Exploration & Drilling

Finding and defining ore: geological mapping, geochemistry, geophysics (magnetics, gravity, IP, EM), drilling methods (RC, diamond core, air-core), core recovery, sampling and length-weighted compositing, and QA/QC with standards, blanks and duplicates.

Exploration turns a geological idea into a drilled, sampled and assayed dataset that supports a resource estimate. Drilling is expensive ($100–$500/m), so programmes are designed carefully and every sample must be trustworthy.

An analogy

Drilling is like taking a few biopsies of a patient: each core is a tiny sample, and the diagnosis (resource model) is only as good as the samples.

Exploration toolkit

Mapping and rock-chip sampling; soil and stream geochemistry; geophysics — magnetics (magnetite), gravity (dense bodies), induced polarisation (disseminated sulphides), electromagnetics (conductive massive sulphides); remote sensing for alteration minerals.

Drilling methods

Air-core/RAB: cheap, shallow, soft rock. Reverse circulation (RC): chips returned inside the rod, fast and cheap, good for grade. Diamond core: intact cylinders (NQ 47.6 mm, HQ 63.5 mm) for geology, structure, density and geotechnics — slowest but most informative.

Sampling and compositing

Samples are split at geological contacts and fixed intervals (1–2 m), crushed, pulverised and assayed (fire assay for gold, ICP for base metals). Composites of equal length prepare data for estimation.

QA/QC

Certified reference materials (standards), blanks (contamination) and duplicates (precision) are inserted at ≈ 5 % each; check assays at an umpire lab. Poor QA/QC is a common cause of resource downgrades and investor losses (e.g. Bre-X).

Formulas

• Core recovery: CR = recovered length / drilled length × 100
• Length-weighted composite: ḡ = Σ(l_i g_i) / Σ l_i
• Grade-thickness: GT = ḡ × true thickness

Worked example: Length-weighted composite

Given: 2 m @ 1.5 %, 3 m @ 0.8 %, 1 m @ 2.4 % Cu

Find: Composite grade.

• Σ l g = 3.0 + 2.4 + 2.4 = 7.8
• Σ l = 6 m → ḡ = 1.30 %

Answer: 6 m @ 1.30 % Cu (simple average 1.57 % would be wrong)

Worked example: Core recovery

Given: 3.0 m run, 2.7 m core recovered

Find: CR.

• CR = 2.7/3.0 = 90 %

Answer: 90 % — acceptable (poor recovery biases grades)

Common mistakes

• Wrong: Averaging grades without length weighting. — Right: Use ḡ = Σlg/Σl.
• Wrong: Skipping QA/QC samples. — Right: Insert standards, blanks, duplicates in every batch.

Applications

• Resource drilling: Infill drilling upgrades inferred to indicated.
• Grade control: RC drilling ahead of pit mining.
• Geotechnical drilling: Oriented core for slope and tunnel design.

Key takeaways

• Geochem + geophysics → drill targets.
• RC for grade, diamond for geology.
• Composite by length weighting.
• QA/QC protects resource credibility.

Going further

• Downhole geophysics: EM and televiewer surveys extend hole information.
• Directional drilling: Wedging and steering for deep targets.
• Assay bias analysis: Statistical tests of standards and duplicates.