Geological Core Samples: Logging, Assay and Cost Guide

A geological core sample is a solid cylinder of rock cut by a diamond-tipped bit, logged by a geologist, sawn in half, and sent to a laboratory for assay (a chemical test that measures how much metal the rock holds). One half goes to the lab and the other half stays in the box as the permanent record. At the RC Gold project in Yukon, a July 2026 NI 43-101 technical report budgets all-in diamond drilling at C$400 per metre in summer and C$700 per metre in winter, and its NQ core is sawn in half and assayed for gold by 50 g fire assay plus a 51-element ICP package. This page follows that same path step by step, with the sizes, the quality checks, the costs and a budget calculator.

It is written for claim owners, consultants, investors and mining-company staff who need to read a drill report, brief a contractor or budget a sampling programme. Where a number comes from one project, the text says so; where a price varies by lab and country, the calculator takes your own quote.

Mineral sampling methods compared: core, chips, channels and soil

Mineral sampling means taking a small, representative piece of ground and measuring what it contains. The method you choose sets what you learn, how deep you see and what a sample costs. Diamond core is the standard method that keeps an intact column of rock, so a geologist can read depth, structure, veins and alteration directly. Rock chips from reverse circulation (RC) drilling are faster and cheaper per metre but lose that structure. Surface samples (soil, stream sediment, rock chips and grab samples) are the cheapest way to decide where to drill at all.

The same ladder applies in every country: start with cheap surface samples, drill RC where the geology is simple, and use diamond core where you need structure, depth or a sample that can support a resource estimate. If you are choosing a rig, our guide to exploration drilling rigs and what each type samples compares rig types and costs.

Core sizes and how much rock you handle

Core comes in standard wireline sizes named by letter. The most common in exploration are NQ (47.6 mm core diameter) and HQ (63.5 mm), according to the standard size table for exploration diamond drilling. Smaller BQ (36.5 mm) and AQ (27 mm) core is lighter and cheaper to drill, while PQ (85 mm) gives the most rock per metre. Larger core costs more per metre but gives a bigger, more representative sample, which matters when gold or other metals sit in a few coarse grains. Our comparison of BQ, NQ, HQ and PQ core sizes goes further into hole diameter and rig choice.

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Weight of one metre of core

Kilograms per metre at a density of 2.7 t/m3

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Standard wireline core diameters

Core (inside) diameter, millimetres

The next chart sets the core diameters side by side. The chart after it shows what that means in weight: a metre of whole NQ core weighs about 4.8 kg, so a 1,000 metre programme produces roughly 4.8 tonnes of core, and half of it (2.4 tonnes) goes to the laboratory. Those weights are calculated from the core diameters in the table, assuming a rock density of 2.7 tonnes per cubic metre, which is typical for many hard rocks; heavy sulphide ore weighs more.

From drill to core box: recovery and logging

Core comes up the rods in runs of a few metres. The driller and a geologist fit the pieces back together in order, mark the depth on a block at the end of each run and place the core in a tray. Two numbers describe how good the run was. Total core recovery (TCR) is the length of core brought up divided by the length drilled. Rock quality designation (RQD) adds up only the sound pieces longer than 100 mm and divides by the run length, which makes it a measure of how broken the rock is; the definitions are summarised in a reference on core recovery parameters. High-quality rock has an RQD above 75 per cent and poor rock below 50 per cent.

Recovery matters for grade as well as for geology. If soft, altered or fractured zones wash out and only hard waste rock comes up, the assay no longer represents the ground. At the RC Gold project the Sitka drilling averaged 96.4 per cent recovery at the Rhosgobel zone over 4,765 core runs, with the poorest zones tied to sulphide oxidation near the top of holes and to faults. That is the pattern to expect: the weak ground is the ground that loses core, and it is where a geologist should read the log with most care.

After recovery the core is logged for rock type, alteration, structure, mineralisation, TCR and RQD, then photographed in the tray before anything is cut. The logging sheet and the photographs are the permanent record. In the Sitka programme the core was logged, tagged and photographed on site before it was cut.

How core goes from rig to laboratory in six steps

The order of work is the same on almost every project, and skipping or reversing a step is what causes disputes about a result later.

Infographic

From rig to laboratory in six steps

1. 1Drill and recoverCore is pulled in runs and fitted back together in depth order.
2. 2Log and photographRock type, structure, TCR and RQD are recorded and every tray is photographed.
3. 3Mark intervalsSample breaks follow rock type, veins and faults, each with a numbered tag.
4. 4Saw and bagOne half goes in the tagged bag; the other half stays in the tray.
5. 5Insert controlsStandards, blanks and duplicates go in the sequence before shipping.
6. 6Prepare and assayThe lab crushes, splits, pulverises and runs fire assay or ICP.

How core is split and sampled for assay

The geologist marks the intervals to sample. They are not a fixed length: breaks follow changes in rock type, faults, veins and zones of alteration. At the RC Gold project, 2021 sample intervals ran from 20 cm to 4 m on NQ core and were generally 2 m, with shorter intervals over narrow veins and faults. Each interval gets a tag with a unique number that follows it to the laboratory.

The core is then sawn lengthwise with a diamond saw. One half goes in a bag with the tag; the other half goes back in the tray as the permanent record. Where a field duplicate is wanted, the half is sawn in half again and each quarter is bagged with its own tag. Splitting along the same line each time, such as always keeping the left half, avoids a systematic bias when veins run at an angle to the core axis. Whole-core sampling is used only when the core is too small or too broken to cut, since it destroys the record.

Keep the sawn half. The half left in the tray can be re-logged, re-sampled or sent to a second laboratory years later, and the standard exploration reference for diamond drilling notes that half of the split core is assayed while the other half is stored for re-assay.

What a laboratory does with a core sample

Laboratory work has two parts: preparation, which turns a few kilograms of rock into a fine, even powder, and analysis, which measures the metal. At the RC Gold project the laboratory crushed each sample to 70 per cent finer than 2 mm, split off 1 kg and pulverised it to 85 per cent finer than 75 micrometres, then ran a 51-element ICP package and a gold fire assay on a 50 g charge (ALS method Au-ICP22). Samples over 10 g/t gold (grams of gold per tonne of rock) were re-run by fire assay with a gravimetric finish, according to the technical report’s sample preparation and analysis section.

Fire assay is the reference method for gold. The sample is mixed with a lead-based flux and heated above 1,000 degrees Celsius so the gold collects in a lead button; the button is then cupelled to leave a gold bead, which is weighed or dissolved and measured. ALS lists 30 g and 50 g charges, with the ICP-finish methods covering 0.001 to 10 ppm gold and the gravimetric finish covering 0.05 to 10,000 ppm. A larger charge samples more rock per test, which helps when gold is coarse. Base metals such as copper, zinc or nickel, and critical minerals such as lithium, are usually measured by acid digestion with an ICP finish or by fusion, and uranium and rare earth elements use their own packages. For methods, turnaround and price ranges by commodity, see our guide to assay services for mining.

Accredited labs publish their preparation steps. The Saskatchewan Research Council’s geoanalytical laboratory, for example, states that it works to ISO/IEC 17025 and lists crushing to 70 per cent passing 10 mesh (1.70 mm), riffle splitting and pulverising about 250 g as its standard rock preparation. Always ask your lab for its preparation specification in writing, because crush size and pulp size change how well the sub-sample matches the original.

QA/QC: standards, blanks and duplicates

Quality assurance and quality control (QA/QC) is the set of checks that tells you whether assays can be trusted. Three kinds of control sample are inserted into the stream without the laboratory knowing which is which:

• Certified reference material (a standard): a powder with a known, certified grade. A result that drifts from the certificate shows the lab is biased.
• Blank: barren rock or gravel. A blank that returns metal shows contamination during crushing or analysis.
• Duplicate: a second sample from the same interval, such as a quarter core, a coarse reject or a pulp. Poor agreement shows the sampling or the rock is highly variable.

Insertion rates differ by project. At the RC Gold project, standards and blanks were alternated at every tenth sample in the diamond drill programmes, and the earlier RC programme used rig duplicates every 15th sample and gravel blanks every 20th. A CIM Magazine article by AMEC geologists says a QA/QC programme usually adds no more than one to two per cent to total exploration costs, and warns that laboratories tend to report the checks that pass their own controls but not the failures. Their conclusion is that relying only on the laboratory’s internal QC is not acceptable practice, so the project should add independent external QC checks of its own.

The practical rule for a claim owner: ask to see the control charts for any assay set you rely on, and ask what happened to batches that failed. A report that gives high grades but no control-sample results is hard to compare with one that does.

What core drilling and sampling cost

Drilling is the biggest line and varies most by country, season, access and rig type. The RC Gold project gives one documented example. Its Phase I budget lists 60,000 m of diamond drilling at an all-in rate of C$400 per metre for summer drilling and C$700 per metre for winter drilling at Blackjack, for a total of C$25.8 million. The report states that the all-in rate includes helicopter support, fuel, analytical work, camp costs, mobilisation and equipment rental, so the assay bill is already inside that figure. A contractor quoting a bare metre rate is quoting something different. Our guide to diamond drilling in Canada covers permits, seasons and contractor rates in more detail.

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Phase I drilling budget by target

C$ million, all-in cost, 60,000 m total

The chart shows how that Phase I budget splits between targets. It is one project, not an average; remote Arctic or alpine ground is more expensive than road-accessible ground, and a drill programme in a place with a developed supplier base can cost far less per metre.

For your own budget, count samples rather than metres. At 2 m intervals, 1,000 m of core makes 500 core samples; adding a control sample at every tenth position adds roughly 50 more. Each sample pays for sawing, bagging, freight, preparation and analysis, and the analysis fee depends on the lab and the element package, so use your lab’s current written quote. Use the calculator below to compare the drilling bill with the assay bill.

Reverse circulation and the drilling sample system

With reverse circulation drilling, compressed air pushes down the outer tube and brings broken rock up through the inner tube. At the surface the cuttings pass through a cyclone, a chamber that slows the air stream and lets the chips drop out, and then through a splitter that cuts the sample down to a bag of a few kilograms. The mass and the grade of that sample depend on the splitter and the dryness of the ground.

The choice of splitter has been tested. In a conference paper to the Australasian Institute of Mining and Metallurgy, Catto and Church found that a cone splitter gave a better split by particle size, more consistent duplicate masses and lower gold scatter than a tiered riffle splitter, and that either type must be held vertical. Wet samples are the common failure: they clog the splitter and smear grade between intervals. If the rig is drilling below the water table, ask how the contractor keeps samples dry or how wet samples are flagged in the log.

Plan the rest of the drilling sample system with the same care: numbered bags matched to the log, a duplicate taken from the splitter reject at a set interval, a field blank, and a bag seal or chain-of-custody sheet that travels with the samples to the lab.

Soil sample drills and small core drills

Soil sampling is the cheapest way to find out where to drill. Where the soil is thin, a trowel and shovel are enough. In the CSIRO UltraFine+ protocol a sample of about 200 g is taken from roughly 2 to 10 cm depth, with material coarser than 2 mm sieved out. Where cover is deeper, a hand-held or vehicle-mounted auger (sometimes called a soil sample drill) takes the sample from the target horizon. The target is usually the B horizon, the mineral layer below the topsoil, and regional surveys such as a USGS B-horizon soil survey of northern Minnesota use that layer.

Small core drills sit at the light end of the range. They take AQ or BQ core (27 mm and 36.5 mm) or use smaller portable machines, which makes them easier to move into steep or roadless ground. The trade-off is sample mass: a metre of half BQ core is about 1.4 kg against 2.4 kg for NQ, so a few coarse gold grains can swing the assay more. Use small core for geological mapping, shallow tests and structure, and repeat or enlarge the sample where grade is erratic.

Storing core and keeping the data

Core is evidence. Store it dry, off the ground, in labelled trays with depth blocks, and keep the logs, photographs, assay certificates and QC charts in one place. Some governments hold public core: the USGS Core Research Center in Denver is a public access repository holding cores from more than 9,800 wells and cuttings from more than 53,000 wells, and visitors who run their own analyses must return the results. In Western Australia, the Geological Survey runs the Perth and Joe Lord core libraries for drill core from mineral and petroleum exploration. Check with the regulator in your area before discarding core, because some jurisdictions ask for samples or reports.

▶ How a mineral management system keeps maps, samples and targets in one place. · 3:21

Choosing hole positions before you drill

Every metre of core costs money, and the cheapest hole is the one you do not drill in barren ground. Satellite imagery cannot replace core: it cannot measure grade at depth. It can map surface alteration, iron staining, clay minerals and structural lineaments over a whole licence in 5 to 20 business days, so surface sampling and the first holes go where the signals agree. The results are exploration targets that sit ahead of a resource estimate, not a resource under NI 43-101 or JORC. Farmonaut offers this kind of screening through its satellite-based mineral detection service, and our write-up of modern drilling technology covers the rig side.

If you want a quote for your licence, you can ask through the mining query form or the contact page.

▶ Why a hole in the wrong place costs the same as one in the right place, and how satellite screening helps. · 4:26

Frequently asked questions

What is mineral sampling?

Mineral sampling is taking a small, representative piece of soil, rock or drill material and measuring its metal or mineral content, so a project can decide where to drill, where the grade is high and whether a deposit is worth developing.

What is ore sampling?

Ore sampling is the same process applied to rock already known to be mineralised. It uses drill core, RC chips, channels cut across veins and, at operating mines, blast-hole and conveyor samples to estimate grade for resource models and for mine control.

What is geological sampling?

Geological sampling covers all the ways of collecting rock, soil or sediment for study: grab samples on outcrop, soil and stream sediment grids, channel and chip samples, and drill samples. The choice depends on cost, depth and what the sample has to prove.

How are geological core samples taken?

A rig turns a hollow diamond bit that cuts a cylinder of rock into an inner tube, which is pulled up on a wireline. The core is laid out in trays, logged, photographed, sawn in half and one half is bagged for the laboratory.

Why is core cut in half?

Half goes to assay and half stays in the tray as a record that can be re-logged or re-sampled later. Cutting also gives a fresh surface that shows veins and mineralisation clearly.

What is a drilling sampling system?

It is the chain from the bit to the lab bag: the core barrel or the RC cyclone and splitter, the numbering and tagging of bags, the control samples and the sealed transport. Weak links usually appear at the splitter and at tagging, not at the bit.

What are drill sample chambers?

The term describes the cyclone and sample box on an RC rig where cuttings drop out of the air stream before they are split. The chamber must be clean between samples to avoid carry-over from one interval to the next.

What drill is used for soil sampling?

Hand augers and trowels work in thin soils; powered augers, hand-held or on a vehicle, reach deeper cover. The sample is usually taken from a set horizon at a set depth on every site so the results can be compared.

What is a small core drill used for?

A small core drill takes AQ or BQ core (27 mm or 36.5 mm) or similar light-rig core for shallow holes, tight sites and early exploration. It is easier to move but gives less rock per metre than NQ or HQ.

How much does a core sample cost?

The cost is mostly the drilling. The RC Gold project budget uses C$400 per metre in summer and C$700 per metre in winter, all-in, which includes the analytical work. A bare assay fee is a small part of that and depends on the lab and the element package.

How many QC samples should be inserted?

There is no single rule. One documented diamond programme alternated a standard and a blank at every tenth sample. The AMEC article in CIM Magazine puts the whole QA/QC programme at one to two per cent of exploration spend, so check the project’s own protocol and the chart results.

How long should core be kept?

Keep it for as long as you rely on the assays: for the life of the project, plus the time needed for any audit or sale. Rules vary by jurisdiction, so check with the mining regulator before discarding any.