Once diamond drilling cores have been recovered, cataloged, and stored safely in core boxes, the work of describing the cores begins. One important part of this work is the description and measurement of geologic structures.
Structures such as faults and cleavage planes serve as conduits for fluids moving underground.These fluids carry and deposit many of the minerals sought by miners.
In order to understand the dynamics of mineralization in a region, a detailed understanding of the local geologic structures is required. Identifying these structures is not always easy given the small diameter of a typical diamond core.
The best way to learn to recognize structures is to gain experience identifying them. Nevertheless, there are some basic rules that can make the work easier:
– Structures look different in a round drill core than in cross section or outcrop. If a planar feature is perpendicular to the core axis, is will appear as a circle. If a feature is parallel to the core axis, its trace appears as a straight line. Of course, most features fall somewhere in between these orientations, resulting in an elliptical trace.
– Identification of folds is not always easy. In the case of small wavelength folds, it might be possible to identify the fold axis within the core sample: the orientation of the axis can be determined by measuring the angle of a line passing through the fold axis on either side of the core.
Often, however, the fold axis cannot be seen within the core. When this is the case, the general orientation of the fold can be resolved by identifying repeating units, which occur when both limbs of the fold intersect the core.
More accurate identification of the core can occur through the measurement of cleavage planes, which form perpendicular to the axial plane of a fold. If they can be identified in the core, their orientations can be used to calculate the orientation of the axial plane.
– Faults can be identified by the presence of fault gouge or extensive fracturing along a planar surface. However, it is not usually possible to identify fault orientation using a single core.
-In order to asses the amount of void space underground, it is critical that accurate labels of the drilling depth represented by a core are kept on the core trays. If the final length of the core is shorter than the drilling distance it represents, void space must have been present in the original rock body.