Occurrence and geochemical nature of phosphatic rocks and Mn-rich carbonate rocks in the Toyoman Series, Kitakami Mountains, Northeastern Japan.

Abstract

Various phosphatic and carbonate rocks are intercalated within black shale of the Upper Permian Toyoman Series. They are divided into four types on the basis of their occurrence and lithology, i. e., 1) nodule, 2) A type: phosphate bearing calcareous shale, 3) B type: coarse grey coloured carbonaceous sandstone, 4) C type: massive greyish sparitic limestone. Petrological and chemical features of four types are transitional with each other. Phosphate is dominant in the nodules and A type rocks, while carbonate is dominant in B and C type rocks. The analytical results of these rocks show the following characteristics: P2O5 and F contents of analysed rocks have wide ranges from 17.73 to 0.09% and 14, 000 to 85 ppm, respectively. Strong correlations between P2O5-F, FeO* (total iron as FeO)-MgO, CaO-CO2, and MnO-CaO are worthy of note. Phosphate-rich rocks are composed mainly of cryptocrystalline fluorapatite. Carbonate in all types contain several% of MnCO3 as solid solution. Judging from an important role of upwelling, the existence of sedimentary phosphatic rocks and Mn-rich carbonate rocks indicates that the sedimentary environments of the Toyoman Series were not a closed inland sea, but a continental shelf or slope.