Citation: Syczewski, M.D.; Siuda, R.; Parafiniuk, J. REE Concentrations in Secondary Uranium Minerals from the Izera Metamorphic Complex (SW Poland). Minerals 2023, 13, 945. https://doi.org/10.3390/ min13070945 Received: 14 May 2023 Revised: 10 July 2023 Accepted: 12 July 2023 Published: 14 July 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article REE Concentrations in Secondary Uranium Minerals from the Izera Metamorphic Complex (SW Poland) Marcin Daniel Syczewski 1,2, * , Rafal Siuda 1 and Jan Parafiniuk 1 1 Faculty of Geology, University of Warsaw, ˙ Zwirki i Wigury 93, 02-089 Warsaw, Poland 2 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany * Correspondence: marcinsyczewski@uw.edu.pl or marcin.syczewski@gfz-potsdam.de Abstract: The subject of this work was supergene uranium mineralization and the YREE concen- trations within. YREE differentiation patterns were used to recreate the prevailing crystallization conditions of abandoned mine dumps in Kromnów, Kopaniec, and Radoniów, located in the Iz- era Metamorphic Complex, Sudetes Mts. The collected samples were investigated using PXRD, SEM-EDS, and EPMA. YREE concentrations were measured using LA-ICP-MS. The secondary ura- nium mineralization from these locations consists of phosphates (meta-autunite, meta-torbernite, metauranocircite-I, saleéite, bassetite, phosphuranylite), arsenates (zeunerite), silicates (uranophane, sklodowskite), and uranyl hydroxides (likely becquerelite). Moreover, in Radoniów, phosphuranylite was found; it had not been found in Poland previously. Uranyl mineral assemblages indicate the diversity of chemistry of their mother solutions and suggest their weakly acidic character. The YREE content in secondary uranium minerals also reflects the pore solutions’ chemistry variation. The negative Y anomaly is observed in all uranyl phases. Similar behavior of Sm is also noted, exclud- ing metatorbernite and torbernite. Among the uranyl minerals studied, only metatorbernite from Kromnów showed a positive Nb anomaly, which was probably related to proximity to weathering in YREE-breeding phases. Nevertheless, the YREE and chemical results suggest that this mineralization originated from the oxidizing solutions generated during the weathering of primary hydrothermal mineralization. In order to better understand the weathering zones in these locations, more detailed studies on pore solution chemistry are needed. Keywords: REY; uranyl phosphates; uranyl silicates; physicochemical conditions; Sudetes Mountains 1. Introduction In the natural environment, uranium occurs in two oxidation states, reduced U 4+ and oxidized U 6+ . Generally, uranium primary mineralization is represented by urani-nite and sometimes by brannerite, coffinite, or davidite. The oxidation of these minerals starts right after the decomposition of less stable phases in such an environment, e.g., ferric sulfide. This process usually causes sharp and rapid decreases in pH values. Due to this, barren minerals and more stable ones in oxidative conditions start to decay and release many different ions into the weathering zone environment. These processes are further accelerated by microbial activity. Under these conditions, uranium changes its oxidation state from U 4+ to U 6+ , forming the highly soluble uranyl ion UO 2 2+ , which is the most stable speciation in an aquatic environment. Uranium complexes with fluorides, sulfates, phosphates, and carbonates exhibit high solubility and may migrate over long distances. This paper presents a mineralogical study of uranyl mineralization in weathering zones developed in uranium mine waste dumps in Kromnów, Kopaniec, and Radoniów (SW Poland, central Europe). Due to the impossibility of collecting pore solutions in these mine dumps, the differentiation patterns of rare earth elements and yttrium (YREE) were used to reconstruct the prevailing physicochemical conditions during their crystallization. Minerals 2023, 13, 945. https://doi.org/10.3390/min13070945 https://www.mdpi.com/journal/minerals