Mineralogy and paragenesis of the cave-in-rock fluorspar district, Hardin County, Illinois

The paragenesis of minerals occurring in the bedding-replacement fluorspar ores in the Crystal, Minerva No. 1 and Hill mines in the Cave-in-Rock district, Hardin County, Illinois, has been studied by underground, binocular, petrographic and ore microscopic examination. More than 300 hand specimens were collected from the mines, 100 thin sections were prepared and 36 polished surfaces were examined to prepare paragenetic diagrams for the Crystal and Minerva No. 1 mines. The minerals determined in the fluorspar ores were: fluorite, sphalerite, galena, chalcopyrite, pyrite, marcasite, calcite, strontianite, witherite, barite, quartz, dolomite, oil and bitumen. Most of these minerals are present both in disseminated form and as crystals deposited in vugs, but calcite, strontianite, witherite and barite were observed to be deposited from the ore solutions only in cavities. The paragenetic sequence of minerals formed in the disseminated ores has a general similarity to that of the minerals formed in vugs, particularly in the Crystal mine, but there are important differences between the two sequences. The latter sequence includes some minerals which are not present in the earlier sequence, but it is especially characterized by repetitive generations of mineral deposition. The repetitive nature is believed to be controlled by slight variations in the chemistry of the ore solutions during deposition. The paragenesis in the Minerva mine No. 1 differs from that of the Crystal mine by the occurrence of strontianite and witherite, by the number of generations and by the presence of important local, late corrosion. These differences in mineralogy and paragenesis are believed to have resulted from slight areal variations in the chemical nature of the ore solutions --Abstract, pages i-ii