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ABSTRACT: Hydrothermal Alteration of the Felsic Volcanic Rocks in the Seongsan Mine, Korea

Injoon Kim

The Seongsan mine is one of the largest dickite deposits in the southwestern part of the Korean Peninsula. The main constituent

minerals of the ore are dickite and quartz with accessory alunite, kaolinite, and sericite. The geology around the Seongsan mine consists mainly of Late Cretaceous felsic volcanic rocks. In the studied area, these rocks make a synclinal structure with an axis of E-W direction dipping to the east.

Most of the felsic volcanic rocks have undergone extensive hydrothermal alteration. The hydrothermally altered rocks can be classified into the following zones: dickite zone, dickite-quartz zone, quartz zone, sericite zone, albite zone, and chlorite zone, from the center to the margin of the alteration mass. Such zonal arrangement of altered rocks suggests that the country rocks, most of which are rhyolite and welded tuff, were altered by strongly acid hydrothermal solutions. It is reasonable to consider that initial gas and solution containing H2S and other compounds were oxidized near the surface and formed hydrothermal sulfuric acid solutions.

The mineralogical and chemical changes of the altered rocks were investigated using various methods, and chemical composition of 56 samples of the altered rocks were obtained by wet chemical analysis and X.R.F. methods. On the basis of these analyses, it was found that some components such as SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, and TiO2 were mobilized considerably from the original rocks.

The formation temperature of the dickite deposits was estimated as higher than 200°C from fluid inclusion study of samples taken from the quartz zone. On the basis of the chemical composition data on rocks and minerals and estimated temperatures, the hydrothermal solutions, responsible for the formation of the Seongsan dickite deposits, were estimated to have the composition: mk+ = 0.003, mNa+ = 0.097, mSiO2aq. = 0.008 and pH = 5.0; here m represents the molality (mole/kg H2O).

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990