Traces of metasomatic episodes and hydrothermal REE fractionation in the vein filling apatite phases of the Jolotca ore field in the Ditrău Alkaline Massif, Romania

Abstract

The Jolotca ore field in the Ditrău Alkaline Massif (Eastern Carpathians, Romania) hosts REE-bearing mineralisation resulting from complex post-magmatic hydrothermal activity and multiple metasomatic overprinting stages. Apatite, a common phosphate mineral in Jolotca, serves as a key indicator of these alterations, with detailed analysis providing insights into the physicochemical conditions of each stage. Jolotca apatites are classified into two main textural groups: groundmass type (G), which forms crystal aggregates in the carbonate gangue, and the less abundant scattered type, which occurs as thin veinlets (SV) or in sheared bands (SS). Cathodoluminescence imaging reveals distinct apatite phases, formed in the following sequence: (1) groundmass apatite: tawny (G_T) → dark orange (G_O) → yellow (G_Y) → green (G_G); (2) scattered apatite in veinlets: violet-blue (SV_V) → yellow (SV_Y); and in sheared bands: greyish-green (SS_G) → yellow (SS_Y). REE content decreases from older to younger apatite generations, with the groundmass apatite showing an increase in MREE + HREE relative to LREEs over time. The G_T phase of the groundmass apatite represents the original phase, while the subsequent ones (G_O, G_Y, G_G) represent stages of metasomatic overprinting. The G_T phase, representing the original apatite, formed from a CO₂-rich (13.3 ± 7.3 mol%), low-salinity (14 ± 8 mass% NaCl_eq.) H₂O-NaCl-CO₂ fluid under 120–250 MPa pressure and 320–380 °C. Subsequent metasomatic stages involved fluids with reduced CO₂ content (2 ± 1 mol%), higher salinity (15.3 ± 8 to 25.8 ± 2 mass% NaCl_eq.), and elevated Ca/Na ratios (X_NaCl: 0.45 to 0.96) at pressures of 140–385 MPa and temperatures of 350–420 °C. Changes in the distribution of rare earth elements (REE) likely resulted from temperature-dependent fractionation, leading to a relative MREE + HREE enrichment in apatite during the crystallization sequence.