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<p>Recently, a large number of worldwide supergene REE-fluorocarbonate deposits were discovered, one of which occurred in the Um Bogma area in SW Sinai, which hosts important bauxite deposits. These bauxite ore deposits occur unconformably on the top of the lower Carboniferous marly carbonate sequence related to the Um Bogma Formation. The bauxite deposits are referred to as karst-type and consist of gibbsite, kaolinite, goethite, and hematite which increase stratigraphically upwards in horizons hosting the bauxitic–lateritic deposits. This study highlights the economic potential and environmental risks of REE-U enrichment in the Um Bogma karst terrain. Some trace elements such as Pb, Cu, Mo, As, Co, Ba, Zr, Cr, Zn, Ni, V, Y, Be, U, and REE occur in significant amounts in these bauxite deposits, in comparison with the worldwide bauxite deposits. The enrichment of REE and U results in significant concentrations reaching up to 1.45% and 0.19% respectively. The transport and adsorption of rare earth elements are related to the ion-adsorption type. SEM, EDX, and XRD investigations revealed that the REE immobilized in the form of authigenic LREE-bearing bastnaesite mineral which occurs as nest-, walnut-, rosy- and irregular-shaped grains adsorbed on iron oxides and clay minerals. The authigenic REE minerals significantly influence the shape of the REE patterns and induce weak positive Ce anomaly in the studied bauxite deposits. The chondrite-normalized REE patterns of the bauxite deposits indicate enrichment of LREE relative to HREE with weak Ce anomaly. The uranium minerals, umohoite, Zn-zippite, soddyite, uranophane, meta-autunite and uranopilite control the mobilized uranium within the karst profiles. The enrichment of REE suggests that oxidation conditions mostly immobilized Ce<sup>4+</sup> and transported REE<sup>3+</sup> downward to the lower horizons of the karst profiles. The high REE contents are mainly enriched in Fe-depleted bauxitic facies. While U and LREE are also concentrated in Fe-rich bauxites due to scavenging by goethite and clay minerals.</p>

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The geochemical constraints on the supergene REE-fluorocarbonate and uranyl mineralization in the karst bauxites: insights from Um Bogma area, Sinai, Egypt

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  • 发表日期：

    2025年9月8日

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  • Ahmed El Sayed Ahmed Shata

摘要

Recently, a large number of worldwide supergene REE-fluorocarbonate deposits were discovered, one of which occurred in the Um Bogma area in SW Sinai, which hosts important bauxite deposits. These bauxite ore deposits occur unconformably on the top of the lower Carboniferous marly carbonate sequence related to the Um Bogma Formation. The bauxite deposits are referred to as karst-type and consist of gibbsite, kaolinite, goethite, and hematite which increase stratigraphically upwards in horizons hosting the bauxitic–lateritic deposits. This study highlights the economic potential and environmental risks of REE-U enrichment in the Um Bogma karst terrain. Some trace elements such as Pb, Cu, Mo, As, Co, Ba, Zr, Cr, Zn, Ni, V, Y, Be, U, and REE occur in significant amounts in these bauxite deposits, in comparison with the worldwide bauxite deposits. The enrichment of REE and U results in significant concentrations reaching up to 1.45% and 0.19% respectively. The transport and adsorption of rare earth elements are related to the ion-adsorption type. SEM, EDX, and XRD investigations revealed that the REE immobilized in the form of authigenic LREE-bearing bastnaesite mineral which occurs as nest-, walnut-, rosy- and irregular-shaped grains adsorbed on iron oxides and clay minerals. The authigenic REE minerals significantly influence the shape of the REE patterns and induce weak positive Ce anomaly in the studied bauxite deposits. The chondrite-normalized REE patterns of the bauxite deposits indicate enrichment of LREE relative to HREE with weak Ce anomaly. The uranium minerals, umohoite, Zn-zippite, soddyite, uranophane, meta-autunite and uranopilite control the mobilized uranium within the karst profiles. The enrichment of REE suggests that oxidation conditions mostly immobilized Ce4+ and transported REE3+ downward to the lower horizons of the karst profiles. The high REE contents are mainly enriched in Fe-depleted bauxitic facies. While U and LREE are also concentrated in Fe-rich bauxites due to scavenging by goethite and clay minerals.