is somewhat lower than that of fluorspar (3.0 to 3.25); and it is rarely intimately intergrown with the fluorspar. Therefore it usually can be separated easily by mechanical means, and, as for most uses it is not considered a deleterious impurity, its presence ordinarily is not a serious matter.

Quartz or silica is present in most deposits and in some is abundant. It occurs in massive fluorspar as small grains, as massive vein quartz, and in intergrown aggregates of quartz and fluorspar; in residual deposits of gravel fluorspar it occurs as sand. In the Great Eagle mine near Lordsburg, N. Mex., these intergrown aggregates form the larger part of the deposit. Specimens are common that show at one end pure quartz which imperceptibly changes to pure fluorspar at the other end. At Tonuco, N. Mex., are found great masses of concentrically banded quartz nodules or bowlders, which toward the outside grade into fluorspar. Such bowlders appear to be very pure fluorspar, but when broken are found to be nearly solid quartz.

The specific gravity of quartz is only 2.65, and when it occurs as free sand in a deposit of residual fluorspar gravel it is easily removed by log washers, jigs, or tables; but when it exists in finely intergrown aggregates it can not be separated and a pure fluorspar product obtained. As fluorspar concentrates containing more than 6 per cent silica are severely penalized, the presence of a large amount of quartz in massive fluorspar may seriously affect the value of a deposit.

Barytes or barite is a relatively common associate of fluorspar in some localities. It occurs as distinct crystals, as crystalline masses, and as small grains so mingled with fluorspar that it can be detected only by analysis. Its specific gravity is 4.3 to 4.6, 1 to 12 points more than that of fluorspar, but in spite of this difference mechanical methods of separation have thus far failed to yield satisfactory products. Barytes is considered an injurious impurity for most uses (more than 1 per cent is prohibited by some steel companies) and its presence in a deposit is very unfavorable.

Galena, or lead sulphide, is often associated with fluorspar, at some places in considerable quantities. The Rosiclare vein, Illinois, was first worked for lead. Galena in quantity in a finished fluorspar product would be objectionable, but as its specific gravity is high (7.4 to 7.6) it may be easily removed by jigging and tabling and afterward sold as a valuable by-product. Most of the galena in the Rosiclare district carries about 7 ounces of silver per ton.

Sphalerite, or zinc sulphide, is found in some fluorspar veins in the Illinois-Kentucky district. It is an objectionable impurity in fluorspar concentrates and, as its specific gravity is only 3.9 to 4.1,

separation is difficult. To make a clean or satisfactory separation by jigging or tabling has been impossible, but the development of successful flotation or electrostatic methods seems probable.

Wall rock, such as shale, sandstone, limestone, quartzite, or granite, or surface materials, such as sand and clay, usually contaminate the fluorspar as mined, but at most deposits they are easily removed by hand sorting or by milling.

Other associated minerals, such as pyrite, chalcopyrite, and celestite, are sometimes found, but usually they are easily separable or occur only locally and in unimportant quantities.

GEOLOGICAL OCCURRENCE

The element fluorine has been estimated to constitute about 0.1 per cent of the earth's crust. Although it is widely distributed in minute quantities, deposits containing a large proportion of the element are distinctly uncommon. Cryolite, a sodium-aluminum fluoride having the formula Na,AIF, contains 54.4 per cent of fluorine and fluorspar 48.9 per cent. Fluorspar and cryolite are the only minerals that contain a large proportion of fluorine. But there is only one large deposit of cryolite known, in Greenland, and the mineral can not be considered as a substitute for fluorspar in large amounts. Thus fluorspar is the only fluorine mineral of great commercial importance, and large deposits of fluorspar are relatively

uncommon.

Fluorspar is formed under a great variety of geological conditions. These have been summarized by Wilson and Clark. The former, in summarizing the theories suggested to explain the origin of fluorspar deposits of the Madoc (Ontario) type, separates the theories into four classes, as follows:

1. The vein material has been concentrated from the adjacent sedimentary rocks and redeposited in fissures through the agency of the ordinary groundwater circulation.

2. The vein material has been derived from the adjacent sedimentary rocks through the solvent action of ascending heated waters.

3. The lime contained in the fluorspar has been derived from the limestone wall rock but the fluorine and other elements composing the vein material have been brought up in solution from a magmatic source.

4. Both the vein material and the solutions from which the vein material has been deposited are of magmatic origin.

It is, of course, probable that all important deposits of fluorspar were not formed in the same way and by the same agencies, but certain general conclusions seem to be applicable to most commercial deposits.

1 Wilson, M. E., Fluorspar deposits of Madoc district, Ontario: Canada Dept. of Mines, Geol. Survey Summary Rept., 1920, part D, pp. 47-48.

2 Clark, F. W., Data of geochemistry: U. S. Geol. Survey Bull, 695, 1920, pp. 331-332.

First, virtually all important deposits are in or near fault fissures. They may exist as fillings of the fissures, as replacements of the walls, as replacements of favorable sedimentary beds in or near zones of faulting, as fillings of solution cavities near faults, and as residual deposits left by the weathering of deposits formed in one of the other ways mentioned.

Second, limestone beds or calcite veins are the most favorable places for the formation of large deposits. Most deposits in igneous rocks are small, narrow, and nonpersistent. In sedimentary beds comprising limestones, shales, and sandstones, as in the IllinoisKentucky district, the widest and best parts of the deposits are nearly always in the limestones or in highly calcareous shales and sandstones, or in those parts of the fissures where the original filling was calcite. Where the fissures cut sandstones or other highly siliceous rocks they show a strong tendency to become narrow and of poor grade. A notable exception to this generalization is the deposit at Wagon Wheel Gap, Colo. This deposit, which is by far the largest and most persistent of those known to be in igneous rocks, occurs in Tertiary volcanic rocks, rhyolite tuff, and some quartz latite and andesite. There, however, as in other fluorspar deposits in igneous rocks such as that near North Gate, Colo., there are known sources of lime near by. At Wagon Wheel Gap the vein, if projected, would pass through a deposit of travertine around a hot mineral spring a short distance away. At North Gate the vein is in granite gneiss separated by a fault from a group of sedimentary beds containing limestone.

Third, some evidence of igneous activity is to be seen at or near most of the important fluorspar deposits. This evidence may be intrusive dikes, as in the Illinois-Kentucky field, or hot mineral. springs, as at Wagon Wheel Gap, Colo., and at the Great Eagle mine near Lordsburg, N. Mex., or it may be the corrosion and etching of fluorspar surfaces as by hot solutions or vapors (observed in the Illinois-Kentucky district). In the Madoc district, Ontario, these evidences of igneous activity seem to be lacking, but Wilson 3 suggests that solutions of magmatic origin may have risen along faults. From the foregoing evidence it seems probable that most commercial fluorspar deposits were formed as given under 3 or 4 in Wilson's summary, and some perhaps by a combination of the two. Unwarranted assumptions have been based on these theories.

Thus it has often been said of the Illinois-Kentucky district that if the fluorspar is of deep-seated or magmatic origin, the deposits may be expected to continue to great depth or at least to the base of the limestone formations. But this deduction is not necessarily valid and developments on the deeper levels in both Illinois and

3 Wilson, M. E., work cited, p. 52.

Kentucky seem to disprove it. Although it is true that those vents, chimneys, or channels which served as feeders might be expected to be mineralized to some depth, it does not follow and is not even probable that the mineralization of the whole vein system continues to the same depth. It seems more likely that a few vents or chimneys served as feeders. Solutions or vapors rising through these channels might not spread much until they encountered some obstacle, such as impervious strata (perhaps removed later by erosion), when under the increased pressure they would spread laterally through fissures or would actively replace limestone or calcite along joints or planes of weakness. According to this theory, the largest and thickest bodies of fluorspar would be nearest the surface and the deposits would become shorter and possibly narrower farther down until at great depth only the chimneys or feeders would be mineralized. Deep workings in the Illinois-Kentucky district, as described more fully on later pages, seem to show this condition, although it is admitted that enough deep mining has not been done to prove or disprove it. In the Derbyshire district, England, where conditions seem to resemble closely those of the southern Illinoiswestern Kentucky district, the fluorspar deposits become shorter and narrower with increasing depth and the vein matter changes to calcite and barite. This change may, however, be due to fundamental differences in rock structure and in mode of occurrence.

Proper determination of the origin of a deposit and correct interpretation of theories of origin are of greatest importance commercially. On them depend intelligent estimation of ore reserves which, in turn, has an important bearing on the method of mining to be followed, the extent and type of development, the nature and size of mining plant and equipment, the size of the mill, and the justifiable investment of capital. If the ore body in its full length and thickness may be expected to continue far down, "pinches and the changing of vein matter from fluorspar to calcite may be considered as only local variations and more extensive development may be expected to reveal ore in workable quantities; but if there is no good reason to believe that the ore body will persist to great depth, the unfavorable conditions noted above should be fairly good evidence that extensive exploration of barren ground by deep levels would be a waste of time and money. Under certain conditions it is evident that too much reliance has been placed on the persistence of fluorspar deposits with depth.

PROSPECTING AND DEVELOPMENT

The most important deposits of fluorspar in the United States lie in fault fissures that cut nearly horizontal sedimentary rocks. The system of faults is very complex. Because of deep weathering and re

cent sedimentation the exact position and nature of the faults can be determined only by careful and detailed prospecting. This is particularly true in the western Kentucky district. On account of various factors mentioned elsewhere in this report, the conduct of prospecting and mining in Kentucky has until recently been irregular, spasmodic, and unsystematic. As the more easily found ore is removed, production must decline and finally stop, or costs (and) therefore prices) must greatly increase, or regular and systematic prospecting must be undertaken.

Accurate maps must be prepared bearing all present reliable data. Careful records of all future prospecting should be kept and the information made available so that needless reprospecting may be avoided. It seems possible that all of the producers in a district could pool their information and jointly pay the cost of compiling and keeping up the records. In any event, the larger companies should adopt some definite system and follow it. Any improvement over the present methods should more than justify the added cost.

GENERAL CONSIDERATIONS

Prospecting may be extremely simple in regions where vegetation is scanty and little or no surface soil covers the rocks, as is generally true in fluorspar districts of the far West. But in the IllinoisKentucky district, particularly in Kentucky, vegetation usually is heavy and solid rock is covered with residual and transported material to a depth of 100 feet in many places, and nearly 200 feet in some places. Of course, some important veins in Illinois outcrop (or did), but most of the solid veins in Kentucky are deeply buried, consequently prospecting is often difficult and uncertain.

Although the southern Illinois and western Kentucky districts are separated only by the Ohio River, the common modes of occurrence in the two fields differ greatly. In Illinois the largest veins outcropped in solid rock, and at only a few relatively unimportant deposits is the outcrop of the vein represented by disintegrated "gravel" in clay or other unconsolidated material. In Kentucky a few solid veins outcrop, but most deposits either show no visible fluorspar at the surface or it exists as "float" or "gravel" mixed with clay. (See fig. 2.)

The country is rolling, but the hills are only high or steep enough to make possible the use of tunnels for either prospecting or mining at a few places.

As prospecting in the far West is usually simple and as the bulk of the Illinois production comes from a few veins that are well known and well defined, prospecting in western Kentucky is given more attention in this report. There the production has always