Geology and hydrothermal alteration of the Milyang pyrophyllite deposit, southeast Korea

The Milyang pyrophyllite deposit, which is embedded in the Late Cretaceous Yuchon Group of the Krongsang Supergroup, is one of the largest hydrotherma l 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 x 3 km in ex tent; we can recognize the concentric arrangement of advanced argillic, pro pylitic, and sericitic alteration zones from the central to peripheral part s of the zone. The Milyang pyrophyllite deposit forms a part of the advanced argillic alte ration zone. The Milyang pyrophyllite deposit is subdivided into the follow ing four zones based on mineral assemblages: the pyrophyllite zones 1, 2, 3 , and the silicified zone. The pyrophyllite zone 1, which occupies the cent ral part of the deposit, comprises mainly pyrophyllite, kaolinite, and dias pore without quartz. Diaspore nodules often concentrate in beds 40-50 cm th ick. Andalusite, dumortierite, and tourmaline locally occur as network vein s, crack-filler, or small spherulitic spots. The Al2O3 content of the ore r anges 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 pyrophyllite zo ne 3 is the hematite-rich marginal facies of the deposit. The silicified zo ne, 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%. Com paring chemical compositions of the high-Al ores with those of unaltered ho st 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 clas sified into the following four stages: 1) extensive sericitic and propyliti c alteration, 2) medium-temperature (200-250 degreesC) advanced argillic al teration, 3) high-temperature (250-350 degreesC or more) advanced argillic alteration, and 3) retrograde low-temperature: alteration. The heat and som e volatile components such as B and S would be derived from the Pulguksa Gr anite intruded underneath the deposit.