IRON 



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as magnetite, which is associated with sulfides, 

 among which pyrite and chalcopyrite are dominant. 

 Actinolite and chlorite are the chief gangue min- 

 erals. Copper, silver, gold, and cobalt are recovered 

 as byproducts (United Nations, 1970). Elsewhere in 

 Pennsylvania, similar ore deposits are found in Pre- 

 cambrian marble and in Triassic limestone conglom- 

 erate in each case in close proximity to a sheet of 

 diabase. The relationship between diabase, lime- 

 stone, and ore is incontrovertible, but the precise 

 origin of the ore-forming fluids is less evident. 



Pyrometasomatic ore bodies related to quartz 

 monzonite are fairly common in the Rocky Moun- 

 tain region of the United States, and some are of 

 large dimensions (Carr and others, 1967; United 

 Nations, 1970). The mined deposits of the Iron 

 Springs district, Utah, occur at or near the contacts 

 of laccolithic bodies of quartz monzonite of Tertiary 

 age with limestone of Jurassic age. Dimensions are 

 on the order of a thousand feet in strike and dip 

 length, with thicknesses of as much as 230 feet. The 

 iron content is about 50 percent, and the iron is in 

 the form of magnetite. Gangue minerals include 

 phlogopite and fine-grained calc-silicates, and sig- 

 nificant amounts of apatite; phosphorus content is 

 as much as 5 percent in places and averages 0.2 

 percent. The ore was deposited by fluids emanating 

 from the intrusive quartz monzonite. 



DEPOSITS FORMED BY HYDROTHERMAL SOLUTIONS (III) 



Many iron deposits are hydrothermal in origin — 

 that is, they were deposited by heated aqueous fluids. 

 These fluids may have originated from relatively 

 distant bodies of crystallizing magma, or they may 

 simply represent waters of any origin that have 

 circulated at depth to acquire the necessary heat. 

 Similarly, the iron may be of primary origin from 

 a magma, or it may have been leached from rocks 

 being traversed by the solutions. The deposits of 

 hydrothermal origin can be divided into two gen- 

 eral groups: those in which the iron has been de- 

 rived from a distant source, then transported and 

 deposited in virtually nonferruginous rocks; and 

 those in which the ores represent a further concen- 

 tration of iron in rocks that are themselves rela- 

 tively iron rich. 



REPLACEMENT DEPOSITS IN NONFERRUGINOUS ROCKS (iII-A) 



Deposits of small to medium size are common in 

 areas of relatively recent magmatic activity, as in 

 the Western United States, and a genetic relation 

 generally is inferred. Most occur as pods, veins, and 

 lenses in volcanic rocks, brecciated igneous rocks, 

 and limestone (Carr and others, 1967; Dutton, 1955; 



United Nations, 1970). Magnetite and hematite are 

 the typical ore minerals and occur mainly in asso- 

 ciation with pyrite and chalcopyrite, but some veins 

 and bedding replacements consist wholly or largely 

 of siderite. The largest production in the Western 

 States — several million tons — from deposits of this 

 category has been from the Buena Vista Hills dis- 

 trict of Nevada. Here the ore occurs as irregular 

 bodies of magnetite and hematite in metavolcanic 

 rocks and diorite of probable Jurassic age. The host 

 rocks are extensively altered to scapolite, a distinctly 

 unusual feature in ore deposits of this type. Indi- 

 vidual ore bodies in the Nevada deposits have maxi- 

 mum strike and dip lengths of several hundred feet, 

 and widths rarely exceed 100 feet. 



An unusual group of deposits assigned to this 

 category is that of Precambrian age in New York 

 and New Jersey (Carr and others, 1967). The ore 

 bodies are tabular to pencillike masses of magnetite 

 and hematite oriented along the dominant linear 

 structure of the enclosing gneiss. The largest known 

 ore body is at the Benson mine. New York, on the 

 overturned flank and keel of a syncline ; it has been 

 mined for a strike length of 21/2 miles and has been 

 traced magnetically for an additional 5 miles. Min- 

 ing widths are several hundred feet. Ores of this 

 group of deposits commonly are relatively low 

 grade; the ore at the Benson mine averages about 

 23 percent iron, and contacts with the country rock 

 are gradational. Gangue minerals are dominantly 

 those of the enclosing gneiss: quartz, potassium 

 feldspar, sillimanite, garnet, and ferromagnesian 

 minerals. Some of the ore contains appreciable 

 amounts of titanium minerals. Structurally, these 

 deposits have features in common with those of 

 category III-B (see below), but no convincing evi- 

 dence for an original bed of iron-rich rock has been 

 presented ; if one existed, the high titanium content 

 suggests that it was deposited as a black sand rather 

 than as a chemically precipitated sedimentary iron- 

 formation. 



ENRICHMENTS OF PREEXISTING FERRUGINOUS ROCKS (III-B) 



In several places in the world, most notably in the 

 Quadrilatero Ferrifero of Brazil, very large depos- 

 its of high-grade ore occur within beds of Precam- 

 brian iron-formation (United Nations, 1970). The 

 ore consists of crystalline hematite (specularite) 

 with minor magnetite, locally with retention of the 

 thinly layered structure of the original iron-forma- 

 tion. Some of the ore is loose and friable. The iron 

 content approaches 70 percent, the theoretical con- 

 tent of pure hematite, and reserves exceed a billion 

 tons. The deposits are believed to have originated by 



