Chemistry and Sulfur Isotopes in a Chert-dominant Sequence around the Stratiform Manganese Deposit of the Noda-Tamagawa Mine, Northern Kitakami Terrane, Northeast Japan: Implication for Paleoceanographic Environmental Setting

Abstract

. Chemistry and sulfur isotopes are analyzed for a series of rocks in the chert-dominant sequence around the stratiform manganese ore deposit of the Noda-Tamagawa mine in the northern Kitakami Terrane, northeast Japan. The sequence is litholog-ically classified into six units in ascending order: lower bedded chert, lower black shale, massive chert, manganese ore, upper black shale, and upper bedded chert. The rocks around the manganese ore deposit exhibit anomalous enrichment in Ni (max. 337 ppm), Zn (102) and U (30) in the upper part of lower bedded chert, Mo (122), Tl (79) and Pb (33) in the lower black shale, MnO, Cu (786) and Co (62) in the manganese ore, and As (247) and Sb (17) in the upper black shale. The aluminum-normalized profiles reveal zonal enrichment of redox-sensitive elements around the manganese bed: Zn-Ni-Fe-Mo-U(-Co), Tl-Pb(-Mo), Mn-Fe-Cu-V-Cr-Co(-Zn) and As-Sb in ascending order. The uppermost part of the lower bedded chert and black shale exhibit negative Ce/Ce* values, whereas the massive chert, manganese ore and lower part of the upper bedded chert display positive values. The isotopic δ34S values are 0±6 % in the lower part of the lower bedded chert, -19 to -42 % in the upper part of the lower bedded chert, -36 to -42 % in the lower black shale, -28 to -35 % in the massive chert, manganese ore and upper black shale, and -23±5 % in the upper bedded chert. Thus, there is a marked negative shift in δ34S values in the lower bedded chert, and an upward-increasing trend in δ34S through the manganese ore horizon. The present data provide evidence for a change in the paleoceanographic environmental resulting from inflow of oxic deepwater into the stagnant anoxic ocean floor below the manganese ore horizon. This event is likely to have triggered the precipitation of manganese oxyhydroxides. The redistribution of redox-sensitive elements through the formation of metalliferous black shale and manganese carbonate ore may have occurred in association with bacterial decomposition of organic matter during early diagenesis of initial manganese oxyhydroxides.