Evolution Of Ore-Forming Fluid In The Huangjindong Gold Deposit,Jiangnan Orogenic Belt

The Jiangnan Orogenic Belt is the most important gold-concentrated area in South China.As a representative Huangjindong gold deposit,is strictly controlled by the Changsha-Pingjiang fault,with reserves of more than 85 tons,the fluid evolution and gold precipitation mechanism of the deposit have always been controversial.Based on the detailed arrangement and comprehensive analysis of previous work,as well as detailed field and indoor work,the study on the evolution of the metallogenic fluid was carried out in the Jintang block,which accounts for 60%reserves of the Huangjindong gold deposit.The following understanding are based on the comprehensively analyzed and discussed the metallogenic mechanism:?1?Huangjindong gold deposit hydrothermal mineralization can be divided into three stages:quartz-arsenopyrite?less?-pyrite?less?-scheelite stage???;quartz-gold-pyrite-arsenopyrite-chalcopyrite-sphalerite-sericite stage???;quartz-calcite-chlorite stage???.Stage???is the main stage of gold mineralization.?2?The initial ore-forming fluid is a CO₂-rich fluid,which containing H₂O-CO₂-CH₄ volatile gas,medium to high temperature?195-330??,low salinity?2.2-8.9 wt%NaCl.eqv?,and low density?0.745-0.988 g/cm³?.The main temperature of gold mineralization ranges from 200 to 240?.From the early stage to the late stage,the temperature of ore-forming fluid tends to decrease,and the density tends to increase.However,the salinity does not change significantly under the process.The pH value of the fluid shows a trend of decreasing first and then increasing.?3?Al is the main element to replace Si in quartz.Monovalent alkali metals such as Li⁺,Na⁺,K⁺,Rb⁺and H⁺enter into the defect structure of quartz together with Al as compensation for electricity price.With the quartz precipitation during the each stage of fluid evolution,the total amount of Al³⁺,Li⁺,Na⁺,K⁺and Rb⁺enter into quartz defect lattice from the fluid increase before they decrease.?4?Gold migrates as Au?HS?₂^-in fluid.?5?The decompression boiling of fluid and sulfuration reaction are the main mechanisms of gold precipitation in Huangjindong gold deposit.Intense fracturing leads to sudden drop of fluid pressure and immiscibility of fluid,which is the main mechanism leading to the precipitation of visible gold in breccia-type ore rock.The strong sulfurization reaction quickly consumes the sulfur in the fluid,leading to the instability of the gold bisulfide complex,which is the main precipitation mechanism to induce the invisible gold and a small amount of visible gold precipitation in the gold-bearing sulfide.