Composite hydrothermal alteration during Late Miocene and since the Latest Pleistocene at the Itaya kaolin deposit, Yamagata Prefecture
1977 Volume 27 Issue 144 Pages 263-276
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1977 Volume 27 Issue 144 Pages 263-276
Hydrothermally altered rocks at the No. 1 Orebody of the Itaya kaolin deposit, one of the largest hydrothermal clay deposits in Japan, have been divided into the following four types of alteration zones on the basis of indicative mineral assemblage and characteristic occurrence: Alteration Type I-kaolinite-sericite-K-feldspar zone, which occurs exclusively within acid tuffs of the Itaya Formation(Funakawa Stage, Late Miocene). Alteration Type II-breccia dike and pyrite-disseminated sheared zone ("Black Fault"); masses of alteration products filling sheared parts of the altered Miocene tuffs (Type I). Alteration Type III-montmorillonite zone, which is limited to mud flow deposit derived from the eruption of the Azuma Volcano in the Latest Pleistocene. Alteration Type IV-native sulfur-opal-alunite-kaolinite zone, which replaces some parts of the altered Itaya Formation (Type I) and the altered Azuma mud flow deposit (Type III).
From the correlation of the stratigraphic range of their distribution to the stratigraphy of the original rocks, these types of alteration appear to have taken place in the order mentioned above. On the basis of radiometric age determination, two major periods of hydrothermal alteration have been recognized at the No. 1 Orebody. The earlier one (Type I) was appeared during the Late Miocene, and may have been associated with the terrestrial acid volcanism which produced the tuffs of the Itaya Formation. The later one (Types II to IV), on the other hand, took place since the Latest Pleistocene to present, as the result of geothermal activity related to the andesitic Azuma volcanism. This study has shown that the general features of the No. 1 Orebody were initially formed by Miocene sericitic alteration, while the northwestern part of the Orebody was later replaced by sericite-free clay during geothermal alteration on or near the surface since the Latest Pleistocene.
