Geochemical Variation of Hwangsan Volcanic Complex by Large Hydrothermal Alteration

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. diagram. The pattern that () content decrease with increasing content results from sericitic alteration, in which hydrothermal fluids continually provide into country rocks but remove and of feldspars within country rocks. The decrease of () with decreasing 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+chloritepyritc 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 , substituting for in feldspars, appears to be resulted from removal of , 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.