The Beiminghe skarn iron deposit, eastern China: Geochronology, isotope geochemistry and implications for the destruction of the North China Craton

Abstract

The Beiminghe (BMH) iron ore in the southern part of the Taihang Mountain (TM), Hebei province, is one of the largest skarn iron deposits in China. Here we report phlogopite 40Ar–39Ar and zircon U–Pb age data, as well as sulfur, lead, and He–Ar isotope geochemistry of pyrite from the ores and skarnitized rocks in the deposit in an attempt to constrain the timing and mechanism of formation of the mineralization. The phlogopite 40Ar–39Ar and LA-ICP-MS zircon U–Pb data show markedly consistent ages constraining the timing of ore formation as 136–137Ma. The presence of several inherited zircons with late Archean or Paleoproterozoic ages indicates the participation of the basement rocks during the ore-forming process. The δ34S values of pyrite from the ores range from 12.2 to 16.5‰, with 206Pb/204Pb=17.84–18.79, 207Pb/204Pb=15.46–15.62, and 208Pb/204Pb=37.93–39.75, suggesting that continental crust is the major contributor. This is further confirmed by the He–Ar isotope data (3He/4He=0.0648–0.1886 Ra, mean 0.1237Ra; 40Ar/36Ar=311.7–22909.4; and 40Ar⁎/4He=0.036–0.421). The Mesozoic magmatism and metallogeny in the BMH correlate well with the peak event of lithospheric thinning and destruction of the North China Craton during this process, the early Precambrian lower crustal rocks in the region were re-melted through underplating of mantle magmas, leading to the formation of the Beiminghe monzodioritic pluton. Minor mantle input occurred during the evolution of the monzodiorite magma, which scavenged the ore-forming materials from the lower crust. Interaction of the magmas and fluids with the surrounding rocks resulted in the formation of the Beiminghe skarn iron deposits. The magmatism and metallogeny in the Taihang Mountain are signatures of the extensive craton destruction and lithospheric thinning in the eastern part of the North China Craton during Mesozoic, probably associated with Pacific slab subduction.