Research On The Fluid Evolution And Mineralogy Of The Diaoquan Ag-Cu Deposit, Shanxi Province, North China

The Diaoquan deposit is a Ag-Cu skarn polymetallic deposit, which is located inthe north of Taihang Mountains. Combined with previous studies, this paperresearched the geological settings, skarn and ore characters of the Diaoquan Ag-Cudeposit in detail and divided the mineralization stage. Studies on fluid inclusions arecarried out to discuss exsolution of fluid from magma and the physical and chemicalconditions of fluid evolution. EPMA results help us get the mineralogicalcharacteristics of typical metallic minerals and skarn minerals.The mineralization of the Diaoquan Ag-Cu deposit are divided into3periods andcan be further divided into5stages, showing typical skarn mineral assemblages. Ⅰskarn period includes Ⅰ1early skarn stage and Ⅰ2late skarn stage. Ⅱquartz-sulphide period is made up of Ⅱ1Mo mineralization stage and Ⅱ2Cu-Agmineralization stage. The last stage is Ⅲ carbonate stage. According to the phase ofinclusions at room temperature, fluid inclusions in the Diaoquan deposit can bedivided into3categories and5sub-categories: type1liquid-rich inclusions, type2vapor-rich inclusions (2A type and2B type), type3inclusions containing daughterminerals (further divided into3A type and3B type).Ore-forming fluids in the Diaoquan Ag-Cu deposit are mainly NaCl-H2O system.Fluids of skarn period are composed of one with high temperature (481~587℃) andhigh salinity (51~64%) and the other with relatively high temperature (339~587℃)and medium salinity (11~18%). As for quartz-sulphide period, decompression andboiling occurred in Mo mineralization stage, leading to the coexistence of vapor-richinclusions and liquid-rich inclusions. The temperature (170~510℃) and salinity (35~58%,2-20%) of this period decreased significantly compared to these in skarnperiod. Then copper and silver minerals precipitate. The temperature (132~323℃)and salinity (<8%) of the fluid inclusions in carbonate period continue to reduce anddisplay centralized distribution. Minimum of trapping pressure of fluid inclusions inskarn period range from23to66Mpa. The corresponding depth is0.9~2.6km inaccordance with lithostatic pressure. Pressure of quartz–sulphide period is5~ 46MPa, and the depth is0.5to4.6km on the basis of hydrostatic pressure. Momineralization occurs mainly in1~3km, while the Cu-Ag mineralization occursmainly in0.5~1.5km. The pressure of carbonate stage is less than15MPa and thedepth less than1.5km according to hydrostatic pressure. The Diaoquan Ag-Cu depositis a skarn deposit related to hydrothermalism with low temperature andshallow-seated features.EPMA results of pyrite indicate that it is formed in a relatively shallowenvironment and magmatic water dominated in the early fluid, while atmosphericwater added to the fluid during late period. Bornite is one of the main carriers of Agminerals. Ag content in galena and Fe content in sphalerite is relatively volatile,indicating that the environment during this period is still turbulent. Garnet of thisdeposit in skarn is maily andradite, followed by grossularite. MgO content in biotiterange from8.74%to10.40%, showing the nature of crust-mantle transition. Themagma may originate from deep magma chamber, and crustal substance join into themagma during the evolution process. Fluid pH and oxygen fugacity play importantroles in leading to mineral precipitation.