EXPERIMENTAL QUANTIFICATION OF THE PARTITIONING OF COPPER BETWEEN DOLOMITE AND SEDIMENTARY BRINE AT TEMPERATURES OF 150 TO 200° C

Dolomite is a common ore-stage gangue mineral in sediment-hosted base metal deposits. Although base metals are precipitated mainly as sulfide minerals, base metals may also be incorporated into dolomite in proportion to their concentration in the parent fluid. Therefore, if a partition coefficient can be known for the exchange of base metals between a fluid and the dolomite that the fluid precipitates, the base metal concentration in the fluid can be calculated if the Ca or Mg concentration in the fluid is also known. This study aims to determine empirical partition coefficient (D) values for the exchange of copper between brine and dolomite under conditions that form sediment-hosted base metal deposits. In this study, dolomite is experimentally precipitated from a synthetic sedimentary brine containing about 21 weight % total dissolved solids, which is modeled after the ore fluids of Mississippi Valley-type deposits in the US mid-continent, but with a higher content Mg concentration to promote dolomite precipitation. Copper concentration varies from 100 to 1000 ppm and pressure is maintained at 100 bars. The experiments are conducted in two 100 mL vessels, one made of Hastelloy and the other made of titanium. The experiments are conducted for durations of 10, 20, and 40 days as needed until equilibrium is reached. To date, the experiments have been successfully carried out at 200° C and 175° C for copper concentrations of 100 and 1000 ppm, and at 150° C for a copper concentration of 1000 ppm. To confirm the formation of ordered dolomite, the experimental precipitates are examined using X-ray diffraction (XRD). Following this, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is used to analyze the composition of these precipitates, including their copper (Cu) content. This analysis allows for the calculation of the partition coefficient (D), which is 0.90 , 0.986, and 0.96 with standard deviations of 0.30, 0.106, and 0.074 at temperatures of 200, 175, and 150° C, respectively. These partition coefficient values can provide a convenient way of determining Cu concentration in brines under typical sedimentary basinal conditions, and can be particularly useful when Cu concentrations cannot be determined from fluid inclusions.