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 Ar-40-Ar-39 isotope dating

摘要

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 (S(1)m), and the structural framework includes five similar to 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 Ar-40-Ar-39 isotope dating to characterize the structural development and mineralization of the orefield. Three stages of structural deformation are identified; the second stage (S-2) is dominant and included the formation of thrustnappe 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 Ar-40-Ar-39 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 (similar to 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 (similar to 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 (similar to 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.