Fosforiidi ja graptoliitargilliidi üldgeoloogilised uuringud Kirde-Eestis

Estonian phosphorites are among the largest unmined phosphorite deposits in the European Union (EU). The three main phosphorite deposits – Rakvere, Toolse and Aseri – are located close to each other in NE Estonia, ca. 100 km east of Tallinn. The phosphorites occur as yellowish or dark-grey fine- to coarse-grained weakly cemented sandy deposits of the Kallavere Formation. The sandstones are primarily composed of quartz and biogenic apatite. The main apatite forms in these deposits are bioclasts of lingulate brachiopods. Estonian phosphorite deposits were extensively studied during the 1950s–1980s. However, exploration ended in 1989 and no large-scale exploration campaigns have been conducted since. The current report is part of the Geological Survey of Estonia’s project to evaluate phosphorite and associated resources to understand their economic potential and if the resources enable Estonia to be a main actor in the European phosphate industry. In the current study, the following tasks were conducted: • construct a 3D model of the phosphorite deposits based on historical data; • execute drilling campaign; • validate the 3D model by comparing it with the new drill core data; • study the Rare Earth Elements (REE) content and distribution in phosphorites; • study the geochemistry and trace elements composition of black shale. During this assessment, historical drill hole (DH) data of lithological units and main oxides for phosphorite and black shale were digitised and validated. 3D models of three main phosphorite deposits and surrounding prospects were prepared, as well as mineralisation block models for Toolse and Rakvere deposits, enabling grade and tonnage estimations. To validate the models and historical explorations, 37 new full core DHs were drilled and detailed petrophysical and geochemical analyses were executed for the black shale and phosphorite layers. The modelled total phosphorus pentoxide (P2O5) mineralisation in the Toolse deposit is 777 Mt with an average grade of 8.9 wt% of P2O5, estimated within wireframes above 4 wt% P2O5. The modelled total P2O5 mineralisation in Rakvere deposit is 2.2 Mt with an average grade of 10.9% of P2O5, estimated within wireframes with P2O5 above 3 wt%. Provided grades and tonnages are not estimated according to internationally recognised mineral resource and reserve reporting codes and thereby confidence categories are not provided. The average ΣREE concentrations in Toolse, Aseri and Rakvere phosphorite deposits are 364, 340 and 262 mg/kg, respectively. For Toolse and Aseri deposits, there is a strong correlation between P2O5 and ΣREE indicating that the REE are situated in the apatite structure and their concentration can be increased during ore beneficiation. Provisional mineralisation of vanadium (V) in black shale were calculated within the area of phosphorite mineralisation for the Toolse deposit, resulting in 326 Mt at an average grade of 999 mg/kg of V. Vanadium estimate should still be considered preliminary since the sampling density for black shale is by far sparser than for phosphorite and historical geochemical assays were performed with outdated methods.

Maavara: fosforiit, graptoliitargilliit; Tellija: Eesti Geoloogiateenistus; Uuringu läbiviija: Eesti Geoloogiateenistus; Andmed imporditud Geoloogiafondi APIst: https://fond.egt.ee/fond/egf-data/9594