A mixed hydrogenous and hydrothermal origin of the Bahariya iron ores, Egypt: Evidences from the trace and rare earth element geochemistry

Abstract

This study utilizes the trace and rare earth element geochemistry of bulk ores and in situ LA-ICP-MS analyses of mineral grains to clarify the controversy in the origin of the Bahariya sedimentary iron ores, Egypt. Different types of iron ores were identified in the Bahariya Oasis including high grade, high-Mn, high-Ba, oolitic, and ochreous iron ores. Hematite and goethite are the main constituents of the studied ores with some manganese oxide and hydroxide minerals (pyrolusite, bixbyite, cryptomelane, aurorite, romanechite, manjiroite, and pyrochroite). Barite is common in the high-Ba ore, while some quartz, calcite, and halite are detected in the oolitic iron ore. High-Mn iron ores are characterized by higher contents of SiO2, Al2O3, MgO, CaO, Na2O, and K2O, while high-Ba iron ore has higher TiO2 contents. Analyses of bulk ores and in situ mineral grains are characterized by substantially high contents of Ba (average of 9497ppm); also Zn and Sr occur in considerably high concentrations (averages of 4263 and 429ppm, respectively). Mn-rich ores and Mn-bearing minerals show relative enrichments of trace metals compared to the Mn-poor ores probably due to the ability of Mn-bearing minerals to fix such trace metals by adsorption, absorption, and/or replacement. The ΣREE ranges between 2.6 and 80ppm with an average of 23ppm. High-Mn ores show higher ΣREE (average of 61ppm) compared with the low Mn ore (average of 13ppm). The oolitic iron ore shows very low ΣREE content (7ppm). Red and yellow ochers from El Gedida mine have similar ΣREE values (38 and 39ppm, respectively), while red ocher from the Ghorabi area has relatively higher ΣREE (57ppm). Chondrite-normalized REE patterns of all types of iron ores, whether in the form of bulk ores or Fe- and Mn-bearing grains, have LREE enrichment relative to HREE as shown by (La/Yb)N ratios that vary from 1.7 to 29.4. Majority of the bulk samples and mineral grains have negative Eu anomalies with EuN/Eu* ranges from 0.68 to 0.8. However, bulk samples of one high grade and high-Ba ores as well as some of the Mn-bearing grains from El Gedida ores show positive Eu anomalies with Eu/Eu* ranges from 1.1 to 17.7. With few exceptions, all bulk ores and mineral grains show negative Ce anomalies with Ce/Ce* range from 0.28 to 0.96. A seawater precipitation (hydrogenous to hydrothermal exhalite) is proposed for the Bahariya iron ores. The hydrogenous origin is suggested based on the occurrence of high-Mn iron ores in the base of the iron ore succession, oolitic texture of some of these ores, Si–Al plot, low La/Ce ratios, high Y/Ho ratios, and LREE-enriched patterns with negative Eu anomalies of most of the bulk ores and mineral grains. The hydrothermal contribution to the source of these iron ores can be evident from the high contents of some trace elements such as Ba, Zn, and Mo, and plots the analyzed samples in the hydrothermal fields of the Fe–M–(Ni+Co+Cu)∗10 ternary diagram, (Co+Ni)–(As+Cu+Mo+Pb+V+Zn) and (Co+Ni+Cu–Co/Zn) binary plot, low ΣREE concentrations and positive Eu anomalies and high La/Ce ratios in some of the analyzed samples. The hydrothermal contribution looks for local effect and is restricted to El Gedida area.