Metallogenesis And Metallogenic Prediction For Xiongcun Porphyry Copper-gold District,Tibet

Gangdese belt recorded the tectonic-magmatic evolution since the Paleo-Tethysto the new Tethys and India-Asia continental collision. Geologists pay much attationto this belt. Volcano–magma in Gangdese provide abundant ore-forming fluid andmaterial for mineralization. The Xiongcun district is located in the middle of thesouthern margin of southern Gangdese. Its southern margin is Shigatse forearc basin.Its administrative division belongs to Xiongcun Village, Rongma township,Xietongmen County, Rikaze, Xizang.The metallogenic tectonic setting of district is the island arc environment,associated with inner-oceanic subduction of the Neo-Tethyan oceanic crustlithosphere similar to the present South-West Pacific island arc belt. The genetic typeof the deposit belongs to porphyry Cu-Au deposit occures in the island arcenvironment, including:1) Cu-Au mineralization occurs in porphyry and tuff locatedin contact zone, presently, we have found I, II, III orebody, and IV and V mineralizedbody;2) epithermal gold veins occurs in the porphyry mineralization systemperipherals, we have only found the Dongga gold deposit so far. The mineralization ofXiongcun district can be divided into early and late mineralized periods.The earlymineralizition occurred in the early Jurassic (166.9~179.3Ma), related to hornblendequartz diorite porphyry emplaced in170~184Ma,and formed II, III orebody and Vmineralized body, which belong to the oxidized porphyry copper deposit with thecharacteristics of forming a large number of magnetite, gypsum and absent pyrrhotite;late mineralization occurred in the late Jurassic (157.8~165.8Ma), related to the latehornblende quartz diorite porphyrite with big quartz phenocrysts emplaced in162.4~172.9Ma,formed the Xiongcun I orebody, IV mineralized body and the Dongga golddeposit, which belong to reduced porphyry Cu-Au deposit, with the characteristics offorming a large number of pyrrhotite and lack of magnetite and gypsum. The formation of deposit related to inner-oceanic subduction of the Neo-Tethyanoceanic crust lithosphere in the Early-Middle Jurassic.The early mineralized porphyry(hornblende quartz diorite porphyry) originated from the subducted Neo-Tethyanoceanic crust,with the mineral assemblage is garnet, plagioclase, hornblende,pyroxene and titanium minerals, which has the characteristic of saturated water andhigh oxygen fugacity, partial melting form the saturated water, high oxygen fugacityfelsic magma, resulting in the hornblende quartz diorite porphyry has the trend ofsilicon-rich of calc-alkaline series, and condensation, crystallization, fractionation ofoxidised ore-bearing fluids with higher oxygen fugacity, and formed the oxidizedporphyry copper deposit. Late ore-bearing porphyry (late hornblende quartz dioriteporphyry with big quartz phenocrysts) originated from jiuvenile basaltic strata at thebottom of arc. The mineral assemblages of the source region aregarnet,hornblende,pyroxene, plagioclase and titanium minerals, characterized byunsaturated wate and low oxygen fugacity, partial melting form felsic magma withunsaturated wate and low oxygen fugacity, leading to the late hornblende quartzdiorite porphyry with big quartz phenocrysts shows the trend of iron-rich of thetholeiitic series, and condensation, crystallization, fractionation of reducedore-bearing fluids with lower oxygen fugacity, and formed the reduced porphyrycopper deposit. Metallogenic materials in the deposit originated from the porphyryitself, the ore-forming fluids mainly from magmatic water and adding meteoric water.Adding meteoric water and reduction of temperature and pressure are the significantfactors to make mineral precipitate.The oxidized porphyry copper deposit and the reduced porphyry copper depositin the Xiongcun district have great diffirence in the metallogenic characteristics,formed in the early and late mineralized periods, which respectively represent IIorebody and I orebody. Therefore, respectively, comprehensive prospecting modelof the two types of porphyry copper deposits were established, that is comprehensiveprospecting model of I orebody and II orebody. With the instruction of comprehensiveprospecting model, according to the geochemical survey results of the soil and rock inxiongcun district and its peripheral areas, the author have conducted metallogenicprediction for theose area and found many mineralization abnormities zone. Theauthor also proposed methods of prospecting for theose mineralization abnormities.