Under the Lake
In this particular case, the geological situataion of the deposit might be a huge bonus.
The lake is shallow, 3-5 meters, so the dyke is easy to build and seal as the hydrostatic water pressure is very low.
The deposit starts at the southwest near the shore, and gradually deepens, so in effect the excavation can be a ramp starting at the shore and getting deeper as it goes out into the lake. THis is easy to mine. and the strip ratio can be very low.
Since the deposit is very high concentration, the open pit will be safer to mine for health reasons. So the 20-30% concentration pockets can be mined quite safely, as it shoudl be very easy to set up ventilation.
THe dyke just has to surround the deposit with the appropriate slope into the deposit, so we are not talking about draining the whole lake.
Since the host rock is very solid, there should not be water ingress problems like other deep mines, so minimal water inleakage to pump out, and not an environmental issue.
Roscoe Postle are a very good engineering firm so thay will get this all figured out for the potential buyers.