Abstract
We examined mineralogical and geochemical studies of weathered shale and siliceous shale, Miocene Onnagawa Formation, Oga peninsula, to understand the migration of trace and rare earth elements during the chemical weathering of sedimentary rocks under oxic environment.
Pyrite is completely decomposed and ferrous iron hydroxide formed in the weathered sedimentary rocks. P, Mn, Co, Ni, Cu, Mo, Pb, U and REEs contents of weathered rocks are quite different from those of parent rocks. The decrease ratios of P, V, Mo, Pb, U and REEs are small with the increasing of increase ratio of Fe2O3* in the weathered rocks. These mineralogical and geochemical variations suggest that dissolved trace and rare earth elements were reaccumulated in the weathered rock by adsorption onto ferrous iron hydroxides.
However, there is significant difference between in the elemental adsorption ratio of shale and siliceous shale. The adsorption ratios of elements onto ferrous iron hydroxides of shale are generally lower than those of siliceous shale. Based on mineral composition of shale and siliceous shale, we conclude that the difference between in the adsorption ratios of shale and siliceous shale was caused by the changes of dissolved elements species and surface charge of ferrous iron hydroxide, because the pH condition for the weathering of pyrite-rich shale was more acidic than that of pyrite-poor siliceous shale.
