What is a good drill intercept? Grade, width, depth and context

A good drill intercept is one that could plausibly be mined at a profit: enough grade over enough true width, close enough to surface for the mining method it implies, and in a place that adds to what is already known. The same intercept can be excellent in one setting and ordinary in another, so the answer always depends on depth, deposit style and how the hole fits with earlier drilling.

The four numbers in every intercept

A typical line reads: "28m at 3.1g/t Au from 64m (true width about 20m)". It contains four separate facts:

• Width (28m): the length of the mineralised interval along the hole.
• Grade (3.1g/t): the average metal content over that length.
• Depth (from 64m): where the interval starts down the hole. Vertical depth below surface is usually less, because most holes are angled.
• True width (about 20m): the actual thickness of the zone, which is often less than the down-hole width.

Width times grade gives gram-metres, which is the quickest way to compare holes. But two intercepts with identical gram-metres can lead to very different mines, so each number deserves a look on its own.

Grade: what it takes to be economic

There is no universal pass mark, because costs, metal prices, metallurgy and infrastructure all vary. As rough rules of thumb only:

These are starting points for reading announcements, not thresholds any company must meet. A 0.8g/t intercept beside an operating mill can be valuable; a 6g/t intercept 400km from infrastructure may not be.

Width: can it actually be mined?

Mines cannot extract a perfect slice of ore. Underground stopes have a minimum mining width, often a couple of metres or more, and any narrower vein gets mined with waste rock that dilutes the grade.

Example: 1.5m at 40g/t from 600m (60 gram-metres). If the minimum mining width is 2.5m, the mined grade is roughly 60 / 2.5 = 24g/t, still very high. So a narrow high-grade vein can survive dilution. But 0.4m at 40g/t (16 gram-metres) diluted to 2.5m is about 6.4g/t, which is marginal at depth. Width matters most at the narrow end.

For open pits the issue is different: thin zones make the strip ratio (waste per tonne of ore) worse. Broad, continuous zones near surface are what open-pit economics want.

Depth: shallow is cheaper

Depth decides the mining method. Gold within roughly the top 100-200m can often be mined by open pit; below that, underground. A good test is to ask whether the grade suits the mining method the depth implies:

• 40m at 1.2g/t from 15m: 48 gram-metres, shallow, plausibly open-pittable. Interesting.
• 40m at 1.2g/t from 450m: the same 48 gram-metres, but too low grade for most underground mines. Much less interesting on its own.
• 6m at 7g/t from 450m: 42 gram-metres and fewer of them, yet a grade that underground mining can work with.

True width: how thick is it really?

Down-hole width overstates thickness whenever the hole cuts the zone at an angle. A 30m down-hole interval through a zone hit at 30 degrees is only about 15m thick. Good announcements state the estimated true width or say clearly that it is unknown. The full explanation, with a table of angles, is in true width vs down-hole width.

Where the hole sits: discovery, step-out or infill

The same intercept means different things depending on where it was drilled.

Discovery holes

The first meaningful hit in a new area. Even modest numbers can matter because they prove a mineralised system exists. They also carry the most uncertainty: one hole tells you very little about size.

Step-out holes

Drilled some distance (often 50-200m or more) beyond known mineralisation to test whether it extends. A strong step-out can add meaningful size potential. A weak one may mark the edge of the system.

Infill holes

Drilled between existing holes, often at 20-50m spacing, to improve confidence in a resource. A great infill hit is reassuring but expected; it adds confidence more than new metal. A poor infill hit inside a resource can be more important than it looks, because it suggests the zone is less continuous than modelled.

"Ends in mineralisation"

When a hole finishes inside the mineralised zone, the interval is open: the true thickness is unknown and could be greater. Companies flag this as "EOH" (end of hole) or "ends in mineralisation". It is genuinely positive, but remember why it happens: the hole may have been stopped because of drilling difficulties, budget or design. It means "we do not know where it ends", not "it continues".

Compare with previous holes and peers

An intercept only makes sense against a baseline:

1. The company's own history. Is this better or worse than the average of its previous holes on the same prospect? A run of hits followed by a weaker batch can be informative.
2. The deposit style. Compare a porphyry with porphyries and a shear-hosted gold vein with similar veins.
3. Peers. The drill results leaderboard lets you see how a hit ranks against everything recently reported on the ASX, and the announcements page shows each company's prior results.

Worked comparison

A hypothetical gold explorer and its neighbour report on the same day:

A has the bigger headline and more gram-metres, but 82 of its 100.8 gram-metres come from one metre, the true width is not stated and the hole sits inside a known resource. Without that metre, the remaining 8m average about 2.35g/t, which is low for underground. B is shallow, broad, close to its true width and extends the known zone. On this evidence alone B's result looks more meaningful, even though A will probably get the louder headline. This is an illustration of method, not a view on any real company.

The short version

• A good intercept needs grade that suits the mining method its depth implies: very roughly above 1g/t for open-pit gold and 4-5g/t for underground.
• Copper porphyries work at 0.3-0.6% Cu because of scale; high-grade VMS copper runs at several percent.
• Check true width, internal "including" intervals and whether the hole ends in mineralisation.
• Where the hole sits (discovery, step-out or infill) changes what the same numbers mean.
• Always compare with the company's earlier holes and with peers in the same deposit style.