Abstract

The sulphur isotope ratios of barite in many syngenetic barite deposits are shown to be approximately the same as those of the seawater from which they were deposited. Although the stratiform replacement barite-fluorite deposits of northeast British Columbia are epigenetic, their barite sulphur isotope ratios (δ34S) of +24.2 to +30.1‰ may indicate a late Middle Devonian time of mineralization if the sulphur in these deposits was derived from seawater.A model for barite mineralization is proposed in which mixing of euxinic (i.e., reducing) seawater with meteoric groundwater near the seaward edge of a large coastal aquifer, similar to the present-day Floridan Aquifer, caused solution of the shelf carbonates and induced low-temperature precipitation of barite in the resultant solution cavities. The extensive solution collapse that accompanied mineralization, and the coarse crystallinity and low manganese content (< 65 ppm) of the barite in northeastern British Columbia support this interpretation. The postulated Devonian paleoaquifer in northeastern British Columbia coincided with the regional distribution of solution-collapse breccias in the Stone Formation dolomite. The karsted upper surface of the Sulphur Point Formation may have been the landward recharge area for this aquifer.The variation in the mole percent SrSO4 along barite blades suggests that during precipitation there were changes in the rate of circulation of the mineralizing solution. In some places up to 30% of the barium in solution may have been precipitated. The δ34 values of later formed, strontium-enriched barites tend to be lower suggesting an isotope effect during crystallization favouring 34S in the crystalline phase.

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