Origin of radioactive barite sinter, Flybye springs, Northwest Territories, Canada

Cold springs emerging a long the contact between Devonian limestone and shale units in the northwestern Canadian Cordillera are presently depositing a radium-enriched barite sinter. A geological cross section through the springs area shows that groundwaters could circulate through a mainly limestone aquifer to depths of approximately 2 km. Some shales and volcanic rocks associated with the aquifer contain: barium, bound in feldspars; barite, pyrite, and organic matter hosted in shale; and radium in feldspars or produced by the radioactive decay of uranium associated with organic matter hosted in shale. Spring waters are of the [Formula: see text] type characteristic of water that has equilibrated with clay minerals. A subsurface equilibration temperature of 34 °C was determined by silica geothermometry, and 31 °C by magnesium-corrected Na+–K+–Ca2+ geothermometry. Emerging waters are partly mixed with surface runoff and therefore these temperatures represent only minimum values. Assuming a normal geothermal gradient these temperatures indicate minimum groundwater percolation depths of 1 km. The δ34S values of barite sinter samples and one sample of aqueous sulphide range from + 15 to + 23‰, indicating a marine sedimentary rock source for sulphur. The corresponding δ18O values are negative, implying that the bulk of the sulphate oxygen is derived from groundwater during sulphide oxidation. These data suggest that the springs are fed by groundwaters that have percolated to depths of as much as 2 km, passing through an aquifer of Paleozoic marine sedimentary rocks and volcanic rocks. At depth these waters were reducing and probably weakly acidic. They dissolved barium, sulphur, and radium, which were transported to the surface where the water quickly oxidized and precipitated Ba(Ra)SO4.