GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 65-12
Presentation Time: 9:00 AM-5:30 PM


SMITH, Sarah Elisabeth, Department of Geological Sciences, University of Missouri--Columbia, 101 Geological Sciences Bldg., Columbia, MO 65211, APPOLD, Martin S., Department of Geological Sciences, University of Missouri--Columbia, 101 Geological Sciences Bldg, Columbia, MO 65211 and KLYUKIN, Yury I., Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061,

Knowledge of the concentration of Zn in Mississippi Valley-type (MVT) ore fluids is fundamental to understanding MVT ore genesis but has been difficult to determine unambiguously. Published thermodynamic data in theory allow Zn concentrations in MVT ore fluids to be determined from measured Zn concentrations in galena and from measured Mg or Mn concentrations in galena and the ore fluid, e.g. via fluid inclusions. The purpose of the present study was to use this data to try to determine Zn concentrations in the ore fluids of six MVT ore districts in the U.S. mid-continent: the Central Tennessee, Central Missouri, Northern Arkansas, Tri-State, Illinois-Kentucky, and Southeast Missouri districts.

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.