"A zone or intersection should be economic if grade (in ounces of gold) times width (in feet) exceeds the factor of three"

- I assume they mean if the gold is at the surface? Otherwise the formula would have to have some way of putting in the depth. Anything other than at-surface, the depth definitely has to be a factor - the same gold under 1000m of overburden can't be worth the same as at surface. Thanks for the surface formula though.