Distributions, paragenesis and fluid inclusions of hydrothermal minerals in the Kakkonda geothermal field, Northeastern Japan

Abstract

Distributions, paragenesis and fluid inclusions of hydrothermal minerals in the Kakkonda geothermal field, Northeastern Japan, were studied by means of the X-ray diffractometry and the microthermometry. Many veinlets are found in Tertiary sedimentary rocks and welded tuff of the Pliocene-Pleistocene. The mineralization can be divided into three paragenetic stages: Stage IA, Kfeldspar+quartz+pyrite; Stage IB, quartz+calcite+laumontite+yugawaralite+stilbite+mordenite+pyrite; and Stage II, calcite+kaolinite+quartz+pyrite. Minerals in Stage II were formed from recent geothermal fluids. Fluid inclusions in quartz and calcite are generally several microns in size and are liquid inclusions. Homogenization temperatures of fluid inclusions in quartz and calcite range 87-296°C and 83-217°C, respectively. Judging from the distribution of the maximum homogenization temperatures obtained for the samples from geothermal wells (Muramatsu, 1984) and the surface, it is inferred that at least 900 meters of overburden were eroded after the mineralization. Temperature distribution pattern around the Kakkonda geothermal field suggests that high temperature fluids ascended mainly from the deeper zone of the western area, and that some meteoric water descended from the central area at Stage I and Stage II (present day). The data on the modes of occurrence, and the distribution pattern and fluid inclusion data of quartz and calcite, indicate that most of quartz were deposited from ascending fluids in the convective geothermal system, and that calcite precipitation resulted partly from the boiling of ascending hydrothermal fluids in the vicinity of the surface.