Title: Trace Fossils and Sulfur Isotopes in Mudstones around the Kuroko Deposits in the Hokuroku Basin, Northeast Japan: An Attempt to Delineate the Depositional Environment
Abstract: Abstract. A comprehensive investigation was carried out on the distribution of both trace fossils and sulfur isotopes in mud‐stones in the Hokuroku district, northeast Japan, in the hope of delineating the depositional environment of the mudstones in which the Kuroko deposits are embedded. The mudstones are generally massive in structure and usually contain large trace fossils, being indicative of an aerobic biofacies. On the other hand, 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. The δ 34 S values of sulfides in the mudstones above and below the ore horizon range from ‐40 to ‐12 %o, indicating mostly oxic depositional conditions in equilibrium with the inferred aerobic biofacies. In the mudstones in the ore horizon, the δ 34 S values exhibit regionally discriminated variations: ‐44 to ‐12 %o in areas far (>1 km) from the known Kuroko deposits and ‐24 to +6 %o in areas closer to them. The latter high δ 34 S group implies the temporal occurrence of local anoxic basins in the vicinity of the known Kuroko deposits. At the time of late Nishikurosawa Stage (i.e. the currently assumed Kuroko metallogenic epoch), an intense oceanic stagnation is suggested to have taken place to form the local anoxic basins responsible for the formation and preservation of Kuroko deposits. This oceanic environmental event is considered to be most likely due to increasing biological productivity primarily triggered and enhanced by upwelling of NADW in the paleo‐Sea of Japan at that time.