SYNTHESIS OF BARIUM-STRONTIUM SULPHATE SOLID SOLUTION AND SOME CONSIDERATIONS ON THE GENESIS OF BARITE-CELESTITE DEPOSITS IN NEUQUEN PROVINCE, ARGENTINA

Experiments on the synthesis of barium-strontium sulphate solid solution were performed. The results showed that the cell constants a0 and c0 of the solid solution series vary linearly with respect to chemical composition and corresponding changes in b0 follow a curve with gentle slope. Since the synthesized mineral was richer in barium than the solution, the [Sr2+]:[Ba2+] ratio of the solution became gradually higher as crystallization of the mineral progressed. The results also showed that the distribution coefficient kBaSr increased with increasing temperature. It was observed that crystallization of the mineral with intermediate [BaSO4]:[SrSO4] molar ratio ranging from about 1:1 to 1:4 at 270°C and from about 3:1 to 1:9 at 170°C, was extremely more difficult than the minerals outside these ranges. This indicates a possibility that at these ranges an immiscibility area exists. It appears that the area is enlarged with decrease in temperature. Many barite-celestite deposits are found in Neuquen province, Argentina. Some of them are of vein-type traversing stratified rocks. Others have bedded shapes, roughly concordant with stratification and formed by selective replacement of limestone seams by barite and celestite. Measured homogenization temperature of liquid inclusions in barite and celestite, their chemical compositions, wall-rock alterations, the associated sulphide minerals and the results of the present synthesis experiments suggest that these deposits were formed under the following conditions: (1) Some of the ore deposits were formed by hydrothermal solutions poor in strontium at rather high temperatures; (2) Some others were formed by hydrothermal solutions rich in strontium at rather high temperatures; and (3) the rest of the deposits were formed by hydrothermal solutions rich in strontium at rather low temperatures.