300 



UNITED STATES MINERAL RESOURCES 



of sedimentary rocks other than banded iron- 

 formation and ironstone, but these are of little 

 present or potential importance. Bog-iron deposits, 

 which are accumulations of iron oxides in swampy 

 areas or shallow lakes in northern latitudes, have 

 been mined in northern Europe and North America 

 (James, 1966). 



"Black band" and "clay band" are deposits of 

 siderite that occur as thin layers in coal sequences; 

 they once were mined extensively in Great Britain. 

 Possibly of greater potential significance are clastic 

 accumulations of iron minerals (black sands) such 

 as occur in southeastern Alaska and New Zealand; 

 these, however, typically contain abundant ilmenite, 

 the iron-titanium oxide, and recovery of iron de- 

 pends upon the economic feasibility of separation 

 and recovery of titanium (United Nations. 1970). 



DEPOSITS RELATED DIRECTLY TO IGNEOUS ACTIVITY (II) 



Iron may be concentrated during crystallization 

 of igneous rocks in several ways: (1) as a constitu- 

 ent of early formed minerals such as ilmenite that 

 may have settled to the base of the magma cham- 

 ber, (2) as a late-stage magmatic fraction from 

 which iron minerals are precipitated after most 

 other constituents have crystallized, and (3) as a 

 constituent of fluids (gases and aqueous liquids) 

 that escape the magma chamber and deposit iron 

 minerals in surrounding rocks (James, 1966). The 

 first two constitute category II-A; the third (and 

 quantitatively probably most important), category 

 II-B. 



MAGMATIC SEGREGATIONS (II-A) 



Magmatic segregations of iron fall into two main 

 groups: titaniferous and nontitaniferous. 



The titaniferous ores occur as layers and segre- 

 gations in gabbro, pyroxenite, and anorthosite. The 

 deposits in gabbro and pyroxenite commonly are 

 crudely layered lenses of magnetite, ilmenite, and 

 silicates such as pyroxene. Content of iron gen- 

 erally is about 20 percent, and that of titanium is 

 2 percent or more. Many of these deposits, particu- 

 larly in Alaska, are of considerable thickness and 

 extent, but as with the titaniferous black sands re- 

 ferred to previously, their significance as potential 

 sources of iron hinges largely on the feasibility of 

 separating iron from titanium economically (United 

 Nations, 1970). Most of the ores occurring in anor- 

 thosite are of quite different character. These are 

 irregular masses and dikes of coarse-grained ilmen- 

 ite, magnetite or specularite, feldspar, ulvospinel 

 (FezTiOi), and rutile (TiOz), often in complex 

 intergrowths on a microscopic scale. Deposits are 



abundant in the belt of Precambrian anorthosite 

 bodies that extends from upper New York State 

 northward into Quebec and Labrador. The iron con- 

 tent of some deposits, such as the one at Allard 

 Lake, Quebec, is about 40 percent, but the ore is 

 exploited primarily for titanium and iron recovered 

 as a byproduct. 



Nontitaniferous magmatic segregations are not 

 abundant in North America, but they form the 

 great ore bodies of the Kiruna, Rektor, and other 

 districts of northern and central Sweden (United 

 Nations, 1970). These Swedish deposits occur within 

 syenite porphyry and quartz porphyry as massive 

 sheets or irregular veinlike masses composed of 

 magnetite and minor amounts of hematite. The main 

 body at Kiruna is a steeply dipping sheet several 

 hundred feet thick and at least 14,000 feet long. The 

 iron content is 60-65 percent. Phosphorus content 

 is high, commonly 1-2 percent, and this may have 

 been a significant factor in the evident ability of a 

 concentrated iron fluid to separate, remain molten, 

 and ultimately intrude solidified parts of the parent 

 igneous body. 



The important ore bodies of Precambrian age in 

 Missouri, such as those at Pea Ridge and Pilot Knob, 

 are similar in occurrence and association to the 

 Kiruna deposit and are so classed in this report. 

 Some marginal parts of these ore bodies, however, 

 show indications of hydrothermal activity, and there 

 may well be all gradations between a purely mag- 

 matic concentration and a high-temperature hydro- 

 thermal transfer of iron to form replacement ore 

 deposits in peripheral rocks. 



PYROMETASOMATIC DEPOSITS (II-B) 



This category encompasses a rather wide variety 

 of deposits that have igneous (presumably genetic) 

 associations ranging from diabase to syenite. 



The typical pyrometasomatic or contact meta- 

 morphic deposits are replacements, most commonly 

 in limestone, at or near a contact with the parent 

 igneous rock. An important group in the United 

 States is related to Triassic diabase sills that occur 

 in a belt that extends intermittently from Connecti- 

 cut to South Carolina. The known deposits of iron 

 ore, however, are restricted to a 75-mile segment 

 that centers at Cornwall, Pa. (Carr and others, 

 1967). The deposit at Cornwall, which has been 

 mined since 1792, is the type example ; here the ore 

 occurs in two separate bodies replacing limestone 

 of Cambrian age (Button, 1955). The ore bodies are 

 tabular masses about 100 feet thick and half a mile 

 to a mile long, and they extend downdip for as much 

 as 2,400 feet. The ore contains about 40 percent iron 



