Isotope geochemistry and geochronology of the Qiubudong silver deposit, central North China Craton: Implications for ore genesis and lithospheric dynamics.

The Qiubudong silver deposit is a breccia-type silver deposit in the Fuping ore cluster region within the Taihang Mountain region of the central North China Craton (NCC). The cylindrical breccia pipe occurs at the conjunction of a NE trending fracture and a NW trending fracture and along the latter a number of quartz porphyry and felsite dikes are emplaced. The breccia clasts include quartz porphyry, leptite, and felsites, cemented mainly by felsic magmatic and hydrothermal components. The ore formation process can be divided into five stages: magnetite–quartz stage (I), biotite–quartz stage (II), quartz–pyrite stage (III), quartz-polymetallic sulfide stage (IV), and quartz–carbonate stage (V). Hydrothermal alteration is characterized by pyritization, sericitization, silicification, carbonatization, potash feldspathization, epidotization and chloritization with a systematic zoning from the ore body to the margin with transitional contact boundaries. We present S, Pb, He, and Ar isotope compositions of the ore minerals and evaluate the source of the ore-forming materials and fluids. The δ34S values of the pyrite range from 0.5‰ to 3.4‰, with an average of 1.6‰, consistent with a mantle input. The Pb isotope compositions from the sulfide minerals range from 15.799 to 16.049 for 206Pb/204Pb, 15.143 to 15.241 for 207Pb/204Pb, and 36.999 to 37.590 for 208Pb/204Pb, suggesting a lower crustal origin. Helium isotopic compositions of the fluid inclusions trapped in pyrite vary from 0.45Ra to 1.65Ra with an average of 0.98Ra (Ra is the 3He/4He ratio of air equal to 1.4×10−6), tending to crustal contribution with calculated mantle helium proportions ranging between 5% and 20% (average at 12%). Six pyrite samples from the silver deposit yield a Rb–Sr isochron age of 102.04±0.61Ma, whereas crystallization of the quartz porphyry dike is dated by zircon U–Pb LA-ICP-MS chronology at 107.95±0.87Ma. These ages suggest a close link between the silver mineralization and magmatism that generated the quartz porphyry. Combined with previous age data, we infer that the magmatism and metallogenesis in the central NCC took place during 140–100Ma with a duration of 40Ma, accompanied by mantle upwelling and lithospheric thinning. [ABSTRACT FROM AUTHOR]