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UNITED STATES MINERAL RESOURCES 



rather than economic, but it does, however, provide 

 a practical means of grouping characteristics such 

 as mineralogy, tonnage-grade factor, form, and pro- 

 jection in depth, that are of prime importance to 

 assessment of resource potential. The major cate- 

 gories, together with the more significant subdivi- 

 sions, are listed as follows and are discussed in 

 more detail in succeeding paragraphs. 



I. Bedded sedimentary deposits: 



A. Banded iron-formations, almost exclusively 



Precambrian. 



B. Ironstones, mainly post-Precambrian. 



C. Miscellaneous sedimentary deposits, in- 



cluding "black band" siderite, bog iron, 

 and clastic accumulations. 



II. Deposits related directly to igneous activity: 



A. Magmatic segregations. 



B. Pyrometasomatic deposits. 



III. Deposits formed by hydrothermal solutions : 



A. Replacement deposits in nonferruginous 



rocks. 



B. Enrichments of preexisting ferruginous 



rocks. 



IV. Deposits produced by surface or near-surface 



enrichment : 



A. Laterites. 



B. Enrichments of low-grade ores. 



BEDDED SEDIMENTARY DEPOSITS (I) 



The principal concentrations of iron in the earth's 

 crust are products of chemical sedimentation, which 



have been modified by wave and current action, by 

 chemical and mineralogic changes that take place 

 below the depositional interface (diagenesis), and 

 by mineralogic changes induced by heat and pres- 

 sure in burial environments. 



BANDED IRON-FORMATIONS (i-A) 



Banded iron-formations occur as sedimentary 

 units in Precambrian rocks throughout the world. 

 They constitute the greatest single source of iron 

 now being mined and contain the largest potential 

 resources. The rock has been defined as follows: 

 "a chemical sediment, typically thin bedded or lami- 

 nated, containing 15 percent or more iron of sedi- 

 mentary origin, commonly but not necessarily con- 

 taining layers of chert." In some iron-formations 

 the iron is in the form of carbonate ( siderite, with 

 an appreciable content of manganese, magnesium, 

 and calcium) or silicate (greenalite, minnesotaite, 

 stilpnomelane) , and in rare occurrences it is in the 

 form of sulfide (pyrite). The most distinctive and 

 economically most significant rock type, however, 

 consists of iron oxides (magnetite or hematite) and 

 chert (or its recrystallized equivalent) in alternat- 

 ing thin layers, with an iron content commonly in 

 the range of 25 to 40 percent. Each of the sedi- 

 mentary facies — oxide, carbonate, silicate, sulfide — 

 has the distinctive aspects of mineralogy, total iron 

 content, and structure that are summarized in 

 table 57. 



Table 57. — Principal features of the iron- formation facies (modified from. James, 1966) 



