EXPERIMENTAL QUANTIFICATION OF THE PARTITIONING OF ZN BETWEEN DOLOMITE AND BRINE

Many of the world’s Zn deposits were precipitated from sedimentary brines. Knowledge of the Zn concentrations in these brines is fundamental to understanding how the deposits formed. Fluid inclusions offer a potential record of Zn concentrations in the brines. However, measuring Zn concentration reliably in fluid inclusions can be challenging because of the small size of the fluid inclusions, interferences from Zn-bearing accidental mineral inclusions within the fluid inclusions, or interferences from Zn in solid solution in the mineral host.

Element partitioning theory provides an alternate and potentially easier and more reliable means of determining Zn concentration in brine based on the concentration of Zn in a mineral precipitated from the brine. Dolomite is a strategic mineral for such an effort because dolomite is a common ore-stage gangue mineral in sediment hosted Zn deposits and because Zn is known to substitute readily for Mg in dolomite’s crystalline lattice. However, to date, experimental partition coefficients (D) needed to calculate the concentration of Zn in brine based on the concentration of Zn in dolomite have not been available.

The purpose of the current study was to determine D values for the partitioning of Zn between brine and dolomite under conditions typical for the formation of sediment-hosted Zn deposits. To this end, a series of dolomite precipitation experiments was performed at temperatures of 125, 150 and 200° C, and pressures up to 10 MPa using model sedimentary brines that contained 10, 100, or 1000 ppm Zn. Experimental precipitates were first analyzed using XRD to verify that they consisted primarily of dolomite; the dolomite precipitates were then analyzed using LA-ICP-MS to determine elemental composition. The compositions of the coeval fluids were determined using ICP-AES and ICP-MS. The Zn/Mg ratios measured in the dolomite precipitates and the coeval fluids were then used to calculate temperature specific D values. Initial results indicate an equilibrium D value of 75±10 at 200° C and a trend of declining D values at lower temperatures.