Gram-metres explained: how to judge a gold drill hit

Gram-metres are the width of a drill intercept in metres multiplied by its gold grade in grams per tonne, so 10m at 5g/t is 50 gram-metres. That single number lets you compare a thin, rich hit with a long, modest one on the same scale, which is why it is the starting point for every gold intercept DrillScore rates.

The formula

For copper the same idea uses percent instead of grams per tonne, giving %-metres: 150m at 0.6% Cu is 90 %-metres. Everything in this guide applies to copper too, with the numbers on a different scale.

A few quick conversions:

• 12m at 4.2g/t = 50.4 gram-metres
• 2m at 25g/t = 50 gram-metres
• 50m at 1.0g/t = 50 gram-metres
• 85m at 2.4g/t = 204 gram-metres

The second and third lines score the same but describe completely different deposits. One is a narrow, high-grade vein that might suit an underground mine. The other is a broad, low-grade zone that only makes sense as an open pit. Gram-metres put them on one ruler; they do not tell you which one is the better business. You can run your own numbers with the gram-metre calculator.

Why width times grade makes sense

Grade alone is misleading because a single metre of rich rock can hold very little metal. Width alone is misleading because a thick zone of barely mineralised rock is worthless. Multiplying them approximates how much gold the hole passed through.

There is a physical meaning hiding in the number. Take one square metre of ground directly above the intercept. If the rock has a density of about 2.7 tonnes per cubic metre, a 10m-thick zone under that square metre weighs 27 tonnes. At 5g/t that is 135 grams of gold, about 4.3 ounces. That works out to gram-metres multiplied by density: 50 × 2.7 = 135. So gram-metres are, roughly, grams of gold per square metre of footprint divided by rock density. Double the gram-metres and, if the zone continues sideways, you roughly double the metal under each square metre.

The rule of thumb: what counts as good

Australian mining news site Stockhead popularised a simple scale for gold intercepts: anything over 50 gram-metres is good, 100 to 200 is really good, 200 to 500 is excellent, and beyond that is stellar. The lower bands below are added for context; they are not part of the Stockhead rule.

The bands are a conversation starter, not a valuation. A 60 gram-metre hit in a brand-new area 5km from any known gold can be far more significant than a 300 gram-metre hit in the middle of a deposit that has already been drilled hundreds of times. DrillScore's ratings start from gram-metres and then adjust for depth, context and disclosure quality; the details are on the methodology page. You can see how the market's best recent hits stack up on the gold drill results leaderboard.

Worked example: three holes, one announcement

A hypothetical explorer reports three holes from the same prospect:

The headline will almost certainly lead with "31g/t". But hole 1 delivers about the same metal from shallow depth over a width that open-pit equipment could mine. Hole 2 is deeper and narrow, and as the next sections show, its grade may lean heavily on one or two samples. Hole 3 is a reminder that not every hole hits; a good announcement reports it anyway.

Where gram-metres mislead

Depth

100 gram-metres starting at 30m and 100 gram-metres starting at 800m are not the same prize. Shallow gold can be mined cheaply from an open pit; deep gold needs an underground mine, higher grades and more capital. Always read the "from" depth next to the interval. The guide to what makes a good drill intercept covers rough grade thresholds for each mining method.

Down-hole width versus true width

Most intervals are reported as down-hole lengths. If the hole cut the ore zone at a shallow angle, the zone's real thickness can be half the reported width or less, and the gram-metres fall by the same proportion. See true width vs down-hole width.

The nugget effect and smearing

Gram-metres add up every metre, so one very rich sample can carry a long interval. 20m at 2.35g/t is 47 gram-metres, but if one metre in it ran 40g/t, that single metre supplies 85% of the total. The other 19 metres average under 0.4g/t. This is covered in smearing and the nugget effect.

Cut-off grade and internal dilution

The same assays can be reported as 4m at 8g/t or 40m at 1.1g/t depending on the cut-off grade and how much low-grade material the company lets into the middle of an interval. Both are "true", but they score differently. JORC reporting expects the cut-off and aggregation method to be stated; if they are missing, treat the gram-metre figure with caution.

One hole is one hole

A drill hole samples a core a few centimetres across. A spectacular hit with no nearby holes may be a pod, a cross-cutting vein or a fault-offset fragment. Continuity between holes, not any single number, is what eventually supports a Mineral Resource.

Metallurgy and payability

Gram-metres assume the gold can be recovered. Refractory ore locked in sulphides may need expensive processing. That does not show up in the number at all.

Gram-metres for copper and metal equivalents

For copper, %-metres follow the same logic: 300m at 0.45% Cu is 135 %-metres. Bulk porphyry deposits often produce very long intervals at low grade, so %-metres on a porphyry hole tend to look large compared with narrow high-grade copper. Compare like with like: porphyry against porphyry, vein against vein, using the copper drill results page.

Be careful with gram-metres calculated on metal-equivalent grades such as AuEq or CuEq. Those grades depend on assumed prices and recoveries, and the assumptions can lift the number substantially. The metal equivalents guide shows how.

How to use gram-metres sensibly

1. Calculate gram-metres for the headline interval and for every other hole in the announcement, not just the best one.
2. Check the depth. Shallow gram-metres are generally worth more than deep ones.
3. Check whether widths are down-hole or true.
4. Look inside the interval for an "including" figure, and work out what the rest of the interval averages.
5. Compare with the company's previous holes and with peers in the same style of deposit.

This is general information to help you read announcements, not advice about any particular stock.

The short version

• Gram-metres = width (m) × grade (g/t); for copper use %-metres.
• Stockhead's rule of thumb: over 50 good, 100 to 200 really good, 200 to 500 excellent, above 500 stellar.
• The same gram-metres can describe a narrow rich vein or a broad low-grade zone; depth and mining method matter.
• Down-hole widths, single rich samples and generous cut-offs can all inflate the figure.
• Use gram-metres to compare holes, then check continuity, true width and context.