| Basis for focus area |
The Entrada focus area is defined by tabular sandstone uranium occurrences in the Jurassic Entrada Sandstone in the Rifle, Placerville, and Barlow Creek-Graysill districts (Chenoweth, 1980). Uranium and V in sandstones of the upper part of the Entrada, just below its contact with the overlying Wanakah Formation. Due to the overlap of the southern area of this focus area with the Silver Peak-Calico focus area, these have been combined here. Only one reported occurrence of elevated radiation was reported from the Rico mine (Nelson-Moore and others, 1978). |
| Identified resources |
Identified resources and historical production of uranium and vanadium. |
| Production |
Chenoweth (1980) reports that since 1948 the three areas have produced 597,000 tons of ore averaging 0.07% U3O8 and 1.51% V2O5. The Rifle mine produced ~ 57,000 lbs U3O8 and 4,627 tV between 1948 and 1977 (Pool, 2017). Approximately 62 tons of material was mined by 1971 at the Blue Eagle mine at grades of 0.18% U3O8 and 2.36% V2O5, producing 226 lbs U3O8 and 2,929 lbs V2O5 (host rock is reported as the Morrison Formation) (Nelson-Moore and others, 1978). At the Graysill No. 1 mine to the east, by 1971, 171 tons were mined at grades of 0.049% U3O8 and 1.907% V2O5. At the Graysill No. 2, 31,778 tons were mined containing 0.08% U3O8 and 2.41% V2O5 (Nelson-Moore and others, 1978). Additional production is reported in Nelson-Moore and others (1978) for the Barlow Group and South Barlow, reportedly from the Entrada Sandstone. |
| Status |
Past mining; minor surface sampling in 2018. Graysill mine area has received some interest. |
| Estimated resources |
Historical resource estimates (reported as conceptual and not based on verifiable drill data used in modern estimates) by others, estimate exploration targets of 400,000 short tons with 2.5% V2O5 and 640,000 short tons with an average grade of 0.08% U3O8. At Silver Peak-Calico, 40 million short tons of 0.31% Mo; projections may exceed 182 million short tons (Cameron, Barrett and Wilson, 1986). Alunite resources at Calico Peak are estimated at 100 million tons containing 19.9% alunite (Serna-Isaza, 1971). |
| Geologic maps |
Pratt and others (1969), scale 1:24,000; McKnight (1974), scale 1;12,000; Bromfield (1967, plate 1), scale 1:24,000; Bush and Bromfield (1966), scale 1:24,000; Bush and others (1957, 1960, 1961), scale 1:24,000. |
| Geophysical data |
Inadequate aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Jurassic Entrada Sandstone (possibly some in the Navajo Sandstone) (Chenoweth, 1980). For Silver Peak, Proterozoic quartzite/greenstone (Uncompahgre Formation) and overlying Paleozoic sedimentary rocks (Hermosa Group) at Silver Creek especially along the Last Chance fault. Monzonite porphyry intrusion at Calico Peak. |
| Deposits |
Rifle mine (MRDS dep_id: 10090134) was the largest of the producing mines. Small amounts of V were produced from the Barlow Group (MRDS dep_id: 10014147), Blue Eagle mine (MRDS dep_id: 10014214), and Graysill mine (MRDS dep_id: 10014146). The northern portion of this area includes the Placerville, Sawpit, and Lower San Miguel mining districts. These mining areas contain Au and Ag (associated with the limestone member of the Wanakah Formation), Cr and Cr-bearing belts, as well as Au-bearing placers (see the Colorado Geological Survey unpublished historical mine summary for the Lower San Miguel District). Also, mining at the Silver Swan with no production records. Fall Creek Group, Lizzie Group, Wilmarth to the north. Rico Mining District to the west. The Graysill deposit from S&P Mining Property records listed in the eastern portion of this area. |
| Evidence from mineral occurrences |
MRDS; International Atomic Energy Agency (2020b); U.S. Environmental Protection Agency (2006). |
| Geochemical evidence |
No data. |
| Geophysical evidence |
No data. |
| Evidence from other sources |
No data. |
| Comments |
These areas are spread out. Currently, this record includes the Rico mining district to the west (Silver Peak-Calico). Also, this focus area includes the area to the north near Rifle, including the Rifle mine. |
| Cover thickness and description |
Covered by the Wanaka Formation in places (unknown thickness). Cover thickness at Silver Creek is 1 to 2 km. Cover thickness for potential Climax-type deposit at Calico Peak is unknown. |
| Authors |
Susan Hall, Michael K. O'Keeffe. |
| New data needs |
Geologic mapping at 1:24,000 scale, potentially geophysics. |
| Geologic mapping and modeling needs |
New 1:24,000 scale geologic mapping of the Clyde Lake, Groundhog Mountain, Engineering Peak, Sams, and Orphan Butte quadrangles is required to fill geological information gaps. Additionally, the preliminary 1:20,000 scale geologic map of the Hermosa Peak quadrangle would require additional field mapping to upgrade it to Colorado Geological Survey publication standards. Also, the Deep Creek Point quadrangle near the Rifle mine would need to be mapped at 1:24,000 scale. |
| Geophysical survey and modeling needs |
High resolution Rank 1 aeromagnetic and aeroradiometric surveys needed; electromagnetic data also would be useful. |
| Digital elevation data needs |
Lidar is adequate. |
