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Sampling & Assay Practitioner

Core Sampling & Splitting: Half-Core, Quartering & Why It Matters

Core sampling is the process of dividing drill core into representative pieces and sending one portion to the laboratory for assay. Here's how to split core correctly, choose sample intervals, and avoid the mistakes that compromise your data.

Core sampling is the process of dividing logged drill core into measured intervals, splitting each interval to create a representative sub-sample, and dispatching one portion to an accredited laboratory for geochemical analysis. The retained half stays in the core box as the permanent geological record. Every resource estimate, every intercept announcement, every mine plan ultimately traces back to whether the samples collected from core were truly representative of the rock they came from.

This guide covers the full sampling workflow — from where it sits in the logging sequence, through the split methods available and when to use each, to interval length decisions, QAQC insertion, and the marking and labelling practices that keep chain of custody intact. If you haven’t yet read How to Log Drill Core, Step by Step and Lithology Logging: Identifying & Recording Rock Types, complete those first — sampling follows logging, and interval boundaries must align with the lithological contacts you picked during the logging pass.

Why sampling decisions are irreversible

Every assay result your project ever reports is the product of a sequence of decisions made at the core table: where to start and end an interval, how to split the core, which half to send to the lab, and whether a QAQC sample went in next to it. None of these decisions can be fully undone after the fact.

If you split a sample incorrectly — taking a biased cut that captures more mineralisation on one side — the assay result is wrong. If you sample across a lithological contact rather than respecting it, the result mixes two different geological populations into one number. If you forget to insert a blank and the lab has a cross-contamination event, you won’t know. If you send the wrong half to the lab and retain the other, and then later cut a quarter-core duplicate — the duplicate will be drawn from a different face than the original, making the comparison meaningless.

This is why core sampling is treated as a technical activity, not a physical one. It requires the same disciplined adherence to procedure as the logging that precedes it.

Where sampling sits in the core handling workflow

Sampling comes after the full geological log is complete — after recovery and RQD have been measured, after photography, after lithology, alteration, mineralisation, and structural logging. The geologist defines the sample intervals during or immediately after logging; the physical splitting and tagging can then be done by a trained technician under supervision.

The sequence within a sampling run:

  1. Mark sample intervals on the core directly in permanent marker, noting from/to depths at each interval break
  2. Confirm contact alignment — each interval break must align with a geological contact or mineralisation boundary from the log; no interval should straddle two rock types unless there is a deliberate reason
  3. Insert QAQC markers in the tray at the planned insertion points before cutting begins
  4. Split the core using the method appropriate to the project and deposit type
  5. Tag each sample half with the unique sample number; bag, seal, and label the sample half
  6. Complete the sampling sheet (or database entry): sample number, hole ID, from, to, length, split method, and QAQC insertions
  7. Dispatch to laboratory with a sample submission form; retain a copy

The three core split methods — and when to use each

Diamond saw (most common)

A diamond-blade saw cuts the core longitudinally through its axis, producing two halves of similar cross-sectional area. This is the standard method on most exploration programs for good reasons: it produces a clean, flat cut surface, it works across all rock types and hardnesses, and it is fast enough to keep pace with an active drilling program.

Practical notes:

  • Use a water-lubricated blade at all times; dry cutting damages the blade and generates dust
  • The saw should be cleaned between holes and whenever contamination is a concern
  • One drawback: the sawing action generates fine cuttings (up to 200 g per metre of NQ core), and visible gold in the core can be lost to the blade or the water slurry — not a material concern in most base metal or disseminated gold deposits, but significant in coarse-gold systems
  • On particularly soft or friable core (some clays, friable sulphides), the saw can disaggregate material; in these cases a core splitter may be preferable

Mechanical core splitter (chisel splitter)

A mechanical splitter uses a hardened steel wedge to split the core along a plane of weakness. It requires no water, no power, and generates no cuttings — meaning no gold is lost in saw slurry.

When to prefer it:

  • Soft, clay-rich, or friable core that the saw would disaggregate
  • Projects in remote locations without reliable power
  • Where the loss of fine gold to saw cuttings is a demonstrable concern

Drawbacks: Mechanical splitting is slower than sawing, requires individual pieces of core to be removed from the tray and re-inserted one by one (increasing the risk of misalignment or mislabelling), and the split plane follows natural weaknesses in the rock rather than the core axis — meaning the split is sometimes uneven. In schistose or strongly foliated rocks, the split tends to follow foliation rather than dividing the core symmetrically.

Whole core sampling (entire interval crushed)

In some situations the entire core interval — rather than a split half — is sent to the laboratory for crushing and analysis. This is appropriate when:

  • Coarse visible gold is present. Coarse gold particles (sometimes called “nuggety gold”) are so unevenly distributed through a core interval that any longitudinal split will place the particle disproportionately in one half. Sending the whole interval eliminates the split bias and produces a far more representative result. Research on high-nugget gold systems confirms that the effect of segregation during splitting can multiply sampling variance by a factor of 20–100×.
  • Metallurgical test material is required. Metallurgical test programs sometimes require the full mass of a core interval to meet the minimum sample size for a particular test.
  • Very small diameter core. In BQ core (27.7 mm diameter), half-core intervals may not provide enough mass for reliable analysis; the whole core is sometimes submitted instead.

Whole core sampling destroys the reference record for that interval. It should be reserved for specific, documented reasons, and photography of the core before crushing is mandatory.

Half-core vs quarter-core: when does quartering apply?

The default is half-core — one half to the lab, one half retained. Quarter-core arises in two situations:

Check assays and duplicates. Once a primary sample has been assayed and the result is in hand, a quarter-core duplicate can be taken from the retained half and submitted to the same or a different laboratory. The quarter-core duplicate tests the reproducibility of the original assay. Because it comes from the remaining half of the original split, it is drawn from a different face than the primary sample and will capture somewhat different material — that’s part of what the check is measuring.

Metallurgical sub-sampling. A program may take the full half-core for metallurgical testwork and then split a quarter from the remaining portion for geochemical assay.

Quarter-core should not be used as the primary sample method unless there is a compelling mass or material reason. Quarter-core provides half the sample mass of half-core, which reduces representativeness. In low-grade disseminated deposits this is usually acceptable; in heterogeneous or high-nugget systems it is not.

The critical consistency rule: always sample the same face

Whichever half you send to the lab must be consistent throughout a hole — and ideally throughout the program. This rule sounds simple. It is routinely violated.

If a technician sometimes takes the left half and sometimes the right half, the retained pieces of core will not fit back together. Cross-sections comparing core face to face become meaningless. Re-assay of the retained half is compromised because the relationship between the two halves at each interval is unknown.

Standard practice: mark the retained half before cutting. Place a small arrow or angled line on the core at each sample interval boundary on the face that will be retained, with the arrow pointing downhole. This mark survives the cut and makes it unambiguous — after splitting, the arrow is on the archive half; it goes back in the box.

Some programs also paint the cut face of the retained half with a consistent colour (commonly red) to distinguish it from the sampled face. Whatever the convention, it must be documented in the project’s sampling protocol and followed by every person who touches the core.

How to set sample intervals

Sample interval decisions are made by the geologist, not the technician. The geologist defines the interval breaks in the log; the technician marks and cuts them. Intervals are defined by:

Geological contacts

The most important rule in sample interval selection: a new sample interval must start at each lithological contact. A single interval must not straddle two different rock types — if it does, the resulting assay mixes two geological populations into one number, and neither can be interpreted cleanly.

This means some intervals will be shorter than the target length because a contact falls at an awkward depth. That’s correct — the geological boundary takes priority over the target length.

Target interval length

Most programs use a target interval length in the range of 1–2 metres, with common defaults of 1 m or 2 m. The appropriate length depends on:

FactorShorter intervalsLonger intervals
Mineralisation styleNarrow high-grade veins (≤ 0.5 m true width)Broad disseminated mineralisation
Grade variabilityHigh variability over short distancesRelatively uniform grade
Ore body geometryStructurally controlled, discontinuousStratiform or tabular
Project stageResource definition drillingRegional reconnaissance

For narrow vein systems — where a vein of 20 cm width may carry most of the gold in the hole — 1 m or even 0.5 m intervals may be necessary to isolate the vein material from the surrounding waste. For a bulk disseminated porphyry copper deposit, 2 m intervals are standard.

Minimum interval length

Most programs set a minimum sample length (commonly 0.3 m or 0.5 m). This avoids extremely short samples that may be unrepresentative due to low mass, and prevents a proliferation of tiny intervals that slow assay turnaround and increase cost without adding meaningful information.

Barren zones

Not every metre of a hole needs to be sampled. In clearly barren, unaltered waste rock well away from any mineralised zone, some programs use wider sampling intervals or even skip sampling entirely (recording it as “not sampled” with the geologist’s justification). This practice is acceptable when the decision is geological and documented — it is not acceptable as a cost-cutting shortcut in zones of genuine interest.

QAQC insertion: blanks, standards, and duplicates

No sampling batch leaves the core shed without QAQC samples inserted at regular intervals. QAQC insertion is mandatory, not optional — it is the mechanism by which you detect contamination, analytical drift, and sampling bias. JORC (Table 1, Section 2) and NI 43-101 (Form 43-101F1) both require disclosure of QAQC procedures in public reporting of exploration results.

The three QAQC sample types:

Blank samples are clean material with negligible concentrations of the target elements — crushed granite or coarse silica sand are commonly used. A blank inserted into the sample stream tests for contamination introduced during laboratory sample preparation. A blank that returns above detection limit for the target element is a contamination alert.

Certified Reference Materials (CRMs / standards) are commercially prepared samples with assayed concentrations published by the manufacturer. Inserting a CRM into a batch and comparing the returned value to the certified value tests laboratory accuracy. CRMs should be chosen to bracket the anticipated grade range in the program — a low-grade, mid-grade, and high-grade CRM ideally.

Field duplicates are a second split taken from the same core interval as the primary sample, submitted blind to the laboratory. A field duplicate tests sampling precision — how much of the variation between the original and the duplicate comes from the splitting process rather than laboratory measurement. For half-core sampling, the duplicate is typically a quarter-core split from the retained half.

A common insertion rate is one of each type per 20 samples, meaning QAQC makes up roughly 10–15% of any given batch. The insertion order within a batch should be randomised, and the laboratory should not be told which samples are QAQC until after results are received.

Labelling, bagging, and chain of custody

Each sample interval requires:

  • A unique sample number (pre-printed tags, sequential numbering, no gaps)
  • The sample number written on the core tray at the interval boundaries
  • A sample tag inside the bag and another on the outside
  • A sealed, tamper-evident bag
  • A completed dispatch sheet listing every sample number, the hole ID, from/to depths, and the laboratory submission date

The chain of custody is the unbroken record of where a sample has been from the moment it was cut to when the assay certificate is issued. Any gap or undocumented transfer in that chain introduces uncertainty about whether the sample received by the laboratory is the same material that came out of the ground. Most exploration companies use a numbered chain-of-custody form that travels with the sample batch, is countersigned at each handover point, and is retained with the project records.

Common sampling mistakes — and what they cost

Sampling across a lithological contact. An interval that straddles granite and mineralised schist will return an assay that represents neither. The result is uninterpretable and is useless for resource estimation. It cannot be fixed after the fact.

Inconsistent face selection. Taking the left half in some intervals and the right half in others (due to technician convenience or a poorly marked core) means the archive half is no longer a coherent geological record. Subsequent duplicate sampling or check assays on the retained half are compromised.

Losing fine material in saw water. In coarse-gold or visible-gold intervals, gold fines lost to the saw’s water slurry may represent a material portion of the sample’s gold content. Programs in these settings should either switch to a mechanical splitter or whole-core sample the mineralised intervals.

Skipping QAQC insertions during busy periods. There is always a temptation to skip a blank or standard when the drill is running fast and the batch needs to get to the laboratory. The problem is that QAQC events — a contamination incident, a calibration drift at the lab — happen at the worst times. A batch without QAQC samples cannot be validated after the fact.

Intervals that are too long in high-grade systems. A 2 m sample interval that straddles a 0.3 m high-grade vein will dilute the vein’s grade by roughly 6:1 against the surrounding waste rock. The assay result will be low. The high-grade zone won’t show up. You may drill past a deposit without knowing it. In vein-hosted deposits, sample intervals need to be short enough to isolate mineralised structures.

Not photographing core before whole-core sampling. Whole-core intervals are destroyed. If they aren’t photographed beforehand — with depth marks and scale visible — there is no visual record of what was submitted. Photography before whole-core sampling is non-negotiable.

FAQ

What is core splitting and why is it done? Core splitting is the process of dividing a drill core interval longitudinally into two or more portions. It is done because sending the entire core to the laboratory for crushing would destroy the geological record permanently. By splitting the core and retaining one half in the core tray, the project keeps a physical archive that can be re-logged, re-sampled, or displayed to investors and regulators indefinitely while the other half is consumed by geochemical analysis.

When should I use a core saw vs a mechanical splitter? A diamond core saw is appropriate for the vast majority of programs — it produces a clean, consistent cut across hard and moderately competent rock, and it is fast enough to keep pace with active drilling. A mechanical (chisel) splitter is preferable for soft, friable, or clay-rich core that a saw blade would disaggregate, and for remote locations without reliable power. For coarse gold or visible gold mineralisation, consider whole-core sampling rather than either splitting method, to avoid losing fine gold to saw cuttings.

What is the difference between half-core and quarter-core sampling? Half-core sampling sends one longitudinal half of a core interval to the laboratory, retaining the other half in the tray. Quarter-core sampling takes a further split of the retained half — typically for check assays, QAQC duplicates, or metallurgical sub-sampling. Quarter-core should not be used as the primary sampling method unless the core diameter or project design specifically calls for it, because the reduced mass decreases representativeness.

How do I set sample interval lengths? Sample intervals are set by the geologist based on geological contact positions (every interval must start and end at a contact) and a project-specific target length — commonly 1 m or 2 m, adjusted shorter for narrow high-grade veins and wider for broad disseminated systems. Most programs set a minimum interval length of 0.3–0.5 m. The target length should be documented in the project sampling protocol before drilling begins.

What QAQC samples go into a core sampling batch? A standard batch includes blanks (to detect contamination), certified reference materials or CRMs (to monitor analytical accuracy), and field duplicates (to assess sampling precision). A common insertion rate is one of each type per 20 primary samples, giving roughly 10–15% QAQC content per batch. All QAQC insertions and their positions within the batch should be recorded in the sampling database.

How Blue Butterfly supports core sampling workflows

Core sampling is a data-generation step — and the quality of the data depends entirely on the rigour of the decisions made at the core table. Blue Butterfly is built so that the sampling record, the geological log, and the QAQC insertion log all live in the same database, linked by hole ID and depth interval.

Sample intervals are entered against the validated lithology log, so interval breaks that cross a geological contact without justification are flagged before the batch leaves the shed. Sample numbers are assigned sequentially from the project registry — no duplicate numbers, no gaps. QAQC insertions are recorded in the sampling run with their type (blank, standard, duplicate) and their position in the dispatch sequence, creating an auditable QAQC record that exports directly to the assay validation workflow.

When the laboratory returns results, the assay data imports to the same database and attaches to the sampling intervals already on file — no manual matching, no spreadsheet lookup. The complete chain from geological interval to validated assay is intact and queryable from the first drill turn.

See how Blue Butterfly handles sampling and assay data →

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