Subsurface Methods as Applied in Mining Geology 981 



careful driller who is more concerned with core recovery than with footage 

 drilled will recover a high percentage of core even in bad ground. Re- 

 covery in bad ground is important, because so often ore is found associ- 

 ated with fractured and altered rock, and the purpose of most drilling is 

 to obtain representative samples for assay and study. 



The geologist records all possible information from a diamond-drill 

 core. He not only logs geologic data such as depths to contacts, rock 

 descriptions, ore descriptions, and inclination of bedding, structural 

 planes, schistosity, and veins, but he also notes the conditions of the core 

 and the amount of core recovery. The last two observations may indicate 

 zones of structural weaknesses. The desired information obtained from 

 diamond drilling includes dip, strike, width and grade of the ore body, and 

 structural and stratigraphic data. Obviously, one drill hole will not yield 

 all the desired information. Under suitable circumstances with sufl&cient 

 drill holes it is possible to compute the attitude of veins, faults, and bed- 

 ding.^2 With the dip and strike known and the inclination of the hole 

 measurable, the width or thickness of veins and beds can be determined. 

 The assumption that drill holes have the same inclination at depth as at 

 the collar may lead to incorrect calculations. Drill holes deviate for a 

 number of reasons; and, if deviation is suspected, a drill-hole survey may 

 be necessary.^^ ^^ Many deposits are of irregular grade and thickness, and 

 the variation and inaccuracy in sampling can be minimized by closer spac- 

 ing of drill holes. 



To sample the material from diamond-drill holes, both the sludge 

 and the core are assayed, and, if warranted, a weighted average can be 

 calculated which is computed on the basis of the relative amounts of 

 sludge and core. Very seldom is all of the core assayed. Most commonly 

 the core is split longitudinally with one half going to the assay office and 

 the other half being retained for study and record. 



In laying out an exploration program, limitations of the diamond 

 drill should be understood; if they are, diamond drilling offers a rela- 

 tively fast and inexpensive means of exploring unknown ground. For an 

 effective program, a trained geologist should be on the job. The geologist 

 certainly should see that the general drilling schedule is carried out, that 

 the driller is recovering the maximum amount of core, that the driller's 

 records are accurate, and that the samples are properly handled and 

 tagged. In addition, the geologist should have authority to modify the 

 program to fit the pattern developed or changed by the continued drilling 

 and increased geologic knowledge. 



Calyx-Drill Holes: A method somewhat similar to diamond drilling 

 is shot drilling (calyx drilling), in which chilled-steel shot is used for 

 cutting rather than a diamond bit. Calyx holes are vertical but can be 

 drilled up to five feet in diameter. Since calyx holes are drilled vertically, 



'^ McKinstry, H. E., op. cit., pp. 100-103, 1948. 

 "' Forrester, J. D., op. cit., pp. 409-411, 1946. 

 ^* McKinstry, H. E., idem, pp. 97-98, 1948. 



