Lead and Zinc in Hydrothermal Fluids

Lead and zinc mineralization have been documented in low-temperature Mississippi Valley type (MVT), Volcanogenic Massive Sulfide (VMS), and high-temperature porphyry and skarn ore deposits. These deposits are characterized by the precipitation of galena (PbS) and sphalerite (ZnS) from a saline hydrothermal fluid. The direct relationship between metal concentration and the total chloride of the fluid has been documented previously; however, the role of acidity has not been studied extensively. Experiments were conducted in René 41 cold-seal pressure vessels at temperatures of 200, 300, and 500 °C and a pressure of 100 MPa to provide better constraints on the formation of galena and sphalerite in hydrothermal systems spanning a range of fluid acidities (pH). The concentrations of Pb and Zn in the synthetic hydrothermal fluids were determined at galena and sphalerite saturation as a function of HCl and at a total chloride of 15 wt.% NaCleq.. Zn concentrations ranged from 1.7 (±0.3) × 102 µg/g at 200 °C and an HCl concentration of 2.28 × 103 µg/g to 2.55 (±0.5) × 103 µg/g at 500 °C and an HCl concentration of 3.40 × 104 µg/g. Pb concentrations were 1.8 (±0.4) µg/g at 200 °C and a HCl of 2.28 × 103 µg/g and increased to 7.93 (±1.5) × 103 µg/g at 500 °C and a HCl concentration of 3.40 × 104 µg/g. Zn/Pb mass ratios in the fluids at sphalerite and galena saturation decreased with increasing temperature. The experimental data demonstrate that the concentration of Pb and Zn in the fluid increase with both temperature and HCl concentration and, consequently, decrease with increasing pH. These results demonstrate that acidic fluids can transport substantially greater concentrations of Pb and Zn than neutral or basic fluids. Experimentally determined slopes of Pb and Zn concentrations as a function of HCl in the fluid provide empirical measurements of the apparent dependence of metal solubility on HCl at a constant total salinity. These results are consistent with Pb and Zn being transported predominantly as chloride-complexes under acidic, although the experiments do not directly determine aqueous metal speciation. Consequently, the experimentally determined HCl dependencies should be interpreted as empirical measures of apparent metal solubility rather than direct measurements of speciation or ligand coordination. The observed dependence of dissolved metal concentrations on HCl likely reflects the combined effects of chloride complexation, increasing HCl association with increasing temperature, non-ideal solution behavior, and changes in the distribution of dissolved chloride- and possibly sulfur-based complexes. Therefore, the neutralization of an acidic Pb- and Zn-bearing, chloride-rich hydrothermal fluid could produce substantial galena and sphalerite mineralization if sufficient reduced sulfur is available. In hydrothermal fluids depleted in reduced sulfur, H2S must be supplied through sulfate reduction or by mixing with an H2S-rich fluid to promote galena and sphalerite precipitation.