The Zr/Hf ratio as a fractionation indicator of rare-metal granites

Abstract

The Zr-Hf geochemical indicator, i.e., the Zr/Hf ratio (in wt %) in granitic rocks is proposed to be used as the most reliable indicator of the fractionation and ore potential of rare-metal granites. It was empirically determined that the fractional crystallization of granitic magma according to the scheme granodiorite → biotite granite → leucogranite → Li-F granite is associated with a decrease in the Zr/Hf ratio of the granites. The reason for this is the stronger affinity of Hf than Zr to granitic melt. This was confirmed by experiments on Zr and Hf distribution between granitic melt and crystals of Hf-bearing zircon (T = 800°C, P= 1 kbar). The application of the Zr/Hf indicator was tested at three classic territories of rare-metal granites: eastern Transbaikalia, central Kazakhstan, and the Erzgebirge in the Czech Republic and Germany. The reference Kukul’bei complex of rare-metal granites in eastern Transbaikalia (J3) is characterized by a uniquely high degree of fractionation of the parental granitic melt, with the granites and their vein derivatives forming three intrusive phases. The biotite granites of phase 1 are barren, the leucogranites of phase 2 are accompanied by greisen Sn-W mineral deposits (Spokoininskoe and others), and the final dome-shaped stocks of amazonite Li-F granites of phase 3 host (in their upper parts) Ta deposits of the “apogranite” type: Orlovka, Etyka, and Achikan. The Kukul’bei Complex includes also dikes of ongonites, elvanes, amazonite granites, and miarolitic pegmatites. All granitic rocks of the complex are roughly coeval and have an age of 142±0.6 Ma. The Zr/Hf ratio of the rocks systematically decreases from intrusive phase 1 (40–25) to phases 2 (20–30) and 3 (10–2). Compared to other granite series, the granites of the Kukul’bei Complex are enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but are depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From earlier to later intrusive phases, the rocks become progressively more strongly enriched or depleted in these elements, and their Zr/Hf ratio systematically decreases from 40 to 2. This ratio serves as a reliable indicator of genetic links, degree of fractionation, and rare-metal potential of granites. Greisen Sn, W, Mo, and Be deposits are expected to accompany granites with Zr/Hf < 25, whereas granites related to Ta deposits should have Zr/Hf < 5.