Petrogenesis of late Mesozoic high-Ba–Sr granites in the Qiushuwan Cu[sbnd]Mo orefield: Implications for the distribution of porphyry Mo mineralization in the East Qinling area of Central China

Abstract

The Qinling–Dabie orogenic belt of China is well known for its large Mo resources. There have been many studies of the genesis of the Mo deposits, but the reasons for the intensive mineralization during the late Mesozoic and the spatially uneven distribution of the resulting ore deposits are not fully understood. Deposits with reserves of >6 Mt. of Mo occur predominantly in the southern margin of the North China Craton (NCC) and are mostly late Mesozoic porphyry deposits. In comparison, a few relatively insignificant and Cu-dominated porphyry ore deposits have been identified in the North and South Qinling terranes. This study presents new whole-rock geochemical, isotopic, and zircon UPb age data for ore-bearing granites associated with the Qiushuwan CuMo deposit, the largest porphyry deposit in the North Qinling (NQ) terrane, which provide insights into the asymmetric distribution of Mo mineralization in this region. The rocks display high-K calc-alkaline to shoshonitic affinities, and are metaluminous to weakly peraluminous with high Ba and Sr contents, low Rb, Nb, and Ta contents, and depletion in heavy rare earth elements and Y, which are features typical of high-Ba–Sr granites. Zircon UPb dating indicates that the granites crystallized at ca 145 Ma. Their narrow ranges of whole-rock initial 87Sr/86Sr ratios (0.7056–0.7069) and εNd(t) (−7.4 to −9.7), zircon εHf(t) (−2.49 to −5.13), and δ18O (+5.58‰ to +6.49‰) values, together with the presence of significant NdHf isotopic decoupling (ΔHf = +7.61 to +11.3), indicate that the parent magma was derived from partial melting of enriched subcontinental lithospheric mantle with minor assimilation of the lower crust. In contrast, late Mesozoic granitic rocks in the southern margin of the NCC (including both mineralized and barren intrusions) are geochemically associated with the crust of the northern margin of the Yangtze Block (YB). The YB contains Mo-rich shales that are thought to have been subducted beneath the southern margin of the NCC during the Triassic. Melting of these Mo-rich black shales would have mobilized Mo, a process that enabled the formation of the extensive Mo mineralization associated with the late Mesozoic granites of the southern margin of the NCC. This indicates that the extensive Mo mineralization within the southern margin of the NCC is genetically related to a Mo-rich source that was generated by the stagnation of subducted YB continental crust beneath the southern margin of the NCC. In comparison, the less significant Mo mineralization within the East Qinling orogenic belt might reflect the variable removal of the subducted YB continental slab during post-collisional intracontinental orogenesis.