How to Read a Drill Result Without a Geology Degree

Grade, width, true width, gram-metres, cut-offs and metal equivalents, explained with a worked hypothetical drill hole and a list of red flags.

Key takeaways

Grade-thickness, in gram-metres, multiplies grade by width, so 42.0 metres at 2.10 g/t equals 88.2 gram-metres.
True width can be much shorter than core length: at a 45 degree crossing angle it is about 71% of core length.
In the article’s hypothetical hole, a 3.0 metre high-grade interval held 63% of the metal in a 42.0 metre intercept.

A drill result is three numbers: how much metal, over what length, measured how. Most of what matters sits in the fine print under the headline intercept. This walk-through uses a made-up hole to show how to turn a press release into something you can compare.

The hypothetical press release

Everything in this example is invented. The company, the hole and the numbers are for illustration only.

“Hole HX-26-07 intersected 42.0 metres grading 2.10 g/t gold, including 3.0 metres grading 18.50 g/t gold. True widths are estimated at approximately 70% of core length. Intervals calculated at a 0.3 g/t cut-off with up to 3 metres of internal dilution. High-grade assays are uncapped.”

The rest of this article unpacks that paragraph.

Grade, width and gram-metres

Grade is concentration. For gold it is usually grams per tonne (g/t). One g/t is one gram of gold in a tonne of rock. Base metals are usually quoted in percent.

Width is the length of the interval. In most releases it is the length of drill core, which is where the trouble starts.

Grade-thickness multiplies the two. 42.0 metres at 2.10 g/t is 88.2 gram-metres. It is the simplest way to compare holes, because 2 metres of 20 g/t (40 gram-metres) and 40 metres of 1 g/t (40 gram-metres) contain similar metal per unit of area, even though they point to very different mines.

Core length versus true width

A drill hole rarely crosses a mineralized zone at right angles. When it crosses at an angle, the core length is longer than the zone is thick. The geometry: true width equals core length times the sine of the angle between the hole and the plane of the zone. At 90 degrees the two match. At 45 degrees, the true width is about 71% of core length. At 30 degrees, it is 50%.

National Instrument 43-101 requires written disclosure of drill results to include the relevant widths and “to the extent known, the true widths of the mineralized zone.” If a release says true widths are unknown, that is permitted. It also means the headline length may overstate the thickness. Crux Investor cited a real case where 146 metres at 3.33 g/t shrank to a true width of about 20 metres.

In our example, 70% of 42.0 metres is 29.4 metres of true width. The gram-metre figure on a true-width basis is 29.4 x 2.10, or 61.7.

The math: what is inside the “including”

NI 43-101 also requires companies to disclose significantly higher-grade intervals within a lower-grade intersection. Readers should also run the calculation the other way, and check what is left once the high-grade part is removed.

Piece	Length (m)	Grade (g/t)	Gram-metres
Full interval	42.0	2.10	88.2
High-grade “including”	3.0	18.50	55.5
Everything else	39.0	0.84	32.7

Three metres hold 63% of the metal (55.5 of 88.2). The other 39 metres average 0.84 g/t.

Now capping. Say the three high-grade samples were 45.0, 6.0 and 4.5 g/t, one metre each, which averages 18.50. Resource estimates normally apply a top cut to outlier values so a single sample cannot dominate a wide interval. If a hypothetical cap of 15 g/t is applied to the 45.0 g/t sample, the three metres contain 25.5 gram-metres instead of 55.5. The full interval becomes 58.2 gram-metres over 42.0 metres, or 1.39 g/t.

The same hole can be described as 2.10 g/t uncapped over core length, or 1.39 g/t capped over the same length. On a true-width basis, the capped version is about 40.7 gram-metres. That is less than half the 88.2 headline.

Cut-off grade and internal dilution

A reporting cut-off is the grade used to decide where an interval starts and stops. Our example uses 0.3 g/t with up to 3 metres of internal dilution. That means stretches of low-grade rock up to 3 metres long can sit inside the interval.

A low cut-off with generous dilution makes long intervals. A high cut-off makes short, rich ones. Neither is wrong, but results reported at different cut-offs cannot be compared directly. Look for the cut-off in the release. If it is missing, that is information too.

Metal equivalents

For deposits with more than one metal, companies often convert everything into one “equivalent” grade. Take a hypothetical interval of 1.00 g/t gold, 20 g/t silver and 0.30% copper. With assumed prices of US$3,000 an ounce for gold, US$35 an ounce for silver and US$4.50 a pound for copper, chosen for illustration only:

Silver adds 20 x (35 / 3,000), or 0.23 g/t gold equivalent.
Copper at 0.30% is 3 kg, or 6.61 lb, per tonne. That is worth US$29.76, which divided by US$96.45 per gram of gold adds 0.31 g/t.
Total: about 1.54 g/t AuEq at 100% recovery.

Assume recoveries of 90% for gold, 70% for silver and 80% for copper, and the figure drops to about 1.46 g/t. Change the price deck and it moves again. NI 43-101 requires a metal-equivalent grade to be disclosed with the grade of each metal, plus the prices, recoveries and other conversion factors used. It prohibits stating the gross value of metal in a drill intersection.

Red flags

Core length only, with no true-width estimate or a vague one.
A headline driven by one “including” interval, with no statement on capping.
Metal equivalents without individual grades, prices or recoveries.
Only the best holes released, or holes clustered around an earlier hit.
No hole location, azimuth or dip. NI 43-101 requires these.
No lab named, or a lab related to the company, with no QA/QC description.
No qualified person named as responsible for the technical content.

What to watch

The results table in each release: from, to, length, grade, and true width.
Drill plan maps and cross-sections showing where each hole sits in relation to earlier holes.
The QA/QC paragraph: lab name, assay method, standards, blanks and duplicates.
Any later mineral resource estimate on SEDAR+, which will state the capping and cut-off actually used.

Sources

Frequently asked questions

What does g/t mean in a drill result?

g/t means grams per tonne, the usual way gold grade is reported. One g/t is one gram of gold in a tonne of rock. Base metals are usually quoted in percent. Grade is concentration, and it is only meaningful alongside the width of the interval, which in most press releases is the length of drill core.

What is the difference between core length and true width?

A drill hole rarely crosses a mineralized zone at right angles, so the core length is usually longer than the zone is thick. True width equals core length times the sine of the angle between the hole and the zone. At 45 degrees, true width is about 71% of core length. At 30 degrees, it is 50%.

What are red flags in a mining drill press release?

Red flags include core length only with no true-width estimate, a headline driven by one high-grade interval with no statement on capping, metal equivalents without individual grades, prices or recoveries, missing hole location, azimuth or dip, no lab named or no QA/QC description, and no qualified person named as responsible for the technical content.