Multi-Stage Tectonothermal Events in the Nageng Mining Area, East Kunlun, China: Evidence from In Situ U–Pb Dating of Apatite and Zircon

The Nagengkancherer (“Nageng”) silver deposit is the only independent silver mine in Qinghai Province, China. Here, the results of LA–ICP–MS U–Pb dating of apatite and zircon in quartz veins in this area are reported for the first time. The apatite is homogeneous and bright, with characteristics typical of unaltered magmatic apatite that has been unaffected by fluid. The zircons display oscillatory zonation typical of a magmatic origin. Apatite and zircon ages of the Paleoproterozoic Jinshuikou Group are older than those of the Triassic Erashan Formation, with apatite lower-intercept ages of 409.0 ± 3.7 and 376.2 ± 5.6 Ma, respectively, consistent with 207Pb-corrected ages and representing the two formation ages of apatite minerals. The zircon weighted-mean 206Pb/238U age is 425.2 ± 1.8 Ma, with ages of 425–408 Ma being consistent those of late Silurian outcrops on the periphery of the mining area. This indicates the occurrence of a major tectonothermal event in the late Silurian–Early Devonian, revealing the existence of concealed rock at depth in the mining area and providing new information concerning the geological characteristics of the mining area. The age of 376 Ma is reported here for the first time and provides new constraints on the regional tectonic evolution. One sample from the Erashan Formation contained magmatic zircons with a U–Pb age of 220 ± 0.62 Ma, which represents the age of the country rocks (Erashan rhyolite) captured during the ascent of ore-forming hydrothermal fluids.