Geochemistry Of Ore-forming Processes In The Yangshan Gold Belt, West Qinling, Central China

The Yangshan gold belt is structurally located within the Mian-Lue suture zonein the West Qinling orogen. The gold belt is an ideal area to study the origin of golddeposits in the composite orogen. The source of huge amounts of gold and itsaccumulation mechanism are the critical scientific issues that need to be resolved. Thefollowing conclusions could be made after studying the geology and geochemistry ofore-forming processes in the Yangshan gold belt.（1） Disseminated mineralization is commonly developed in all the rock types andthrough the sedimentary-metamorphism-mineralization stages; the vein-stylemineralization is less common than the disseminated style, and is usually developednear the fault belt, but could be found in all the mineralization stages. There arevarious wall-rock alteration types, with widespread development and compositeoverprinting and zoning structure.（2） Five pyrite stages, two arsenopyrite stages, one stibnite stage are recognizedin the thesis. Gold is mainly disseminated in the pyrite and arsenopyrite in the phylliteand acidic dykes. Minor irregular free gold is developed in the late ore stagequartz-calcite vein.（3） Pyrite and arsenopyrite have complete lattice structure without obviousdislocation or deformation or independent gold mineral inclusions. Gold substitutesfor Fe in the pyrite and arsenopyrite structures, leading to the increase of interplanarspacing of pyrite and the minor change of interplanar spacing of arsenopyrite.（4） The Thin the late ore stage is271.3²88.3℃, and thesalinity is3.17wt%NaCl, with pressure of2.877kbar. The phase diagrams show that the pH is3⁵, andthe fS2is10^-10.4, and the fO2is10^-36.310^-34.2. The oxygen isotope indicates ametamorphic fluid origin, which is similar to the orogenic gold deposits.（5） The comprehensive study of S-Pb-Sr-O isotopes shows that the Bikou Groupand the Devonian strata provide fluid and metal source for the mineralization.（6） The EMPA and LA-ICP-MS research shows that the diagenetic pyriteprovides fertile gold resource for the mineralization. The As and Au enrichment in the main ore stage could be related to sulfidation between the ore fluids and the rocks,and reflects a coupled substitution mechanism in which Au³⁺substitutes for Fe²⁺andAsS^3-substitutes for the S2^2-dianion. The late ore stage fluid is enriched in Sb andgold is liberated from the pyrite and/or arsenopyrite structure and forms free gold,indicating the drop in temperature and/or pressure in the late ore stage.（7） Compared with the typical orogenic gold deposits and the Carlin-type golddeposits, the Yangshan gold belt is a orogenic gold belt with disseminatedmineralization.