Diesel will arguably never be cheaper than it is now, and it is a major input. Same for electricity. Labor costs vary.
Miners who have everything ready to go are digging dirt, and refining. Smelting continues at a high pace, and stockpiles are being built while input costs are historically low.
It is a viable long-term strategy, if you are well-enough capitalized, and if you have enough money to warehouse the product in either finished form, or in easily-worked billets. In the shorter term, share price could suffer, but the books will still be OK if it is done right.
They are making hay while the sun is shining, and it doesn't just apply to huge mines--it applies at all levels, and depends only on capitalization relative to mineral reserves, and on the duration of mining rights.