Trader101: "This may be an ignorant question but if the ground is too frozen/hard to drill assays during the winter months, why isn't it too frozen/hard to operate an open-pit mine in during the same time period?"

The issue is not with frozen ground. The issue is snow levels make it difficult to get around, creates issues with avalanche hazard, and flights in and out of camp (safety, costs). Frozen ground is actually really good for moving equipment around and I don't think would make a bit of difference with the drills themselves. Extreme cold (less than minus 40 C (-40F)) creates new problems with engine starting, frostbite and things breaking. Apart from that, the cold is probably a lot better than BUGS!