Geochronological Constraints on the Genesis of the Changshitougounao Gold Deposit, Qinling Orogen

The Western Qinling Orogenic Belt, China’s second-largest Au-metallogenic province, hosts numerous polymetallic deposits, with gold resources particularly concentrated in the northwestern Xiahe–Hezuo area. The Changshitougounao gold deposit, located south of the Xiahe Fault, comprises disseminated ores controlled by near E–W-trending faults and is primarily hosted in quartz diorite and the Lower Triassic Longwuhe Formation. Zircon LA–ICP–MS U–Pb dating of fresh quartz diorite yields an age of 241.8 ± 2.6 Ma. Two generations of monazite were identified: type I magmatic monazite and type II hydrothermal monazite. Type I monazite is intergrown with feldspar, quartz, and biotite, and in situ LA–ICP–MS U–Pb analysis gives an age of 239.2 ± 2.2 Ma. Type II monazite occurs as irregular granular aggregates associated with Au-bearing sulfides and hydrothermal sericite, with an in situ U–Pb age of 230 ± 3.5 Ma. Apatite, also coeval with Au-bearing sulfides and type II monazite, yields an LA–ICP–MS U–Pb age of 230.9 ± 2.5 Ma and 230.7 ± 3.0 Ma. Zircon and type I monazite thus constrain the emplacement of the ore-bearing quartz diorite to ca. 240 Ma, whereas hydrothermal type II monazite and apatite constrain the timing of mineralization to ca. 230 Ma. The ~10 Ma interval between magmatism and mineralization indicates that goldmineralization in the Changshitougounao deposit is decoupled from Early Triassic magmatic activity. Integrating previous studies of the West Qinling geodynamic evolution, we infer that the Changshitougounao deposit formed during collisional orogenesis, in response to the closure of the Paleo-Tethys Ocean. Consequently, the Changshitougounao gold deposit is best classified as an orogenic gold system. Pyrite–arsenopyrite and sericite alteration serve as effective exploration vectors, and the contact zone between quartz diorite veins and slate represents a favorable structural setting for ore prospecting.