Geology of the Illinois-Kentucky Fluorspar District

The geology of the IKFD has been discussed in many previous reports, with those of Bain (1905), S. Weller et al. (1920), Bastin (1931), J.M. Weller et al. (1952), Trace (1974, 1976), and Trace and Amos (1984) being the most notable. Two rounds of geologic mapping of the district in Illinois at 1:24,000 scale have been conducted, the first by Baxter and Des-borough (1965) and Baxter et al. (1963, 1967) and the second by Denny and Counts (2009) and Denny et al. (2008a, 2008b, 2010, 2011, 2013, 2016). Both have been summarized at a county-wide scale by Denny and Seid (2014) and Devera et al. (2016). The discussion below covers selected aspects of the geology of the district as it relates to the genesis of ore deposits.

Ore Deposits

Economic minerals within the IKFD occur in four types of deposits: (1) horizontal bedding replacement or strata bound, (2) veins along vertical faults and fractures, (3) residual gravel (referred to as “gravel spar”) derived from the weathering of either veins or beds, and (4) fluorite breccia. The primary ore mineral was fluorite, but several mines contained significant quantities of sphalerite and galena. Geologists noted that barite was quite abundant along the periphery of the ore shoots (Perry 1973), but calcite was by far the most abundant gangue mineral. At some of the mines, a considerable quantity of barite was not mined because of the price, as well as because of the deleterious effect of finely ground witherite associated with barite in the milling and gravity separation–flotation processes (Bradbury 1959). Sphalerite was abundant in some bedding replacement ore, as documented in both the Minerva and Davis-Deardorff Mines in the Cave-in-Rock Subdistrict. Some deposits contained a small amount of silver in the galena, along with recoverable cadmium and germanium in the fluorspar. Sikich (1959) reported that 6,400 tons of zinc was mined in southern Illinois in 1958. Gustavson and Sikich (1958) also reported that 3,075 ounces of silver was recovered during refining or smelting of the galena. In total, 1,775 tons of lead and 9,225 tons of zinc were produced in southern Illinois as a by-product of fluorspar mining in 1960 (Sikich 1961). Other minerals associated with the fluorite ore in this district included pyrite, chalcopyrite, quartz, celestite, cerussite, greenockite, malachite, smithsonite, witherite, strontianite, benstonite, and alstonite (Goldstein 1997). Paralstonite was identified as a new mineral, with type locality at the Minerva No. 1 Mine (Roberts 1979). Pyromorphite and anglesite were noted in oxidized zones at the Patrick Mine (Weller et al. 1952). Azurite was identified from specimens collected in the Lead Hill area as coatings on quartz and cherty limestone matrix (personal communication reported to R.C. Lillie, X-ray diffraction by Dr. John Rakovan, Miami of Ohio, 2019). The paragenetic sequence of mineralization has been studied by Hall and Friedman (1963), Richardson and Pinckney (1984), and Spry and Fuhrmann (1994). Marketing the ore required separating the host rock and other gangue minerals from the ore minerals. This was accomplished through milling or refining by both mechanical and chemical processes. In the early years, fluorite or galena was handpicked from the ore, and the host rock and gangue minerals were discarded. Modern milling operations use a variety of techniques to separate the valuable minerals from the waste, including crushing, grinding or milling, and separating minerals by gravity and flotation. The raw ore sent to the mill is normally greater than 30% fluorite, 1% to 14% sphalerite, and 1% to 5% galena. The completed, refined, or finished fluorspar is classified, according to the percentage of fluorite, as acid grade (>97% CaF₂), ceramic grade (97% to 85% CaF₂), or metallurgical grade (met spar; 85% to 65% CaF₂).

Stratigraphy of the Host Rock

Mineralization in the IKFD occurs in sedimentary rocks of Paleozoic age. Stratigraphic nomenclature in this region has been updated several times, but the older stratigraphic nomenclature persists in geologic literature from this mining district. Strata-bound or horizontal bedding replacement deposits are primarily hosted in Mississippian carbonates, but they also occur in Ordovician to Devonian carbonates at Hicks Dome. Fluorspar veins throughout the region are associated with Mississippian to Lower Pennsylvanian System rocks. The current stratigraphic nomenclature of the lower part of the Chesterian (Upper Mississippian) rock in Illinois is given in Figure 2. In vein-type mines, the mining companies usually name the mining or working levels as depths below the ground surface. Hence, the 300-foot level would be located 300 feet below the ground surface at the main hoisting shaft. In strata-bound deposits, the mining levels were more commonly named for the stratigraphic formation that formed the roof above the mineralized beds. For example, the “Bethel Level” would be found in the limestone bed below the Bethel Sandstone, which, when using current stratigraphic nomenclature, would be the Downeys Bluff Limestone Member of the Paoli Limestone (Figure 2). Lower Chesterian Series stratigraphic nomenclature in Illinois is complex and has undergone several revisions. Early geologic classifications combined all strata from the top of the Ste. Genevieve Limestone to the base of the Bethel Sandstone into the Renault Formation (Weller et al. 1920). Swann and Atherton (1948) correlated the Rosiclare Sandstone with the Aux Vases. Swann and Atherton (1948) also classified a lower sandy zone in the Ste. Genevieve as the Spar Mountain Sandstone. Both Baxter et al. (1967) and Swann (1963) correlated the Rosiclare with the Aux Vases. They also used the name Spar Mountain for a sandy bed in the Ste. Genevieve Limestone, conforming with the Swann and Atherton (1948) classification. Because the “Renault” of southeastern Illinois includes rocks younger than the type section in Monroe County, Nelson et al. (2002) combined units and reduced several formations to member status (Figure 2). Nelson et al. (2002) discontinued usage of Renault Formation in southeastern Illinois and substituted the Indiana name Paoli Limestone. They also divided the Paoli Formation into the Downeys Bluff Limestone Member (youngest), Yankeetown Member, Shetlerville Limestone Member, and Levias Limestone Member (Figure 2). In addition, previous authors, such as Baxter and Desborough (1965) and Willman et al. (1975), had used the term West Baden Group to encompass the Bethel Sandstone (oldest), Ridenhower Formation, and Cypress Formation. Under current usage (Nelson et al. 2002; Denny and Nelson 2005), the West Baden is a formation (not a group) composed predominantly of sandstone and is restricted to areas where the Bethel, Ridenhower, and Cypress cannot easily be differentiated. Although Nelson et al. (2002) recommended abandoning the Rosiclare Sandstone for the Aux Vases, the term Rosiclare is so entrenched in the historical literature that removing this term would undoubtably create more confusion. However, it should be noted that the Aux Vases is currently the preferred stratigraphic nomenclature. For additional information on the stratigraphy, we recommend the Illinois State Geological Survey (ISGS) ILSTRAT web interface (https://isgs.illinois.edu/ilstrat/index.php/Main_Page). A thin, sandy limestone was mineralized near the base of the Ste. Genevieve Limestone in the Annabel Lee Mine in the Harris Creek Subdistrict. This silty to sandy limestone unit was never more than 1 foot thick and was often challenging to identify in cores. The working level was described by miners as the St. Louis Level. The interval was below the Spar Mountain Member but still within the Ste. Genevieve Limestone. To prevent a confusing name being applied to this level, the name “Cadiz Level” was proposed by Ross Lillie, who worked as a mine geologist for the Ozark-Mahoning Mining Company. Cadiz is a very small, unincorporated community and former site of a Civilian Conservation Corps work camp located near the Annabel Lee Mine. The Cadiz Level was recognized, but not uniquely identified, in a stratigraphic section drawn by Minerva Oil Company geologists (Montgomery et al. 1960, p. 8, fig. 3).

Ore Models

Depositional models or theories concerning emplacement of the strata-bound fluorspar involve low-temperature hydrothermal replacement of carbonate host rock. Acidic fluorine-rich ore fluids are theorized to have ascended along small faults and fractures, and sometimes through cylindrical-shaped breccia pipes, until reaching an upper impermeable boundary or roof rock. The upward flow of the fluids is suggested by the mushroom-type structures observed at some of the mines at Lead Hill (Currier 1944) and the breccia pipes observed in several mines in the Cave-in-Rock Subdistrict (Brecke 1962). The mineralizing fluids ascended until they encountered an impermeable or less permeable roof rock, at which time the fluids flowed laterally along the fault zone or the bedding of the host strata. In bedding replacement deposits, the fluorite replaced the carbonate host, which was usually limestone. It is unclear whether the carbonate host rock supplied calcium to form the fluorite, buffered the pH of the very acidic ore fluid (Kenderes and Appold 2017), or possibly both. Conceptually, a limestone (CaCO₃)-to-fluorite (CaF₂) reaction would proceed as follows: CaCO₃ (solid) + 2HF (ore fluid) → CaF₂ (solid) + H₂O (liquid) + CO₂ (gas). Some ore models proposed the ore fluid ascended in response to an increasing geothermal gradient, which was initiated by Permian igneous activity that underlies the entire IKFD. The age of the mineralization is not certain, given that age determinations from Permian to Early Cretaceous have been suggested by previous authors (Harder 1986; Ruiz et al. 1988; Chesley et al. 1994; Symons 1994; Brannon et al. 1996a). The vein-type ore along fault planes most likely formed in an analogous manner, but the fluorspar crystalized in open spaces or voids along faults. The vein deposits are also usually in contact with or adjacent to a carbonate host rock, and in places have replaced an earlier phase of vein calcite. The amount of displacement along mineralized faults normally ranges between 50 and 500 feet (Weller et al. 1952). Faults with smaller displacement typically did not create enough open space to accommodate the ascending ore fluid, whereas faults with large displacement commonly produced gouge zones or incorporated shale layers that impeded the flow of the ore fluid. However, exceptions to these rules are known. The fluorite breccia beneath Hicks Dome, although somewhat different in form, is most likely genetically and temporally related to the other fluorspar deposits in the region. The Hicks Dome mineralized breccia is described in the Hicks Dome section of this document.

Mississippi Valley-Type Relationship

Geochemical investigations indicate that the chemistry and temperature of the ore-forming fluids in the IKFD have much in common with other Mississippi Valley-type (MVT) deposits, and this district has been considered an MVT district (Ohle 1959). The issue with an MVT classification for the IKFD relates to the fact that MVTs are generally not associated with igneous rocks, whereas numerous ultramafic intrusive rocks, breccias, and diatremes occur within the IKFD. However, much evidence exists that the ore deposits were formed in a similar manner as other MVT deposits. Hall and Friedman (1963) proposed that ores in the Cave-in-Rock Subdistrict were sourced from several different fluids. Their work with oxygen and carbon isotopes suggested that the ore probably formed through a combination of connate, hydrothermal, and magmatic fluids. The mixing of several types of fluids is also supported by geochemical analyses of the ore (Richardson et al. 1988; Spry et al. 1990; Spry and Fuhrmann 1994). Fluid inclusion microthermometry results from Spry and Fuhrmann (1994) also suggested the mixing of three fluids, with two being relatively saline and relatively hot and the third being cooler and less saline. Pelch (2011) likewise proposed the mixing of at least three fluids in the Cave-in-Rock Subdistrict, based on an analysis of the elemental concentrations and microthermometry of fluid inclusions. Although consensus is lacking on the proper classification of these deposits, as a whole, the ore-forming solutions were acidic, with pH as low as 0 to 1.4 (Kenderes and Appold 2017), saline (typically >20% NaCl equivalent), and epithermal or low-temperature hydrothermal (80 to 150 °C). A few workers have proposed a relationship between the underlying ultramafic igneous activity and mineral deposition (Plumlee et al. 1995; Kendrick et al. 2002). Although mineralized breccia-type deposits have not been well studied, rare earth minerals, niobium, and possibly thorium are associated with the fluorite mineralization within breccia beneath Hicks Dome (Pinckney 1976). The identification of microcrystalline aggregates of xenotime in fluorite (Wall and Mariano 1996) is very suggestive of carbonatite intrusion beneath Hicks Dome (Mariano 1989). Morehead (2013) concluded that magmatic carbon is most likely present at Hicks Dome and that more than one igneous phase may be present in the region. Long et al. (2010) also discussed the presence of rare earth elements at Hicks Dome in their compilation of U.S. rare earth element deposits.

Illinois-Kentucky Fluorspar District Ore Model

Considering the available data, we suggest that IKFD mineralization formed through the mixing of several fluids, including possibly NaCl-rich “MVT brines,” hydrothermal fluids, and magmatic fluids associated with the ultramafic intrusions (Denny et al. 2017). The ultramafic igneous rock was most likely a key factor in supplying a higher geothermal gradient, uplifting and fracturing the host rock, and providing at least a portion of the fluorine to the ore-forming solutions. The pH of the acidic mineralizing fluids was buffered in contact with the limestone host rock, which led to the disequilibrium and crystallization of the ore minerals.

Faulting and Tectonic History

Mineralized veins are mostly aligned along northeasterly striking high-angle normal faults, which are assumed to be the primary conduits for the ascending ore fluid. Although most faulting occurred before the deposition of the minerals, some postmineralization movement has been documented along faults in the Rosiclare Subdistrict. In particular, postmineralization movement along the Daisy Vein is very well documented. The overprinting of multiple tectonic events along with the Permian igneous uplift and subsequent subsidence have created a very complex geologic fabric. The region has undergone tectonic activity spanning more than 500 million years of Earth’s history. Three major episodes of tectonism took place in response to plate movements during the assembly and diassembly of supercontinents:

1. Failed rifting produced normal faults during the breakup of the supercontinent Rodinia in the latest Proterozoic Era and early Cambrian Period.
2. Northwest-directed compression induced reverse faulting and ultramafic igneous activity during the Alleghenian orogeny, related to the assembly of the supercontinent Pangea during the late Paleozoic Era.
3. Crustal extension induced a second episode of normal faulting (with a probable left-lateral component) during the breakup of Pangea, culminating during the Jurassic Period.

Cambrian Failed Rifting

Setting the stage for the IKFD was the failed rifting that took place during the breakup of the supercontinent Rodinia during the latest Proterozoic to the middle Cambrian Period. The term “failed rifting” defines the partial breakup of a tectonic plate without complete separation and the development of an ocean basin. The failed rift segments relevant to the IKFD are the northeast-trending Reelfoot Rift (Ervin and McGinnis 1975) and the east-trending Rough Creek Graben (Soderberg and Keller 1981), which join in southern Illinois and adjacent Kentucky (Figure 3). Regional seismic reflection profiles (Bertagne and Leising 1991; Potter et al. 1995, 1997) image some of the large normal faults that compose the Reelfoot Rift and Rough Creek Graben. Penetrating the Precambrian crystalline basement, these faults outline grabens that contain thick Cambrian sedimentary rocks absent elsewhere in the region. Late Paleozoic reverse faulting originating within the Precambrian is interpreted on the flank of the Rock Creek Graben, and Permian or younger extensional faulting is suggested as overlying the fluorspar-area faults (Potter et al. 1997). Deep oil and gas test holes have partially penetrated these rift-filling strata, yielding clues to the tectonic succession. The failed rifting is significant in that it created zones of crustal weakness that were reactivated during later tectonic episodes. As is evident in the New Madrid Seismic Zone, portions are still undergoing movement today. These same crustal fracture zones also provided pathways for subsequent hydrothermal mineralization.

Late Paleozoic Compression

As Rodinia broke apart, the Reelfoot Rift and Rough Creek Graben became a narrow, elbow-shaped arm of the sea. By late Cambrian time, crustal subsidence became more general, producing an elongate trough that became the Illinois Basin (Kolata and Nelson 1991, 2010). The sedimentary rocks that would later host IKFD orebodies accumulated in the basin. During the Pennsylvanian Period, tectonic plates that had rifted away earlier began to collide with the eastern and southern margins of the North American plate. The Appalachian and Ouachita Mountains rose in response to collisions that were part of the process of assembling all the major landmasses of the globe into the supercontinent of Pangea. Collisions related to the Appalachian and Ouachita Mountain-building events transferred compressional stresses toward the northwest into the present area of the IKFD. This crustal compression had two main consequences. First, some of the normal faults that had formed during Cambrian rifting became reverse faults, hanging walls moving upward. This action chiefly affected the Lusk Creek, Shawneetown, and Rough Creek Fault Systems, which today generally delimit the western and northern boundaries of the IKFD. Second, a series of northwest-trending fractures served as pathways for ultramafic magma derived from the Earth’s mantle. Chaotic reflectors near Hicks Dome were modeled as “a combination of intrusive brecciation, intense faulting, and alteration of carbonate strata by acidic mineralizing fluids, all of which occurred in the Permian” (Potter et al. 1997, p. 537). Evidence of and theories regarding whether Hicks Dome also formed during the compressional episode remain in conflict.

Mesozoic Extension

The third great tectonic episode shaping the IKFD commenced with the breakup of Pangea, which began late in the Triassic Period and is continuing to the present. New ocean basins, including the Atlantic Ocean and Gulf of Mexico, are products of the breakup, which also reactivated many older zones of weakness within the North American continent. Many older faults within the IKFD underwent normal movement, and many new normal faults developed (Potter et al. 1995). These faults displaced the Permian-age dikes of the district (Hook 1974; Trace 1974) and overprinted reverse faults of the Lusk Creek, Shawneetown, and Rough Creek Fault Systems (Nelson 1991, 1995). The age of mineralization in the district is controversial, but most authors agree it is Permian or younger. Chesley et al. (1994) reported an early Permian date based on the 147Sm/144Nd isochron for fluorite. Symons (1994) settled on a Late Jurassic age based on paleomagnetism. Using strontium isotope data, Ruiz et al. (1988) reported an Early Jurassic date. Brannon et al. (1996b) also reported an Early Jurassic age based on ore-stage calcite U-Pb and Th-Pb isotopes. Assuredly, a Jurassic age is attractive from the tectonic viewpoint, although Permian mineralization of northeast-trending normal faults is less compatible with the northeast–southwest compressional stress regime that prevailed at that time. The age of Hicks Dome is also controversial. Its association with Permian-dated intrusions has led many authors to assign Hicks Dome to the Permian Period. However, Ruiz et al. (1988) suggested a Mesozoic age based on strontium isotope data. Shallow intrusions on the dome are conceivably Permian age, whereas deeper breccias that formed the dome are younger. As Potter et al. (1995) showed via seismic reflection data, the major breccias and intrusions that raised the dome reside within the Precambrian basement, not the Paleozoic sedimentary succession.

Role of Strike-Slip Faulting

Geologists dating back at least to Bastin (1931) have reported evidence for an element of strike-slip faulting in the IKFD. Hook (1974) stated, “The northwest-trending [dike] faults seem to have predominantly strike-slip movement” (p. 81). Many authors, ourselves included, have observed striations and mullion indicative of a strike-slip component on the northeast-trending faults that bear vein deposits. However, those who have studied the structure closely concur with Trace (1974) and Hook (1974) that the horizontal component cannot be large because Permian dikes display little or no lateral offset where they intersect the younger northeast-striking faults. That said, a modest strike-slip element may have been sufficient to play a large but unappreciated role in orebody genesis. In the Rosiclare Subdistrict, host to the richest fluorspar mines in the United States, the complex fracture zone that bounds the southeast side of the Rock Creek Graben bends from its typical northeast trend to nearly due north. Bounding the east side of the zone is the widest vein in the IKFD, the Rosiclare Vein, which follows a nearly vertical fault that is sinuous in dip direction. Bastin (1931), who spent more time underground in these mines than any other publishing geologist, reported multiple examples of wide-open caverns and fissures along the veins of the Rosiclare Subdistrict. These included “a channel large enough for a man to crawl into” (p. 42) in the Daisy Mine and a cavern lined with crystals that was 60 feet long, 30 feet high, and as wide as 15 feet in the Argo Vein. The veins themselves contained jumbled, angular, and corroded clasts of wall rocks as large as about 3 feet across. Such conditions led to speculation of pull-apart or transtensional structures. Bastin (1931) also noted striations and mullion indicative of strike-slip. At the Hillside Mine, slickensides and grooves are pitched 20° to 30° S on a fault having the west side downthrown, a configuration consistent with left-lateral slip. In the Daisy Mine, the plunge of striations varied from 10° S to 80° N, again consistent (for the most part) with a sinistral component. Elsewhere, Hook (1974) observed, “The Gaskins Mine in the Empire District on the west side of Hicks Dome has predominantly horizontal slickensides and tension fractures indicating left-lateral movement” (p. 83). However, right-lateral indicators also have been reported. Citing a personal communication with Don Saxby, Hook (1974) noted, “Both left-lateral and right-lateral movement have been observed in the Cave-in-Rock area of the district” (p. 83). In 1982, John Nelson observed prominent striations and mullion plunging northwest on the West Vein in Ozark-Mahoning’s Barnett Mine. Being on a normal fault with the northwest side down, these features indicate right-lateral slip. Structural movement may currently be occurring, as evidenced at the Knight Mine, where residual “stress relief ” deformed circular diamond drill holes into ellipses (personal communication from R.L. Perry to Ross Lillie, 2015; R.L. Perry was the Chief Geologist with the Ozark-Mahoning Company at the time of this observation). Just southwest of the IKFD in southern Pope and Massac Counties in Illinois, segments of the Fluorspar Area Fault Complex displace sediments as young as Pleistocene. The faults that offset Cenozoic units strike N 20°–40° E, more northerly than the trend of the fault complex as a whole. They outline narrow, linear grabens into which sediments have dropped. As in the Rosiclare area, the fault pattern fits left-lateral transtension (Nelson et al. 1997, 1999). However, such a pattern is not consistent with the contemporary stress field, in which the principal compressive stress axis is oriented N 60°–90° E and should induce right-lateral transpression (Zoback and Zoback 1980, 1991; Nelson and Bauer 1987). Although several explanations are possible, the most appealing is that the stress regime has changed through time (Nelson et al. 1999). Such changes in the stress field orientation would account for observations of right-lateral slip in the IKFD. In summary, the IKFD has undergone three major episodes of deformation and probably several lesser episodes during the last 500 million years of Earth’s history. Failed rifting during the breakup of Rodinia in the latest Proterozoic and early Cambrian Period set the stage for deep structural control of the faults. Compression associated with continental collision during the assembly of Pangea in the late Paleozoic Era induced reverse faulting and ultramafic intrusions. Renewed extension as Pangea broke apart during the Jurassic Period created the northeast-trending faults and fractures that served as pathways for mineralizing fluids. Indicators are strong that the phenomenally rich Rosiclare Vein System formed at a releasing bend under left-lateral transpression. However, the stress regime has changed through time since the Jurassic and continues to evolve to the present day.

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Circular 604 | Production History | Geology of the Illinois-Kentucky Fluorspar District | Mining Methods | Mineral Subdistricts and Individual Mines | Conclusions and Acknowledgments

References

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