The Zr/Hf ratio as an indicator of granite magma evolution of rare metal deposits related to post-orogenic granites

Abstract

Using the example of three classical regions of rare metal granitoid occurrences (Eastern Transbaikalia, Russia, Central Kazakhstan and Erzgebirge-Krusne Hory of Germany and Czech Republic) it was shown that biotite granites, leucogranites and Li-F granites are the products of granitic melt fractionation. The bulk Zr/Hf ratio in granitic rocks regularly decreases from 35 to below 5 during granite magma evolution. Simultaneously in the residual melt the accumulation of incompatible elements such as: F, Li, Rb, Cs, and rare metals: W, Mo, Sn, Bi, Be, Hf, Ge, Ga, U, Th, Tl, Nb, Ta occurs. The greisen deposits of W, Mo, Sn, Bi, Be are associated with leucogranites and Li-F granites with a Zr/Hf index in the range 25 to 10. Commercial Ta, Nb, Li concentrations require fractionating granite melts with a decreasing Zr/Hf index towards values below 5.