via The Canadian Institute Of Mining - Recovery Rates

Rates of recovery and grade controls are a problem at depth because you're going to cut corners and hastily recover as much ore as possible from the depths of the mine.

GBN.V has mining prospects that start at surface, with the exception of the Roy Lloyd mine. Decade and Jolu(probably) have adits. Komis will definitely start at surface.

The senior mines in Ontario and Quebec have all seen decades of service and extract ore at depths sometimes greater than 1km.

http://bit.ly/1AN1KwF

- F6