Indosinian Orogenic Gold Metallogenic Deposits In The Southern Belt Of The West Qinling, Central China

The West Qinling which contains numerous Indosinian orogenic gold deposits is one of the most important gold provinces in central China. The occurrence and mineralization type of these deposits vary from each other. However, timing of the mineralization, geodynamics background, metamorphic grade of host terrane,ore-fluid characteristics and ore-forming process are identical as a whole. Based on field geology and geochemical research of the three typical orogenic gold deposits,the main achievements are summarized below:(1) On the regional scale, geological characteristics of gold deposits show common features. Isotopic dates suggest that gold metallogenic events about 221 to200 Ma were generally synchronous with or postdate metamorphism. The timing of gold introduction was associated with post-collisional extension setting. These deposits hosted in green-schist facies rocks are normally sited in second or third order structure associated with large-scale Mianlue faults at convergent plate margins. The gold mineralization shows strong correlation with sulfidization, silicification and carbonation. Disseminated mineralization and quartz–carbonate lode mineralization are commonly developed in all the rock types.(2) On the goldfield scale, geological characteristics of gold deposits show diversity. Within this gold belt, goldfields occur in a wide variety of host rocks,including phyllite, spilite and limestone in the Devonian Sanhekou group and the Triassic slates. On the basis of crosscutting relationships of the mineralized veins or veinlets, we identified three paragenetic sequences,including pyrite-quartz( stage 1),quartz-polysulfide(stage 2) and quartz-carbonate(stage 3) in the Huachanggou deposit and four sequenes, including quartz-pyrite-arsenopyrite(stage 1),quartz-carbonate-stibnite(stage 2), quartz-realgar-cinnabar(stage 3) and quartz-carbonate(stage 4) in the Manaoke deposit, respectively.(3) The LA-ICP-MS research shows that the sulfide mineral in the early and main stage exhibit variable enrichments in Au, As, Te, Bi, Sb, Ag; Cu, Pb, Zn, Co and Ni concentrations are generally only slightly elevated above regional backgrounds.The late ore stage stibnite in Yangshan and Manaoke gold deposit exhibit relatively higher Ag concentrations, much poorer Au. Such is probably the case that hydrothermal fluids that have been depositing gold along crustal-scale fault zones at depth, must still advect along these faults to the surface where their relatively low temperature hinders significant gold transport and bisulphide complexes still may carry significant Sb and Hg into the upper few kilometres of crust.(4) The comprehensive study of Sr-Nd-Pb isotopes shows the apparent conflict between diversity on a goldfield-scale and the presence of a linear trend on a province-scale, and that the crystalline basement in the middle and lower crust and the host rock provide fluid and metal source for the mineralization together.(5) The post-collision stage of orogeny involved large-scale crust-mantle interaction and magmatism raise geothermal gradients within the lithosphere in Late Triassic epoch, initiate and drive long-distance hydrothermal fluid migration into the crustal-scale fault system. A series of coherent genetic orogenic gold deposits are emplaced over a unique depth range through depositional mechanisms including wall rock sulphidation, phase separation and fluid mixing.