The Study Of The Metallogenic Characteristics And Metallogenic Mechanism Of The Huachanggou Gold Deposit In Shaanxi, China

Located in the south part of Qinling fold system, the Huachanggou gold depositis between two tectonic units, the Motianling Caledon fold belt in the south andKangxian-lveyang Variscian fold belt in the north. It is an alteration type gold deposit.Through systematic study of the geological and geochemical characteristics of theHuachanggou gold deposit, conclusions concerning the mineral composition, theoccurance of gold, the genesis and the mineralization mechanism of this deposit havebeen drawn as follows:(1) On the basis of detailed field geological survey and systematic laboratorystudy including microscope observation, electron probe scanning, X-ray surfacescanning, the mineral composition and the occurance of gold has been identified. Theresults show that Huachanggou gold deposit has a small variety of minerals withpyrite as the dominant mineral and minor chalcopyrite, bornite, sphalerite, galena,pyrrhotite and so on. The gangue minerals are mainly quartz, dolomite and albite. Thegold in the ores mainly exists in the form of independent mineral phase. The carriersare pyrite, quartz, dolomite, chalcopyrite and tennantite. The gold minerals are nativegold. Neither argentiferous native gold nor electrum is found. Gold is embodied asinclusion, fissure-filling, intergranular and interlocking form. The gold granularity ismainly characterized by micro particulate, micro fine grained and extra microparticulate.(2) The altered spilite gold ore and the limestone-quartz gold ore have obviouslydifferent isotopes and trace elements. The sulfur isotope and the lead isotope in thealtered spilite gold ore indicate that the sulfur and lead came from deep source, whilein the limestone gold ore, they show that they were derived from the crust. The leadisotope shows that the lead in the spilite gold ore is the transitional product betweensubduction zone lead and orogenic belt lead while the lead in the limestone gold orecame from upper crust, both of which were affected by magmatism. The carbon isotope also shows that carbon in the deposit is a mix of deep source carbon andmarine carbonate. The Q type cluster analysis of the trace elements shows that the twotypes of gold ores are closely connected to their wall rocks. The rare earth elements inthe two types of gold ores are originated from their own wall rocks seperately,showing the close relation between the gold ores and their wall rocks. The obviousdifference of the geochemical characteristics between these two types of gold ores andtheir close relation with their own wall rocks indicate that the ore-forming materialwas derived from the host rocks.(3) The study of fluid inclusions shows that the ore-forming fluid ishigh-to-middle in temperature, low-to-middle in salinity and low in density. Theore-forming depth is shallow. The ores formed in a reduced environment which isfavorable to the precipitation of gold. The hytrogen and oxygen isotope indicate thatthe ore-forming fluid is mainly composed of formation water, partly associated withmagmatic water and organic water.(4) In the early Devonian, an E-W extentional rift occurred in the north rim of theYangtze plate, which caused masssive submarine volcanic sedimentation, leading tothe formation of the source bed of gold. In the late Paleozoic, the Yangtze platestarted to subduct beneath the Qinling plate. The collision between the north andsouth plate during the Indosinian movement in the late Triassic period causedintensive tectonic-hydrothermal activity, which activated the ore-forming materialinto the hydrothermal fluid and finally formed the gold ore body.