:Studies on Geology and Ore Deposits in the Yanahara Mining District-Part I

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The Yanahara Mining District is occupied mainly by the Upper to Middle Maizuru Group, the Lower to Middle Triassic Fukumoto Group, the Upper Triassic Nakaiso Formation, the Latest Paleozoic Yakuno Intrusives and the Cretaceous rhyolitic and granitic rocks. The Maizuru Group in the district is divided into the following four formations; the Lowest Formation mainly consisting of pelitic schist, the Lower Formation basic schist, the Middle Formation slate and slaty sandstone with several layers of rhyolitic pyroclastic rocks and Upper Formation alternation of slate and sandstone often showing graded bedding. The Yakuno Intrusives, mainly composed of meta-granodiorite and meta-gabbro, intrude into the Maizuru Group concordantly. The Yanahara ore deposits occur within the rhyolitic pyroclastic strata of the Middle Formation of the Maizuru Group and form massive and lense-shaped bodies consisting mainly of pyrite ore. In the ore bodies massive ore is most frequently observed, and graded ore and disseminated ore are less abundant. Some ore bodies are intercalated with thin layers of slate and rhyolitic tuff. The Maizuru Group in this district was folded at least three times. During the first stage of the deformation the Maizuru Group and the Yakuno Intrusives were folded and the Lowest and Lower Formations of the group became schistose and the Middle and the Upper Formations slaty. The fold axes and the lineation of this stage trend northerly. During the second stage of deformation the Maizuru and Fukumoto Group and the Yakuno Intrusives were folded. The folds of this stage trend northerly, subparallel to the structural trend of the first stage and show the characteristics of the flexural slip folding. During the third stage of deformation not only the strata and rocks mentioned above but also the Nakaiso Formation were folded to form flexural slip folding. This crustal deformation formed a large synclinorium with the wave length of ten odd kilometers whose axes trend easterly and whose axial part is occupied by the Upper Triassic formation. The older structures north of the axial part plunge to the south and those in the south plunge to the north. The Yanahara ore deposits seem to have been formed concordantly with the stratification of the Maizuru Group, but later, strongly deformed by the above three crustal movements. During the first and the second stages of deformation the ore bodies were folded, and some part of ore bodies were crushed to show breccia-like structure. During the third stage the Yanahara Main Ore Body was divided into four displaced bodies as observed today. Contact metamorphic rocks are widely distributed in the mining district. Metamorphic minerals occur zonally not around the Yakuno Intrusives but around the Cretaceous granitic rocks. The Yanahara ore deposits are also influenced by the contact metamorphism.
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:Studies on Geology and Ore Deposits in the Yanahara Mining District-Part I

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