Research On The Mineralogy And Fluid Inclusions Of The Xinzhuang Au Deposit In Fanshi County, Shanxi Province

Xinzhuang Gold Deposit is located in the northem part of the North China Craton intermediate belt,and in a Mesozoic tectonic-magma activation area which is embedded by the two tectonic units — the northwestem Wutai uplifts of Shanxi platform and the southwestern Yanshanian subsidence belt. This paper gave an overview of the geological setting and features of the Xinzhuang Gold Deposit and a detailed analysis of the characteristics of mineral ores. Based on these, the period and stage of mineralization were identified according to ore genesis theories for gold.Electron microprobe analysis(EMPA), measurement of fluid inclusions temperature,and Raman spectroscopy were conducted to investigate how the deposit formed and analyze the mineralogical properties of the typical metal sulfide minerals in the deposit.The mineralization of the Xinzhuang Gold Deposit are divided into 2 periods and can be further divided into 5 stages. ⅰmagmatic hydrothermal period includes Ⅰoxidation stage, Ⅱ1 pyrite-quartz mineralization stage,Ⅱ2quartz-pyrite mineralization stage, Ⅲ quartz-polymetallic sulfide mineralization stage and Ⅳ carbonate stage.The last stage is ⅱsupergene stage.EMPA of pyrite: It was observed that Fe and S were obviously depleted, while the abundances of Co and Ni were relatively high in pyrite. This, together with the results of Co/Ni, Au/Ag, and S/Se calculations, indicated that pyrite in the deposit was formed by magmatic-hydrothermal process. Pyrite in the deposit was rich in As and the mean value of Fe/S+As was 0.867. The content of As and the value of Fe/S+As,which are important typomorphic indicators, implied that the mineral was deposited in a shallow environment. The spectral image of Co, Ni, and As showed that pyrite was distributed in the range between the summit of Co and the summit of As, implying that magmatic water prevailed the early stage and atmospheric water was present at the late stage of the genesis process. Pyrite, chalcopyrite and quartz are the main Au-bearing minerals; and Au occurred in the forms of inclusion gold, intergranular gold, and fissure gold.According to the phase of inclusions at room temperature, fluid inclusions in the Xinzhuang Gold Deposit can be divided into five types: type Ⅰ — liquid-rich inclusions,type Ⅱ — vapor-rich inclusions,type Ⅲ — inclusions containing daughter mineral, type Ⅳ — pure gas inclusions and type Ⅴ — carbon dioxide three-phase inclusions. The gas content of fluid inclusions is carbon dioxide and the daughter mineral which is cubic and colorless transparent is most likely halite.Homogenization temperatures of fluid inclusions were in the range of 127.2℃~407.5℃. There are two peaks in the temperature histogram, at 260℃~280℃ and200℃~240℃, respectively. The salinity histogram also has two summits, which, at7%~8% and 3%~5%, represent the two main metallogenic stage which are the quartz-pyrite stage and the quartz-multi-metal stage, respectively. During the carbonate stage, the homogenization temperatures varied from 127.2 to 190.3℃; the salinity was 0.3%~4.2%, suggesting a medium-low temperature and low salinity mineralizing fluid. The fluid density during the whole magmatic-hydrothermal process ranged between 0.568 g/cm3 and 0.963 g/cm3, with an average of 0.818g/cm3,indicating a medium density fluid. The mineralization occurred under pressures of28.73MPa~201.89 MPa and the depth of main metallogenic stage focusing on the3.1Km~5.3Km, implying a shallow mineralization setting. Therefore, it was concluded that the Xinzhuang Gold Deposit is a quartz vein-type deposit formed by a magmatic-hydrothermal process in medium-density, low-salinity, and medium-low temperature conditions.