Origin and Evolution of Ore-Forming Fluid and Gold-Deposition Processes at the Sanshandao Gold Deposit, Jiaodong Peninsula, Eastern China

The Early Cretaceous Sanshandao gold deposit, the largest deposit in the Sanshandao-Cangshang goldfield, is located in the northwestern part of the Jiaodong peninsula. It is host to Mesozoic granitoids and is controlled by the north by northeast (NNE) to northeast (NE)-trending Sanshandao-Cangshang fault. Two gold mineralizations were identified in the deposit's disseminated and stockwork veinlets and quartz-sulfide veins, which are typically enveloped by broad alteration selvages. Based on the cross-cutting relationships and mineralogical and textural characteristics, four stages have been identified for both styles of mineralization: Pyrite-quartz (stage 1), quartz-pyrite (stage 2), quartz-pyrite-base metal-sulfide (stage 3), and quartz-carbonate (stage 4), with gold mainly occurring in stages 2 and 3. Three types of fluid inclusion have been distinguished on the basis of fluid-inclusion assemblages in quartz and calcite from the four stages: Pure CO2 gas (type I), CO2-H2O inclusions (type II), and aqueous inclusions (type III). Early-stage (stage 1) quartz primary inclusions are only type II inclusions, with trapping at 280-400 degrees C and salinity at 0.35 wt %-10.4 wt % NaCl equivalent. The main mineralizing stages (stages 2 and 3) typically contain primary fluid-inclusion assemblages of all three types, which show similar phase transition temperatures and are trapped between 210 and 320 degrees C. The late stage (stage 4) quartz and calcite contain only type III aqueous inclusions with trapping temperatures of 150-230 degrees C. The delta S-34 values of the hydrothermal sulfides from the main stage range from 7.7 parts per thousand to 12.6 parts per thousand with an average of 10.15 parts per thousand. The delta O-18 values of hydrothermal quartz mainly occur between 9.7 parts per thousand and 15.1 parts per thousand (mainly 10.7 parts per thousand-12.5 parts per thousand, average 12.4 parts per thousand); calculated fluid delta O-18 values are from 0.97 parts per thousand to 10.79 parts per thousand with a median value of 5.5 parts per thousand. The delta D-water values calculated from hydrothermal sericite range from -67 parts per thousand to -48 parts per thousand. Considering the fluid-inclusion compositions, delta O-18 and delta D compositions of ore-forming fluids, and regional geological events, the most likely ultimate potential fluid and metal would have originated from dehydration and desulfidation of the subducting paleo-Pacific slab and the subsequent devolatilization of the enriched mantle wedge. Fluid immiscibility occurred during the main ore-forming stage due to pressure decrease from the early stage (165-200 MPa) to the main stage (90-175 MPa). Followed by the changing physical and chemical conditions, the metallic elements (including Au) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. Water-rock sulfidation and pressure fluctuations, with associated fluid unmixing and other chemical changes, were the two main mechanisms of gold deposition.