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

•The timing of ore forming at ~ 120 Ma•Ore fluids derived from dehydration and decarbonisation of paleo-Pacific oceanic slab•Ore fluids were medium-high temperature, CO2-rich, and low-salinity metamorphic fluids.•Fluid immiscibility occurred at 200-40 MPa and 334-230 °C.•Fluids/wallrock reaction and fluid immiscibility were two main precipitation mechanisms.•Gold mineralization in the Jiaojia goldfield was a large-scale unified event.

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 > 60 t gold at a grade of 4.07 g/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 85 MPa and 334 to 300 °C for stage 1 and 200 to 40 MPa 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 (~ 8 mol%) with minor CH4 (1-4% in carbonic phase), and low salinity (3.38-8.45 eq. 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.

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