Origin and evolution of ore fluid, and gold-deposition processes at the giant Taishang gold deposit, Jiaodong Peninsula, eastern China

Abstract

The Early Cretaceous Taishang gold deposit is the largest deposit in the Linglong goldfield, Jiaodong Peninsula, with a pre-mining endowment of 326t of gold. It is hosted in the 165–150Ma Linglong biotite granite and controlled by the NEE- to NE-trending Potouqing Fault near the northern end of the regional Zhaoping Fault system. Hydrothermal alteration is well developed in the footwall of the Potouqing Fault and is characterized by a narrow zone of sericitization, followed by a wider zone of silicification, and an even wider zone of potassic alteration. The main orebodies (No. I orebody) comprise disseminated and stockwork-veinlet systems, although about 5% of the resources are in larger quartz veins (No. II orebody). Four stages have been identified for both styles of mineralization, on the basis of cross-cutting relationships and mineralogical and textural characteristics: pyrite–quartz–sericite (stage 1), quartz–pyrite (stage 2), quartz–pyrite–base-metal-sulfide (stage 3) and quartz–carbonate (stage 4). Gold was mainly deposited in stages 2 and 3, with minor amounts in stage 1.Petrographical, microthermometric, and laser Raman spectroscopic studies on fluid-inclusion assemblages in quartz and calcite from the four stages reveal three types of primary fluid inclusions: type 1 H2O-rich aqueous-carbonic, type 2 CO2-rich aqueous-carbonic, and rare type 3 carbonic inclusions. Stage 1 quartz primary inclusions are only type 1 inclusions, with an estimated composition of 88% H2O, 10% CO2, 4.5 wt.% NaCl equiv., and 0.5% CH4, with trapping at ~336°C and ~1.7kbars. The gold-rich stages 2 and 3 from both orebodies typically contain primary fluid-inclusion assemblages with both type 1 and 2 inclusions, which show similar phase-transition temperatures and were trapped between 246° and 294°C. The stage 4 quartz and calcite contain only primary type 1 inclusions, which are estimated to have a composition of 93% H2O, 6.0% CO2, 3.6 wt.% NaCl equiv., and trace amount of CH4, and were trapped at temperatures of >236°C.The δ34S values of hydrothermal pyrite from the four stages have a narrow range from 4.5‰ to 8.0‰, and are within the ranges for whole-rock sulfides from the Archean Jiaodong Group, and magmatic pyrite from Mesozoic granitoid and intermediate-basic dikes. The δ18O values of hydrothermal quartz range mainly from 10.9 to 12.5‰ and remain constant for all four stages; calculated fluid δ18O values are 1.3–10.0‰. The δDwater values calculated from hydrothermal sericite range from −60 to −45‰. Considering the fluid inclusion compositions, δ18O and δD compositions of ore-forming fluids, and regional geological events, the most likely ultimate potential fluid and metal reservoirs would be the Paleo-Pacific oceanic slab and its overlying sediments, which were thrust below the high-grade metamorphic rocks of the Jiaodong Peninsula.The initial ore-forming fluids were medium-high temperature, CO2-rich, and low salinity H2O–CO2–NaCl±CH4 homogeneous fluids. Over the duration of the hydrothermal system, the fluid remained fairly consistent in P–T–X, although becoming slightly more water-rich during final post-ore activity. Fluid immiscibility occurred during stages 2 and 3 ore deposition at pressures that fluctuated strongly from 1700 to 580bars during hydrofracturing. The (HS)2− ion was the most probable gold-transporting complex at Taishang. Wall-rock sulfidation and episodic pressure drops, with associated fluid unmixing and other chemical changes, were the two main mechanisms of ore deposition.