46 EXPLORATION GEOPHYSICS 



however, an active electrical center is not obtained over a magnetic high, 

 the chances are that the magnetic work has merely mapped a basic igneous 

 rock or a concentration of magnetite. 



Two types of geophysical measurement often can be made during the 

 same survey without an appreciable increase in cost. For example, magnetic 

 and gravity measurements can be made at each station with an increased 

 time for reading which adds about 20 per cent to the total cost. Comple- 

 mentary measurements allow more reliable interpretation, and in many 

 cases actually result in a reduction in overall cost of the work by decreasing 

 the detail which would be necessary if only one method were applied. 



A complementary method which has received little attention, but which 

 can be of considerable value, is the electrical logging of drill holes. 

 Although diamond and core drills usually are not considered as geophysical 

 equipment, their use often is closely related to the geophysical work. The 

 function of geophysics is to show where subsurface anomalies exist. Drill- 

 ing is a means of investigating these anomalies and of obtaining the most 

 direct type of data at minimum cost. Information from such a direct 

 method possesses three components which are most desirable : ( 1 ) direct 

 determination of the structural conditions existing at a specific point or lo- 

 calized area; (2) specific information regarding the material traversed by 

 the drill hole, by means of cores which may be examined or assayed; (3) 

 accurate depth measurements. Electrical logging will greatly expand the 

 range of this data. Prevalent in mining folklore are stories of ore bodies 

 that have been missed a few inches by the drill hole. By means of electrical 

 logging, the effective radius of the hole may be extended to from ten to 

 fifteen feet, with reliable results. Electrical logging is extensivly employed 

 in petroleum work ; it provides a rapid, economical means for determining 

 the depth and thickness of strata penetrated by the drill hole, and allows 

 correlation of drill holes. Its use in mining exploration should prove to be 

 of equal value. 



The usual classification of ore deposits with respect to a consideration 

 of geophysical methods comprises the genetic divisions: (1) mechanical 

 concentration deposits, (2) chemical concentration deposits. f 



TABLE 4 



CLASSIFICATION OF ORE DEPOSITS WITH RESPECT TO THE 

 APPLICATION OF GEOPHYSICAL METHODS 



A. PLACER DEPOSITS 

 L Gold Placers 



a. Eluvial placers 



b. Stream placers (alluvial) 



c. Bench or terrace placers 



d. Marine placers 



e. Buried stream placers 



2. Placer deposits of other heavy minerals 



3. Detrital deposits 



t Waldemar Lindgren, Mineral Deposits (McGraw-Hill). 



