Hicks Dome

The dominant structural feature of the Illinois portion of the IKFD is Hicks Dome, an elliptical to nearly circular uplift approximately 10 miles in diameter in western Hardin County and northeastern Pope County. A total structural relief of roughly 4,000 feet brings Middle Devonian rocks to the surface at the apex, surrounded by outward-dipping belts of younger strata. Igneous diatremes are scattered throughout the region, and nearly vertical dike-like intrusive breccia bodies radiate from the center of Hicks Dome (Baxter et al. 1967). Most geologists concur with Bradbury and Baxter (1992) that the dome is the product of large-scale igneous intrusion and brecciation at depth.

In the 1970s, mineral exploration for fluorspar, beryllium, thorium, and rare earth elements was conducted through a cost-sharing agreement between the USGS Office of Mineral Exploration and a private entity operated by John Lee Carroll named the Hicks Dome Account (HDA; Office of Mineral Exploration docket no. 6873). The geologist in charge of the Hicks Dome project for Carroll was Joe Porter. Several test holes drilled near the crest of Hicks Dome disclosed significant mineralization in the Ordovician strata over a thickness of more than 200 feet. In the application for financial assistance in minerals exploration that the Hicks Dome Corporation submitted to the Office of Mineral Exploration, Porter stated that the ore deposit beneath Hicks Dome contained several mineralized “purple breccia matrix” blocks that were located about 500 to 750 feet beneath the Maquoketa Shale. Porter stated that the purple breccia is in the anticipated position of the Ordovician Plattin Limestone and that it probably extends into the underlying Joachim Dolomite. The deepest exploration hole was more than 2,500 feet and the shallowest was 2,200 feet. The Pankey No. 2 hole was drilled to 2,485 feet, and the core was composed of breccia cemented by white calcite, with sparse fluorite in the upper portion, but fluorite averaged between 2% and 8% from 2,000 to 2,240 feet. Hamp No. 3 was composed of brecciated limestone or “crackled” limestone cemented by 4-millimeter-thick bands of purple fluorite and calcite, with the percentage of fluorite increasing at 2,000 feet. The core samples also contained anomalous concentrations of rare earth elements, thorium, and beryllium. Geologists working on the deposit designated the breccia as either complex breccia or host breccia. Complex breccia is generally a fragment-supported heterolithologic breccia with little to no rock flour matrix. Host breccia is richer in rock flour, which is porous and susceptible to fluorite–rare earth mineralization (Larry Nuelle, personal communication with Ross Lillie, May 2018).

Although their findings are highly speculative, Staatz et al. (1979) postulated that a large potential reserve of thorium (ThO₂) was present at Hicks Dome. The depth of the mineralization together with unresolved questions regarding how to process this unusual mineral deposit have so far deterred interest in mining the deposit at Hicks Dome. Other mines and prospects in the area of Hicks Dome are described below.

Mines in the Hicks Dome

Lacey Prospect

Located southwest of Hicks Dome, this prospect comprises several surface pits that follow veins striking N 40°–47° E (Weller et al. 1952). Development work by the Big Creek Fluorspar Company and A.B. Mann was ongoing here in 1939 (Davis and Trought 1940). Shafts 24 to 154 feet deep were driven to a vein that averaged 4 feet wide, but no commercial production is on record (Weller et al. 1952). Denny et al. (2017) reported the geochemical analysis of a sample composed of a mixture of calcite and fluorite collected from the mine dump in this area. Results were inconclusive because of poor fluorine results along with total weight percentage calculations of 68%. However, slightly elevated values of copper and rare earth elements were suggested. More Information: 10083005

Rose Mine

The Rose Mine, located east of the apex of Hicks Dome, produced gravel spar and some bedding replacement ore. It was operated by W.M. Rohrer (Weller et al. 1952) and may have been owned by D.C. Peyton before 1917 (Pogue 1918). This mine, which was abandoned before 1931, consisted of two pits no more than 20 feet deep and a shaft no more than 100 feet deep (Bastin 1931). A small steam shovel dug the pits to extract gravel spar (Bastin 1931). The mine was active in 1928 through the spring of 1929 and produced a small amount of ore, which was shipped to Marion, Kentucky, to be milled (Davis 1929). In 1969 and 1970, the Industrial Minerals Company excavated additional pits and recovered approximately 436 tons of ore grading 53% CaF2. Evidence of bedding replacement ore is present in the tailings at this site in Devonian limestone. In 1982, Ross Lillie examined the tailings of the Rose Mine. Numerous fluorspar fragments were collected and easily cleaved into octahedrons, a property commonly associated with replacement deposits. Lillie surmised that the fluorspar, although colorless, appeared greenish because of copious inclusions of chalcopyrite. More Information: 10083024

References

Bastin, E.S., 1931, The fluorspar deposits of Hardin and Pope Counties, Illinois: Illinois Geological Survey, Bulletin 58, 116 p.

Baxter, J.W., G.A. Desborough, and E.W. Shaw, 1967, Areal geology of the Illinois Fluorspar District: Part 3—Herod and Shetlerville Quadrangles: Illinois State Geological Survey, Circular 413, map, 1:24,000; report, 41 p. and 1 pl.

Bradbury, J.C., and J.W. Baxter, 1992, Intrusive breccias at Hicks Dome, Hardin County, Illinois: Illinois State Geological Survey, Circular 550, 23 p.

Davis, H.W., 1929, Fluorspar and cryolite, in O.E. Kiessling, ed., Mineral resources of the United States 1926, Part II—Nonmetals: U.S. Department of Commerce, Bureau of Mines, Division of Mineral Statistics, p. 17–49.

Davis, H.W. and M.E. Trought, 1940, Fluorspar and cryolite, in H.H. Hughes, ed., Minerals yearbook 1940 [year 1939]: U.S. Department of the Interior, Bureau of Mines, Economics and Statistics Branch, p. 1333–1351.

Denny, F.B., A.H. Maria, J.L. Mulvaney-Norris, R.N. Guillemette, R.A. Cahill, R.H. Fifarek, J.T. Freiburg, and W.H. Anderson, 2017, Geochemical and petrographic analysis of the Sparks Hill Diatreme and its relationship to the Illinois-Kentucky Fluorspar District: Illinois State Geological Survey, Circular 588, 51 p.

Pogue, J.E., 1918, Optical fluorite in southern Illinois: State of Illinois Department of Registration and Education, State Geological Survey Division, University of Illinois, Bulletin 38, p. 424.

Staatz, M.H., T.J. Armbrustmacher, J.C. Olson, I.K. Brownfield, M.R. Brock, J.F. Lemons, L.V. Coppa, and B.V. Clingan, 1979, Principal thorium resources in the United States: U.S. Geological Survey, Circular 805, 42 p.

Weller, J.M., R.M. Grogan, and F.E. Tippie, 1952, Geology of the fluorspar deposits of Illinois: Illinois State Geological Survey, Bulletin 76, 147 p.