This Including That
A number of companies report broad intersections of significant mineralisation and then state that this includes a narrow high grade intersection, but with no further information.
For example: 20 metres at 8g/t 1 gold including one metre at 140g/t gold2 .
Intersections are reported this way because a broad, good grade is more likely to be continuous and an ultimately mineable resource rather than a narrow intersection, which is more likely to be small, discontinuous and of little economic interest. But back to the example.
Firstly, the one metre interval contains 87.5% of the reported gold, the remaining 12.5% of the gold lies somewhere within the other 19 metres of the intersection. It could be a separate one metre intersection at 20g/t or it could be 19 metres at 1.05g/t gold.
In the absence of any additional information, it is most likely that the 20 metre interval includes sub-economic grades and in fact the only interval of real interest is the one metre.
Secondly, if the gold mineralisation is within a vein-style system then the one metre intersection could be a sampling artefact. The actual intersection width could be 0.5 metres or even 0.1 metres at a higher grade. This, of course, would be even less interesting than the one metre intersection, regardless of grade.
For example if the intersection is from rotary percussion drilling the resulting rock chip samples are usually collected at arbitrary intervals such as one metre. Similarly some companies sample diamond drill core at regular intervals although good practice is to sample according to the observed geology.
Final Word
Be cautious of broad intersections that contain most of the mineralisation in a narrow subset. Narrow high grade occurrences are occasionally economic, but usually only as a subset of a larger mineralised body.
1 g/t or grams per tonne of gold. There are one million grams in a tonne so a grade of 1g/t is equivalent to one part of gold per million parts of rock. Precious metals are usually measured in troy ounces, each of which weighs 31.1 grams. Thus, at a gold price of USD1,700/ounce, a tonne of rock at a grade of 1g/t gold is worth $54.70.
2This is not a reported result to the author’s knowledge, however it is comparable to many.