Fluids Geochemical Characters Of The Magma Domal Structure Gold Deposit In Jinchang, Heilongjiang

Jing chang gold deposit in Dong ning county, Hei longjiang province, one of the biggest gold deposit in China, lies in the easternmost part of Song len block of northern orogenic belt and it is a significant part of Ji hei magmatic ore area. Regionally it experienced intensive tectonic magmatic activity. Many experts have worked in this area and they thought the magmatic dome in the deposit is close to porphyry deposit according to its overall characteristics of the ore and alternation zoning (a large scale of potash feldspathization in the early stage, phyllic alteration in the middle stage and siliconization in the late stage ). The author reckons this deposit as porphyry cooper-gold deposit.Through field work, analysis of the bore hole and slice and study of the fluid inclusion in metallogenetic section within the deposit, it is realized that the total fluid inclusions have a wide range of Th(homogeneous temperature), relatively high salinity and gas-rich and daughter mineral-rich inclusions, all of which further confirmed former opinion that this deposit is of porphyry cooper-gold type.This article mainly worked on metallogenetic fluid inclusions and did much lab work including the microscopic observation, temperature testing and composition analysis. It is concluded from the analysis of the results that quartz in the magmatic dome of Jinchang gold deposit well developed fluid inclusions ranging from silicated inclusions and melt-fluid inclusion to gas-fluid inclusion, which indicates the primitive magma experienced a full magma-fluid evolvement process, during which tantalum were fully enriched in solution. The deposit is characteristic of daughter mineral inclusions with multi-phase and lack of CO₂ inclusions. daughter minerals mainly include halite, sylvite, subsequently pyrite, chalcopyrite and rare hematite. Up to 4-5 daughter minerals can be seen in a single inclusion in some cases, and the presence of so many daughter minerals suggests a high concentration of tantalum in the metallogenetic fluid. By calculating ht pressure, the metallogenetic depth appeared to be smaller than 2.5 km. So this deposit falls into the catalogue of epithermal deposit, moreover, it underwent ebullience several times, which contributed to the enrichment and precipitation of the tantalum. By testing the composition of the inclusion in metallogenetic section, we found that the metallogenetic fluid contains a large number of volatile phase, and these volatile content plays an important role in the enrichment and precipitation of the metallogenetic elements. Additionally the content of CO₂ and CH₄ in the volatile phase decreases with the increase of the depth at first and the increase rapidly, and all of these also indicate that there is an orebody deeply down. Therefore, the deposit within the magmatic dome is porphyry and was formed by the superposed effect of hydrotherm and atmospheric hydrotherm and the primitive porphyry magma was the main source of the metallogenetic elements and the metallogenetic fluid is subacid and lower reducibility.