In-situ pyrite trace element and sulfur isotope characteristics and metallogenic implications of the Qixiashan Pb-Zn-Ag polymetallic deposit, Eastern China

The Qixiashan Pb-Zn-Ag deposit (2.6 Mt at 13.4% Pb + Zn and 3.9 Kt at 236.5 g/t Ag) in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRB) is the largest Pb-Zn-Ag polymetallic deposit in Eastern China. The stratabound orebodies are hosted in the Carboniferous carbonate units. Whether this deposit and other sediment-hosted deposits in MLYRB are associated with Late Mesozoic magmatism or Carboniferous sedimentary exhalative events is still disputed, due to the poor constraints on their ore-material source and metallogenic processes. In-situ pre-/syn-ore pyrite S-isotopic and trace-element data from Qixiashan favor a seawater sulfate origin over a magmatic-hydrothermal one. Framboidal pyrite (Py1) is likely diagenetic, as evidenced by its low Co/Ni ratio (< 1), size (~4 mu m), and negative delta 34S values (-27.4 to-2.2%o) caused by bacterial sulfate reduction (BSR). Pre-ore colloform (Py2) and massive (Py3) pyrites have high Mn concentrations, whereas syn-ore pyrites (Py4 and Py5) have elevated Pb, Zn and Cu grades. The low Co/Ni values of the pre-/syn-ore pyrites indicate that both the Fe and base metals are of non-magmatic diagenetic origin. Sulfur isotope fractionation in the fluid may have reached equilibrium in the Pb-Zn-Ag metallogenic stage. Equilibrium fractionation calculation suggests that the positive shift of delta 34S values from Py2 to Py5 (10.2 to + 22.2%o) is primarily caused by thermochemical sulphate reduction (TSR). Pre-ore pyrites were likely formed in early carboniferous diagenesis, and then enclosed by epigenetic pyrites. Iron in the syn-ore pyrites was most likely recycled from sediments or pre-ore pyrites via dissolution-leaching or replacement processes. During the Qixiashan Pb-Zn-Ag mineralization, the magmatic-hydrothermal fluid is unlikely a major base metal source.