304 



UNITED STATES MINERAL RESOURCES 



Philippine Republic, Sierra Leone, Spain, and Tu- 

 nisia. 



The identilied iron-ore resources of the United 

 States are shown in table 60. 



Table 60. — The identified iron-ore resources of the United 

 States, in millions of metric tons (United Nations, 1970) 



(Figures rounded. Include estimates for some deposits for which precise 

 data are not available] 



Identified re- 

 sources - ( re- 

 Region Reserves ^ serves -j- po- 

 tential ores) 



Lake Superior 6,500 78,000 



Northeastern 200 1,400 



Southeastern 100 9,400 



Central-Gulf 600 700 



Central-western 1,100 2,300 



Western 600 1,200 



Alaska 6,600 



Hawaii 1,000 



Total 9,100 100,600 



^ Reserves : Identified deposits from which minerals can be extracted 

 profitably with existing technology and under present economic con- 

 ditions. 



= Identified resources: Specific, identified mineral deposits that may or 

 may not be evaluated as to extent and grade, and whose contained 

 minerals may or may not be profitably recoverable with existing tech- 

 nology and economic conditions. 



HYPOTHETICAL RESOURCES 



Because of the great amounts of identified iron- 

 ore resources, no attempt is made to estimate quan- 

 tities of hypothetical iron-ore resources beyond 

 stating that they are enormous. The identified iron- 

 ore resources in the world are a sampling of the 

 surface and near-surface parts of the geologically 

 explored land areas of the earth. Most of the identi- 

 fied resource material is within half a mile of the 

 surface of the earth. It is reasonable to expect that 

 in many areas the zone between a depth of half a 

 mile and 1 mile beneath the surface will contain 

 hypothetical resources proportional in amount to 

 the near-surface identified quantities. This concept 

 is especially applicable to the great expanses of 

 Precambrian iron-formation and younger ironstone 

 formations that exist on all the continents. It also 

 applies to areas in which deposits are the results of 

 processes related to magmatic activity. Allowance 

 must be made, however, for some deposits that are 

 not resources because they did not undergo surficial 

 enrichment and also for some flat-lying iron-forma- 

 tions that do not extend to depths of more than 

 half a mile. 



In North America, and in the geologically ex- 

 plored parts of other continents, bedded Precam- 

 brian sedimentary iron-formations and younger 

 sedimentary ironstone deposits extend for lengths 

 totaUng hundreds of miles (United Nations, 1970). 

 Undoubtedly these formations, in many areas, main- 

 tain their basic characteristics for considerable dis- 



tances in the subsurface. In view of the existence 

 of very large quantities of identified iron-ore re- 

 sources, and because setting upper limits to the 

 amounts of hypothetical iron-ore resources would 

 involve many questionable assumptions, detailed 

 estimates of the existing quantities of hypothetical 

 iron-ore resources can have little meaning. Factors 

 such as the amounts of ore that must be left in 

 place as mine pillars and physical limitations upon 

 depths to which mining activities can be carried on 

 in various types of rock impose limitations upon 

 the practical estimation of hypothetical resources at 

 great depths. Advances in the state of the art of 

 mining can extend the depths to which ore can be 

 mined, but can also extend the competitive exploi- 

 tation of low-grade shallow ore deposits. 



SPECULATIVE RESOURCES 



Although speculative iron-ore resources are im- 

 possible to quantify in any meaningful way, they 

 are worthy of consideration. In the past two decades 

 most of the identified iron-ore resources of Australia 

 became known, and Australia has become one of 

 the major iron-ore producing nations (Inst. Geol. 

 Sci., 1971; United Nations, 1970). Similarly, large 

 high-grade deposits, measuring in thousands of mil- 

 lions of tons, have been identified in recent years in 

 Brazil. Large areas in South America, Africa, Asia, 

 and Antarctica remain to be explored geologically. 

 On the other continents large areas in which Pre- 

 cambrian basement rocks are under a relatively 

 thin cover of younger rocks offer possibilities for 

 the discovery of major iron deposits. Manganese- 

 and iron-rich nodules and other sediments on the 

 sea floor remain to be assessed as potential re- 

 sources . 



In the United States, the most promising regions 

 for iron-ore discovery have largely been identified 

 and at least partially explored. The principal need 

 is to obtain more detailed information concerning 

 these regions. Because most of the major iron de- 

 posits or iron-bearing formations have associated 

 magnetic anomalies, major programs of aeromag- 

 netic surveying to delineate the zones of magnetic 

 rocks and potential ore bodies should be continued. 

 Such surveys have been made for many areas and 

 have proved to be successful. In some cases further 

 geologic studies and interpretations of the mag- 

 netic and geological data are needed. 



EXPLORATION 



Exploration for iron ores differs significantly 

 from that involved in the search for most other 

 metals, in part because of the low value of ore — 



