Iron Hill deposit, White Earth Mining District (Powderhorn Mining District), Gunnison County, Colorado, USAi

Regional Level Types
Iron Hill deposit	Deposit
White Earth Mining District (Powderhorn Mining District)	Mining District
Gunnison County	County
Colorado	State
USA	Country

A REE-V-Fe-Mn-F-Nb-Ti-P-Ba-Pt-Cu-Mica-Th occurrence located in secs. 11 & 12, T46N, R2W, NMM, on patented (private) land, on located claim(s) and on private lease. Discovered in 1885. Operated by Du Pont (1957-1958). Owned by Reunion Energy Co. (100%), Houston, Texas (1994). Owned by Teck Resources (1990-2010).

Mineralization is a massive carbonatite stock (Mineral occurrence model information: Model code 24; USGS model code: 10; deposit model name: carbonatite), that forms the core of the Iron Hill carbonatite complex. The orebody is funnel-shaped, pear-shaped in plan view. The ore is disseminated in the pipe. It is 200 meters thick, 460 meters wide, 900 meters long, with a depth-to-top of 0 meters, and covering an area of 1,300 ha. It is irregular in form. The primary mode of origin was magmatic differentiation. Primary ore control was igneous activity, and the secondary control was lithology. There is no wall rock alteration. It is hosted in nepheline syenite, pyroxenite and carbonatite intrusive. There is one main orebody located on and around Iron Hill. Three smaller orebodies occur around the main orebody. 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 Th is in veins in the country rock. Biotite-bearing zones in the pyroxenite contain 0.2% Cu. Carbonate rocks are rich in Sr (to 2%). Pods of apatite follow flow bands in the carbonatite. Associated rocks include granite. Local rocks include intrusives.

The Iron Hill carbonatite complex is exposed for 31 km² (12 square miles) near the small town of Powderhorn, about 35 km (21.75 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. This intrusive complex was described by Olson and Hedlund 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); 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 and the ridge to its northwest, and it is exposed throughout an area of about 3.7 km (2.3 miles) long by 1.9–0.8 km (1.2–0.5 mile) 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.

Geological structures include concentric structures (Concentric Structures formed by arcuate shapes of different rock types, cone sheets of carbonatite; radial carbonate dikes; and later faults and dikes)

Workings include unspecified underground openings.

Reserves/resources: Type: in-situ (estimate year = 1998): Total resources 1,600,000,000 metric tons of ore. Titanium (TiO2): 10.9 weight percent.

Analysis test results: Pt = 39 ppb (Field No. WP-741). Analyses averages: 0.026% Cu, 0.025% Zn, 0.010% Ni, 1.65% Ba, 0.27% Sr, 2.9% Ti, 0.049% Zr, 0.071% Nb, 0.41% Mn, 0.23% La, 0.59% Ce, 0.008% Y (see Temple and Grogan). Also reported: V, U, As, Ta. Ta grade is not known, and Thompson states Ta2O5 grade occurs in geochemical amounts (less than 10 ppm).

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Link to USGS MRDS:	10142743

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References