In Situ U-Pb Dating of Hydrothermal Monazite From Metasediment-Hosted Orogenic Au Deposits: Constraints on the Age and Geotectonic Setting of Au Mineralization in the West Qinling Orogen, Central China

The general lack of absolute ages for most Au deposits in the well-endowed West Qinling orogen (central China) has hampered establishment of the exact temporal framework of regional Au mineralization during the geotectonic evolution of this orogen. This study presents new in situ U-Pb ages on hydrothermal monazite in native gold-bearing ores from three world-class Au deposits (Liba, Baguamiao, and Shuangwang) in the West Qinling orogen. The dating results indicate that these deposits formed synchronously at ca. 220 Ma. In all cases, these ages overlap with new U-Pb dates for felsic and mafic magmatism in the deposit settings. Our new age data, when combined with independent geologic evidence and previous geochronological studies, suggest orogen-wide Au mineralization was controlled by the Triassic orogenic processes involving westward diachronous subduction and closure of the Mianlue ocean and subsequent oblique continental convergence of the Yangtze craton and the South Qinling terrane. During ca. 250 to 225 Ma, Au mineralization in the western part of the West Qinling orogen is attributed to oceanic subduction, whereas in the eastern part it is related to incipient continental collision. At ca. 225 to 210 Ma, continental collision culminated in the final formation of the West Qinling orogen, during which slab tearing induced upwelling of the asthenosphere and melting of lithospheric mantle and continental crust, as is reflected by the widespread granitic magmatism and synchronous mafic dikes. These latter events facilitated extensive crustal fluid flow that generated the significant Au endowment in the West Qinling orogen as represented by the dated three deposits.