Jurassic and Cretaceous Granitic Rocks in South Korea

Abstract

Recognition of the importance of chlorine complexing in enhancing the solubility of metal sulfides in aqueous solution has led to the view that the chlorine content of intrusive rocks could govern the ability of a magma to separate a metal-rich hydrothermal phase. Relatively high chlorine contents were obtained for apatites, biotites, and amphiboles from Cretaceous granitic rocks in South Korea, whereas the same minerals from the Jurassic granitic rocks are essentially chlorine-free.
The distribution coefficient for chlorine-hydroxyl exchange between apatite and biotite depends on biotite composition and the distribution coefficient for fluorine-hydroxyl exchange between apatite and amphibole depends on amphibole composition. Detailed consideration of amphibole chemistry shows predominance of edenitic and tschermakitic substitution schemes, as well as coupling between substitution of Ti in octahedral sites and Al in tetrahedral sites.
Although the most of the Cretaceous granitic rocks in the Kyongsang basin belong to the magnetite-series, the Jurassic granitic rocks belong in part to the magnetite-series and in part to the ilmenite-series. For the Jurassic granitic rocks, Fe/(Fe+Mg) for biotites and amphiboles increased with an increase in host-rock SiO₂ content; for the Cretaceous granitic rocks in the Kyongsang basin, Fe/(Fe+Mg) for the same minerals decreased with an increase in host-rock SiO₂. It is suggested that the trend followed by Fe/(Fe+Mg) for mafic minerals with increasing host-rock SiO₂ was largely dependent on the presence or absence of early magnetite crystallization in these magmas.
From the mode of occurrence and textures of the granitic rocks and from the chemistry of rock-forming minerals, especially of mafic silicates, it is suggested that the Cretaceous granitic rocks in the Kyongsang basin are high level intrusions compared to the Jurassic granitic rocks. If hydrothermal transport of base metals in chloride complexes is accepted, it is suggested that most of hydrothermal base metal deposits in South Korea were associated with the Cretaceous granitic magmas. Sulfur isotopic composition of base metal deposits in South Korea also supports this conclusion.