PREDICTION OF MVT ORE FLUID METAL CONCENTRATIONS FROM SOLID SOLUTION METAL CONCENTRATIONS IN ORE-STAGE CALCITE

Mississippi-Valley-type (MVT) deposits contain some of the greatest enrichments of Pb, Zn, Ba, and F in the Earth’s crust. Fundamental to understanding how these elements were transported and precipitated to form MVT deposits is knowledge of their concentrations in the ore fluids. Recent research aimed at determining the concentrations of Pb, Zn, and Ba in the ore fluids that formed the MVT deposits of the U.S. mid-continent, the type examples for the MVT deposit class, has focused on using LA-ICP-MS to analyze fluid inclusions. This research has shown U.S. mid-continent MVT ore fluids to have Ba concentrations on the order of 10’s of ppm. However, LA-ICP-MS results for Pb and Zn concentrations are equivocal due to interferences from Zn and Pb in the host mineral matrix and uncertainties about whether the measured Pb and Zn signals represent aqueous solute or “accidentals”, i.e. Pb or Zn solid particulates entrained within the fluid inclusions. In light of these limitations, this study sought to determine metal concentrations in MVT ore fluids instead by calculating them theoretically based on their solid solution concentrations in ore-stage calcite.

Using experimental distribution coefficients from Rimstidt et al. (1998) at 100° C and measured compositions of ore-stage calcite from the Illinois-Kentucky MVT district, concentrations of Mg, Mn, Fe, Sr, Ba, and Pb in the ore fluid were predicted. The predicted ore fluid concentrations of Mg and Mn, which form carbonate minerals (magnesite and rhodochrosite) with the calcite structure, were in good agreement with available fluid inclusion data for these elements. Thus, the predicted ore fluid concentrations of 1’s of ppm Zn and 0.1’s to 1’s of ppm Fe, which also form carbonate minerals (smithsonite and siderite) with the calcite structure, are likely to be accurate. These Zn concentrations are typical of modern sedimentary brines and high enough to allow efficient Zn ore formation. In contrast, the predicted ore fluid concentrations of Sr and Ba, which form carbonate minerals (strontianite and witherite) with the aragonite structure, were in poor agreement with available fluid inclusion data for these elements. Thus, the predicted 1’s of ppm ore fluid concentration of Pb, which also forms a carbonate mineral (cerussite) with the aragonite structure, is unlikely to be accurate.