Geochronological constraints on the Baguamiao gold deposit, West Qinling orogen, central China: Implications for ore genesis and geodynamic setting

摘要

The Baguamiao gold deposit, located in the Fengxian-Taibai ore district, is one of the largest gold deposits in the West Qinling orogen, central China. The poor age constraints on the gold mineralization event at the Baguamiao has caused strong debate on ore genesis and associated geodynamic controls. We carried out Sm-Nd and Ar-40-Ar-39 geochronological studies to determine more precise age constraints on the god deposition. The gold mineralization at Baguamiao deposit was developed during four hydrothermal stages that formed sequentially: (I) NW-trending ankerite-quartz veins, (II) deformed and folded NW-trending sulfide-ankerite-quartz veins, (III) joint-hosted NE-trending sulfide-ankerite-quartz veinlets, and (IV) NW- and NE-trending quartz fissure veins. Stages II and III define the main gold mineralization. The Sm-Nd dating of dolomite, ankerite, and calcite yielded isochron ages of 209.3 +/- 4.2 Ma (MSWD = 1.80) and 208.1 +/- 3.1 Ma (MSWD = 0.50) for gold mineralization of stages II and III, respectively. The Ar-40-Ar-39 dating of muscovite intergrown with sulfides of stage II yielded a plateau age of 209.5 +/- 1.4 Ma (MSWD = 0.30). The age data obtained from the two geochronological methods are consistent with each other within uncertainties, indicating clearly the Baguamiao gold deposit formed at ca. 209 Ma. Integrating deposit geology from this and previous studies, geochronological data, and regional tectonics, we classify Baguamiao as an orogenic gold deposit, which was resulted from a number of fluid pulses but during a single ore-forming episode in late Norian-Rhaetian time of the Late Triassic. The ore-forming fluids and metals are suggested to have been derived from both metamorphic and magmatic processes in a post-collisional regime of the West Qinling orogen.