:K-Ar dating of alunite from Kasuga, Iwato and Akeshi
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Gold-bearing massive silicified rocks occur in the volcanic piles of the Neogene Tertiary age in the Makurazaki district, southern Kyushu and are termed as the Nansatsu type gold deposits. Currently three mines, Kasuga, Iwato and Akeshi, are operating. The argillized zones consisting mainly of quartz, kaolinite and minor goethite surround the silicified rocks. Alunite series minerals occur in and around the silicified rocks.<BR>Ages of mineralization have been discussed for many years but were unanswered. K-Ar dating of selected alunite and alunite-bearing rocks from three mines yields ages of 5.5-3.7 m.y. K-Ar ages at Mt. Sonomi of the Kasuga mine (5.5±0.4m.y.) and at the Arabira orebody of the Iwato mine (4.7±1.0m.y.) probably represent the ages of gold mineralization. Slightly younger ages at Mt. Iwato of the Iwato mine (4.4±0.7m.y.) and at the No. 1 orebody of the Akeshi mine (3.7±1.1m.y.) might reflect possible changes in chemical composition of alunite during weathering after mineralization.<BR>Gold-silver mineralization in southern Kyushu took place in the Pliocene to early Pleistocene, except minor silver-rich vein type deposits in the middle Miocene time. This study shows that four values are concordant with each other and indicate the latest Miocene to early Pliocene ages for mineralization of the Nansatsu type gold deposits in the Makurazaki district. The ages are comparable with the vein type gold deposits of the early Pliocene time such as Kushikino (4.0±0.3m.y.) and Hanakago (4.8±2.9m.y.). Another gold mineralization were known in the early Pleistocene time such as Hishikari (1.5±0.3m.y.) and Ora (1.8±0.2m.y.). At present there seems to be only minor gold mineralization between 4 and 2 m.y.
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