Chap. 8] MAGNETIC METHOD 419 



(see Fig. 8-65), Krahmann*^^ on a Kimberlite pipe in southwestern Trans- 

 vaal. In this survey the — 1200 gamma (Southern Hemisphere) anomaly 

 followed very closely the outline of the pipe. Strong magnetic anomalies 

 in the contact zone due to magnetized shales were also recorded. 



The stratigraphic association of sedimentary nonmagnetic beds of com- 

 mercial value with magnetic members in the geologic section has been 

 utilized to advantage in the Lake Superior region where strike, dip, and 

 faults of the copper-bearing lava flows have been mapped by tracing mag- 

 netic members in the series,*^' and in South Africa in connection with 

 exploration for the gold conglomerates of the Witwatersrand system 

 (see Fig. 8-66). These strata show evidence of considerable metamor- 

 phism and consist of quartzites, slates and shales, the gold-bearing con- 

 glomerates, and volcanic beds. Several dikes and faults cut through the 

 area. In the west Witwatersrand the accurate stratigraphic relation of 

 the magnetic beds (magnetic slates) to the main reef could be well estab- 

 lished. During the experimental period a number of magnetic profiles 

 were run on outcropping sections. Fig. 8-66 shows vertical intensity 

 anomalies for the section near the main reef. The curves are quite irregu- 

 lar because of changes in magnetite content. In the upper profile, at the 

 point of greatest irregularity, magnetic material (extracted by electro- 

 magnets) varied from to 80 per cent, and susceptibilities from to 

 10,000 10"^. When the overburden becomes comparable with the dis- 

 tance of the magnetic shale members, their eflfects overlap, resulting in one 

 major negative (Southern Hemisphere) anomaly (see Fig. 8-67). While 

 in this particular area the magnetite shales were normally magnetized, 

 syenite, dolerite, and granophyre dikes cutting through the area were 

 abnormally magnetized. In another area^^ abnormal polarization was 

 found on the magnetite shales (anomalies of 20,000 7 for a depth of 477 

 feet). The susceptibilities determined in the laboratory were not sufficient 

 to explain the magnitude of the anomalies.^** It has been estimated that 

 as a consequence of the magnetic surveys the potential gold production 

 has been increased by at least one-eighth of its present amount. 



In the location of gold placer deposits, the stratigraphic association of 

 placer channels with magnetic formations other than the concentrates can 

 be helpful in locating the channel itself. Edge and Laby^" have demon- 

 strated this at Gulgong, New South Wales, where the channels are filled 

 with basalt flows, producing positive (Edge-Laby definition) anomalies 



«2Min. and Met., 16, 342 (June, 1935). 



»" Stearn, op. ciL, 187. 



1" Krahmann, Geol. Soc. S. Afr. Trans., 39 (1936). 



»" See F. Hahnemann, A.I.M.E. Contrib. No. 79, 1935. 



"' See discussion on p. 401. 



1" Op. cit., 189. 



