[H-RE11] 資源地球科学

[HRE11-P08] Nitrogen isotope geochemistry of ion adsorption-type REE mineralization: Insights from the weathered granitoid rocks in the Sancheong district, south Korea

*Jaeguk Jo¹、寿朗 山中²、Dongbok Shin¹ (1.公州大学、2.東京海洋大学)

キーワード：アンモニウムー窒素、窒素同位体、カオリナイト、希土類、イオン吸着型

Ion adsorption-type rare earth deposits formed by the weathering of igneous rocks have drawn attention in recent years as an important source of REEs. We studied for the mechanism of REE enrichment on clay fractions formed in weathered crusts of granitic orthogneiss, gabbroic anorthosite, and syenite rocks in the Sancheong district, South Korea, based on trace element and ammonium-nitrogen behavior. Ion adsorption-type REE mineralization commonly occurs with kaolinite and halloysite in the B-horizon layer, which reflects a series of pH changes from wholly weathered surface soil (A-horizon) to slightly weathered rock (C-horizon). The REEs of clay fractions separated from weathered crusts under weakly acidic conditions (pH 5-7) were higher than those of the bulk rocks. In contrast, the REEs of the clay fractions decreased sharply under more acidic conditions (pH < 4). The enrichment of REEs in both the bulk rocks and clay fractions correlate with negative Eu/Eu^* and a positive Th_(n) and U_(n) trend, which indicates that the REEs were gradually concentrated during igneous and weathering processes. Ammonium-nitrogen concentrations correlate with the enrichment of REEs as well as the increasing δ¹⁵N values in the weathered crusts of each rock type. The δ¹⁵N values of the clay fractions (av. 5.1‰, av. 3.5‰, av. 1.1‰), which are heavier than those of the bulk rocks (av. 4.0‰, av. 1.4‰, av. -0.7‰) for granitic orthogneiss, gabbroic orthogneiss, and syenite rocks, respectively, are mostly attributed to the influence of sedimentary organic matter, in addition to nitrogen isotope fractionation. The geochemical characteristics and the features of nitrogen behavior in the bulk rock and its clay fraction contribute to our understanding of the genesis of ion adsorption-type REEs mineralization on the weathered crusts of granitoid rocks.