Nitrogen isotope geochemistry of ion adsorption-type REE mineralization: Insights from the weathered granitoid rocks in the Sancheong district, South Korea

Abstract

Ion adsorption-type rare earth element (REE) deposits formed by the weathering of igneous rocks have drawn much attention in recent years as an important source of heavy REEs including yttrium (Y). Our study investigated the mechanism of REE enrichment on clay fractions formed in weathered crusts of granitic orthogneiss, gabbroic anorthosite, and syenite 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 extracted 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 correlates negatively with Eu/Eu* but positively with Th(n) and U(n), 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, 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.