POLLUCITE AND THE CESIUM-DOMINANT ANALOGUE OF POLYLITHIONITE AS EXPRESSIONS OF EXTREME Cs ENRICHMENT IN THE YICHUN TOPAZ-LEPIDOLITE GRANITE, SOUTHERN CHINA

摘要

The Yichun topaz-lepidolite granite is the youngest and most evolved unit of the Yichun granitic complex, southern China, and is well known by virtue of the unusual development of Ta-Nb-Li mineralization and enrichment in cesium in a granite. Three petrographie zones are recognized from the bottom upward: K-feldspar-rich facies, albite-rich fades, and albite- and K-feldspar-rich facies. The Cs contents increase with fractional crystallization of the magma, and reach a maximum in the middle part of the albite-rich facies (1709 ppm). Two principal carriers of Cs are observed in this facies: pollucite and the Cs-dominant analogue of polylithionite. In the middle part of this facies, both minerals occur as small inclusions within quartz or K-feldspar phenocrysts, indicative of a magmatic origin. However, in the upper part of this facies, pollucite appears interstitially among tabular crystals of albite around K-feldspar phenocrysts, suggesting that this type of pollucite postdates the rock-forming minerals, but formed prior to complete crystallization of the silicate melt. In addition, Cs-rich replacement zones typify the rim of lepidolite flakes in the groundmass. Lepidolite may contain up to 25.8 wt percent Cs_2O, or approx 0.9 Cs apfu (based on O = 11). Such lepidolite may be considered a Cs-dominant analogue of polylithionite. According to their distribution and compositional characteristics, pollucite and the Cs-dominant analogue of polylithionite seem to have formed at the late-magmatic or magmatic to hydrothermal transition stage of evolution of the leucogranitic magma at Yichun.