METALLOGENIC EPOCHS IN THE INNER ZONE OF SOUTHWEST JAPAN

The principal metallogenic epochs in the Inner Zone of southwestern Ja pan are classified by considering mainly radiometric and microfossil a ge data, as follows-(1) Early Cambrian Epoch: Characterized by podifor m-type chromite deposits hosted in ophiolite complexes; these are the earliest-formed mineral deposits in the Japanese Islands, and probably formed in the shallow mantle of island-are settings; (2) Carboniferou s to Permian Epoch: Characterized by Besshi-type base metal sulfide de posits mainly associated with submarine lavas and tuffs of basaltic co mposition including their metamorphosed equivalents, and by bedded cup riferous pyrite deposits associated with submarine acidic tuffs. Both types of deposits would have been formed in a back-are basin environme nt, geochemically influenced by an island-are; (3) Middle Triassic to Jurassic Epoch: Characterized by most of the bedded manganese deposits associated with bedded cherts, including their metamorphosed equivale nts, which would have formed by hot spring activity in an ocean basin environment with a limited current circulation. The manganese deposits and their host rocks are the result of seafloor accretion; (4) Late C retaceous to Paleogene Epoch: Characterized by extensive continental-a re magmatism (volcanic and plutonic), resulting in formation of variou s mineral deposit types, related mainly to ilmenite series (Late Creta ceous) or magnetite series (Paleogene) granitic rocks. Related to the former are polymetallic skarns and veins containing Cu, Sn and W; also related to this metallogenic activity are stratabound hydrothermal cl ay deposits of kaolin, pyrophyllite, and sericite associated with pre- ilmenite series granitoids subaerial felsic pyroclastic rocks. Molybde nite-bearing veins and replacement-type sericite deposits are characte ristics of the magnetite series terranes, with local skarns and vein o ccurrences. As with ilmenite - series,granitoids occurring in southwes tern Japan, eastward younging is also recognized in these mineral depo sits, regardless of ore type; (5) Neogene Epoch: Characterized by inte nse submarine volcanism of acidic composition and associated Kuroko de posits in the Green Tuff region, a major tectonic division and a major metallogenic province of the Neogene Tertiary of Japan. These mineral deposits would be related to Japan Sea opening. In the 'Non-Green Tuf f region,' important mineralization events are represented by sediment ary uranium deposits, and veins characterized by Au, Sb or Hg occurren ces; (6) Plio-Pleistocene Epoch: Characterized by epithermal Au-Ag and Au-Ag-Cu veins associated with subaerial andesitic to dacitic volcani c rocks of Plio-Pleistocene age. Volcanism of this and following epoch s is characteristic of an island-are setting; and (7) Quaternary Epoch : Characterized by volcanic sulfur deposits and minor iron sand deposi ts. (C) 1998 Elsevier Science B.V. All rights reserved.