Fluorspar Area Fault Complex
Location
Hardin and Pope Counties, Illinois (J, K-6, 7); Crittenden and Livingston Counties, Kentucky and parts of adjacent counties
References
(selected) Bain 1905, S. Weller et al. 1920, Bastin 1931,Currier and Hubbert 1944, Clark and Royds 1948, J. Weller et al. 1952, Williams et al. 1954, Clegg and Bradbury 1956, Palmer 1956, Heyl and Brock 1961, Baxter et al.1963, Brecke 1962, 1964, McGinnis and Bradbury 1964, Baxter and Desborough 1965, Heyl et al. 1965, Baxter et al. 1967, Grogan and Bradbury 1968, Hook 1974, Trace 1974, Pinckney 1976, Treworgy 1981, Trace and Amos 1984, Nelson and Lumm 1987, Bradbury and Baxter 1992, Sargent et al. 1992, Whitaker et al. 1992, Nelson and Harrison 1993
Description
(pl.2) The term Fluorspar Area Fault Complex was first used by Treworgy (1981) for the complexly faulted area associated with the Illinois-Kentucky Fluorspar District. The complex contains a variety of structural elements, reflecting a long and complicated history of recurrent tectonic, igneous, and mineralizing activity.
The boundaries of the Fluorspar Area Fault Complex more or less coincide with the limits of commercial fluorspar deposits. These limits can be taken as the Lusk Creek Fault Zone on the west, the Rough Creek-Shawneetown Fault System on the north, and the Tabb and westernmost Pennyrile Fault Zones on the south in Kentucky (fig. 2). To the southwest, the Fluorspar Area Fault Complex continues beneath sediments filling the Mississippi Embayment. Eastward, the faults of the complex die out in Webster, Hopkins, and western Union Counties, Kentucky.
Structural elements of the Fluorspar Area Fault Complex include Tolu Arch, Hicks Dome and associated ultramafic intrusions and diatremes, faults radial and concentric to Hicks Dome, northeast-trending block faults, and northwest-trending strike-slip faults.
Tolu Arch is a southeast-trending anticlinal feature located mainly in Kentucky but extending into Hardin County, Illinois. It is in line with Hicks Dome and greatly broken by northeast-trending block faults.
Hicks Dome is a roughly circular uplift centered in western Hardin County. It is approximately 10 miles (16 km) in diameter and has structural relief of about 4,000 feet (1,200 m). Many diatremes or explosion breccias have been encountered near the dome at the surface and in a well drilled at the apex (Brown et al. 1954, Bradbury and Baxter 1992). Also, numerous dikes and small stocks and sills of lamprophyre and mica peridotite occur near Hicks Dome. The dikes radiate from the dome and are concentrated along a northwest-southeast axis that includes Tolu Arch on the southeast and the Cottage Grove Fault System on the northwest. Radiometric dating places the time of intrusion as Early Permian (Zartman et al. 1967). Diatremes contain fragments of sedimentary and igneous rocks in a fine grained calcareous matrix. Palmer (1956) reported fragments of "slate, quartzite, and granite" in two diatremes south of Hicks Dome in Section 31, T12S, R8E. The granite probably is Precambrian. The nature and origin of the slate and quartzite are unknown. Most geologists now attribute Hicks Dome to explosive igneous activity (Bradbury and Baxter 1992).
Faults radial and concentric to Hicks Dome were mapped by Baxter and Desborough (1965) and Baxter et al. (1967). They are concentrated in a belt 3 to 4 miles (5-6.5 km) out from the center of the dome. Most apparent are high-angle normal faults, having displacements of a few hundred feet at most. They evidently are products of the doming process.
Northeast-trending block faults dominate the Fluorspar Area Fault Complex and carry rich vein deposits of fluorite, galena, sphalerite, and barite. They are high-angle faults that outline complicated horsts and grabens (fig. 35). Most are normal faults but some are reverse, and many faults bear obliquely plunging striae and corrugations indicative of strike-slip movement (fig. 36). Maximum dip-slip displacement may reach 3,000 feet (900 m); strike-slip displacement generally is less than dip-slip. Many faults show evidence of multiple periods of movement before, during, and after mineralization.
Small northwest-trending strike-slip faults have been encountered in underground mines of the district. Some of these faults contain ultramafic dikes. Dikes and faults are offset by northeast-trending block faults.
The sequence and timing of structural events in the Fluorspar Area Fault Complex have not been worked out entirely. Most of the movements occurred after early Pennsylvanian and before Late Cretaceous time. No rocks from the intervening period exist to help date the movements. As noted above, the ultramafic dikes are Early Permian, as determined from radiometric dating. Hicks Dome, its radial and concentric faults, and the northwest-trending strike-slip faults probably also are Early Permian (Bradbury and Baxter 1992).
Regional relationships point to an early episode of compressional deformation and igneous activity followed by an episode of extension, normal faulting, and mineralization. The compressional episode probably took place during the Permian Period in conjunction with the Alleghenian Orogeny. In this event the principal stress axis was oriented northwest to southeast. High-angle basement faults, including the Rough Creek-Shawneetown Fault System and the [[Lusk Creek Fault Zone]], were reactivated as reverse faults. Ultrabasic magma was injected upward along northwest-trending tension fractures, and Hicks Dome was formed. Following the compression came an episode of northwest-southeast extension, probably during early Mesozoic time. Earlier formed reverse faults underwent normal movement, and many new northeast-trending normal faults were created. These faults provided pathways for recurrent mineralization with fluorite and associated minerals.
The last movements on faults in the Fluorspar Area Fault Complex extended well into the Tertiary Period, and possibly into the Quaternary. Rhoades and Mistler (1941) documented displacements of Cretaceous strata along northeast-trending faults near the Tennessee and Cumberland Rivers in western Kentucky. Their findings were confirmed by geologic quadrangle mapping in the same area (Amos 1967, Amos and Finch 1968). In Illinois, Ross (1963, 1964) reported tectonic deformation of Cretaceous through Eocene strata of the [[Mississippi Embayment]]. Kolata et al. (1981) reexamined the evidence and reported that all known post-Cretaceous deformation in Illinois could be attributed to nontectonic causes such as landsliding or solution collapse. I have recently discovered, however, several examples of faults offsetting units as young as the Mounds Gravel in the Dixon Springs Graben near the edge of the Mississippi Embayment. The age of the Mounds is believed to be Pliocene to possibly early Pleistocene (Willman and Fry 1970). The Dixon Springs Graben is directly in line with the zone of most intense earthquake activity in the New Madrid Seismic Zone immediately south of Cairo, Illinois (Nelson and Harrison 1993).
In terms of cumulative production, the Illinois-Kentucky district is the largest fluorspar-producing region in the United States. Metallic sulfides provide lead, zinc, some copper, and silver (from argentiferous galena) as byproducts; barite is produced when markets permit. Deposits are classified as veins (found along faults) and bedded replacement (mainly in upper Valmeyeran and lower Chesterian limestones).
References
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