In terms of rare earth potential, the Ditrău Alkaline Massif (Romania) is one of the most prospective formations in Europe. The REE mineralization occurs as mineralized ore veins and is most prominent in the northern (Jolotca) and eastern (Belcina) parts of the Massif. The ore veins of Jolotca area outcrop over a larger area than the Belcina ones, and are predominantly rich in LREE minerals. The most important REE minerals in the Jolotca veins are phosphates, of which monazite (Mnz) (group) is by far the most abundant, while xenotime (Xtm) occur only sporadically. The monazite-xenotime mineral assemblages are the products of at least three successive mineralization stages: (1) Mnz1 followed by Xtm1 (the vast majority of REE phosphates formed in this stage); (2) Mnz2 + Xtm2 (as inclusions in apatite) and Mnz3 + Xtm3 (as interstitial phases), the two assemblages are texturally distinct but their synchronous formation is likely; (3) Mnz4 scattered in magnetite veins (no xenotime formed in this stage). The chemical composition of the phases formed in each stage supports their hydrothermal origin. The alteration characteristics, reaction patterns and internal textural features of the monazite-xenotime assemblages reflect the metasomatizing effect of each hydrothermal stage. Three fluid types with different compositions were identified in the earliest Mnz1 phase: An H2O-NaCl-CO2 type (2–4 mol% CO2, 10–22 wt% NaCleq., Th: 225–350 °C), an H2O-NaCl type (10–22 wt% NaCleq., Th: 160–330 °C) and an H2O-NaCl-CaCl2 type (23–25 wt% NaCleq., Th: 150–250 °C). Combining Y-in monazite thermometry data with isochores of primary originated fluid inclusions indicate <500 MPa pressures and 200–420 °C temperature conditions for formation of Mnz1. The observed REE patterns of monazites were most probably resulted by a temperature derived fractionation of REEs carried as carbonate complexes in hydrothermal fluids by the effect of successive metasomatic flushing.
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