EXPERIMENTAL QUANTIFICATION OF THE PARTITIONING OF CU BETWEEN DOLOMITE AND SEDIMENTARY BRINE

Many sediment-hosted base metal deposits contain dolomite that paragenetically overlaps the stage of base metal sulfide mineralization. Thus, if the base metal content of the dolomite can be quantified, the base metal content of the parent hydrothermal fluid can also be quantified, provided that an appropriate partition coefficient is available. The present research is focused on obtaining empirical partition coefficient (D) values for the partitioning of Cu between brine and dolomite at conditions typical of the formation of sediment-hosted base metal deposits. In this research, dolomite is experimentally precipitated from a synthetic sedimentary brine containing about 21 weight percent total dissolved solids, modeled after the ore fluids of Mississippi Valley-type deposits in the U.S. mid-continent, but slightly enriched in Mg to promote dolomite precipitation. The Cu concentration of the brine varies between 100 and 1000 ppm. The dolomite precipitation experiments are carried out in 100 mL Hastelloy and titanium reaction vessels at temperatures between 125° and 200° C and a constant pressure of 100 bars over 10, 20 and 40 days. Experimental precipitates are analyzed using XRD to confirm that they are ordered dolomite. The precipitates are then analyzed using LA-ICP-MS to determine their composition, including their Cu content, from which D can be calculated. So far, experiments have been completed at 200° C for Cu concentrations of 100 and 1000 ppm. The calculated D values are very consistent as a function of Cu concentration in the brine and experimental duration, with a mean of 0.90 and a standard deviation of 0.03. This indicates that the experiments reached equilibrium very quickly, well within the 40-day maximum duration. This D value could then be used to determine the Cu concentration of a natural brine that precipitated dolomite at a temperature of about 200° C and a pressure of about 100 bars, provided that the Cu and Mg content of the dolomite could be measured, as well as the Mg content of the brine, e.g., from fluid inclusions hosted by a cogenetic mineral that does not contain Mg in its matrix.