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

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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 elements oxides and about 23,000 metric tons (25,300 tons) of ThO2.

Mineral List

Aegirine Åkermanite Albite Analcime Anatase Andradite var: Melanite Augite Baryte 'Bastnäsite' Bastnäsite-(Ce) Biotite Britholite-(Ce) Britholite-(Y) Brookite Brugnatellite Calcite Cancrinite Cebollite (TL) Cerite-(Ce) Chalcopyrite 'Chlorite Group' Columbite Diopside

Dolomite Fluorapatite Fluorite Galena 'Garnet' Gehlenite Goethite Grayite Gunnisonite Hastingsite Hematite var: Martite var: Specularite Hydroxylapatite var: Carbonate-rich Hydroxylapatite Ilmenite Juanite (TL) Leucoxene Limonite Magnesio-arfvedsonite Magnesiohornblende Magnesite Magnetite 'Melilite'

Microcline Monazite-(Ce) Monticellite Muscovite Natrolite Nepheline Norsethite 'Olivine' Orthoclase Parisite-(Ce) Perovskite Phlogopite Piemontite Pyrite 'Pyrochlore' Pyrochlore Group Pyrophanite Quartz Rhabdophane-(Ce) Richterite Riebeckite Rutile Schorlomite Siderite

Siegenite Sphalerite Spinel Strontianite 'Synchysite' Synchysite-(Ce) Tainiolite Thorite Thorogummite ? Titanite Tremolite Vanadinite Vermiculite Vesuvianite Vishnevite ? 'Wad' Winchite Wollastonite Xenotime-(Y) Zircon Zoisite

93 entries listed. 72 valid minerals. 2 type localities (others).

References