540 hawkes. GEOCHEMICAL PROSPECTING FOR ORES [Ch. 30 



however, have an immediate bearing on our present problems and 

 deserve special consideration. 



Glacial Dispersion 



Glacial erosion and transport is predominantly a physical process. 

 The preglacial soil and weathering products, together with a variable 

 amount of material plucked from the unweathered rock, are moved 

 physically in the direction of the ice movement. Under ideal con- 

 ditions the material removed from a given locality will be deposited 

 in the ground moraine in a fan-shaped pattern converging toward the 

 locality where the material originated. The relative abundance of such 

 material in the till will fall off rapidly with distance from the source. 

 This, of course, presupposes the absence of multiple ice movements in 

 several different directions and excessive complication by glaciofluvial 

 action. 



Studies of the distribution of glacial boulders of distinctive rock 

 types illustrate the characteristic fan-shaped distribution pattern 

 (Goldthwait, 1925; Lundqvist, 1935) and show that most of the mo- 

 rainal material is of relatively local origin (Salisbury, 1900; Alden, 

 1918). Following these principles, Scandinavian investigators have 

 used systematic mapping of ore boulders as a method of locating de- 

 posits buried beneath the glacial cover (Sauramo, 1924; Sederholm, 

 1922; Odman, 1947), and in Sweden, particularly, "boulder hunting" 

 has become a standard method of prospecting in both governmental 

 and private organizations. 



The same problem may be attacked by chemical rather than min- 

 eralogical methods, by analyzing the fine-grained fraction of the till. 

 A single relatively small sample of the fines may contain many 

 particles of ore material that will be indicated in the chemical analysis, 

 whereas many thousands of barren boulders must be laboriously ex- 

 amined for every ore boulder that is found and mapped. The difference 

 in the two approaches is thus primarily one of scale. 



A chemical survey of the distribution of copper, lead, and zinc in 

 organic surface soil derived from till near the Lossius deposit in the 

 R0ros district of Norway showed relatively high metal concentrations 

 (Cu 100+, Pb 20+, and Zn 1,000+ parts per million) for a distance of 

 250 meters from the ore, measured in the direction of ice movement 

 (Vogt and Bergh, 1946, 1947). A preliminary study by the U. S. 

 Geological Survey of the zinc content of till in the vicinity of the 

 Hyatt zinc mine, St. Lawrence County, New York, showed high con- 

 centrations (over 300 parts per million compared with background of 

 less than 100 parts per million) in the surface till 800 feet to the "lee" 



