Origin of the bedding replacement deposits of fluorspar in the Illinois field

The banded fluorspar deposits of the Cave In Rock district are attributed to replacement of limestone and the preservation of bedding and cross-bedding of the rock. The solutions contained hydrofluoric acid which reacted with CaCO ₃ . The replacement was stoichiometrical, with consequent reduction of volume, but continued deposition of calcium fluoride, brought in from other points, partly or completely filled the voids. Thus, the pure bands display a comb structure, and represent replacement of pure limestone laminae. The less pure granular bands, devoid of comb structure, reflect the alternating, impure bands of the rock in which the fine-grained impurities (ferriferous carbonate, clay, et al) were disseminated. Recrystallization of clastic quartz grains in some calcareous sandstone layers resulted in the formation of bands of quartz grains. The writer dissents from the views of Bastin who believes the banding is due to rhythmic precipitation. Chemistry and controlling structural conditions are discussed.