Core Recovery

Core recovery describes how much drilled material has been recovered as core under a stated measurement convention. It helps assess the completeness of the physical record and identify intervals requiring investigation. It does not, by itself, establish rock-mass quality, sample representativeness or the reliability of an ore-grade estimate. These questions need additional observations and checks.^[1][2]

The distinction matters because missing material may be geologically significant. A small loss concentrated in a weak or mineralised zone can be more consequential than the same total loss distributed through otherwise well-understood ground. Recovery should therefore be examined by interval and geological context, not only as a hole-wide percentage.^[3]

Defining the Measurement Basis

For length-based recovery over a drilling run, the basic calculation is:

Recovery (%) = 100 × recovered core length / drilled run length

Both lengths must use the same unit, and the drilled run length must be greater than zero. The numerator represents the recovered material attributed to that run under the project's documented measurement procedure. The denominator is the length drilled during the run, not the capacity of the core barrel or the distance between convenient box labels.^[1][3]

The metric is linear, not a direct measurement of recovered mass or volume. It does not establish that all constituents were retained in their original proportions. The procedure needs to explain how broken or fragmented core is measured and reconstructed. Empty spacing or loss markers in a box are not recovered material and cannot be included in the numerator.

Recovery has other meanings elsewhere in mining. Mining recovery concerns extraction of material from the mining design, while processing recovery concerns a component recovered into a product. Neither is the quantity measured by the drilling-run calculation above. Identifying the basis prevents a percentage from acquiring a meaning it was not designed to express.

Measuring and Recording Runs

Run boundaries connect the drilling record to the box layout. Records should identify the hole, depth origin, start and end depths, recovered length, missing material and relevant drilling or handling events. Reconciliation with drilling records is important before interpreting an apparent loss or gain.^[3]

Fitting broken pieces can restore their physical sequence where matches are reliable. It must not erase evidence of breakage or justify assigning an uncertain fragment to a precise position. If material is known to be absent but its location within the run is unknown, the record should distinguish the known amount of loss from the unknown position. A conventional loss marker at a run boundary is a storage convention, not proof that the loss occurred there.^[3]

Material intentionally removed later for analysis or testing is another category. The original recovery record should not be retrospectively reduced because a specimen has been sampled. Instead, the box and archive record should identify what was removed, from where, for which test and under which sample identifier. Drilling loss and later consumption of recovered material represent different events.^[3]

Combining Runs Without Hiding Local Loss

An aggregate length-based recovery uses the sum of recovered lengths divided by the sum of corresponding drilled lengths. Runs must cover non-overlapping intervals on a consistent measurement basis, without counting the same recovered material twice. The result is equivalent to weighting each run's recovery by its drilled length. A simple average of percentages can give the wrong result when run lengths differ.

For illustration only, consider one run of 1.00 m returning 0.80 m, and another of 3.00 m returning 2.85 m. Their individual recoveries are 80% and 95%. Combined recovery is 100 × (0.80 + 2.85) / (1.00 + 3.00) = 91.25%, rather than the unweighted mean of 87.5%. These are hypothetical lengths used solely to demonstrate the calculation, not acceptance criteria.

Even a correctly weighted result can obscure the location of a problem. The combined percentage does not show which lithology was lost or whether either run contains a critical contact. Aggregation should therefore retain access to the run-level record and any geological subdivisions used in interpretation.^[3]

Investigating Loss and Apparent Gain

Loss may arise from ground conditions, drilling equipment or technique, retrieval, handling, or errors in depth and length measurement. Weak, weathered or intensely fractured material may behave differently from competent rock. Reclamation's guidance also discusses apparent gains associated with material retained from a preceding run, measurement problems, swelling or expansion of the recovered material.^[3]

An apparent recovery above 100% should therefore trigger reconciliation rather than automatic deletion or silent capping. Original measurements and subsequent corrections should remain distinguishable. Any reassignment to a previous interval needs evidence; a mathematically convenient allocation is not proof of provenance. When material from an earlier run is recovered later, the revised depth attribution and run totals must be reconciled together so the same core is not credited twice.^[3]

Likewise, low recovery does not establish its own geological explanation. Drilling behaviour, photographs, adjacent core and other investigations may support competing interpretations. Where the missing portion could change an engineering decision, the unresolved possibilities need explicit treatment rather than replacement by a confident lithological description.

Recovery, Rock Quality and Sample Bias

Rock Quality Designation, or RQD, uses the lengths of sound core pieces meeting the adopted procedure’s length criterion, divided by the drilled run length rather than the recovered length. It is not total core recovery. Treatment of mechanical breaks depends on identifying them reliably; fitting a natural fracture together does not make it a drilling-induced break. A run can have high recovery but consist largely of short natural fragments; the two observations describe different aspects of the recovered material.^[1]

Neither metric independently proves sample representativeness. Selective loss can affect weak minerals, fines or particular geological zones. Depending on what is lost, the surviving sample may overstate or understate the constituent of interest; the direction cannot be inferred from the recovery percentage alone. JORC Table 1 specifically calls for consideration of relationships between recovery and grade and of preferential loss or gain of fine or coarse material.^[2]

Analytical quality control tests another part of the evidence chain. Repeatability alone does not establish absence of analytical bias. Even an accurate analysis of the submitted material cannot demonstrate that the material represents the complete drilled interval. Missing core is not an assay result of zero, and a precise assay does not supply the composition of the missing portion. Rescaling an assay by length recovery alone cannot correct an unknown compositional bias.^[4]

Using Recovery in Engineering Decisions

The review should identify which decisions depend on affected intervals. For structural work, loss may remove a discontinuity surface; for geotechnical assessment, it may conceal weak material; for mineral estimation, it may affect sample support and grade interpretation. The same percentage can therefore have different consequences for different uses.

A defensible response links the observed problem to its likely mechanism, documents unresolved uncertainty and assesses whether further evidence is needed. Changes to drilling or handling can improve subsequent information, but cannot recreate material already lost. Acceptance should be justified by the investigation purpose and evidence, rather than an unsupported universal recovery target.^[4][3]