You are correct about maximum amount of impairment over...

You are correct about maximum amount of impairment over goodwill.

To judge whether a stock undervalued or not, future earnings will be more appropriate.
Based on recent announcements, I calculated ARI would generate EBITDA between $491m - $644m p.a. That would be far enough to support current market cap of $602m.

AGO, is currently producing at loss, as break even point is US$68/t IO62%.
MGX, would be producing at break even point at best, and unknown bill on broken sea wall.
BCI, would be producing at approx. 2/t marginal profit. But production life and scale is limited.