How to Read Assay Results: Grades, Intercepts and Units Explained

What is an assay?

An assay is a laboratory analysis that measures how much of a particular element is present in a sample of rock. In exploration, samples come from drill core or drill chips, split into intervals typically between half a metre and two metres long. Each interval is crushed, pulverised and analysed. The result is a grade: the concentration of the metal, expressed as a percentage, in grams per tonne, or in parts per million. Companies then group consecutive intervals into the "intercepts" you see in an RNS announcement.

Anatomy of a drill intercept

Hole XD-014

the drill hole's identifier. The release's collar table should give its location, azimuth (compass direction) and dip (angle).

14.2 m

the length of core over which the grade is averaged. This is downhole length unless the release says it is true width.

@ 1.35% Sn

the length-weighted average grade across those 14.2 metres.

from 312.0 m

where the interval starts, measured along the hole. This is not vertical depth unless the hole is vertical.

including 3.6 m @ 4.10% Sn

a higher-grade zone inside the headline intercept. It is already counted in the 14.2 metres, so never add the two together.

The "including" figure tells you a lot. In this example, the 3.6 m zone holds about three-quarters of the metal in the whole intercept. The remaining 10.6 m averages only about 0.42% Sn. A headline intercept can look broad and consistent when most of its metal sits in one narrow zone.

Grade units and conversions

Different metals are reported in different units, depending on how concentrated they usually are.

Element or oxide?

Many strategic metals are reported as oxides rather than as the pure element, because that is how they are traded or how the industry has always quoted them. For the same rock, the oxide grade is always higher than the element grade. So before comparing two companies, make sure both figures are on the same basis. Tin is the main exception: it is usually reported as Sn.

To go from oxide to element, divide by the factor. For example, 0.80% WO₃ is about 0.63% W.

Reading rare earth results: TREO, NdPr and the basket

Rare earth results are usually given as TREO, total rare earth oxides: the sum of the individual rare earth oxide grades, in ppm or %. Check whether the company includes yttrium oxide in its TREO figure. Practice varies, and including it can noticeably lift the total.

TREO on its own can mislead, because the rare earths differ enormously in value. Lanthanum and cerium often make up most of a deposit by weight but are worth relatively little. Neodymium and praseodymium (NdPr), used in permanent magnets, and the heavy rare earths dysprosium and terbium usually carry most of a deposit's value. Look for NdPr as a percentage of TREO, or a "magnet rare earth oxide" (MREO) figure, and check how the company defines it.

Remember too that an assay tells you what is in the rock, not whether it can be recovered economically. The host mineral matters: monazite, bastnäsite, xenotime and ionic clays each need very different processing.

Downhole length versus true width

Drill holes rarely cross mineralisation at right angles. When a hole cuts a zone at an angle, the length of core recovered is longer than the zone is thick. True width is roughly the downhole length multiplied by the sine of the angle between the hole and the zone. A 14.2 m intercept crossing a zone at 45° represents a true width of only about 10 metres. A good release states the estimated true width, or says plainly that it is not yet known.

Grade × width: comparing intercepts fairly

A short, very high-grade intercept can contain less metal than a longer, moderate one. Multiplying grade by width gives a metal accumulation: metre-percent (m%) for grades in %, or gram-metres (g·m) for grades in g/t. Our example, 14.2 m @ 1.35% Sn, gives about 19.2 m%. A flashier 2.0 m @ 6.0% Sn gives 12.0 m%: a higher grade, but less metal. Use true width where you can. Also bear in mind that very narrow zones may be diluted with waste rock when mined, which lowers the grade that reaches the plant.

Cut-off grades and how intercepts are built

Intercepts are built by grouping consecutive samples above a cut-off grade. Usually a limited amount of lower-grade material is allowed inside the interval ("internal dilution"). Lower the cut-off and the intercept gets longer and lower grade; raise it and the intercept gets shorter and richer. The same hole can therefore be reported in several honest ways. A reputable release states its cut-off and dilution rules, and any top-cut applied to extreme high-grade values.

Metal-equivalent grades

Deposits with more than one valuable metal are often summarised as a single "equivalent" grade, such as SnEq, CuEq or AuEq. The by-product metals are converted into the main metal using assumed prices and, ideally, assumed metallurgical recoveries. Check which prices were used, whether recoveries were applied, and how much of the equivalent grade comes from by-products. An equivalent grade can change substantially when metal prices move.

Sample types: not all results are equal

• Diamond core: a continuous cylinder of rock. The most reliable sample type.
• Reverse circulation (RC): rock chips brought up by air. Cheaper and faster, with some risk of contamination between intervals.
• Channel samples: cut continuously across an exposed rock face. Reasonably representative if taken properly.
• Rock chip and grab samples: individual pieces, often picked because they look mineralised. Useful for deciding where to drill, but not representative of grade. "Grades of up to…" usually refers to the single best sample.

How laboratories analyse samples

Samples are crushed, split and pulverised before analysis. The right method depends on the metal:

• Fire assay with an atomic absorption or gravimetric finish: the standard for gold and platinum group metals.
• Four-acid digestion with ICP analysis: common for base metals and multi-element suites.
• Fusion methods with XRF (fused bead) or peroxide fusion: used for tin and tungsten, because cassiterite and wolframite resist acid digestion and acid-based methods can under-report them.
• Lithium borate fusion with ICP-MS: the usual approach for rare earths and other refractory elements.

The release should name the laboratory and the method. Independent laboratories accredited to ISO/IEC 17025 are the norm for credible results.

Quality control: standards, blanks and duplicates

Good exploration programmes insert control samples into every batch sent to the laboratory:

• certified reference materials (standards of known grade), to check accuracy;
• blanks (barren material), to detect contamination;
• duplicates, to measure how repeatable the results are.

Some companies also send a share of samples to a second "umpire" laboratory. A well-prepared release describes the QA/QC performed and whether the results passed.

Reporting codes and the Competent Person

Exploration results and resources are reported under codes in the CRIRSCO family:

• JORC (Australasia, widely used by AIM companies);
• NI 43-101 (Canada);
• PERC (Europe);
• SAMREC (South Africa);
• S-K 1300 (United States).

AIM-quoted companies also follow the AIM Note for Mining and Oil & Gas Companies. It requires technical information in announcements to be reviewed by a suitably qualified person, and resources to be reported to a recognised standard. Releases name this person and their professional body. Under JORC, the accompanying "Table 1" sets out sampling, drilling and QA/QC details, and is often the most useful part of the announcement for due diligence.

From exploration results to resources and reserves

Exploration results describe individual drill holes. A Mineral Resource is a geological estimate of tonnage and grade with reasonable prospects for eventual economic extraction. It is classified as Inferred, Indicated or Measured, in increasing order of confidence. A Reserve is the part of a resource shown by technical and economic studies to be mineable, classified as Probable or Proved (Proven under NI 43-101). A strong intercept matters most in context. Ask whether it sits inside or outside the existing resource, and whether its grade is above or below the resource average.

Reading an assay RNS in five minutes

1. Find the collar table and the map: where are these holes relative to the known deposit?
2. Find the table of all intercepts, including holes with no significant results.
3. Check the widths: downhole or true?
4. Note the cut-off grade and dilution rules.
5. Compare the grades with the deposit's resource grade or with peer projects.
6. Check the lab, the method and the QA/QC statement.
7. Confirm the Competent Person sign-off.

Red flags

• Only the best holes are reported, with no full table of results.
• No true width, and no statement that it is unknown.
• Cut-off grade and internal dilution rules are not stated.
• Metal-equivalent grades are given without the prices and recoveries used.
• "Up to" grades from grab or rock chip samples sit alongside drill results.
• Oxide and element figures switch between announcements.
• The laboratory and assay method are not named, and there is no QA/QC statement.
• Long intercepts are reported at grades unlikely to be economic for that deposit type.
• Selected holes are announced while related results are still pending, without saying so.

Frequently asked questions

Glossary

Assay

a laboratory analysis measuring the concentration of an element in a sample.

Intercept

a reported length of drill core at an average grade.

Grade

the concentration of a metal in rock, expressed as %, g/t or ppm.

Cut-off grade

the minimum grade used to define a reported interval or resource.

True width

the actual thickness of a mineralised zone, as opposed to the length measured along the hole.

Collar

the point where a drill hole starts at the surface.

Azimuth and dip

the compass direction and angle of a drill hole.

TREO

total rare earth oxides, the combined grade of the rare earth elements expressed as oxides.

Metal equivalent

a combined grade converting several metals into one, using assumed prices and recoveries.

Certified reference material

a sample of known grade inserted into a batch to check laboratory accuracy.

Related reading

• NdPr
• Rare earth elements
• Neodymium
• Lanthanum
• Samarium
• Cerium
• Yttrium
• Praseodymium
• Dysprosium
• Terbium

For company announcements and market context, visit investors.assay.earth.