EXPERIMENTAL CONSTRAINTS ON LEAD-ZINC HYDROTHERMAL DEPOSITS

Lead and zinc mineralization has been documented in low-temperature Mississippi Valley Type (MVT) deposits, moderate temperature massive sulfides (VMS) and higher-temperature porphyry systems. These deposits are characterized by the presence of galena (PbS) and sphalerite (ZnS) that formed from a salt-bearing hydrothermal fluid. The Pb and Zn are most likely transported as PbCl₂ and ZnCl₂ before precipitating as PbS and ZnS through a decrease in temperature, an increase in pH, or the addition of reduce sulfur. The HCl concentration (pH) of the hydrothermal fluid plays a major role in the concentration of Pb and Zn carried in the fluid, however, there are few data detailing the solubility of PbS and ZnS in acidic hydrothermal fluids at temperatures applicable to MVT and porphyry systems.

Experiments were conducted in René 41 cold-seal pressure vessels from 200 to 500°C and 100 MPa to determine the concentrations of Pb and Zn in hydrothermal fluids as a function of HCl. Platinum capsules were loaded with natural galena and sphalerite and an aqueous fluid of 13-15 wt.% NaCl (eq.) containing HCl + NaCl + H₂O. The aqueous fluids were captured at the conclusion of the experiment and Pb and Zn concentrations were determined by using AA and ICP-OES.

Pb concentrations were 10.8 ± 1.1 μg/g (HCl = 4700 μg/g), 16.2 ± 1.6 (HCl = 54000 μg/g), and 6153 ± 1226 μg/g (HCl = 54000 μg/g) at 200, 300, and 500 °C, respectively. Zinc concentrations were found to be 1266 ± 127 μg/g (HCl = 9900 μg/g), 1790 ± 628 μg/g (HCl = 54000 μg/g), and 2175 ± 380 μg/g (HCl = 54000 μg/g), respectively. The data demonstrate that the concentration of Pb and Zn in the fluid increased directly with temperature, total Cl^- and HCl concentration. Therefore, decreasing temperature and HCl (increase pH) are efficient at inducing the precipitation of galena and sphalerite in MVT and porphyry systems. The variable Pb:Zn ratios observed in some ore-bearing systems are possibly the result of differences in the rate of change of temperature, acidity, and/or reduced sulfur and not from separate pulses of distinct mineralizing fluids.