Geology, Geochemistry And Genesis Study On Xingjiashan W-Mo Deposit In Jiaodong Mesozoic Metallogenic Area, China

Xingjiashan Mo-W deposit, presently, is the largest molybdenum deposit inJiaodong where is known by bunch of gold fields in China. Compared with othermineral deposits in this area, Xingjiashan Mo deposit exhibits many unique propertiesincluding distinctive type related to skarn, eld in ore-forming age, huge ore reservesand special location surrounded by gold deposits. In order to find out the ore-formingmechanism of this deposit, systematic geological investigation, mineralogicobservation and geochemical analyses were proceeded in Xingjiashan Mo deposit.The mineralization occurs in skarn bodies which are comprised dominantly ofdiopside-hedenbergite, grossular, vesuvianite, tremolite, hornblende, biotite, potassicplagioclase, and quartz, with economically important amounts of molybdenite andscheelite. Scapolite and apophyllite, which are rare in skarn deposit, especially injiaodong area, are also detected by EPMA technique. Two periods of ore formingprocesses are recognized, namely hydrothermal period and epigene period. Thehydrothermal period can be subdivided into three stages including early skarnstage,quartz-sulfides stage and carbnification stage,respectively. Molybdenite existsmainly in quartz-sulfides stage, mostly as molybdenite-quartz veins and dissminationor films in garnet-diopside skarn.Geochemical data indicate that substances like SiO2, Al2O3, TiO2and Fe2O3migrated into the skarn from Xingfushan monozie granite, while CaO and MgO werebrought outward. Molybdenite Re-Os dating and LA-ICP-MS Zircon U-Pb datingshow that the timing of Mo mineralization is about163Ma, and the emplacement ofXingfushan pluton is166.6±1.0~167.1±1.0Ma, respectively. Therefore, XingjiashanMo deposit might be chronologically related to the emplacement of the Xingfushanintrusion. These data may also indicate this magmism take place in a thicken crustsettings due to slab subduction in late Jurassic period. Fluid inclusions and laser-man tests suggest that the ore forming fluid inXingjiashan Mo deposit belongs to a H2O-NaCl±CO2system, with high temperature,moderate-low salinity. The mainly temperature and salinity ranges in each stage are:300~390℃,0.97~47.44%(including one daughter crystal inclusion, average10.31%)in the skarn stage;180~330℃,0.62%~49.68%(various type fluid inclusions coexist)and91-180℃,0~1.48%in the carbonates stage,respectively. Moreover, both H2S andCH4were confirmed in quartz in quartz-sulfides stage, suggesting the reducedcondition of this stage. Meanwhile, the result of C-H-O-S isotope test favor themagmatic origin of the oreforming fluid.Thus, taking field geology features and these data into consideration,Xingjiashan Mo deposit is a typical skarn deposit formed by the Xingfushan stockemplacing into the Fenzishan formation at Late Jurassic period. It is pressure releaseboiling that cause the huge molybdenite precipitate from the magmatic fluid inXingjiashan area, and contact metasomatism, pH-Eh change and meteoric waterdilution accelerate sulfides accumulation. This study also reveals good explorationpotential in the surrounding area and depth.