REE systematics in hydrothermal fluorite

REY and Trace Element Chemistry of Fluorite from Post-Variscan Hydrothermal Veins in Paleozoic Units of the North German Basin

“…Variations in complexation behaviour/chemistry and fluid-rock interaction may also have led to the distinctly different REY CN patterns, that is, a carbonate buffered system in the Permian shelf-carbonate dominated LSB versus Paleozoic volcanics and siliciclastic sedimentary rocks in the ABB, FCB and Harz. The minor but noticeable Eu CN anomalies in our samples could be due to (a) an inherited signature that reflects the source REE (i.e., Rotliegend volcanics, Figure 6) e.g., [98], (b) adsorption features and crystallographic controls (see Section 5.3), (c) strongly reducing and or oxidizing conditions, or temperatures above 200 • C-thermochemical reduction of Eu 3+ to incompatible Eu 2+ [30,36,38], (d) analytical fractionation or bias due to the poor ablation behaviour of fluorite, or (e) a feature induced by the C1 chondrite normalization (i.e, seawater would also display a negative Eu anomaly when normalized to C1 chondrite). Option (c) is unlikely because there is no evidence for these conditions in the literature or in our investigation.…”

“…Schwinn and Markl [36] pointed out that petrographically late fluorite from several of their investigated deposits display geochemical characteristics that reflect new fluid influx rather than remobilization. However, they also acknowledged that variable degrees of supersaturation and fluid/fluorite ratios as well as fluid flux lead to a complex precipitation process that reflects these variable conditions.…”

“…Moreover, our petrographic observations and characteristic growth patterns detailed in Sections 4.2 and 5.3 strongly suggest that both fluorite generations formed during one hydrothermal event and reflect micro-scale adsorption features and thermodynamic and compositional variations of the precipitating fluid. Schwinn and Markl [36] proposed that relative LREE enrichment in the outer zones of fluorite crystals may represent a sub-micron LREE-enriched phase or can be attributed to LREE clustering in later, cooler fluids. However, adsorption favours HREE, whereas LREE are more easily held in solution by chemical complexation [32,100,101].…”

“…In general, fluorites preserve or inherit the REY signature of the hydrothermal fluid from which they precipitated and hence offer an insight into the source of the REY in the hydrothermal fluid (i.e., intrusive or volcanic rocks) e.g., [36]). In particular in the ABB, where a suite of volcanogenic units of variable compositions [7] is part of the basin's Paleozoic rock inventory, the possibility that percolating basinal brines migrated through these volcanic rocks along steep and deep-reaching faults [94] and picked up their distinct REY signature, including a typical negative Eu anomaly, is a plausible explanation for the observed REY CN signatures.…”

“…Following Möller's interpretation, the fluorite populations in this study that show relatively lower LREE concentrations would therefore have to be considered as a later or secondary (remobilized) fluorite generation (II) compared to the fluorite (generation I) with relatively elevated LREE contents. However, adsorption, chemical complexation, changes in temperature and salinity can lead to highly localized variations in fluorite compositions including the REE [30,31,36,38], making it difficult to attribute different REE signatures to single controls. Moreover, our petrographic observations and characteristic growth patterns detailed in Sections 4.2 and 5.3 strongly suggest that both fluorite generations formed during one hydrothermal event and reflect micro-scale adsorption features and thermodynamic and compositional variations of the precipitating fluid.…”