GEOLOGIC AND ECONOMIC BACKGROUND 51 



has taken place subsequent to original deposition. Important represen- 

 tatives of this class of deposit are the so-called "porphyry coppers." * 

 In the typical "porphyry copper" deposit, primary mineralization con- 

 sists of copper and iron sulphides widely distributed through a large mass 

 of intrusive rock, generally monzonitic in character. Oxidation processes 

 in the upper portion of this mass have removed a large part of the 

 original metallic content and redeposited it at greater depth in the 

 form of secondary enrichment. Electrical methods may be employed to 

 locate such zones of secondary sulphide concentration. In addition, they 

 may sometimes be employed to outline the lateral extent of low-grade 

 primary mineralization. 



Bedded Deposits 



Bedded mineral deposits are probably of greater commercial impor- 

 tance than any other general class. These deposits are found in all parts 

 of the world and include a wide range of metallic and non-metallic ores. 

 The most prominent auriferous deposits belonging to this class are the 

 well-known gold conglomerates of the Witwatersrand in South Africa. 

 Important representatives of the base metal ores are the sedimentary 

 iron and sulphide ores. 



Bedded Deposits — Metallic 



Gold-Bearing Conglomerates. — Magnetic methods have been ex- 

 tensively applied in prospecting for the gold-bearing conglomerates in 

 the Witwatersrand and other areas of South Africa. In the Witwaters- 

 rand area alone, magnetic surveys have been conducted over several 

 thousand square miles, t From the standpoint of geophysical exploration, 

 the steeply dipping Witwatersrand conglomerates are similar to veins and 

 lodes. Because of their steep dips and large lateral extents, the outcrop- 

 ping deposits have been called reefs. Even though magnetite is practically 

 absent in the conglomerates,. the magnetic methods have been useful in 

 locating certain magnetic formations which are genetically and structurally 

 associated with the gold series.:]: 



Sedimentary Iron Ores. — The use of magnetic methods in the ex- 

 ploration for iron ores may be either direct or indirect. Direct location 

 of such ore deposits can be accomplished only when magnetite or other 

 magnetic material is a constituent of the ore. Only certain iron ores are 

 magnetic. (Compare Chapter III.) In particular, some of the ores of 



* E.g., in Bingham, Utah, and Ely, Nevada. 



F. M. Galbraith, "The Magnetometer as a Geological Instrument at Sudbury," A.I.M.E. Trans., 

 Vol. 164, pp. 98-103, 1946. 



t Noel H. Stearn, "Geomagnetic Exploration in 1938," Geophysics, Vol. IV, No. 2, March, 1939. 

 A. Frose, R. Mclntire, E. Papenfus, and O. Weiss, "The discovery and prospecting of a poten- 

 tial gold field near Odendaalsrust in the Orange Free State, Union of South Africa," Chem., Met. 

 Mill. Soc. South Africa Jour., Vol. 47, pp. 107-141, 1946. 



t R. Krahman, "The Geophysical JMagnetometric Investigations on West Witwatersrand Areas," 

 Trans. Geol. Soc. of S. Africa, Vol. 39, 193G. 



