North-Central Section - 54th Annual Meeting - 2020

Paper No. 29-4
Presentation Time: 8:30 AM-5:30 PM


KERSHAW, Charles Thomas1, HUMMER, Daniel R.1 and DENNY, F. Brett2, (1)School of Earth Systems and Sustainability, Southern Illinois University, Carbondale, 1259 Lincoln Dr., Carbondale, IL 62901, (2)Illinois State Geological Survey, Prairie Research Institute, University of Illinois, Champaign, IL 61820

Hick’s Dome is a crypto-volcanic feature within the south-central portion of the Illinois Basin. Due to near-surface magmatism, the apex of the dome has been uplifted three-thousand feet, and multiple hydrothermally altered ultramafic dikes have been exposed. Due to occurrences of rare earth minerals, radioactive metals such as Th, and minerals commonly found in diamond-bearing kimberlites, the region has potential for producing economically viable ore deposits. The samples observed during this study were collected from numerous dikes around the uplifted area, and each contained a wide variation in levels of hydrothermal alteration. The overall goal was to observe mineral assemblages and textures within each sample in order to make inferences about the makeup of the original melt, along with methods of emplacement for any rare metals observed. Techniques utilized include observation under a petrographic microscope, scanning electron microscope, and X-ray powder diffraction. Mineralogically, most of the samples exhibiting hydrothermal alteration were composed of brecciated Paleozoic country rock that had undergone large amounts of silicification. Veins of quartz and amorphous silica glass indicate movement of silica-rich hydrothermal fluid. Samples from two of the dikes that underwent heavy hydrothermal alteration showed occurrences of rare earth minerals. One sample from the southwestern region of the dome contains widespread euhedral grains with hexagonal and tetragonal cross section, similar to those seen within other samples from Hick’s Dome known to contain rare earth minerals. The tetragonal grains in particular are morphologically similar to regional occurrences of the mineral xenotime-(Y). Within the less altered samples, what appear to be brecciated pieces of Precambrian granite are present within an ultramafic matrix. Results of this study have yielded a more widespread presence of rare earth elements than previously documented, signs of multiple hydrothermal events of varying compositions, and the presence of granitic basement rock clasts that indicate an explosive upwelling of partially melted upper mantle material.