EXPERIMENTAL CONSTRAINTS ON LEAD-ZINC DEPOSITS OF THE ILLINOIS BASIN

Extensive lead and zinc mineralization have been documented in low-temperature Mississippi Valley Type (MVT) deposits of the Illinois basin. These deposits are characterized by the presence of galena (PbS) and sphalerite (ZnS) that formed from a saline hydrothermal fluid. The precipitation of galena and sphalerite is in response to either a change in temperature (not thought to be important in these deposits), pH of the fluid (nominally HCl) or reduced sulfur (nominally H₂S). The precipitation mechanism, reduced HCl or increased H₂S, control where the ore is located and is one of the controls on the ore’s Pb/Zn ratio. To provide better constraints on the formation of galena and sphalerite, we conducted experiments in René 41 cold-seal pressure vessels at 200, 300, and 500 °C. The concentrations of Pb and Zn in synthetic hydrothermal fluids were determined as a function of HCl and at H₂S. Pb concentrations were 11±1 μg/g (HCl = 4.7x10³ μg/g), 16±2 (HCl = 5.4x10⁴ μg/g), and 7(±1)x10³ μg/g (HCl = 5.4 x10⁴ μg/g) at 200, 300, and 500 °C, respectively. Zinc concentrations were found to be 1.3(±0.1)x10³ μg/g (HCl = 9x10³ μg/g), 1.8(±0.6)x10³ μg/g (HCl = 5.4x10⁴ μg/g), and 2.2(±0.4)x10³ μg/g (HCl = 5.4x10⁴ μg/g), respectively. The data demonstrate that the concentration of Pb and Zn in the fluid increased directly its HCl concentration and with temperature. Equilibrium constants were determined from the data at each temperature and used to model the formation of galena and sphalerite in natural environments. Decreasing HCl (increase pH) and increasing H₂S are both efficient at inducing the precipitation of galena and sphalerite in MVT systems. The presence of carbonate host rocks will favor precipitation due to HCl neutralization as long as reduced sulfur is carried in the fluid or present in the host rock. A fluid depleted in reduced sulfur must obtain H₂S from another source (fluid mixing or from the host rock) for the formation of galena and sphalerite. Additionally, the variable Pb:Zn ratios observed in some Illinois basin deposits are partially from differences in the rate of change of acidity and/or reduced sulfur due to host rock reactions.