Isotope geochemistry and geochronology of the Niujuan silver deposit, northern North China Craton: Implications for magmatism and metallogeny in an extensional tectonic setting.

The Niujuan breccia-type silver deposit forms part of the North Hebei metallogenic belt along the northern margin of the North China Craton. The Hercynian Baiyingou coarse-grained granite and the Yanshanian Er’daogou fine-grained granite are the major Mesozoic intrusions exposed in this region. Here we investigate the salient characteristics of the mineralization and evaluate its genesis through zircon U-Pb and fluorite Sm-Nd age data, and Pb, S, O, H, He and Ar isotope data. The orebodies of the Niujuan silver deposit are hosted in breccias, which contain angular fragments of the Baiyingou and Er’daogou granitoids. The δ 34 S values of pyrite from the silver mineralized veins range from 2.4‰ to 5.3‰. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios of the sulfide minerals show ranges of 16.837–16.932, 15.420–15.501 and 37.599–37.950, respectively. The 3 He/ 4 He and 40 Ar/ 36 Ar ratios of the fluids trapped in pyrite are 0.921–4.81Ra and 299.34–303.84, respectively. The δ 18 O and δ 18 D w values of the ore-forming fluids range from 0.6‰ to −4.15‰ and from −119.4‰ to −98.7‰, respectively. Our isotopic data suggest that the ore-forming fluids were originally derived from the subvolcanic plutons and evolved into a mixture of magmatic and meteoric water during the main hydrothermal stage. The ore-forming materials were primarily derived from the lower crust with limited incorporation of mantle materials. The emplacement time of the Er’daogou granite is constrained by LA-ICP-MS zircon U-Pb geochronology at 145.5 ± 2.1 Ma. Five fluorite samples from the last hydrothermal stage yielded a Sm-Nd isochron age of 139.2 ± 3.8 Ma, indicating the upper age limit for the silver mineralization. These ages correlate with the formation of the Niujuan deposit in an extensional setting associated with the closure of the Mongol-Okhotsk Ocean and the subduction of the Paleo-Pacific oceanic plate beneath the North China Craton. [ABSTRACT FROM AUTHOR]