A fluid inclusion study of an amethyst deposit in the Cretaceous KyongsangBasin, South Korea

The Eonyang amethyst deposit is thought to be spatially and temporally asso ciated with the biotite granite of the Cretaceous Kyongsang Basin, South Ko rea. The euhedral quartz crystals in cavities in the aplite which intrudes biotite granite are colour-zoned from white at the base to amethystine at t he top. Fluid inclusions from rock-forming quartz in granitic rocks and euh edral quartz crystals in cavities were examined. Three types of primary inc lusions were observed and three isochores for inclusions representing each type are constructed to constrain the trappings conditions and fluid evolut ion involved during the formation of the amethyst. The intersection of the isochore representing the early fluid inclusions with solidus temperature o f the host granite indicates initial quartz formation at similar to 600 C a nd 1.0-1.5 kbar. Intermediate quartz formation. associated with the high-sa linity inclusions. Occurred at somewhat lower temperatures (400 C) and pres sures of similar to1 kbar. The amethystine quartz formed front H2O-CO2-NaCl fluids at temperatures between 280 and 400 C, and pressures of similar to1 kbar. Based on the texture and mineralogy of host minerals and on the flui d inclusion characteristics. the euhedral quartz began growing at near soli dus conditions of the granite and the pressure did not vary significantly u ntil the end of crystallization of amethystine quartz crystals in cavities. Early quartz in cavities formed from moderately saline fluids that either exsolved from or were in equilibrium with the granite, whereas the amethyst ine quartz apparently grew from fluids of at least partial sedimentary orig in. The granite crystallized at considerable depth tinder relatively low wa ter pressures probably in the root zones of porphyry-type systems. Hydrothe rmal activities, fluid compositions and erosion factors combined to provide favourable conditions for the formation of the Eonyang amethyst deposit an d its presence near the Earths surface today.