Magmatic–hydrothermal processes in Sangdong W–Mo deposit, Korea: Study of fluid inclusions and 39Ar–40Ar geochronology

Abstract

The Sangdong scheelite-molybdenite deposit in northeast South Korea consists of strata-bound orebodies in intercalated carbonate-rich layers in the Cambrian Myobong slate formation. Among them, the M1 layer hosts the main orebody below which lie layers of F1-F4 host footwall orebodies. Each layer was first skarnized with the formation of a wollastonite + garnet + pyroxene assemblage hosting minor disseminated scheelite. The central parts of the layers were subsequently crosscut by two series of quartz veining events hosting minor scheelite and major scheelite-molybdenite ores, respectively. The former veins associate amphibole-magnetite (amphibole) alteration, whereas the latter veins host quartz-biotite-muscovite (mica) alteration. Deep quartz veins with molybdenite mineralization are hosted in the Cambrian Jangsan quartzite formation beneath the Myobong formation. In the Sunbawi area, which is in close proximity to the Sangdong deposit, quartz veins with scheelite mineralization are hosted in Precambrian metamorphic basement. Three muscovite Ar-39-Ar-40 ages between 86.6 +/- 0.2 and 87.2 +/- 0.3 Ma were obtained from M1 and F2 orebodies from the Sangdong deposit and Sunbawi quartz veins. The Upper Cretaceous age of the orebodies is concordant with the published ages of the hidden Sangdong granite, 87.5 +/- 4.5 Ma. This strongly suggests that the intrusion is causative for the Sangdong W-Mo ores and Sunbawi veins. Fluid inclusions in the quartz veins from the M1 and F2 orebodies, the deep quartz-molybdenite veins, and the Sunbawi veins are commonly liquid-rich aqueous inclusions having bubble sizes of 10-30 vol%, apparent salinities of 2-8 wt% NaCl eqv., and homogenization temperatures of 180-350 degrees C. The densities of the aqueous inclusions are 0.70-0.94 g/cm(3). No indication of fluid phase separation was observed in the vein. To constrain the formation depth in the Sangdong deposit, fluid isochores are combined with Ti-in-quartz geothermometry, which suggests that the M1 and F2 orebodies were formed at depths of 1-3 km and 5-6 km below the paleo-surface, respectively. The similarity of the Cs (cesium) concentrations and Rb/Sr ratios in the fluid inclusions of the respective orebodies indicate an origin from source magmas having similar degrees of fractionation and enrichment of incompatible elements such as W and Mo. High S concentrations in the fluids and possibly organic C in the sedimentary source likely promoted molybdenite precipitation in the Sangdong orebodies, whereas the scheelite deposition in the deep quartz-molybdenite veins hosted in the quartzite is limited by a lack of Ca and Fe in the hydrothermal fluids. The molybdenite deposition in the Sunbawi quartz-molybdenite veins hosted in the Precambrian metamorphic basement rocks was possibly limited by a lack of reducing agents such as organic C.