Thirtynine Mile Mountain Ranch
Mineral Remote Assessment

Regional Geology — Thirtynine Mile Mountain lies within the Southern Rocky Mountains of central Colorado and is situated in the south-central part of the State. This region includes a complex geologic history, with bedrock ranging from Precambrian-age (~1.4- to 1.8-billion years) metamorphic granites, migmatites, gneisses, and schists; Cambrian- to Paleogene-age (~530- to 40-million years) sedimentary rocks; and Late Cretaceous- to Paleogene-age (~64- to 10-million years) igneous intrusions and volcanic fields. Near the end of the Cretaceous Period and during the early to middle part of the Tertiary Period, widespread disturbances in the North American continental plate generated most of the major mountain uplifts and basins of the present Rocky Mountain region.

This period of mountain building in the Rocky Mountains is known as the Laramide Orogeny (approximately 80- to 35-million-years ago). During Laramide mountain building, continental tectonic extension helped to develop intermountain structural basins that include thick sequences of Cretaceous- and Tertiary-age sedimentary rocks, some of which contain important accumulations of coal, oil, and gas. Most of the sedimentary rocks deposited earlier were eroded from the uplifts, leaving scattered remnants of the once immense lateral extents of sandstone, limestone, and shale. Toward the end of the Laramide Orogeny, Tertiary-age volcanic centers formed throughout the region, including the Thirtynine Mile volcanic area, which represents the largest remnant of the Central Colorado volcanic field.

Local Geology — Local geology exposed at and within proximity of the subject parcel consists of the Oligocene-age Thirtynine Mile volcanic rocks (Tvtm), Oligocene-age Guffey Mountain volcanic rocks (Tvgm), and Paleoproterozoic (Precambrian) metamorphic rocks (Xmu) overlain by Quaternary-age alluvium and terrace deposits (Epis et al., 1979; McIntosh and Chapin, 2004; Barkman et al., 2015; BLM, 2018).

Proximal Metallic and Critical Mineral Deposits — Between 1890 and the early 1900’s, insignificant deposits of (1) tungsten, zinc, copper, cobalt, silver, and gold were mined found associated with skarns within Proterozoic-age granitoids, gneisses, and  schists; (2) copper, gold, tungsten, and rare earth minerals including niobium, bismuth, uranium, thorium, tantalum, beryllium, titanium, and tin found in Proterozoic-age pegmatites; and (3) uranium and vanadium found in stratabound Oligocene-age volcaniclastic and Pennsylvanian-age clastic host rocks; were mined in the Guffey Mining District. Mine workings were small and production records unavailable (Burnell, 2015; Dunn, 2015; Scarbrough, 2001; BLM, 2018; USGS, 2025c).

References —

Barkmann, P.E., Sebol, L.A., Johnson, E.P., Fitzgerald, F.S., and Curtiss, W., 2015, Geology and Groundwater Resources of Park County, Colorado: Colorado Geological Survey, OF-15-11, 82 p., 20 fig., 38 plates, GIS database. https://bit.ly/3MH2HYk

Burnell, J.R., 2015, Historic Metal Mining Districts: Colorado Geological Survey, ON-007-08D, v20201112, 565 p.  https://bit.ly/3DBs0Dk

Dunn, L.G., Colorado Mining Districts (revised), 2015, Arthur Lakes Library, Colorado School of Mines, p. 135.   http://dx.doi.org/10.25676/11124/1703

Epis, R.C., Wobus, R.A., and Scott, G.R., 1979, Geologic map of the Guffey quadrangle, Park County, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1180.  https://doi.org/10.3133/i1180

McIntosh, W.C., and Chapin, C.E., 2004, Geochronology of the central Colorado volcanic field, in Cather, S.M., McIntosh, W.C., and Kelley, S.A., eds., Tectonics, geochronology, and volcanism in the southern Rocky Mountains and Rio Grande rift: New Mexico Bureau of Geology and Mineral Resources Bulletin 160, p. 205–237.  https://bit.ly/3NTfUef

Scarbrough, Jr., L.A., 2001, Geology and Mineral Resources of Park County, Colorado: Colorado Geological Survey Resource Series 40.  https://doi.org/10.58783/cgs.rs40.mitw1961

U.S. Bureau of Land Management (BLM), 2018, Mineral Potential Report for the Royal Gorge Field Office, Colorado.  https://bit.ly/3va1a4m