A relict mound of Holocene barite (BaSO 4 ) tufa underlies the Flybye Springs, a small, barium-rich, cold sulphur spring system in the Northwest Territories of Canada. The tufa is composed of relatively pure barite with £0AE34 wt% Ca 2+ and £0AE77 wt% Sr 2+ . The mound is made up of coated bubble, raft, undulatory sheet, stromatolitic, coated grain and detrital conglomerate barite tufa. Although previously unreported in barite, these lithotypes are akin to facies found in many carbonate spring deposits. Raft and ooid-coated grain tufa was formed via 'inorganic' barite precipitation in spring water ponds and tributaries where rapid oxidation of sulphide to sulphate established barite supersaturation. Undulatory sheet tufa may have formed by the reaction of dissolved barium with sulphate derived from the oxidation of extracellular polysaccharide-rich colloidal sulphur films floating in oxygenated, barite-saturated spring water ponds. Coated bubble, oncoid-coated grain and stromatolitic tufa with filamentous microfossils was formed in close association with sulphur-tolerant microbes inhabiting dysoxic and oxygenated spring water tributaries and ponds. Adsorption of dissolved barium to microbial extracellular polysaccharide probably facilitated the development of these 'biogenic' lithotypes. Detrital conglomerate tufa was formed by barite cementation of microdetrital tufa, allochthonous lithoclasts and organic detritus, including caribou hair. Biogenic textures, organic artefacts and microfossils in the Flybye barite tufa have survived diagenetic aggradational recrystallization and precipitation of secondary cements, indicating the potential for palaeoecological information to be preserved in barite in the geological record. Similarities between the Flybye barite tufa and carbonate spring deposits demonstrate that analogous textures can develop in chemical sedimentary systems with distinct mineralogy, biology and physiochemistry.

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