Hydrothermal Alteration of the Fushime Geothermal Field, Kyushu, Japan.

Abstract

Petrographical and geochemical studies on cores and cuttings from the Fushime geothermal wells were carried out using microscopy, XRD, XRF, and EPMA. The petrographical studies revealed that the alteration mineral assemblage in the geothermal reservoirs is characterized by the occurrence of K-feldspar and sericite. This assemblage is subdivided into three mineral zones: a K-feldsparquartz assemblage, a sericite-quartz assemblage, 3 ) an albite-chlorite assemblage. The geochemical study shows that potassium and rubidium are enriched in the K-feldspar-quartz assemblage zone. The authigenic K-feldspar observed with EPMA indicates that precipitation of K-feldspar and sericite contributed to enrichment of the alkali elements. In comparison with the temperaturerecovery survey, the alkali-enriched zone corresponds to a hydrothermal discharge.