True width vs down-hole width: why drill intervals can overstate thickness

A down-hole width is the length of the mineralised interval measured along the drill hole; the true width is the actual thickness of the mineralised zone. They are only the same when the hole hits the zone at right angles, and at shallower angles the down-hole figure can overstate thickness by half or more, inflating gram-metres by the same amount.

Why the two widths differ

Picture a tilted slab of ore, like a book leaning on a shelf. A drill hole that pierces the cover at right angles travels exactly the book's thickness. A hole that enters at a glancing angle travels much further through the same book before it comes out the other side. The book has not got any thicker; the path through it has got longer.

Ore zones are rarely horizontal. Veins, shear zones and lodes usually dip steeply, so geologists drill angled holes to cross them as close to perpendicular as they can. But the geometry is often uncertain early on, holes deviate as they go deeper, and some holes are deliberately drilled at awkward angles because of site access or to test something else. The result is that many reported intervals are longer than the zone is thick.

How to estimate true width

If you know (or can estimate) the angle between the drill hole and the plane of the mineralised zone:

Equivalently, using the angle between the hole and a line perpendicular to the zone, multiply by the cosine of that angle instead. Both give the same answer.

Notice that the first 30 degrees away from perpendicular cost very little. It is at shallow intersection angles that down-hole widths become seriously misleading.

Worked examples

A vertical hole into a steep zone

A hypothetical explorer drills a vertical hole into a lode that dips at 60 degrees. The angle between the vertical hole and the lode is 90 - 60 = 30 degrees. The hole reports 30m at 3.0g/t, or 90 gram-metres. True width is 30 × sin(30) = 15m, so the zone holds about 15m at 3.0g/t: 45 true-width gram-metres. Half the headline.

A well-designed angled hole

The same explorer drills its next hole at -60 degrees towards the east, into a lode that dips 60 degrees to the west. In cross-section the hole and the lode meet at about 60 degrees, so true width is about 87% of the down-hole figure. A 30m interval represents roughly 26m of real thickness. This is the kind of geometry companies aim for.

Drilling down-dip

If that hole had instead been drilled towards the west, roughly parallel to the dip of the lode, it would run along the zone rather than across it. The down-hole interval could be enormous while the zone is thin. Reputable companies avoid reporting such holes as if the width were meaningful, or they flag it clearly.

What good disclosure looks like

JORC Table 1 (Section 2) has a specific item called "relationship between mineralisation widths and intercept lengths". In summary, the JORC Code 2012 asks companies to:

• Explain the geometry of the mineralisation with respect to the drill hole angle, if it is known.
• Say clearly when only down-hole lengths are reported and the true width is not known, for example "down-hole length, true width not known".

The better announcements go further, with a true-width column in the intercept table, a cross-section showing the holes against the interpreted zone, and a stated estimate such as "true widths are estimated at 70-80% of down-hole widths". If you see collar dip and azimuth in the drill-hole table, plus the dip of the zone in the geology description, you can make your own estimate. The JORC Table 1 guide explains where to find each piece.

Oriented core and alpha angles

Diamond drilling can recover oriented core, where the bottom of the core is marked so geologists can measure how veins and contacts cross it. The "alpha angle" is the angle between the core axis and the plane of a structure. An alpha angle of 90 degrees means the hole crossed it perpendicular; 30 degrees means true width is about half the down-hole length. When an announcement says structures were "measured from oriented core at alpha angles of 50-70 degrees", the hole crossed the structures at a reasonable angle (roughly 77-94% of down-hole width).

Reverse circulation (RC) drilling produces rock chips, not core, so it cannot measure orientations directly. Early RC programs often have the least certain true widths.

When missing true widths are a red flag

None of these prove anything is wrong. In a genuine first discovery hole the geometry may be honestly unknown. The issue is when an announcement leans heavily on a long width while being silent on how that width relates to the zone. DrillScore flags missing true-width statements on the red flags page and takes them into account in the rating, as described in the methodology.

Why it matters for resources and valuation

Tonnage scales directly with thickness. If an explorer's first ten holes all overstate thickness by 40%, any early "size potential" talk is overstated by the same proportion. Resource estimates use modelled true thickness, which is one reason a first Mineral Resource can disappoint investors who had been adding up down-hole gram-metres. See Mineral Resources vs Ore Reserves.

Width corrections also stack with other effects. A 30m interval at a 30 degree intersection, carried by one very rich sample, may contain a fraction of the metal its gram-metres suggest. Smearing and the nugget effect covers the second half of that problem.

A quick checklist

1. Is the width labelled "down-hole" or "true"? If neither, assume down-hole.
2. Find the hole's dip and azimuth in the drill-hole table.
3. Find the dip and strike of the mineralised zone in the text or sections.
4. Estimate the angle between them and apply the sine rule above.
5. Recalculate gram-metres on the true width with the gram-metre calculator.

This is general information to help you interpret announcements; it is not advice about any security.

The short version

• Down-hole width is the length along the hole; true width is the zone's real thickness.
• True width = down-hole width × sin(angle between hole and zone); at 30 degrees it is half.
• Grade stays the same, but gram-metres and tonnage potential shrink with true width.
• JORC Table 1 expects companies to explain the geometry or say true width is not known.
• A long interval with no true-width statement, no hole angles and no section deserves extra scepticism.