Precambrian history of the eastern Ferris Mountains and Bear Mountain, south-central Wyoming Province

Abstract

The eastern Ferris Mountains and Bear Mountain area of south-central Wyoming contain a complex assemblage of Archean and Proterozoic rock units, including a metasedimentary and metavolcanic supracrustal sequence named the Spanish Mine metamorphic suite, three granitic plutons (Turkey Creek, Ferris Mountains, Bear Mountain), and at least three sets of mafic dikes. The Spanish Mine metamorphic suite was deposited, intruded by mafic sills and (or) dikes, and underwent amphibolite-grade metamorphism and folding just prior to, or synkinematic with, the intrusion of the Turkey Creek metaplutonic suite, U–Pb dated at 2733.5 ± 2 Ma. A second set of mafic dikes intruded the Turkey Creek metaplutonic suite prior to mylonitic shearing and late-stage folding along the Miners Canyon shear zone. These events were followed by intrusion of the Ferris Mountains plutonic suite at ~2717 Ma. The emplacement of the yet undated granite of Bear Mountain records the last phase of Neoarchean magmatism. The timing of magmatism and deformation support a model that the Ferris Mountains basement rocks formed in an arc terrane distinct from the older Wyoming craton and were accreted to the province in the Neoarchean. Intrusion of a third mafic dike set is dated at 2113 ± 15 Ma and may date the silicified, epidote-rich zones that crosscut all of the Archean rock units. Lead isotopic compositions of galena within the Spanish Mine metamorphic suite indicate additional Proterozoic activity and mineralization.