Evidence from fission track thermochronology for the mineralization ages of sandstone copper deposits in the Fenghuoshan area, Qinghai

Abstract：The Paleogene and Neogene sediments in the Fenghuoshan basin include the continuously deposited early Eocene to early Oligocene Fenghuoshan Group and early Oligocene Yaxicuo Group in the lower part and their overlying early Miocene Wudaoliang Group. All the sandstone copper deposits in the Fenghuoshan area occur in gray (green) sandstone of the first formation of the Fenghuoshan Group. Three apatite samples collected from the ore-bearing and barren beds of the first formation of the Fenghuoshan Group yielded fission track ages of 22.5±2.3, 33.1±6.0 and 28.0±3.6 Ma respectively. Of these ages, the age of 33.1-28.0 Ma is the mineralization age of the sandstone copper deposits of the area, while the 22.5±2.3 Ma age is the age of the post-mineralization tectono-thermal event. The mineralization ages of the sandstone copper deposits are in the range of ages of deposition of their overlying early Oligocene Yaxicuo Group, indicating that the sandstone copper deposits are the product of compaction and dehydration of the early-stage sediments in the basin.