Magnesiofoitite from the uranium deposits of the Athabasca Basin, Saskatchewan, Canada

Two generations of tourmaline have been observed in the matrix of altered breccias associated with the uranium deposits of the Athabasca Basin, Saskatchewan, Canada. The earlier tourmaline occurs as isolated crystals, whereas the later stage is represented by radial (or spherulitic) aggregates of fibrous crystals or overgrowths on earlier tourmalines. Tourmaline in associated veins closely resembles the late-stage tourmaline in the matrix of brecciated rocks. Atom proportions (based on electron-microprobe data) normalized to 15 Y + Z + T cations suggest that the earlier tourmaline is partially deprotonated, whereas the late-stage tourmaline is highly alkali-deficient. Assuming a stoichiometric content of boron, the structural formulas calculated for early and late-stage tourmalines from Rabbit Lake (sample R81) are: Na0.48K0.01Ca0.51 (Mg2.48Fe0.213+Mn0.01Ti0.24) Al5.34Fe0.663+ (Si-6.06)O-18(BO3)(3)(OH)(2.68) and square 0.76Na021K0.01Ca0.02 (Mg2.15Fe3+ Al-0.05(0.80)) Al-6.00 (Si5.89Al0.11)O-18(BO3)(3)(OH)(4.01). Similar compositions have been observed at the Second Link Lake and Key Lake uranium deposits. Early-stage tourmalines consist of proton-deficient dravite, whereas the late-stage tourmalines are close to the magnesiofoitite end-member of the tourmaline group.