Mineral resource estimation is the step where scattered drill holes and assays become a statement of tonnes and grade that investors, lenders and mine planners can use. It is also where the most expensive mistakes in mining begin. A resource is not “all the mineralisation we found”. The JORC Code calls it “a realistic inventory” of material with reasonable prospects for eventual economic extraction, estimated from specific geological evidence, and it must be classified by how confident the estimator is.

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This guide follows the path of a mineral resource estimation from exploration data to a Measured resource. It leans on three primary documents: the JORC Code 2012 edition, the CIM Definition Standards that Canada’s NI 43-101 incorporates, and the CIM best practice guidelines for estimation. Where the codes use slightly different words, we say so. For the reporting side, see our JORC Code explainer.

“A Mineral Resource cannot be estimated in the absence of sampling information.” JORC Code, Clause 20

What a mineral resource is, and what it is not

JORC Clause 20 defines a Mineral Resource as “a concentration or occurrence of solid material of economic interest in or on the Earth’s crust in such form, grade (or quality), and quantity that there are reasonable prospects for eventual economic extraction.” The CIM Definition Standards, adopted on May 10, 2014, use near-identical wording. Three parts of that definition do most of the work.

• ✔ Specific geological evidence. Location, quantity, grade and continuity must be known, estimated or interpreted from evidence that includes sampling. No samples, no resource.
• Reasonable prospects for eventual economic extraction (RPEEE). JORC glosses this as “more likely than not”. Material with no such prospects must be left out, and the basis for the assumption, including the cut-off, must be disclosed.
• “Eventual”. The time horizon depends on the commodity. JORC notes that for some bulk commodities such as coal, iron ore and bauxite, eventual extraction may reasonably cover more than 50 years.
• ⚠ Not ore. JORC Clause 28 bans the words “ore” and “reserves” for resource estimates, because those words imply feasibility has been shown.

One more rule shapes every resource statement you will read. Clause 25 says estimates “are not precise calculations” and should be rounded to reflect their uncertainty, usually to two significant figures. The Code’s own example: 10,863,000 tonnes at 8.23 per cent becomes 11 million tonnes at 8.2 per cent.

The mineral resource estimation workflow, step by step

The CIM Estimation of Mineral Resources and Mineral Reserves Best Practice Guidelines, adopted on November 29, 2019, lay out the work in a sequence that most estimators follow whatever the code they report under. The order matters in any mineral resource estimation: everything after step 3 inherits the quality of the database and the geological model.

Steps 1 to 3: data and geology

The guidelines put the database and the geology first for a reason. Collar and downhole surveys place every sample. Logging records which rock each sample came from. QA/QC proves the assays are right. Bulk density, which CIM calls “as important a part of the MRMR estimation as determination of the volume or grade”, turns volume into tonnes. The geological interpretation then decides the shape of the mineralised body, and no interpolation method can correct a wrong shape.

Steps 4 to 6: domains, statistics and continuity

Estimation domains split the deposit into zones whose samples belong to one population, such as oxide and fresh rock, or a high-grade shoot and its halo. Composites of uniform length go into each domain, and composites must not cross domain boundaries. Outliers are handled by capping (top cutting), by restricting the reach of high assays, or by methods such as indicator kriging. Variograms then describe how quickly grade similarity decays with distance in each direction, which later sets search ellipses and kriging weights.

Steps 7 to 9: estimate, check, classify

The block model interpolates grade into regular blocks. CIM lists polygonal, nearest neighbour, inverse distance, several kriging methods and conditional simulation, and says the choice should follow geology, data and end use. Our guide to polygon, triangle and cross-section methods works through the classical ones. Validation compares the model with the drill data visually, globally and locally (swath plots), and against production where it exists. Only then does the estimator assign categories.

Inferred, Indicated and Measured: what each category needs

Classification is the part of mineral resource estimation that readers see most. The three categories describe confidence, not size. JORC and CIM use the same verbs to separate them, and those verbs are the most useful thing to remember from this article: geological evidence is enough to imply continuity for Inferred, to assume it for Indicated, and to confirm it for Measured.

Two further rules matter for anyone reading an Inferred-heavy statement. JORC expects that “the majority of Inferred Mineral Resources could be upgraded to Indicated Mineral Resources with continued exploration”, but that is an expectation, not a promise. Where an Inferred estimate is extrapolated beyond the drilling, JORC Clause 21 requires the report to state the maximum extrapolation distance, the share of the resource based on extrapolated data, the basis for it, and a diagram showing the extrapolated part.

Classification is not purely about drill spacing. The CIM standards describe a case where sampling and testing are good enough to show continuity for Indicated or Measured, but the QA/QC does not meet industry norms for those categories. There, they say, it may be reasonable for the Qualified Person to report the material as Inferred instead. Weak assay checks can hold back a mineral resource estimation as surely as wide drilling can, which is one more reason to get QA/QC right from the first soil sample.

Converting all of this into a statement is simple arithmetic, and the calculator below does it with the rounding the codes expect.

What it takes to report a measured mineral resource

A measured mineral resource is the top of the confidence ladder. JORC Clause 23 and the CIM standards both say its quantity, grade, densities, shape and physical characteristics are estimated with enough confidence to support detailed mine planning and final evaluation of economic viability. The evidence must be detailed and reliable enough to confirm continuity between points of observation.

The test in the guidance is strict. JORC says material may be classified as Measured when the data leave “no reasonable doubt, in the opinion of the Competent Person”, that tonnage and grade can be estimated “to within close limits”, and that any variation would be unlikely to affect economic viability significantly. CIM adds that the category “requires a high level of confidence in, and understanding of, the geology and controls of the mineral deposit.”

In practice, Measured usually means

1. Closer drilling than the Indicated parts of the same deposit, often infill or grade-control spacing.
2. Mapped geological controls: the estimator knows which structures and rock types carry grade.
3. Density, recovery and QA/QC data that match the confidence claimed.
4. For operating mines, reconciliation showing the model predicts what the mill receives.

Neither code sets a drill spacing for Measured in a mineral resource estimation, and that is deliberate. The right spacing depends on the deposit’s continuity, which the variograms describe. A drill spacing that proves continuity in a flat coal seam may prove nothing in a nuggety gold vein.

Gold resource estimation: nuggets, capping and sample size

Gold resource estimation is harder than mineral resource estimation for most other commodities, for one physical reason: gold often sits in a small number of discrete grains. Two samples a metre apart can differ by an order of magnitude, and a single coarse grain in a small aliquot can make an ordinary sample look spectacular.

That is why gold is assayed on bigger samples than other elements. In the lab procedures lodged with the Northern Territory Geological Survey, fire assay uses a 30 g or 50 g charge, against 0.25 g for a four-acid digestion. The same document warns that coarse, malleable native gold is “not representatively characterized” by small samples. Estimators then deal with the remaining high values by capping, by limiting how far a high assay can influence nearby blocks, or with indicator kriging, all options named in the CIM guidelines.

National statistics show what a mature gold inventory looks like. Geoscience Australia’s AIMR 2025 reports that at 141 operating gold mines, as at 31 December 2024, Measured and Indicated resources held 7,256 t of gold (inclusive of 3,369 t in Ore Reserves), with a further 2,666 t Inferred. Mine production in 2024 was 284 t.

Gold-specific checks on any estimate

• ✔ Charge size and method: fire assay on 30 g or 50 g, or screen fire assay where gold is coarse.
• Field duplicates: how well do repeat samples of the same interval agree? Poor repeatability is a nugget warning.
• Capping level and its effect: how much metal does the cap remove, by domain?
• ⚠ High-grade influence: is any single hole driving a large share of the ounces?

“Australia’s 141 operating gold mines produced 284 tonnes of gold in 2024.”

If you are still choosing where to drill for gold, our satellite gold exploration guide explains how targets are ranked from imagery before any assay exists.

Who signs a mineral resource estimation, and how often

A mineral resource estimation is only as credible as the person who takes responsibility for it. Both major codes name that person and set minimum standards, and both put the company’s board, not the consultant, in charge of what is published.

The JORC Competent Person

Under JORC Clause 11, a Competent Person must be a Member or Fellow of the AusIMM, the Australian Institute of Geoscientists or a Recognised Professional Organisation on the list kept on the JORC and ASX websites. Those bodies must have enforceable disciplinary processes, including the power to suspend or expel a member. The person needs at least five years of experience relevant to the style of mineralisation and to the activity. For a mineral resource estimation, that means experience in the estimation, assessment and evaluation of Mineral Resources. Clause 9 requires the company to name the Competent Person, say whether they are an employee, and publish the report only with their written consent to its form and context.

The NI 43-101 Qualified Person

Canada’s National Instrument 43-101 uses a Qualified Person: an engineer or geoscientist with at least five years of experience in exploration, mine development or operation, or project assessment, relevant experience in the subject matter, and good standing with a professional association that can discipline its members. Section 2.1 requires every disclosure of scientific or technical information on a material property to be prepared by, supervised by or approved by a Qualified Person. Our NI 43-101 guide covers the Qualified Person rules and technical reports.

How often estimates are refreshed

JORC Clause 15 requires companies to review and publicly report their Mineral Resources and Ore Reserves annually, to nominate the review date, and to show the effective date of each statement. Material changes since the previous statement must be discussed. In practice, a mineral resource estimation is rerun whenever enough new drilling arrives to change the model, and at least once a year for listed Australian companies.

Reading a resource statement: a mineral estimation checklist

Most people meet mineral estimation as a table in a press release. The table is the end of a long chain, and a few checks reveal most of what matters.

1. Which code, and who signed it? A JORC statement names a Competent Person, who needs at least five years of relevant experience and membership of AusIMM, AIG or a Recognised Professional Organisation. An NI 43-101 statement names a Qualified Person.
2. Effective date. A resource is a snapshot of one drill database and one set of price and cost assumptions.
3. Cut-off grade and its basis. JORC requires the RPEEE discussion to include the technical and economic support for the cut-off.
4. Inclusive or exclusive? Are Measured and Indicated reported inclusive of reserves or additional to them? The statement must say.
5. Categories kept apart. NI 43-101 section 2.2 says an issuer must not add Inferred resources to the other categories. JORC likewise forbids adding reserves to resources.
6. Rounding. Precision should match confidence.

For a sense of how the categories typically split, the same Geoscience Australia table can be expressed as shares of each commodity’s total resource at operating mines:

Geoscience Australia refreshes these tables each year with data as at 31 December, so use the latest edition rather than this snapshot. If you are raising money on a resource, our guide to finding mining investors covers what they will ask to see.

Where satellite targeting fits before mineral resource estimation

Every mineral resource estimation needs drill holes, assays and a qualified estimator. Nothing replaces them. What can be improved is where the first holes go. Our satellite-based mineral detection analyses multispectral and hyperspectral imagery, where each mineral and alteration zone has its own spectral signature, and flags likely mineralised target zones, alteration halos and structures before anyone disturbs the ground.

• ✔ What you send: coordinates, a KML/KMZ file or a polygon, plus the target mineral.
• What you get: prospectivity heatmaps, high-potential zones with estimated location and depth ranges, indicative quantity, geological interpretation, and PDF plus georeferenced GIS files, in 5 to 20 business days.
• Premium+: TargetMax™ drilling intelligence with drilling-angle recommendations and interactive 3D subsurface models.
• Why do it first: up to 80 to 85% lower early-exploration cost and timelines cut from months to days, compared with starting on the ground.
• ⚠ The limit, stated plainly: our reports describe exploration targets. Any quantity in them is conceptual. They are not Mineral Resources under JORC, CIM or NI 43-101, they are not public reports under any code, and no Competent Person or Qualified Person signs them.

Start at mining.farmonaut.com (Map Your Mining Site) or ask for a quote through the mining query form. The mineral exploration guide places targeting, drilling and estimation in one sequence, and the satellite-driven 3D mineral prospectivity mapping overview shows a sample deliverable. We have scanned 100,000+ hectares for 20+ mineral types across 25+ countries.

Key terms

RPEEE

Reasonable prospects for eventual economic extraction: the test every Mineral Resource must pass, “more likely than not” in JORC’s words.

Estimation domain

A volume of the deposit whose samples share one grade population and are estimated together.

Compositing

Combining assay intervals into equal lengths so samples have similar support before estimation.

Capping (top cutting)

Reducing extreme assays to a set value so a few outliers do not drive the estimate.

Variogram

A model of how differences in grade grow with distance between samples, used to set kriging weights and search ranges.

Competent Person / Qualified Person

The named, accountable professional under JORC and NI 43-101 respectively.

Frequently asked questions

Reviewed September 2026 against the JORC Code 2012 edition, the CIM Definition Standards for Mineral Resources and Mineral Reserves (May 10, 2014), the CIM Estimation of Mineral Resources and Mineral Reserves Best Practice Guidelines (November 29, 2019), the ALS procedures lodged with the Northern Territory Geological Survey, and Geoscience Australia’s AIMR 2025.

Code wording is quoted from the editions named above; the JORC Code is under review, so check jorc.org for the edition in force. National figures are Geoscience Australia’s as at 31 December 2024 and are updated annually. Satellite targets are exploration targets, not mineral resources or ore reserves. This article is general technical information, not investment advice.