Timing and mechanism of gold mineralization at the Wang'ershan gold deposit, Jiaodong Peninsula, eastern China

Abstract

The Wang'ershan gold deposit, located in the southern Jiaojia goldfield, is currently the largest gold deposit hosted within the subsidiary faults in Jiaodong Peninsula, with a gold reserve of >60t gold at a grade of 4.07g/t Au. It is hosted in the Late Jurassic Linglong biotite granites and controlled by the second-order, N- to NNE-trending Wang'ershan Fault (and its subsidiary faults) which is broadly parallel to the first-order Jiaojia Fault in the goldfield. Gold mineralization occurs as both disseminated- and stockwork-style and quartz–sulfide vein-style ores, mainly within altered cataclasites and breccias, and sericite–quartz and potassic alteration zones, respectively. Mineralization stages can be divided into (1) the pyrite–quartz–sericite stage, (2) the quartz–pyrite stage, (3) the quartz–sulfide stage, and (4) the quartz–carbonate stage.Two sericite samples associated with the main ore-stage pyrites from pyritic phyllic ores of the deposit with weighted mean plateau 40Ar/39Ar age of 120.7±0.6 Ma and 119.2±0.5 Ma, respectively, were selected for 40Ar/39Ar geochronology. On the basis of petrography and microthermometry, three types of primary fluid inclusions related to the ore forming event were identified: type 1 H2O–CO2–NaCl, type 2 aqueous, and type 3 CO2 fluid inclusions (in decreasing abundance). Stage 1 quartz contains all three primary fluid inclusions, while stages 2 and 3 quartz contain both type 1 and 2 inclusions, and stage 4 quartz contains only type 2 inclusions. The contemporaneous trapping, similar salinities and total homogenization temperature ranges, and different homogenization phases of type 1 and type 2 inclusions indicate that fluid immiscibility did take place in stages 1, 2 and 3 ores, with P–T conditions of 190 to 85MPa and 334 to 300°C for stage 1 and 200 to 40MPa and 288 to 230°C for stages 2 and 3. Combined with the H–O–C–S–Pb isotopic compositions, ore-forming fluids may have a metamorphic-dominant mixed source, which could be associated with the dehydration and decarbonisation of a subducting paleo-Pacific plate and characterized by medium–high temperature (285–350°C), CO2-bearing (~8mol%) with minor CH4 (1–4% in carbonic phase), and low salinity (3.38–8.45eq.wt.% NaCl). During mineralization, the fluid finally evolved into a medium–low temperature NaCl–H2O system. Au(HS)2− was the most probable gold-transporting complex at Wang'ershan, due to the low temperature (157–350°C) and near-neutral to weakly acidic ore fluids. The reaction between gold-bearing fluids and iron-bearing wall-rocks, and fluid-immiscibility processes caused via fluid–pressure cycling during seismic movement along fault zones that host lode-gold orebodies, which led to breakdown of Au(HS)2−, are interpreted as the two main precipitation mechanisms of gold deposition.In general, the Wang'ershan deposit and other deposits in the Jiaojia camp have concordant structural system and wall-rock alteration assemblages, nature of orebodies and gold occurrence conditions, as well as the similar geochronology, ore-forming fluids system and stable isotope compositions. Thus gold mineralization in the Jiaojia goldfield was a large-scale unified event, with consistent timing, origin, process and mechanism.