Abstract
The mode of occurrences of gageite from manganese ore deposits in outer zone of the Southwest Japan were investigated. Twenty six occurrences of gageite, beside locations reported by Fukuoka and Hirowatari (1988) and Matsubara and Kato (1979), were newly found from high P/T type (pumpellyite-actinolite facies to zeolites facies) manganese ore deposits distributed in the southern and northern zone of Chichibu belt in the Kii Peninsular, Shikoku and Kyusyu district. Gageite occurs as a principal constituent mineral of altered braunite designated as chocolate ore, which is chocolate-colored massive ore composed of various minerals with high Mn content, locally as veinlets, and form tabular and fibrous crystals of 2-3 mm order in length. Two occurrences of gageite are distinguished. Gageite is commonly accompanied by rhodo-chrosite, alleghanyite and hausmannite with minor ribbeite, pyrochroite, kellyite, barite, cymrite, and sussexite. Gageite, also, occurs rarely as thin laminated layers with caryopilite in the banded rhodochrosite ore. EPMA analyses indicated that gageite is chemically characterized by low Mg content, however their Mg contents are generally larger than those in closely coexisting alleghanyite. The occurrence of gageite suggests that gageite in chocolate ore have been formed later than alleghanyite as an alteration products from braunite ore under the high f H2O condition. On the other hand, it is assumed that laminated gageite in rhodochrosite ore is primary mineral formed under the low grade regional metamorphism.
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