How to read drill results and intercepts
A drill headline combines length and grade. To understand it, check what was averaged, how the hole crosses the mineralised zone and what the company has left out of the headline.
Educational content, not investment advice
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Read the intercept line
Consider this hypothetical result: 10 m at 2.0 g/t Au from 100 m, including 2 m at 6.0 g/t Au. It describes an interval from 100 m to 110 m along the hole. Au means gold. Unless stated otherwise, the 10 m is downhole length, not true width. The 2 m interval lies inside those 10 m.
A company-announcement intercept usually combines several consecutive samples. Check the hole identifier, collar position, dip and azimuth, plus downhole surveys where relevant. The headline does not tell you the zone’s vertical depth or its distance from an existing resource boundary. For laboratory measurements, units, analytical methods and QA/QC, use our guide to reading assay results.
Calculate a length-weighted grade
For intervals combined on a length-weighted basis, multiply each sample grade by its length, add those products and divide by total length. This is a reporting composite; resource estimation may require additional treatment of density, geological boundaries and extreme values.
| Sample length | Gold grade | Length × grade |
|---|---|---|
| 2 m | 6.0 g/t | 12 g/t·m |
| 8 m | 1.0 g/t | 8 g/t·m |
| 10 m total | 2.0 g/t average | 20 g/t·m |
Here the 2 m higher-grade interval contributes 12 of the total 20 grade-metres. The remaining 8 m averages 1.0 g/t. Adding 2 m to the headline 10 m would count that interval twice. A broad headline can conceal an uneven distribution of grade.
Downhole length is not true width
True width is the thickness of the mineralised zone measured perpendicular to its boundaries. An oblique hole can travel through much more mineralised rock than that thickness. For a simple planar zone, true width equals downhole length multiplied by the sine of the angle between the hole and the zone’s plane.
A 10 m intercept at 30° to that plane represents 5 m true width. The calculation is not justified if the zone’s orientation is unknown or the geometry is complex. JORC reporting calls for the relationship between mineralisation widths and intercept lengths to be explained; if true width is unknown, that should be stated.
Check cut-offs, dilution and top cuts
- Cut-off: the threshold used to select reported intervals. It need not be an economic mine cut-off.
- Internal dilution: lower-grade material allowed inside an intercept; check the maximum length and rules.
- Top cut: a limit applied to extreme assay values, which can materially alter the composite.
- Minimum interval: the shortest result reported. Different rules can make the same samples produce different headlines.
Compare like with like: the same commodity, units, reporting basis, width convention and broadly similar geology. Metal-equivalent grades also require stated prices and recoveries. Our guide to reading assay results covers units, oxides and equivalents in more detail.
What a strong intercept does not prove
Grade multiplied by width is a limited comparison tool, most useful for geometrically comparable zones using true widths. It is not a measurement of contained metal: tonnage also needs geometry and density. It says nothing by itself about recovery, mining dilution, continuity or costs.
One strong hole does not establish a continuous orebody, a compliant resource or a viable mine. Look for sections and plans, neighbouring holes, geological boundaries and balanced reporting of lower-grade or barren results. A deep isolated intercept has a different significance from an infill result in a well-understood deposit.
At an operating mine, grade control addresses shorter-term material-routing decisions. Exploration intercepts and operational grade-control results have different objectives and should not be compared as if they were interchangeable.
A practical reading checklist
- Locate the hole on the plan and section, not just in the headline.
- Check units, sample lengths, compositing rules and any included intervals.
- Find the true-width estimate or explicit statement that it is unknown.
- Follow the assay-results guide for the laboratory checks behind the reported grades.
- Check whether reporting is balanced and how the result relates to existing geological evidence.
Sources
Related reading
This guide is educational, not investment advice or a site-specific operating procedure. Numerical examples are hypothetical.