| Basis for focus area |
Jamestown fluorite district was a major producer in Colorado. Mineralization cuts the northern granitic phase of a Cenozoic intrusive complex in Jamestown. Tentative outline of focus area encompasses the granitic phase of the Jamestown intrusive suite, and broadly extends northwest-southeast to outline MRDS fluorite records. Tellurium hosted in Au-Ag veins that formed relatively late and in the outer parts of the district (Lovering and Goddard, 1950). |
| Identified resources |
Historical production of copper, fluorite, gold, lead, silver, tellurium, and uranium. |
| Production |
1 to 1.5 million tons of fluorite (MRDS dep_id 10008720, Jamestown). Mining of U in the Jamestown district began about 1955, and by 1960 the output as reported to the U.S. Bureau of Mines totaled 7,993 tons of ore that contained 84,025 lbs U308 (Sims and Sheridan, 1964). |
| Status |
Some MRDS records (for example, Buena Mine, dep_id 10017417) refer to Sunshine Mining Co. exploration work that was abandoned in 1992. Noranda exploration drilled numerous cores in the early 1980s. |
| Estimated resources |
Unknown. |
| Geologic maps |
Gable (1980), scale 1:24,000; Cole and Braddock (2009), scale 1:100,000. |
| Geophysical data |
Inadequate Rank 4 aeromagnetic and Rank 3 aeroradiometric coverage. Rank 4 aeromagnetic coverage. |
| Favorable rocks and structures |
Northwest-trending zone of veins and breccias that cuts northern portion of the Jamestown intrusions. Wallace (2010) suggests temporal association with 54 to 56 Ma granitic unit. Also, acid-grade fluorspar produced in 1973 from depths below 1,400 feet (425 m) in the Burlington mine where in the lower levels of the mine the smaller high-grade fluorspar veins coalesce to form a larger vein zone of lower grade, and brecciated wall rock is less common at depth (Brady, 1975). Brady (1975, page 6) indicates , "Further exploration of the more significant deposits at depth seems to be warranted. The potential of the district is enhanced by the recovery of trace amounts of base-metals in the fluorspar ores. |
| Deposits |
Argo (MRDS dep_id: 10018054), Brown Spar (MRDS dep_id: 10108205), Chancellor, Burlington, Emmet, Yellow Girl. |
| Evidence from mineral occurrences |
MRDS. |
| Geochemical evidence |
Fluorine rich granitic rocks and numerous assays of fluorite veins (see Wallace, 2010). Grades > 60% CaF2 in numerous mines, in some cases mines targeted > 85% CaF2 (Lovering and Goddard, 1950). Fluorspar concentrations in the Jamestown district averaged 60 to 90% CaF2 that occur within the central zones of the deposits, and are commonly enclosed by lower grade brecciated ore containing 5 to 60% CaF2 (Goddard, 1946: Brady, 1975). Uranothorite identified in fluorite breccia bodies (Phair and Onoda, 1951). |
| Geophysical evidence |
A regional magnetic low corresponds to the area of the granitic rocks. The area is located at the edge of a very large regional gravity high. |
| Evidence from other sources |
USGS Core Library (numbers R835, R838, R839, R841, R844, R873). Fluorite mineralization, Quartz-molybdenite veins, silicic and phyllic alteration and brecciation intercepts documented in various core logs from Noranda Exploration. |
| Comments |
The principal fluorspar deposits lie within the breccia zones along the southern and western borders of the granite-quartz monzonite intrusive. The breccia zones are from 10 to 70 feet (3-21 m) wide, and from 150 to 1,000 feet (45-305 m) long (from Brady, 1975, page 5). Although U is not currently listed as a critical element for alkalic porphyry systems, the Jamestown district did produce small amounts of U3O8. Data regarding the abundance of other potential critical commodities (Bi, Ga, Ge, and In) in the district are scarce, and their presence is inferred from the mineral system/deposit type table (Hofstra and Kreiner, 2020). |
| Cover thickness and description |
Good exposure. |
| Authors |
Joshua M. Rosera, Michael K. O'Keeffe. |
| New data needs |
1:24,000 scale geologic mapping; Geochemistry and characterization of historical ore to determine potential for by product critical minerals (Ga, Ge, etc.). |
| Geologic mapping and modeling needs |
Gold Hill/Lyons/Boulder quadrangles are mapped at 1:24,000 scale, however most of the late granitic phase associated with fluorite mineralization is located to the north in the Raymond 1:24,000 scale quadrangle which is not mapped at 1:24,000 scale. Also, the Greeley 1:250,000 geology map covers this area. |
| Geophysical survey and modeling needs |
New high resolution aeromagnetic and radiometric surveys would help delineate areas of the granitic phase of the igneous intrusion. |
| Digital elevation data needs |
Lidar is adequate. |
