The Goshogake hydrothermal field (northern Japan): purely geothermal or hybrid sediment-hosted?

The Goshogake hydrothermal field in Tohoku, northern Japan, is located on the western flank of the Akita Yakeyama volcano. This N-S elongated field extends over ~20-25 acers and is placed over a tectonic discontinuity that controls the migration of hydrothermal fluids. A variety of surface degassing manifestations can be distinguished including small hydrothermal lakes, clustered bubbling pool fields, active mud erupting gryphons, sulfur-rich fumaroles and localized apparently oil-rich pools. So far it remains unclear if this system and the released gases are purely magmatic or if is also involved the migration of mantle-derived fluids interacting with buried sedimentary deposits. Here we report the result of field observations and fluid samples analyses. The measured temperatures of the active seepage sites range from 33 to 97 °C (in large part higher than 90 °C), while pH is overall low at all sites (i.e. ~2.5). Gas analyses reveal that all the active sites are CO2-dominated with slightly higher CH4 content in the colder and mud seeping sites. While CO2 has a distinct mantle-derived isotopic signature, the origin of methane is still debated. The apparent d13C thermogenic signature of methane, could also be related to an abiotic origin which would be consistent with a CO2-dominated geothermal system. The presence of oil, if confirmed, could be related to shallow-sourced hydrothermal oil (e.g. similar to that described by Didyk and Simoneit (1989), or, more interestingly, could indicate that this active site is part of a petroleum system potentially linked with deeply buried lacustrine sediments that fill a 1-Ma-caldera formed after a ignimbrite eruption. The possible presence of these deposits in the erupted mud has been suggested by Komatsu et al., (2019) based on mineralogical analyses. To further test this hypothesis, we are now conducting multiple analyses on the samples recovered from 6 locations at the Goshogake field, including the isotope analysis of water, gas, oil and mud compositions to unravel the source of these fluids as well as the reactions that took place at this site. If confirmed, Goshogake could potentially be the first example of known sedimentary hosted geothermal system in Japan. We speculate that targeted fieldworks would likely identify other hybrid systems in the country.