Fractional Crystallization and Partial Melting of the Paleoproterozoic Gneisses and Pegmatite in the Giant Husab Uranium Deposit, Namibia

Abstract

The giant Husab uranium deposit is located in the Paleoproterozoic Abbabis Metamorphic Complex, which was highly partially melted and metamorphosed during the Damara Orogenic Event. The timing of magma emplacement has been investigated; however, the petrogenesis is unclear. Here we reported petrology, geochemistry, and monazite U-Pb age data from biotite granitic gneisses, syeno-granite, syeno-granitic pegmatites, syeno-granitic gneiss, granitic syenite and biotite quartz monzonites of this complex. Geochemical data suggest that these Paleoproterozoic rocks show high SiO2, Al2O3, and K2O, moderate Na2O, low CaO and Fe2O3, and MgO abundance. The alkali-calcic to alkalic, peraluminous, low Fe-number, depletion in HFSE (Nb-Ta, Ti) and enrichment in LILE (e.g., Rb, Pb) characteristic correspond with I- and S-type granite. Major and trace elements are strongly fractionated with the increase of SiO2, which, together with strongly fractionated LREE patterns and high (La/Yb)N ratios of the biotite granitic gneiss and syeno-granitic gneiss, suggest that the magma was highly evolved and fractionated. Monazite U-Pb data show three metamorphic age groups of 581–535 Ma, 531–522 Ma and 518–484 Ma. The increasing trend of La/Sm and La/Yb with the increase of La, suggest these rocks most likely experienced a partial melting process during the late Palaeozoic metamorphism. We, thus, propose a fractional crystallization model for the generation of the Paleoproterozoic Abbabis Metamorphic Complex basement rock, which was metamorphosed and melted during the late Palaeozoic Damara Orogenic Event and provided the magma sources for primary uranium mineralization.