556 EXPLORATION GEOPHYSICS 



The surrounding country rock is composed chiefly of an old series of 

 sediments, portions of which had been highly silicified. These rocks in all 

 probability are of pre-Cambrian age. 



Prior to the geophysical investigations, a tunnel was driven 770 feet 

 parallel to, and west of the ledge outcrop in an effort to cut the ledge at 

 the 2250 foot elevation. Because ore was not encountered, the mine 

 operators believed that the vein might dip farther to the west. A cross 

 cut was driven westward for 310 feet, and failing to encounter the vein, 

 an eastward cross cut was driven for 300 feet. At this stage of the devel- 

 opment program, the geophysical work was employed. 



The cross section in the lower portion of the figure is drawn from the 

 results of the geophysical survey and shows the depth to which the ledge 

 extends and its relation to the surrounding rock and structure. The limited 

 amount of sulphide ore disclosed by the geophysical survey made the 

 property of doubtful value due to high shipping and operating costs. 



Lateral Extension and Depth of Ore Body. — The application of 

 geophysical work over known mineralized areas, where some development 

 has already been done, allows important information to be obtained relative 

 to the lateral extension of known ore bodies. Should the measurements 

 indicate the possible extension of such bodies, it is economical to use a 

 few well-placed drill holes to check values. In many instances it has been 

 found that where the geoelectrical work shows fairly constant magnitudes, 

 the commercial values found in the drill holes may, for purposes of 

 preliminary estimates, be used for estimating the deposit. Occasionally, 

 the geophysical work is of value in indicating the nature of the deposit 

 and may throw light on whether the deposit is of primary or secondary 

 origin. 



The most familiar geoelectrical result is that obtained over a highly 

 conductive ore body of the sulphide type. When an electric current is 

 passed through an area containing a conductive ore body, a decided 

 change in values will be noted when in the vicinity of the ore body. A 

 typical example is illustrated in Figure 342, which shows the general 

 surface conditions and the locations of the resistivity stations. The oxi- 

 dized portion of a mineralized fracture zone between the diabase and 

 the diorite porphyry outcropped at A. This vein showed good values 

 in copper but was too narrow for economical mining, and no assurance 

 could be had that commercial quantities of sulphide ores would be found 

 at depth. A geoelectrical study of the area gave the results shown by the 

 curves on the left of the figure. Because of the lower resistivity of the dia- 

 base, curve B starts with a lower initial value than either A or C. Because 

 of the relatively low conductivity of the diorite porphyry, curve C has a 

 higher initial apparent resistivity value than curves A or B, and maintains 

 this higher value over the entire range of measurement. Under the geo- 

 logic conditions existing in this district, it appeared most logical to assume 



