Geochemistry and Mineralogy of Rare-Earth Elements in Ferruginous Deposits and Bottom Sediments of the Laptev Sea

Abstract

––The geochemical and mineralogical aspects of the distribution of rare-earth elements (REE) in ferruginous deposits (FD) and bottom sediments of the southeastern Laptev Sea are analyzed. The FD are dominated by biomorphs developed after polychaete tubes. The REE patterns depend on the proportion of ore and non-ore substances. The ore substance is represented mainly by iron hydroxides (limonite). It is a product of suboxic diagenesis enhanced by bioturbation and determines the slight enrichment of FD in MREE and cerium deficiency (Cean = 0.94). The non-ore substance comes from terrigenous sediments and has an (alumino)silicate composition. It controls the scandium content and is the source of REE mineral grains, among which monazite-(Ce) prevails. The sediments demonstrate a common similarity in REE patterns to shales and suspended material transported to the Laptev Sea by the Lena River, with elevated LREE and MREE contents and the value of Ce anomaly almost equal to unity (Cean = 1.06). In the sediments, REE minerals occur mostly in the silt fraction (<63 μm in size). The high content of organic matter (Corg of up to 2.15%) of predominantly terrigenous origin (OMter of up to 85%) in the sediments explains the oxygen deficiency and weak diagenetic mineral formation with low accumulation of trace elements, including REE, in the FD. The total REE content in the FD is lower than that in the sediments (on average, 173 ppm against 206 ppm).