What is Ore grade, cut-off grade, tonnage?

The ore grade of a volume of rock is the mass fraction of the valuable element or mineral in it: in per cent for base metals, in grams per tonne for gold, silver and the platinum-group elements. The cut-off grade is the lowest grade at which a block of rock is counted as ore; below it, the block is waste. The tonnage of a deposit is the mass of rock above the cut-off grade; with its mean grade $\bar g$ it gives the contained metal, $M = T\bar g$.