Slide 18 of the Macassa update shows SMC spanning.....the Main Break across to the Amalgamated Break. The traditional mining at (the town of) Kirkland Lake/Teck was down the Main Break, at the MB's much steeper 65 degree angle from flat. The SMC bends out from the Main Break, at a much shallower angle. Better pooling, like a saddle? In any case, SMC is richer than even the 15-20 typical of the Main Break. Not explored earlier, because it was difficult & expensive to explore & mine at these depths.
The whole idea of shaft #4 is to span that difficultly, between the depths where the rich ore is, and the surface/mill; making deep mining safer and less difficult, and making it cheaper to move larger quantities of ore to the mill. $325M is an expensive upgrade; but it's a state-of-the-art multi-lift system. Analogous to replacing a $10k milling machine, on which an operator can produce 5 widgets an hour, with a $200K automated CNC machine, on which the same operator can produce 30 widgets an hour, with lower defects.
None of this is to disparage Fosterville, where repeats of visible gold and similar geology have already been found at Harrier & Robbin's Hill. But Macassa & the unexplored areas under the historic mines in Kirkland Lake/Teck have tremendous potential, which that new shaft is ideally-configured to leverage.