GENERAL FEATURES
Iron and steel and their alloys are the most generally used of the metals. The raw materials necessary for their manufacture include a wide variety of minerals.
Iron is the principal element in this group; but in the manufacture of iron and steel, manganese, chromium, nickel, tungsten, molybdenum, vanadium, zirconium, titanium, aluminum, uranium, magnesium, fluorine, silicon, and other substances play important parts, either as accessories in the furnace reactions or as ingredients introduced to give certain qualities to the products.
Nearly all parts of the world are plentifully supplied with iron ores for an indefinite period in the future, but their abundant use has thus far been confined mainly to the countries bordering the North Atlantic,—the United States, Germany, and England,—which, possessing ample coal supplies, have had the initiative to develop great iron and steel industries. China has abundant coal, moderate quantities of iron ore, and a large population, but a low per capita consumption of iron and steel products. Development of its iron and steel industry is just beginning. Japan has neither coal nor iron in sufficient quantities, and hence the Japanese effort in recent years to control the mineral resources of China and other countries. As a result of the war Germany has been largely deprived of its iron ores, and France may assume somewhat the rank in iron ore production once held by Germany. Sweden and Spain have been considerable producers of iron ore, but both lack coal, with the result that their ores have been largely exported to England and Germany. With increase of per capita consumption in outlying parts of the world, iron and steel industries are beginning to develop locally on a small scale, as in India, South Africa, and Australia. Russia has had sufficient supplies of coal and iron, but the stage of industrial development in that country has not called for great expansion of its iron and steel industry.
There has been a tendency for iron and steel manufacture to become concentrated at a comparatively few places on the globe favored by the proper combinations of coal, iron, transportation, proximity to consuming populations, initiative and capacity to take advantage of a situation, and other factors. Even though on paper conditions may seem to be favorable in outlying territories for the development of additional plants, this development is often held back by competition from the established centers. On the west coast of the United States, there are raw materials for an iron and steel industry and there has been discussion for years as to the possibilities of starting a successful large scale steel industry. The consuming power of the local population for all kinds of iron and steel would seem to be great enough to warrant such action. However, the demand is for an extremely varied assortment of iron and steel products; and to start an industry, making only a few of the cruder products such as pig iron and semi-finished forms, would not meet this demand. All varieties of finishing plants and associated factories would also need to be started in order to meet the situation. This would require large capital. Furthermore the local demand for some of the accessory finished products might not warrant the establishment of the accessory plants.
Throughout the history of the iron and steel business there has been a marked tendency for the iron ore to move to regions of coal production rather than for the coal to move to the iron ore regions. The coal or energy factor seems ultimately to control. This is due in considerable part to the fact that coal furnishes the basis of a great variety of industries for which iron ore is only one of the feeders, and which are so interrelated that it is not always easy to move the iron and steel industry to a spot near the sources of iron ore where iron and steel alone could be produced.
In regard to iron ore supplies of proper grade and quantity, the United States is more nearly self-sufficing than any of its competitors. It imports minor amounts of ore from Cuba and Canada, and even from Chile and Sweden, to border points, in the main merely because these imported ores can compete on a price basis with the domestic ores. The entire exclusion of these ores, however, would make comparatively little difference in the total volume of our iron and steel industry; though it would probably make some difference in distribution, to the disadvantage of plants along the coast. There is only one kind of iron ore in which the United States has anything approaching deficiency, and that is ore extremely low in phosphorus, adapted to making the so-called low-phosphorus pig which is needed for certain special steels. Ordnance requirements during the war put a premium on these steels. While some of these extremely low-phosphorus ores are mined in the United States, additional quantities have been required from Spain and Canada and to a lesser extent from North Africa and Sweden. Also the Spanish pyrite, imported ordinarily for its sulphur content, on roasting leaves a residue of iron oxide extremely low in phosphorus which is similarly used. The elimination of pyrite imports from Spain during the war, therefore, was a considerable contributing factor to the stringency in low-phosphorus iron ores. War experience showed that the United States was dependent on foreign sources for 40 per cent or upwards of its needs in this regard. Certain developments in progress, notably the project for concentration of siliceous eastern Mesabi Range ores, make it likely that future domestic production will more nearly be able to meet the requirements.
The equivalent of 15 per cent of the iron ore mined in the United States is exported as ore to Canadian ports on the Great Lakes and in the form of crude iron and steel products to many parts of the world. England and Germany are almost the sole competitors in the export trade.
When we turn to the minerals used for making the alloys of iron and as accessories in the manufacture of iron, it appears that no one of the principal iron and steel producing countries of the world is self-supporting, but that these "sweeteners" must be drawn in from the far corners of the earth. The importance of these minor constituents is altogether out of proportion to their volume. For instance, only fourteen pounds of manganese are necessary in the making of a ton of steel, yet a ton of steel cannot be made without manganese. The increasing specialization in iron and steel products, and the rapidly widening knowledge of the qualities of the different alloys, are constantly shifting the demand from one to the other of the ferro-alloy minerals. Each one of the ferro-alloy minerals may be regarded as being in the nature of a key mineral for the iron and steel industry, and the control of deposits of these minerals is a matter of international concern. Control is not a difficult matter, in view of the fact that the principal supplies of practically every one of the alloy minerals are concentrated in comparatively few spots on the globe,—as indicated on succeeding pages.
Nature has not endowed the United States, nor in fact the North American continent, with adequate high-grade supplies of the principal ferro-alloy minerals,—with the exception of molybdenum, and with the exception of silica, magnesite, and fluorspar, which are used as accessories in the process of steel making. With plenty of iron ore and coal, and with an iron and steel capacity amounting to over 50 per cent of the world's total, the United States is very largely dependent on other countries for its supplies of the ferro-alloy minerals. The war brought this fact home. With the closing of foreign sources of supplies, it looked at one time as if our steel industry was to be very greatly hampered; and extraordinary efforts were made to keep channels of importation open until something could be done in the way of development, even at excessive cost, of domestic supplies. The result of war efforts was a very large development of domestic supplies of practically all the ferro-alloy minerals; but in no case, with the exceptions noted above, did these prove sufficient to meet the total requirements. This development was at great cost and at some sacrifice to metallurgical efficiency, due to the low and variable grades of the raw materials. With the post-war reopening of importation much of the domestic production has necessarily ceased, and large amounts of money patriotically spent in the effort to meet the domestic requirements have been lost. These circumstances have resulted in the demand in Congress from producers for direct financial relief and in demand for protective tariffs, in order to enable the new struggling industries to exist, and to permit of development of adequate home supplies. Such tariffs might be beneficial to these particular domestic industries if wisely planned; but also, in view of the limited amounts of these particular ores in this country, their general low grade, and the high cost of mining, tariffs might very probably hasten exhaustion of our limited supplies and might handicap our metallurgical industries both in efficiency and cost (see pp. 365-366, 393-394).
IRON ORES
Economic Features
Technical and commercial factors determining use of iron ore minerals. Popularly, an iron ore is an iron ore, and there is little realization of its really great complexity of composition and the difficulty of determining what is or is not a commercial ore. Percentage of iron is of course an important factor; but an ore in which the iron is in the mineral hematite is more valuable than one with an equivalent percentage of iron which is in the form of magnetite. Substances present in the ore in minor quantities, such as phosphorus, sulphur, and titanium, have a tendency to make the iron product brittle, either when it is cold or when it is being made, so that excessive amounts of these substances may disqualify an ore. Excessive quantities of silica, lime, or magnesia may make the ore undesirable. Where an acid substance, like silica, is balanced by basic constituents like lime and magnesia, considerable amounts of both may be used. Excessive moisture content may spoil an ore because of the amount of heat necessary to eliminate it in smelting.
The metallurgical processes of the iron and steel industry are essentially adapted to the principal grades of ore available. The cheapest of the steel-making processes, called the acid Bessemer process, requires a very low-phosphorus ore (usually below .050 per cent in the United States and below .030 per cent in England.) The basic open-hearth processes, making two-thirds of the steel in the United States, allow higher percentages of phosphorus, but not unlimited amounts. The basic Bessemer (Thomas) process, used for the "minette" ores of western Europe and the Swedish magnetites, may use an ore with any amount of phosphorus over 1.5 per cent. The phosphatic slag from this process is used as fertilizer. The supply of low-phosphorus Bessemer ore in the United States is at present limited as compared with that of the non-Bessemer ores, with the result that steel-plant construction for many years past has been largely open-hearth. The open-hearth process is favored also because it allows closer control of phosphorus content in the steel.
Small but increasing amounts of steel are also made in the electric furnace; for the most part, however, this process is more expensive than the others, and it is used principally for special alloy steels.
Iron ores are seldom so uniform in quality that they can be shipped without careful attention to sampling and grade. In the Lake Superior region the ores are sampled daily as mined, and the utmost care is taken to mix and load the ore in such a way that the desired grades can be obtained. Ordinarily a single deposit produces several grades of ore. When ores are put into the furnace for smelting the mixtures are selected with great care for the particular purpose for which the product is to be used. The mixture is compounded as carefully as a druggist's prescription. An ore salesman, after ascertaining the nature of the iron and steel products of a plant, has to use great skill in offering particular ores for sale which not only will meet the desired grade in regard to all elements, but also will meet competition in price. In some respects, the marketing of different grades of iron ore is as complex as the marketing of a miscellaneous stock of merchandise. With ores, as with merchandise, custom and sentiment play their part,—with the result that two ores of identical grade mineralogically and chemically may have quite a different vogue and price, simply because of the fact that furnace men are used to one and not to the other and are not willing to experiment.
The geologist is ordinarily concerned merely with finding an ore of as good a general grade as possible; but he often finds to his surprise that his efforts have been directed toward the discovery of something which, due to some minor defect in texture, in mineralogical composition, or in chemical composition, is difficult to introduce on the market. There is here a promising field, intermediate between geology (or mineralogy) and metallurgy, for the application of principles of chemistry, metallurgy, and mineralogy, which is occupied at the present time mainly by the ore salesman. Both the mineralogist and metallurgist touch the problem but they do not cover it. With increasingly precise and rapidly changing metallurgical requirements, this field calls for scientific development.
Geographic distribution of iron ore production. Iron ores are widely distributed over the world, but are produced and smelted on a large scale only in a few places where there is a fortunate conjunction of high grades, large quantity, proximity of coal, cheap transportation to markets, and manufacturing enterprise. Over 90 per cent of the iron ore production of the world is in countries bordering the North Atlantic basin. The United States produces about 40 per cent, France about 12 per cent, England about 10 per cent, Germany before the war 15 to 20 per cent, and Spain, Russia, and Sweden each about 5 per cent. Lesser producing countries are Luxemburg, Austria-Hungary, Cuba, Newfoundland, and Algeria; and insignificant amounts are produced in many other parts of the world. Of the world's iron and steel manufacturing capacity, the United States has about 53 per cent, Germany 16 per cent, England 14 per cent, France 10 per cent, the remainder of Europe (chiefly Russia, Austria-Hungary, and Belgium) 7 per cent. The absence of important iron ore production and of iron and steel manufacture either in the southern hemisphere or in any of the countries bordering the Pacific is a significant feature, when we remember what part iron plays in modern civilization. Japan, however, is beginning to develop a considerable iron and steel industry, which promises to use a large amount of ore from China, Manchuria, and Korea, and possibly to compete in American Pacific Coast markets.
In the United States about 85 per cent of the production, or one-third of the world's production, comes from the Lake Superior region, a large part of the remainder from the Birmingham district, Alabama, and smaller quantities from the Adirondacks. For the rest of the North American continent, the only largely producing deposit is that at Belle Isle, Newfoundland, which is the basis of the iron industry of eastern Canada. Cuba supplies some ore to the east coast of the United States.
In Europe there are only three large sources of high-grade iron ore which have heretofore been drawn on largely,—the magnetite deposits of northern Sweden, the hematites and siderites of the Bilbao and adjacent districts of northern Spain, and the magnetite-hematite deposits of southern Russia. The first two of these ores have been used to raise the percentage of iron in the low-grade ores which are the principal reliance of western Europe. The Swedish ores have also been necessary in order to raise the percentage of phosphorus and thus make the ores suitable for the Thomas process; on the other hand the Spanish ores and a small part of the Swedish material have been desired because of their low phosphorus content, adapted to the acid Bessemer process and to the manufacture of low-phosphorus pig. The Russian ores have largely been smelted in that country.
The largest of the western European low-grade deposits is a geographic and geologic unit spreading over parts of Lorraine, Luxemburg, and the immediately adjacent Briey, Longwy, and Nancy districts of France. The ores of this region are called "minette" ores. This unit produces about a fourth of the world's iron ore. Low-grade deposits of a somewhat similar nature in the Cleveland, Lincolnshire, and adjacent districts of England form the main basis for the British industry. There is minor production of iron ores in other parts of France and Germany, in Austria-Hungary, and in North Africa (these last being important because of their low phosphorus content).
Comparison of figures of consumption and production of iron ores indicates that the United States, France, Russia, and Austria-Hungary are self-supporting so far as quantity of materials is concerned. Certain ores of special grades, and ores of other minerals of the ferro-alloy group required in steel making, however, must be imported from foreign sources; this matter has been discussed above. Great Britain and Germany appear to be dependent on foreign sources, even under pre-war conditions, for part of the material for their furnaces. During the war there was considerable development of the low-grade English ores, but this does not eliminate the necessity for importing high-grade ores for mixture. Belgium produces a very small percentage of her ore requirements and is practically dependent on the Lorraine-Luxemburg field.
The principal effect of the war on iron ore production was the occupation of the great French mining and smelting field by the Germans, thereby depriving the French of their largest source of iron ore. Since the war the situation has been reversed, France now possessing the Lorraine field, which formerly supplied Germany with 70 per cent of its iron ore. As the German industrial life is largely based on iron and steel manufacture, the problem of ore supplies for Germany is now a critical one. It has led to German activity in Chile and may lead to German developments in eastern Europe and western Asia, particularly in the large and favorably located reserves of southern Russia. It seems likely, however, that arrangements will also be made to continue the export of ore from the Lorraine field down the Rhine to the principal German smelting centers. France needs the German coal for coking as badly as Germany needs the French iron ore. The Rhine valley is the connecting channel for a balanced movement of commodities determined by the natural conditions. These basic conditions are likely in the long run to override political considerations.
The Lake Superior deposits, the Swedish magnetites, the Spanish hematites, and the Russian ores carry 50 to 65 per cent of metallic iron. The Birmingham deposits of southeastern United States, the main British supplies, and the main French and German supplies contain about 35 per cent or less. It is only where ores are fortunately located with reference to consuming centers that the low-grade deposits can be used. For outlying territories only the higher-grade deposits are likely to be developed, and even there many high-grade deposits are known which are not mined. The largest single group not yet drawn on is in Brazil. Others in a very early stage of development are in North Africa and Chile.
World reserves and future production of iron ore. The average rate of consumption of iron ore for the world in recent years has been about 170 million tons per year. At this rate the proved ore reserves would last about 180 years. If it be assumed that consumption in the future will increase at about the same rate as it has in the past, the total measured reserve would still last about a century. These calculations of life, however, are based only on the known reserves; and when potential reserves are included the life is greatly increased. And this is not all; for beyond the total reported reserves (both actual and potential), there are known additional large quantities of lower-grade ores, at present not commercially available, but which will be available in the future,—to say nothing of expected future discoveries of ores of all grades in unexplored territories. Both geological inference and the history of iron ore exploration seem to make such future discoveries practically certain. Iron ore constitutes about 4 per cent of the earth's shell and it shows all stages of concentration up to 70 per cent. Only those rocks are called "iron ores" which have a sufficiently high percentage of iron to be adapted to present processes for the extraction of iron. When economic conditions demand it, it may be assumed that iron-bearing rocks not now ordinarily regarded as ores may be used to commercial advantage, and therefore will become ores.
Not only is an indefinitely long life assured for iron ore reserves as a whole, but the same is true of many of the principal groups of deposits.
The question of practical concern to us, therefore, is not one of total iron ore reserves, but one of degrees of availability of different ores to the markets which focus our requirements for iron.
The annual production of ore from a given district is roughly a measure of that ore's ability to meet the competitive market, and therefore, of its actual immediate or past availability. Annual production is the net result of the interaction of all of the factors bearing on availability. It may be argued that there are ores known and not yet mined which are also immediately available. On the whole, they seem to be less available than ores actually being produced; otherwise general economic pressure would require their use and actual production.
In considering the future availability of iron ores, it is obvious that tables of past production afford only a partial basis for prediction. Presumably districts which have produced largely in the past may be expected to continue as important factors. In these cases production has demonstrated availability. Continued heavy production may thus be expected from the ores of the Lake Superior region, from the Clinton hematites of Alabama, from the ores of the Lorraine-Luxemburg-Briey district, from the Cleveland ores of England, from the Bilbao ores of Spain, from the high-grade magnetites of northern Sweden, and (assuming political stability) from the ores of southern Russia.
Similarly, also, recent increases in production from certain districts are probably significant of increased use of such ores in the future. Among these developments are the increasing production of Swedish ores and their importation into England and Germany, and the increasing use of Clinton hematites and Adirondack magnetites in the United States. Low-grade ores from the great reserves of Cuba are being mined and brought to the east coast of the United States in increasing amounts, and it is highly probable that they will take a larger share of the market. A similar project in Chile, which lay dormant during the war because of restricted shipping facilities, is expected in the near future to yield important shipments to the United States. In none of these cases will production be limited in the near future by ore reserves. Increased production and use of iron ores are also to be looked for in Newfoundland, North Africa, China, India, Australia, and South Africa.
On the commercial horizon are ores of still newer districts, the availability of which may not be read from tables of production. Their availability must be determined by analysis and measurement of the factors entering into availability. Availability of iron ore is determined by percentage of iron, percentages of impurities, percentages of advantageous or deleterious minor constituents, physical texture, conditions for profitable mining, adaptability to present furnace practice, distance from consuming centers, conditions and costs of transportation, geographical and transportational relation to the coal and fluxes necessary for smelting, trade relations, tariffs and taxes, inertia of invested capital, and other considerations. All of these factors are variable. A comparison of ores on the basis of any one of these factors or of any two or three of them is likely to be misleading. A comparison based on the quantitative consideration of all of the several factors seems to be made practically impossible by the difficulty of ascertaining accurately the quantitative range and importance of each factor, and by the difficulty of integrating all of the factors even if they should be determined. However, their combined effect is expressed in the cost of bringing the product to market; and comparison of costs furnishes a means of comparing availability of ores. A high-grade ore, cheaply mined and favorably located with reference to the points of demand, will command a relatively high price at the point of production. The same ore so located that its transportation costs are higher will command a lower price; or it may be so located that the costs of mining and bringing it to places where it can be used are so high that there is no profit in the operation. There are known high-grade iron ores which, because of cost, are not available under present conditions.
The availability of an ore, then, depends on its relation to a market,—whether, after meeting the cost of transportation, it can be sold at prevailing market prices at the consuming centers, and can still leave a fair margin of profit for the mining operation. The price equilibrium between consuming centers affords a reasonably uniform basis against which to measure availability of ores.
Figures of cost are obtainable as a basis for comparison of availability of iron ores of certain of the districts, but not enough are at hand for comparison of the ores of all districts. Careful study of costs has demonstrated the availability in the near future of the Brazilian high-grade Bessemer hematites; and projects which are now under way for exportation to England and the United States will doubtless make this enormous reserve play an important part in the iron industry. Iron ore is known but not yet mined in many parts of the western United States and western Canada. With the increasing population along the west coast of North America, projects for smelting the ore there are becoming more definite. Establishment of smelters on the west coast would make available a large reserve of ore (see also, however, p. 155).
The list of changes now under way or highly probable for the future might be largely extended. The use of iron and steel is rapidly spreading through populous parts of the world which have heretofore demanded little of these products. This increased use is favoring the development of local centers of smelting, which will make available other large reserves of iron ore. The growth of smelting in India, China and Australia illustrates this tendency.
Iron ore reserves are so large, so varied, and so widely distributed over the globe, that they will supply demands upon them to the remote future. Reserves become available and valuable only by the expenditure of effort and money. Ores are the multiplicand and man the multiplier in the product which represents value or availability. Iron ore can be made available, when needed, almost to any extent, but at highly varying cost and degree of effort. The highest grade ores, requiring minimum expenditure to make them available, are distinctly limited as compared to total reserves. Any waste in their utilization will lead more quickly to the use of less available ores at higher cost. One of the significant consequences of the exhaustion of the highest grade reserves will be an increased draft upon fuel resources for the smelting of the lower grade ores. Availability of iron ores is limited, not by total reserves, but by economic conditions.
Geologic Features
Iron rarely exists in nature as a separate element. It occurs mainly in minerals which represent combinations of iron, oxygen, and water, the substances which make up iron rust. Very broadly, most of the iron ores might be crudely classified as iron rust. In detail this group is represented by several mineral varieties, principal among which are hematite (Fe2O3), magnetite (Fe3O4), and limonite (hydrated ferric oxide). Iron likewise combines with a considerable variety of substances other than oxygen; and some of these compounds, as for instance iron carbonate (siderite), iron silicate (chamosite, glauconite, etc.), and iron sulphide (pyrite), are locally mined as iron ores. While an ore of iron may consist dominantly of some one of the iron minerals, in few cases does it consist exclusively of one mineral. Most ores are mixtures of iron minerals.
Fully nine-tenths of the iron production of the world comes from the so-called hematite ores, meaning ores in which hematite is the dominant mineral, though most of them contain other iron minerals in smaller quantities. About 5 per cent of the world's iron ores are magnetites, and the remainder are limonites and iron carbonates.
Iron ores are represented in nearly all phases of the metamorphic cycle, but the principal commercial values have been produced by processes of weathering and sedimentation at and near the surface.
Sedimentary iron ores. Over 90 per cent of the world's production of iron ore is from sedimentary rocks. The deposits consist in the main either of beds of iron ore which were originally deposited as such and have undergone little subsequent alteration, or of those altered portions of lean ferruginous beds which since their deposition have been enriched or concentrated sufficiently to form ores. A minor class of iron ores in sediments consists of deposits formed by secondary replacement of limestones by surface waters carrying iron in solution.
1. Deposits of the first class,—originally laid down in much their present form,—are usually either oölitic, i. e., containing great numbers of flat rounded grains of iron minerals like flaxseeds, or consist in large part of fossil fragments of sea shells, replaced by iron minerals. The Clinton ores of the Birmingham district, the Wabana ores of Newfoundland, the minette ores of the Lorraine district in central Europe, and the oölitic ores of northern England are all of these types. Their principal iron mineral is hematite, although the English ores also contain considerable iron carbonate or siderite. The cementing or gangue materials are chiefly calcite and quartz, in variable proportions.
The large reserves of high-grade hematite in the Minas Geraes district of Brazil are also original sediments, but lack the oölitic texture.
An insignificant proportion of the world's iron is obtained from "bog ores," which are sedimentary deposits of hydrated iron oxide in swamps and lakes. These ores have been used only on a small scale and chiefly in relatively undeveloped countries. They are of particular interest from a genetic standpoint in that they show the nature of some of the processes of iron ore deposition as it is actually going on today.
None of the ores of this class, with the exception of the iron carbonates, have undergone any considerable surface enrichment since their primary deposition. Neither, with the exception of the Brazilian ores, have they undergone any deep-seated metamorphism. The shapes, sizes, and distribution of the deposits may be traced back to the conditions of original deposition. In England and western Europe the principal deposits have been only slightly tilted by folding. In the United States the Clinton ores have partaken in the Appalachian folding. In Brazil, the ores have undergone close folding and anamorphism.
2. Deposits of the second class, which owe much of their value to further enrichment since deposition, are represented by the hematite ores of the Lake Superior district. These may be thought of as the locally rusted and leached portions of extensive "iron formations," in which oxidation of the iron, and the leaching of silica and other substances by circulating waters, have left the less soluble iron minerals concentrated as ores. The Lake Superior iron formations now consist near the surface mainly of interbanded quartz (or chert) and hematite, called jasper or ferruginous chert or taconite. These are similar in composition to the leaner iron ores of Brazil, called itabirite, but differ in that the silica is in the form of chemically deposited chert, rather than fragmental quartz grains.
Fig. 11. Alteration of Lake Superior iron formation to iron ore by the leaching of silica.[ToList]
When originally deposited the iron was partly hematite (perhaps some magnetite) and largely in the form of iron carbonate (siderite) and iron silicate (greenalite), interbanded with chert. The original condition is indicated by the facts that deep below the surface, in zones protected from weathering solutions, siderite and greenalite are abundant, and that they show complete gradation to hematite in approaching the surface. The ore has been concentrated in the iron formation almost solely by the process of leaching of silica by surface or meteoric waters, leaving the hematite in a porous mass. Figure 11 illustrates this change as calculated from analyses and measurements of pore space. During this process a very minor amount of iron has been transported and redeposited. In short, the Lake Superior iron ores are residual deposits formed by exactly the same weathering processes as cause the accumulation of clays, bauxites, and the oxide zones of sulphide deposits. The development of an iron ore rather than of other materials as an end-product is due merely to the peculiar composition of the parent rock. The solution of silica on such an immense scale as is indicated by these deposits has sometimes been questioned on the general ground that silica minerals are insoluble. However, there is plenty of evidence that such minerals are soluble in nature; and the assumption of insolubility, so often made in geologic discussions, is based on the fact that most other minerals are more soluble than silica minerals, and that in the end-products of weathering silica minerals therefore usually remain as important constituents. Iron oxide, on the other hand, is less soluble even than silica,—with the result that when the two occur together, the evidence of leaching of silica from the mixture becomes conspicuous.
The fact that these deposits are almost exclusively residual deposits formed by the leaching of silica has an important bearing on exploration. If they have been formed by the transportation and deposition of iron from the surrounding rocks, there is no reason why they should not occasionally be found in veins and dikes outside of the iron formation. As a matter of fact they do not transgress a foot beyond the limits of the iron formation. Failure to recognize the true nature of the concentration of these ores has sometimes led to their erroneous classification as ores derived from the leaching and redeposition of iron from the surrounding rocks.
The distribution and shapes of ore deposits of this class are far more irregular and capricious than those of the primary sediments, as would be expected from the fact that their concentration has taken place through the agency of percolating waters from the surface, which worked along devious channels determined by a vast variety of structural and lithological conditions. The working out of the structural conditions for the different mines and districts constitutes one of the principal geologic problems in exploration. These conditions have been fully discussed in the United States Geological Survey reports, and are so various that no attempt will be made to summarize them here.
One of the interesting features of the concentration of Lake Superior iron ores is the fact that it took place long ago in the Keweenawan period, preceding the deposition of the flat-lying Cambrian formations, at a time when the topography was mountainous and the climate was arid or semi-arid. These conditions made it possible for the oxidizing and leaching solutions to penetrate very deeply, how deeply is not yet known, but certainly to a depth below the present surface of 2,500 feet. At present the water level is ordinarily within 100 feet of the surface, and oxidizing solutions are not going much below this depth. This region, therefore, furnishes a good illustration of the intermittent and cyclic character of ore concentration which is now coming to be recognized in many ore deposits.
Subsequent changes far beneath the surface have folded, faulted, and metamorphosed some of the Lake Superior iron ores but have not enriched them. The same processes have recrystallized and locked together the minerals of some of the lean iron formations, making them hard and resistant, so that subsequent exposure and weathering have had little effect in enriching them to form commercial ores.
The weathering of limestones containing minor percentages of iron minerals originally deposited with the limestones may result in the residual concentration of bodies of limonite or "brown ores" associated with clays near the surface. This process is similar in all essential respects to the concentration of the Lake Superior ores. Such limonitic ores are found rather widely distributed through the Appalachian region and in many other parts of the world. Because of the ease with which they can be mined and smelted on a small scale they have been used since early times, but have furnished only a very small fraction of the world's iron.
3. In a third class of sedimentary ores, the iron minerals are supposed to have been introduced as replacements of limestones subsequent to sedimentation. Such ores are not always easy to discriminate from ores resulting primarily from sedimentation. This class is represented by the high-grade deposits of Bilbao, Spain, Austrian deposits, and by smaller deposits in other countries. The Bilbao ores consist mainly of siderite, which near the surface has altered to large bodies of oxide minerals. They occur in limestones and shales and are not associated with igneous rocks. The deposits are believed to have been formed by ordinary surface waters carrying iron in solution, and depositing it in the form of iron carbonate as replacements of the limestones. The original source of the iron is believed to have been small quantities of iron minerals disseminated through the ordinary country rocks of the district. The action of surface waters, in thus concentrating the iron in certain localities which are favorable for precipitation, is similar to the formation of the lead and zinc ores of the Mississippi valley, referred to in the next chapter. Deposits formed in this manner may be roughly tabular and resemble bedded deposits, or they may be of very irregular shapes.
The sedimentary iron ores in general evidently represent an advanced stage of katamorphism, and illustrate the tendency of this phase of the metamorphic cycle toward simplification and segregation of certain materials. The exact conditions of original sedimentation present one of the great unsolved problems of geology, referred to in Chapter III.
Iron ores associated with igneous rocks. About five per cent of the world's production of iron ore is from bodies of magnetite formed in association with igneous rocks. These are dense, highly crystalline ores, in which the iron minerals are tightly locked up with silicates, quartz, and other minerals, suggestive of high temperature origin. The largest of these deposits is at Kiruna in northern Sweden; in fact this is the largest single deposit of high-grade ore of any kind yet known in the world. Here the magnetite forms a great tabular vertical body lying between porphyry and syenite. In the Adirondack Mountains of New York and in the highlands of New Jersey, magnetites are interbedded and infolded with gneisses, granites, and metamorphic limestones. In the western United States there are many magnetite deposits, not yet mined, at contacts between igneous intrusives and sedimentary rocks, particularly limestones (so-called "contact-metamorphic" deposits). The ores of the Cornwall district of Pennsylvania and some of the Chilean, Chinese, and Japanese ores are of the same type.
Magnetites containing titanium, which prevents their use at the present time, are known in many parts of the world as segregations in basic igneous rocks. They are actually parts of the igneous rock itself (p. 34). Among the large deposits of this nature are certain titaniferous ores of the Adirondacks, of Wyoming, and of the Scandinavian peninsula.
In all of these cases, it is clear that the origin of the ores is in some way related to igneous processes, and presumably most of the ores are deposited from the primary hot solutions accompanying and following the intrusion of the igneous rocks; but thus far it has been difficult to find definite and positive evidence as to the precise processes involved. None of these deposits have undergone any important secondary enrichment at the surface. Their sizes, shapes, and distribution are governed by conditions of igneous intrusion, more or less modified, as in the Adirondacks, by later deformation.
Iron ores due to weathering of igneous rocks. A small part of the world's iron ores, less than 1 per cent of the total production, are the result of surface alteration of serpentine rocks. These ores are mined principally in Cuba (Fig. 12). Here they have been developed on a plateau-like area on which erosion is sluggish. The process of formation has been one of oxidation of the iron minerals and leaching of most of the other constituents, leaving the iron concentrated near the surface in blanket-like deposits. The minerals of the original rock contained alumina, which, like the iron, is insoluble under weathering conditions, and hence the Cuban iron ores are high in alumina. They also contain small quantities of nickel and chromium which have been concentrated with the iron. A large part of the iron minerals, especially where close to the surface, have been gathered into small shot-like nodules called pisolites. It is thought that the solution and redeposition of the iron by organic acids from plant roots may be at least a contributing cause in the formation of this pisolitic texture.
Fig. 12. Representing in terms of weight the mineralogical changes in the katamorphism of serpentine rock to iron ore, on the assumption that alumina has remained constant, eastern Cuba.[ToList]
The Cuban iron ores are similar in their origin to laterites, which are surface accumulations of clay, bauxite, and iron oxide minerals, resulting from the weathering of iron-bearing, commonly igneous, rocks. The typical laterites carry more clay and bauxite than the Cuban iron ores, but this is due merely to the fact that the original rocks commonly carry more materials which weather to clay. In fact the Cuban iron ores are themselves, broadly speaking, laterites.
Iron ores due to weathering of sulphide ores. A relatively minute portion of the world's iron ore comes from the "gossans" or "iron caps" over deposits of iron sulphides. The gossans are formed by oxidation and leaching of other minerals from the deposits, leaving limonite or hematite in concentrated masses (see pp. 46-47).
MANGANESE ORES
Economic Features
Manganese ores are used mainly in the manufacture of steel, the alloys spiegeleisen and ferromanganese being added to the molten steel after treatment in the Bessemer converter and open-hearth furnace in order to recarburize and purify the metal. The alloy ferromanganese is also used in the production of special manganese steels. Manganese ore is used in relatively small amounts in dry batteries, in the manufacture of manganese chemicals, in glass making, and in pigments. Steel uses 95 per cent of the total manganese consumed, batteries and chemicals 5 per cent. On an average each ton of steel in the United States requires 14 pounds of metallic manganese, equivalent to 40 pounds of manganese ore.
With manganese ores, as with iron ores, the percentage of minor constituents,—phosphorus, silica, sulphur, etc.,—determines to a large extent the manner of use. Low-grade manganese ores, ranging from 10 to 35 per cent in manganese, 20 to 35 per cent in iron, and containing less than 20 per cent of silica, are used mainly in the production of the low-grade iron-manganese alloy called spiegeleisen or spiegel (16 to 32 per cent manganese). The higher-grade ores, ranging from 35 to 55 per cent in manganese, are used mainly in the production of the high-grade alloy called ferromanganese or ferro, in which the manganese constitutes 65 to 80 per cent of the total. To a very limited extent manganese is smelted directly with iron ores, thus lessening the amount to be introduced in the form of alloys; this, however, is regarded as wasteful use of manganese, since its effectiveness as so used is not very great. Steel makers usually prefer to introduce manganese in the form of ferromanganese rather than as spiegel. On the other hand, the ores of the United States as a whole are better adapted to the manufacture of spiegel. With the shutting off of foreign high-grade supplies during the war, resulting in the increased use of local ores, it became necessary to use larger amounts of the spiegel which could be made from these ores. Metallurgists stated that it was theoretically possible to substitute spiegel for the higher grade alloy up to 70 per cent of the total manganese requirement, but in actual practice this substitution did not get much beyond 18 per cent.
The principal manganese ore-producing countries in normal times are Russia, India, and Brazil. Relatively little ore is used in these countries, most of it being sent to the consuming countries of Europe and to the United States. The Indian ore has been used largely by British steel plants, but much of it also has gone to the United States, Belgium, France, and Germany. The Russian ore has been used by all five of these countries, Germany having a considerable degree of commercial control and receiving the largest part; a small quantity is also used in Russia. Brazilian ore has gone mainly to the United States, and in part to France, Germany, and England.
Smaller amounts of manganese ore have been produced in Germany, Austria-Hungary, Spain, and Japan. This production has had little effect on the world situation. That produced in Austria-Hungary and Germany is used in the domestic industry. That from Spain and Japan is in large part exported.
The highest grade of manganese ore comes from the Russian mines, especially those in the Caucasus region. Most of the ore used for the manufacture of dry batteries and in the chemical industry, where high-grade ores are required, has come from Russia. By far the larger part of the Russian production, however, has gone into steel manufacture. Indian and Brazilian ores have likewise been used mainly in the steel industry. Some Japanese ore also is of high grade and is used for chemical and battery purposes.
Nature has not endowed the United States very abundantly with manganese ores, and such as are known are widely scattered, of relatively small tonnage, and of a wide range of grade. The principal producing districts are the Philipsburg district of Montana and the Cuyuna Range of Minnesota; there are also scattering supplies in Virginia, Arizona, California, and many other states. The use of domestic ores has sometimes been unsatisfactory, because of frequent failure of domestic producers to deliver amounts and grades contracted for. It has been, on the whole, cheaper, easier, and more satisfactory for the large consumers to purchase the imported ore, which is delivered in any desired amount and in uniform grades, rather than to try to assemble usable mixtures from various parts of the country.
Before the European War, the United States produced only 1 to 2 per cent of its needed supply of manganese, the rest being imported mainly from India, Russia, and Brazil, in the form of ore, and from England in the form of ferromanganese (about half of the total requirement). The partial closing of the first two and the fourth of these sources of supply under war conditions made it necessary to turn for ore to Brazil and also to Cuba, where American interests developed a considerable industry in medium-grade ores. At the same time steps were taken to develop domestic resources; and with the high prices imposed by war conditions, the domestic production, both of high- and low-grade ore, was increased largely, but still was able to supply only 35 per cent of the total requirements of manganese.
At the close of the war sufficient progress had been made—in the discovery of many new deposits in the United States, in the use of low-grade domestic ores, which before had not been able to compete with imported ores, and in the increased use of spiegel, allowing wider use of low-grade ores,—to demonstrate that, if absolutely necessary, and at high cost, the United States in another year or two could have been nearly self-sufficing in regard to its manganese requirements. The release of shipping from war demands resulted immediately in larger offerings of foreign manganese ore and of ferromanganese from England, at prices which would not allow of competition from much of the domestic or Cuban ore production or from the domestic manufacture of alloys. The result was a rather dramatic closing down of the manganese industry, with much financial loss, the passage of a bill for reimbursement of producers, and a demand on the part of the producers, though not of consumers, for a protective tariff. In the questions thus raised it is desirable that geologists and engineers professionally connected with the industry thoroughly understand the basic facts; for they are liable to be called upon for advice, not only on questions relating to domestic supplies affected by possible future foreign policies, but on the formulation of the policies themselves. Conservation, cheaper steel, and future trade relations of the United States all require consideration, before action is taken to protect this one of several similarly situated mineral industries, in the effort to make the country self-supporting. These questions are further dealt with in Chapters XVII and XVIII.
Manganese production was also developed during the war in the Gold Coast of West Africa, in Costa Rica, in Panama, in Java, and elsewhere; but with the possible exception of Java and Chile, none of these sources are likely to be factors in the world situation. The war-developed manganese production of Italy, France, Sweden, and United Kingdom is also unlikely to continue on any important scale.