The isotopic composition of noble gases in gold deposits and the source of ore-forming materials in the region of North Hebei, China

The source of ore-forming materials has long been a controversial focus both in metallogenic theory and in ore-searching practice. This study deals with the helium and argon isotopic characteristics of pyrites from 11 gold deposits and some country rocks in the gold mineralizationconcentrated areas within the three mantle-branch structures in the region of North Hebei Province. It is indicated that3He/4He ratios in the gold deposits are within the range of 0.93 × 10−6 –7.3 × 10−6, with an average of 3.55 × 10−6; R/Ra = 0.66−4.93, averaging 2.53;40Ar/39 Ar ratios vary between 426 and 2073, with the average value of40Ar being 8.32; and the average of4He/40 Ar ratios is 2.17.3He/4He ratios in gneiss and granite in the periphery of the mining district are within the range of 0.001 × 10−6–0.55 × 10−6, reflecting significant differences in their sources.3He and4He fall near the mantle, as is shown in the He concentration diagram. Studies have shown that the ore-forming materials in this region should come from the deep interior of the Earth. With the multi-stage evolution of mantle plume, ore-forming fluids in the deep interior were moving upwards to shallow levels (crust). Under such circumstances, there would be inevitably occur crust/mantle fluid mixing, so their noble gas isotopic characteristics are intermediate between the mantle and the crust.