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process which enables the treatment of low-grade ore is another factor in enlarging the reserves of zinc ore in the United States and other countries.
The ability of American mines greatly to increase their output was demonstrated in 1915 to 1917. In spite of the enormous drain upon resources in that period of extraordinary production, the known reserves of zinc ore were actually increased. A few districts show some depletion, and all over the United States the cream of the highgrade ore has probably been skimmed; but new districts have been and are being opened up. Domestic resources are very extensive and the ore bodies are moderately cheap to mine. In the Mississippi Valley zinc regions (Joplin-Miami and Wisconsin) zinc ore is the chief product with only subordinate quantities of lead. But elsewherenotably in the Rocky Mountain region-zinc mining is generally linked with the production of other metals. Sometimes zinc concentrate is the main product, perhaps more frequently it is a by-product; but in nearly all the complex ore districts it is an important joint product.
Transportation is a large factor in the zinc business. There are few other ores that are transported such long distances, since reduction works are necessarily located in populous industrial centers adjacent to adequate supplies of fuel. This condition is general throughout the world. Australian and African concentrates are smelted chiefly in Europe. A large part of the American zinc ore is produced in States west of the Mississippi River and must be brought to the Mississippi Valley for smelting, while the metal is carried still farther eastward to the consuming centers in Pennsylvania and the neighborhood of New York City. The difference in the case of the United States is that this transportation is by rail, whereas in the case of the European works, which are nearly all situated at or near seaboard, transportation is almost exclusively by water.
The zinc-ore situation in the United States is much like that of lead ore, which also must be transported long distances by rail. Most of the zinc-ore hauls, however, are longer and because of the lower value of the material to the miner freight rates are an even more important factor. While zinc metal is quoted at a slightly higher price than lead, a ton of zinc ore is seldom worth as much to the miner as a ton of lead ore. The reason is that smelting charges are greater, a larger amount of the metal content is lost in the smelting operation, and, whereas lead ores nearly always contain precious metal which adds to their value, zinc ores frequently do not contain recoverable values other than zinc.
It may be stated that most foreign mines are more favorably situated as regards transportation of their product to reduction works than are the mines of the United States. The chief oxceptions are the mines of Canada and Mexico. Zinc mining has not yet been actively prosecuted in either of these countries, but may be expected to become much more important in the next few years. By virtue of their geographical position these countries are almost of necessity forced to market their product in the United States. Until very recently they were the only countries that shipped ore to the United States, and, with the exception of South American countries, whose resources can not be accurately gauged as yet, Canada and Mexico are the only countries that are likely to compete at all actively in the American zinc-ore markets with the domestic product.
Few of the Canadian mines have any particular advantage as compared with those located in adjacent parts of the United States. The cost of labor and supplies is very nearly the same on both sides of the border. The Canadian mines are not much richer and are a little farther away from the smelting centers.
In Mexico, however, the situation is different. Here we find large deposits of high-grade zinc ore that have not been worked. As soon as there is adjustment of the unrest in those regions these deposits can furnish enormous quantities of desirable zinc ore at extremely low cost. The freight from these mines to the American zinc smelteries is high, but in view of the low cost of producing the ore it can be delivered at Mississippi Valley points for less cost than much of the output from the mines even of that section of the United States.
The tariff problem is concerned chiefly with the Mexican ore. Mexico has the largest undeveloped resources of zinc ores that are likely to be imported into the United States. The development of Mexican mines was hampered by the high duties of the act of 1909 and political disturbance has prevented any considerable importation under the low duties of the act of 1913. Since 1912 the operation of most of the Mexican zinc mines has been very uncertain, as these regions have suffered from bandit warfare. Eventually, however, it is possible that more or less normal conditions will prevail and that the mines can again be operated. In the past this ore was shipped mostly to the Mississippi Valley, although a little was shipped to Germany and Belgium. The resumption of Mexican mining would again cause quantities of this ore to be thrown on the American market and it is quite possible that the result would be to close down many of the mines and smelteries of the Mississippi Valley and a transfer of the smelting business to the Rio Grande. The latter location would be better adapted to the treatment of Mexican ore. Coal supplies might then be drawn from Colorado and New Mexico. Oil from California and Texas and possibly natural gas from the latter State would favor such a change.
1 Neglecting the possible resumption of electrolytic zinc production on a larger scale in British Columbia. A bounty guaranty of 8 cents per pound is offered by the Canadian Government to remain in effect for two years after the end of the war.
ZINC MINING IN THE MISSOURI-KANSAS-OKLAHOMA AREA.
The region centering in the southwest Missouri and extending into adjacent parts of Kansas and Oklahoma is commonly known as the Joplin district.” It embraces about 3,000 square miles of known zinc productive area and its furthest limits are not yet defined. This district, reaching from Springfield, Mo., to Miama, Okla., a length of over 100 miles and a width of approximately 30 miles, is situated on the extreme western edge of the Ozarks. In general, the topography is gently rolling, much flatter than that of most mining districts.
The first workable ore was discovered at Joplin and Granby about 1850, and these immediate areas are still producing. The development of mining camps at various points has resulted in a number of “booms,” the most spectacular of which was that following the discovery of rich deposits in the Indian lands of Oklahoma.
When the district started over 60 years ago, a man with $50 or $100 capital could lease an acre of ground. If he struck ore in one or more of his gopher holes, he would set up a hand jig; and if this proved profitable, he bought a steam mill of 50 to 100 tons capacity on day shift (12 hours). These mills are of standard design, and are moved from lease to lease as the latter were successively exhausted or abandoned. The terms of many leases require the erection of a mill on each 40-acre tract.
The early lots under the leasing system, which is still in vogue, were only about an acre in size (200 by 200 feet); but to-day they run from 5 to 160 acres, averaging between 10 and 40 acres. The leases are generally let for a period of 10 years. On account of the short terms of these leases and the small size of the tracts there has never been much incentive to build large plants until quite recently. The upper ore bodies are very erratic, and may last six years or six months. The sheet ground that in recent years has furnished a greater and greater proportion of the output is more dependable; the result has been that one or two mills with capacities of 750 tons in 10 hours' have been built. The adverse conditions in 1917–18, however, caused the closing down of many of these sheet ground mines.
Speculation in leases has been rife, so that by far the greater number of operators are working under a second or even a third or fourth lease. Subleasing increases the royalties. The final operator must often pay as high as 35 per cent. The average royalty is between 10 and 15 per cent, and is never less than 7 per cent. According to the terms of most of the contracts the owner of the ground has the privilege of putting the operator off at the expiration of the lease. The latter has the right to remove his machinery, but gets no credit for work done or permanent improvements made.
Under these conditions plant investment must be kept at a minimum and all complication of treatment avoided. Better mill recoveries have been gotten in recent years by the treatment of the finer material, but the Joplin jig, although å marvel of simplicity, has proved to be a most efficient machine for the treatment of these relatively easily concentrated ores, and the district in general shows a fairly high degree of efficiency in spite of the seeming crudity of the operations.
The machinery is largely made in Joplin and is standardized. is cheap and quickly obtainable, relatively so even during the war period. Under normal conditions the labor supply is generally more plentiful than in most mining regions. Wages are on a sliding scale, ranging up from $3 a day when blende (60 per cent concentrate) is quoted at $50 to $4.50 with blende at $100 à ton.
The principal ores of the district are sphalerite, calamine, galena. Minor quantities of smithsonite, cerussite, and other oxidized zinc and lead minerals are found, but never predominate as ore. A few of the shallower deposits yield a larger amount of lead, but a rough average of the ore from deeper workings shows that the ratio of zinc to lead is 17 to 1.
Most of the ore bodies are found in the Boone formation, a series of limestone and chert beds of Mississippian age. Two main classes may be distinguished. The sheet ground areas are the lower beds of the series. They are mostly chert, rather uniformly mineralized over large areas, and are best represented in the Webb City-Carterville district. These deposits carry a lower percentage of ore than the upper soft ground, which is the other distinctive type of deposit, but their greater dependability makes possible the mining of comparatively large tonnages.
The soft ground deposits are found in the upper strata of the series. They are very irregular in size, but are generally rich. Sometimes the ground is so soft that considerable timbering is required, while in other mines the entire ore body can be worked out with comparatively few pillars. Near Miami the shallow ore deposits carry a considerable amount of tarry material that is a handicap to concentrating. The deeper ore does not.
Like most deposits of the Ozark region, the Joplin district ore minerals are thought to have been leached from overlying rocks and deposited in cavities by descending meteoric waters. The irregularity of the occurrences is attributable to the distribution of the old water channels.
Churn drilling is the usual method of prospecting. The ordinary amount of drilling necessary is from 10 to 30 holes for each 40-acre tract (the area of the average lease), but many of the older operators in the field are satisfied if they find two or three holes near each other and carrying good ore. The amount of drilling necessary for sheet ground is considerably less than in the case of narrow ore bodies. In the latter case, the holes are often spaced as close as 15 to 20 feet. In general throughout the district, the holes will vary from 100 to 400 feet deep, averaging about 300. Prospecting depths less than 100 feet is done by shafts.
After the ore body is reached, cuttings taken every 2 or 3 feet are sampled and assayed. Permanent records of cuttings showing the various formations passed through, are kept in glass jars or glued on large cards. Drilling costs in the early part of 1917 ranged from $1.25 to $1.50 a foot as compared with $0.75 to $1 in prewar times.1 The cost of a 300-foot hole is from $450 to $5002 in the Miami section where costs run a little above those in the older sections of the district.
After proving the property, the usual practice is to put shafts down to the ore body at once. These shafts are usually vertical. The cost of a typical 5 by 7 foot shaft 200 to. 250 feet deep and handling an average of 100 to 500 gallons of water per minute is from $25 to $30 a foot. Usually two or more such shafts are sunk on each operation and are connected for the purpose of ventilation by air drifts 5 by 7 feet. The cost of driving this drift varies from $6 to $9 a foot.
The actual methods used in the underground work vary with the nature of the deposit and are altered to suit the local conditions. In most of the sheet ground mines where the ore is found in only one horizon and is seldom over about 16 feet thick, the room and pillar system of mining is used. About 10 feet can be mined in one lift and the remaining footage shot off the back.
In mines where the ore body is higher than 16 feet, underhand stoping is used, starting from a drift or heading at the top of the ore body and carrying one or more high benches back of the breast.
In some parts of the district, however, two or more distinct levels of ore strata are found, separated by a relatively thick stratum of hard, barren rock. As this barren stratum is almost invariably strong enough to stand, each horizon can be mined separately by whichever of the above systems is applicable.
As a rule the ground is worked in small tracts, by small operators. More recently, especially in the Miami field, there has been a tendency for the working of the area in larger units than the 40-acre or smaller units that had been worked by leasers."
C: A. Wright' gives detailed cost figures for several of the larger companies. These figures cover the whole scope of the operations in 1914 to 1916, inclusive. Average detail costs of mining and milling covering the whole area would be difficult to secure, as each company has its own system of accounts and mining conditions and methods of treatment vary rather widely in different sections.
1 H. I. Young. Trans. A. I. M. E., Vol. LVII (1918), p. 671. 2 Weed. Mines Handbook, Vol. XIIS, 1918. 8 Ore Dressing Practice in the Joplin District. Trans. A. I. M. E., LVII (1918).