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UNITED STATES MINERAL RESOURCES 



made mainly in existing mining districts through 

 direct physical exploration of undeveloped areas 

 that are geologically similar to the environments of 

 the known ore deposits. In previously unproductive 

 but geologically favorable terranes and in small or 

 relatively inactive districts, the preliminary tech- 

 niques of exploration employed in advance of drill- 

 ing, shaft sinking, or drifting are less direct and 

 may be divided into three categories: geologic, geo- 

 chemical, and geophysical. These techniques may be 

 used separately or in any combination or sequence. 

 Prior to World War II and the era of accelerated 

 exploration that followed it, some new discoveries 

 were chance finds or were made through serendipi- 

 tous observations during unrelated efforts. As noted 

 by McKinstry (1948, p. 413), however, such lucky 

 accidents often make entertaining anecdotes but are 

 not the source of many mine discoveries. Most mod- 

 ern discoveries result from scientifically directed 

 prospecting, a costly undertaking. 



Geologic techniques for prospecting for lead de- 

 posits range in magnitude from the regional delinea- 

 tion of mineral belts, greenstone belts, and other 

 metavolcanic terranes and areas of domal uplift on 

 cratonic platforms to the solution of the problem 

 of fault displacement of part of a minor ore body — 

 all based on geologic mapping at appropriate scales. 

 Features to be recognized and evaluated, especially 

 in the search for concealed deposits or deposits 

 with unimpressive surface manifestations, are struc- 

 tural and stratigraphic localization of ore, chemical 

 and mineralogical changes in the adjacent wallrocks, 

 zonation, and temperature and pressure regimes. 

 Among many examples of mineral discoveries based 

 largely on geologic reasoning are the lead deposits 

 of the Viburnum area, Missouri, the lead and zinc 

 deposits of central Tennessee, and on a smaller scale, 

 the polymetallic Bulldog Mountain mine, Creede dis- 

 trict, Colorado (Steven and Ratte, 1960; Cox, 1965). 

 Geochemical exploration is generally used in con- 

 junction with geologic studies in several ways: to 

 help eliminate unmineralized tracts within broad 

 areas that are geologically favorable, to identify 

 halos of dispersed metals in rocks or in residual sur- 

 ficial deposits above and adjacent to concealed de- 

 posits, to delineate the source areas of ore-metal 

 dispersion trains in transported materials such as 

 stream sediments and glacial till, and to determine 

 possible patterns of compositional zonation of known 

 deposits or groups of deposits. The techniques used 

 include chemical, spectrographic, radiometric, and 

 atomic-absorption analysis, heavy-mineral dispersion 

 studies, and analysis of geobotanical relations 

 (Hawkes, 1957). Examples of lead-producing mines 



that were discovered, at least in part, by geochemical 

 techniques, include among others the Laisvall, Rak- 

 kejaur, and other deposits in Sweden (Grip, 1953), 

 the Burgin deposit, Utah (Bush and others, 1960), 

 and the Tynagh ore body in Ireland (Clark, 1965) . 



Geophysical prospecting techniques are based on 

 certain physical characteristics of ore minerals, ore 

 bodies, or ore host rocks that contrast with their 

 enclosing country rocks and may be detected, meas- 

 ured, and evaluated rapidly, cheaply, and usually at 

 some distance from the body being sought or through 

 a concealing cover. Such characteristics include den- 

 sity, magnetism, electrical conductivity, radioactivi- 

 ty, and seismic response. In the search for metallic 

 mineral deposits, including galena ore bodies and 

 especially polymetallic sulfide deposits containing 

 galena in association with high proportions of pyrite 

 and pyrrhotite, the most successful methods during 

 the past decade or so have been airborne magnetic 

 and electromagnetic surveys that establish the posi- 

 tion, size, and trend of anomalies over large areas of 

 till-, forest-, or swamp-covered terrane, followed by 

 detailed ground surveys that employ other geophysi- 

 cal, geochemical, and geologic techniques. These 

 methods have been especially successful in the Pre- 

 cambrian shield areas of the world where large 

 massive sulfide deposits are concealed beneath a rela- 

 tively thin cover of surficial material. The aeromag- 

 netic surveys delineate greenstone belts, intrusive 

 bodies, major faults, and other geologic features that 

 are commonly concealed. The electromagnetic (EM) 

 systems are then used to create a low-frequency in- 

 ductive field in the vicinity of the low-flying aircraft, 

 inducing eddy currents in the conductive ore bodies ; 

 the secondary fields radiating from the conductive 

 bodies are then detected by receivers. The applica- 

 tion of EM techniques has been greatly enhanced in 

 recent years by the development of rapid data- 

 processing equipment coupled directly to the trans- 

 mitting and receiving equipment. Other techniques 

 that may be used concurrently with airborne mag- 

 netic and electromagnetic methods include infrared 

 and radar imagery, color and multiband photogra- 

 phy, and side-viewing radar. 



Geophysical techniques used on the ground in the 

 evaluation of anomalies discovered by airborne meth- 

 ods include a variety of EM systems that are similar 

 to the airborne EM systems, the long-established and 

 widely used induced polarization (IP) methods, and 

 the audiofrequency magnetic (AFMAG) method, 

 which utilizes natural currents of the earth that arise 

 chiefly from thunderstorm activity in the equatorial 

 regions. Locally, resistivity and self-potential meth- 

 ods also continue to be used. Each of these systems 



