: Hydrothermally altered areas forming pyrophyllite‐kaolin‐sericite‐alunite deposits are distributed in Chonnam and Kyongsang areas, Cretaceous volcanic field of the Yuchon Group. The Chonnam alteration area is located within depression zone which is composed of volcanic and granitic rocks of late Cretaceous age. The clay deposits of this area show the genetic relationship with silicic lava domes. The Kyongsang alteration area is mainly distributed within Kyongsang Basin comprising volcanic, sedimentary and granitic rocks of Cretaceous and Tertiary age. Most of the clay deposits of this area are closely related to cauldrons. Paleozoic clay deposit occurs in the contact zone between Precambrian Hongjesa granite gneiss and Paleozoic Jangsan quartzite of Choson Supergroup. Cretaceous igneous rocks of the both alteration areas belong to high K calc‐alkaline series formed in the volcanic arc of continental margin by subduction‐related magmatism. Chonnam igneous rocks show more enrichment of crustal components such as K, La, Ce, Sm, Nd and Ba, higher (La/Yb)cn ratio, and higher initial 87Sr/86Sr ratio (0. 708 to 0. 712) than those of Kyongsang igneous rocks. This might be due to the difference of degree of crustal contamination during Cretaceous magmatism. The most characteristic alteration minerals of Chonnam clay deposits are alunite, kaolin, quartz, pyrophyllite and diaspore which were formed by acidic solution. Those of Kyongsang clay deposits are sericite, quartz and pyrophyllite which were formed by weak acid and neutral solution. The formation ages of the clay deposits of two alteration areas range from 70. 1 to 81. 4 Ma and 39. 7 to 79. 4 Ma, respectively. The Daehyun clay deposit in Ponghwa area of Kyongsang province shows the alteration age range from 290 to 336 Ma. This result shows the different alteration episode from the hydrothermal alteration of Cretaceous to early Tertiary in the Kyongsang and Chonnam alteration areas. These data indicate, at least, three hydrothermal activities of Tertiary (middle to late Eocene), late Cretaceous (Santonian to Maastrichtian) and Paleozoic Carboniferous Periods in South Korea.

The Milyang pyrophyllite deposit, which is embedded in the Late Cretaceous Yuchon Group of the Kyongsang Supergroup, is one of the largest hydrothermal clay deposits in the Kyongsang basin, southeast Korea. Host rocks of the deposit are porphyritic andesite lava and minor andesitic lapilli tuff. In the Milyang district, a hydrothermally altered zone is about 2 × 3 km in extent; we can recognize the concentric arrangement of advanced argillic, propylitic, and sericitic alteration zones from the central to peripheral parts of the zone. The Milyang pyrophyllite deposit forms a part of the advanced argillic alteration zone. The Milyang pyrophyllite deposit is subdivided into the following four zones based on mineral assemblages: the pyrophyllite zones 1, 2, 3, and the silicified zone. The pyrophyllite zone 1, which occupies the central part of the deposit, comprises mainly pyrophyllite, kaolinite, and diaspore without quartz. Diaspore nodules often concentrate in beds 40–50 cm thick. Andalusite, dumortierite, and tourmaline locally occur as network veins, crack‐filler, or small spherulitic spots. The Al2O3 content of the ore ranges from 27 to 36 wt%. The pyrophyllite zone 2, which constitutes a major part of the deposit, comprises mainly pyrophyllite, kaolinite, and quartz. The Al2O3 content of the ore ranges from 15 to 24 wt%. The pyro‐phyllite zone 3 is the hematite‐rich marginal facies of the deposit. The silicified zone, which occurs as beds and septa, is mostly composed of quartz with minor pyrophyllite and kaolinite; the SiO2 contents range from 79 to 90 wt%. Comparing chemical compositions of the high‐Al ores with those of unaltered host andesite, the Fe, Ca, alkalis, HFSE, and HREE contents are significantly depleted, whereas S, B, As, Sr, and LREE are enriched. The hydrothermal alteration of the Milyang pyrophyllite deposit can be classified into the following four stages: 1) extensive sericitic and propylitic alteration, 2) medium‐temperature (200–250°C) advanced argillic alteration, 3) high‐temperature (250–350°C or more) advanced argillic alteration, and 4) retrograde low‐temperature alteration. The heat and some volatile components such as B and S would be derived from the Pulguksa Granite intruded underneath the deposit.