PARTITIONING THEORY AS A TOOL FOR DETERMINING BASE METAL CONTENT IN HYDROTHERMAL FLUIDS: EXAMPLES FROM SEDIMENT-HOSTED MINERAL DEPOSITS IN THE NORTH AMERICAN MIDCONTINENT, CORDILLERA, AND SOUTH AUSTRALIA

The metal content of hydrothermal fluids is among the most important characteristics to know in order to determine how these fluids precipitate mineral deposits. In many mineral deposits, fluid inclusions can provide metal content information, commonly through analysis by LA-ICP-MS. However, in many other mineral deposits, suitable fluid inclusions for analysis either do not exist or yield equivocal results. Partitioning theory can provide an easier, alternative way for determining metal content in hydrothermal fluids, provided that a pertinent experimental partition or thermodynamic distribution coefficient is available, the concentration of a major element common to both a mineral and the fluid is known, and the concentration of the metal in the mineral can be measured. Partition coefficients as a function of temperature for the partitioning of several base metals between calcite and water have been published and experiments are currently underway for dolomite. For calcite, studies show that the partition coefficients that give the most accurate results are for metals like Zn and Fe that form carbonate minerals with the calcite structure. Partition coefficients that give the least accurate results are for metals like Pb and Ba that form carbonate minerals with the aragonite structure. Application of this theory to ore-stage calcite from the Illinois-Kentucky and Central Tennessee Mississippi Valley-type districts predicts the hydrothermal ore fluids to have had Zn and Fe concentrations of up to 10’s of ppm. These concentrations are typical of those in modern sedimentary brines. In contrast, Zn concentrations calculated for hydrothermal fluids in the Lemhi Pass (Idaho-Montana) thorium and rare earth element district are much greater than those in sedimentary brines, ranging between about 200 and 30,000 ppm. These concentrations fit better with a magmatic, and perhaps specifically a carbonatite origin for the ore fluids. Zinc concentrations calculated for the hydrothermal ore fluid in the Beltana hypogene non-sulfide zinc deposits in South Australia are on the order of tens of thousands of ppm. This is consistent with the high theoretical solubility of willemite, the main zinc ore mineral, in silica saturated fluids and suggests that very sulfide-poor fluids are needed to form willemite-rich Zn deposits.