Chap. 8] 



MAGNETIC METHOD 



415 



and somewhat irr^ular anomalies were observed by Grohskopf^^ on iron 

 ores in sink holes in sandstone and dolomite. The soft red hematite ore 

 was found to be nonmagnetic, the irregular anomalies being due to blue 

 specular hematite. In Wisconsin Stearn* could trace oxidized iron ore 

 by the maximum found on the hanging and foot wall over unoxidized por- 

 tions of the Cuyuna iron formation. Likewise, Rossiger and Puzicha"* 

 found that hematite (red iron ore) was virtually nonmagnetic, but that its 

 action changed completely when altered to magnetite. Fig. 8-62o shows a 

 typical example. The iron ore in the N-W of the profile produces little 

 or no effect, whereas the "cuk" formation (a Devonian sihceous slate with 



I I i J 



# M 4V Mm 



Fig. 8-63. Magnetic anomalies on brown iron ore pockets in basalt (after Meyer). 



red iron ore, partially reduced to magnetite [see microphotograph]) is 

 accompanied by a pronounced anomaly. 



2. Surveys of pyrrhotite deposits and of nonmagnetic ores associated with 

 magnetite or pyrrhotite. The pyrrhotite deposit of Falconbridge (Sudbury, 

 Ontario) has been the subject of intense study with virtually all major 

 geophysical methods — gravitational, electric, magnetic. Various traverses 

 made across the ore body with a dip needle have been pubUshed by Slich- 

 ter and clearly reveal the effect of variations in the thickness of glacial 

 drift. 



1" J. G. Grohskopf and C. O. Reinoehl, Mo. State Geol. Rep., App. Ill, 146-157 

 (1931) ; App. IV, 5-20 (1933). 

 ^^^Op. cit., 188 (1932). 

 ^" Loc. cit, 

 1" Op. cit., 254. 



