The Flybye Springs, Northwest Territories, consist of 10 active vents and numerous small seeps that discharge sulphide- and barium-rich spring waters at an average temperature 8.5 °C. Oxidation of sulphide to sulphate drives precipitation of stellate and platy barite microcrystals in the proximal flow paths. Downstream, and in vent- and tributary-fed ponds, barite is precipitated among streamer and mat forming colonies of sulphur-tolerant microbes, including Thiothrix, Beggiatoa, Thioploca, Chromatium, Oscillatoria, fungi (dominantly Penicillium), and unicellular sulphate reducing bacteria. These microbes mediate barite saturation by adjusting redox gradients and via passive adsorption of barium ions to cell surfaces and extracellular polymeric substances. Passive biomineralization produces barite laminae in floating microbial mats, nanometric coatings, and micrometric encrustations around microbial cells and filaments, and local permineralization of Thiothrix, Beggiatoa, and Oscillatoria outer cell walls. Intracellular barium enrichment and (or) metabolic sulphur oxidation may be important to “active biomineralization” that produces nanometric barite globules on the tips of fungal hyphae, barite-filled cell cavities in Beggiatoa and Thiothrix, and baritized sulphur globules. Degradation of biomineralized cells generates detrital “microfossils,” including barite tunnels, layered cylinders, solid cylindrical grains and chains of barite beads. The diversity of inorganic and biomineralized barite in the Flybye Springs flow path highlights the influence of ambient chemistry, microbial metabolism, and cellular structure on barite solubility and on the taphonomy of microfossils preserved in barite.