Abstract
The Illinois-Kentucky fluorspar district has yielded most of the United States fluorite production. The fluorspar ore occurs as epigenetic replacement deposits in Mississippian limestone and as vein deposits. Paleomagnetic analysis was done on 324 specimens from 33 sites in fluorspar ore and in host limestones and cap-rock sandstones both adjacent to and remote from ore using alternating field and thermal step demagnetization. Saturation isothermal remanence methods were used to identify magnetization carriers in typical specimens and mineral crystals. An A characteristic remanent magnetization (RM) component was found outside the district in stratigraphically equivalent limestones that record a Pennsylvanian Kiaman remagnetization, and a C characteristic RM was found in surface weathered specimens that records the Recent earth's magnetic field. At 28 sites in ore and nearby host rocks a B characteristic RM was found that gives a Late Jurassic pole position of 152.4 degrees E, 78.0 degrees N (dp = 3.0 degrees , dm = 4.4 degrees ). Late Jurassic genesis for the fluorspar fits within the permissible geologic window from Early Permian to Late Cretaceous and agrees with fission track and strontium isotope dating on the fluorites but does not agree with a recent Sm/Nd date. It is suggested that a Late Jurassic hydrothermal event, here termed the "Rosiclare event," formed the fluorspar deposits, probably remagnetized Kiaman-remagnetized host rocks, probably was controlled geographically by faults of the Reelfoot-Wabash rift system, and probably coincided with the substantial uplift in the region that formed the Pascola arch.
