Diamond core drilling uses a diamond bit, which rotates at the end of drill rod (or pipe) (Figure 16 – 1).  The opening at the end of the diamond bit allows a solid column of rock to move up into the drill pipe and be recovered at the surface.  Standard core sizes are 7/8 inch (EX), 1 3/16 inch (AX), 1 5/8 inch (BX) and 2 1/8 inch (NX).  Most drill rods are 10 feet long.  After the first 10 feet is drilled, a new section of pipe is screwed into the top end, so the combination of pipes can be driven another 10 feet into the ground.  The diamond bit is rotated slowly with gentle pressure while being lubricated with water to prevent overheating.  The drill depth is estimated by keeping count of the number of drill rods. 

Figure 16 – 1.  Diamond bit.

The driller listens to the drill very carefully to evaluate the condition of the drilling below.  He will adjust the rotation speed, pressure and water circulation for different rock types and drilling conditions so as to avoid problems, such as getting the bit stuck or overheated.  Highly fractured rocks (often encountered near the surface), in addition to the risk of a stuck bit, allow the fluids to escape, leading to overheating.  The problem is minimized by injecting “drilling mud” (or sawdust or other materials) into the drill hole to “plug” the fractures and prevent escape of the fluids.

Inside the drill pipe is a “core tube”, which has a latching mechanism attached to a cable.  At the end of each 10 foot run, the cable is used to winch the core tube containing the new rock core to the surface where it can be recovered.  The drill core is stored in especially designed core boxes containing compartments to hold sections of the core.  Standard core boxes are 2.5 feet long and contain four compartments, so ten feet of core can be stored in each box. 

The drill core is first washed and logged by a qualified geologist, and then split in half to provide a sample for geochemical analysis.  Since so much time, effort and money is expended to obtain the drill core, it is worthwhile to study and log the core very carefully.  A standardized log form is used to log the core.  The form has columns for each of the types of information which will be recorded, with tick marks indicating the footage.  The information typically shown includes the recovery %, lithology, alteration, mineralization, rock quality data (RQD), and structural details.  Although the strike and dip of planar features such as bedding, foliation, faults and veins is not known, the angle of these features with respect to the axis of the drill core axis is noted, because it still provides valuable information about the geometry of the features.  Mineral testing may also be done, including testing for fluorescence (scheelite), testing for effervescence with dilute HCl (carbonate alteration), or mineral staining (feldspars or carbonates).  Often the core will be photographed as well.  The recovery % is the ratio of the actual core length compared to the drill interval indicated.  Voids and fracture zones cause poor recovery.  For example, if a drill run of 10 feet obtains 8 feet of drill core, the recovery is 80 %.  The drill logs used to construct drill sections (cross sections showing the drill holes) which illustrate the subsurface geometry of the ore body.  The current trend is to create drill logs in digital or spreadsheet format, which facilitates the construction of drill sections by computer. 

Core splitting is done using a rock saw or an impact core splitter.  There is always the problem of obtaining a representative split of the core.  Great care must be taken to avoid this problem.  Sometimes the entire core is analyzed to avoid this problem, but usually only if logging is extremely thorough.  In some cases a series of small chips are collected along the length of the core to form a “skeleton core” for archiving purposes.