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Authigenic, euhedral tourmaline crystals are present in a core from a productive shallow gas reservoir in the Upper Cretaceous Eagle Sandstone of the Tiger Ridge field, north-central Montana. Detrital tourmaline is a common constituent of sands and sandstones, and although overgrowths on detrital grains have been reported elsewhere, discrete euhedral crystals of authigenic tourmaline formed in an unmetamorphosed sandstone have not been described. The tourmaline occurs as acicular or prismatic crystals of dravite which are 10 to 20 µ in length and 1 to 5 µ in diameter. The dravites usually occur in intergranular pore spaces and exhibit typical tourmaline crystal habit. An authigenic origin for these crystals is suggested by their (1) delicately euhedral morphology, (2) unabra ed appearance, (3) occurrence in growth positions, (4) similarity to dravite overgrowths within the same rock, and (5) close association with other authigenic phases.
The presence of igneous rocks in the vicinity of the well, combined with the absence of tourmaline in several wells not associated with igneous rocks at Tiger Ridge field, suggests that Eocene volcanism in the Bearpaw Mountains was the source for boron, which facilitated tourmalinization. This interpretation has implications relative to the timing of gas emplacement in the Eagle Sandstone at Tiger Ridge. The gas was generated by bacteria during Late Cretaceous time; then adjacent Eocene volcanism caused tourmalinization. The gas was then remigrated into gravity-slide fault blocks in response to the Bearpaw Mountains intrusion and uplift.
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