Study On The Geological Characteristics And Enrichment Regularities Of Mineralization Of Jitoushan-Xiaoyanzigou Gold Deposits In Ningqiang County, Shaanxi Province

Jitoushan-Xiaoyanzigou gold deposits is situated in Ningqiang County, ShaanxiProvince. It is also located geotectonically in the Bikou Block, which is the east edge ofMotianling fold belt，Songpan-ganzi fold system.The deposits lies at the south part ofMian-lue-ning ore belt. And this region has a good geological condition of Mineralzationand also is rich in mineral resources.Most of the exposed strata in this area are the second subgroup of mid-upperProterozoic Bikou group. The orientation of these stratum are almost EW, leaning to South. Protolith of Bikou group are intermediate-basic volcanic rocks, and geologically formed inisland arc. Regional metamorphism made this rocks transformed to greenschist (e.g.sericite chorite quartz schist, chorite schist).The ore district is regionally located in Bahai ductile shear zone that lies with theorientation of NEE. Ore bodies lie in brittle. Up to now，there are four ore veins werefound in this deposits, such as AuⅠ, AuⅡ, AuⅧ and Au C2.These ore veins becomethicker when their orientation changed in horizontal direction and their inclination havebeen flatter. The fault controlled the orientation of ore bodies forming like “double Y”type vertically.Metallic minerals in Jitoushan-Xiaoyanzigou gold deposits are mainly pyrite， chalcopyrite, haematite, magnetite and a little gold. Gangue mineral are mainly quartz, chlorite, epidote, calcite and so on. Major textures of ores are cataclastic texture, euhedral-subhedral granular texture, poikilitic texture. The major structure is disseminatedstructure. The country rock alterations are mainly silicatization， chloritization,epidotization and sericitization. Silicatization is closely related to the mineralization.Microthemometer of fluid inclusion in Xiaoyanzigou gold deposits demonstrate thatthe homogenization temperatures of mineralization are180~220℃and260~320℃, thepeak value of fluid salinity is2~6%NaCl, the depth of mineralization is5.5~7.5km. While,homogenization temperatures of mineralization in Jitoushan deposits are160~200℃，thepeak value of fluid salinity is2~4wt%NaCl，the depth of mineralization is5~6.5km. Bothof them are characterize by mid-high temperature，low salinity. We conclude that they arethe same fluid which forming from high temperature to low. The fluids, which formed inneutral environment or neutral-acid, is the mixture of magmatic water and atmosphericwater.The ignous rock that closely related to the mineralization is Yangba granitic mass,which lies outside of the deposits. It provides material for mineralization. Zircon U-Pbdating result is215.5±2.1Ma，shows that the rock is the production of magmation inIndo-Chinese epoch. Geochemical characteristics show that the rock belongs to the seriesof low magnesium,high-K calc-alkali and aluminous. It is the I type granite. The traceelements rich in large ion lithophile element e.g. Rb，Sr，Ba，K etc. and deplete in highfield intensity elements e.g.Nb，Ta，P，Ti etc. The REE pattern shows that the rock richesin LREE，depletes in HREE. Geodynamics setting that forming Yangba granodiorite is thetransition from sys-collision to post-collision.The forming process of Yangba graniticmass is that the subductive slabs make the mantle uplift through metasomatism. Upliftingmantle trigger the partial melting of lower crust, and then, adakite formed. Meanwhile，lower crust become heavier. It makes the delamination of lithosphere more severe. Mantlemagma mix with lower crust magma, and Yangba granitic mass is the result from thereplacement of mixture magma.According to the tectonic background, geological characteristics.and themineralizing condition of Jitoushan-Xiaoyanzigou gold deposits, we conclude that the deposits is the type of mesothermal hydrothermal vein deposit hosted in metamorphicrocks. It also can be classified as mesozonal orogenic gold deposits, proposed byGroves(1998).