REE+Y distribution in Tremadocian shelly phosphorites (Toolse, Estonia): multi-stages enrichment in shallow marine environment during early diagenesis

Historically exploited for fertilizer production, phosphorite deposits are potential rare earth element (REE) source as by-products. Thus, news mineral exploration research will provide news insights into REE resources and the origin of these ores. Tremadocian Estonian phosphorites are among Europe's most extensive phosphate rock resources, with a tonnage of approximately three billion metric tons. The ore consists of sandstone and phosphatic brachiopods fragments, with downward-increasing content, up to 50-60 w.t% authigenic carbonate-bearing fluorapatite (CAF); deposited in a shallow peritidal environment, with an extension from Estonia to NW Russia.

Throughout the Toolse deposit, REE+Y patterns are homogeneous and present little variability. The REE and Yttrium (Y) contents indicate a distinctive medium-REEs (MREEs) bell-shaped pattern with also an enrichment for neodymium, praseodymium, dysprosium and Y. In the lower part of the deposit, the ∑REEs+Y concentration is up to twelve times enriched relatively to post-Archaean Average Australian shale (PAAS). The profiles indicate a multi-stage REE+Y intake during deposition and early diagenesis associated with a stratified redox porewater developed during marine regressions. The upwelling of deep-water rich in precursor carriers has developed a REE and nutrient-rich system in the porewater. The positive Y-anomalies indicate an initial REE+Y intake by hydroxyapatite through Ca-substitution in an active oxic environment in the near-water surface; the presence of dolomite even indicates episodes of brine reflux. The sediment burial induced CAF-apatite formation, shift to absorption-intake, and transition to a suboxic environment that initiated a reductive dissolution of Mn-oxyhydroxides and the release of light-REEs (LREEs), resulting in an overprint of the original REE pattern to the extent of leading to positive Ce anomalies. A late enrichment in MREE occurred during the early diagenesis due to developing anoxic conditions and desorption of Fe-oxides and organic-rich particles, accentuated by the gradual transition to a marine transgression regime and deposition onset of organic-rich argillites in an anoxic environment. Overall, new study findings reflect a predictable REE+Y resource and consistent enrichment patterns.