IRON 



305 



less than 1 cent a pound — and the general abundance 

 of deposits, and in part because most ores are mag- 

 netic or have some surface expression and hence 

 are comparatively easy to discover. 



In unexplored areas, now relatively few in the 

 United States, the primary tool in the search for 

 new deposits is the airborne magnetometer. Surveys 

 can be made rapidly and inexpensively; an area of 

 1,000 square miles can be adequately searched at a 

 cost less than $20,000. Once favorable areas are 

 located, detailed geologic mapping and ground mag- 

 netic surveys, possibly supplemented by gravity sur- 

 veys, serve to delineate target areas for drilling. 



Most exploration in the United States except that 

 for buried magmatic, pyrometasomatic, or hydro- 

 thermal deposits, is directed not toward discovery 

 but toward definition of deposits that meet the eco- 

 nomic criteria of geologic location, large size, suit- 

 ability for low-cost mining and beneficiation, and 

 low content of objectionable impurities. The major 

 developments of the past 20 years in the United 

 States and Canada and many that are anticipated 

 in the future concern the bedded iron-formations 

 (taconites) in the Lake Superior and the Rocky 

 Mountain regions. The location and extent of vir- 

 tually all such deposits probably are now known, 

 but intensive geologic mapping, drilling, and sam- 

 pling are necessary to appraise the actual economic 

 potential. Investments for a major taconite mine and 

 processing plant are so large that a minimum proved 

 reserve of about 100 million tons of minable low- 

 grade ore commonly is required. 



Exploration for buried high-grade deposits of 

 magmatic, pyrometasomatic, or hydrothermal ori- 

 gins presents problems more analogous to those en- 

 countered in base-metal exploration. The deposits 

 are likely to be irregular in form and size, and they 

 lack the geologic predictability of bedded ores. 

 Nevertheless, those within a few thousand feet of 

 the surface generally will yield measurable aeromag- 

 netic anomalies that provide a basis for restricting 

 the area in which detailed geologic and ground mag- 

 netometer surveys are needed for definition of 

 drilling targets. 



The geologic and geophysical techniques are used 

 to select targets, but physical sampling and analyses 

 of samples are required to prove the existence of a 

 mineral deposit of economic value. Such sampling 

 commonly involves drilling to obtain representative 

 specimens of the ore from locations sufficiently 

 spaced so as to give a three-dimensional view of the 

 ore zone and thus locate a volume large enough to 

 meet the minimum size requirements for an eco- 

 nomically minable deposit. Additional drilling and 



sampling may be done to determine the existence 

 of greater quantities of ore for larger scale opera- 

 tions, to determine the limits of the ore zone, for 

 planning of mining techniques and pit design, and 

 for selection of plant sites that will not interfere 

 with the extraction of the ore. Additional factors 

 particularly applicable to the economic evaluation 

 of iron-ore deposits have recently been discussed by 

 Ohle (1972). 



PROBLEMS FOR RESEARCH 



Specific regions that ought to be studied or reap- 

 praised geologically are: (1) Parts of the Adiron- 

 dack Mountains area of northern New York, for 

 titaniferous and nontitaniferous iron deposits; (2) 

 selected parts of the Lake Superior region, particu- 

 larly the South Cuyuna Range and adjoining areas, 

 for iron-formation in the subsurface; (3) the Ce- 

 bolla Creek area, Colorado, for titaniferous deposits ; 

 (4) iron-ore districts in southwestern Utah, for 

 magnetite and hematite deposits; (5) iron-ore dis- 

 tricts in parts of Nevada, for magnetite and hema- 

 tite deposits; and (6) the Iliamna Lake area in 

 southwestern Alaska, for titaniferous magnetite 

 deposits. Some of these areas have been recon- 

 noitered or studied in more detail by private inves- 

 tigators, but sufficient data for the appraisal of 

 their iron-ore resource potential are not publicly 

 available. The fact that some areas have been inves- 

 tigated in the past does not eliminate the possibili- 

 ties for significant discoveries in these areas. 



Perhaps the most meaningful studies, from the 

 long-range point of view, would be those directed 

 toward the objective of utilizing the entire iron- 

 rich parts of major iron-formations such as the 

 Biwabik Iron-formation in Minnesota (Button, 

 1955). Such studies would involve sampling, min- 

 eralogic investigations, beneficiation tests, and ex- 

 perimentation directed toward new iron-ore smelting 

 techniques. In many places only selected parts of 

 the iron-formation are mined as ore, and large 

 quantities of iron-rich rock must be moved in order 

 to recover the selected parts. For example, the mag- 

 netic taconite layer mined may represent only about 

 150 feet of a total thickness of 500 feet or more of 

 iron-formation. In places, as much as 550 feet of the 

 iron-formation contains more than 23 percent in 

 total iron. Such material would contain about 335 

 million tons of iron per square mile in a layer 550 

 feet thick. 



The need to minimize environmental problems 

 resulting from mining and processing ores, while 

 assuring adequate domestic supplies of iron ore, 

 gives added incentive to efforts toward use of more 



