ABSTRACT

Experiments on glassy rhyolite- and phenocrystic andesite-seawater interactions were carried out at 300°C and 1, 000b using Dickson-type gold bag equipments. Chemical reactions in rhyolite-seawater interaction proceeded very rapidly and a steady state was attained within about 48h. Andesite-seawater interaction, however, proceeded very slowly and a steady state was not attained within the run period of 7, 090h. Rhyolite and glass, augite and hypersthene in andesite were altered into smectite. Seawater lost most Mg²⁺ and SO₄^2- and become acid Na-K-Ca-Cl-type solution through the reactions. The experimental result suggests that two stage alteration episodes existed in Kuroko-type mineralization. The present experimental solutions reproduced the chemical composition of a deep hot water from a bore hole drilled into andesitic geothermal area of southern Kyushu. However, shallow thermal waters discharging from andesitic volcanic rocks along some coastal areas of Japan cannot be explained as direct products of high-temperature interaction of seawater and fresh acid volcanic rocks.