Supergene enrichment of rare earth elements in weathered carbonatite systems:Insights from the Sokli deposit, northeastern Finland

Carbonatites are major sources of rare earth elements (REE) and, through weathering, significant upgrading of REE concentrations can be achieved. Here we study weathering profiles at the Sokli carbonatite-phoscorite P-Nb-REE deposit, NE Finland, to investigate the mineralogical and geochemical processes contributing to REE enrichment during weathering. Weathering at Sokli consists of three stages: incipient, karstic collapse and advanced stages. During incipient weathering, dissolution of calcite and dolomite facilitates passive REE enrichment through residual apatite, pyrochlore and minor monazite-(Ce) accumulation. However, some main REE hosts in the bedrock, ancylite-(Ce) and kukharenkoite-(Ce), likely dissolve at this stage, leading to REE loss. Volume loss from carbonate dissolution eventually results in the collapse of the saprolite. Dissolution of apatite and precipitation of carbonate fluorapatite, monazite-(Ce) and rhabdophane-(Ce) first takes place under more carbonate-rich circumneutral soil solution conditions, forming a lithified phosphatic crust. With progressively decreasing pH towards the surface with increased organic acid input, apatite and carbonate fluorapatite, and later monazite-(Ce) and rhabdophane-(Ce) dissolve in the advanced stage. The released REE are largely fixed through precipitation of neoformed monazite-(Ce), rhabdophane-(Ce) and crandallite. Pyrochlore also loses REE and is replaced by crandallite and goethite. Weathering of phosphates and vermiculite results in mass removal and further REE enrichment. However, vertical REE mobilisation and fractionation is limited. The enrichment manifests as passive enrichment of neoformed secondary REE-bearing minerals. Compared to the initial REE concentration of the bedrock, availability of REE depositors is more important in fixing the REE and upgrading the REE concentration in regolith.