Geochemistry and geothermobarometry of the Um Eleiga Neoproterozoic island arc intrusive complex, SE Egypt: genesis of a potential gold-hosting intrusion

Abstract

Gabbro-diorite-tonalite±granodiorite complexes, derived from subduction-related, calc alkaline magmas represent a distinct unit in the Precambrian basement of Egypt. These intrusive complexes are commonly intruded into ophiolitic mélange, schistose or gneissic rock assemblages. Well-constrained geological mapping of the Um Eleiga complex, combined with geochemical studies, should enable a better understanding of its genesis, especially its potential as a host for gold mineralisation. This complex represents a composite pluton intruded into the allochthonous, ophiolitic Abu Dahr massif and its associated undifferentiated metavolcano-sedimentary mélange matrix in the southern Eastern Desert of Egypt. Field and petrographic investigations have revealed a compositional continuum from gabbro to granodiorite through diorite and subordinate tonalite. Petrography and bulk rock geochemical data have been used to determine the magma type and tectonic setting of the Um Eleiga complex, whereas the electron microprobe data were valuable for the purpose of determining the pressure–temperature and oxygen fugacity conditions. The compositional continuum, pattern of zonation, shallow magma origin and marked common negative Nb anomalies suggest that a close petrogenetic relationship between the gabbro, diorite, tonalite and granodiorite units of the Um Eleiga complex. Subducted upper mantle wedge is proposed as being the source region for the gabbroic magma, whereas zoning of the complex is attributed to interplay of fractional crystallisation and assimilation. Pressure estimates based on the Al(iv)-in-hornblende geobarometry indicate that the gabbroic and dioritic rocks crystallised under 4–5 kbar, whereas tonalite and granodiorite were formed at 3–4 kbar. The hornblende-plagioclase thermometer indicates a formation temperature range of about 720 to 880°C for the complex. Varied oxidation states are inferred from the mineral assemblages, including magnetite, titanite and ilmenite and checked by Fe/(Fe+Mg) ratios of biotite in the granitoid rocks. The tectonic setting, mineralogical, geochemical characteristics and varied redox state of the Um Eleiga intrusive complex are typical for arc-related intrusives potential for hosting intrusion-related gold in a collisional environment.