RARE-EARTH ELEMENT MOBILITY IN MINERALIZED AREAS OF THE SUB-ARCTIC BOREAL FOREST IN EAST-CENTRAL ALASKA

Interest in rare-earth element mobility has recently increased, due in large part to the advent of sensitive and rapid analytical chemical methods such as ICP-MS. Lanthanide series rare-earth elements (REE) were considered to be relatively immobile in many geologic environments. However, aqueous concentrations of REE at the sub-ppb level are now frequently observed. REE concentrations in rock, water, sediment, and soil samples from two mineralized watersheds in eastern Alaska were determined. North American Shale Composite (NASC)-normalized patterns for sediments from the Goodpaster drainage were light-REE (LREE) enriched, while Fortymile drainage samples tended to be flat or only slightly LREE enriched. Soils from the Fortymile had higher concentrations of heavy-REE than those from the Goodpaster and were LREE depleted relative to the NASC. In both drainages total REE in soils samples increased with depth. Of the two watersheds examined, aqueous REE concentrations generally were greater in the Fortymile River drainage than the Goodpaster River drainage. NASC-normalized patterns varied from LREE depleted to enriched in the samples from the Fortymile and were LREE depleted in the Goodpaster. Both areas had NASC-normalized patterns with negative Ce anomalies and both positive and negative Eu anomalies. Aqueous REE concentrations increased with increasing Fe and Al. Neither the Eu anomalies nor the (La/Yb)n, a measure LREE depletion, show a relationship to Al, Fe, or pH but the Ce anomaly did. Differences in the Fe and Al between the two drainages can account for the differences in the aqueous REE concentrations and Ce anomaly. The correlation between the aqueous REE concentration and the Fe and Al concentration indicate that colloidal material may be controlling the aqueous REE transport and Ce fractionation in this system.