CHARACTERIZATION OF SEAFLOOR HYDROTHERMAL DEPOSITS BY GAMMA RAY LOGGING IN THE IZENA HOLE, THE MID-OKINAWA TROUGH

During the CK16-05 cruise from Nov. 16 to Dec. 15, 2016 aboard the D/V CHIKYU, we conducted the gamma ray logging by using the PPS71*¹ at a total of five sites in the active hydrothermal area as part of the SIP*² drilling project. The survey focuses on constraining the distribution of the massive sulfide ore bodies beneath sediments and investigating the relationship between the ore bodies and the massive sulfide mound on the seafloor in the Izena Hole, the mid-Okinawa Trough. The total gamma ray intensity, temperature and pressure data were successfully collected in the three boreholes (C9025, C9026 and C9032) located along the east-west transect crossing the hydrothermal mound at the Hakurei Site and the other boreholes (C9029 and C9030) to the north of the site. The downhole logging was performed for each site from close to the bottom hole (generally 5 m above the total depth) to ~5 meter below the seafloor (mbsf). The gamma ray profile in the boreholes by the PPS71 was correlated with the discrete natural gamma ray data measured from the core samples by the Multi Sensor Core Logger (MSCL). This profile suggests that the obtained gamma ray amplitude and variation patterns are reliable and effective for in-situ formation evaluation. By comparing these gamma ray data with the core samples, the sulfide ore bodies with low gamma ray intensity (less than ~15 API) were identified. The intensity variation coincided with changes in lithology were interpreted as an occurrence of the pumice layer, sulfide ore body, silicified rock and K-rich altered clay layer. Along the transect, the top of the ore bodies were almost the same depth (~30 mbsf), but the bottom depth became shallower to the east (from ~78 to ~46 mbsf), showing that the thickness of ore bodies became thinner with increasing distance from the sulfide mound. At the other two sites, the ore bodies were not identified and the gamma ray profile was correlated with the porosity and density measured from the core samples by the MSCL. Our results demonstrate that the gamma ray logging in the hydrothermal area is a powerful tool for the identification and characterization of seafloor hydrothermal deposits, especially in depth intervals with poor or no core recovery.

*1: Geothermal well logging tool (Pioneer Petrotech Services Inc.), *2: Cross-ministerial Strategic Innovation Promotion Program