A crash in Australian thermal coal prices is raising fresh questions about the viability of a controversial US$4 billion coal mine just a week ahead of a national election in which climate change is a key issue.Final approval of the Carmichael coal mine in Queensland, owned by India’s Adani Enterprises, should come in “a matter of weeks, not months” following nearly a decade on the drawing board, the company’s mining chief executive, Lucas Dow, said last month.But a 40 per cent slump in…