LARAMIDE-AGE FAULTING & SYNKINEMATIC MINERALIZATION IN UNAWEEP CANYON, CO

Unaweep Canyon is an exhumed Laramide-age extensional graben system found on the Uncomphagre Plateau of west-central Colorado. The Uncompahgre Plateau is a 150 km long, Laramide-age, NW-SE striking anticlinal uplift. The NE-SW striking Unaweep Canyon cuts orthogonally across the entire width of the Uncompahgre Plateau. Laramide-age deformation along the Uncompahgre Plateau is controlled by a series of WNW-ESE striking, left-lateral strike-slip faults. These controlling faults bend right into compressive orientations resulting in the classic Colorado Plateau-style reverse faults & monoclines as seen in the Colorado National Monument. Conversely, in Unaweep & subparallel Big Dominguez Canyons, the controlling left-lateral strike-slip faults bend left into extensional grabens that display a lazy ‘S’ pattern in map view. In Unaweep Canyon, deeply-cutting left-lateral strike-slip faults & associated normal faults opened gaps in the Earth’s crust that were filled by hydrothermal fluids that formed mineralized breccias. These breccias are matrix-supported, with no grain-size grading, suggesting that they formed as implosion breccia. Outcropping strike-slip faults, up to 150m wide, cut across Unaweep Canyon & are characterized by spectacular vertical ‘walls’ of mineralized & altered implosion breccia. Elevation differences of the ‘Great Unconformity’ across Unaweep Canyon (<50m) suggest the presence of normal faults within the canyon.

Hydrothermal mineral veins within Unaweep Canyon formed in at least two phases synkinematic with Laramide deformation. Phase 1 includes coarsely crystalline calcite-hematite & Mn-bearing quartz (dark purple amethyst) followed by phase 2 fluorite-quartz (clear, milky to pale purple amethyst). Phase 2 alteration minerals occur as both open space fillings along faults & as partial replacements of phase 1 minerals. Phase 1 & 2 minerals are found along faults as well-preserved veins with massive crystals or pulverized into implosion breccias due to synkinematic deformation. Late- to post-kinematic mineralization includes sulfides (galena, pyrite, auriferous chalcopyrite) & supergene Cu-carbonates (azurite, malachite) & Fe-oxides (limonite, goethite) forming along veins that cut the implosion breccia.