Styles of Fe–Ti–V ore deposits in the Neoproterozoic layered mafic-ultramafic intrusions, south Eastern Desert of Egypt: Evidence for fractional crystallization of V-rich melts

Abstract

The Korab Kansi and Abu Ghalaga Neoproterozoic mafic-ultramafic intrusions in the South Eastern Desert (SED) of Egypt host economic Fe–Ti–V oxide deposits, ∼41 million tons for Abu Ghalaga ores. The Korab Kansi deposits are composed of titanomagnetite with subordinate ilmenite layers in dunites, gabbros and troctolites. The Abu Ghalaga ores consist of hemo-ilmenite and ilmenite with subordinate magnetite and titanomagnetite lenses in norites, gabbros and anorthosites. The difference in calculated oxygen fugacity (fO2: ∼ ΔFMQ–1.24 to −3.28 for Korab Kansi ores and +0.21 to −0.3 for Abu Ghalaga ores) during subsolidus re-equilibration is consistent with different dominant iron and titanium oxides, magnetite and hematite, respectively. Ilmenite in both deposits is enriched in Nb, Ta, Zr, Hf and V, but is poor in Cr and Ni relative to coexisting magnetite, which belongs to magmatic Fe–Ti–V deposits. The Korab Kansi and Abu Ghalaga ore deposits are rich in Ti, Fe, V, Nb, Ta and Hf with subordinate S, Cu, Ga and Zn at high fO2. The Abu Ghalaga gabbroic intrusion crystallized from ferrobasaltic magmas of tholeiitic affinity at lower temperature (∼1082 °C) and pressure (5.1 kbar) than the Korab Kansi intrusion (∼1180 °C, 8.3 kbar). The Korab Kansi ore deposits mainly formed by fractional crystallization of olivine followed by in situ crystallization of Fe–Ti oxides from ferropicritic/ferrobasaltic parent melts at the floor of the magma chamber. The Abu Ghalaga ores formed as a result of more advanced fractional crystallization of ferrobasaltic parent melts. The Abu Ghalaga Fe–Ti oxides in massive ores at the base of the intrusion grew in situ, whereas most ore lenses in the middle to the top of the intrusion precipitated from immiscible oxide melts that separated from the parental magma in association with floatation of plagioclase and sulfide crystals at a late magmatic stage. Both Fe–Ti ore deposits formed from similar mantle-derived magmas but Korab Kansi ores formed as early fractionates, whereas Abu Ghalaga ores formed late. The Korab Kansi ilmenite is rich in vanadium (up to 3.8 wt% V2O5) relative to that of other magmatic deposits. Its parental melt may have formed from locally Fe–Ti–V enriched part of the Neoproterozoic mantle due to interaction with upwelling asthenosphere or a mantle plume. Controlling factors for formation of economic SED Fe–Ti–V deposits were the V-rich ferropicritic/ferrobasaltic compositions of the parental magmas, and addition of H2O to cause high magma ƒO2.