In the southern part of the Korean peninsula, hydrothermal clay deposits are mainly associated with the late Cretaceous Yuchon group of the upper Gyongsang system, which principally comprises rhyolitic to andesitic lavas and pyroclastic rocks. In particular, the pyrophyllite/kaolin deposits in the southern Chollanamdo province tend to occur as a largescale sheet in pyroclastic rocks of rhyolitic composition. The pyrophyllite deposits of Nohwado Island at the southwestern extremity of Korea are a representative example in the province. To examine the typical field occurrence and ore mineralogy of hydrothermal clay deposits in Korea, this study has focused the No. 1 deposit of the Nohwa mine of the Minkyung Industry Co., Ltd. (under ground exploitation) on the island.
The pyrophyllite deposit was formed by intensive hydrothermal alteration of rhyolitic lapilli tuff and interbeds of tuffaceous sandstone and shale. The ore body occurs as, a sheet of 700 m (major axis)×250m (minor axis)×65m (max. thickness), that is almost concordant with the host strata. It is divided into a pyrophyllite and a diaspore zone on the basis of mineral assemblages.
Diaspore zone: This zone has an elongated shape and occurs in the central part of the ore body. Its mineral assem-blage is pyrophyllite+quartz+diaspore+kaolinite with subordinate alunite and rutile. Diaspore occurs as an aggregate of, 0.2-1.5 mm diameter grains; its content is usually 1-2 vol% (partly>20 vol.%). Pyrophyllite, alunite, and kaolinite com-monly replace diaspore along rims and cleavage.
Pyrophyllite zone: This zone is characterized by the mineral assemblage pyrophyllite+quartz, and constitutes a main part of the ore body. Alunite and rutile occur as accessory phases.
Both zones are partly silicified (SiO2>80wt%). The irregular shaped silicified parts tend to occur in and around the boundaries between the two zones and in the marginal parts of the ore body. A weakly altered zone, which is composed of quartz and illite with subordinate albite, kaolinite, chlorite, and biotite, occurs in the periphery of the ore body. Although the ore mineral assemblages of the zones are different, their chemical composition is similar.
Ore texture and thermodynamic considerations suggest that the diaspore zone may have been secondarily formed by the alteration of the central pyrophyllite zone due to the ascent of steam. The high permeability of the sedimentary host rocks enabled the passage of the hydrothermal fluid, leading to the formation of the large sheet pyrophyllite deposit.
Several illite veins are observed along fractures in the ore body. One illite sample from such a vein yielded a K-Ar age of 71.8±3.6 Ma.
