Genesis and evaluation of heavy minerals in black sands: A case study from the southern Eastern Desert of Egypt

Abstract

Black sands in the southern Eastern Desert (SED) of Egypt contain substantial reserves of heavy minerals (up to 5 %), and are found mainly in three basins namely: Hodein, Ibib and Diit between Shalateen and Halayeeb cities. The heavy minerals in these black sands include ilmenite-leucoxene (31 %–44 %), magnetite (15–18 %), zircon (11–21 %), garnet and green silicates (11–15 %), rutile (6–12 %) and monazite (2–4 %). Cassiterite, thorite, uranothorite, gold, xenotime and chromian spinel are minor quantities (<1 %). Magnetite (FeO: 75–93.5 wt%) and ilmenite (TiO2: 42.7–56.9 wt%), hosting high Mn, V, Zr, Zn, Cr, Nb and Co, were probably derived from gabbroic rocks. The detrital chromian spinel composition (Cr#, 0.51–0.61; Mg#, 0.5–0.63; TiO2 < 1.0 wt%) and its morphology are similar to those of spinels in fore-arc peridotites from the SED of Egypt, suggesting dominance of fore-arc basins for peridotite emplacement. These basins were formed during arc-arc or arc-oceanic crust collision and encolsed ophiolites, gabbroic rocks and I-type granites as sources of the SED black sands. The studied garnets are mostly almandine in composition with few grossularite and spessartine; they might have been derived from I-type granites and gneisses sources. The rutile and monazite show enriched LREE relative to HREE, and display marked defeciency in Eu, suggesting highly fractionated granitic rocks as a main source. Two distinct types of zircon are recorded: radioactive (Hf: 1578–8770, Y: 319–1335, U: 36–114 and Th: 40–64 ppm) and non radioactive (Hf: 427, Y: 44, U: 2 and Th: 2 ppm); they were probably derived from different granitic sources. Compositions and P-T conditions (T: 655–970 °C, P: 1.18–9.53 kbar) of magmatic amphiboles are similar to those derived from I-type granitoids. Bulk analyses of the economic heavy mineral assemblages show significant concentrations of Fe (393 kg/ton), Zr (183 kg/ton) and Ti (129 kg/ton) with minor Cr (14 kg/ton), Ba (7 kg/ton), Hf (4.9 kg/ton), Th (up to 3.34 kg/ton) and U (0.29 kg/ton). The elevated contents of Th and U could be related to the occurrence of monazite and zircon with subordinate thorite, uranothorite and xenotime. The total REE contents of these bulk analyses range from 1 to 4 kg/ton, where LREEs form 80–90 % of total REEs. Monazite (ΣREEs: 443604 ppm on average), garnet, zicon (ΣREEs: 421 ppm) and rutile (ΣREEs: 309 ppm) are the main host of REEs in the investigated black sands. Tonnages of raw sands, to a depth of one meter, are estimated per 10 km2 in each basin, giving 18 million tons for Ibib basin and 19 million tons for both Diit and Hodein basins. Economic heavy minerals constitute 6–26 % of the total heavy minerals and around 1.0 % of total raw sands. Calculated reserves of these economic minerals, per 10 km2 of black sands, range from 0.1 million ton in Ibib and Hodein basins to 0.2 million ton in the Diit basin.