PALEOCEANOGRAPHIC ENVIRONMENTAL SETTING OF KUROKO DEPOSITS, NORTHEAST JAPAN

The Kuroko deposits are considered to have been formed by submarine hydrothermal activities in association with spreading of the back arc basins. In order to understand the paleoceanographic environmental setting of the deposits, fossils and the mode of occurrences of the minerals in siliceous black ore from the Ezuri Kuroko deposit, and trace fossils and sulfur isotopes in mudstones around the Kuroko ore horizon in the Hokuroku basin are examined.

The siliceous black ore from the Ezuri deposit contains sponge spicules, radiolaria, and foraminifers. Structural observations suggest that the siliceous component of the ore constitutes autochthonous proto-Kuroko sediment. The foraminiferal assemblage is composed predominantly of agglutinated species (83%) with subordinate calcareous species (17%), and is assigned to the Cyclammina Assemblage. The Assemblage is different from the assemblages in the vicinity of present deep-sea hydrothermal vents, but is identical to those found in Miocene black mudstones of Japan. The proto-Kuroko sediment is assumed to have been deposited in an oxygen-deficient environment within a closed, deep-seated basin. The existence of siliceous microfossils suggests that silica in the ore did not from silica sinter deposits produced by submarine hydrothermal activities, but from a biogenic siliceous ooze, probably composed of diatoms. The later mineralization containing fluid inclusions with homogenization temperatures of ca. 250°C cuts across the siliceous masses and foraminiferal septa, and may have been formed after consolidation of the siliceous ooze.

The mudstones in the Hokuroku basin are generally massive in structure and usually contain large trace fossils, being indicative of an aerobic biofacies. Some mudstones in and above the Kuroko ore horizon are partly laminated and usually contain smaller trace fossils, being assignable to an anaerobic or dysaerobic biofacies. In the mudstones in the ore horizon, the d³⁴S values exhibit regionally discriminated variations: -44 to -12 ‰ in areas far (>1 km) from the known Kuroko deposits and -24 to +6 ‰ in areas closer to them. The latter high d³⁴S group implies the temporal occurrence of local anoxic basins in the vicinity of the Kuroko deposits. A paleoceanographic environmental evolution model will be constructed and discussed.