i wouldn't keep that as a strict rule?20m @ 5g/t will have as...

i wouldn't keep that as a strict rule?

20m @ 5g/t will have as much gold as 5m @ 20g/t but it will cost more to extract and process simply due to the volume of dirt

grade (control) becomes more critical in an underground mine.. if the operators are sloppy (or paid on a gross tonnage, not recovered basis) you will often find an u/g mine with supposedly high grade not being profitable - simply because they are shifting far too much waste dirt and not focusing on following the good stuff