418 



MAGNETIC METHOD 



[Chap. 8 



Surveys of intrusive bodies have been helpful in connection with prospect- 

 ing for chromite. From the published data it appears that some occur- 

 rences of chromite are magnetic and others are not. Deposits in Russia 

 on which magnetic tests were made were definitely magnetic and also 

 showed usable anomalies. Where the chromite is nonmagnetic, a survey 

 of the associated intrusions may, nevertheless, jdeld useful results. 



Intrusive dikes associated with gold values may often be traced by a 

 magnetometer through the use of positive and negative anomalies. Mal- 



Insfr. /heading 

 Scale DiV 



Folded Paleozoic 



Scale mUlles 

 <6 1/g 



— ■<^^^/' — 



/\ • :*o^.~'ir 5or/;ue of fiehdoti/e 



Peridolile Plug 



Fig. 8-65. Section across peridotite plug in Arkansas, showing depth finding by 

 anomalous vectors (after Stearn). 



amphy traced gold-bearing pegmatite dikes by their increased magnetiza- 

 tion (negative anomalies, Southern Hemisphere), while Stearn^*" surveyed 

 gold-bearing rhyolite dikes that were intruded into monzonite and caused 

 negative anomalies. 



In diamond-mine exploration, the magnetometer can be helpful in trac- 

 ing the igneous plugs or pipes in which the diamonds occur. Steam*" 

 applied the magnetic method to the location of peridotite plugs in Arkansas 



"9 A.I.M.E. Geophysical Prospecting, 325 (1934). 

 "" hoc. cii. 

 "• Ibid., 197. 



