Characteristics And Evolution Of Ore-forming Fluids Of Hadamengou Gold Deposit In Inner Mongolia

The Hadamengou gold deposit in Inner Mongolia,located on the gold margin of the northern margin of the North China Craton,is one of the representative super-large gold deposits in China.Quartz(vein)is very common in the mining area,and some of them are obviously mineralized.This paper divides the stages of quartz in the mining area and the quartz veins related to mineralization are identified.Secondly,the fluid inclusions of each stage were studied systematically in petrography,microthermometrical and LA-ICP-MS.On these basis,this article explores the characteristics of ore-forming fluids,the evolution of ore-forming fluids,the origin of the fluid system,and the migration and precipitation of gold in the Hadamengou gold deposit.There are five stages quartzs can be observed in Hadamengou gold deposit.The first stage is milky white microcrystalline quartz(Q1)distributed in the potassium-altered rock.The second stage is the metamorphosed and deformed wavy quartz veinlets(Q2).The third stage is a white quartz vein(Q3)arranged in an angle-like arrangement.The fourth stage was a reticular vein of smoky grey quartz(Q4).The fifth stage is a large milky white quartz vein(Q5).Fluid inclusion petrography and microscopic measurement show that the inclusions are mainly three types:gas liquid two phase brine inclusions(?),pure liquid H₂O inclusions(?)and CO₂inclusions(?).From early to late mineralization,the homogenization temperature of inclusion was from 375?to 215?to 307?to 291?,and the corresponding salinity was from 16.40%NaCl_eqvto 11.98%NaCl_eqvto 13.72%NaCl_eqvto 9.50%NaCl_eqv?As the mineralization progresses,the temperature and salinity of ore-forming fluid show a declining trend.In the main metallogenic stage,the ore-forming fluid temperature is 270??400?,the salinity is 13%?19%NaCl_eqv,and the density is 0.69?0.91g/cm³.It belongs to the H₂O-NaCl-CO₂ system with medium high temperature,medium low salinity and medium density.In situ trace results of a single fluid inclusion showed that the main contents of the fluid inclusion were Na,K,Mg,Al,Fe,Cu in high concentration,and significant Zn and Pb.It also contains Rb,Sr,Ag,Sn and Cs.Fluid inclusions in different ore-forming stages have different compositions.Exspecially,although the degree of mineralization is different,Cs/(Na+K)and Rb/(Na+K)in the fluid were distributed in a small numerical range,which displays that the ore-forming fluid in Hadamengou gold deposit originated from a geochemical homogeneous magmatic source region.The ore-bearing hydrothermal fluid mainly migrates in the form of Au(HS)⁰ and precipitates at the appropriate structural sites.The main geological processes that lead to gold precipitation in Hadamengou gold deposit include water-rock reaction,temperature reduction,pressure reduction and boiling,and fluid immiscibility in H₂O-NaCl-CO₂ system.Among them,the water-rock reaction related to alkaline gold-bearing magmatic rocks may be the most effective mechanism for causing gold precipitation.