RARE EARTH ELEMENTS IN YELLOWSTONE’S SILICEOUS SINTER DEPOSITS

Hydrothermal fluids erupting from geysers contain REEs that were dissolved from surrounding rocks and are then incorporated into hydrothermal deposits. Fractionation between the fluids and the deposits can yield insight into mineralization processes. We report REE concentrations for sinter deposits from Giant and Castle Geysers in the Upper Geyser Basin of Yellowstone National Park. At each geyser, we sampled sinter following the stratigraphy from the older terrace to the younger cone. Sinter REE concentrations were analyzed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The highest total REE concentration is <6 ppm. In all samples, HREEs have lower concentrations than LREEs. REE concentrations in sinter are more than two orders of magnitude lower than that in the underlying Biscuit Basin and Mallard Lake rhyolite flows. The relative REE concentrations in the sinter and rhyolite are similar, resulting in moderately flat rhyolite-normalized profiles; these profiles also exhibit positive Eu, Gd, Tm, and Yb anomalies. The Yb and Tm anomalies are more pronounced in samples with lower total REE concentrations. In contrast, REE concentrations in sinter are an order of magnitude greater than in their associated thermal waters. Thermal water normalized REE profiles display LREE enrichment and pronounced positive Gd and Yb anomalies. This suggests that some metals may be preferentially incorporated into sinter as a result of metal-chloride complexing. Additionally, elevated concentrations of REEs in sinter containing high amounts of organic matter may suggest that organic matter facilitates the adsorption of REE.