Determination of the Au background value and Au geochemistry in the Devonian of the West Qinling and their geological significance

Abstract: Geological and geochemical studies indicate that the Au background value of the Qinling orogen is 2.89×10-9. The Au background values of the Fengtai basin, Xicheng basin and Limin basin, which are important gold polymetallic geochemical metallogenic provinces, are 3.1×10-9, 3.2×10-9, 4.7×10-9 respectively. From east to west, the Au background values gradually decrease from the Limin basin→Wenkang basin→Xicheng basin→Fengtai basin→Bansha basin→Zhenxun basin→Zhashan basin. The distribution pattern of Au in strata is controlled by the lithology and sedimentary environment. The Devonian is the source bed of gold, and phyllite, slate, schist and siltstone are important source rocks. Compared with other sedimentary covers in the area, the Mid-Upper Devonian in the Qinling orogen is rich in Au, Ag, Cu, Co, Ni, Cr, Pb and Zn, and compared with the upper crust, it is rich in Au, As, Sb, Ag, Co, Ni, Cr, V, Pb and Zn. The hydrothermal sedimentary rocks developed in the Devonian have higher Au contents. Study indicates that gold preconcentration took place in the hydrothermal sediments. It is suggested that Au was concentrated during Devonian sodic hydrothermal deposition along submarine contemporaneous faults to form the source bed.