the main problem is the narrow widths in my opinion. I'm no expert and could be wrong...But most finds that do anything to SP are a few meters wide. And mind you, the last NR i read here a few months ago about the golden ext...everything is so narrow and before that the NRs were all taking about 0.2-0.5M stuff.
Maybe with all the spider viens things will turn around...making open pit possible. But digging down 300M for an individual vein 0.5M wide would be to costly IMO.
and here again i place my disclaimer that i aint no expert.
sw