IRGWIA
The SOURCES of OUR IRON ORES.'
I
ERNEST F. BURCHARDt United States Geological Survey, Washington, D. C.
Iron, the most commonly used metal, although second in and chlorine. I?. W. Clarke,' who has made most perabundance in the earth's crust, is d e l y distributed in sistent and penetrating studies of the composition of most of the continents, and North America, fiarticularly the earth, states: in the eastern half of the United States, is well s u ~ f i f i d It is although not universally supposed that the with this indispensable basis for industry. This Paper ear& mnsists mainly of a ,,"dens of nickel-iron, surrounded by endeavors to indicate the relations between the iron of the an of igneous rocks. The dominant minerals of these interior and crust of the earth, the common iron minerals rocks could have crvstallized onlv from a state of fusion, as is and ores, and the& geographic and geologicdistribution also true of the min&als found in meteorites. The presence of in the northeastern and southeastern United States, glass in the rocks and also in meteoric stones tells the same story. The crystallie structure of meteoric iron is also attributable t o reserving for a later instalment the discussion of deposits the cooling of a melt under p r e s s u r e a fact which is e m ~ h a in the Lake Superior and western States. sized by presence of minute diamonds in the iron of Canyon Diablo. . . . I n the cooling of the molten earth the iron separated
A
question is suggested in Our so broad indeed, that astronomical researches would be necessary to trace the iron of the earth back to its ultimate source. It is generally believed, however, that ironexistsin the stars, in the sun, in the planets, and it has been found in the that accidentally reach the surface of the earth. It is through study of meteorites that much has been learned about the probable of the earth as a whole. A comparison of average compositions of meteoric and natural terrestrial irons shows marked similarity. Both contain more than 90 per cent. of iron with nickel by far the largest minor constituent, and quantities, generally less than 1 per cent., of cobalt, copper, chromium, phosphorus, sulfur, carbon,
* Published by permission of the Director, United States Geo-
logical S m y . t Geologist, United States Geological Survey, Washington,
D.C .
from the stony material just as i t separates from the slag in a blast furnace. This discussion of the relations between meteorites and the earth bears directly upon the problem of the relative abundance of the chemical elements. ~tis now possible by more than one method t o determine with a high degree of probability the dimensions and mass of the terrestrial nucleus and of its envelope. . . Without going into details, which can be found in Professional Paper 132-De2i t is easy t o show that in the earth as a whole iron is by far the most abundant element, with oxygen second, silicon third, and nickel next in order. The percentages found by me are iron 67.2, oxygen 12.8, silicon 7, and nickel 4. The percentages of the other elements are all small.
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one can have little experiw i t h the earth as a ence and consequently its have less cance than those of the n u s t of the earth which can be observed. With regard to this phase Clarkea believes
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1 CLARKE,F. W., "Data of geochemistry," U.S. Gcol. Survey Bull., No. 770, pp. 4 2 3 (1924). CLARKE,F. W.. "The evolution and disintegration of matter," U.S. Geol. Suwey Prof. Paper, No. 132, pp. 51-86 (1924). 8 C L ~F. W., , see footnote (I), pp. 3%.
195
that if certain factors such as porosities of rocks are kept in mind i t is possible to compute the relative abundance of the chemical elements in all known terrestrial matter, and gives a table showing the mean composition of the lithosphere, the hydrosphere, and the atmosphere, in which iron forms 5.06 per cent. of the lithosphere (itself 93 per cent. of the 10-mile crust) with an average of 4.71 per cent. for the lithosphere and hydrosphere combined. The figure of 4.71 per cent. iron
is the one with which we are most familiar as it has been commonly quoted and shown in diagrams. Iron is rarely found in its native, or metallic state, on account of its chemical affinity for oxygen, especially in the presence of water. The known occurrences are generally as minute grains in eruptive rocks, such as basalt, although a t Ofivak, Greenland, masses weighing as much as 20 tons have been weathered out from basalt. In combination with other elements, such as aluminum, arsenic, bismuth, boron, calcium, carbon, chlorine, chromium, copper, hydrogen, lithium, mag-
nesium, manganese, nickel, oxygen, phosphorus, potassium, silicon, sodium, sulfur, tantalum, tellurium, tin, titanium, tungsten, uranium, yttrium, and zinc, iron is found in varying quantities in nearly all kinds of rock and in hundreds of mineral species. Dana's "Mineralogy" lists about 225 of these species, most of them minerals seldom heard of by any one save the most technical mineralogists. Many of these minerals are complex silicates, antimonates, tantalates, phosphates, arsenates. minerals in the formulas of which there may be several radicals with iron playing a very unimportant part. There is, of course, iron in sea water andin nearly all surface water and ground water, for certain iron salts are easily soluble. With so widespread a distribution of iron it is not surprising that soils, the product of disintegration of rocks, should be more or less femginous. The foregoing comments leave no doubt that the world is fundamentally well supplied with iron, but make it equally clear that only such deposits as are conveniently situated in the outer crust of the earth are available for the use of man. The modes of transference of iron from the nucleus of nickel-iron to the envelope of igneous rocks and thence to the outer portions of the lithosphere would constitute a fascinating subject for research and compilation but is far too extensive and complex to be dwelt upon here except incidentally. Naturally a great deal of iron has been brought from the depths of the crust of the earth in volcanic rocks, in molten magmas and in magmatic waters, forming iron carbonates, sulfides, and oxides within the igneous rocks and a t their contacts with sedimentary rocks, and these deposits have been reworked into the sediments and soils, often in very concentrated forms. The concentration of iron compounds into valuable deposits is likewise the subject of a voluminous literature which i t would be impossible to even outline here, but the classifications of iron ore deposits on page 200 may indicate some of the processes through which the deposits attained their present condition. IRON ORES
A few iron minerals are amenable to the economical extraction of metallic iron, but the great majority are not. The ones which yield iron readily to present commercial metallurgical practice are termed iron ores. The important ores consist of simple compounds of iron with oxygen, carbon, and water; other iron-bearing minerals in which sulfur or silicon may be present play a very minor part as a commercial source of iron. There are four important types of ores of iron-red iron
ore or hematite; brown iron ore, a mixture of hydrated iron oxides, represented by gothite, turgite, and limonite; magnetic ore or magnetite; and carbonate ore or siderite. These types of ore may easily be remembered through their most common colors, which are, respectively, red, brown, black, and gray. Other iron minerals, pyrite and pyrrhotite, sulfides, are indirectly the sources of iron ore. These are first used for the extraction of sulfur for the manufacture of sulfuric acid and the residue, mainly iron oxide, then becomes available. Certain iron silicates, such as thuriugite, chamosite, and glauconite, have been used as commercial sources of iron in Europe but not in the United States. Hematite.-Hematite is by far the most important ore of iron, constituting about 90 per cent. of total iron ore mined in the United States. It includes all varieties of the anhydrous sesquioxide or ferric oxide (Fe20a). This is known locally as red hematite, specular ore, gray ore, fossil ore, oijlitic ore, etc. Hematite is composed of two parts of the metal iron united with three parts of oxygen. Iron forms 70 per cent. by weight of pure hematite and oxygen 30 per cent., and the mineral should yield 1568 pounds of metallic iron -per pross ton of theoretically pure ore. Hematite may occur as a rock in beds, in veins, or in masses of irregular shape, and it may occur in a soft, earthy state or as loose debris in the soil, and yet in all these forms be profitably mined if the controlling conditions are favorable. Hematite is easily recognized in the hand specimen by its colors (red to steel-gray and black), its red streak, and by its high specific gravity. The old-fashioned "keel" or red chalkstone formerly used by carpenters for marking lumber is an impure hematite. Finely ground hema-
tite is used in the manufacture of red paint and other red coloring materials. Brown Ore.-Brown ore is composed of iron, oxygen, and water and might be said to be hematite combined with water. A general formula may be expressed by 2Fez03.nHz0, in a series of minerals which differ from each other in the proportion of water chemically combined with iron oxide. Brown ore may contain water in several proportions so that the percentage of irou likewise varies, but the latter ranges generally from 5% to 66 per cent. in the pure mineral, as indicated in Table I. Thus brown ore cannot yield so much metal as hematite. One of the common hydrates is g6thite which occurs as a crystalline mineral and may be the only definite compound of the series.4 Its formula may be expressed as 2Fe2O3.2H20,and it contains 62.9 per cent. of iron, 27 per cent. oxygen, and 10.1 per cent. water. Brown ore deposits often approximate the composition of limonite (2Fe2O3.3Hz0),which contains 59.9 per cent. of iron, 25.7 per cent. of oxygen, and 14.4 per cent. of water, and should yield 1342 pounds of irou per gross ton of theoretically pure ore. Brown ore occurs in a large variety of forms, nearly all of which have in common a characteristically yellowish brown to chocolate-brown color and yield a brown streak and powder. Exceptionally, certain hydrated sesquioxides of iron may appear bright red, and some may show black surfaces; but these exceptional forms are generally to be found closely associated with a mass of ordinary brown ore, and familiarity with the deposits soon dispels any perplexity regarding the nature of the ore. Brown ore commonly occurs in irregular masses and lumps, and as ore gravel and sand, embedded in banks of clay, sand, and stone gravel. In places the ore takes POSNJAK, E. am, MERWIN, H. E., Am. I. Sci. (41, 47, 311 (1919). -
A HOLLOW CONCRETION OR "POT"OF BROWN ORE BIRMINGHAM DISTRICT
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the form of hollow concretions or "pots" which inclose sand or clay, and also it appears as honeycombed masses ramifying through the inclosing ground. In some deposits brown ore has replaced beds of limestone or has formed by oxidation of iron sulfides. Brown ore is used chiefly as a source of iron, and constitutes about 5 per cent. of the total iron output of the United States. The soft yellow, less pure form of brown ore, called ocher, is used for paint and for making oilcloth and linoleum. A porous variety is used in the purification of illuminating gas, and also as a contact material in the production of hydrogen gas for inflation of dirigible balloons and for precipitation of fats from oils. Many iron compounds are used in medicines. Magnetite.-Magnetite (FeaO,) is a black ore of iron which is attracted by the magnet. I t contains iron and oxygen in the proportion of 3 parts of iron to 4 parts of oxygen, which by weight gives 72.4 per cent. of iron and 27.6 per cent. of oxygen, and should yield 1622 pounds of iron per gross ton of theoretically pure ore. Magnetite contains the highest percentage of iron of all the iron ores. I t usually occurs in masses or in granules and crystals in veins in comparatively old crystalline rocks, although it may be found in comparatively young volcanic rock. Magnetite is also found in the form of sand along streams and on the shores of lakes or seas where it has been concentrated by the action of water, but in this form it has not proved of much commercial value as an ore of iron. Magnetite is used chiefly for the production of iron, but the nncommon variety known as lodestone,* which is naturally magnetic, is more valuable for museum exhibits and for laboratory purposes. Magnetite in places contains titanium in such percentages as to render the ore of no metallurgical value. Such ore containing one or more per cent. of titanium is known as titaniferous magnetite. The deleterious effect of the titanium is to render the ore difficult to fuse. The titanium is present chiefly as the mineral ilmenite either fairly distinctly crystallized or as intergrowths, and rutile may also be present in intergrown masses. Where distinctly crystallized the iron and titanium oxides are easily separated by magnetic means but where microscopically intergrown this is impossible. Siderite.Siderite, or iron carbonate (FeC03), is composed of 1part of iron, 1part of carbon, and 3 parts of oxygen and thus contains by weight, where pure, approximately 48.3 per cent. of iron, 10.3 per cent. of carbon, and 41.4 per cent. of oxygen, and should yield 1082 pounds of iron per gross ton of theoretically pure ore. Iron carbonate, where fresh, is generally grayish in color, and occurs in nodules and in layers principally in sedimentary rocks that contain coal and lignite. The carbonate is generally unstable and rapidly changes to hydrated iron oxide on exposure to moist air. Siderite is known locally as spathic iron ore, kidney ore, clay ironstone, and blackband ore.
Certain of these names are derived from the appearance of the ore in its different forms. For instance, a bed of iron carbonate ore that is found on the top of a coal bed in eastern Kentucky is striped with fine black carbonaceous bands. Iron Sulfides.-Pyrite, or iron disuliide, FeS2, contains 46.7 per cent. of iron and 53.3 per cent. of sulfur, while the monosulfide, pyrrhotite, FeS, contains 60.5 per cent. of iron and 39.5 per cent. of sulfur. These ores are not used directly as a source of iron but, after being roasted for the recovery of their sulfur content, the residue, impure oxide of iron, is available for the extraction of iron. Iron Silicates.-Only three of the many hydrous iron silicate minerals have been regarded as possible sources of iron, thuringite, chamosite, and glauconite. These minerals all contain aluminum and the latter contains potassium. According to Dana, the apparent iron content of thuringite is 36 per cent.; of chamosite it is from 31 to 47 per cent.; and of glauconite it is between 16 and 20 per cent. The first two minerals have been used for the extraction of iron in Germany and AnstroHungary and pig iron has been made experimentally in the United States by E. C. Eckel from glauconite as a by-product after recovery of potash.
* Lodestone occurs a t Magnet Cove, Arkansas, and has been reported from Shasta County, California, and near El Paso.
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COMPOSITIONS OF IRON ORES
In the following table by Ecke16 the theoretical formulas and compositions of the principal iron ore minerals have been summarized. The table also shows the series of hydrated oxides of iron with their inneasing proportions of water of crystallization. N*M~
TABLE 1 C~nsrrcm FORMULA
CDUPOS~ON
Carbon Metallic Suljw Dioxide Oxydrz Watn
Magnetite Hematite Tugte G0thite Limonite Xanthosidetite Limn& Siderite Pyrrhotite pyrite
Imfiurities in Iron Ore.-Consideration should be given to some of the principal impurities that accompany iron ores in order to appreciate, first, why a deposit of ore never yields the full percentage of iron that the predominant iron mineral contains, and, second, the relation of the ore to the operation of the blast furnace. Deposits of iron ore are generally so closely associated with the rocks which inclose them that they contain either mixed with the deposit or chemically combined within it some of the other rockforming minerals. Among the most common of these minerals are silica (or sand), lime, magnesia, alumina, and other clay minerals, manganese oxides, and small quantities of sulfur and phosphorus. The presence of varying proportions of these and other minerals 6 E ~E. ~C., ~"Iron ~ ores," , McGraw-Hill Book Co., New York City, 1914. p. 27.
tends, of course, to reduce the percentage of metallic iron that the deposit will yield, and they must be removed as far as practicable in mining and as completely as possible in the process of making iron. Therefore, it is of the utmost importance that the average chemical composition of a deposit of iron ore be ascertained before an attempt is made to mine it. The average composition may be termed the "quality" of the deposit. The quality is determined chiefly by the prospector and the chemist. The prospector digs trenches and pits or drills holes in the ore and takes samples of it. The chemist analyzes these samples and finds the percentages of metallic iron, silica, lime, alumina, manganese, sulfur, phosphorus, water, etc., contained in average samples of the ore. On the quality of the ore depend the quantities of fluxingstone and fuel that must be used in the hlast furnace and the kind of iron that can be produced.
hematites contain considerable quantities of lime and are therefore workable where a highly siliceous ore carrying no more metallic iron and little lime would not be available under present conditions of metallurgic practice. At present ores averaging less than 25 per cent. of metallic iron are not charged into domestic hlast furnaces in any considerable quantities, and ores as lean as this cannot be used economically in the United States unless they carry more than enough lime to flux them and are used in connection with richer ore. In using limestone as a flux a stone casrying 15 to 20 per cent. of femc oxide (10.5 to 14 per cent. metallic iron) might be more desirable than one containing only 3 to 5 per cent. ferric oxide, on account of the higher iron yield of the former. Nevertheless, the more ferrnginous limestone would not commercially be classed as an ore; yet a bed carrying 35.5 per cent. ferric oxide (25 per cent. metallic iron) and 16 per
Range in Composition.-Many chemical analyses of commercial grades of these iron ores indicate a range in composition given in the following table:
cent. lime, CaO, although in itself not rich in iron, might be conceded to be an "ore." There are other exceptions to be noted in this table. Some deposits of non-titaniferous magnetite can be so treated by means of crushing and electromagnetic separation that, although they may contain as little as 25 per cent. metallic iron, they will yield a high-grade concentrate. Such ores may be used to advantage, especially where they can be concentrated along with richer grades. Considerable phosphorus is present in the mineral apatite associated with Adirondack magnetite, and this and silica are largely eliminated by electromagnetic concentration. The possibilities of various methods of concentration or beneficiation must, therefore, be taken into consideration in appraising an ore, and this involves consideration of the physical as well as the chemical character of the ore. The content of water is a widely variable item. Some ores contain much absorbed water, which can be driven off by drying. Hematite is found in all stages of hydration, and thus may contain water up to the point where it should
~ N O IN B
TABLE 2 C I B ~ C AC L O U P O S ~ OOFN COYMBBCIAL GEADBS0s IRON OIB B&h Bmvn O m Mognclilc Carbonale per cent.
Mctnliie iron Silica Alumina Lime Mang~nese Sulfvt Phosphorus Titanium Water
per cent.
per cent.
Per cent.
25-85 35-58 4970 2649 2-35 3-23 1-25 0.6-20 0.2-9 1.4-12 0.74.5 1-5 Traee to 27 0-0.6 0.64.5 0-7 Trace to 5 0.16-5 0.07-1.5 0.14.7 Trace to 1.5 Trace to 0.5 T-e to 1.2 0.01-1.8 0.013-1.23 O.W1 Trace to 1.76 0.042-0.7 W.9 Trace lt1.5 0-0.04 0.3-6 4.615 M.5 0.260.5
Under exceptional conditions ores are mined that contain more or less of the common constituents indicated above, but local conditions may determine whether an ore that falls within the limits shown may be mined. For instance, hematite carrying as little as 25 per cent. metallic iron would not be considered of value as an ore unless i t contained enough lime to flux the silica and alumina present. Most southern red
notes on distribution of iron ore deposits brief mention of such classifications may be justified here. Many genetic classifications of ore deposits may be found in works on economic geology, most of them long and involving too many hair-splitting distinctions to be practically applied to iron ore alone so it seems desirable to bear in mind the following statement by Eckel! There are many ways in which iron ore deposits can originate but only a few of these possible modes of origin have given rise to deposits of serious commercial importance. Practically all the known iron ore supply of the world is, and will be, derived from (a) sedimentary basin deposits, from ( b ) replacement deposits, or (c) residual deposits. Of these, the sedimentary ores are of by far the greatest importance.
In addition to the three general classes mentioned under (a), (b), and ( 6 ) there is a fourth class (d) comprising igneous deposits. Each of these classes is. susceptible of being considerably subdivided. Lindgren7 discusses iron ores under detrital, sedireally be classed as a brown ore. Hydrated ores are mentary, residual, pyritic replacement of lower vein concentrated by heating to drive off the chemically zones, contact-metamorphic, igneous, and metamorcombined water. Carbonate ores are calcined to drive off both carbon dioxide and water. Some iron oxides phosed classes of deposits. The above classifications have no reference to the are subjected to a partial reduction and are thus renage of the deposits, and since it is often necessary to dered magnetic, and the separation of non-metallic refer to certain iron ore deposits in terms of their geoimpurities is thereby greatly facilitated. One brown ore deposit that averages in the raw state not more than logic age, it may be well to recall to the reader that 30 per cent. of metallic iron is thus, through a compli- rocks are differentiated in ascending rank into formacated process of bendciation, rendered of commercial tions, groups, series, and systems. The terms presented value. The question of benefiaation is therefore be- in Table 3 are at present recognized, the youngest at coming increasingly important as the higher grades of the top of the column: ore are becoming depleted; and there must also be TABLE 3 G~OLOCIC EPAS.SYSTBMB, AND SBBI~S taken into consideration the proximity to transportaEm System (aPeriod) Snim (or E m k ) tion, to blast furnaces, to water-power sites for possible electric reduction, as well as the prevailing conditions Pliocene of the market. The laws of supply and demand that Cenozoic..... . Tatiary .... . . . . . prevail at certain periods make available lower grades Oligocene Eocene of ore than would be considered desirable in times when Cretaceous... . . .. lupper and Lower Crefaeeau. low prices for iron and steel are current. The question Triarnie........ . . (Upper, Middle, and Lower T.iassie of quantity available is also of importance, for iron ore permian is so bulky a product, is relatively so low in value even Mississippian (.'Sub-CarboniferouoI') at the best, and involves so large an expenditure for ,,,,.. Devonian ........ (Upper, Middle, nod Lower Devonian equipment for mining on a satisfactory scale and ior Ordovician...... . [Upper, Middle, md Lower Ordovician transportation that any deposit should contain several Cambrian.. . . .. . . (Upper, Middle, and Lower Cambrian years' supply to be worthy of development. m t m z o k . . .. ( ;?"' To summarize, then, the ore should occur in a deposit containing several hundred thousand tons of merchant- GEOGRAPHIC DISTRIBUTION OF IRON ORE DEPOSITS able grade, susceptible of economical mining and conThe major features of the distribution of iron ore venient to transportation and markets, if its develop- deposits in the United States are now fairly well known, ment is to be seriously considered. as most of the ore-bearing districts that are advantageously situated with respect to transportation facilities, CLASSIPICATIONS OF IRON ORE DEPOSITS fuel supplies, manufacturing centers, and markets have For purposes of description iron ores may be sub- been studied in more or less detail by the United States jected to various classifications aside from the simple Geological Survey or by state organizations, including mineralogical divisions indicated above. For instance, tax commissions, and a number, of publications and a genetic classification, one in which the mode of origin maw have been issued on the subject.* of the ore is the underlying feature, may be of much ECKEL. E. C., loc. cit., p. 40. interest to the mining geologist, but of little or no value ' LINDGREN,W A L D E ~ R"Mine, , nl deposits," 3rd ed., Mcto persons who have to buy, sell, or use the ore. Be- Graw-Hill Book Co.. New York Citv-,,1928. * A bibliography bf publications on iron ores by the Federal cause certain of the terms applied in a genetic classi- and State Surveys will be furnished upon applicatim to the fication of iron ore deposits will be used in subsequent United States Geological Survey. Wahington, D. C.
Fifteen to thirty states are producers of iron ore, and several others contain deposits of value. The states in which commercially important deposits of iron ore occur may be grouped into six geographic divisions under which the principal deposits will be noted. 1. Northeastern States: Massachusetts, Connecticut, New York, 'New Jersey, and Pennsylvania. Magnetite, hematite, and brown ore. 2. Southeastern States: Maryland, Virginia, West Virginia, Tennessee, North Carolina, Georgia, and Alabama. Hematite, brown ore, and magnetite. 3. Lake Superior States: Michigan, Wisconsin, and Minnesota. Hematite, brown ore, and magnetite. 4 Mississippi Valley and GulfStates: Ohio, western West Virginia, Kentucky, western Tennessee River Valley, Iowa, Missouri, Arkansas, Mississippi, South Dakota, and Texas. Brown ore, hematite, magnetite, and siderite. 5. Rocky Mountain States: Idaho, Montana, Wyoming, Colorado, New Mexico, Utah, and Nevada. Hematite, magnetite, and brown ore. 6. Paci$c States: Washington, Oregon, and California. Magnetite, brown ore, and hematite. Northeastern States.-A map of the distribution of iron ore deposits in the United States shows, as the most easterly, a small area of magnetite at Iron Hill, Rhode
Island. Unfortunately this ore is low in iron and contains titanium in too large a quantity and too intimately intergrown with the magnetite to render it available for making iron8 under present conditions. A belt of disconnected iron ore areas extending through western Connecticut, Massachusetts, and Vermont, and a short distance into eastern New York supplied the ore for the earliest blast furnaces in the United States. The first successful blast furnace is reported to have been built at Lynn, Massachusetts, in 1645, followed in 1648 by an iron forge at thesame place. This marked the beginning of the manufacture of iron in the New England colonies. The first iron article made from native ore in America, cast at Lynn, Massachusetts, in 1645, and still preserved, is an iron pot havinga capacity of nearly one quart. The first discovery of iron ore in the Atlantic colonies was, however, in North Carolina in 1585, and probably the first attempt to make iron in the colonies was undertaken on Falling Creek, seven miles below Richmond, Virginia, in 1619, but in 1622, before the process was perfected, the iron master and his operatives were slain and the works destroyed in an Indian mas~acre.~
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6 SINGEWALD, J. T., JR., "The titaniferous iron ores in the United States." U.S. Bureeu of Mines Bull. No. 64, p. 46 (1913). S,WAWK,JAMES M., "History of the manufacture of iron in all ages, Am. Iron and Steel Assoc., Philadelphia, Pa., 1892, pp. 104-8.
The brown iron ore deposits first utilized in New England were formed in bogs and often found below water in shallow ponds and lakes near the ocean. When the bog deposits were exhausted ores associated with hard rocks were used. They were largely formed by replacement by iron carbonate of shale or limestone beds and later oxidized to brown ore, or else they may have been deposited directly as limonite in basins or in cavities in limest~ne.'~ These deposits are indicated on the map, p. 201. New York State has, in addition to the brown ore area along the Connecticut border, four distinct areas of iron ore: magnetite in the Adirondacks and in the southeastern highlands and hematite in the northwestern and west-central part. The Adirondack magnetite area is the most important, but in a few places contains titaniferous ore. Magnetite mines near Lake Cbamplaiu have yielded ore for many years. The magnetite occurs in lenticular masses associated with gneisses and schists of pre-Cambrian age. The ore is usually mixed with crystalline gangue rock from which it must be separated by crushing, sizing, and magnetic concentration. The hematite of west-central New York occurs as a thin, even bed, oolitic in places, cropping out in a nearly east-west direction for about 225 miles and dipping about 50 feet to the mile toward the south. This bed is of interest geologically because of the widespread distribution in various parts of the country of a similar ore of nearly equivalent age. It is known as the Clinton ore, of Silurian age, and may be traced southwestward through the Appalachian States to central Alabama. In New York this red ore is mined at present only for making paint. The magnetite belt that crosses the highlands of southeastern New York and northern New Jersey extends into southeastern Pennsylvania. A few magnetite mines are active in New Jersey and the Pennsylvania production is mainly from deposits near Corn-
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'"CKEL, E. C.. "Limonite deposits of eastern New York and western New England," U. S. Geol. Survey Bull. No. 260, pp. 335-42 (1905).
wall. The iron content of this ore is low, 35 to 40 per cent., hut it can be beneficiated cheaply and yields a small percentage of copper besides. The deposits of magnetite are associated with intrusions and flows of igneous rock, diabase, and basalt, which have invaded Mesozoic sandstones and Paleozoic limestones. A few brown ore areas also are scattered along nearly parallel to the magnetite belt in this region. Thin beds of Silurian hematite crop out in narrow strips in central Pennsylvania. In southwestern Pennsylvania siderite beds, altering to limonite on the outcrop, are present in large areas of Coal Measures rocks. In the early days of the iron industry in Pennsylvania local ores of all four varieties formed a large part of the blast furnace supply. With the opening of the Lake Superior iron ore fields, the iron and steel industry in western Pennsylvania expanded so rapidly that the smaller deposits of local ore could no longer be operated on a large enough scale to make their operations profitable and all but a few mines were abandoned. Ores imported from foreign countries also began to supplement the domestic supply to the seaboard furnaces. Southeastern States.-The Appalachian region, southwestward from Pennsylvania, lies chiefly within Maryland, West Virginia, Virginia, North Carolina, Tennessee, Georgia, and Alabama. The proximity in this region of deposits of iron ore, coal, limestone, and dolomite, all of which are essential in making iron, had, of course, a large influence in the early development and continuation of the iron industry here. Moreover, this region was one of the earliest in the United States to be settled and together with the other Atlantic Coast States farther north constitutes a large iron-consuming area. The South contains some of the largest as well as some of the smallest units of the iron and steel industry. Districts in which iron mining and manufacturing has developed extensively are in the Valley of Virginia, and near Chattanooga, Tennessee, and Birmingham, Alabama, but in the early days there were hundreds of small forges and furnaces scattered throughout the valleys. At times foundry and basic pig iron
OPEN-CUT
Bnom One MINE W O ~ I N ON G T W O LEVELS,
B~NGRAM DIsrnrcr
EXPLANATION
Outcrop of red iron ore bearing formetion
Area of probably workable red iron ore
Limestone and dolomite
El
Coal fields
Are-
worked forbmwn ore
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.
8Lsl furnace Stebl plant %-
Mine
have been made in this region at costs lower than elsewhere in the United States. The three principal types of ores of iron-hematite, brown ore, and magnetiteare all found in abundance in certain southern ore fields, and the residue from the extraction of sulfur from iron and copper pyrites in southern Virginia and eastern Tennessee is marketed as a low-phosphorus iron oxide. The most valuable deposits of hematite in the South are the beds of Clinton or nearly equivalent Silurian age that crop out intermptedly in west-central Virginia and almost continuously from Big Stone Gap southwestward along the base of the Cumberland escarpment through eastern Tennessee and northwestern Georgia and terminate in the great deposits of the Birmingham district in Alabama. Deposits of brown ore are common in Virginia, Tennessee, Georgia, and Alabama, particularly in valleys underlain by limestone and in the clays and
sands of the Coastal Plain. Magnetite occurs in commercial quantities in the Appalachian region of Virginia, North Carolina, and eastern Tennessee and in the Piedmont region of Virginia and North Carolina. Maryland has long since ceased production of iron ore. Scattered deposits of brown ore occur in the mountain valleys and hematite beds of low grade occur in the Silurian rocks in northern Maryland. Small deposits of siderite partly altered to limonite are situated between Washington and Baltimore, and at one time supplied a small local blast furnace. The large iron and steel furnaces at Sparrows Point, near Baltimore, now depend principally upon ores from Cuba, Chile, Morocco, Russia, and Australia. In eastern West Virginia the Appalachian belt of brown ores is represented by scattered deposits and the Silurian bedded hematite is present in a few unimportant outcrops. This part of West Virginia is
the South, is second in rank in ore production in the United States; it compares favorably as a producer of iron and steel with Pittsburgh, Youngstown, and Chicago; and may prove to he the longest lived ironore mining district in the corn*. Its industrial history is, however, one of the shortest, as its beginnings date hack only to the late sixties, or since the Civil War. Noteworthy steps in the development of this district were the successful manufacture of pig iron with coke for fuel in 1876, the opening of mines of coking coal in 1879, the making of open-hearth steel in 1899, and the present-day manufacture of many iron and steel The Birmingham district is an elliptical area about 75 miles long by about 40 miles wide. It includes extensive deposits of red hematite, large though less extensive deposits of brown ore, and enormous areas of coking coal and fluxing dolomite and limestone. The city and its suburbs are in the valley between two coal fields. Red Mountain, on the southeast, contains the not a factor in the iron industry at present except as beds of hematite, which dip into the mountain at angles coking coal may he supplied to furnaces in other states. of 15' to 45' and gradually become flatter as they exBrown ore, which comprises the well-known Devo- tend southeastward. The ore beds are of sedimentary nian, Oriskany, replacement type, as well as the "valley" origin and have not been altered essentially since their and "mountain" ore, is the most abundant iron ore in deposition. The workable ore ranges from 7 to 20 Virginia, but there are also deposits of hematite and feet in thickness and its outcrop extends for about 20 magnetite. The brown ore and hematite occur in the miles. Drilling has indicated the presence of ore of Appalachian region; the magnetite is found on the commercial grade at depths not exceeding 4000 feet Piedmont Plateau. The deposits of iron ore in Vir- underlying probably more than 50,000 acres-a quanginia are so discontinuous and scattered that no great tity of 1'/8 to 2 billion gross tons. The slopemines have concentration of the iron industry has developed, al- penetrated to distances of nearly a mile and a half from though it has had a long and interesting life, dating the outcrop and to vertical depths of 1500 to 2000 feet. from colonial days. Most of the ore was used locally, The brown iron ores occur in fine to coarse grains but some from the southern counties went to Tennessee and lumps in irregular masses, inclosed in clay, sand, furnaces. Little or none is now being mined. and gravel of Cretaceous and Tertiary age that overlie In the Appalachian region of western North Carolina Cambrian and Ordovician limestones and dolomites. two types of iron ore have been mined-magnetite and Large deposits occur in Birmingham Valley, and in brown ore. The largest deposits of magnetite are near the vicinity of Russellville, in northwest Alabama. Cranberry where ore has been' mined at intervals since Some deposits are 100 feet or more in thickness and 1820. This ore is notable because of its extremely low underlie many acres. The brown ore must be washed, content of phosphorus and sulfur, and low-phosphorus picked, and jigged to free it of clay, gravel, and sand pig iron has long been made from it. The brown ores The brown ore is mined from open pits, and the deoccur chiefly in the valleys of Cherokee and Madison posits are gradually becoming depleted as Alabama has counties. They are of relatively high grade and have long held first rank in the production of this type of been shipped to blast furnaces in Tennessee. ore. Chattanooga, on the Tennessee River, occupies an The Birmingham district ranks second to the Lake advantageons position for iron and steel manufa&ning. Superior district as a producer of iron ore. During The bedded red hematite tributary to its blast furnaces the World War the output of iron ore increased apunderlies parts of the great plateau areas of eastern preciably but not in as large a proportion as in the Lake Tennessee, northeastern Alabama, and northwestern Superior district, because of the physical limitations Georgia, and crops out in the bordering valleys. The inherent in underground mining of red ore and washing ore outcrops in the three states aggregate more than of hrown ore. I n 1925 the output constituted more 200 linear miles. Most of the outcrop has been worked than 10 per cent. of the total for the United States and for its soft, or leached, ore and much will be worked exceeded that of any of the Lake Superior iron ranges eventually for the hard, or calcareous, ore although only except the Mesabi. The range in production is bethe thicker deposits have thus far been mined. Coking tween 3 million and 6 million tons of ore annually, and in coal is mined within this district, and at Rockwood, 1931 the Red Mountain group of mines at Birmingham Tennessee, the limestone, iron ore, and coal are situated was the largest single producer of hematite in the United close together. States. Birmingham, Alabama, the greatest iron center in (Part N wiU appear in the May issue.)
