State regulators have approved exploratory drilling near the headwaters of the rugged flow that inspired ‘A River Runs Through It,’ which is still recovering from a mining disaster more than 50 years ago.By Mosabber HossainMore than half a century after one of Montana’s most destructive mining disasters poisoned the upper Blackfoot River, a new gold exploration project near the river’s headwaters is sparking concerns about whether its decades of environmental recovery could be at risk.