52 EXPLORATION GEOPHYSICS 



the Lake Superior district contain sufficient magnetic materials to allow 

 their direct detection by means of magnetic measurements. In the 

 majority of cases, however, the ores do not differ sufficiently from the 

 adjacent formations and hence an indirect means of locating favorable 

 areas is used : viz., the mapping of structure in the iron - bearing 

 formations. § 



In general, the chief constituent of sedimentary iron ores is hematite; 

 hence, the ores are often less magnetic than the adjacent iron formations 

 from which they may have been derived. In the limonite and hematite 

 ores of direct sedimentary origin, for example the Clinton iron ores, 

 the magnetic methods ordinarily can only be applied indirectly: that is, 

 to determine structural conditions associated with the ore occurrence. 



Electrical and gravitational methods have also been used in some of 

 the iron ore districts, particularly at Lake Superior, but to date they have 

 had little utility in direct iron ore location. 



Residual Deposits. — Residual iron ores are representative of this 

 class of ore deposits. The deposits have various forms and com- 

 position and are generally the result of rock decay and weathering. 

 Frequently the individual deposits are quite irregular, although occur- 

 rence and form are generally somewhat controlled by bedding in sedi- 

 mentary rocks. Geophysical methods have not been applied very ex- 

 tensively or successfully to this class of deposit. In certain cases, it is 

 probable that magnetic and electrical methods may be useful in deter- 

 mining structure related to these deposits. 



Copper-Bearing Conglomerates and Lavas. — The Lake Superior 

 copper ores are a unique occurrence and are mentioned here to illus- 

 trate the use of magnetic methods for the location of ore bodies 

 occurring in lava flows. In one type of ore body in this district, 

 native copper occurs in amygdaloidal basalt flows which dip from 

 20° to 40°. Magnetic methods have been used to trace: (1) the flows 

 in which the copper ore is concentrated and (2) certain flows 

 which bear structural and genetic relationships to the copper deposits, t 



Bedded Deposits — Non-Metallic 



Geophysical prospecting in the field of non-metallic mining has ex- 

 panded greatly in the past few years. The general class of sedimentary, 

 bedded deposits includes representatives of most of the valuable non- 

 metallic deposits. Because many non-metallic minerals and ores extend 

 through a broad genetic range, it is not possible to restrict them to a strict 

 classification as to form or mode of occurrence. Phosphate deposits, by 

 way of illustration, may occur in the following ways :t ( 1 ) marine con- 



§ Noel H. Stearn, loc. cit. 

 C. O. Swanson, "Use of Magnetic Data in Michigan Iron Ranges," A.I.M.E. Geophysical 

 Prospecting, 1931. 



t N. H. Stearn, loc. cit. 



X Lindgren, loc. cit., p. 277. 



