230 



EXPLORATION GEOPHYSICS 



gradient extended in a northwest-southeast direction. The concentration of placer 

 materials is well shown by the magnetic highs.* The magnetic conditions in this par- 

 ticular area are almost ideal for application and interpretation of magnetometer studies. 



A magnetic survey was carried out at the Roscoe Placer of the Humphreys Gold 

 Corporation, on Clear Creek, Jefferson County, Colorado.! The rather tortuous course 

 of the deepest portion of the old stream channel was traced by magnetic mapping of 

 the concentrations of black sand, which carried the best gold values. It was found that 

 magnetic lows were associated with areas of shallow bed rock or local bed rock ridges. 

 The results of the magnetic work and resistivity determinations for bed rock depth 

 were critically tested as bed rock was exposed in the course of operations. 



H. R. Joestingt describes results of a number of magnetic and resistivity surveys 

 on placer deposits in Alaska (1939-1940). When supported by geologic and mineral- 



FiG. 114. — Geological cross section AA' with magnetic profile. (After Galbraith, 

 A.I.M.E. Gecphysics Volume 164, 1945.) 



ogical data, the magnetic method was valuable in preliminary prospecting for about half 

 of the gold placers in the interior of Alaska. Magnetic work was most useful where 

 the placer ground contained appreciable amounts of magnetite concentrated in pay 

 streaks. It was of no value where the placers carried little or no magnetite, as would 

 be expected, or where large bed rock anomalies were present. While the magnetic 

 method did not permit an evaluation of placer ground in all cases, it often obviated 

 much costly drilling and shaft sinking in barren areas. The depth and areal distribution 

 of permanently frozen ground and of thawed unconsolidated deposits were determined 

 successfully by the direct current resistivity method, as part of the program of 

 investigation. 



* In the application of magnetic measurements to placer gold location, the interpre- 

 tation must be made from the viewpoint that the magnetic methods are not locating the 

 gold itself, but only the placer magnetic materials with which the gold is associated. 



t D. Wantland, "A Comparison of Geophysical Surveys and the Results of Operations at the 

 Roscoe Placer of the Humphreys Gold Corporation, Jefferson County, Colo.", Colorado School of 

 Mines Quarterly, Vol. 32, No. 1, Jan., 1937, pp. 87-115. 



t H. R. Joesting. "Magnetometer and Direct Current Resistivity Studies in Alaska , A.I.M.E. 

 Tech. Paper 1284. Feb., 1941. 



