announcement - nabanga has more gold, page-12

The laterite and oxide layers would most likely be free dig,
Once they get into a transition layer there may be some supergene enrichment which will improve grade even though the digging will get harder. Its likely they need to drill and blast once they get into the sulphides which adds to costs both in mining and milling as higher bond mill index (harder ore) and slower leach rates in agitation tanks increase both CAPEX and OPEX, it could even be refractory ore.
On the flip side the harder ore means a higher repose angle can be used on the pit walls in the sulphide layer as improved stope stability
Guessing once they get under 75 metres (down th that level guessing a strip of 10:1 ratio) the economics deteoriate exponentially though due to the above reasons plus as you pointed out the strip ratio increasing due to a much wider trench needed plus greater cost in getting overburden/waste/ore to surface.
Using a SG figure of say 1.8 one could work out the total tonnage roughly for say 2 km of strike 75 metres deep guees
2 metres wide ie 1.8 * 2M * 75M * 2000M =540,000 tons
That would be an uneconomic tonnage to mill they really need around 3Million tons for say a 400KTPA CIL plant to make it viable IMHO.
I just plugged in figures I guessed at the geos would have a far better idea I am not a geo so the above is an unqualified opinion taken 5 minutes to do they spend months if not years coming to a conclusion !
If the oxide ore was conducive for heap leaching(need to see recovery rates from column tests above 60%) a lower tonnage lower CAPEX lower OPEX plant may work.