Ann: December 2018 Quarterly Activities Report, page-12

Here is my take on NST's strategy of mining the lower grade ore when justified by the gold price.

The first rule of capitalism is that when you are out of money you are out of the game.

If you have low grade ore that you can choose to mine on the way to exposing higher grade ore, then it makes sense to mine the low grade  ore when the gold price makes that profitable. That way you have less chance of the first rule ever applying to you.

Mining the low grade ore is an investment in improving the overall mine that is subsidized by the current favourable gold price. That way if the gold price declines, you still ore to mine that is "in the money". On the other hand, If the gold price improves further, nothing is lost, the gold is still there ready to be mined at a high margin.

This might sound like the opposite of "a bird in the hand is worth two in the bush" approach, but an identified block of rich ore accessed and ready to be mined is pretty much the same as a bird in the hand. 

I wish that I had stocked up more this morning when the horses took fright.
Bob's the man. He will soon have Pogo jumping.
.
Cheers