On the subterraneous geological structure in the Nansatsu plateau

With special reference to an interpretation technique of geophysical data

Abstract

The Nansatsu plateau is largely covered by Quaternary pyroclastic flow deposits. The Shimanto supergroup of Mesozoic to Paleogene age, the Nansatsu group of Neogene age and the post-Nansatsu volcanics of Quaternary age are distributed around the plateau and make a semibasin structure which is open southwards. Gold-bearing massive silicified rocks are hosted in the Nansatsu group and partly in the post-Nansatsu volcanics, and make small hills in the periphery of the plateau because of their resistance to erosion.
Ministry of Agriculture, Metal Mining Agency of Japan (MMAJ), Gold Mining Promotion Corporation (GMPC), Mitsui Kushikino Mining Co., Miyauchi Akeshi Mining Co. and Kasuga Mining Co. conducted integrated water and mineral resources explorations, using geological survey, drilling, electric surveys and gravimetric surveys in the Nansatsu plateau during a period of about 7 years starting in 1969.
The authors re-examined the data of these surveys, and estimated the shallow geological structure based on the results of drilling and electric survey in the plateau. The gravimetric data were treated with the manual dataprocessing technique, which was proposed by TERASHIMA & TSUCHIYA (1976a·b) and TERASHIMA & YOSHIZAWA (1976), after a revision to make it applicable to the geological circumstances of the area, and with the threedimentional simulation technique proposed by CORDELL & HENDERSON (1968) in order to compare the results each other.
On the basis of the local gravimetric anomalies on the plateau, many faults or shear zones are considered to be developed parallel and perpendicular to the caldera-like structure which is situated on the south coast of the plateau. Moreover, the paleo-hills of the pre-pyroclastic flow deposits and the dome structure of the low resistivity, which includes the gold-bearing massive silicified body, are in parallel and perpendicular to the caldera-like structure on the plateau. These facts may indicate that there is some mutual relation between the silicification accompanied by the gold mineralization and the tectonic movement accompanied by volcanic activities.
Through such discussions, it was proved that the manual filtering operation is very useful to explain a reasonable geological structure. Moreover, even in the case of electric surveys, the authors pointed out that the method of interpretation and data-processing is very important to clarify the detailed two-layered structure in the area which is covered by pyroclastic flow deposits.