IRON 



299 



Many iron-formations have undergone conditions 

 of higher temperature and pressure (metamorph- 

 ism) which have changed preexisting minerals to 

 silicates such as grunerite, pyroxene, and olivine 

 and which have resulted in coarsening of grain size 

 of quartz, magnetite, and hematite. Oxide facies of 

 iron-formation that has been so metamorphosed is 

 the most suitable as a low-grade ore (taconite) be- 

 cause the iron oxides are more readily concentrated 

 by milling. In contrast, the more weakly metamor- 

 phosed carbonate and silicate facies, generally not 

 usable at the present time in the United States as 

 low-grade ore, are more susceptible to alteration by 

 surface and ground waters and commonly give rise 

 to ore bodies of soft limonite and hematite (direct- 

 shipping ores) and to deposits classed in the Lake 

 Superior region as "wash ores" or "semi-taconite" 

 (Button, 1955; United Nations, 1970). 



Chemically, the iron-formations are marked by 

 extraordinarily low contents of alumina, sodium, 

 potassium, and minor elements. Iron and silica gen- 

 erally dominate, but in some districts, notably the 

 Cuyuna district of Minnesota, the manganese con- 

 tent is several percent (Button, 1955). 



The banded iron-formations commonly are ex- 

 posed as steeply to moderately dipping beds in belts 

 a few miles to several tens of miles long (Gross, 

 1965). A few, such as those in Labrador, are in 

 belts several hundreds of miles long. Thicknesses 

 commonly are on the order of a few hundred feet 

 but range from less than 25 to more than 2,000 feet. 

 With some prominent exceptions, notably the Me- 

 sabi Range of Minnesota and the Hamersley Range 

 of Australia, the iron-formations are tightly, and 

 often very complexly, folded. Bips generally are 60° 

 or more, a fact of considerable importance to eco- 

 nomic assessment. Aggregate tonnages of rock con- 

 taining 20 percent or more iron are enormous, but 

 factors such as location, mineralogy, and structure 

 are of profound importance in assessment of actual 

 potential. 



Prominent examples of this class of iron deposits 

 are the iron ranges of the Lake Superior region of 

 the United States — Mesabi, Cuyuna, Gogebic, Mar- 

 quette, and Menominee; the Michipicoten, Lake 

 Albanel-Mistassini, and Labrador districts of 

 Canada; the Quadrilatero Ferrifero of Brazil; the 

 Krivoi Rog and Kursk districts of Russia; and the 

 Hamersley Range of Australia (James, 1966; United 

 Nations, 1970). 



IRONSTONES (I-B) 



The strata classed as ironstones contain iron in 

 grades comparable to or greater than that of Pre- 



cambrian banded iron-formations — ^that is, in the 

 general range of 20^0 percent — but they differ 

 profoundly in many ways, notably in much younger 

 age, in the absence of interlayered chert, and in the 

 much smaller dimensions (James, 1966). 



Considerable variety exists within the class, but 

 the most common type is thick-bedded rock con- 

 sisting of small pellets (ooliths) of limonite, hema- 

 tite, or chamosite in a matrix of chamosite, siderite, 

 or calcite. Grains of clastic quartz and fossil frag- 

 ments form the cores of many ooliths and are dis- 

 persed in the matrix, and the acceptability as an 

 ore depends in part on the relative amounts of 

 quartz and calcite. Rock containing abundant calcite 

 may be self-fluxing, whereas content of abundant 

 quartz is deleterious. Phosphate minerals are also 

 common contaminants in ironstones. As with banded 

 iron-formations, ironstones may be divided into ox- 

 ide, carbonate, silicate, and sulfide facies, depending 

 upon the dominant iron mineral. The rocks originate 

 as chemical sediments, but the oolite types — eco- 

 nomically the most important — have been pro- 

 foundly modified by wave action and transport so 

 that chemically heterogeneous assemblages of 

 limonite (or hematite), chamosite, and siderite are 

 common. 



Thicknesses of ironstone formations range from 

 at least a foot to several tens of feet. Beds tend to 

 be lenticular on a scale of a few thousand feet or a 

 few miles, but a belt of deposits having approxi- 

 mately the same age may continue for hundreds of 

 miles, as for example those in the Clinton Forma- 

 tion of Silurian age that extend from Alabama to 

 New York. In general, thicknesses and lateral ex- 

 tents of ironstones are an order of magnitude less 

 than those of banded iron-formations, consequently 

 potential tonnages probably are less than 1 percent 

 of those contained in the older rocks. 



Because of the prime importance of location, ease 

 of beneficiation, and local near-surface enrichment, 

 many ironstones have been and are being mined. 

 The relative importance of these ores in world pro- 

 duction, however, is decreasing. The most important 

 region of economic extraction was in northern 

 Europe — England, France, Luxembourg, and Ger- 

 many — where the ironstones of Jurassic age have 

 been mined for generations. In North America, the 

 most important examples have been the Clinton ores 

 of Silurian age in the Birmingham district of Ala- 

 bama and the Wabana ores of Ordovician age in 

 Newfoundland (United Nations, 1970). 



MISCELLANEOUS SEDIMENTARY DEPOSITS (l-C) 



Iron has been extracted in the past from a variety 



