Hydrothermal W-Mo mineralization of the Cheongyang mine, Republic of Korea: A fluid inclusion and stable isotope study.

Abstract

The Cheongyang W-Mo mine, about 120km south of Seoul, is composed of fissure-filling quartz veins within Cretaceous granite porphyry (70.5±1.6 Ma) and Precambrian banded biotite gneiss. The spatial distribution of mineralized veins exclusively within or near the granite porphyry and related acidic dykes indicates that W-Mo mineralization was associated with intrusion of the granite porphyry. The major ore minerals are wolframite, scheelite and molybdenite. Ore mineralization can be divided into three distinct stages: greisen, vein (early W-Mo, and late base-metal sulfides), and vug (carbonates). Fluid inclusion data indicate that the hydrothermal system at Cheongyang evolved from initial high temperatures (near 400°C) to later lower temperatures (near 150°C) from fluids with salinities between 0 and 6 wt. % eq. NaCl. Deposition of W-Mo minerals occurred at temperatures between 300° and 400°C mainly as a result of pH increase due to vapor loss by fluid boiling. Later deposition of base-metal sulfides and carbonates was the result of increasing influx of cooler meteoric waters. Fluid inclusion evidence of boiling during the early W-Mo mineralization indicates pressures 100 to 260 bars. Fluid inclusion data for the Cheongyang W-Mo deposits are similar with those of W-Mo deposits in the Hwanggangri district, and are quite different with those of deposits in the Pusan-Yangsan district. This difference in temperature and compositions of hydrothermal fluids for Korean vein-type W-Mo deposits may reflect different pressure-depth conditions of W-Mo mineralization associated with late Cretaceous granitic activity in Korea. Sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a δ34SΣs value near 4 per mil. There is a systematic decrease in calculated δ18Owater values with increasing paragenetic time in the Cheongyang hydrothermal system, from values of ≈8 per mil for greisen formation and W-Mo mineralization, to ≈3 per mil for base-metal sulfide mineralization, to ≈-2 per mil for carbonate mineralization in vugs. There is a small increase in δ18Owater values from greisen formation (6.3-6.8‰) to W-Mo mineralization (7.0-7.8‰), likely indicating boiling of ore fluids during W-Mo mineralization. The trend of overall decrease in δ18Owater values with time is interpreted to indicate progressive meteoric water inundation of an early magmatic W-Mo hydrothermal system.