Rare earth element (REE) enrichment and mobilization in the parent granites of ion-adsorption REE deposits in South China

The initial enrichment and mobilization of rare earth elements (REEs) in granites are of crucial importance in the generation of widely distributed ion-adsorption REE deposits in South China. In this paper, we compile geochronological, geochemical, and mineralogical data on granites from ion-adsorption REE deposits, as well as geochemical data from global granites, to investigate the mechanisms of REE enrichment and mobilization in granites and their association with specific tectonic settings in South China. The granites associated with ion-adsorption REE deposits are all high-K calc-alkaline to shoshonitic and were generated by crustal reworking in high-temperature extensional settings. In conjunction with the observation that REE concentrations in granites demonstrate significant positive correlations with crystallization temperatures as well as Th concentrations and Th/U ratios, it is deduced that the high temperature conditions in South China’s extensional settings promoted initial REE enrichment in the granites through the melting of REE- and Th-enriched accessory minerals. In addition, the F-rich conditions, as evidenced by the occurrence of F-rich minerals in the granites, may have further enhanced REE concentrations by increasing the solubility of REEs in the melts. The parent granites of heavy REE (HREE) deposits are all high-silica muscovite granites that belong to the fractionated granites and exhibit flat REE patterns. The occurrence of metamictized zircons indicates that these granites evolved to the magmatic-hydrothermal transition stage. Magmatic evolution led to decreasing light REE (LREE) contents, whereas HREE contents remained consistent and even sometimes increased, resulting in the relative enrichment of HREEs in the evolved granites and thereby contributing to the generation of HREE deposits. The high geothermal gradient and F-rich conditions in the extensional settings in South China also favored magmatic evolution. There are two types of parent granites for LREE deposits. The first type contains abundant primary REE-rich accessory minerals of allanite, titanite, and apatite, which account for a substantial proportion of the REE budget in these granites. The common phenomenon in these granites is the alteration of these minerals and the formation of secondary REE-fluorocarbonates, which favors LREE mineralization during weathering because the altered minerals and secondary REE-fluorocarbonates are more easily weathered and release LREEs for mineralization. The second type contains few of these accessory minerals but still exhibits high REE contents with an obvious negative Ce anomaly. It is concluded that the high-oxygen-fugacity meteoric water carrying the REE ions derived from the weathering profiles resulted in REE enrichment within the rocks.