Mississippi Valley-type (MVT) deposits serve as important sources of lead-zinc metals, requiring abundant reduced sulfur for their formation. High density H2S fluids greatly influence the efficiency and scale of mineralisation, although they are rarely observed in sedimentary basins. In this study, we report high density (0.68-0.76 g/cm3), high purity H2S (similar to 100 %) fluid entrapped in quartz cement in the Ediacaran Doushantuo Formation dolomite, South China. In situ Raman spectroscopy identifies diagnostic peaks for coexisting liquid (similar to 2580 cm-1) and vapour (similar to 2610 cm-1) H2S phases in quartz hosted inclusions at 21 degrees C. Microthermometry shows that the triple point temperature (-85.1 to-83.2 degrees C) is consistent with pure H2S system. The high trapping temperature (134-158 degrees C) for H2S fluids and extremely negative delta 13C values of calcite cement indicate that the H2S fluids were formed through thermochemical sulfate reduction (TSR). The precipitation of calcite removes the TSR-derived CO2 to purify the H2S fluids. The formation of high density, high purity H2S fluids provides new insights for the metal-sulfide mineralisation in MVT deposits.