OFFICIAL The 16 REE found at Fen include, Cerium (Ce), Dysprosium (Dy), Erbium (Er), Europium (Eu), Gadolinium (Gd), Holmium (Ho), Lanthanum (La), Lutetium (Lu), Neodymium (Nd), Praseodymium (Pr), Samarium (Sm), Scandium (Sc), Terbium (Tb), Thulium (Tm), Ytterbium (Yb), Yttrium (Y). Deleterious elements include Lead (Pb), Thorium (Th), and Uranium (U). 3. GEOTECHNICS AND ROCK MASS QUALITY Rock mass classification for the Fen deposit supports mine design using ~7,300 m of geotechnical core logging and three schemes (RMR, Barton Q, GSI). The steep, massive Fe-dolomite carbonatite (FDC) forms a competent rock mass, but brecciation, veining, dyke intrusion and alteration/fenitization create localized weak zones. Oriented core defined borehole-specific joint sets underpinning RMR/Q inputs and kinematic/stability checks. Short intervals captured RQD, fracture frequency/spacing, joint condition, groundwater and defects. Deposit averages show high quality (RMR ~74, GSI ~76, Q ~273); variability is controlled by discrete weak intervals. Point-load and ISRM hammer tests indicate medium-strong to very-strong intact rock. 4. MINING METHODS – GENERAL Favorable geology and geotechnical conditions, together with the relatively shallow planned mining depth, make the Fen deposit attractive for a mining complex targeting 5 Mt of ore per year. The study for this paper compares two mining methods that maximize resource extraction, ensure high safety standards, minimize workforce radiation exposure, and reduce environmental impacts by using processing-plant tailings as backfill to avoid large surface tailings dams. Additional objectives include high mechanization, automation, and digital/remote operation, strong equipment reliability, high labor productivity, and flexibility under changing conditions through large primary excavations that are subsequently backfilled. Methods based on large primary stopes followed by cemented paste backfill made from dewatered plant tailings are considered most suitable under these conditions. Mining would typically start at the lowest horizon and advance upward. Stopes are mined, backfilled via belt or pipelines with cement added near the stope, then allowed to gain strength before adjacent ore pillars are extracted. Success depends strongly on backfill quality, supply reliability, and backfill operational performance. 5. LONGHOLE OPEN STOPING WITH CEMENTED BACKFILL (LHOS) 5.1 Mining method concept and layout logic Sublevel stoping is suited to competent rock masses where open stopes can be maintained during extraction. For Fen, a backfilled variant is considered to enhance stability and reduce deformation/subsidence risk. The deposit is subdivided into multiple sublevels; stopes are mined transversely, requiring two drifts per stope (top and bottom).
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