A diamond drill rig can recover a perfect, intact rock core — but that core only becomes useful data once it’s been sampled correctly. Core sampling is the step where geology turns into numbers: grade, mineral content, and the chemical assays that ultimately justify (or rule out) further investment in a project. Get it wrong, and even a technically flawless drilling program produces unreliable results.
This article walks through how core sampling works in practice, from the moment core comes out of the drill to the point it’s ready for assay sampling at the lab.
What Is Core Sampling?
Core sampling is the process of selecting, dividing, and preparing sections of drill core so they can be analyzed for grade and composition without destroying the entire record of the hole. Because a core sample is also the only physical evidence of what exists at depth, sampling has to strike a balance: enough material has to be removed for accurate laboratory analysis, while enough of the original core is preserved for future reference, re-assay, or verification.
This is a deceptively difficult balance. Take too little material and the assay may not be representative; consume the entire core and there’s nothing left to check the result against, or to revisit if a deposit is reevaluated years later.
How Drill Core Is Sampled
Once a core has been recovered, washed, and conditioned, it’s divided lengthwise along its axis — most commonly into two halves. One half is sent for assay, and the other is retained in the core tray for future reference, verification, or additional testing. This is the essence of core splitting: preserving a complete physical record while still enabling destructive lab analysis.
A few technical parameters govern how this is done correctly:
- Sample interval length typically shouldn’t exceed 1.5 meters or fall below 20 cm, keeping individual assay samples manageable and representative.
- Consistent interval length matters especially when the data will feed geostatistical studies — comparing grades across a deposit only works if sample lengths are consistent across the hole and between different holes.
- Recovery percentage is tracked closely throughout: recovery below roughly 75% is generally considered a red flag, since it can introduce significant error into how the deposit is ultimately evaluated. Missing core means missing (or unrepresentative) data.
From Core to Assay-Ready Sample: The Preparation Steps
Once a core sample has been split and selected for assay, it goes through a defined preparation sequence before it ever reaches a chemical analyzer:
- Drying — removes moisture and organic material so the sample is stable for chemical analysis.
- Crushing — reduces particle size, typically down to a coarse mesh, to make the material manageable and homogenous.
- Splitting — reduces the overall sample quantity without compromising its representativeness, ensuring the smaller portion carried forward still reflects the composition of the original material.
- Pulverizing — grinds the sample down to a fine mesh appropriate for chemical analysis, maximizing surface area and ensuring homogeneity.
- Storage and labeling — the prepared sample (and any retained pulp or reject material) is stored and coded correctly to prevent contamination and preserve traceability.
Each of these steps exists to solve the same underlying problem as core splitting itself: reducing the sample down to something a lab can analyze accurately, without losing the representativeness of the original material.
Where Core Sampling Commonly Goes Wrong
Assay sampling is only as reliable as the discipline behind it. Some of the most common sources of error include:
- Poor definition or delimitation of the vein or mineralized zone being sampled.
- Improper handling that alters or contaminates the sample.
- Damaged or poorly maintained sampling equipment.
- Inadequate labeling or bag/tray identification, leading to mix-ups.
- Human error in recording sample location or depth.
- Insufficient cleaning of equipment between samples, introducing cross-contamination.
Most of these failure points have nothing to do with the drilling itself — they happen in the handling, labeling, and storage steps that surround the actual sampling. Which is exactly why the physical container the core sits in throughout this process matters as much as the sampling protocol itself.
Why the Core Tray Is Part of the Sampling Chain
Every stage of core sampling — splitting, selecting intervals, tracking recovery, retaining reference halves — depends on the core staying in correct order and correctly labeled from the moment it leaves the barrel until it’s logged, split, and sent for assay. A tray that shifts, mixes up compartments, or degrades in the field puts all of that discipline at risk.
Coreplast’s core trays are built with this in mind: standard HQ, NQ, and PQ formats with separators that keep split core halves in order, so the retained reference half stays exactly where the geologist expects it — even after the assay half has been removed and sent to the lab. A tray that holds up through washing, splitting, handling, and long-term storage isn’t a minor detail in the sampling chain; it’s what keeps the sampling record intact and trustworthy over the life of a project.
FAQs
1. Why is a core sample typically split into two halves instead of sampled whole?
Splitting the core preserves a complete physical record while still allowing destructive lab analysis. One half is sent for assay sampling, while the other remains in storage for future reference, re-assay, or verification — a balance that would be impossible if the entire core were consumed during testing.
2. What’s the difference between core splitting and the sample preparation done at the lab?
Core splitting happens at the drill site or logging facility, dividing the physical core into a retained half and an assay half. Sample preparation happens afterward, at the lab, and includes drying, crushing, splitting (cuarteo) to reduce quantity, and pulverizing — steps that convert the assay half into a homogenous sample small enough for chemical analysis.
3. How does core recovery percentage affect the reliability of a core sample?
Recovery percentage measures how much of the drilled interval was actually retrieved as core. When recovery drops below roughly 75%, the missing material can introduce significant uncertainty into grade estimates, since the recovered core may no longer be fully representative of the interval drilled.
