Apatite as a fingerprint of granite fertility and gold mineralization: Evidence from the Xiaoqinling Goldfield, North China Craton

The relationship between granitic magmatism and lode gold mineralization has remained as a controversial topic among economic geologists. In this paper, we report in situ elemental and Sr isotopic signature of apatite from the Wenyu and Huashan granitic plutons in the Xiaoqinling goldfield, southern margin of the North China Craton (NCC), intending to provide further constrain the relationship between magmatism and metallogeny. The formation ages of Huashan and Wenyu granitic plutons are coeval with the gold mineralization, but most of the gold deposits are clustered nearing the Wenyu pluton, and only a few small gold deposits are found adjacent to the Huashan pluton. Although apatite from both plutons is all fluorapatite, characterized by negative Eu anomaly and enrichment of LREEs, the apatite from the two plutons shows different signatures. Based on these differences, the apatite from the Huashan pluton is divided as group A (HS-A) and group B (HS-B). The delta Eu, delta Ce values and MnO and Ga contents of the Wenyu and HS-A apatite indicate high oxygen fugacity range from MH to NNO for the magma, whereas the HS-B apatite shows low oxygen fugacity range from NNO to FMQ. The Sr-87/Sr-86, REE, Th/U, and Ce/Y values of the apatite suggest that the magma for the Wenyu pluton incorporated more mantle-derived materials in contrast to Huashan pluton. Sr/Y ratio and REE content of the apatite indicate that WY apatite crystallized earlier than feldspar and Huashan apatite was affected by feldspar crystallization. The fluid-volatiles exsolution texture and S content of the apatite indicate higher content of volatiles in the Wenyu pluton magma, where the magma experienced more intense fluid exsolution than that of the Huashan pluton during apatite crystallization. The characteristics of many mantle-derived materials, high oxygen fugacity, and higher volatiles as revealed by the apatite data show that the magmatism of Wenyu pluton was more conducive to the mineralization of gold deposits than that of the Huashan pluton and is more closely related to gold deposits.