EXPERIMENTAL QUANTIFICATION OF THE PARTITIONING OF COPPER BETWEEN DOLOMITE AND SEDIMENTARY BRINE AT TEMPERATURES OF 125 TO 200° C AND 100 BARS

The formation of sediment-hosted base metal ore deposits is significantly influenced by the composition of their parent sedimentary brines, particularly their concentrations of base metals. Dolomite is a common ore-stage gangue mineral in sediment-hosted base metal deposits and thus may provide insights into the ore-forming brines’ composition. This study aims to calculate empirical partition coefficient values for copper exchange between brine and dolomite under conditions typical of the formation of sediment-hosted base metal deposits. Because dolomite incorporates copper in proportion to its concentration in the brine, by knowing the pertinent partition coefficient, the concentration of copper in the dolomite, and the concentration of magnesium in both dolomite and the brine, it is possible to calculate the concentration of copper that was in the brine. Synthetic sedimentary brine containing approximately 21 weight % total dissolved solids, super-saturated with respect to dolomite, and with copper concentrations ranging from 100 to 1000 ppm, was used to precipitate dolomite. Experiments were performed in 100 mL vessels made of Hastelloy and titanium at a constant pressure of 100 bars. Experiments have so far been conducted for periods of 10, 20, 40, and 90 days, with the goal of achieving equilibrium, at temperatures of 200°, 175°, and 150° C for copper concentrations of 100 and 1000 ppm, and at 125° C for a copper concentration of 1000 ppm. The formation of ordered dolomite was confirmed by powdered X-ray diffraction (PXRD) analysis. The dolomite composition, including its copper content, was analyzed through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The average partition coefficient values found from the experiments were 200, 860, and 78 with standard deviations of 48.2, 590, and 9.5 at temperatures of 200°, 175°, and 150° C, respectively. The partition coefficient values determined thus far have relatively low precision and have not yet stabilized as a function of experimental duration, indicating that still longer experiments are needed to reach equilibrium. Nonetheless, these results represent valuable progress toward producing a versatile and convenient tool for determining copper concentration in ancient brines that precipitated dolomite.