Nb-Ta bearing minerals in a metasomatized granite, Eastern Desert, Egypt

摘要

A variety of complex Nb-Ta-Sn mineralizations occur in a rare-earth-elements-bearing metasomatized granite. The minerals bearing these elements occur characteristically as inclusions in medium grained granular to platy feldspars, smoky quartz and medium flaked yellowish green mica. The main phases of REE bearing minerals are manganocolumbite-manganotantalite, wodginite, cassiterite, microlite and mon-tebrasite in decreasing order of abundance. This assemblage is related to the late stage of granitic magma differentiation. The presence of wodginite indicates that a marked albitization has occurred, while greisenization is evidenced by presence of Li-mica. The source rock was most likely enriched in Fe, Sn and Ti, at the latest stages of fractional crystallization, a strongly peraluminous granitic magma rich in Nb and Ta is produced. This melt was also relatively enriched in Li, Rb, Cs, P and F. With increasing fractionation, the ratio Ta/Ta+Nb increases from 0.03 to 0.72. In contrast, Mn/Mn+Fe increases only from 0.74 to 0.99.