| Basis for focus area |
Focus area was selected as part of Earth MRI Phase 2 on the basis of Al, K2SO4, H2SO4, ± Ga in alunite. Focus area includes other mining districts, such as the Galena, Lake, Burrows Park, and Sherman districts, that were historically mined for precious and base metals. Lithocap alunite at surface, concealed porphyry Mo mineralization. Focus area based on a 10 km buffer on Red Mountain alunite deposit (MRDS dep_id: 10096873). Included here because numerous Ag-Au mines associated with caldera system contain tertiary product Sb. |
| Identified resources |
Identified aluminum (alunite) resource; historical production of copper, gold, silver, and zinc. |
| Production |
Unknown. |
| Status |
Past mining of distal Ag-Au deposits in around the caldera. Exploration drilling encountered subeconomic high fluorine, Mo-porphyry mineralization in potasically altered rocks at a depth of ~600 m. |
| Estimated resources |
Red Mountain. is the second largest alunite resource in the country with 225 Mt ore at 30% alunite (Hall, 1978); >70 Mt alunite (Bove and Hon, 1990). Unknown for other deposits. |
| Geologic maps |
Lipman (1976), scale 1:48,000; Hon (1987), scale 1:24,000; Luedke and Burbank (1987), scale 1:24,000. |
| Geophysical data |
Inadequate Rank 3 aeromagnetic coverage and adequate Rank 2 aeroradiometric coverage-the Rank 3 aeromagnetic coverage may actually be adequate in this case because alteration has destroyed the magnetic minerals. |
| Favorable rocks and structures |
Miocene (23.1 Ma) dacite lavas and porphyritic intrusions on the eastern margin of the Lake City caldera affected by brecciation and advanced argillic alteration of the same age (23.1 Ma). Potassic alteration at depth associated with Mo mineralization is inferred to be of the same age. |
| Deposits |
Red Mountain alunite (MRDS dep_id: 10096873), Ocean Wave mine, Pelican mine, Ute-Ulay mine. |
| Evidence from mineral occurrences |
MRDS. |
| Geochemical evidence |
Surveys published in Steven and others (1977) demonstrate anonymously high Sb abundances in Ag-Au mines. Anomalous Mo occurs in potassic altered rocks at depth. The occurrence of anomalous Mo coupled with the overall pattern of alteration, presence of multiple intrusions, and high F intercept values of sericite and secondary biotite, suggest the presence of a relatively high F, low silica Mo-porphyry system at depth. Plots comparing F versus F/Cl intercepts illustrate that micas in the Red Mountain porphyry system fall in a region between a quartz monzonite-granodiorite Cu-porphyry system such as Santa Rita, and a high-silica granite Mo-porphyry system as exemplified by the Henderson deposit (Bove and others, 1990). Numerous analyzed grab samples from mines presented in Steven and others (1977) yield Sb abundances between 500 to 1000 ppm. |
| Geophysical evidence |
Aster and Landsat alteration maps of the area (Rockwell, 2012, 2013). |
| Evidence from other sources |
Alunite alteration is evident in ASTER mineral maps of the area. State-wide trend of similar occurrences. |
| Comments |
Given the size and grade of the alunite lithocap, related Mo porphyry and polymetallic mineralization may be present at depth. |
| Cover thickness and description |
Alunite is exposed at the surface. Drilling shows it extends to a depth of about 250 m. Weak porphyry-type mineralization is found throughout one drill-hole (LC-4, total depth 2,777 ft), with molybdenite present mostly along hairline fractures in association with late pyrite ± sericite ± quartz. The highest molybdenite concentrations (150 to 1150 ppm) occur over a roughly 1,500 ft interval and are generally coincident with the zone of potassic alteration (Bove and others, 1990). |
| Authors |
Albert H. Hofstra, Joshua M. Rosera, V.J.S. (Tien) Grauch. |
| New data needs |
1:24,000 scale geologic mapping. 1:24,000 scale geologic mapping is complete within Lake City Caldera (Hon, 1987). |
| Geologic mapping and modeling needs |
Need 1:24,000 scale geologic mapping for [at the Lake City, Uncompahgre Peak, Redcloud Peak, Lake San Cristobal and] portions of Slumgullion Pass/Cannibal Plateau quadrangles. |
| Geophysical survey and modeling needs |
Not a high priority for new geophysical data collection because the alteration is evident from the Rank 3 aeromagnetic and rank 2 radiometrics may be sufficient. |
| Digital elevation data needs |
Lidar in progress. |
