| Basis for focus area |
Polygons extracted and merged from the National Assessment. National Assessment tracts GB19, GB20 and some GB21. Previously categorized as Massive sulfide Cyprus, Massive sulfide Besshi, and a few polygons categorized as Massive sulfide, Sierran Kuroko (U.S. Geological Survey National Mineral Resource Assessment Team, 2002). |
| Identified resources |
Historical production of copper, gold, and silver. |
| Production |
Rio Tinto (Mountain City) mine (1932-1949): 1,109,878 short tons of ore averaging 9.74% Cu, 0.274 oz/t Ag, and 0.0057 oz/t Au. |
| Status |
Past mining. |
| Estimated resources |
Unknown. |
| Geologic maps |
Stewart and Carlson (1978), scale 1:500,000; Crafford (2010), scale 1:500,000. |
| Geophysical data |
Inadequate Rank 4 aeromagnetic and Rank 5 aeroradiometric coverage. |
| Favorable rocks and structures |
Parts of the Roberts Mountains and Golconda allochthons include thick pillow basalts, chert, and turbidites and are permissive for massive Cyprus- and Besshi-type massive sulfide deposits. The Jurassic Black Rock and Triassic Koipato assemblages are permissive for Kuroko-type massive sulfide deposits. |
| Deposits |
Rio Tinto (Mountain City) (MRDS dep_id: 10310316). |
| Evidence from mineral occurrences |
MRDS. |
| Geochemical evidence |
Unknown. |
| Geophysical evidence |
Unknown. |
| Evidence from other sources |
Unknown. |
| Comments |
The Rio Tinto mine is developed in the Ordovician Valmy Formation. Several plutons of Cretaceous age intrude this and other Paleozoic formations; this sequence was eroded and covered with Miocene, Pliocene, and possibly older volcanic rocks. The primary mineralization of the area is believed to postdate the Mississippian Nelson Formation and pre-date the Cretaceous intrusives. The primary orebodies are lenticular in shape and are composed largely of quartz, pyrite, and chalcopyrite. The ore lenses, in general, strike northwestward and dip northward; they are in shales with associated minor quartzite lenses. The ore is epigenetic, and the principal orebody was leached to the 200-ft level; supergene copper sulfide ore was immediately below the barren gossan. The supergene enrichment of the ore may have required a large part of Tertiary time. |
| Cover thickness and description |
Unknown. |
| Authors |
Allen K. Andersen, Deborah A. Briggs. |
| New data needs |
Geologic mapping, geophysics. |
| Geologic mapping and modeling needs |
Updated geologic mapping. |
| Geophysical survey and modeling needs |
High-resolution, Rank 1 aeromagnetic and aeroradiometric surveys. |
| Digital elevation data needs |
Lidar variable over large focus area; some complete, some in progress, some inadequate. |
