50 EXPLORATION GEOPHYSICS 



application of geophysical methods to the discovery and mapping of veins, lodes, 

 and dikes. § 



Irregularly Concentrated Deposits 



Magnetic and electrical methods have found extensive application in 

 prospecting for irregular ore deposits such as those found in shear zones, 

 pipes and stocks, replacements, contact-metamorphic deposits, and mag- 

 matic segregations, tt Spontaneous polarization methods are particularly 

 applicable to vertical pipe-like bodies of sulphide ore.* In cases where 

 the pipe-like form of the ore bodies is the result of their deposition in 

 shattered volcanic plugs, magnetic methods often may be used to locate 

 the intrusive plug, tt 



Replacement ore bodies are of many types and are widely distributed in occurrence 

 and varied in mineralogical assemblage. Replacement processes occur to a certain 

 extent in all cases of ore deposition. Common types are replacement bodies of base 

 metal sulphides. These are often accompanied by gold and silver values deposited in 

 limestone by ascending solutions ** or by percolating meteoric waters.*** The forms 

 of these replacement ore bodies are usually very irregular, depending, in many cases, 

 upon the texture and other properties of the host rock. In certain cases, replacement 

 in fairly flat dipping limestones has extended laterally for sufficient distances to result 

 in a bedded form of deposit. 



Electrical methods are preferred for locating the various types of replacement 

 ore bodies which have a high electrical conductivity. Occasionally, however, this 

 method has not been successful, as, for example, in the upper Mississippi Valley where 

 there is a large percentage of poorly conductive sphalerite in the ores, t 



Contact-metamorphic deposits frequently contain the ore minerals : magnetite and 

 chalcopyrite. Magnetic methods have been used for direct location of the magnetite ores 

 of contact deposits and for outlining the intrusive igneous rocks responsible for the 

 ore deposition. Electrical methods are applicable when the conductive base metal sul- 

 phides are the prominent ore constituents. 



Ore deposits resulting from segregation and separation of ore minerals from. 

 magmas during the processes of solidification are common in many parts of the world. 

 The Sudbury Canada nickel ores are prominent in this group. The chief application 

 of geophysical methods to this class of deposit consists in locating and outlining the 

 igneous rock in which the segregation may have taken place. When such deposits 

 have a high content of magnetite or metallic sulphides, they can sometimes be located 

 directly by magnetic or electrical methods, t 



Disseminated Deposits 



The class of disseminated deposits covers many genetic occurrences. 

 They are important chiefly to the extent that concentration or enrichment 



^ A.I. M.E. Geophysical Prospecting, 1929, 1952. 19Si. 

 A. B. Broughton Edge and T. H. Laby, Geophysical Prospecting. The Report of the Imperial 

 Geophysical Experimental Survey (of Australia), Cambridge University Press, 1931. 



tt Hans Lundberg, "Recent Results in Electrical Prospecting for Ore," A.I.M.E. Geophysical 

 Prospecting, 1939. 



* See Chapter V. 



tt Compare Lindgren, loc. cit., pages 153 and 183. 



** E.g., lead-silver ores of Park City, Utah. 

 *** E.g., lead-zinc ores of the Mississippi Valley. 



t Hans Lundberg, loc. cit. 



t Max Mason, "Geophysical Exploration for Ores," A.I.M.E. Geophysical Prospecting, 1929. 



