Question 7 of 7: Diamond-Core Drilling in Mineral Exploration

Question 7: Diamond-Core Drilling in Mineral Exploration (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Diamond-core drilling: method, conditions, advantages and disadvantages

In diamond-core drilling, a rotating, diamond-impregnated annular bit cuts a cylindrical plug ("core") of rock rather than grinding the whole hole face to cuttings; the core is captured intact inside a core barrel, periodically brought to surface (conventionally, or continuously via a wireline system that retrieves the inner barrel without pulling the whole drill string), and the borehole is kept cool and clear of cuttings by continuously circulating drilling fluid (water or mud) down the drill rods and back up the annulus. The method is most effective in COMPETENT, cohesive rock at essentially any depth or dip, and is the standard choice whenever an oriented, geologically-intact sample is required for structural, geotechnical or detailed geological logging purposes.

Its principal ADVANTAGES are that it recovers a continuous, oriented, geologically intact rock sample – the only drilling method that preserves true structural (dip/strike, vein-width, contact) information – and that assay results from solid core are the most reliable and least contamination-prone sample available. Its principal DISADVANTAGES are that it is slow and expensive per metre drilled compared with rotary/RC methods, it performs poorly in loose, broken, or highly fractured/unconsolidated ground (core loss and poor recovery), and it requires a significant volume of circulating fluid and a larger drill rig footprint, both of which can be problematic in water-scarce or environmentally sensitive/remote terrain.

b) Drilling parameters and their effect on the overall project

1. Hole diameter (core/bit size). A larger diameter recovers more sample volume per metre (better for metallurgical testwork and for confidence in erratic, coarse-gold-style mineralization) but drills more slowly and costs more per metre; a smaller diameter is faster and cheaper but risks poorer recovery in broken ground and provides less material for check assays or duplicate testwork.
2. Hole depth/length. Determines which rig class, rod strength and drilling-fluid capacity are needed; longer holes are progressively more prone to deviation (see below) and cost rises non-linearly with depth as deeper holes need higher pump pressures and more trip time to change bits.
3. Hole orientation (azimuth and dip/inclination). Must be planned to intersect the target structure(s) as close to PERPENDICULAR as practical – a hole drilled sub-parallel to a vein or bed either misses it or grossly over-estimates its true (drilled) thickness, so orientation directly controls both the probability of intersecting the target and the accuracy of any thickness/grade estimate derived from the core.
4. Drill spacing/pattern (grid density). Wider spacing is cheaper per unit area covered but yields lower geological confidence between holes; the required spacing tightens as the project moves from reconnaissance through to a resource category (Inferred → Indicated → Measured) that can support mine planning, per NI 43-101/CIM guidelines.
5. Downhole deviation. No hole drills perfectly straight; deviation must be regularly surveyed (downhole survey tools) and accounted for when plotting collar-to-toe hole traces, since an un-surveyed or badly deviated hole can systematically mislocate intersections and corrupt the 3-D geological model built from it.
6. Drilling fluid (type and circulation rate). Fluid cools the bit, flushes cuttings, and stabilizes the borehole wall; too little circulation causes bit overheating and hole collapse, while fluid loss into permeable/fractured ground can be costly, can contaminate downhole geochemical/hydrogeological readings, and (in environmentally sensitive settings) raises the project's water-use and containment obligations.

Every one of these parameters ultimately trades DATA QUALITY against COST and SPEED – the drilling program is designed by working backward from the resource-confidence category the project needs to achieve, and only then selecting the diameter, spacing, and orientation that deliver that confidence at the lowest defensible cost.