Rare-metal mineralization in granitic rocks of the Tongolo Anorogenic Complex — Northern Nigeria

The Tongolo Anorogenic Complex consists of peraluminous biotite granites and peralkaline riebeckite granites in which mineralization is spatially associated with the peraluminous biotite granites. Metallization is dominated by Nb-Sn and Sn-W types. Geochemical analyses of fresh bedrock samples indicate that the Tongolo biotite granites are characterized by enhanced values of a suite of trace elements (Sn, Nb, W, Zn, Rb, Li, F, Th, Y, U) which readily identify them as “specialized” granites. These geochemical data are also examined by R-mode factor analysis with the primary objective of isolating the significant factors accounting for the sample composition as derived from mineralization, alteration and lithology. The resulting orthogonal varimax solution yields a three-factor model that accounts for 79.7% of the total variance. These granite series are marked by what is interpreted as the “lithophile factor” (heavily loaded by Li, Rb, F, Th, Ga, Y, U) dominated by magmatic processes and metallization factors (Nb, Zr, Ga, U, Zn, Li and Sn, W, Rb, F, Th) which are dominated by postmagmatic processes. The two dominant types of mineralization (Nb-Sn and Sn-W), although characterized by the same pattern of trace-element enrichments, can be discriminated on the basis of Rb/Zr and Sn-Li-F relationships.