Origin of lamprophyres in the giant Poshan silver deposit from the Tongbai orogen, central China: Implications for tectonics and ore genesis

• Zircon, titanite, apatite U-Pb dating reveals Poshan lamprophyre emplacement (136 to 131 Ma) and hydrothermal Ag mineralization (∼125 Ma) occurred in the Early Cretaceous. • Lamprophyres were derived by partial melting of an EMI-type enriched lithospheric mantle. • Volatile and metal-enriched fluids were exsolved from the evolved metasomatized mantle-derived mafic magma chamber. The metasomatized lithospheric mantle is proposed to be conducive to the gold mineralization. However, genetic links between silver deposits and the metasomatized mantle remain poorly understood. The giant Poshan silver deposit, located in the Tongbai Orogen of central China, has a remarkably high average Ag grade of 146.21 g/t and is supposed to be associated with a magmatic hydrothermal system. In this study, integrated zircon, titanite and apatite U-Pb dating, elemental, Sr-Nd-Hf isotopic analyses were conducted on fresh and mineralized lamprophyre dykes in the giant Poshan silver deposit to clarify the relationship between deep-seated magmatism and silver mineralization. Magmatic zircons, titanite and apatite from the lamprophyres yielded U-Pb ages of 136.0 ± 1.6 Ma to 134.2 ± 2.4 Ma, 134.1 ± 9.2 Ma, and 131.0 ± 8.5 Ma, respectively, indicating the emplacement age of the lamprophyres. Hydrothermal titanite and apatite from the silver-mineralized lamprophyres yielded LA-ICP-MS U-Pb ages of 125.7 ± 7.9 Ma and 125.0 ± 6.0 Ma, respectively, constraining the timing of silver mineralization. The fresh lamprophyres have high K 2 O (4.78 to 5.76 wt%) and MgO (6.50 to 7.05 wt%) contents, high Mg # values (65 to 67), and belong to calc-alkaline series. They are characterized by relative enrichment in LREE and weakly negative Eu anomalies ( δ Eu = 0.89 to 0.94). They have initial 87 Sr/ 86 Sr ratios ranging from 0.707283 to 0.707456, negative ε Nd(t) values varying from − 12.8 to − 12.6, with zircon ε Hf(t) values of − 11.0 to − 6.7. These results suggest that these lamprophyres were generated by the partial melting of the enriched lithospheric mantle resembling EMI, and underwent minimal fractionation within intra-crustal processes. The hydrothermal titanite at Poshan yielded ε Nd (t) values of − 13.6 to − 11.2, which are comparable to those of enriched mantle. Sulfides from the mineralized lamprophyre have δ 34 S V-CDT values of 3.40‰ to 4.01‰ and Pb isotopic ratios similar to those in silver ores, with Pb isotopic compositions are consistent with coeval mantle-derived mafic rocks. These results indicate that volatiles, fluids and metals were likely exsolved from a mantle-derived mafic magma chamber. The mafic magmas, derived from partial melting of the enriched metasomatized lithospheric mantle during 135 to 125 Ma, migrated upward and accumulated in magma reservoirs within an extensive extensional setting, and eventually evolved and released volatiles and metals leading to the shallow giant silver mineralization. The results demonstrate genetic links between lithospheric extension, metasomatized mantle-derived magmatism, and the formation of the giant silver deposit in the Tongbai orogen.