GEOLOGIC AND ECONOMIC BACKGROUND 49 



Two criteria serve to indicate the subsurface location of such chan- 

 nels : t ( 1 ) the difference in electrical conductivity between the water- 

 bearing gravels and the bedrock and (2) the lowest point in the 

 bedrock profile. 



Veins and Lodes 



Magnetic and electrical methods have been employed extensively in 

 exploration for commercial mineralization in veins and lodes, while 

 gravimetric methods have been employed to a much lesser extent. A great 

 variety of minerals and ores occur in the form of veins and lodes. Promi- 

 nent in these classes are: (1) sulphide veins (usually intermixed sulphides 

 of copper, iron, lead, zinc, etc.), (2) gold-pyrite veins, and (3) gold- 

 quarts veins. Electrical methods are useful for locating all of these types 

 of deposits, but are especially applicable for deposits of electrically 

 conductive base metal sulphides (ores of chalcopyrite, pyrite, galena). 

 In the latter cases, the methods are used to locate the vein directly or to 

 outline shoots or other concentrations within the vein. A problem which 

 is frequently encountered is the location of possible zones of primary sul- 

 phides or of secondary enrichment beneath oxidized cappings. The 

 electrical methods have been especially useful for locating non-out- 

 cropping veins in areas adjacent to outcropping veins of known char- 

 acteristics and for locating veins beneath coverings of alluvium, glacial 

 till, tundra, etc. t 



The use of electrical methods in prospecting for gold-bearing veins and lodes 

 is an indirect process. Such deposits may be located indirectly by means of the occur- 

 rence of pyrite or other metallic sulphides in genetic association with the gold. Gold- 

 quartz veins, under favorable conditions, may be located by reason of their poor 

 electrical conductivity relative to the country rock.§ 



Igneous dikes: Ore bodies are found in association with igneous dikes only 

 infrequently. However, because such dikes have a structural significance in many 

 mineralized areas, their occurrence is mentioned here. Dikes of basic material may be 

 located by magnetic methods due to their high content of minerals of large magnetic 

 susceptibility, ft 



Pegmatite dikes : Valuable ore and mineral concentrations are frequently asso- 

 ciated with pegmatic dikes. Where such dikes have a sufficiently lower electrical 

 conductivity than the country rocks, they may be located by electrical methods, as in 

 the case of quartz veins. Magnetic methods are applicable when, as is frequently 

 the case, magnetite or other magnetic material is an accessory mineral in the peg- 

 matite. The literature of geophysical methods contains many descriptions of successful 



t J. J. Jakosky and C. H. Wilson, "Geophysical Studies in Placer and Water Supply Problems," 

 A.I.M.E. G'eoftiysical Prospecting, Tecli. Pub. No. 515, 1934. 



t "Summary of Results from Geophysical Surveys at Various Properties," A.I.M.E. Geophysical 

 Prospecting, 1932. 



§ Folke H. Kihlstedt, "Electrical Methods in Prospecting for Gold," A.I.M.E. Geopliysical 

 Prospecting, 1934. 



Sherwin F. Kelly, Theodor Zuschlag and Bela Low, "Discovering Gold-Quartz Veins Elec- 

 trically," Mining and MctaUurgx, Tune, 1934. 



tt W. R. Johnson, Jr., G. R. MacCarthy, J. C. Campbell, and H. W. Straley, III, "Tracing a 

 basic dike by geoelectrical and geomagnetic methods," A.I.M.E., Geopliysics, 1940. 



