1992 Volume 42 Issue 233 Pages 165-173
Strata-bound manganese oxide deposits are distributed in the Setana area, southwestern Hokkaido, Japan. The pre-Tertiary sedimentary rocks and plutonic rocks are located in the marginal region of the area to form a sedimentary basin. The Kunnui and Yakumo formation which consist of volcanic breccia and hard shale respectively, heaped up on the basin by neogene green tuff activity. Manganese deposits lie in the most upper part of the Kunnui formation or lie between the Kunnui and Yakumo formation. Therefore manganese deposits are considered to be formed in a same geological age. These deposits are located on the rim of the basin and regarded to have been formed by submarine hydrothermal activities based on the studies of field occurrence and geologic setting. Pyrolusite and manganite are the dominant manganese mineral. Groutite and todorokite are common but found in minor amounts.
The concentration in manganese ores of major and trace elements are measured by X-ray fluorescence method and rare earth elements (REE) are measured by neutron activation method. The data shows; (1) the data on manganese ores from the area fall in the hydrothermal area in the Mn-Fe-(Cu+Ni+Co)X10 ternary diagram. The trace elements concentration of several deposits show an influence of hydrogenous activity. (2) The concentration of Cu and Co vary widely among each locality, whereas Ni and Zn do not vary widely. The result is similar to that of the Tokoro area; the behavior of these elements originates from the tendency of adsorption of each transition element to manganese. (3) The total amount of REE is same to that of submarine hydrothermal deposits. The REE pattern normalized by chondrite shows positive Ce anomaly in the northern side of the basin and suggests the more oxic environment than the southern side of the basin. (4) The variation of trace elements concentration are relatively small in the Setana area. The data suggest that the manganese deposits in this area were formed by the hydrothermal activity under a nearly same condition as inferred from the geological study.
