Ann: December 2018 Quarterly Activities Report, page-18

Sorry but I think your analysis (@dangar the many who fell in the same trap) is mathematically (really only arithmetically) flawed, although it is an understandable mistake the way NST presented it.  Since the higher grade ore also produces more gold, it is best to sell the most gold when the prices are high and sell less when the prices are low.  See this example.

When Gold $1300 mine 1 thousand tons at 1oz/ton gives 1,000 oz = $1.3M
When Gold drops to $1200 mine 1 thousand tons at 10oz/ton gives 10,000 oz = $12M
Total $13.3M

Other way
When Gold $1300 mine 10 thousand tons at 1oz/ton gives 10,000 oz = $13M
When Gold drops to $1200 mine 1 thousand tons at 1oz/ton gives 1,000 oz = $1.2M
Total $14.2M

You can assume that your costs are the same since mining the same tons each quarter, but the AISC goes up if you sell less gold.  So even if you need to sell the low grade gold at lower price than costs, you are still better off selling the higher grade producing the most gold at the best price.  As most goldies try to do.

If you think the price of gold will go down when you will mine your lower grades then hedge it at the current price.

If this decision was NST strategy then to me it makes sense only if they expect the price of gold to go up further, the opposite of what they say.  Maybe they just don't want to look bad mining the low grades at a loss or maybe there are other strategic decisions or problems. There may be factors I am overlooking, but if it were that simple then I am sure all goldies would have figured it out!

I hope this helps, don't feel over criticized, even simple math is often overlooked when an someone with authority makes an argument that sounds reasonable.