Contrasting whole-rock and mineral compositions of ore-bearing (Tongchang) and ore-barren (Shilicun) granitic plutons in SW China: Implications for petrogenesis and ore genesis

摘要

A common perception is that oxidized magma is critical for the formation of a porphyry Cu +/- Mo +/- Au ore deposit We have used an ore-beating pluton (Tongchang) and an ore-barren pluton (Shilicun) in the western rim of the South China block to test this idea and to determine other important controls on ore genesis. Zircon U-Pb ages indicate that the Tongchang and Shilicun plutons were emplaced at 36.3 +/- 0.2 Ma and 35.2 +/- 0.4 Ma, respectively, broadly coinciding with strike-slip faulting in the region and continental collision that occurred similar to 300 km to the west. These two plutons are all characterized by significant light REE enrichments and pronounced negative Nb-Ta anomalies, similar to the bulk crust. Apatite separates from these two plutons all have elevated initial Sr-87/Sr-86 (Tongchang, 0.70690 to 0.70796; Shilicun, 0.70703 to 0.70726) and negative epsilon(Nd) (t) (Tongchang, -6.2 to -7.3; Shilicun, -4.5 to -5.8). The mean epsilon(Hf)(t) and delta O-18 of zircon from these plutons are -1.4 and 6.8 parts per thousand for Tongchang, and -0.3 and 6.5 parts per thousand for Shilicun. The Sr-Nd-Hf isotope compositions are all within the ranges of the Neoproterozoic mafic arc lower crust in the region. Whole-rock U-Nb-Ta systematics indicate that the Tongchang and Shilicun plutons originated from an amphibole-bearing source and a garnet-bearing source, respectively, implying a shallower and more H2O-rich mafic source for the former than the latter. Quartz trace element data indicate that the Tongchang pluton was emplaced at a shallower depth than the Shilicun pluton. Zircon Ce4+/Ce3+ and whole-rock V/Sc ratios, and apatite SO3 and MnO contents indicate that the parental magmas of both plutons are as oxidized as those of some porphyry Cu deposits in northern Chile, but the parental magma of the Tongchang pluton is less oxidized than that of the Shilicun pluton. Apatite Cl-F-OH systematics and higher abundant hydrous silicate minerals such as amphibole and biotite in the Tongchang pluton than the Shilicun pluton indicate that the parental magma of the Tongchang pluton has higher Cl and H2O content than that of the Shilicun pluton. Our results show that oxidized magma is important but this alone cannot make a porphyry Cu +/- Mo +/- Au ore deposit. High contents of H2O-Cl in magma and shallow depth of emplacement are also important. (C) 2019 Elsevier B.V. All rights reserved.