A currently forming petroleum associated with hydrothermal mineralization in a submarine caldera, Kagoshima Bay, Japan

Petroleum-like products have been found in hydrothermally altered recent sediments of the submarine Wakamiko Caldera (depth = 200 m) in the northern Kagoshima Bay, southern Kyushu, Japan. High concentrations of polynuclear aromatic hydrocarbons (PAHs) of pyrolytic origin and biomarkers signatures representing diverse degrees of maturation have indicated that the associated bitumen is a typical hydrothermal petroleum. This is the first report on a significant content of hydrothermal petroleum related with a submarine volcanism of island-arc. The petroleum was generated by pyrolysis of organic matter in recent sediments deposited in the caldera. The hydrothermal petroleum and massive sulfide deposits are concluded to be currently forming within the caldera by hydrothermal activities below the seafloor where presently forming Kuroko-type sulfide deposits were also reported. Considering simultaneous formation of the hydrothermal petroleum and the Kuroko-type deposits in the Wakamiko Caldera, we propose a hypothesis that, during the middle Miocene, the same phenomena happened extensively in northeast Japan in association with the large-scale marine volcanisms, resulting in the formation of both petroleum and sulfide ore deposits (Kuroko) in the same geological horizons.