Source Of Ore-forming Materials Of The Quxi Gold Deposit In The Jiangnan Orogen

The Huangjindong gold ore field in northeastern Hunan is located in the middle part of the Jiangnan orogenic belt,which is strictly controlled by the Changping fault.It has proved more than 80 tons of gold reserves and has great prospecting potential.It is an ideal selection area for studying the source of ore-forming materials and fluids of orogenic gold deposits.The newly discovered Quxi gold deposit in this ore field is the closest gold deposit to the Changping fault.In this paper,three ore types of the deposit,quartz-sulfide vein type,altered slate type and quartz-breccia type,are selected to determine the ore-forming material source of the deposit through the analysis of EPMA and LA-ICP-MS trace elements of pyrite and arsenopyrite.The following results are obtained:The mineralization stage of Quxi gold deposit can be divided into three stages:quartz-pyrite-arsenopyrite stage,quartz-polymetallic sulphide-natural gold stage and quartz-calcite-stibnite stage.Gold mainly occurs in gold-bearing pyrite and arsenopyrite in the form of visible and invisible gold.The Au-As input points of pyrites of different ore types all fall below the Au solubility curve.It is presumed that the occurrence form of invisible gold in pyrites is lattice gold.Pyrite in quartz-sulfide vein ores enriches Au?1.45 ppm?,Co,Ni,Zn,As and PB elements;pyrite in altered slate ores enriches Au?11.15 ppm?,Co,Ni,Cu,As and Pb elements;pyrite in quartz-breccia ores enriches Ni,Cu,Sb,As,Pb and Bi elements,with Au content of 1.90 ppm.The arsenopyrite enriches Au,Co,Ni,As,Sb,Te,Pb and Bi elements.The contents of Mo,Cd,Pt and Tl in arsenopyrite are lower,and the content of Au is 224.81 ppm,which is obviously higher than that of pyrite.According to the Mo/Ni ratio of pyrite,stratigraphic lithology and sulfur isotope composition of gold-bearing arsenopyrite and pyrite in Huangjindong gold field,it is inferred that the ore-forming material originated from deep metamorphic rocks with higher metamorphic degree than those of the Neoproterozoic Lengjiaxi Group slate.According to the comparison of trace element content in pyrite with that in typical ore-forming fluids of orogenic gold deposits,it is inferred that the ore-forming fluids originate from the devolatilization diversion bodies of sedimentary rocks,and that the ore-forming fluids in the main stage are rich in trace elements Au,Co,Ni,Cu,As,Sb,Te,Pb and Bi.The development of pyrrhotite in Quxi gold deposit indicates that the deposit was formed in a reducing environment.According to the metallogenic temperature and mineral assemblage,it is presumed that gold migrates mainly in the form of Au?HS?₂^-1 in the ore-forming fluid.According to the structural ore-controlling characteristics of the deposit and the sulfide content and fabric characteristics of the ore body,it is inferred that sulfidation is the dominant precipitation mechanism of invisible gold,and phase separation or liquids immiscibility of multi-phase H₂O-CO₂-CH₄ is the dominant metallogenic mechanism of visible gold.