Western Kentucky Fluorspar District.—The presence of fluorspar in the Kentucky-Illinois district was recognized early in the eighteenth century, attention being attracted by its brilliant colors, and was referred to by Owen in his early publications. The first important mining venture was apparently in Crittenden County. Other operations followed, but in all of these early attempts lead, in the form of galena, was the objective. Lead mining was stimulated by the Civil War and continued into the early seventies. With the extensive use of fluorspar in the steel industry production has followed the industrial expansion of the past several decades. The Kentucky-Illinois district constitutes the greatest source of fluorspar in the United States and one of the most important in the world. In Kentucky it takes in Crittenden, Livingston, and Caldwell counties and scattered prospects in Trigg and Lyon counties. Numerous studies of these deposits have been made including those by Ulrich and Smith (1905), Fohs (1907), Stuart Weller (1920a, 1921a, 1923, 1927a), Currier (1920, 1923), and Bastin (1931, 1939).
In 1937 Kentucky produced 87,296 tons of the various grades of fluorspar with a total value of $1,710,122, an average of $19.59 per ton. The greater part was gravel ores (78,163 tons) including flotation concentrates for hydrofluoric acid and cement and run of mine for flux in steel plants. Lump ore totaled 2,967 tons at $26.82 per ton and ground ore 6,166 at $27.21. The latter included flotation concentrates for the glass and enamel trades. Demand in this year was great. Domestic shipments were the largest since 1920 and imports were the largest since 1930. In the same year the Illinois part of the field produced 44,120 tons. Production in both Kentucky and Illinois was hampered for some weeks by floods.
The fluorspar region is intensely faulted (p. 149) and is intruded by peridotite dikes (p. 130). The mineral occurs in veins, also in bedding (blanket) deposits in Illinois, and as residual weathered material (gravel). The bedding deposits, unknown in Kentucky, are more or less horizontal ore bodies resulting from replacement of limestone, particularly the oolitic limestone underlying the relatively impervious shale at the base of the Rosiclare sandstone. This shale apparently blocked upward movement of the mineral bearing solutions which spread laterally beneath it following bedding planes and minor fractures, selectively replacing the limestone.
The veins have been formed along fault zones which are usually zones of intense brecciation many feet in width, in which mineralization has been accomplished in part by simple fissure and crevice filling and in part by replacement of the limestone fragments and wall rock. In this there was a selective process, some limestones being more susceptible than others. Replacement (Bastin, 1931) seems to have been the dominant process. As evidence of replacement Bastin listed ragged limestone remnants, diffusion banding, and the preservation of original sedimentary structures such as stylolites, oolites, and fossils.
Evidence of an igneous source for the mineralizing solutions is found in the "presence of basic dikes and sills, and of fragmental acid igneous material which may be explosive products of volcanism. These opposite chemical types of igneous rock in the same province make possible the postulation of magmatic differentiation at depth, and indicate that the region was one of more or less pronounced igneous activity, that is, it overlies a mass of igneous rock of which the dikes, etc., are offshoots. The proximity of a large igneous intrusion is also suggested by a slight doming of the strata and the complex pattern of block faulting" (Currier, 1920, p. 278). Some dikes have been faulted. Mineralization is known to follow dike contacts in only a few instances and only one such occurrence is mined (Currier, 1923). Folding, igneous activity, and faulting preceded the Cretaceous.
The veins in Kentucky vary in width from a few inches to 12 feet and occasionally more. Their trend is that of the faults. The lenticular form is due to varying susceptibility to replacement of the different limestone beds and the "structural sinuosity of the faults" (Currier, 1923, p. 53). The more important veins are found in areas of faulting where at least one wall is the Ste. Genevieve or St. Louis limestone. Conditions are not so favorable where the walls consist of the thinner Chester limestones or of sandstone or shale. Amount of displacement has little bearing on the degree of mineralization. There is some evidence of post-mineralization movement.
"The continuance of some fluorspar veins, chiefly along master fault zones to depths of 1,000 to 1,500 feet is considered probable. ... It cannot yet be predicted with assurance that any of these metals (lead or zinc) will increase at great depths" (Currier, 1923, p. 7). Bastin (1931) has observed that the Rosiclare (Illinois) vein pinches at moderate depths, where it becomes almost wholly calcite.
The same author listed the following vein minerals:
oil and bitumen
Calcite is the most abundant mineral and the first to have been deposited. Some galena and sphalerite are recovered in milling, and the former is the more abundant.
A considerable quantity of fluorspar has been recovered from surface pit mining of the loose residual material (gravel ore) in the clay soil blanketing veins. This is the weathered product of the vein and accumulates directly over or downhill from the vein. Mining of such deposits is usually the first step in the exploitation of a property. Metallic minerals are uncommon in this surficial material, but lead and zinc appear as cerrusite and smithsonite. Barite is not an important element of the veins but is conspicuous in these deposits.
Fluorspar is an important flux. About 80 per cent of production is used in the manufacture of steel where it serves as a temperature flux in the basic open hearth process. It is used in the making of enamels for ceramic wares, in the making of opal or opaque and colored glass, and in the manufacture of cement. It is also used in foundries and in the making of steel and ferro-alloys in the electric furnace.
More than 60 per cent of the crude barite consumed in the United States (Lilley, 1936) is used in the manufacture of lithopone. This is a mixture of about 70 per cent BaSO4, 26 to 28 per cent ZnS and one to three per cent ZnO. It finds wide use as a white pigment in paints, rubber goods, paper, linoleum, oil cloth, and window shades. Twenty to 25 per cent is ground and used as a filler and weight producer in paint, rubber, linoleum, paper, and cloth goods.
Central Kentucky Mineral Veins.—Fluorspar, barite, galena, sphalerite, and calcite occur in veins of some size in central Kentucky. They have been extensively prospected and have been commercialized on a small scale. The early history of their development is also a record of the search for lead and silver. In 1866 a Wisconsin company attempted to develop the Lockport vein in Henry County. Miller (1905) reported: (a) a mining and milling plant of the Chinn Mining Company (calcite) at Mundy's Landing in Mercer County, (b) a plant at Kissinger in Franklin County, (c) mines at Lockport, Henry County, and (d) a lead plant at Gratz in Owen County. In 1907 a barite mill was built at Nicholasville. Fohs (1913) reported the Kissinger plant with a concentrating mill, a lead fume works, and a barium sulphate-sodium sulphide plant. The Gratz plant was in operation, as was also the lead operation at Lockport.
|FIG. 40. Principal faults of
central Kentucky: (A) Lexington-Maysville fault;
(B) Kentucky River fault; (C) Switzer fault (McFarlan, 1939b)
Up to 1914 only small amounts of barite were produced. The development of the industry resulted in a maximum production in 1916 of 11,068 short tons marketed for $54, 995. The industry more or less died out in the years following 1923. The veins occur mainly in Woodford, Fayette, Jessamine, Lincoln, Bourbon, Scott, Boyle, and Garrard counties of the central Blue Grass.
Fluorspar has been mined on a small scale. The chief producers have been the Twin Chimney Mine (northern Mercer County) and the Faircloth Mine (southern Woodford County) both at Mundy's Landing. Another operation worked the Smitha vein on Boones Creek in southern Fayette County.
The only commercial calcite operation has been the Chinn calcite mine on the south side (Mercer County) of the Kentucky River at Mundy's Landing. The calcite was sold pulverized and some optical calcite was present.
Central Kentucky mineral operations have been dormant from about 1923 to a few years ago. The Faircloth fluorspar mine was in operation for a couple of years recently. At the present time operations on a larger scale than formerly are under way at Lockport and Gratz with lead and zinc concentrate the main objective. However barite is the dominant mineral. These operations are along a north-south vein involving no faulting.
The dominant central Kentucky vein mineral is barite and calcite ranks second. Others are relatively uncommon, though locally the quantity of galena and sphalerite has been such as to cause an attempt at commercialization. The minerals occur as veins occupying fissures either along fault planes and fractures associated with them or joints not closely connected with faulting. They involve both simple fissure filling and replacement of the limestone wall rock, the latter not important quantitatively (Robinson, 1931). Fault breccias with cementation of the limestone fragments only, or with partially replaced fragments, are common. Most of the veins intersect the limestones of the Lexington and High Bridge formations, and Miller (1905, 1919) observed that the barite veins are characteristic of the Lexington limestone, and the calcite and fluorspar veins are characteristic of the High Bridge rocks. Where occupying fault fissures, the barite veins in places involve higher beds. Near Burdett Knob (Garrard County) the Garrard sandstone forms one wall locally, and just north of the Knob a barite vein occupies a fault with wall rock of Fairmount limestone on the downthrow side. The majority of the veins strike in a general north-south direction. In the case of the Mundy's Landing occurrence there is no noticeable displacement, nor do any of the major fault zones occur in that vicinity.
|FIG. 1. Distribution of vein minerals in Kentucky-fluorite, barite, calcite, galena, and sphalerite.|
|FIG. 2. Fluorspar production in Kentucky, 1921-1937|
The veins are nearly vertical and the walls as a rule are clean cut but this is varied by replacement, brecciation, and the mineralization of small fractures in the wall rock. There is great variation in width horizontally and with depth in the same veins. The Hayden vein, one and a half miles south of Danville, is eight to 14 feet wide. Miller reported a 24-foot vein but did not describe the deposit. A width up to six feet seems to be the characteristic range (Currier, 1923). The Lockport-Gratz vein has been traced for a distance of 10 to 11 miles (Miller, 1905). It is not known that any significant change occurs with depth, but mining has almost all been above drainage. It has been reported by one operator that there was marked change in the composition of the Twin Chimney fluorspar vein at river level but no details are available.
Various investigators have expressed their views on origin.
1. Norwood (1877a, p. 20) referring to the Henry County zinc sulphides stated: "The lodes in this district may be regarded as the result of lateral secretion, the minerals having been gathered from the enclosing rocks."
2. Shaler (l877i) was of the same opinion.
3. Siebenthal (1915) regarded the zinc and lead deposits of the Joplin region as being formed by an artesian circulation that originated in the central Pre-Cambrian core of the Ozark uplift. The source of the metals was the disseminated sulphides in the Cambrian and Ordovician limestones, dissolved by carbon dioxide-bearing waters. On Kentucky he commented (p. 17):
"In this report the suggestion is made that the lead and zinc ores of the Kentucky-Illinois fluorspar district have in part been derived from artesian circulating waters from the Cincinnati and Nashville uplifts, which passed beneath the Carboniferous rocks of western Tennessee and Kentucky and rose through the complicated faults of the low-lying fluorspar district. The central Kentucky well and spring waters are zinciferous, as is shown by a great number of analyses. The common occurrence of barite, the abundance of fluorspar, and the entire absence of dolomite show that these deposits are related to the veins of the Cincinnati and Nashville uplifts. On the other hand, the argentiferous character of the galena, the abundance of fluorspar, and the occurrence of igneous intrusions in the region suggest a connection between the ores and igneous activity. If this connection can be established, it would seem that there is in this region not a transition type of ore deposit but a juxtaposition of types—one type due to normal artesian circulation and the other due to igneous action."
4. Fohs (1907), discussing the same deposits, discounted the enclosing rocks as a possible source and placed the deposits in the hydrothermal class.
5. Currier (1923, p. 161-163) said:
"The mineralogic, geologic, and structural features of the veins1 are such as to support the idea of deposition largely or entirely from waters of low temperature, possible of meteoric origin, though scarcely of pronounced artesian circulation. . . .
"Aside from fluorspar there is no mineralogic association that could be considered suggestive of igneous affiliations any more strongly than of non-igneous connections. Concerning fluorspar itself there is, in general, considerable difficulty in assigning large concentrations of this mineral to be possible from ordinary ground waters traversing limestone areas and gathering their content of calcium fluoride from these country rocks. The question has been briefly considered in an earlier chapter. On the other hand, the total volume of that mineral appears to be very small indeed when compared with the deposits of western Kentucky or with other vein materials of the Bluegrass. It would appear that the fluorspar of these veins was transported from sources other than immediate limestone areas, but the nature of the solutions and the adequate sources are certainly in question. . . .
"The complete absence of any known phenomena that can be safely attributed to igneous activity at depth in the district imposes considerable stress on the imagination to postulate such an origin. It could be, of course, that the faulting in the Bluegrass region was a concomitant effect of deep-seated igneous activity, the adjustment of the "crust" having resulted in such breaks, or that the crustal disturbance accompanying the general upbowing along the Cincinnati arch could cause either faulting or faulting with deep-seated igneous adjustment, or that the phenomenon is more directly related to the general Appalachian revolution,"2
6. Spurr (1926, p. 735) referred to the central Kentucky fluorspar veins.
"A smaller and restricted fluorspar area (the Mercer County area) lies in Kentucky some miles east-northeast of the main district, and this Mercer County district is also characterized by localized faulting and hence it is probably a magmatic district. And continuing east-northeast in a straight line to Elliott County, peridotite dikes and pipes outcrop. The magmatic belt, characterized at intervals by faulting, by peridotitic pipes and dikes, and by deposits of fluorite, with barite, lead and zinc, traverses the whole of Kentucky."
7. Robinson (1931) regarded the hypotheses of meteoric origin and from artesian waters untenable and accepted Spurr's conclusion. Three modes of origin have been mentioned:
1. Deposition by meteoric waters in which the vein materials have been leached from the enclosing rock.
2. Deposition by artesian waters in which the vein materials have been dissolved from deep-lying beds or from surface or near surface rock in the vicinity of outcrop of the water-bearing horizon.
3. Deposition by magmatic waters from deep-seated intrusives, rising along fissures.
Although local evidence is not conclusive, some facts bearing on the question are given below.
No igneous rocks are known in outcrop in central Kentucky. They do occur in western Kentucky in Caldwell, Crittenden, and Livingston counties and in the east in Elliott County. In connection with the former are the noteworthy fluorspar veins of Kentucky. Although no intrusives are known in central Kentucky and vicinity, the cryptovolcanic structures (p. 152) are highly suggestive. The Jeptha Knob structure is pre-Laurel in origin. The Serpent Mound (Ohio) structure is post-Waverly. The Wells Creek (Tennessee) structure is post-St. Louis and the Flynn Creek (Tennessee) structure is pre-Chattanooga. Another similar structure in Indiana involves only the Ordovician and is thus in-determinate in age. At least two periods of vulcanism are indicated in a region otherwise devoid of evidence of igneous activity.
Miller (1905) mentioned a common association of calcite-fluorspar veins with the High Bridge limestones and barite with the Lexington limestones.
There is an artesian circulation in the St. Peter sandstone and other deep beds. This artesian circulation apparently originates in the Ozarks and the Wisconsin uplifts with their exposed Pre-Cambrian cores, and particularly in the latter, the St. Peter outcrops at sufficient elevation to supply adequate head. The amount of head must have varied in the past with varying difference in structural elevation of the regions involved. That there have been a number of uplifts is known. At the same time it must be recognized that these "St. Peter" waters are usually salt sulphur, indicating a lack of active circulation.
The absence of barium, fluorine, lead, and zinc in analyses of central Kentucky limestones has been given as evidence against these sediments being a source of the vein materials. The numerous analyses of water by the Kentucky Agricultural Experiment Station which have been offered as evidence were analyses made for agricultural purposes and are not complete. Traces of such metals were not looked for. Siebenthal (1915) mentioned that well and spring waters of central Kentucky are zinciferous. Practically all of the vein minerals, including fluorite and barite, occur as vugs in almost every Ordovician limestone, occasionally also as crusts on joint faces. This in itself indicates some functioning by meteoric waters. Further, there must have been some change in the composition of ground water with the removal of a very great thickness of sedimentaries formerly covering the area.
The writer has found fibrous quartz veins cutting the Ohio shale at two places along the Kentucky River fault: (a) at Burdett Knob (not in place), and (b) where the fault crosses the Danville-Stanford highway (in place). Barite is associated with the quartz at Burdett Knob. The significance of these veins is open to question.
At Burdett Knob scattered residual geodes of Keokuk or Warsaw derivation are found. Some of these have cores of fluorspar. At a number of places along the Kentucky River loose geodes of the Irvine formation lined with chalcedony cubes are found. These cubes are interpreted as pseudo-morphs after fluorite. The writer has not seen any in place, though they came from limestones of the Keokuk or Warsaw.
The occurrence of barite only along certain faults of a fault zone may be due to presence or absence of gouge or again the depth of penetration of a fault plane (or joint). Its presence along faults involving shale formations (Cynthiana and Eden) tends to minimize the significance of gauge.
There is a definite trend toward minimizing the importance of meteoric waters in the forming of ore veins and particularly is this true of fluorspar. There is good reason to consider the source of some or much of central Kentucky vein materials as magmatic but the agent of distribution is in part at least meteoric. Granting a probability of igneous intrusion, and this seems to be indicated by the cryptovolcanic structures, and also a magmatic source for fluorspar and some of the other vein minerals, meteoric waters, not excluding artesian, may well have played a part, receiving these materials from magmatic emanations.
1 In central Kentucky.
2 Bucher (1921) described the first of the cryptovolcanic structures of this region.