Age, igneous petrogenesis, and tectonic setting of the Bilihe gold deposit, China, and implications for regional metallogeny

The Bilihe gold deposit, along the northern margin of the North China Craton (NCC), is hosted in Permian intrusions that were emplaced into Bainaimiao Group greenschist- to amphibolite-facies Mesoproterozoic basement rocks, the early Paleozoic Bainaimiao Arc, and late Paleozoic sedimentary and volcanic cover rock sequences. The Middle Ordovician to Early Silurian calc-alkaline arc comprises volcanic sequences of basalt, andesite, and felsic lavas, and an intrusive complex ranging from gabbro through granite. The deposit is genetically related to ilmenite-series and highly fractionated quartz diorite porphyry host rocks. The most fractionated variety of the quartz diorite porphyry is granitic, which contains dendritic quartz phenocrysts and occurs at the top part of the intrusions. Gold mostly occurs as Au grains (> 990 fineness) in trails in dendritic quartz, and the gold trails typically occur along certain crystallographic elements of dendritic quartz. The gold is believed to have crystallized directly from magma at temperatures of at least 750–800 °C. The Fe₂O₃/(Fe₂O₃ + FeO) ratio of the quartz diorite ranges from 0.21 to 0.29, and there is significantly more ilmenite than magnetite, suggesting that the magma was moderately reduced. There are also post-ore syenogranite porphyries in the deposit area. Zircon U-Pb LA-ICP-MS dating in this study shows that two samples of the volcanic wallrock have ages of 274 ± 3 Ma and 270 ± 4 Ma, two samples of gold-forming quartz diorite porphyries have ages of 261 ± 2 Ma and 259 ± 3 Ma, with 440–400 Ma and 1344–1316 Ma cores, and one syenogranite porphyry sample is dated at 253 ± 3 Ma.

The mineralized quartz diorite porphyries exhibit a high-K calc-alkaline composition, high Mg# (46–51), and a high content of compatible elements (V: 114–164 ppm; Cr: 80–93 ppm; Ni: 37–42 ppm) and Rb, Th, and U, as well as depletions in Ba, Sr, and HFSE (e.g., Ti, Nb, and Ta), suggesting melt from a slab fluid-metasomatized enriched mantle. The LREE enrichment, the intermediate oxidation state, and the presence of the Silurian and Mesoproterozoic zircon cores indicate that crustal materials also played a significant role in porphyry formation. In addition, the rocks have slightly negative ε_Nd(t) (− 2.5–0.1), but variable (⁸⁷Sr/⁸⁶Sr)_i (0.70328–0.70573). Based on the above features, we propose that quartz diorite porphyries were produced by partial melting of the metasomatized mantle coupled with the assimilation of crustal materials of the Bainaimiao Group, Late Ordovician-Silurian subduction-related plutons, and Late Carboniferous-Early Permian marine sedimentary rocks. The assimilation of rocks of the Bainaimiao Group, which have an elevated gold content of 26.3 ppb, is speculated to have played important roles in the formation of a rarely documented magmatic gold deposit. In contrast, syenogranite porphyries are characterized by high contents of K₂O + Na₂O (10.3–10.6 wt%), Al₂O₃ (16.3–16.5 wt%), K, Hf, and Zr; low CaO (0.6–1.2 wt%), Mg# values (17–21), compatible elements (V: 16–21 ppm; Cr: 4.9–7.9 ppm; Ni: 2.8–4.5 ppm), and Eu, Sr, and HFSE; and positive ε_Nd(t) (1.4–1.5) with low (⁸⁷Sr/⁸⁶Sr)_i (< 0.70431), indicating that the porphyries were formed by partial melting of the subduction-modified lower crust with a minor input of asthenospheric materials.

It is concluded that the quartz diorite porphyries formed during the southward subduction of the Paleo-Asian ocean underneath the northern margin of the NCC. This confirms that the northern margin was not a passive continental margin, but an active margin with subduction and development of a continental arc. In addition to Bilihe, there are at least four more gold deposits (Hadamiao, Changshanhao, Saiyinwusu, and Chaihulanzi) that formed at the northern margin of the NCC from Middle Permian to Early Triassic. Such a timing indicates that they may also be ultimately related to the tectonic regime imposed by southward subduction of the Paleo-Asian ocean. Based on the new understanding of the tectonic setting, with significant known mineral occurrences along the length of the margin, it is inferred that the > 1000-km-long northern margin of NCC may have a large potential for subduction-related mineralization.