Depends on if "diorite" means "it all contains gold, every bit of it". If there's no gold in it, it's waste rock. You can average down your grade by including the waste rock around the gold-bearing deposits, no big deal... but average your grade down too much and your operation begins bleeding money.

I'll be honest and admit I don't know anything about the geo - if I had real money in this game I'd go look at their core photos, or even go up and visit the core shack to get an idea of how the gold presents itself, and how much of the diorite actually hosts gold. I guess that would be your real determinant of what you can mine.