Petrogenesis of neoproterozoic Mo-bearing A-type granites in the Gattar area, northern Eastern Desert, Egypt: Implications for magmatic evolution and mineralization processes

Abstract

Three Mo-bearing granitic intrusions, namely: Gattar, Abu Harba and Abu Marwa occur at the Gattar area in the North Eastern Desert of Egypt. These late tectonic granites intrude Neoproterozoic rocks of island arc metavolcanics and metagabbro-diorite complexes in addition to post-collision molasse-type sediments (Hammamat Group) and Dokhan-type volcanic rocks. U–Pb zircon geochronology yielded ages of 630 ± 27 Ma, 600.1 ± 8.5 Ma, and 601.1 ± 2.4 Ma for the Gattar, Abu Harba, and Abu Marwa granite intrusions, respectively.Whole-rock geochemical data showed the granites to be alkaline to peralkaline post-collision A-type granites. The three granite intrusions are considered co-magmatic as indicated by their similar ages and the continuous variation of major and trace elements on the variation diagrams. The inferred crystallization temperatures, as calculated from zircon geothermometry according to Watson et al. (2006), fall in the range of 616–690 °C, (average = 662 °C), 592–720 °C, (average = 653 °C), and 592–894 °C, (average = 706 °C) for the Gattar, Abu Harba, and Abu Marwa granite intrusions, respectively. This indicates that the Abu Marwa granite is the least evolved, followed by the Gattar and Abu Harba granites. The calculated logfO2 values for zircons from all the studied granites show highly oxidized magmatic features. The Abu Marwa granite (average logfO2 = −12.62) has the strongest oxidized state, whereas Gattar granite (average logfO2 is −15.46) and Abu Harba granite (average logfO2 = −15.99) have moderate logfO2 values implying moderate oxidized states. Guided by the tectonic evolution of the post-collision stage in the Arabian-Nubian Shield, it is suggested that the original magma of the granites in the Gattar area was formed by partial melting of an underplated crustal source, which had been fertilized by alkali-, HFSE-, and REE-rich fluids released and metasomatized by an underplating mantle-derived magma.Fractional crystallization and fluid differentiation/metasomatism were major processes in the petrogenetic evolution of these granite intrusions. Fluorine-complexing of HFSEs and REEs was involved in the early stages of fractional crystallization (i.e., crystallization of the Abu Marwa granite intrusion), resulting in a HFSE- and REE-rich residual melt that form the more evolved alkali feldspar granite of Gattar and Abu Harba intrusions. The late magmatic ore-forming fluids enhance the Gattar granite intrusion’s ability to become enriched in molybdenum relative to Abu Harba and Abu Marwa granites. The reason of the different degrees of molybdenite enrichment in the three intrusions depended on many factors (e.g., degree of fractionation and oxygen fugacity of the granitic magma).