Host Rock Geochemistry and Tectonic Setting of Some Volcanogenic Massive Sulfide Deposits in Japan:Examples of the Shimokawa and the Hitachi Ore Deposits

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The Shimokawa and the Hitachi Cu-Zn type volcanogenic massive sulfide deposits occur in Cretaceous-Paleogene and late Paleozoic accretional complexes, respectively. The results of geochemical studies including major and minor element analyses by XRF of the host rocks suggest that the Shimokawa basic rocks and massive sulfide deposits formed at a pull-apart basin as the Gulf of California and the Andaman Sea. The pull-apart basin was possibly developed at Hokkaido during Cretaceous to Paleogene age, connected with the migrated ridge-trench-trench triple junction formed by the collision of Kula-Pacific ridge with Japanese arc. The Hitachi accretional complex consists mainly of basic to acidic volcanic rocks of calc-alkali series, basic to acidic back-arc volcanic rocks of tholeiitic and calc-alkali series, and boninite. It seems to be a supra-subduction zone ophiolite (PEARCE et al., 1984) originally formed at a back-arc-arc-trench system on the Farallon oceanic plate in late Paleozoic age. The Hitachi massive sulfide deposits were formed by the hydrothermal solution associated with acidic and intermediate volcanic activity at the spreading axes in the back arc basin of the island arc. Considering the ore characteristics, tectonic settings and associated volcanic rocks, the Hitachi massive sulfide deposits are more analogous to the Noranda-type and the Scandinavian Caledonide Cu-Zn deposits than the Kuroko deposits.
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