Role of Magmatism and Related-Exsolved Fluids during Ta-Nb-Sn Concentration in the Central Eastern Desert of Egypt: Evidences from Mineral Chemistry and Fluid Inclusions

The rare metals of Abu Dabbab area in the Central Eastern Desert of Egypt have been investigated for their mineralogy and conditions of precipitation using combination of EMPA and fluid inclusions studies, in order to delineate the source, mechanism of formation and evolutionary model for these economic metals. The (Ta-Nb-Sn) -bearing minerals at the Abu Dabbab area include columbite group minerals (CGMs), wodginite and cassiterite. In both granitic intrusion and its enclosed quartz veins, most of zoned CGMs and cassiterite grains are commonly characterized by a well-developed two-stage texture. Hence, columbite-(Mn) (CGM-I) represents the early formed phase of CGMs that is characterized by high Mn# values (0.64–0.92) with quite low Ta# values (0.13–0.49). It was invaded by Ta-rich phases including tantalite-(Mn) (CGM-II; Ta# = 0.13–0.49) and wodginite, which contain high Ta2O5 and SnO2 (17.91 wt.%). In regard to cassiterite, there are distinct compositional differences between the early-phase cassiterite (Cst-I) and the late-phase one (Cst-II), where the latter is enriched in Ta2O5, Nb2O5 and FeO. The chemistry and textural criteria of the early stage CGM-I and Cst-I, all are indicative of magmatic origin. While, the latter CGM-II, wodginite and Cst-II were influenced by the late magmatic Ta-rich fluids. Fluid inclusions microthermometry shows criteria of phase separation represented by both boiling and fluid immiscibility. The initial fluid was supposed to be of magmatic origin (magmatic CH4), that was consequently influenced by fluid mixing/dilution with post-hydrothermal/meteoric water with respect to the decompression process during uplift. Isochore construction gave rise to an estimate P-T conditions (T = 330–370 °C, P = 22–50 MPa). The fluid inclusions’ microthermometry supports a transition between magmatic and late to post-hydrothermal activities in addition to surface-derived fluid (meteoric fluid?) in a part as main source for the polymetallic deposits.