Origin of the Jiaodong-type Xinli gold deposit, Jiaodong Peninsula, China: Constraints from fluid inclusion and C–D–O–S–Sr isotope compositions

Abstract

The Xinli gold deposit within the Sanshandao–Cangshang gold belt in the Jiaodong gold province is a typical “Jiaojia type” ore deposit, with high-grade, quartz–sulfide vein/veinlet stockworks that cut Mesozoic granodiorite. Four ore stages are distinguished in the following sequences: I) quartz–K–feldspar–sericite–pyrite; II) quartz–pyrite; III) quartz–polymetallic sulfide; and IV) quartz–calcite–pyrite. Fluid inclusions in quartz and calcite in the ores contain C–O–H fluids dominantly of six types with varied liquid/vapor ratio. Ore fluid in the main ore stage is characterized by medium–low temperature, low salinity, reducing condition, and NaCl–H2O–CO2±CH4 in composition. The δ18Owater (‰)SMOW of quartz in ore stages II and III range from 2.82 to 5.34‰, and the δD (‰)SMOW are between −69.6 and 88.3‰. Both suggest that the ore fluid was dominantly magmatic water. The δ13CPDB of the calcite are from −6.4‰ to −2.4‰, indicating mantle contribution to the ore fluid. The pyrite δ34SCDT from four ore stages are basically similar varying from 9.42 to 11.62‰, which implies significant involvement of crustal materials. The initial 87Sr/86Sr ratios in the calcite ranging from 0.710657 to 0.711542 also denote a crustal contribution. The fluid composition and systematic H–O–C–S–Sr isotopic data are explained by a model of slab-subduction induced gold mineralization. In the model, the ore fluid and metals were considered to associate with the dehydration and desulfidation of the subducting paleo-Pacific slab and the subsequent devolatilization of enriched mantle wedge. The H2O in the fluid might have been released from the devolatilization of enriched mantle wedge; the CO2 could be from mantle wedge or subcontinental lithospheric mantle; and the sulfur and strontium would be initially released from the subducted seafloor sediments. This process was further supported by the geochemical features of intermediate-basic dykes contemporaneous to the gold ore deposits. The deposit features and ore-forming process of the Xinli gold deposit hardly concur those of the orogenic gold deposit and the pluton-related gold deposit. The study supports the argument of that the gold deposits in the Jiaodong belong to a unique gold type related to a complex slab subduction-related process. The geodynamic engine of the Jiaodong-type gold deposit is specific, i.e., the slab subduction coupling with the consequent lithospheric thinning and structure adjustment.