VERTICAL VARIATIONS IN THE MINERALOGY OF THE YICHUN TOPAZ LEPIDOLITE GRANITE, JIANGXI PROVINCE, SOUTHERN CHINA

Abstract

The Yichun topaz–lepidolite granite is the latest and most evolved unit of the Yichun granitic complex, South China, and is well known by virtue of its unusual Ta–Nb–Li mineralization. A drill hole down to 300 m has intersected the granite from top to bottom, and revealed three petrographic zones: K-feldspar-rich facies in the lower part, albite-rich facies in the middle part, and Ab- and Kfs-rich facies in the upper part. Geochemically, the Yichun granite belongs to the phosphorus-rich type of rare-element-enriched granite. The phosphorus is concentrated in the feldspars, which contain up to 0.93 wt% P2O5. It is less enriched in feldspars in the middle part, where primary amblygonite is abundant. Principal accessory minerals include columbite–tantalite, cassiterite, zircon, and wodginite in all facies, and microlite in the middle facies. The composition of the columbite-group minerals corresponds to manganocolumbite and manganotantalite. Zircon contains mostly 4 to 8 wt% HfO2, but the HfO2 contents may reach up to 22 wt% at the rim. From the lower part to the middle part of the drill hole, the progressive increases in Ta/(Nb + Ta) of columbite and Ta-rich cassiterite, and in Hf content of zircon, are strongly suggestive of magmatic differentiation. On the other hand, in the upper part of the drill hole, the decrease in Ta/(Nb + Ta) of columbite and cassiterite, in Hf content of zircon, and the increase in Fe of lepidolite, and especially marked enrichment in Fe and W in columbite (5.76% FeO, 7.28% WO3) at the uppermost part of the drill hole, indicate another environment at the final stage of crystallization of the granite, influenced by fluid probably derived from the surrounding mica schist.