Citation: Klötzli, U.; Burda, J.; Tibuleac, P. Phosphate Petrochronology of the Belcina REE Mineralization (Ditrău Alkaline Massif, Romania). Minerals 2023, 13, 739. https://doi.org/10.3390/ min13060739 Academic Editor: Stefano Salvi Received: 3 May 2023 Revised: 17 May 2023 Accepted: 23 May 2023 Published: 30 May 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 Phosphate Petrochronology of the Belcina REE Mineralization (Ditrău Alkaline Massif, Romania) Urs Klötzli 1, * , Jolanta Burda 2 and Paul Tibuleac 3 1 Department of Lithospheric Research, University of Vienna, Josef-Holaubek-Platz 2, 1090 Vienna, Austria 2 Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia in Katowice, 41-200 Sosnowiec, Poland; jolanta.burda@us.edu.pl 3 Department of Geology, University Alexandru Ioan Cuza of Iasi, Boulevard Carol I, 20 A, 700506 Iasi, Romania; paul.tibuleac@uaic.ro * Correspondence: urs.kloetzli@univie.ac.at; Tel.: +43-1-4277-53460 Abstract: A notable REE mineralization in Europe is associated with the Ditrău Alkaline Massif (DAM) in the Eastern Carpathians (Romania). It is an expression of the latest hydrothermal phase in the DAM and is found in the form of mineralized carbonate veins cross-cutting the complex in the NW (Jolotca region) and the SE (Belcina region) parts of the DAM. In the Belcina veins monazite-(Ce), xenotime-(Y) and apatite, together with Fe-Mg-rich carbonate, thorite, thorogummite, gedrite and plagioclase are rock-forming. Three different textural and chemical types of the monazite-(Ce) and the xenotime-(Y) document a three-stage evolution. The relative phosphate age succession (from older to younger) thereby is xnt1 > xnt2 (>)+ mnz1 + Fe 2 O 3 + Fe-gedrite > mnz2 + Fe-dolomite (+ plagioclase) > mnz3 + xnt3 + apatite. Phosphate chemistry shows that these crystallized from hydrothermal fluids, whereby each phosphate type follows a separate evolutionary path suggesting growth from (at least) three independent and successive hydrothermal fluids. Chemistry and pathways within the DAM suggest that these hydrothermal fluids could be derived from a subsurface carbonatitic intrusion. Mnz1,2 and xnt1,2 ages are tightly clustered at 215.8 ± 0.7 Ma (Norian, Upper Triassic). The third-generation phosphate ages are younger, but are associated with large analytical uncertainties and did not deliver geologically useful ages. The mean age of ca. 216 Ma is interpreted as the timing of the Belcina REE mineralization, which together with the fluid chemistry, supports a model of the presence of a late-stage, independent carbonatitic intrusion about 10 Ma after the main igneous activity (ca. 235–225 Ma) forming the DAM, synchronous with extension-related magmatism in the region. Keywords: phosphate petrochronology; Ditrău Alkaline Massif; Eastern Carpathians; REE mineralization; metasomatism; hydrothermal fluids 1. Introduction Alkaline rocks are a minor but important component of the continental crust. They usually contain significant amounts of rare metals such as Li, Be, Nb, Ta, Zr, Th, REE and volatile components, mainly F and Cl. One of the most notable REE mineralization in Europe, having significant economic importance, is associated with the ca. 235–225 Ma Ditrău Alkaline Massif DAM) in the Eastern Carpathians of Romania (Figure 1,[1,2]). REE-rich mineralized veins are found within the alkaline complex at Jolotca, and outside the complex in the Belcina region (Figure 2). The Belcina carbonate vein REE mineralization is localized in the ESE of the DAM within the metamorphic Cambrian Putna Nappe (Figure 2; e.g., [3–6]). The first mineralog- ical description of the Belcina carbonate vein REE mineralization was given by [7] that was subsequently complemented by [8], and references therein. A thorite ((Th,U)SiO 4 ), thorogummite ((Th,U)(SiO 4 ) 1−x (OH) 4x ), xenotime-(Y) (YPO 4 ) paragenesis represent the characteristic mineral association of the Belcina mineralization. Other common minerals are Minerals 2023, 13, 739. https://doi.org/10.3390/min13060739 https://www.mdpi.com/journal/minerals