MAGNETIC METHOD [Chap. 8 



In the Zeehan field in Tasmania, Edge and Laby'^^ investigated copper- 

 nickel deposits, accompanied by basic dikes. The anomalies were rather 

 indefinite."® In the Renison-Bell tin field, flat-lying pyrrhotite bodies 

 containing cassiterite, near the surface and outcropping, were surveyed 

 with the vertical magnetometer, and anomalies as high as ±4000 7 were 

 observed. A core hole brought down at the point of greatest magnetic 

 anomaly encountered massive pyrrhotite at a depth of from 8 to 25 feet. 

 Other magnetic indications were confirmed in a similar manner. 



In the Lake Superior copper country, magnetic measurements have been 

 used extensively because of the association of magnetite with the copper- 

 bearing lava flows. The copper is found in the brecciated tops of certain 

 of the flows, and the traceabihty of individual flows within the sequence is 

 due to variations in their magnetite content. ^*^ In igneous basic rocks an 

 association of magnetite concentration with gold values sometimes is 

 encountered. Where this relation is consistent, magnetic measurements 

 can be useful. Other appHcations of magnetic measurements to the 

 location of nonmagnetic ores will be discussed under the heading of struc- 

 tural or stratigraphic associations. 



3. Surveys of placer deposits (noble minerals associated with magnetite by 

 mechanical concentration) have been conducted in California, Colorado, 

 British Columbia, Alaska, and elsewhere. The surveys in British Colum- 

 bia were described by Laylander,"^ those in Colorado by Heiland and 

 Courtier^ and by Wantland."^ On shallow placers, anomalies may be 

 quite large (up to 300 7) (see Fig. 8-64) which explains why these effects 

 were originally discovered with less sensitive instruments. Recent placer 

 investigations in California and Nevada are described by Jakosky.^** 

 Magnetic measurements are said to have been applied in platinum placer 

 prospecting in Russia but details are not available. Where placer channels 

 are filled with flows of igneous material they may be located without diffi- 

 culty, but then the relation between magnetite and gold concentrates is 

 masked and the efi'ect is one of a stratigraphic or structural association. 



4. Surveys of mineral deposits associated structurally or stratigraphically 



"8 Op. cit., p. 84. 



1'^ Authors do not state whether the anomalies are negative or positive. In the 

 southern hemisphere, vertical intensity anomalies are negative when ore bodies are 

 normally magnetized. When plotting anomalies in the same way as in the northern 

 hemisphere, a maximum would indicate abnormal polarization. To avoid confusion, 

 negative anomalies should be plotted upward. 



"oStearn, op. cit, 187 (1932). 



"» Heiland, Terr. Mag., 37(3), 343 (Sept., 1932), 



i« Eng. and Min. J., 121, 325 (1936). 



»"A.I.M.E. Geophysical Prospecting, 369-370 (1929). 



1" Colo. Sch. Mines Quart., 32(1), 87-115 (1937). 



i« A.I.M.E. Tech. Publ. No. 515, Dec, 1933. 



