Gold deposition and resource potential of the Linglong gold deposit, Jiaodong Peninsula: Geochemical comparison of ore fluids

The Linglong gold deposit is a super large deposit in the Linglong goldfield, northern Jiaodong Peninsula. The orebodies consist of hundreds of auriferous quartz veins that surrounded by relatively narrow K-feldspar alteration halos. Four mineralization stages have been identified on the basis of cross-cutting relationships, and mineralogical and textural characteristics: milky quartz-pyrite (stage 1), grey-white quartz-pyrite (stage 2), quartz-pyrite-base-metal sulfide (stage 3), and quartz-carbonate (stage 4). Native gold was mainly deposited in stages 2 and 3. Petrographical and microthermometric studies on quartz from stages 2 and 3 ores reveal three types of fluid inclusions associated with gold mineralization: Type 1 H2O-CO2- NaCl +/- CH4 inclusion that consisting FIA 1 and represent a homogeneous fluid trapped at the same pressure and temperature; coexisting Type 2a H2O-rich H2O-CO2-NaCl +/- CH4 and Type 2b CO2-rich H2O-CO2-NaCl +/- CH4 inclusions that constitute FIA 2 and are interpreted to be products of a heterogeneously trapped fluid. The ore-forming fluids belong to a H2O-NaCl-CO2 +/- CH 4 system with medium temperatures (similar to 290-340 degrees C), low salinity (similar to 4-7 wt% NaCl eq.), and are CO2-rich (similar to 15% mol) with minor CH4 (similar to 2-5% mol). The delta D-delta O-18 values of ore-forming fluids vary from -63 to -57 parts per thousand and 6.0 parts per thousand to 9.3 parts per thousand, respectively, overlap both the magmatic and metamorphic fluid fields. The delta S-34 values of hydrothermal pyrite have a narrow range from 5.6 parts per thousand to 7.9 parts per thousand, and are within the ranges of Neoarchean Jiaodong Group, and Mesozoic Linglong granite, Guojialing granitoid and intermediate-basic dikes. Combined with H-O-S isotope data, fluid inclusions compositions, and regional geological events, the oreforming fluids may have a metamorphic-dominate origin, but a mantle or crustal contribution cannot be excluded. Fluid immiscibility played a major role, and fluid-wallrock interaction may be a subordinate reason for gold deposition in Linglong. The stage 2 quartz grains contain more FIAs 2 and volatiles than those in stage 3 quartz, indicating a more extensive fluid-immiscibility process which has a positive-correlated higher gold grade. Based on the data of FIAs 2, the P-T conditions of gold depositions generally range from 242 to 54 MPa and 348 to 269 degrees C, with a depth of metallogenesis of 5.4-9.0 km. Considering the depth of regional post-ore exhumation has been calculated as about 5-7 km, the space with deep gold exploration is expectable at Linglong.