Chap. 



MAGNETIC METHOD 



417 



with magnetic formations involve the mapping of intrusions, dikes, magnetic 

 slates, and similar formations. An example of the first application is the 

 work done on the bauxite deposits in central Arkansas, believed to have 

 been formed by weathering of syenite plugs. In the Tri-State district^ 

 lead zinc ores were found to follow the magnetic lows and the flanks of 

 magnetic highs, which in turn reflect topographic highs of the pre-Cambrian 

 porphyry surface. Apparently the circulation of mineraUzed solutions 

 was controlled by structure of and faults in the flanks of the buried uplifts. 

 Contact zones with associated ore bodies may be traced by magnetic 

 measurements of igneous intrusions where iimonite and hematite com- 



5680 





FiG. 8-64. Horizontal and vertical intensity anomalies and anomalous vectors on 

 gold placer near Golden, Colorado. 



pounds in adjacent formations have been changed by reduction into mag- 

 netite. Examples were discussed before. 



Frequently the intrusive bodies are magnetic themselves, and ores asso- 

 ciated with intrusives may thus be indirectly located. Examples are the 

 surveys near the Abana mine, where a quartz diabase dike in the vicinity 

 of sulfide (replacement) deposits was subjected to intensive study with 

 various geophysical methods,"^ and a survey published by Stearn 

 showing the effect of a monzonite dike associated with seams of galena. 



"* Grohskopf and Reinoehl, loc, cit. 



1" Canad. Geol. Survey Mem. 170, 52-73. 



^*<'Op.cit., 193 (1932). 



