The behaviour of rare metals in Piaoac granite massif, Vietnam

Abstract

The Piaoac granite massif is Paleogene in age and genetically related to surrounding rare metal mineralization. The massif consists of two-mica granite and aplite granite. The Piaoac granite is different from normal granite in its geochemical characteristics. Content of uranium is about 3–4 times more and the ratio U/Th is lower. Besides the radioactive elements, many rare metals have been found with high content in comparison with normal granite. W, Ag, Li, Sn and Nb are 200, 60, 20, 10 and 10 times higher, respectively. Hydrothermal metasomatism is widespread in the Piaoac granite massif, especially in its periphery (in endocontact). In addition, there is widespread quartz-muscovite greisenization of high temperature (T = 550° C) . In this process W- and Sn-bearing quartz veins had been formed with a new typical assemblage of accessory minerals, such as: topaz, rutile, cassiterite, wolframite, etc. In a detailed section, across the greisenized granite we note the gradational characteristics adjacent to quartz-wolframite veins or blocks. Study of the relation between the grade of greisenization and the content of rare metals shows that the radioactive elements are inversely, and W, Sn, Li, Be are linearly, correlated with greisenization. In the northwest contact of the Piaoac massif with limestone, quartz lepidolite greisen of medium temperature ( T = 450° C ) is widespread in the granite side. Overlaying it are fluorite-hydromuscovite-bertrandite metasomatites of low temperature.