Setting and formation of the earliest Neoproterozoic rifted arc Pingshui VMS deposit, South China

Abstract

The Neoproterozoic Pingshui volcanogenic massive sulfide (VMS) deposit (Cu and Zn metals of 0.40 Mt) represents one of the best preserved VMS deposits in the northeastern margin of the Jiangnan Orogen, South China. This study reconstructs its magmatic and metallogenetic history and geodynamic evolution using new geological, whole-rock geochemical, and zircon U–Pb–Lu–Hf isotopic analyses. Two lithotectonic units are identified: the ore-hosting Pingshui Formation composed of a bimodal basalt-rhyolite suite and pyroclastic rocks, and the post-ore extrusive-intrusive assemblage including high-Mg basalt, Nb-enriched basalt, high-Mg diorite and felsic granitoids. The formation age of Pingshui Formation basalt to andesite is newly constrained at ∼977 ± 15 Ma by the least radiation-damaged zircon. The new and compiled data suggest that these post-ore rocks were generated during ∼932–904 Ma after a possible magmatic quiescence. In terms of geochemical characteristics, both the Pingshui Formation basalt to andesite (∼977 Ma) and late high-Mg basalt (∼921 Ma) are enriched in light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, P, Ti, Zr and Hf). The late high-Mg diorite (∼932 Ma, MgO = 5.84–6.91%) has moderate (La/Yb)N ratios (13.7–14.1), resembling those of high-Mg andesites (HMA). These geochemical signatures indicate that they originated from a heterogeneous mantle wedge metasomatized by slab-derived fluids and melts. The Nb-enriched basalt (∼916 Ma) was derived from a mixture of OIB- and arc-like mantle sources as evidenced by relatively high Ti/V, Ti/Zr and Zr/Y ratios. The geochemical changes from a dominant subduction influence to asthenospheric involvement, corresponding to the mafic-intermediate rocks (977–921 Ma) and the Nb-enriched basalt (916 Ma) respectively, is indicative of a possible slab roll-back that triggered asthenospheric upwelling. Furthermore, the felsic rocks originated from partial melting of juvenile mafic lower crust or fractional crystallization of parental mafic melts. Their anomalously high temperatures of formation (over 950 °C), which are indicated by zircon saturation thermometry, suggest a long-lasting high heat flow environment. In summary, subduction of young oceanic crust and intra-arc rifting are likely mechanisms to account for the formation of the high temperature magmatic rocks and massive sulfide ores at Pingshui.