Temporal And Spatial Evolution Of Ore-forming Fluid And Metallogenic Mechanism In The Jinwozi Gold Deposit,Beishan Orogen

The Jinwozi gold deposit is located in the central Beishan area,southern margin of the subduction-collision zone between the Kazakhstan and Tarim Plates.The ore genesis is the orogenic type.The wall rock of the Jinwozi gold deposit is granodiorite of the ore district.In order to explore the temporal and spatial evolution of ore-forming fluid and the metallogenic mechanism,the samples from different mineralization stages and different depths were analyzed by petrography,microscopic temperature measurement,Laser-Raman spectrum,isotope geochemistry.The hydrothermal ore-forming process can be divided into three stages from early to late,i.e.,pyrite-quartz stage?early stage?;quartz-pyrite polymetallic sulfide stage?middle stage?;quartz-calcite stage?late stage?.The gold mineralization mainly occurred in the middle stage.Two types of fluid inclusions are identified:NaCl-H₂O inclusions?W-type?and CO₂-H₂O-NaCl inclusions?C-type?.Both of the two FI types can be observed in the first two stages;while only the W-type inclusions occur in the late stage.The ore-forming fluid system evolved from a CO₂-H₂O-NaCl system to a NaCl-H₂O system.The homogenization temperatures of early stage FIs range from 200?and 300?,with salinities of 1.4¹4.8 wt%NaCl eqv.The FIs of middle stage are homogenized between 160?and 260?,with salinities of 0.4¹4.5 wt%NaCl eqv;and in late stage they are 120¹80?and of 0.2⁷.6 wt%NaCl eqv.From early to late stage,the homogenization temperature and salinities decreasing gradually.The results show that the ore-forming fluid system has evolved from the mesothermal,medium-low salinity,CO₂-rich metamorphic water to the mesothermal-epithermal,low salinity and CO₂-poor meteoric water.The metallogenic depth of the gold deposit is 5 to 11 km using the C-type inclusion trapping pressure.From the early stage to the late stage,the?¹⁸O_H2O values ranged from 4.8‰to7.0‰,-0.4‰to 4.8‰,-15.5‰,the?D_H2O values of varied from-129‰to 96‰,-133‰to-101‰,-114‰,respectively.The?¹⁸O_H2O value gradually decreased,and the ore-forming fluid was evolved from metamorphic fluid to atmospheric precipitation,and the middle stage had obvious mixing of atmospheric precipitation.The?¹³C values ranged from-5.4‰to 3‰with an average of-3.0‰,indicating that the CO₂ in the ore-forming fluid was probably sourced from the decarbonization of carbonates which is hosted in the Jinwozi Formation.The?³⁴S values of sulfides in ores are concentrated in the range of 5.5 to 7.5‰.The values of ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb for sulfides are 18.430 7¹8.461 0,15.683 9¹5.713 9,38.628 5³8.714 5.The metallogenic materials of the Jinwozi gold deposit are mainly provided by the Jinwozi Formation.From the shallow to deep of the orebody,the homogenization temperature and salinity firstly increase and then decrease,which might be caused by the multi-superposition of ore-forming fluids.From the southwest to the northeast area at Jinwozi gold deposit,the homogenization temperature of fluid inclusions and ore-forming depth gradually increases,the lead isotope ratio of sulfides and the age of lead model gradually increases.It is reflected that the northeastern part of the mining area may be a heat source center,and ore-forming fluids migrate from the northeast to the southwest of the mining area.Therefore,it is prospected that there will be a good metallogenic potential in the northeastern mining area.The physicochemical and hydrogen-oxygen isotopic data of fluid inclusions show that fluid mixing might be the dominant mechanism of gold deposition,and there may be local fluid boiling.