Tectonic transition from a compressional to extensional metallogenic environment at ∼120 Ma revealed in the Hushan gold deposit, Jiaodong, North China Craton
Abstract The world-class giant gold province (>4000 t) in the Jiaodong Peninsula has attracted global attention and extensive studies for decades. However, most of the works have been focused on the western Zhaoyuan-Laizhou and eastern Muping-Rushan gold metallogenic belt, leaving the middle Penglai-Qixia belt largely unexplored. The Hushan gold deposit in the Penglai-Qixia belt shows a unique two-stage gold mineralization, i.e., an early-stage pyrite-sericite-quartz altered-rock type mineralization and a late-stage pyrite-pyrrhotite vein type mineralization. Hydrothermal monazite coexisting with Au-bearing pyrite is observed in the early-stage mineralization. The widespread “circular zoned” metasomatic relict structures indicate that monazite was formed by the strong interaction between (Ca, P, REE)-rich fluids and feldspars (plagioclase and K-feldspar). Therefore obtained in-situ U-Pb age of 120.0 ± 3.1 Ma for the “pure” monazite without mineral inclusions (such as feldspars) can represent the timing of the early-stage gold mineralization. The in-situ monazite e Nd (t) (t = 120 Ma) values vary from −20.6 to −18.4, well corresponding to Nd isotopes of the late Jurassic Linglong granite. It is inferred that the REE-rich components in the ore-forming fluids were extracted during the process of primary mineralizing fluids passing through the lower Linglong granite body, and that the Nd isotopic compositions were altered due to the strong fluid-rock interaction. In this regard, Linglong granite contributes some ore-forming materials to the mineralizing fluids during fluid-rock interaction. The early-stage altered-rock type gold mineralization indicates a compressional (closed) ore-forming system, whereas the late-stage pyrite-pyrrhotite vein type mineralization and the occurrences of fracture, geode and hole in the ore samples reflect an extensional (open) ore-forming system. The presence of barite and magnetite in the late-stage mineralization also suggests an increase of oxidation state during the process. Furthermore, pyrites in the late stage generally have lower δ 34 S values (5.69–6.98‰, av. = 6.47‰) than pyrites in the early stage (δ 34 S values = 7.06–7.85‰, av. = 7.42‰), which can also be well explained by the increase of oxidation state. Therefore, the Hushan gold deposit uniquely records a metallogenic environmental transition from compressional, reduced to extensional, oxidized at ∼120 Ma. In addition, the mineralizing fluids with higher oxidation state in the late-stage is supposed to superimpose on the early-stage mineralization, dissolve and rework previously crystalized pyrite to form the high-grade gold mineralization. Unlike the gold deposits in the western and eastern belts, the Hushan gold deposit temporally preserves two types of mineralization in one single deposit, as a response to regional stress field transition.