"High grade nickel core (5%) is much heavier the lower grade (0.9%) due to the ~100% volume of sulphide minerals present (pentlandite, pyrrhotite and pyrite + pms). All of these minerals are almost twice as heavy as barren rock."

My point was and is that it is not significant enough to mitigate extraction. I was also comparing apples to apples in that we're talking about the difference within the same deposit where one part of the core may have for example, 1.5% at 5m and another might be 4% at 6m. It is all extracted the same way and does not pose an issue for the equipment used.