Ba/Rb and Rb/Sr ratios as indicators of magmatic fractionation, postmagmatic alteration and mineralization—Afu Younger Granite Complex, Northern Nigeria

Abstract

The Afu Younger Granite Complex is the southernmost of the Nigerian Younger Granite Complexes and is composed mainly of biotite granites. They are granites enriched in volatile elements, Li, Rb, F and ore elements such as Sn, W, Zn and Nb. Tin-tungsten — niobium mineralization is associated with the younger phases of the biotite granites. Alkali elements variation particularly Ba/Rb and Rb/Sr ratios indicate the degree of fractionation of the successive phases of the biotite granites, postmagmatic alteration and mineralization in the complex. The granites associated with mineralization generally have higher Rb/Sr and lower Ba/Rb ratios as a result of intensive fractionation and increasing hydrothermal volatile activities. Hydrothermal alteration and mineralization result in even higher Rb/Sr ratios and lower Ba/Rb ratios. It is concluded that the use of Ba/Rb and Rb/Sr ratios offers a more consistent guide to the ore bearing potential of granitic rocks in the Younger granite region, than the use of absolute concentrations of Sn, W and Nb. © 1981, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.