Mineralogic and stable isotope studies of cassiterite greisen mineralization in the Uljin area, Korea

Abstract

Vein-type, greisen-related cassiterite deposits occur at the Uljin, Yuchang, and Dongseog mines in eastern Korea within a Precambrian pegmatite dike swarm, and locally, within granite gneiss and metasedimentary rocks. Paragenetic stages of mineralization and alteration are as follows: greisen I, albitization, K feldspathization, and greisen II. A late-stage of quartz-fluorite vug filling is present locally. K-Ar age dating of muscovite from the greisen I stage mineralization indicates a crystallization age of 158 to 179 + or - 6 Ma, in close agreement with previously determined ages of 151 to 179 Ma (Chon et al., 1993). Two samples of muscovite from the pegmatites give ages of 696 + or - 15 and 778 + or - 14 Ma. Greisen-related tin mineralization occurred during the Middle Jurassic to Early Cretaceous and is much younger than either the pegmatite (Late Proterozoic) or the granite gneiss (Early Proterozoic based on Rb/Sr studies of Hong, 1985, and Lee, 1988).Temperatures of greisen I stage mineralization, determined from fluid inclusion homogenization studies, arsenopyrite composition, and quartz-muscovite delta 18 O values, range from 450 degrees to 600 degrees C. The coexistence of sulfide and oxide minerals suggest that during ore deposition f (sub S 2 ) increased and f (sub O 2 ) was constant or decreased slightly. Fluid inclusion analyses indicate that the ore fluid was of low to moderate salinity (2.89-8.55 wt % NaCl equiv), with variable CO 2 content. Oxygen and hydrogen isotope values of quartz and muscovite decrease along a distance of 1,500 m from the northeast (Uljin mine) to the southwest (Dongseog mine) and suggest that fluid delta 18 O and delta D values decreased from approximately 11 to 9 permil, and -55 to -61 per mil, respectively. A similar decrease in fluid delta 18 O and delta D values is recorded temporally with the paragenetic stage at the Uljin mine. Sulfur isotope values of arsenopyrite and molybdenite range from -1.2 to 1.2 per mil.A genetic model of fluid derivation from an underlying Mesozoic granitic pluton, accompanied with the involvement of meteoric water in later stages, is proposed. Isotopic data are consistent with temporal and spatial variations in fluid delta D and delta 18 O values caused either by tapping of a magmatic fluid during various stages of chemical and isotopic evolution, or by reaction and isotopic exchange with high 18 O sedimentary rocks along a fluid flow path. Variations in delta 18 O values of carbonate country rocks from 19.3 to 12.3 per mil are suggestive of selective fluid transport through particularly permeable units in the limestone. Low delta 18 O values of vug-filling quartz suggest the involvement of low-temperature meteoric water in the latest hydrothermal stages of mineralization.