Deep ore‑forming fluid characteristics of the Jiaodong gold province: Evidence from the Qianchen gold deposit in the Jiaojia gold belt

The Jiaodong gold province hosts world-class gold deposits with giant resources of >5000 t, where the Jiaojia-Xincheng fault belt and the Sanshandao-Cangshang fault belt occupy more than two-thirds of the gold deposits. Recent developments of exploration have advanced in tracking deep mineralization. The nature of the ore-forming fluid, source, and the extreme enrichment mechanism of the gold in this region remain debated. Here, we investigate the deep ore body of the Qianchen gold deposit at the southern part of the Jiaojia gold belt based on mineralogy, fluid inclusion, elemental and isotopic composition of pyrite, and the primary halo geochemistry. Natural gold is the important form of Au in this deposit with a relatively large (up to 200 μm) grain size, suggesting the prospect of mineralization at deeper domains. The accompanying PGE minerals (Hexaferrum) hint at contributions from mantle material. The ore-forming fluids of the Qianchen gold deposit are characterized by medium–low temperatures and salinities and have a general H2O–NaCl–CO2 ± CH4 composition, which indicates that the Jiaodong gold province has a unified ore-forming fluid system from shallow to deep. However, the trace elements and S isotopes of pyrite at different elevations show slight differences, suggesting the composition and properties of the fluid changed during the process of ascending, affected by ore-forming conditions such as temperature, pressure, and oxygen fugacity. Furthermore, the evidence of a decrease of salinity with depth and temperature and fluid immiscibility indicates that fluids from different sources continuously mixed during the upward migration of fluid, with the mineralization triggered possibly by phase separation. The primary halo model suggests promising prospecting potential. We formulate a comprehensive metallogenic model for the mineralization process in the Jiaodong gold province. The Au-bearing hydrous magma derived from the partial melting of the metasomatized SCLM, underwent further magmatic-hydrothermal differentiation and fluid exsolution, and thus the Au was enriched and partitioned into exsolved fluids that evolved to auriferous fluids. During the upward migration of ore-forming fluids (magmatic-hydrothermal), many factors affected mineralization, mainly including fluid-rock interaction and mixing of the metamorphic-hydrothermal and meteoric fluids. The fluid activities, multiple sources of Au in the mantle and ancient basement, together with the large fault structures resulted in the formation of the giant Jiaodong gold province.