"The study of the geology and mineralization of the Crystal Mine was undertaken at the suggestion of Mr. Dodson G. Gibson of the Crystal Fluorspar Company, who allowed access to the mine and its records for this purpose. The problem was to develop a system of prospecting the so-called bedded-replacement fluorspar deposits of Hardin County, Illinois. It is believed that an economically feasible method has been developed. This method involves not only the actual finding of fluorspar, but also serves as a guide in revealing structural trends suggestive of mineralization.
The fluorspar deposits of the Crystal Mine are representative of the mineralized areas in the Cave In Rock district. They are composed, principally, of crustified, “coontail” and disseminated fluorspar which is associated with very small amounts of calcite, barite, sphalerite, galena, chalcopyrite, pyrite and hydrocarbons. The fluorspar occurs in horizontal bladed* [*Refers to geologic structures having dimensions approximately 100:10:1] deposits along definite stratigraphic horizons. The principal mineralized zones are at the base of the Bethel sandstone and at the bases of the Rosiclare and Sub-Rosiclare members of the Ste. Genevieve limestone. These zones occur in fossiliferous limestone and are associated with trough-like structures which are superimposed upon a regional structure having a strike of N. 73° W. and a dip of about 4° N. 17° E. The trends of the fluorspar deposits are N. 50° E. and from N. 40° W. to N. 70° W. They are, in general, parallel and normal to the Peters Creek fault which marks the northern boundary of the district. The principal mineralization is parallel to the fault.
As the trough-like structures which accompany the mineralization are readily evident, their careful delineation should aid in locating the fluorspar deposits. Because they are minor structures, superimposed upon the regional dip and somewhat obscured by it, the writer conceived the idea that they might best be shown by drawing structure contour maps at various well defined horizons, using as a datum, not sea level, but an inclined plane parallel to the regional dip. The delineation of these trough-like structures is based upon well logs of approximately 400 diamond and churn drill holes.
Although these trough-like structures are indicative of mineralization, other features considered important in localization of the fluorspar are: suitable lithology, high permeability, fracturing, faulting and the presence of preexisting solution channels.
Because of heavy flooding by rains, only about 25 percent of the mine workings were accessible. For these workings, the geology was mapped in detail and correlated with the data obtained from the well logs. It is believed that enough of the mine was mapped and compared to the structure to predict the geology throughout the rest of the mine"--Introduction, pages 1-2.
Grawe, Oliver R. (Oliver Rudolph), 1901-1965
Geosciences and Geological and Petroleum Engineering
M.S. in Geology
Missouri School of Mines and Metallurgy
vi, 53 pages
Hardin County, Illinois
© 1950 John Amsden Emery, All rights reserved.
Thesis - Open Access
Fluorspar -- Illinois -- Hardin County -- Analysis
Geology -- Illinois -- Hardin County
Geology, Stratigraphic -- Mississippian
Print OCLC #
Electronic OCLC #
Link to Catalog Record
Emery, John Amsden, "Geology of the Crystal Mine, Hardin County, Illinois" (1950). Masters Theses. 3050.
Geologic Map of the Rosiclare Workings
Emery_John_1950_Plate_5.tif (1952 kB)
Geologic Map of the Bethel Workings
Emery_John_1950_Plate_6.tif (97495 kB)
Structure Contour Map of the Bottom of the Rosiclare Sandstone
Emery_John_1950_Plate_7.tif (98053 kB)
Structure Contour Map of the Top of the Rosiclare Sandstone
Emery_John_1950_Plate_8.tif (104079 kB)
Structure Contour Map of the Adjusted Surface of the Bottom of the Rosiclare Sandstone