• Economic and Environmental Geology
• /
• v.44 no.2
• /
• pp.95-107
• /
• 2011

The Hwangsan volcanic rocks, hosting the Moisan epithermal Au-Ag deposit arc widely distributed throughout the Seongsan district, and associated with large hydrothermal alteration. They were analyzed as the Moisan and around voleanic rocks, and most of them show dacitic to rhyolitic compositions. Hydrothermal alteration related to epithermal system causes the host rocks to show the geochemical variation due to high mobility of alkali elements. These features can be applied for quantitative estimates of alteration intensity. Alteration intensity of volcanic rocks from the Moisan ranges from subtle to intense, based on AI vs. $Na_2O$ diagram. The pattern that ($CaO+Na_2O$) content decrease with increasing $K_2O$ content results from sericitic alteration, in which hydrothermal fluids continually provide $K^+$ into country rocks but remove $Ca^{2+}$ and $Na^{2+}$ of feldspars within country rocks. The decrease of ($CaO+Na_2O$) with decreasing $K_2O$ in some samples from the Moisan may be caused by advanced argillic alteration that all alkali elements are entirely removed from country rocks by acid hydrothermal fluids. Two alteration trends, based on Al and CCPI alteration indices suggest both sericitic alterations of feldsaprs to illite and sericite+chlorite$^{\circ}{\ae}$pyritc alteration of high Mg and Fe activities. Trace and Rare Earth Elements patterns show the similar geochemical variation related to hydrothermal alteration. Of LIL elements, strong depletion of $Sr^{2+}$, substituting for $Ca^{2+}$ in feldspars, appears to be resulted from removal of $Ca^{2+}$, during replacement of feldspars to alumino-silicates or phyllo silicates minerals by hydrothermal fluids. Relatively low total REEs contents (Moisan: 119-182 ppm; Seongsan: 111-209 ppm) and gently negative slopes suggest that significant mobility of LREEs appear to occur during hydrothermal alteration.

• The Journal of the Petrological Society of Korea
• /
• v.1 no.1
• /
• pp.58-70
• /
• 1992

From the K-Ar age determinations for the clay deposits and their surrounded rocks in southwest Korea, the ages of the ore formation in all clay deposits fall in very narrow range from 78.1 to 81.4 Ma. K-Ar ages of clay deposits are slightly younger than those of the Cretaceous volcanic rocks (Hwangsan Formation, 81.4 to 86.4 Ma) and are slightly older than those of the Cretaceous granitic rocks (77.1 to 81.5 Ma). These results indicate that clay deposits were formed with genetical relation to late Cretaceous felsic magmatism. Weolgagsan granite, which has been previously considered to be Cretaceous, is proved to be formed its age in Jurassic (140.9 and 144.8 Ma). The close relationships of K-Ar ages between the clay deposits and Cretaceous granitic rocks suggest that the clay deposits were formed during the hydrothermal alterations caused by the thermal effects (hydrothermal circulation) of the granitic intrusions rather than by the hydrothermal activities associated with volcanic activities.

• Journal of the Mineralogical Society of Korea
• /
• v.7 no.1
• /
• pp.14-24
• /
• 1994

The Sungsan clay deposits have been formed by the hydrothermal alteration of volcanic and volcanoclastic rocks of the Hwangsan Formation of Cretaceous age. Claystones are mainly composed of dickite, alunite, illitic minerals and tosudite. The mineralogical properties of clay minerals have been studied using X-ray diffraction analysis, electron microscopy, electron microprobe analysis, and infrared absorption analysis. The physicochemical condition for the clay deposits also have been studied by the activity diagrams and mineral assemblages. Dickite, the dominant mineral in clay deposits, occurs generally as massive aggregates. It shows book-structure of well-defined hexagonal plates. Chemistry of dickite agrees with its ideal formula. Peak depth ratios in infrared absorption spectra were used for discrimination between pure and mixture of kaolin minerals. Five hydrothermal alteration zones are divided according to the mineral assemblages. From center to margin, alunite, dickite, illite and albite zones are discernible. Quartz zone occurs as small lenticular form in dickite zone. The formation of dickite and illite zones are promoted by decreasing $a_{k^+}$. An increase in $a_{H_{2}SO_{4}}$ or $a_{K_{2}SO_{4}}$ is required for the formation of alunite zone. Estimated temperature of formation ranges 110-270 $^{\circ}C$