This study investigates unique weathering crust samples from the most altered sections (30-43 m) of the weathering profile within the Souktal Plutonic Complex, Northern Kazakhstan. The samples, obtained from two drill cores, consist of quartz, kaolinite, microcline, muscovite, and plagioclase, as identified through polarized light microscopy and confirmed by X-ray diffraction analysis. Sequential extraction of rare earth elements (REE) was performed using inductively coupled plasma mass spectrometry (ICP-MS) following a two-step leaching procedure with hydroxylamine hydrochloride (0.2 mol NH₂OH·HCl) and sodium hydroxide (1 mol NaOH) solutions. The extraction process effectively recovered REE, indicating their presence in an ion-exchangeable form, with total extraction rates (REE + Sc + Y) ranging from 4.1 to 7.8 ppm. The total light REE content varies from 3.5 to 5.9 ppm, while heavy REE content ranges from 0.2 to 0.7 ppm across all samples. Petrological and geochemical analyses suggest that the studied area represents an ion-adsorption-type REE weathered deposit. These findings enhance the understanding of ionic-adsorbed REE within weathering crusts and highlight the effectiveness of sequential extraction methods for REE determination. Moreover, the study suggests that this area holds promising potential as a future REE ion-adsorption site, contributing to the development of Kazakhstan’s national REE industry.

Carbonate-silicate rocks of unclear origin have been observed in granulites of the Porya Guba of the Lapland-Kolvitsa Belt within the Fennoscandinavian Shield. The present work aims to reconstruct possible protoliths and conditions of metamorphic transformation of these rocks based on oxygen and carbon isotopic ratios combined with phase equilibria modeling. Isotope analysis and lithochemical reconstructions suggest that carbonate-silicate rocks of the Porya Guba represent metamorphosed sediments (possibly marls) with the isotopic composition corresponding to the Precambrian diagenetically transformed carbonates (δ18O ≈ 17.9 ‰, SMOW and δ13C ≈ –3.4 ‰, PDB). The chemical composition varies depending on the balance among the carbonate, clay, and clastic components. Significant changes of the isotopic composition during metamorphism are caused by decomposition reactions of primary carbonates (dolomite, siderite, and ankerite) producing CO2 followed by degassing. These reactions are accompanied by δ18O and δ13C decrease of calcite in isotopic equilibrium with CO2 down to 15 ‰ (SMOW) and –6 ‰ (PDB), respectively. The isotopic composition is buffered by local reactions within individual rock varieties, thus excluding any pronounced influence of magmatic and/or metasomatic processes.