CSAMT and MT Investigations of an Active Gold Depositing Environment in the Osorezan Geothermal Area, Japan

The Osorezan geothermal area is located in the centre of the Shimokita Peninsula, the northernmost tip of the island of Honshu, Japan. Osorezan is a long-lived composite volcano having a caldera lake. There are many active geothermal manifestations at the north shore of the lake. High-grade gold mineralisation is found in the hot-spring sediments at concentrations as high as 400 g/t (6,500 g/t max.).

Controlled-source audiofrequency magnetotelluric (CSAMT) surveys were carried out at 374 sites to investigate the detailed resistivity structure of the active gold depositing zone. The apparent resistivity contour maps show that resistivity anomalies correspond to the geothermal manifestations. Two-dimensional models analysed along west-east profiles indicate that most of the sites are underlain by a conductive layer. An extremely low-resistivity anomaly in the central part of the area is probably due to high-salinity hot water and intense argillic alteration. A high-resistivity anomaly in the western part of the area is interpreted as a silicified zone or a vapor-dominated zone.

In order to investigate deep resistivity structure, magnetotelluric (MT) surveys were carried out at 46 sites in and around the Osorezan geothermal area. A two-dimensional model analysed along a west-northwest-east-southeast profile indicates that the area basically has a two-layer structure; an upper conductive layer and a lower resistive layer. The lower resistive layer correlates well with the basement interpreted from gravity, which reflects pre-Tertiary basement rocks or intrusive rocks. However, the basement beneath the Osorezan geothermal area itself is relatively conductive. This may be due to fractures containing hot water and associated hydrothermal alteration. It seems likely that fracture systems forming circulation paths of deep geothermal fluids are developed in the basement beneath this area.