Zircon U-Pb age of ore-bearing rock in the Qiaomaichong gold deposits on the southern margin of the Qinling orogenic belt and its geological significance

Abstract: On the southern margin of the Qinling orogenic belt, there exists a multitude of Early Paleozoic mafic volcanic rocks, and the syenite porphyry veins in the Qiaomaichong gold deposit were intruded in NW direction into mafic volcanic rocks in the study area. By means of studying new zircon LA-ICP-MS U–Pb age and Hf-isotope, whole-rock major and trace elements of syenite porphyry veins in the Qiaomaichong gold deposit and the mafic country rock, the authors tried to constrain the age and petrogenesis and geodynamic background of the siliceous veins. It is revealed that the intrusion of the syenite porphyry can be divided into two phases, and the corresponding two zircon samples of syenite porphyry have 206Pb / 238U weighted average ages of (415±7) Ma and (477±6) Ma respectively. The former age belongs to the Late Silurian, whereas the latter to the Early Ordovician. Veins have a high SiO2 (>75%) content, σ=1.59，and A/CNK=1.28, which suggests peraluminous alkaline siliceous shear veins. The εHf (t) values are all positive, and vary in a large range ( 3.4 to 11.8) , which suggests that almost all the matter came from the depleted mantle source, exhibiting a crust-mantle mixing feature. The mafic rocks have a high amount of REE (149.62×10-6-321.55×10-6), and are strongly enriched in light rare earth elements , showing a high degree of differentiation. Research shows that this area was within plate tectonic cleavage in the early Paleozoic, so the mantle asthenosphere upwelling of magma formed mafic rocks. Then the veins brought the Au from deep crust into the surface crust in the invasion process, thus providing material of Au sources for pre-concentration of Au and later shear mineralization. The determination of the age of the formation of veins has a great significance for the study of late stage gold enrichment.