2022 年 72 巻 1 号 p. 21-55
The Shimokawa cupriferous iron sulfide deposit located in the Cretaceous to Paleogene Hidaka Belt produced 6.8 Mt of crude ore, with an average grade of 2.3% Cu and 1% Zn, until it’s closure in 1983. The sulfide deposit occurs in the Shimokawa greenstone, comprises dolerite sills and conformably intercalated argillites, and is preferentially hosted within argillites. The major ore minerals of the deposit are pyrite, chalcopyrite, pyrrhotite, and sphalerite, which form platy and/or lenticular bodies with low aspect ratios. The mineralization stages in the argillites can be divided into early and late stages. The early-stage mineralization is characterized by pyrite ores consisting of previous framboidal pyrites and scab-like fine pyrite aggregates, subsequent overgrown pyrites such as colloform pyrites, euhedral pyrites on the fringe of previously formed fine pyrites, and euhedral pyrites in carbonate and argillite substrates. This kind of low-temperature mineralization likely took place in a mixing zone of thermogenic hydrocarbon and carbon dioxide degraded from organic matter, low-temperature hydrothermal fluids, and unconsolidated sediments on or beneath the seafloor. The late-stage mineralization process forms a banded and stockwork ore primarily hosted by argillites; it is characterized by high-temperature mineral assemblages such as chalcopyrite-pyrrhotite-sphalerite with a minor amount of cubanite, mackinawite pentlandite, magnetite, and electrum. With hydraulic fracturing, the late-stage hydrothermal fluids diffuse laterally in unconsolidated sediments and are superimposed on the early-stage ore as a sub-seafloor replacement deposit. Earlier dolerite sills and early-stage sulfide ores act as cap rocks to control lateral fluid diffusion. Argillites show the chemical composition of the matured continental island arc in the Th-Sc-Zr diagram and the local anoxic to suboxic environment indicated by proxies of redox-sensitive elements. The occurrence of pillowed dolerite sills with quenched glassy selvages and micro-scale peperites suggests that dolerite sills were emplaced into unconsolidated sediments. Some dolerite sills penetrate massive sulfides; however, on rare occasions, sulfides disseminate in dolerite sills. Overall, basic magma activity, sedimentation of fine-grained terrigenous clastic sediments, and sulfide mineralization might have occurred simultaneously. Dolerite sills mainly comprise “N-type” dolerite with characteristics of N-MORB-like (normal-type mid-oceanic ridge basalt) to C-MORB-like (contaminated-type mid-oceanic ridge basalt) geochemistry, as indicated by a crustal contamination vector in the Th/Yb-Nb/Y diagram; they also include small quantities of “E-type” dolerite with an enriched character. The Pb isotope characteristics indicate that Pb of sulfide ore originated from both dolerite and argillite. Unlike the Besshi-type sulfide deposits in the Sanbagawa metamorphic belt that formed at the sediment-free mid-oceanic ridge and suffered eclogite facies metamorphism at the deeper subduction zone level, the Shimokawa deposit associated with dolerite sills and argillites might have been formed in the sedimented spreading center of the Izanagi–Pacific plate located in the paleo-Hidaka sea between paleo-northeast Japan and the paleo-Kuril arc.
