Genesis of alkaline porphyries and associated Cu-Au-Pb-Ag polymetallic mineralization in an intracontinental transpression setting: Example from the Yao'an volcano-plutonic complex in western Yangtze Craton, SW China

The Yao'an Pb-Ag-Au deposit is located far from the present -day margin of the Yangtze Craton, providing an excellent example for studying the genesis of porphyries and associated polymetallic mineralization in intracontinental transpression settings. In situ zircon U-Pb ages and Sr-Nd-Hf-O isotopic data of the Yao'an porphyries in the intracontinental zone, combined with research data published for the Machangqing and Beiya orebearing porphyries in the cratonic margin, were used in this paper. Four-phase felsic rocks (-33 Ma) and later mafic rock (-31 Ma) were recognized in the Yao'an district. Of these, the third-phase syenite porphyry had a genetic link with Pb-Ag-Au mineralization, which occurred within the second-phase volcanic rocks and Mesozoic strata. In addition, the Yao'an porphyry showed high initial 87Sr/86Sr ratios (0.7087-0.7094) and enriched Nd (epsilon Nd(t) = -10.53 to -12.60) isotopic composition, with a wide range of zircon Hf (epsilon Hf(t) = -8.39 to -13.2) and delta 18O values (6.28 %o-9.20 %o). These features suggest that the Yao'an porphyries were produced by partial melting of an ancient lower crust mixed with continental lithospheric-mantle-derived melt, which are differed from the magma source of the Beiya and Machangqing porphyries. Given the nature sources of the porphyries, we conclude that the spatial variation of metal associations changing from the Cu +/- Au +/- Mo system at the cratonic margin to the Pb-Ag-Au deposits in the intracontinental zone in the western Yangtze Craton was likely controlled by different magma sources. The delamination of an overthickened continental lithospheric mantle in an intracontinental transpression setting was a geodynamic trigger for porphyry magma generation, ascent, and emplacement.