POTENTIAL FOR THE PREDICTION OF ZN CONCENTRATIONS IN MISSISSIPPI VALLEY-TYPE ORE FLUIDS FROM SOLID SOLUTION ZN CONCENTRATIONS IN GALENA
Galena samples from these deposits were analyzed using electron microprobe and laser ablation inductively coupled plasma mass spectrometry. None of the galena samples contained detectable concentrations of Mg or Mn, preventing absolute Zn concentrations from being determined. However, Zn was consistently detected in the galena samples on the order of tenths of ppm, allowing aqueous Zn/Pb ratios in the ore fluids to be predicted. Median Zn/Pb mass ratios varied from 0.13 to 1.3 for the six MVT ore districts considered. This variation in ore fluid Zn/Pb ratios is much smaller than the variation in Zn/Pb ratios of the solid ores of the six MVT districts, which range from a low of 0.08 for the Southeast Missouri district to a high of 16 for the Northern Arkansas district. Ore fluid Zn/Pb concentrations predicted in the present study are much lower than the Zn/Pb concentration of about 5 in modern sedimentary brines, and of about 4 to 5 in gangue mineral-hosted fluid inclusions in the Tri-State and Northern Arkansas districts. The consistently low ore fluid Zn/Pb ratios predicted in the present study relative to Zn/Pb ratios in modern sedimentary brines, MVT fluid inclusions, and MVT ores raises doubts about the reliability of determining Zn/Pb ratios in MVT ore fluids from Zn/Pb ratios in galena. The predicted low ore fluid Zn/Pb ratios may indicate the presence of solvus (known to exist at high temperatures ~1040 °C but not demonstrated at MVT temperatures) that limits the solubility of Zn in galena to concentrations too low to allow accurate quantification of Zn in the ore fluid.