Late mesozoic magmatism in the Gucheng district: Implications for REE metallogenesis in South China

Regolith-hosted rare earth element (REE) deposits are common in South China. The newly-discovered Gucheng REE deposit in the western part of Guangdong Province, is characterized by HREE enrichment (with SHREE oxides of 55%). Three types of granites have been identified in the Gucheng ore district, a REE-fertile coarsegrained biotite granite (CGBG), a weakly REE-fertile fine-grained biotite granite (FGBG) and the barren mediumgrained biotite granite (MGBG). New LA-ICP-MS zircon U-Pb dating reveals that the MGBG (107.7 +/- 0.4 Ma), CGBG (103.2 +/- 0.6 Ma) and the FGBG (98.9 +/- 0.7 Ma) were all formed in the Late Cretaceous. Mineralogical and geochemical data indicate that the CGBG is an unfractionated I-type granite, whereas the FGBG and MGBG are fractionated I-type granites. The CGBG and FGBG have (87Sr/86Sr) i and eNd(t) ranging from 0.7121 to 0.7140 and 8.6 to 8.0, and 0.7125 to 0.7131 and 8.2, respectively. The source of these granites was mainly composed of two end-members: one having 90% Paleoproterozoic basement and another 10% enriched mantle metasomatized by subduction-related melts. The barren MGBG is also derived from partial melting of a Paleoproterozoic source with a minor contribution of enriched mantle. The involvement of enriched mantle can provide REE into the magmas, which plays an important role in REE enrichment. Geochemical data show that allanite have been crystallized during magma evolution process of all the three types of granites. However, no allanite has been discovered in the MGBG, and the FGBG contains lower abundance of allanite than that of the CGBG, which is due to different degrees of fractionation. The crystallization and separation of allanite has greatly affected the LREE of the three types of granites, which further influences their potential for LREE mineralization. Lower K/Rb and Nb/Ta ratios of the CGBG, FGBG and MGBG indicate that these three types of granites were affected by hydrothermal events after their crystallization. Zircon geochemical data show that the MGBG was formed in a lower oxygen fugacity environment than the CGBG and FGBG. Above all, involvement of enriched mantle, a relatively high oxygen fugacity, crystallization and separation of allanite as well as involvement of hydrothermal events after magmatic crystallization are important factors controlling REE mineralization at Gucheng, of which the hydrothermal events may play the most important role.