A genetic model of the giant Sangdong W–Mo skarn deposit in the Taebaeksan Basin, South Korea

Abstract

The Sangdong deposit is a giant W–Mo skarn deposit located in the southern Taebaeksan Basin, South Korea. It consists of stratabound orebodies hosted by intercalated limestones of the Myobong Formation and massive limestones of the Pungchon Formation, part of the Cambrian–Ordovician Joseon Supergroup. The Cretaceous Sangdong granite is associated with the W–Mo mineralization. This highly evolved calc-alkaline S-type granite intruded the Precambrian basement and is currently concealed ∼1 km below the surface. F-rich ore-forming fluids exsolved from the Sangdong granite and infiltrated the intercalated limestone of the Myobong Formation via a NE–SW-striking fault system within the Taebaeksan Basin, where the magmatic and meteoritic fluids caused multiple stages of skarn formation, ranging from the early prograde skarns to the final vein stages. Regarding the spatial distribution of the Sangdong deposit, a prograde skarn appears in the outer part of the orebody while a retrograde skarn is observed in the central part of the orebody. This spatial distribution of the multiple-stage skarn is produced by the effect of multiple hydrothermal injections in the Sangdong deposit. Slates of the Myobong Formation prevented the escape of ore-forming fluids toward the surface, and channelized the fluid flow within the intercalated limestone layers. The skarns comprise prograde skarn stages I and II, retrograde skarn stages I and II, and vein stages. Prograde stage I is represented by wollastonite–garnet–clinopyroxene zones with the lowest scheelite contents at the margins of the orebody. The pyroxene–garnet assemblage of prograde stage II replaces the wollastonite–garnet–clinopyroxene assemblage. An amphibole–biotite assemblage (retrograde stage I) replaces the pyroxene–garnet assemblage, and is replaced by a quartz–mica assemblage (retrograde stage II) in the center of the orebody. The vein stage is represented by scheelite and/or wolframite-bearing quartz veins (W vein) and molybdenite-bearing quartz veins (Mo vein) that crosscut the earlier skarns. The final vein stage is also subdivided into two stages: early scheelite–wolframite quartz veins and late molybdenite–quartz veins, with both the vein stages crosscutting the prograde and retrograde skarns. The scheelite contents are the lowest in the marginal wollastonite–garnet–pyroxene zone and the highest in the central retrograde quartz–mica zone and veins. The mineral assemblages and clinopyroxene and garnet compositions within the prograde skarns indicate the mineralizing environment evolved from oxidizing to partly reducing conditions during prograde stage I. The carbon and oxygen isotope compositions indicate the prograde stage skarn mineralization occurred at about 400 °C, within an open system with a fluid XCO2 of 0.1. The growth of fluorite and muscovite during retrograde stage II and that of wolframite during the vein stage, as well as the negative Eu anomalies recorded by the skarns, reflect an ore-forming fluid with high F contents, which enhanced the capacity of hydrothermal fluids to transport and deposit ore metals during skarn formation and greisenization at the Sangdong deposit.