302 



UNITED STATES MINERAL RESOURCES 



hydrothermal replacement of silica layers along fa- 

 vorable structures in the enclosing iron-formation, 

 but the source of the solution is unknown. Deposits 

 of similar character occur in Liberia, Mauretania, 

 India, Australia, and in the now dormant Vermilion 

 district of Minnesota. 



DEPOSITS PRODUCED BY SURFACE OR NEAR-SURFACE 

 ENRICHMENT (IV) 



Many iron deposits owe their origin to surface or 

 near-surface enrichment of rocks of lesser iron con- 

 tent. Preexisting iron-bearing minerals are attacked 

 by surface and ground waters, and the iron is 

 altered to limonite and hematite, often with the loss 

 of other elements in solution. In localities where 

 surface erosion is minor, or where the oxidation 

 occurs below the surface, the result is a progressive 

 concentration of iron oxides. Many of these residual 

 deposits have been mined in the past, and some 

 that are of major economic or potentially economic 

 importance remain. 



Two general groupings are convenient: Laterites, 

 which are characteristic products of weathering in 

 humid tropical climate, and residual enrichments of 

 lower-grade iron deposits. 



LATERITES (iV-A) 



The process of lateritization is one of deep chemi- 

 cal weathering under tropical conditions with alter- 

 nating wet and dry seasons. The nature of the prod- 

 uct varies with that of the underlying rock ; bauxite, 

 for example, is produced by lateritization of rock 

 high in alumina relative to quartz and iron min- 

 erals. The iron-rich laterites most commonly are 

 derived from serpentine, as in Cuba and the Philip- 

 pine Islands (Button, 1955; United Nations, 1970). 

 The material is porous and brown, and it may accu- 

 mulate to depths of many tens of feet. Surface areas 

 are measured in square miles, and so the potential 

 tonnage is very large. The dominant ore mineral is 

 limonite (impure goethite), and the iron content of 

 the rock commonly is 40-50 percent. Nickel, cobalt, 

 and chromium also are concentrated in laterite. The 

 major impurity is clay which, along with chromium, 

 cobalt, and nickel, inhibits the greater use of laterite 

 as an iron ore. Many iron-rich laterites now are 

 being mined for nickel alone, but future improve- 

 ment of metallurgical techniques may result in 

 greater use of laterite as an iron ore, with nickel 

 and cobalt, and possibly chromium, as coproducts. 



ENRICHMENTS OF PREEXISTING LOW-GRADE ORES (IV-B) 



Most of the iron ore used in the world to date has 

 been produced from deposits arising from the near- 



surface enrichment of lower grade material. Before 

 the development of a method for processing taco- 

 nites, virtually all the production from districts 

 such as the Mesabi in the United States and Krivoi 

 Rog in Russia was of soft limonite and hematite. 

 These direct-shipping ores, with iron contents of 

 50-60 percent, and the so-called wash ores and 

 semitaconite of the Lake Superior region are prod- 

 ucts of deep residual enrichment of primary iron- 

 formation, in which oxidation of ferrous minerals 

 was accompanied by partial to complete leaching 

 and replacement of chert (James, 1966). The en- 

 richment extends to considerable depths in many 

 parts of the world — commonly to many hundreds of 

 feet and in some districts to as much as 5,000 feet. 

 The deposits have yielded immense amounts of ore; 

 the Lake Superior districts alone have produced 

 more than 3 billion tons (Aim and Trethewey, 1971 ; 

 see also data in U.S. Bur. Mines Minerals Year- 

 books 1932-1970 and U.S. Geol. Survey and U.S. 

 Bur. Mines Mineral Resources of the United States 

 1882-1931). 



Many other examples exist of ores that are prod- 

 ucts of enrichment. The famous Bilbao deposits of 

 Spain consist of massive limonite and hematite de- 

 rived by enrichment of preexisting siderite bodies 

 (United Nations, 1970). The brown ores of Texas 

 and the southeastern United States were formed by 

 oxidation and enrichment of Tertiary strata con- 

 taining siderite and glauconite. The hard ores of 

 the Marquette district of Michigan, which occur at 

 the top of the Negaunee Iron-formation, probably 

 represent in part a former enrichment and in part 

 a clastic accumulation, now profoundly modified by 

 metamorphism. 



SUMMARY OF TYPES OF DEPOSITS 



Table 58 presents a general summary description 

 of types of iron-ore deposits. Characteristics such 

 as iron content and form vary widely in each type, 

 but the descriptions given apply reasonably well to 

 the major deposits. 



RESOURCES 



IDENTIFIED RESERVES AND RESOURCES 



Vast quantities of iron-ore resources are known 

 to exist throughout the world. A recent survey of 

 world iron-ore resources by the United Nations 

 (1970) indicated world reserves totaling approxi- 

 mately 250,000 million metric tons and an addi- 

 tional 530,000 million tons of potential iron-ore re- 

 sources. The estimated iron-ore resources of various 

 parts of the world, slightly modified from the 

 amounts listed above, are shown in table 59. 



