Role of Alkaline Magmatism in Formation of Porphyry Deposits in Nonarc Settings: Gangdese and Sanjiang Metallogenic Belts

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Abstract The recent discovery of large Cenozoic porphyry copper deposits in the Tibetan Plateau has revealed atypical features. Their formation all postdate the India-Asia collision at 55 ± 10 Ma, and therefore they are not affiliated with normal arc magmatism. Three major nonarc porphyry copper belts or provinces in Tibet comprise the Gangdese porphyry Cu-Mo belt (>45 Mt Cu, 1.79 Mt Mo), the Yulong porphyry Cu-Mo belt (8.75 Mt Cu,1.04 Mt Mo), and the western Yunnan porphyry Cu-Mo-Au polymetallic province (~1 Mt Cu, ~1 Mt Mo, and 310 t Au). Alkaline volcanic rocks (lamprophyres, shoshonites, and potassic-ultrapotassic volcanic rocks) are common in these metallogenic belts and provinces, but the temporal, spatial, and genetic relationship between this magmatism and deposit formation remains enigmatic. There are two episodes of porphyry mineralization in the Tibetan Plateau, 45 to 35 and 22 to 11 Ma, and alkaline volcanic rocks are both contemporaneous with and spatially close to porphyry mineralization. Evolved Nd-Hf isotope compositions, and high Mg#, Cr, and Ni contents of Tibetan alkaline volcanic rocks suggest that they are derived from phlogopite-bearing lithospheric mantle, whereas the adakitic property and hybrid geochemical and isotopic features of the high Sr/Y granitoids suggest they are derived from partial melting of lower crust by mantle-derived alkaline mafic melt, with subsequent mixing. The mantle-derived alkaline magmas: (1) triggered water-flux melting of the thickened lower crust and generation of fertile high Sr/Y magmas with high water contents; (2) that dominate the source of ore-related magmas are more Au rich; (3) have variable oxidation states and some can oxidize residual sulfide in the lower crust to release Cu and Au for porphyry deposit formation; other lower crustal melts became oxidized via amphibole and/or garnet fractionation; and (4) provide higher S and Cl contents that are essential volatiles for deposit formation. We conclude that mantle-derived alkaline melts are vital to form porphyry deposits in nonarc settings, thus explaining the close spatial and temporal association of alkaline volcanic rocks and porphyry deposits in Cenozoic Tibet.