Timing of structural deformation and age of mineralization in the northern Shiquan–Hanyin gold orefield, South Qinling belt, China: Constraints from zircon U–Pb and biotite 40Ar–39Ar isotope dating

Abstract

The northern Shiquan–Hanyin gold orefield in southern Shaanxi, China, is located within the South Qinling belt–North Dabashan thrust-nappe system. The ore-bearing rocks in the orefield comprise low-grade metamorphic rocks and intensely deformed schist of the lower Silurian Meiziya Formation (S1m), and the structural framework includes five ~E–W-trending brittle–ductile shear zones. However, important aspects of the orefield are still uncertain, such as the genetic type of the deposits, the characteristics and development of brittle–ductile shear zones, the division of structural stages, and the ages of structural deformation and mineralization. The present study uses large-scale structure–alteration mapping, zircon U–Pb chronology, and biotite 40Ar–39Ar isotope dating to characterize the structural development and mineralization of the orefield. Three stages of structural deformation are identified; the second stage (S2) is dominant and included the formation of thrust-nappe ore-controlling brittle–ductile dextral shear zones. A zircon U–Pb age (180.2 ± 3.6 Ma) of a granite dike in a brittle–ductile shear zone and biotite 40Ar–39Ar plateau ages (191.0 ± 1.1, 178.4 ± 0.8, 170.4 ± 0.9, and 168.4 ± 0.9 Ma) of lamprophyre and altered mineralized rocks indicate that the main structural–magmatic–mineralization event occurred during the Early–Middle Jurassic, coeval with intracontinental orogenesis in the South Qinling belt. Ore formation occurred in three stages. In stage 1 (~190 Ma), the upper crust underwent a transition from a compressive to an extensional tectonic environment, and the initial deposition of gold likely occurred. In stage 2 (~180 Ma), under an intracontinental orogenic collapse regime, several brittle–ductile shear zones and a series of extensional strike-slip normal faults developed, making stage 2 the main stage of mineralization. In stage 3 (~170 Ma), the late stage of intracontinental orogeny, the main tectonic style gradually transformed from extensional collapse to tectonic-extrusion-related nappe-fold deformation, inducing late alteration–mineralization that superimposed and enriched the existing mineralization.