Subsurface Methods as Applied in Mining Geology 979 



Geology may aid in finding ore, may show that ore exists, and may indi- 

 cate a certain size of deposit, but to satisfy the investor and, incidentally, 

 the geologist, the deposit must be explored and samples taken. Depend- 

 ing upon the kind and size of deposit, the exploration work may consist 

 of trenching, stripping, sampling from drill holes, or underground min- 

 ing operations. 



Trenching and Stripping — ^As is often the case, the surface outcrops 

 may be covered at critical spots with vegetation or rock detritus. If the 

 cover is not too great, structure, rock, and possible ore of any type may 

 be uncovered by a trench dug across areas of interest. Trenches are 

 treated as outcrops, and the location and geology are shown on the sur- 

 face maps. If the cut exposes ore, sampling across the ore zone may be 

 necessary; in which case a uniform sample can be taken either by deepen- 

 ing the trench or sampling the wall of the trench. It should be remem- 

 bered that the exposed rock is near the surface and most surface ores and 

 rocks have been attacked and changed by ground waters. 



Overburden can also be removed with power machinery, most com- 

 monly with a bulldozer. ^^ A good operator can economically remove 

 small shrubs and trees and several feet of overburden in a very short 

 time. Such exposures can be mapped as surface outcrops and sampled 

 by taking cuts at random, on a grid, or by trenching at a known angle 

 across the general trend of the exposed structure. In sampling an area 

 uncovered with a bulldozer, caution must be exercised to be certain that 

 incompletely removed soil does not dilute the sample. 



Drilling — Auger and Hammer Holes: Drill holes put down from 

 the surface can explore greater depths than can be reached by trenching 

 or stripping. If relatively shallow holes in soft material such as clay or 

 concentrator tailings are needed, a hand auger is satisfactory. Two men 

 can easily drill 50 feet in this type of material, and 100-foot holes are 

 not uncommon. In auger drilling, as it is possible to recover 100 percent 

 of the cuttings, the material removed is an accurate sample of the hole. 

 Depending upon the depth of hole and character of cuttings, samples may 

 be taken at regular or irregular intervals, or the entire hole may be con- 

 sidered as one sample. 



Short holes from the surface to prospect any type of deposit may 

 be put down by pneumatic hammers to a depth of 30 to 50 feet. Sampling 

 this type of hole is more diflficult in that the cuttings must either be washed 

 out if drilling is done wet or must be blown out if drilling is done dry. 

 In either case, difficulty may be encountered in removing and collecting 

 the cuttings for accurate sampling. 



Churn-Drill Holes : Churn drilling ^^ is a common, noncoring method 

 of prospecting by means of vertical surface holes and is effective for ex- 

 ploring relatively widespread sedimentary deposits, disseminated epigen- 



*' New Prospectors in the Hills: Mining World, vol. 11, no. 3, p. 38, Mar. 1949. 

 **McKinstry, H. E., op. cit., pp. 171-182, 1948. 



