Geochronology, geochemistry and zircon Hf-O isotopic composition of ore-bearing volcanic rocks at Dapingzhang VMS Cu-Zn deposit, SW China: Petrogenetic, metallogenic and tectonic implications

Abstract

The Dapingzhang Cu-Zn deposit is exposed in the eastern Proto-Tethys Ocean (PTO) tectonic domain. The mineralization is hosted by felsic volcanics that occur in bimodal association with minor basalt. Styles of mineralization include syngenetic banded massive and fragmented sulfides and epigenetic veins, stockworks and disseminations. The deposits are classified as bimodal-felsic (Kuroko type) VMS deposit. Two rhyolite rock samples yielded zircon U-Pb ages of 413.2 ± 1.7 Ma and 430.7 ± 4.1 Ma, revealing that the ore-related magmatism took place in the mid-late Silurian. Geochemically, the submarine sodium-rich volcanic rocks are strong peraluminous and belong to the calc-alkaline series. The rocks are characterized by enrichment in large-ion lithophile elements (LILEs, Ba, Th, and U), depletion in high-field-strength elements (HFSEs, Nb, Ta, Ti, Th, and P) and LREEs, and have affinities with subduction-related arc setting. The zircons yield εHf(t) values of +8.17 to +15 and two-stage model ages of 457–881 Ma. Oxygen isotopes of zircon grains show a mean δ18Ozircon of 5.20 ± 0.23‰ with variations from 3.67‰ to 7.77‰, most of which is similar to the mantle-like value of 5.3 ± 0.6‰ (2σ). The felsic volcanic rocks of the Dapingzhang deposit can be attributed to partial melting of the juvenile lower crust, while minor amounts of asthenospheric mantle components were introduced into their magma sources. Taking into account previous data, we propose that the volcanic rocks were generated in a back-arc extension setting, which provides reliable evidence for the Early Paleozoic an-back-arc system in the northeastern margin of Gondwana. The PTO may have been bipolar-subducted beneath the Sibumasu and Indochina blocks.