Comes down to the economics, its meant to b an open-pit, so they can always have a plan-B for the future. The pit-model can always b changed to capture a higher resource, sitting in-ground.
For now, that cross-section shows the 50-60% Fe grades continuing downwards, so they've only scratched a bit here, IMO. As they haven't found the bottom yet, means the overall resource could go up further in the future.
Should b interesting to see how they handle this part down the road. Just my 18 cents worth, for now :)
SDL Price at posting:
18.5¢ Sentiment: Hold Disclosure: Held