GEOLOGIC AND ECONOMIC BACKGROUND 45 



successfully. The magnetic and electrical methods have enjoyed the 

 greatest general application and success because of the relatively prominent 

 variations in magnetic and electrical properties typical of the majority of 

 types of ore and rock deposits commonly encountered. Complex geology 

 and rough terrain usually hinder or prevent the application of seismic 

 and gravitational methods. However, seismic methods have been used 

 successfully in determining thickness of overburden and bedrock con- 

 tours. Successful location of ledges of pyrite and other relatively heavy 

 materials by means of the torsion balance and gravity meter has also been 

 reported. f Rugged terrain characteristic of most mining areas is a 

 specific deterrent to wider application of the torsion balance, because under 

 these conditions extraneous near-surface anomalies are excessive.! Ter- 

 rain and topographic corrections required are almost prohibitively laborious 

 and time-consuming. 



The magnetic or electrical efifects produced at the surface by an ore 

 body or a structure depend primarily upon: (1) the difference between the 

 magnetic susceptibility or electrical conductivity of an ore body, or the 

 structure, and the same property (susceptibility or conductivity) of the 

 surrounding country rock; (2) the size, form, and orientation of the sub- 

 surface ore body or structure ; and (3) the effective depth of the subsurface 

 ore body or structure. 



The complexity of geological conditions which exists in a majority of 

 mining areas usually precludes the use of any one single geophysical 

 method, if the most useful information is to be obtained. It is seldom that 

 a clean-cut prediction of subsurface conditions can be made with the data 

 from only one type of geophysical measurement, for it must be borne in 

 mind that the resolving power of any one geophysical method is often low, 

 while the complexity of subsurface conditions is high. Therefore, geo- 

 physical methods preferably should be chosen to give complementary data. 

 For instance, the successful use of the magnetometer in reconnaissance 

 exploration for the nickel-bearing ores in Canada is predicated upon the 

 fact that pyrrhotite, a magnetic sulfide mineral, occurs with the nickel. 

 There are, of course, many other conditions which can cause magnetic highs 

 in that area, such as disseminated magnetite or intrusive basic rocks. To 

 differentiate between the sulfide ore desired and some other magnetic 

 condition, a traverse or two with one of the electrical methods will give the 

 necessary complementary information. The self -potential method, when 

 ground water conditions are correct, will detect sulfide bodies. If, there- 

 fore, a self -potential survey discloses an active electrical negative center 

 over the same general area where a magnetic high was mapped, we can 

 be fairly certain that this dual condition is caused by pyrrhotite ore. If, 



t "Studies of Geophysical Methods, 1930," Memoir 170, p. 108 (Geological Survey, Canada De- 

 partment of Mines, Ottawa, Canada). 



V. E. Barnes, R. W. Mathis, and F. Romberg, "Gravity prospecting for lead and zinc. New 

 Mexico," Internat. Geol. Cong., 18th sess., 1948. 



t P. W. George, "Experiments with Eotvos Torsion . Balance in the Tri-State Zinc and Lead 

 District," A.I.M.E. Geophysical Prospecting, 1929. 



