainly precipitated during this stage.Less Au enrichment stage is characterized by highly transparent minerals,including quartz and chalcedony.Calcite and pyrite are also commonly occurred during this stage.Minerals formed during this stage often occurred along the fissure of earlier formed minerals.Gold occurs in three occurences in the Jinchang Au deposit,inclduing grain gold,lattice gold and wrapped gold.The gold-bearing minerals are mainly pyrite and arsenopyrite,and are commonly hosted by gersdorffite and polybasite.Gold has preliminary enriched during the sedimentary period,which was altered by the following magmatic hydrothermal fluid and led to significant Au precipitated during this period.（2）Fluid inclusion study of the Jinjiang Au deposit suggests that the mean temperature of the Au-enrichment stage is 218.9°C.The mean temperature of the Au-rich stage is 162.5°C.The mean temperature of the less Au enrichment stage is 138.2°C.with medium and low temperature,low salt,low density characteristics.The temperature of magmatic fluid decreases gradually from early to late stage.（3）In-situ sulfur isotope analysis of the Mojiang gold deposit,that indicated the normal distribution characteristics of the value ofδ³⁴S near-5‰,as low as-23‰.It indicates that both mantle-derived sulfur and sedimentary sulfur exist at the same time.H-O isotope analysis shows that ore-forming fluids are probably mainly the mixed source of mantle fluids and formation water.So fluid source is mixing source.（4）In summary,the Jinchang Au deposit in Yunnan belongs to the medium-low temperature sedimentation-hydrothermal superposition deposit.