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.