METALLOGENY OF A PALEOPROTEROZOIC COLLISIONAL OROGEN THROUGH TIME—THE GREAT FALLS TECTONIC ZONE, MONTANA AND IDAHO

The northeast-trending Great Falls tectonic zone, is a rejuvenated, largely concealed Paleoproterozoic collisional orogen that has subsequently localized late Cretaceous and Tertiary epigenetic metal deposits in east-central Idaho and adjacent southwestern Montana. The collisional orogen marks a lithospheric plate boundary between the Archean Wyoming Province on the southeast and the Medicine Hat subprovince of the Archean Hearn Province of southern Canada. The orogen consists of the Missouri River shear zone, which marks the boundary between the two Archean cratons and the strongly deformed cratonic rocks of the Wyoming Province, which include from north to south: northeast-trending linear zones of accreted terrane and juvenile crust; a metamorphic-plutonic terrane; a metamorphosed fold and thrust belt with basement crustal duplex structures; Paleoproterozoic foreland basin deposits.

These ancestral Proterozoic structures, which were reactivated in Late Cretaceous-Early Tertiary time, provide zones of weakness that guided the emplacement of mineralizing intrusions and provided conduits for hydrothermal fluids at the regional and district scale. Although the tectonic zone is more than 200 km wide, most of the Late Cretaceous and Tertiary ore-related magmas were emplaced within the 60-km-wide zone of Paleoproterozoic juvenile crust, magmatic arc terrane, and adjacent metamorphic-plutonic terrane. We infer that these plutons concentrated metals from basement sources emplaced during Paleoproterozoic collisional tectonism. The principal epigenetic mineral deposits in the orogen include: mesothermal polymetallic veins and carbonate-replacement deposits; epithermal precious-metal veins and disseminations and polymetallic deposits; alkaline epithermal precious-metal deposits; porphyry copper and skarn deposits. A pronounced gravity low, defined by the -180 mGal complete Bouguer contour, encloses 90% of the deposits in the orogen. These deposits are aligned in northwest-, northeast-, and north-northwest-trending fracture systems interpreted to have resulted from upward propagation of stress along basement structures.