The Isotopic Composition of Noble Gases in Gold Deposits and the Source of Ore-Forming Materials in the Region of North Hebei, China

摘要

The source of ore-forming materials has long been a controversial focus both in metal-logenic 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 mineralization-concentrated areas within the three mantle-branch structures in the region of North Hebei Province. It is indicated that ~3He/~4He ratios in the gold deposits are within the range of 0.93 x 10~(-6) -7.3 x 10~(-6), with an average of 3.55 x 10~(-6); R/Ra = 0.66 - 4.93, averaging 2.53; ~(40)Ar/~(39)Ar ratios vary between 426 and 2073 , with the average value of ~(40)Ar being 8.32; and the average of ~4He/~(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 x 10~(-6)- 0.55 x 10~(-6), reflecting significant differences in their sources. ~3He and ~4He 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.