| Basis for focus area |
Focus area includes fluorite-producing districts off Browns Canyon and Poncha Springs on the eastern flank of the Rio Grande rift. Fluorite (+ some Mn) mineralization is associated with northwest-trending faults. Fluorite mineralization cuts the Dry Union Formation within the rift and was estimated to be between 11 and 14 Ma (Wallace, 2010). Boundary created from volcanic rocks and Quaternary cover from the statewide geological map. |
| Identified resources |
Identified resources of fluorite; historical production of copper, fluorite, gold, and silver. |
| Production |
Estimated 130,000 tons of fluorite production from Browns Canyon district (Van Alstine and Cox, 1969). Small production from Poncha Springs district. More than 260,000 tons of ore were mined, yielding 130,000 tons fluorite at an estimated value of more than $5 million (Brady, 1975). The grade of the product ranged from 85% CaF2 (metallurgical grade-59% of shipments), 92 to 97% CaF2 (ceramic grade-39% of shipments), and 98% CaF2 (acid grade-2% of shipments) (Van Alstine and Cox, 1969). Four mills operated in the district at the height of its production. |
| Status |
Past mining; Poncha Springs prospected for geothermal resources. |
| Estimated resources |
Van Alstine and Cox (1969) estimated the Browns Canyon district contains 2 million tons of fluorite (1.65 Mt Inferred and 0.4 Mt Measured and Indicated). |
| Geologic maps |
Kellogg and others (2017), scale 1:50,000; Minor and others (2019), scale 1:24,000; Van Alstine and Cox (1969), scale 1:24,000; Van Alstine (1974), scale 1:24,000; Wallace and Lawson (2008), scale 1:24,000; Wallace and others (1997), scale 1:24,000. |
| Geophysical data |
Partly (<50%) adequate aeroradiometric coverage. Inadequate aeromagnetic coverage over north part of Browns Canyon, Rank 1 aeromagnetic coverage available for close to all of the rest of the area. |
| Favorable rocks and structures |
Fluorite at Browns Canyon and Poncha Springs is associated with northwest-trending rift-related faults. Most of the fluorspar veins were 3 to 6 feet in width, but some reached 40 feet. Individual veins are traceable for 1000 feet or more (Cox, 1945). |
| Deposits |
Browns Canyon district has numerous producers, including Poncha Springs, Aksarben (MRDS dep_id: 10263138), Lady Luck #1-5, Evert #1-17 (MRDS dep_id: 10017007). |
| Evidence from mineral occurrences |
MRDS. |
| Geochemical evidence |
Fluorite mineralization in Browns Canyon: spatially restricted (within a few inches) replacement alteration (silicification and fluoritization) along ore-hosting veins. Preliminary soil surveys near Browns Canyon suggest F enriched 6 to 20 times above background, above and downslope from fluorite veins (Van Alstine and Cox, 1969). Hot springs at Poncha Springs contain anomalously high F (> 10 ppm; Cappa and Hemborg, 1995; Wallace, 2010). |
| Geophysical evidence |
Resolving fluorite mineralization directly with geophysical data may not be possible, however, high resolution aeromagnetic data from Poncha Springs can be used to delineate rift-related faults and structures and identify shallow buried basement features (Grauch and Drenth, 2009). |
| Evidence from other sources |
No data. |
| Comments |
The focus area includes the Sedalia mine, which was mined intermittently between 1881 and 1923. This mine produced Cu, Zn, Ag, and Au from a Proterozoic Cu-Zn skarn (Heinrich, 1981). A few MRDS locations (Bonus Extension Group) also indicate that Be, Nb, and Ta may have been mined from pegmatites in the area. |
| Cover thickness and description |
Estimated 11 to 14 Ma fluorite mineralization is truncated by younger faulting (Wallace, 2010). Fluorite mineralization is also observed in the Dry Union formation within the rift basin. Exposed mineralization occurs in faults that cut basement rocks. Grauch and Drenth (2009) identified a block of shallow basement rocks (< 300 m) beneath basin fill just north of Poncha Springs. |
| Authors |
Michael K. O'Keeffe, Joshua M. Rosera, V.J.S. (Tien) Grauch. |
| New data needs |
Aeromagnetic data for Browns Canyon, aeroradiometric survey. |
| Geologic mapping and modeling needs |
Browns Canyon recently part of map by Kellogg and others (2017). The Poncha Pass block was recently mapped by Minor and others (2019), where much structural information was collected. The area has been mapped at 1:24,000. |
| Geophysical survey and modeling needs |
New high resolution aeromagnetic and aeroradiometric surveys would help delineate bedrock units and alteration as well as rift-related faults. Most of the area has already been flown for aeromagnetic coverage. |
| Digital elevation data needs |
Lidar in progress. |
