MINERALOGIC AND STABLE-ISOTOPE STUDIES OF CASSITERITE GREISEN MINERALIZATION IN THE ULJIN AREA, KOREA

Vein-type, greisen-related cassiterite deposits occur at the Uljin, Yu chang, and Dongseog mines in eastern Korea within a Precambrian pegmat ite dike swarm, and locally, within granite gneiss and metasedimentary rocks. Paragenetic stages of mineralization and alteration are as fol lows: greisen I, albitization, K feldspathization, and greisen II. A l ate-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 +/- 6 Ma, in close agreement with previously determined ages of 151 to 179 Ma (Chon et al., 1993). Two s amples of muscovite from the pegmatites give ages of 696 +/- 15 and 77 8 +/- 14 Ma. Greisen-related tin mineralization occurred during the Mi ddle 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 homoge nization 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(S2 ) increased and f(O2) was constant or decreased slightly. Fluid inclus ion analyses indicate that the ore fluid was of low to moderate salini ty (2.89-8.55 wt % NaCl equiv), with variable CO2 content. Oxygen and hydrogen isotope values of quartz and muscovite decrease along a dista nce of 1,500 m from the northeast (Uljin mine) to the southwest (Dongs eog mine) and suggest that fluid delta(18)O and delta D values decreas ed from similar to 11 to 9 permil, and -55 to -61 per mil, respectivel y. A similar decrease in fluid delta(18)O and delta D values is record ed temporally with the paragenetic stage at the Uljin mine. Sulfur iso tope 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 chemica l and isotopic evolution, or by reaction and isotopic exchange,ae with high O-18 sedimentary rocks along a fluid Bow path. Variations in del ta(18)O values of carbonate country rocks from 19.3 to 12.3 per mil ar e suggestive of selective fluid transport through particularly permeab le units in the limestone. Low delta(18)O values of vug-filling quartz suggest the involvement of low-temperature meteoric water in the late st hydrothermal stages of mineralization.