Hydrothermal Alteration of the Pungchon Limestone and the Formation of High-Ca Limestone

Pungchon Formation in the Lower Chosun Supergroup consists of originally fine-grained and blackish to dark grey in lithology and color. The upper part of the limestone bed, however, have altered partly into, so called, light-colored 'high-Ca limestone' with more than nearly 98% of calcite, formed by the effects of low-temperature (100 similar to 200 degrees C) epithermal alteration (189 similar to 205 Ma), which has undergone roughly in the mode of a basinal hydrothermal system from the northwestern part of the basin. The early hydrothermal alteration was superimposed by subsequent geologic events, i.e., thermal metamorphism and high-temperature (300 similar to 400 degrees C) hypothermal alteration (76 similar to 135 Ma) facilitated by granite intrusions of the Jurassic and Cretaceous ages, respectively. Thus, the hydrothermal alteration of the limestone formation is somewhat extensive in the western part of the basin, but locally different in the mode of occurrence. In the Taeback-Samcheok areas, i.e., southeastern part of the basin, the original lithology is mostly preserved because of lacking in the hydrothermal alteration. In the upper part of the Pungchon Formation, through these hydrothermal regimes occurred more than twice, comparatively pure limestone was altered to the micro-crystalline (0.1 similar to 0.4 mm) calcite type with the color of greyish white to milky white, and thereafter, in places, they were subsequently replaced by the mega-crystalline (1 similar to 15 cm) calcite type and micro-crystalline marble type ores from the lower part of the formation. The original composition and fabric of the limestone was changed and obliterated by virtue of recrystallization resulting from these hydrothermal alterations, and thereby, characteristic hydrothermal minerals and calc-silicates such as illite, pyrite, clinochlore, tremolite, and garnet were included in the altered limestone. Dolomitization causing the formation of dolomite zone during diagenesis stage might play a major role in facilitation of a ground preparation for the subsequent formation of high-Ca limestone. The hydrothermal alteration zone of the limestone is locally and genetically correlated with the mineralization zone of Pb-Zn-Cu skarn deposits and Au-Ag deposits. The altered part of the Pungchon Formation can be deciphered in overall as a outer hydrothermal zone of these metal deposits. Thus, pre-existing studies on the limestone formation and current exploration guideline for high-ca limestone, depending on a stratigraphic correlation, should be reconsidered based on the knowledge of hydrothermal alteration of the Pungchon Formation.