5 



GEOPHYSICAL METHODS IN MINING 



Selecting the correct geophysical method in mining is not so simple as 

 in oil exploration because of the greater complexity and geologic variety 

 of ore deposits. The choice is often facilitated by the fact that the type 

 of ore body to be located is already known. Conditions prevailing in 

 most mining areas (see page 6) eliminate the possibility of gravitational 

 and seismic methods, so that the choice is likely to be magnetic or electrical 

 exploration. Which of these is preferable depends much on the type of 

 ore and its origin. 



In the genetic classification of Fig. 5-1 mineral deposits are arranged 

 largely in accordance with the scheme suggested by Lindgren. Geological 

 and geophysical "type" locations, schematics of the geologic form, and 

 suggestions as to choice of direct and structural geophysical methods 

 are given for six groups: (I) magmatic differentiation deposits, which are 

 formed by crj'-stallization within magmas and oc^^ur, therefore, mostly in 

 intrusive igneous rocks; (II) heterogenetic solution deposits, formed by 

 infiltration of solutions from without, usually derived from adjacent ig- 

 neous formations; (III) autogenetic solution deposits, originating from 

 chemical concentration of rock substance ; (IV) sedimentation deposits, such 

 as salt, coal, and limestone; (V) dynamo-metamorphic deposits, formed by 

 concentration and chemical transformation of rock substance in conse- 

 quence of diastrophic forces; and (VI) mechanical concentration deposits 

 in gravels, conglomerates, and the like. 



Metal mining is concerned primarily with groups I-III and V-VI. 

 Nonmetallic minerals are derived largely from deposits in group IV. 



I. METAL MINING 



Surprisingly enough the greatest number of geophysical methods are 

 applicable to location of mechanical concentration (placer) deposits. 

 They may be worked by magnetic methods where a definite relation 

 between gold concentration and black-sand content can be established. 

 For determination of depth to bedrock, resistivity and refraction-seismic 



49 



