If you're mining the gold at surface, you want long intervals close to the surface that average out to >1g/t to keep your strip ratio down and keep your head grade constant.

If you're going to mine it underground, you need intervals over 5m long showing >5-10g/t. Intervals need to be big enough that they don't have to excavate a lot of waste to get at the good stuff, and the grade has to be a lot higher to pay for the expense of undergroud mining.

Just because there's "visible" gold doesn't mean there'll ever be a mine. This is because we haven't yet invented nanobots that can tunnel through rock homing in on the "visible" gold flecks, collecting them in a tiny basket and sending them to the surface. Instead, we dig big fat holes with excavators. So we need grade and width.

That's sort of the nice thing about underground bulk sampling: it'll definitively determine whether or not this can be a mine. If Tilsley can pick a location, pull up a few thousand tons of rock, and get a certain g/t out of it, then the people operating a mine there in the future should be able to do so as well.