LEAD ORE.

SUMMARY.

Lead occurs with silver, zinc, and other metals. Few mines produce only lead. A large part of the domestic output of gold, and especially silver, is directly dependent upon the supply of lead ore, which provides a "collector" for the precious metals in the smelting operation. Zine is almost universally associated with lead in ore deposits: but, unlike silver, gold, and copper, can not be recovered in lead smelting. It is frequently separated mechanically, however, and may form an important and valuable product of the mine.

The United States is the largest producer of lead ore in the world. The next largest producers are Australia, Spain, Germany, and Mexico, in the order named. These five countries produce fully SO per cent of the total output of the world.

The United States is also the largest consumer of lead metal. For many years prior to the war in Europe, the consumption practically equaled the production from domestic ore. In addition to this production, however, American smelting and refining works produced refined lead derived from foreign (chiefly Mexican) ore and base bullion for export. The output of this lead of foreign origin and destined for foreign consumption amounted to about 25 per cent of the purely domestic business. By virtue of the provision for bonded smelting, this business, although frequently carried on side by side with the smelting of domestic ores, was absolutely distinct as far as the tariff is concerned.

The lead imported in the form of ore for treatment in bonded works for export does not compete with the product of domestic mines except in foreign markets. Until the outbreak of the war, however, practically no lead of domestic origin was exported. The large exports of domestic lead during the war period were a result of war conditions, the curtailment of European supplies from the normal sources Australia and Spain), and the unprecedented demand. But the ore imported for treatment in bonded works does form a welcome addition to the smelting operations at American works and assists greatly in the economical treatment of American ores, especially those containing little or no lead and valuable especially for their content of precious metals (dry ores).

Spain and Mexico are the only important producers of lead ore whose product comes on the world market. Germany, in spite of its large output, has to import lead to meet its own requirements. Only a part of the Spanish ore is exported, although most of the lead produced in Spanish smelters goes to other countries. But almost all of it is consumed in Europe and it is not even a potential factor in the American market. All of the lead ore produced in Australia is smelted and refined in Australian works. Only refined lead is now exported.

11913 (Geological Survey).

Mexico, therefore, is the only country from which large supplies of lead can come into the United States in the form of ore. Canada, the next largest country of origin of the lead ore imported into the United States, does not produce enough lead for her own needs. Mexico, also, is the only country in which there are large known deposits of lead that are not producing to practically their full economic limit, and which can, therefore, largely increase production.

Domestic lead deposits will be able to supply American lead requirements for many years. The cost of mining lead, however, in the United States is greater than in most foreign countries, due not so much to the higher wage scale as to the poverty of the deposits in many of the producing sections and the unusually long railway hauls, which average longer in this country than in almost any other important lead-producing region. Conditions in Canada are not far different from those in the United States, but with the return of favorable political conditions Mexico will again become a notably low-cost producer.

The tariff problem on lead presents many complex features because of its interrelation with other mining and smelting industries, especially silver and zinc. With reference to the importation of lead in the form of ore for treatment in American smelting establishments, the problem separates clearly into two main phases, depending upon whether the lead metal recovered from the foreign ore is destined (1) for ultimate domestic consumption or (2) for foreign consumption. The first phase practically resolves itself into the decision as to whether or not lead produced from Mexican ore shall be permitted to displace a large part of the product of domestic mines in the home market and force many gold and silver, as well as lead, mines to shut down. The second is the question of bonded smelting. Since bonded smelting applies with equal force to zine ore and to lead bullion, as well as to any other dutiable metal or mineral commodities that may conceivably be refined in this country for export, it seems advisable to treat that subject separately. It is therefore touched upon only briefly in this unit.

* Negligible.

* Includes imports entered for consumption and imports entered free of duty for smelting in bonded works. (Values of imports are given only for those upon which duty was paid.)

Estimated.

Calculated on dutiable imports only, without regard to lead in ore imported for smelting in bonded works and exported.

Includes lead in lead ore and in other ore.

7 Not available.

GENERAL INFORMATION.

DESCRIPTION.

According to the character of the lead minerals, lead ores are divided into two classes: (1) Sulphide ores (galena), and (2) oxide ores (anglesite, cerussite, etc.). The latter are commonly called carbonate ores. Viewing the ores from the manner in which they must be fluxed in the blast furnace, they are classed as (1) silicious, (2) basic, or (3) neutral; but this second classification depends upon the impurities and accessory constituents of the ore rather than upon the condition of its lead contents.

Lead ores almost invariably contain silver and frequently contain appreciable amounts of gold. The chief exception to this rule are the ores of Missouri and the other sections of the Mississippi Valley region of the United States. The precious metals are recovered in the smelting process with the lead metal, and need not be separated from the lead minerals before being sent to the smeltery. Zinc, which is also a very common associate of lead minerals in an ore is not recovered in the smelting process, and must be separated from the lead minerals before being sent to the smeltery, or subject the ore to penalties proportionate to the zinc content. Copper is another frequent but by no means so nearly universally associated constituent of lead-bearing deposits.

The typical lead ore, therefore, is complex, and the profitable exploitation of a lead mine depends not only upon the sale of the lead, but also upon the precious and other base metals that occur associated with it.

USES.

In addition to their obvious function as a source of metallic lead, lead ores are used as carriers in the smelting process for the recovery of gold and silver from their ores. Whereas formerly the precious metals were recovered chiefly by separate processes they are now quite generally smelted with lead or copper ores and recovered from the base bullion so produced. This economic evolution has been practically complete in the case of silver which, except for one plant in Utah, and to a small extent elsewhere (when present as an accessory constituent of gold ores), is no longer extracted by a separate process in the United States.

Lead sulphate (basic) and similar lead pigments are made to an increasing extent direct from the ore.

DOMESTIC PRODUCTION.

The United States is the largest producer of lead ore in the world. In 1913, the last year for which complete estimates are available, this country furnished 36 per cent of the world output as compared with 19.8 per cent produced by Australia, the next largest producer. In 1918 the total production of lead from domestic ore was 550,729 short tons, valued at the average quotation of the year at more than

About two-thirds of the silver produced in the United States is a by-product of lead or copper ores. Mineral Resources, U. S. Geological Survey, 1915, p. 729.

These ores are called "nonargentiferous. The metal recovered from them is called "soft lead" to distinguish it from "desilverized lead," recovered from "argentiferous" (silver-bearing) ores.

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$80,000,000, as compared with 581,716 short tons, valued at over $102,000,000 in 1917. In order to estimate the value of all the metals produced from lead ore mined in this country, the above figures should be increased by from 50 to 75 per cent.

The mine output of lead in the United States decreased greatly in 1918. The output of soft lead (from mines in the Mississippi Valley and the Eastern States) was about 250,000 short tons, and that of argentiferous lead by mines of the Western States was about 313,000 tons, a total of 563,000 tons, as compared to 273,046 tons and 378,110 tons, respectively, and a total of 651,156 tons in 1917. After a further decline in 1919, the mine output of lead again increased in 1920 to about 511,000 tons, of which 275,000 tons were soft lead.

Location of pro 'uction and resources.-Missouri is the largest producing State, followed in order of output by Idaho, Utah, and Colorado. These four States furnished 85 per cent of the total domestic production in 1917. An appreciable amount of lead, however, was mined in other States.

About one-third of the American production of lead in ore comes from southeast Missouri. This district is undoubtedly the largest lead-producing area in the world. It is unique, not only as regards the character of its ore, which contains too little silver to warrant its separation and is extraordinarily free from undesirable impurities, but because it is characterized by the permanent control by large operators. The deposits are generally low grade, but fairly dependable in extent and character, cheaply mined, and easily concentrated to a high-grade product. In other sections of the Mississippi Valley region lead is generally a by-product of operations conducted especially for the recovery of zinc ores. In the Joplin (southwest Missouri) district the ratio of zinc to lead is about 17 to 1. All the ores of the Mississippi Valley region are classed as nonargentiferous, and, although except in southeast Missouri the lead occurs in connection with zinc ores, a clean high-grade concentrate can be made with little difficulty.

All the lead ores of the Western States are argentiferous and most of them are associated with considerable amounts of zine. They are typically complex and much more difficult to concentrate than the simpler ores of the Valley States. Broadly speaking, the high-grade ores are exhausted and the production comes largely from ores comparatively poor in lead and valuable chiefly for their precious metal content. The large deposits of Idaho, containing valuable amounts of zinc as well as lead and silver, and a fraction of the Utah production are the important exceptions to this general statement.

Equipment. In common with most mining industries, American lead mines and concentrators have, if anything, an advantage over foreign producers in securing equipment, both as to cost and delivery. This advantage, however, is offset to a degree by the larger amount of equipment required in this country because the ore bodies are frequently much smaller and the ores are generally more complex and have a lower lead content. The bulk of the domestic ore must be concentrated before shipment and the ratio of concentration is

Engineering & Mining Journal of Smelter Production (revised to January, 1919). Corresponding figures, according to the Geological Survey, were 562,637 tons in 1918 and 649,282 tons in 1917. Because of smelter charges, etc., the value of the ore to the miner is probably not more than one-half the above figures.

A summary of the resources and the lead mining in the more important districts has been prepared for the Auxiliary File at office of the U. S. Teriff Commission.

high as compared to most of the foreign operations. Only a small fraction of the output from domestic mines can be shipped direct to the smelter. Complicated milling machinery and equipment are necessary to prepare the ore for market.

Mining. Lead mining does not differ essentially from the mining of other metals. In general it may be said that the large scale operations characteristic of the "porphyry" copper mines, either on surface or underground, are not applicable. The nearest approach to it is in southeast Missouri, but such uniform and extended deposits of lead ore are the exception. As a rule lead mines are of medium size

or small.

On account of the relatively small units, the cost of mining per ton is considerably higher than if larger tonnages could be handled with the same overhead and if labor-saving equipment were more generally warranted. This disadvantage as compared with the mining of some of the other major metals is partially offset by light drilling expense, easy breaking of the ore, and low powder cost. Machine mining, however, is less common than in mining other metals.

Lead veins are usually not very wide and the item of timbering is a minor factor. Pumping, hoisting, and other minor operations are fairly comparable to those required in mining other metals.

Concentration. On account of its high specific gravity and its cubical cleavage, the gravity separation of galena (sulphide) from nonmetallic gangue is a simple matter.

But the separation from zinc and other sulphides is not so easy. The development of treatment methods for the complex ores of the Western States is one of the most important chapters in the history of ore dressing, and it was the introduction of the Willey table (an American device) that made possible the economic operation of a large part of the domestic resources of lead, zinc, and copper, especially in complex ores. In spite of the advances in treatment processes, however, the production of a very high-grade concentrate from complex ores is uneconomical. In the Mississippi Valley region concentrates containing 80 per cent lead are the standard grade. Few of the western mines are able to make as high as a 50 per cent product. Flotation has been the latest factor in making available the lowgrade deposits of this country. The adoption of this comparatively new process has greatly increased recoveries. Ten years ago a lead content of 2 per cent left in the tailing of lead mills was considered satisfactory work. Now, however, present methods permit the profitable utilization of ores containing not more than that amount to begin with.

Organization. The greater part of the lead mined in this country is mined by large and powerful corporations. There are many small operators, but even in the aggregate their production is not very great. Large operators frequently are highly integrated (owning their own smelting works, etc.) and have varied activities in addition to the business of mining lead ore. Small operators, due to limited. capital and equipment, are at a distinct disadvantage in the economical extraction of their ore from the ground and of the values from their ore and are unable to make as favorable contracts with the smelting companies as the larger operators who can guarantee a steadier and more dependable output.