414 



MAGNETIC METHOD 



[Chap. 8 



5500 7, several hundred feet long. A shaft sunk at the point of maximum 

 anomaly to a depth of 60 feet encountered much magnetite in the schist 

 country rock but no commercial ore. These are examples of "stray" 

 anomalies produced by magnetite stringers and magnetite slate in areas of 

 contact and dynamo-metamorphism. They often interfere with the effects 

 of commercial ore bodies but may be useful in tracing nonmagnetic ores. 

 Magnetic anomalies of fairly regular type were observed on hematite and 

 magnetite deposits of the Lahn-Dill district in Germany by Kegel. 

 Rossiger and Puzicha^^^ examined the magnetite deposit of the Spitzenberg 

 in the Harz mountains. Correlations of anomalies with susceptibility 

 measurements in the laboratory indicated that the deposit was more 



cu tmd Db Df cuk 

 (a) 



(b) 



Fig. 8-62. (a) Difference in anomalies on two iron deposits in same section (after 

 Rossiger). (6) Magnetite (black) pseudomorphism after hematite (hatched), (cm, 

 Lower Carboniferous; imd, Middle Devonian, Stringocephalus beds with hematite; 

 Dh and £>/, diabase; cuk, lower Carboniferous siliceous slates with hematite.) 



strongly magnetized than corresponded to induction in the earth field. 

 The deposits of the Harz mountains originated from contact-metamorphic 

 alteration of hematite and specular hematite to magnetite (see Fig.8-62b). 

 Sedimentary iron ores containing hematite and brown iron produce small 

 magnetic anomalies (unless later subjected to contact- or dynamo-meta- 

 morphism) . Pockets of brown iron ore in basalt, formed by decomposition 

 of basalt, could be located by Meyer*'* (see Fig. 8-63). In Missouri small 



"" The curves indicate a depth of about 80 feet. 



"1 Geol. Landesanst. Berlin Sitzungsberichte, 4, 60 (1929). 



132 Beitr. angew. Geophys., 3(1), 45-108. See also G. Beyer, Beitr. angew. 

 Geophys., 3(3), 337-363 (1933). 



133 Beitr. angew. Geophys., 1(4), 420^31 (1937). 



