Measurement of physical properties of drilling samples obtained by the Boring Machine System in a hydrothermal activity area

Abstract

　Collecting samples, measuring their physical properties, and analyzing their constituent minerals are essential to understand the spatial scale of mineral resources on the deep seafloor and the origins of their formation. In particular, vertically continuous samples obtained by drilling surveys are indispensable to assessing the effectiveness of geophysical explorations. In this study, sub-seafloor drillings using a seafloor-seated drilling system were conducted at a promising location of a seafloor hydrothermal deposit. The main objectives of this study are to expand the sample information of submarine hydrothermal deposits and to evaluate the usefulness of the self-potential survey method for buried hydrothermal deposits. Drillings were conducted at two sites, one in an area with both hydrothermal signatures and self-potential anomalies and one in an area with hydrothermal signatures but no self-potential anomalies. Sulfide mineral precipitations were confirmed in the core collected in the area of self-potential anomaly. In contrast, the core collected in hydrothermal signatures was mainly composed of volcaniclastic material and showed minor sulfide mineralization. Detailed physical property measurements and chemical analyses were performed on the core of the self-potential anomaly area. First, electrical resistivity measurements were performed on the entire core using a multi-sensor core logger. Then precise electrical property measurements were performed using the spectral induced polarization method at one point per section. Elemental concentration analysis by X-ray fluorescence spectrometry and mineral identification by X-ray diffraction analysis was also conducted, and mineral assemblage was determined in conjunction with drilling descriptions. The results showed the intermittent occurrence of conductive sulfide minerals such as galena and pyrite over an area of approximately 20 m. The distribution of concentrations of these conductive sulfide minerals corresponds well with the distribution of points where the induced polarization effect is strong. On the other hand, the distribution of high induced polarization effect points did not coincide with that of low electrical resistivity points. In conclusion, this study shows that hydrothermal signatures do not necessarily indicate the embryo of seafloor hydrothermal deposits and that self-potential surveys can detect buried mineralization zone. The induced polarization effect was again shown to be effective in detecting sulfide minerals. In contrast, the insufficiency of electrical resistivity alone was suggested for estimating the scale of submarine hydrothermal deposits by geophysical surveys.