Apatite geochronology and geochemistry of Gucheng granites: Implications for petrogenesis and REE metallogenesis in South China

Regolith-hosted rare earth element (REE) deposits are common in South China, and occurred dominantly in regolith of granite. The Gucheng deposit in western Guangdong Province is one example of HREE-dominated hosted in weathered coarse-grained biotite granites (CGBG). We conducted petrographic observations, in-situ U-Pb dating, EPMA, and LA-ICP-MS in situ compositions analyses on apatite grains from the CGBG to gain in-sights into the petrogenesis of CGBG and implications for REE mineralization at Gucheng. U-Pb geochronology of 50 apatite grains yields 206Pb/238U age of 102.1 +/- 3.1 Ma, representing the crystallization age of CGBG, which suggests the CGBG was formed in the late Early Cretaceous.LA-ICP-MS data show that the CGBG apatite is characterized by enrichment of LREE ((La/Yb)N = 6.3 to 11.4) and obvious Eu anomalies (delta Eu = 0.12 to 0.16), indicating crust-mantle mixing in the magma source. Meanwhile, the mantle source was indicated to be metasomatized by both sediment melting and slab melting components triggered by the subducted Paleo-Pacific slab. In conjunction with LREE enrichment, no Eu anomaly and ratios (e.g., (Sm/Nd)N, (La/Sm)N, (La/Yb)N and Sr/Y) in apatite, the CGBG is classified as I-type granite. Meanwhile, although apatite is low in the CGBG (0.6 mod. %), it is an important REE-carrier mineral in the CGBG (5194 to 6255 ppm) and contains 30.5 times higher REE concentrations than those of the CGBG. Moreover, it contribute 19.5 % of REE concentrations for total REE of the CGBG, and its decomposition during weathering has a great contribution for REE mineralization in regolith. However, the decomposition of apatite and release of REE cannot account for the HREE-dominated mineralization in regolith of CGBG that are characterized by enrichment of LREE.