Dyke rocks around the Kamioka mining area, especially on so-called granite porphyries

in relation to the chronology of the Kamioka deposits, Hida metamorphic region, central Janpan

Abstract

Petrographical properties of dyke rocks observed in the Kamioka Pb-Zn Mining area, Hida metamorphic region, central Japan, are described, based on their mode of occurrences. These are classified into andesites, quartz porphyries, Tochibora and Mozumi granites (so-called granite porphyries), metabasic dykes (synplutonic dykes associating with granites) and so-called aplites (aplitic to leuco-granitic facies of the Inishi-type migmatitic rocks).
So-called granite porphyries have been assumed to be the ore-bringer dykes occurring within the core of the concentric zonation of the Kamioka deposits and closely associated with the late Cretaceous igneous activities, such as the Nohi Rhyolites. Petrographical analysis, however, shows that they are not porphyrires but granitic rocks having holocrystalline equigranular and hypidiomorphic to granoblastic granitic texture and usually granodioritic to granitic mineral compositions. The geological situations, mode of occurrences, petrological characteristics, x-ray properties of potash-feldspars and magnetic susceptibility suggest that the Tochibora and Mozumi granites are lithologically correspondent with the leuco-granitic facies of the adjacent Funatsu granites (170-180 m.y.) and different from the Shirakawa granites of the late Cretaceous to Paleogene ages. The Rb-Sr isochron age (90 m.y.; which has been presumed to be the age of so-called granite porphyries) is defined by the quartz porphyries not by the granite porphyries, and that of the Tochibora and Mozumi granites seems to be 140-160 m.y. or much older.
It will be assumed that the age of mineralization of the Kamioka deposits is not the late Cretaceous but the Funatsu-stage (pre-middle Jurassic) or much earlier.