Different characteristics of barren and W–Mo–productive Cretaceous granites in the Hwanggangri mineralized district, South Korea

Abstract

The Hwanggangri mineralized district (HMD) in South Korea contains a variety of ore deposits such as porphyry or skarn (Mo, Cu–Pb–Zn), carbonate replacement, and fissure–filling vein types (W–Mo, Au–Ag, fluorite). The HMD is located on the boundary of the Okcheon metamorphic belt and the Taebaeksan Basin. Most of the deposits are hosted in limestones/dolomitic limestones of the Joseon Supergroup, and the age of mineralization is Late Cretaceous (96.5–76.2 Ma) including Geumseong, Jungbo, Kwangsin, and Geumsil deposits. These granites are mainly magnetite-series biotite granites, and they can be categorized into early- and late-stage on the basis of mineral assemblage, texture, grain size, and magnetic susceptibility. Magnetite is present in some of the granites, and some of the early-stage granites retain the high magnetic susceptibility values typical of the magnetite series. However, the late-stage granites show a wide range of magnetic susceptibility values, including low ones due to fractionation. The emplacement ages of the Sokrisan, Weolaksan, Muamsa, and Susan granite bodies are 94–88 Ma, similar to the ages of mineralization in the study area. The Cretaceous granites reported to be related to W–Mo mineralization in the HMD show miarolitic, porphyritic, and micrographic textures indicating considerable hydrothermal fluid activity in a shallow environment. The rocks of the Sokrisan granite are mostly early stage, those of the Weolaksan and Muamsa granites include both early and late stages, and the studied samples of the Susan granite are entirely late stage. These four Cretaceous granites have more than 70 wt% SiO2, high Rb/Sr ratios, depletions in compatible elements, enrichments in incompatible elements, and tetrad patterns with depleted LREEs, enriched HREEs, and strong negative Eu anomalies. They all belong to the high–K calc–alkaline series, are dominantly peraluminous, and can be classified as strongly fractionated I–type granites. The Sokrisan, Weolaksan, Muamsa, and Susan granites have similar emplacement ages and geochemical characteristics that indicate the same magma source. However, the Sokrisan granite, with its mainly early-stage rocks, consistently higher magnetic susceptibility values, and relatively high Nb/Ta values, differs from the other Cretaceous granites in the HMD. This suggests that the widely exposed Sokrisan granite of batholith type has been eroded substantially, removing the late-stage rocks and ore deposits. In contrast, the stock–type Susan granite has undergone relatively little erosion, and it represents well the late-stage parts of the Cretaceous granites in the HMD. The Cretaceous granites in the HMD have mostly highly-fractionated geochemical characteristics, and the late-stage rocks that are more associated with W–Mo mineralization are more fractionated than the early-stage rocks. The respective productivities of the Susan, Muamsa, Weolaksan, and Sokrisan granites range from high to low, and the more-productive late-stage granites are still present, as they have undergone less erosion.