REE systematics in hydrothermal fluorite

REE patterns of fluorite from 63 post-Variscan hydrothermal veins of the Schwarzwald area, Germany, can be divided into two major groups. Type A fluorite shows roof-shaped PAAS normalized patterns with enriched MREE, the turning point of the curvature is at Ho or Er position. All patterns of type A show a distinct positive Eu anomaly. Type B fluorite shows roof-shaped patterns with enriched MREE and strongly depleted HREE and a characteristic, steep slope between Nd and Sm. The turning point of the curvature is at Gd to Dy, all patterns of type B show negative or no Eu anomalies. Laser ablation-ICP-MS traverses through single grains combined with cathodoluminescence imagery revealed either (i) identical REE patterns and total REE concentrations from core to rim, (ii) identical REE patterns, but concentration gradients or (iii) variable REE patterns and concentration gradients from core to rim. These features are explained by different degrees of supersaturation or different fluid/fluorite ratios during crystallization and by complexly controlled precipitation processes under closed system conditions. Petrographically late fluorites from several deposits show the characteristics of new fluid batches rather than the expected remobilization features. LREE enrichment close to the crystal faces in some grains may result from the copreciptiation of a submicrometer-sized LREE-enriched phase or from LREE clustering in late, cool fluids. By comparison with literature data on natural waters from gneissic and granitic aquifers and of leachates of granites and gneisses, we show that the hydrothermal vein fluorite preserves the characteristic, source-related REE feature of the hydrothermal fluid that migrated through either gneissic (type A) or granitic (type B) host rocks. The immediate host rocks of the veins, however, are not the source rocks of the REE, which indicates large-scale fluid convection in the basement. Gneiss-related type A fluorite shows Tb/Ca–Tb/La trends of primary hydrothermal crystallization whereas granite-related type B fluorite indicates remobilization of possibly pegmatitic or Variscan fluorite.