Iron Hill Carbonatite Complex

Location: Colorado, United States (38.235, -106.709 approx.)

Commodities: rare earth elements; niobium; uranium; vanadium; thorium; titanium; iron; vermiculite

Deposit model: Fe-Ti-REE pyroxenite · carbonatite · carbonatite deposits · unclassified

Geology

The complex of alkalic rocks of Iron Hill occupies 31 km2 (square kilometers) and is composed of pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite, in order of generally decreasing age. Fenite occurs in a zone, in places more than 0.6 km (kilometer) wide, around a large part of the margin of the complex and adjacent to alkalic dikes intruding Precambrian host rock. The alkalic rocks have a radioactivity, chiefly due to thorium, greater than that of the surrounding Powderhorn Granite (Proterozoic X) and metamorphic rocks. The pyroxenite, uncompahgrite, ijolite, and nepheline syenite, which form more than 80 percent of the complex, have fairly uniform radioactivity. Radioactivity in the carbonatite stock, carbonatite dikes, and the carbonatite-pyroxenite mixed rock zone, however, generally exceeds that in the other rocks of the complex. (Olson and Hedlund (1981)) 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. (Long and others (2010)) The complex at Iron Hill contains a minor element assemblage characteristic of many alkalic rocks, including titanium, phosphorus, strontium, barium, rare earths, and vanadium. Among these, the niobium and rare earths are sufficiently valuable and widespread in the thorium veins to be considered in discussing the economic potential. (Staatz and others (1979)) The thorium veins contain an appreciable amount of rare earths, which is generally in excess of the amount of thorium. The amounts of various rare earths was determined by semiquantitative spectrographic analysis. The proportions of total rare earth oxides to Th02 varies considerably but the average is 2.4. The ratio of the two is greatest in veins near the south and west sides of the complex at Iron Hill where the total rare-earth oxides are about five times as abundant as the Th02. The cerium group of rare earths is approximately 2.7 times more abundant than the yttrium group of rare earths; they also vary considerably in proportion to one another from sample to sample. the cerium group rare earths is proportionately greater near the south and west sides of the complex at Iron Hill where the ratio of the cerium group to the yttrium group is approximately 10 to 1. This ratio is least in an area within 5 mi of the northern side of the complex where most of the veins are somewhat richer in yttrium group rare earths than in cerium group rare earths. (Staatz and others (1979)) “Discrete resistive zones consisting almost entirely of boxwork ankerite and limonite with vugs and veins of fine to coarse euhedral calcite occur as prominent features extending from the sides of Iron Hill. These zones appear to represent the most highly-altered areas of the stock and were observed up to 15 meters-wide by 10 meters-tall. The carbonatite displays foliation and lineation along shear planes. Where the carbonatite intruded the pyroxenite and uncompahgrite, a mixed-rock zone formed. Temple and Grogan (1965) and Nash (1972) presented evidence of melt mixing in this unit, and the mixed zone displays textures and compositions between those of the pyroxenite and carbonatite stock (Lowers, 2005; Van Gosen and Lowers, 2007).” (Erickson (2015)) “The last intrusive unit in the complex is the carbonatite stock. The carbonatite ranges from coarse to fine-grained, and is dominantly composed of ankeritic dolomite, with lesser aegirine, potassium feldspar, iron oxides and hydroxides, biotite, fluorapatite, rutile, barite, calcite, and quartz. The carbonatite makes up the majority of the prominent hill at Iron Hill. This unit displays a variety of weathering patterns in outcrop. The stock forms a craggy outcrop consisting of massive, coherent, ankeritic-dolomite-dominant carbonatite. This appears to grade into less-resistant zones of carbonatite that has a crumbly, clastic-textured rind of euhedral to subhedral dolomite crystals between 1 mm and 1 cm in width. Beneath this centimeter-thick rind, the carbonatite stock has similar carbonate crystal grain size to the more coherent zones, but contains considerably more iron oxides. This zone also contains abundant euhedral pyrite that has been partially to fully replaced by hematite without destroying the pyrite crystal shape.” (Erickson (2015)) “Roughly 70 percent of the exposed bedrock at Iron Hill is the pyroxenite unit (Temple and Grogan, 1965; Olson and Hedlund, 1981; Van Gosen and Lowers, 2007). This rock unit has been dated at roughly 570Ma by Olson et al. (1977) by K-Ar on biotite and hornblende and Rb-Sr whole rock, 603+/-21Ma by Premo and Lowers (2013) by Pb/Pb whole rock, and 617 +/- 35Ma by Premo and Lowers (2013) by Rb-Sr whole rock. The pyroxenite consists primarily of augite, with lesser amounts of biotite, phlogopite, perovskite, ilmenite, aegirine, fluorapatite, sphene, and magnetite. At least three subunits have been interpreted to occur, including (I) a coarse biotite and magnetite-rich pyroxenite, (II) a perovskite and apatite-rich pyroxenite, and (III) a melanite garnet (Ti-andradite)-rich pyroxenite. The pyroxenites also display evidence of layering and recrystallization that may suggest that this unit formed from multiple pulses of magma (Temple and Grogan, 1965; Van Gosen, 2009). Temple and Grogan (1965) noted pegmatitic zones within the pyroxenite that contain pyroxene crystals up to two meters long, and Olson and Hedlund (1981) described pyroxene crystals up to 15 cm long. In outcrop, the pyroxenite unit is highly weathered, resulting in a crumbly regolith rich in biotite and phlogopite that have been altered to form pods of vermiculite near the surface. The numerous carbonatite dikes that crosscut the pyroxenite appear to have significantly aided in the weathering of this unit.” (Erickson (2015)) “The pyroxenite was intruded by the uncompahgrite unit. This lithology is primarily composed of melilite-group minerals and diopside, with lesser amounts of melanite garnet, magnetite, apatite, and perovskite. Nash (1972) observed 50 cm melilite crystals within pegmatitic zones of the uncompahgrite. The pyroxenite and uncompahgrite were in turn intruded by an ijolite, consisting of medium-grained augite in a fine-grained groundmass of nepheline that has been altered to cancrinite, with lesser amounts of biotite, apatite, magnetite, ilmenite, and melanite garnet. In the northeastern portion of the complex, nepheline syenite intruded at the boundary between the pyroxene and the Proterozoic country rocks. The nepheline syenite is composed of roughly 80 modal percent perthitic microcline, with lesser nepheline, aegirine-augite, biotite, magnetite, and ilmenite. A subunit exists where the nepheline syenite intruded and brecciated the pyroxenite. This subunit consists of brecciated pyroxenite with many small dikes of nepheline syenite (Armbrustmacher, 1981; Van Gosen and Lowers, 2007).” (Erickson (2015))

Published resource estimates

As compiled by USGS USMIN from the cited studies. Resource classes follow the source; "approximate" flags values USMIN reports as approximations. These are not current company reserve statements.

Exploration and site history

• Exploration — USGS studies included geologic mapping, petrology, and geochemical analyses of the rock units that focused on their Th, REE, Nb, and Ti content, 1940–1980 (Van Gosen (2009))
• Discovery — Cebolla Creek Iron Deposit (explored for iron) was considered impractical for smelters of the time (Ti interferred in the processing of Fe), 1880–1889 (Van Gosen (2009))
• Exploration — Teck Resources enters joint venture with Butte Gas & Oil Co. to explore for Ti at Iron Hill, 1990 (Van Gosen (2009))
• Exploration — Pyroxenite unit is explored for titanium during the mid 1970's by Buttes Gas and Oil Co., 1975 (Olson and Hedlund (1981))
• Development — DuPont conducts exploration drilling and excavation of the Du Pont Adit, 1957–1962 (Van Gosen (2009))
• Development — Small attempts to mine vermiculite deposits in the pyroxenite occurred in the mid-1930s to mid-1940s, 1935–1945 (Van Gosen and Lowers (2007))
• Evaluation — DuPont increases Nb2O5 reserves by underground work and additional drilling, 1958 (Olson and Hedlund (1981))
• Exploration — Drilling by DuPont Co. in 1957 and 1958 indicated reserves, 1957–1958 (Olson and Hedlund (1981))
• Exploration — DuPont stakes claims and acquired properties and mineral rights to explore for niobium at Iron Hill, 1956 (Van Gosen (2009))

Record imported from USGS USMIN ver. 4.0 (June 2019). Developments after that date appear on facility records, not here; see the methodology.