Geologic and Fission-Track Evidence for Late Cretaceous Faulting andMineralization, Northeastern Flank of Blacktail Mountains, Southwestern Montana

摘要

The Jake Canyon reverse fault trends northwest for more than 18 mi along thenortheastern flank of the Blacktail Mountains. Archean metamorphic rocks are displaced over Paleozoic carbonate strata along the northwestern part of the fault and are displaced over other Archean metamorphic rocks along the southeastern part. The Jake Canyon fault is separate and distinct from the Cenozoic Blacktail normal fault that marks the common boundary of the range and the valley to the northeast. Mineralizing hydrothermal fluids caused argillic and propylitic alteration along the Jake Canyon fault in the middle and southeastern parts of the mountain range. Fission-track ages were determined on apatite recovered from nine samples of propylitized gneiss and gneiss beyond the zone of hydrothermal alteration. The combined geologic and geochronologic evidence indicate that the ages of fault movement and the hydrothermal event that produced the mineralized, altered rock are Late Cretaceous.