Rare earth elements and origin of Fe[sbnd]Mn ores from the Lower Carboniferous Um Bogma Formation, Southwest Sinai, Egypt: A mixed source of metals and multiple formation processes

Abstract

The economic FeMn ores of the Lower Carboniferous Um Bogma Formation, Southwest Sinai are an important mineral resource, and are also of interest because of their significant enrichment in Cu and U compared to other Phanerozoic Mn deposits. However, their origin remains controversial. Genetic theories range from sedimentary, to hydrothermal, to karst and to laterization origins. This study integrates geological, sedimentological, mineralogical, and geochemical data, particularly rare earth elements, to elucidate the origin of these deposits. Field relations showed the occurrence of FeMn deposits as lenses of Mn-rich material surrounded by Fe-rich clastic deposits, which then are overlain by dolostones, which, in the subsurface, are known to also contain MnFe lenses. The occurrence of the largest of these deposits at a major transgressive surface is consistent with syndepositional Mn mineralization, but not of ironstones. Geochemical proxies, based on modern hydrothermal, diagenetic and hydrogeneous Mn deposits, are mixed: some suggest a hydrogenous and some a hydrothermal source of Fe and Mn. The sweater (hydrogenous) source of Mn and Fe is indicated by the plotting of these ores in the hydrogenous field of the SiAl discrimination plot, by high ΣREE (42–327 ppm), low La/Ce ratios (0.261.5), relatively high Y/Ho ratios for Mn but not for Fe (averages of 35 and 28), and positive YSN anomalies (1–1.8). The ores plot in the sweater fields of the (La/Sm)SN-(La/Yb)SN and (Sm/Yb)SN-Y/Ho diagrams, which also supports this interpretation. However, the hydrothermal input to the source of these ores cannot be ignored and is indicated by high Ba, Cu, Zn, and by high LREE/HREE ratio as well as the plotting of the FeMn samples in the hydrothermal field of the Fe-Mn-(Ni + Co + Cu)*10, (Co + Ni)-(As+Cu + Mo + Pb + V + Zn), (Co + Ni + Cu)-Co/Zn, CeSN/CeSN*-YSN/HoSN and CeSN/CeSN*-NdSN diagrams. In addition, lower Er/Nd ratios than that of seawater and similarity of REE parameters such as ΣREY, Nd and CeSN/CeSN* between the diagenetic FeMn nodules and the FeMn ores of Um Bogma Formation suggest a post-depositional overprint on the REEs geochemistry of these ores. We propose, as most consistent with the evidence from tectonic subsidence history, sequence stratigraphy, and our geochemical-mineralogical data combined with that of others, an initial synsedimentary deposition of Mn, derived from a distally-located back-arc spreading center where modified seawater at relatively low temperature was the hydrothermal fluid released into the surrounding seawater. This plume of Mn-rich water, which can be far-travelled, deposited Mn on the unconformity surface during a rapid transgression. This accumulation of Mn was subsequently modified and significantly upgraded during karstification of the host dolostones soon after deposition of the dolostones. It is likely that the Fe was introduced at this stage via low-oxygen ground water flow along with As and possibly Ba. Finally, much later, there was a flow of warm, saline oxidizing water through the dolomitic aquifer that precipitated Cu, U and Zn in the weathered material accumulated in the karst cavities.