52 GEOPHYSICAL METHODS IN MINING [Chap. 5 



amples are: lead sulfides, iron and copper pyrites, cobalt ores associated 

 with other sulfides, and gold-bearing quartz veins. It is often possible 

 to trace the ore-bearing faults or fissures in which mineralized waters are 

 circulating. This method has been applied on tungsten deposits in Colo- 

 rado and fluorite deposits in Illinois. Magnetic prospecting will frequently 

 furnish valuable structural information in such cases. In the Tri-State 

 district referred to under group II, various methods were tried but no 

 satisfactory way of locating the ore itself was found. Torsion balance 

 measurements appeared to be capable of tracing chert zones associated 

 with the ore and magnetic observations were used to outline highs in the 

 underlying porphyry to which the major ore accumulations appeared to 

 be related. The principal reason for the failure to locate the Tri-State 

 ores directly is that zinc sulfide, unlike other sulfides, is a nonconductor 

 of electricity. 



Magmatic-differentiation deposits have been worked principally with 

 magnetic and electrical methods. In part, these applications have been 

 of an indirect nature, such as the location of diamond-bearing intrusions in 

 Arkansas and of platinum ores in intrusive rocks in the Urals by magnetic 

 methods. Chromite has been worked indirectly by its association with 

 igneous rocks and with magnetite. Magnetite deposits of the magmatic- 

 differentiation type have been surveyed primarily with the magnetometer, 

 though torsion-balance and electrical methods have been applied occa- 

 sionally. Much work has been done on nickel sulfide ores in the Sudbury 

 district with electrical and magnetic methods. 



Magnetite deposits of the dynamo-metamorphic type (group V) have 

 been surveyed primarily with magnetic and gravity methods (Kursk). 

 Magnetic exploration likewise has had some success in the location of 

 sedimentary ores, such as iron (hematite) and manganese. 



A moderate amount of geophysical work has been done on deposits re- 

 sulting from concentration of rock substance (group III). Examples are: 

 magnetic surveys of bauxite deposits, radioactive measurements on ura- 

 nium and vanadium ores, and electrical prospecting for sulfide veins in 

 intrusive rocks. Considerable attention has been given to the possibilities 

 of geophysics in locating copper deposits in the lava flows of the Lake 

 Superior region, but only the magnetic method was found to be successful 

 for determining structural relations of the flows by tracing suboutcrops, 

 faults, and the like. 



II. MINING OF NONMETALLICS 



Nonmetallic mining is chiefly concerned with mineral deposits formed 

 by chemical precipitation and mechanical sedimentation in surface waters 

 (group IV, Fig. 5-1). In discussing the applications of geophysics in this 



