Late Jurassic to Early Cretaceous magmatism in the Xiong'ershan gold district, central China; implications for gold mineralization and geodynamics

The Xiong'ershan area is the third largest gold-producing district in China. The Late Jurassic to Early Cretaceous magmatism in the Xiong'ershan area can be divided into two episodes: early (165-150 Ma) and late (138-113 Ma). Laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) zircon U-Pb dating yields ages of 160.7 ± 0.6 Ma and 127.2 ± 1.0 Ma for the Wuzhangshan and Huashan monzogranites in the Xiong'ershan area, respectively, representing the two magmatic episodes. The Wuzhangshan monzogranites exhibit adakite-like geochemical features (e.g. high Sr/Y ratios, low Yb and Y contents). Their Sr-Nd-Hf isotopic compositions are consistent with those of the amphibolites of the Taihua Group, indicating that the Wuzhangshan monzogranites were formed from partial melting of the Taihua Group metamorphic rocks. Compared to the Wuzhangshan rocks, the Huashan monzogranites have higher MgO, Cr, Co and Ni contents, but lower Sr/Y and Fe3+/Fe2+. All the samples from the Huashan monzogranites plot in the area between the Taihua Group amphibolite rocks and the mantle rocks in the (87Sr/86Sr)t vs εNd(t) and age vs εHf(t) diagrams, suggesting that the Huashan monzogranites were probably generated by mixing of mantle-derived magmas and the Taihua Group metamorphic basement melts. The gold mineralization (136-110 Ma) is coeval with the emplacement of the late-episode magmas, implying that crustal-mantle mixed magma might be a better target for gold mineralization compared to the ancient metamorphic basement melt. The data presented in this study further indicate that the transformation of the lithosphere from thickening to thinning in the Xiong'ershan area probably occurred between ∼160 Ma and ∼127 Ma, and that the gold mineralization in this area was probably related to lithospheric thinning.