Conditions for the formation of REE-bearing minerals in Neoproterozoic carbonaceous metamorphic rocks of the Engane-Pe Ridge, Polar Urals

This article presents data on rare earth element (REE) minerals, highlighting their typochemical characteristics and the specific geological conditions that promote their formation. The minerals investigated in this study originate from the metamorphic carbonaceous volcanogenic-sedimentary and sedimentary deposits found in the Manyukuyakhinskaya suite (RF 3 ), located in the Engane-Pe Ridge within the Arctic regions of the Ural Mountains. The study identifies several REE-bearing minerals across all examined rock types, including allanite-(Ce), monazite-(Ce), and xenotime-(Y). Notably, within the metapelites of the Engane-Pe Range, the rare earth fluorocarbonate synchysite-(Ce) and a rare variety of hydrated thorite with increased contents of REE, phosphorus (P), and arsenic (As) have been documented for the first time. The chemical formula for this specific variety of thorite is represented as follows: (Th_0,44–0,55Y_0,21–0,24Сa_0,11–0,12Zr_0,08–0,09Се_0,04Fе_0,03–0,04Nd_0,03U_0,05Yb_0,01–0,02Gd_0,01)_{Σ1,05–1,11}(Si_0,70–0,72P_0,19–0,20As_0,06V_0,03)_Σ1O₄OH. Moreover, a rare-earth water-containing silicate, known as krizeyite , has been discovered in association with carbonates in volcanic-sedimentary rocks from the Engane-Pe Ridge. In addition, an aqueous silicate containing copper, lead, and iron, referred to as krizeyite, has been observed on the surface of chalcopyrite. The investigation of typomorphic and crystallochemical characteristics has enabled the development of a schematic representation illustrating the evolutionary processes that influence the compositions of REE-bearing minerals within the studied formations. The primary minerals that concentrate REE and thorium (Th) in the sedimentary and volcanogenic-sedimentary carbonaceous rocks of the Manyukuyakhinskaya suite include zircon, apatite, allanite-(Ce), monazite-(Ce), and xenotime-(Y). Hydrothermal-metamorphic transformations of the rocks resulted in the redeposition of REE, leading to the formation of distinct mineral forms such as thorite, synchysite-(Ce), and kainosite-(Y), along with hydrothermally modified grains and rims of monazite-(Ce) and xenotime-(Y).

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