Why would it be profitable to reopen copper mines to mine ore that's just 0.5 percent copper?

A low grade copper mine might be re-opened if:

1) If copper prices go up.

2) It's a massive deposit, easily extracted, close to rail lines or shipping.

3) If it's found with something else, like silver or gold, that boosts the value.

4) During war, or major copper shortage.

5) If cheap new refining methods are discovered.