I am not sure why you would say that minerals a half mile down are generations away from being mined? Half mile being approximately 800 metres.

Shafts can be sunk at rates of 3 metres today once infrastructure is in place, without too much difficulty. Ramps can be driven at faster rates, with multiple headings for future workings.

Whomever decides to put their money into this place will do so based on those deeper metals as well, and will not just focus on the the stuff within 300 meters. Once you get 300 meters of depth, it is easy to keep going. It is the initial problems of setup holding things up, not the depth of the minerals.