Permian magmatic sequences of the Bilihe gold deposit in central Inner Mongolia, China: Petrogenesis and tectonic significance

The Bilihe gold deposit is located in the eastern section of the Ondor Sum-Yanji Suture at the southern margin of the Xing'an-Mongolian Orogenic Belt (XMOB) and the northern margin of the North China Craton (NCC), central Inner Mongolia. The magmatic rocks in the ore district are generally high-K calc-alkaline, enriched in LREE, Zr, and Hf, and depleted in HREE, Nb, Ta, and P. The magmatic evolution sequences are norite gabbro -> granodiorite porphyry granite or norite gabbro -> andesite -> dacite porphyry -> granodiorite, which show a trend of decreasing TiO2, FeO, MgO, CaO, and P2O5 with increasing SiO2. In the Bilihe ore district, hydrothermal processes were coeval with granitic magmatism for a period of similar to 17 Myr (272-255 Ma). The ages of the granite, granodiorite porphyry, granodiorite, and dacite porphyry are 271.5-264.1 Ma, 269.8-255.8 Ma, 268.3 Ma, and 268.6-259.4 Ma, respectively. The magmatic rocks contain magmatic, hydrothermal, and magmatic-hydrothermal zircons. The magmatic zircons have delta Ce > 4, La <3 ppm, and Sm-N/La-N >2.5; the hydrothermal zircons have delta Ce <4, La >3 ppm, and Sm-N/La-N <2.5. The Nb/Ta and Zr/Hf ratios of granodiorite are 12.7-14.99 and 40.2-46.56, respectively. The Zr/Hf ratios successively increase in the sequence of granite (27.4-29.02) -> granodiorite porphyry (29.19-32.18) -> dacite porphyry (33.54-38.5) -> norite gabbro (36.75-3837), and their Nb/Ta ratios are 9.09-12.38. Zircons in granodiorite yield epsilon(Hf) (t) values of 029 to 56 (n=13) and 2.07-7.62 (n=5), and they give a Hf two-stage model age (t(DM2)) of 807-4765 Ma. The epsilon(Hf) (t) values of the zircons in granite, granodiorite porphyry, and dacite porphyry are -0.46 to 8.03, 3.17 to 10.32, and -0.78 to 6.58, respectively, and their Hf t(DM2) ages are 787-1324 Ma, 638-1091 Ma, and 868-1343 Ma, respectively. Dehydration partial melting of subducted oceanic crust resulted in the formation of dacite porphyry; partial melting of depleted mantle resulted in the formation of norite gabbro; mixing of depleted mantle and lower crust resulted in the formation of granodiorite porphyry; partial melting of lower crust resulted in the formation of granite; and mixing of lower crust and old upper crust resulted in the formation of granodiorite. Magmatic rocks in the ore district with ages of 272-255 Ma were formed during the late stages of closure of the Paleoasian Ocean; i.e., during the transformation from a collisional to extensional setting. (C) 2015 Elsevier BM. All rights reserved.