For example if mine cost is 200M, and acquistion cost for zen is $600M, and cash flow is $400m/year, then payback is only 2 years, after which for a 25 yr mine, the next 23 years cash flow is pure gravy.

And if a Company issues shares for the purchase, they often ignore this in the payback calculation or they look at it more like 'cash' payback. In your example they would only expend $200M in cash (purchase done with shares) so the payback from a cash perspective would be 6 months.