48 EXPLORATION GEOPHYSICS 



deposit and will serve as an example for the broad class of placer 

 deposits. The purpose and utility of geophysical methods in gold 

 placer exploration are usually twofold : ( 1 ) indirect location of gold 

 concentrations, (2) determination of depth to and contour of the under- 

 lying bedrock. 



No geophysical method can locate placer gold directly, in the normal concentra- 

 tions usually found in placer deposits. In many placers, however, concentrations of 

 "black sands" (magnetite, ilmenite, etc.) are associated with gold, thereby providing 

 an indirect means for its location, for, under favorable conditions, these magnetic 

 "black-sand" concentrations can be located by magnetic methods. However, such 

 magnetic data are capable of a unique interpretation only when simple conditions 

 prevail in the area. When the bedrock and the deeper basement complex have fairly 

 uniform or low magnetic permeabilities, the magnetic studies will usually show the 

 main concentrations of placer materials by magnetic "highs" superimposed over the 

 regional average of the underlying rocks. 



Such favorable conditions do not usually exist. Variations in thickness of the 

 gravel will cause magnetic anomalies. These variations may be caused by irregular 

 surface topography (which can be seen) and irregular bedrock contour (that can not 

 be seen). Similarly, the unknown variations in bedrock permeability will produce 

 anomalous magnetic variations. Most of these factors cannot be properly evaluated 

 or recognized by geological and magnetic studies alone, so that much supplementary 

 work is generally necessary. 



Magnetic methods may also be employed for the various other types of placer 

 deposits. They usually are not applicable, however, when the placer deposits are capped 

 by lava flows — as is typically the case in the Sierra Nevada placers of California, in 

 Australia, and in other parts of the world. In such cases, magnetic methods may 

 sometimes be utilized for evaluating the thickness and extent of the lava flows, but 

 they are seldom useful for locating "black-sand" concentrations. 



Electrical and seismic methods have both been employed for deter- 

 mining the depth of bedrock in placer deposits ; the former far more 

 extensively than the latter. These methods usually furnish a reliable 

 and rapid means of determining the thickness of gravels and the con- 

 tour of the underlying bedrock. The electrical methods will also indicate 

 important structural features in the bedrock, such as faults and dikes, 

 as well as the subsurface distribution of water. 



Determining bedrock depth and contour is an important part of the 

 preliminary exploration of placer ground. Ordinarily, depth determina- 

 tions are made over a grid work of stations or along sectional traverses. 

 From these studies it is possible : ( 1 ) to plot bedrock contours, and 

 thereby locate and trace stream channels in the bedrock, (2) to estimate 

 yardage of gravel, overburden, etc., and (3) to determine suitable loca- 

 tions for mining and sluicing operations to take advantage of existing 

 slope of bedrock. Electrical methods have been employed successfully 

 in locating and tracing gold-bearing stream channels below lava caps. 



