CRM potential of EU sedimentary basins: Insights on Estonian phosphorites and black shales

The importance of critical raw materials (CRMs), such as rare earth elements (REEs), in high-tech industries and the growing demand for alternative sources have spurred interest in exploring unconventional deposits. Unconventional resources include a wide range of low-grade, high-tonnage ores. In the Baltic Paleobasin, two Lower-Ordovican formations bear significant CRM potential. Estonia's phosphorites, among Europe's most extensive phosphate rock reserves with a tonnage of 3Bt, are considered as REE prospects with extraction as a P by-product. The overlaying black shales, analogue of the Swedish Alum shale, are prospective for V, Mo and U. Assessing these poorly understood ores' economic potential and characteristics is essential to secure sustainable access to a wide range of elements and plan extraction procedures. Detailed investigations were conducted in the Toolse and Aseri deposits.

REE in Estonian phosphorites are carried by fragments of brachiopod shells, mineralised in carbonate fluorapatite (CFA). The shells are complex objects, apatite originating from the crystallisation of organic tissues and precipitation of secondary phosphate during burial, and later REE uptake during diagenesis, leading to diverse ΣREE signatures. Despite this variability, profiles present overall homogenous REE patterns, MREE-enriched up to 12-fold compared to PAAS, with positive Y and Ce anomalies indicative of a diagenetic overprint. Average ∑REE+Y concentration is 600±200ppm with 27% of apatite. In the lower part of the ore, ∑REE+Y reaches up to 1234 ppm with 60% of apatite. New LA-ICP-MS quantitative technique was applied to explore shell properties. The extent of diagenetic overprint and enrichment was found to vary locally, with specific REE grades on apatite edges. In Toolse, shells show less recrystallised textures - the average REE content is 1847±880ppm, with a maximum of 4150±1000ppm. In Aseri, LA-ICP-MS sorting first reveals a higher and more homogenous grade, 2440±450ppm, and a second alteration-driven enrichment stage in which the edges present a ΣREE up to 7020±3500ppm. Despite differences in enrichment level, REE distributions are similar. Main REEs are Ce(33%), Y(21%), La(12%), Nd(16%) and Dy(3%). The outlook coefficient was calculated to determine ore prospectivity (Seredin, 2010). It models the percentage of CR in total REE (REEdef) by the ratio of economical against excessive REE (Koutl). Both deposits fall in the 'promising for economic development' range. Therefore, the ratio of valuable REEs is relatively high.

Black shales were explored through two drill cores. They comprise thermally immature, organic-rich black, grey shales and siltstones. Study reveals maximum hyperenrichment in the basal part, with V content reaching up to 2349ppm and Mo up to 4500ppm, then gradually decreasing. The U content reaches up to 500ppm; however, elements present mismatching patterns, indicating a complex redox environment and specific enrichment mechanisms. Further experiments were conducted for particle fractionation, indicating that the main V carriers are the finest fractions (<2µm, illite-smectite), with V up to 4832ppm, while in organic-rich fractions, V content remained low (200ppm). Strong correlation between V-Cr-Al-Ti further confirms that hyperenrichment of V was largely controlled by clays and associated with dissolved OM.