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Iron Hill (Iron Hill carbonatite complex), White Earth District (Powderhorn District), Gunnison Co., Colorado, USA

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Latitude & Longitude (WGS84): 38° 14' 4'' North , 106° 42' 30'' West
Latitude & Longitude (decimal): 38.23472,-106.70861
Köppen climate type:Dfc : Subarctic climate

A massive carbonatite stock forms the core of the Iron Hill carbonatite complex. The carbonatite stock is enriched in rare earth elements, niobium, and thorium; the adjacent pyroxenite unit is enriched in these elements also and in substantial amounts of titanium.

The Iron Hill carbonatite complex is exposed for 31 km2 (12 mi2) near the small town of Powderhorn, about 35 km (22 mi) south-southwest of Gunnison, Colorado. The intrusion is alkaline with a prominent carbonatite stock at its core. This intrusive complex is noteworthy because of its classic geology and its mineral resource potential (Van Gosen and Lowers, 2007). This intrusive complex was described by Olson and Hedlund (1981, p. 5) as “the best example of the carbonatite-alkalic rock association in the United States and is one of the outstanding occurrences in the world, comparable to many of the classic areas in Africa and other continents.” The primary rock types of the complex are, from oldest to youngest, pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite (Olson, 1974; Hedlund and Olson, 1975; Olson and Hedlund, 1981; Armbrustmacher, 1983). Substantial titanium concentrations have been measured in the pyroxenite unit, which is thought to host the largest titanium (Ti) resource in the United States (Thompson, 1987; Shaver and Lunceford, 1998; Van Gosen and Lowers, 2007). The carbonatite stock is enriched in rare earth elements (REE), niobium (Nb), and thorium (Th); the pyroxenite unit is also enriched in these elements plus vanadium (V). Thus, it may be economic to extract several resources from this complex with a well-coordinated mine and mill plan. Thus far, none of these resources has been developed at Iron Hill.
A dolomitic carbonatite stock was the last major igneous phase of the Iron Hill intrusive complex. The stock forms Iron Hill (fig. 9) and the ridge to its northwest, and it is exposed throughout an area of about 3.7 km (2.3 mi) long by 1.9–0.8 km (1.2–0.5 mi) wide, making it the largest exposed carbonatite mass in the United States. Staatz and others (1979) estimated that the carbonatite stock of Iron Hill contains 655.6 million metric tons (722.7 million tons) of carbonatite. On the basis of the averaged analytical results of 28 samples of the carbonatite stock—0.4 percent for total rare earth oxides and 0.004 percent ThO2—Staatz and others (1979) calculated potential reserves within the stock of 2.6 million metric tons (2.865 million tons) of rare earth elements oxides and 28,190 metric tons (31,080 tons) of ThO2. Recent sampling of the Iron Hill carbonatite stock by Van Gosen (2008) found median values (from 13 samples) of 0.19 percent total rare earth oxides (table 17) and 0.0035 percent ThO2; this result represents estimated resources within the stock of about 1.22 million metric tons (1.34 million tons) of rare earth element oxides and about 23,000 metric tons (25,300 tons) of ThO2.

Mineral List

76 valid minerals. 2 (TL) - type locality of valid minerals.

Rock Types Recorded

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Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Miocene - Eocene
5.333 - 56 Ma

ID: 3190650
Cenozoic volcanic rocks

Age: Cenozoic (5.333 - 56 Ma)

Stratigraphic Name: San Juan Mountains

Comments: Southern Rocky Mountains

Lithology: Intermediate-felsic volcanic rocks

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

23.03 - 33.9 Ma

ID: 2759840
Intra-ash flow quartz latitic lavas

Age: Oligocene (23.03 - 33.9 Ma)

Lithology: Major:{quartz latite}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

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USGS Professional Paper 197A.
Nash, W.P. (1972) Mineralogy and petrology of the Iron Hill carbonatite complex, Colorado. Geological Society of America Bulletin 83, 1361-1382.
Armbrustmacher, T.J. and Brownfield, I.K. (1979) The carbonatite stock at Iron Hill, Gunnison County, Colorado - chemical and mineralogical data. USGS Open-File Report 79-537, 13 pp.
Armbrustmacher, Theodore J. (1980) Abundance and distribution of thorium in the carbonatite stock at Iron Hill, Powderhorn district, Gunnison County, Colorado. U.S. Geological Survey Professional Paper 1049-B, B1-B11.
Tschernich, R. (1992) Zeolites of the World, 65.
Van Gosen, B.S. and Lowers, H.A. (2007) Iron Hill (Powderhorn) carbonatite complex, Gunnison County, CO - a potential source of several uncommon mineral resources. Mining Engineering 59, 56-62.
Van Gosen, B.S. (2009) The Iron Hill (Powderhorn) Carbonatite Complex, Gunnison County, Colorado - A Potential Source of Several Uncommon Mineral Resources. USGS Open-File Report 09-1005.
Berger, V.I., Singer, D.A., and Orris, G.J. (2009) Carbonatites of the World. Explored Deposits of Nb and REE - Database and Grade and Tonnage Models. USGS Open-File Report 09-1139.
USGS Open-File report 2010-5220.

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