Sulfide Scaling of Deep-Geothermal Well at Kakkonda Geothermal Field in Japan

Abstract

Sulfide rich scales in the Kakkonda geothermal area, Northeastern Japan, are investigated. They are precipitated from the geothermal fluid from the deep reservoir that exists at the boundary between Quaternary Kakkonda granite and Pre-Tertiary formations. The scales are classified into two types based on sulfide mineralogy, which are Pb-Zn rich type and Cu rich type. Pb-Zn rich scale is found in Well-19 located at the marginal part of the Kakkonda granite. It is mainly composed of amorphous silica, galena (PbS), sphalerite (ZnS) and pyrite (FeS2). Cu-rich scale is found in Well-13, located at the central part of the Kakkonda granite. It is mainly composed of amorphous silica, chalcocite (Cu2S), bornite (Cu5FeS4), chalcopyrite (CuFeS2), galena (PbS), loellingite (FeAs2) and native antimony (Sb). It is also rich in Au, Ag, As, Cr, Ni and Mo compared to Pb-rich scale. The metal contents of the scale in Well-22 show intermediate features between the two types of scales. The existence of both Pb-Zn rich scale and Cu-rich scale is a characteristic feature of the Kakkonda geothermal area. This is probably due to the difference in the parent fluid. The brine in Kakkonda granite may be an important metal source of the scales.