Precipitation mechanisms of barite in sulfate-sulfide deposits in back-arc basins

Abstract

Barite was synthesized at 150°C using native sulfur and barium chloride solutions. The degree of supersaturation with respect to barite was changed by varying the barium chloride concentrations. Dendritic (rod-like, spindle-like, star-like, and cross-like) barite crystals with rough surfaces, where the crystal growth is controlled by the diffusional transport of solute species from the bulk of the solution to the surfaces of the growing crystals, formed from aqueous solutions with high degrees of supersaturation. In contrast, well-formed rectangular, rhombohedral, and polyhedral crystals with smooth surfaces, where a surface reaction mechanism predominates, precipitated from solutions with low degrees of supersaturation. The morphology of barite from sulfate-sulfide deposits (Kuroko and Mariana deposits) was investigated by scanning electron microscopy measurement. No dendritic crystals occurred in the ore deposits, while rectangular and tabular large crystals which consisted of fine-grained disk-like barite microcrystals were dominant. The morphologies of barites in the sulfate-sulfide deposits, and the experimentally determined relationship between morphology and S.I. (saturation index) indicate that the barites precipitated from hydrothermal solutions with S.I. < ca. 100. The estimated S.I. is in agreement with that estimated by Ohmoto et al. (1983) based on isotopic (Sr, O, S) data on barites from the Kuroko deposits and on a fluid mixing model.