Al - economic grades of bauxite have 25% to 35% AlC
Cu - 0.25% for open cut mines or >2% for undergound
C - can vary from 0.05 to 2 carats pt
Au - 4g/t for open cut, undergound can vary up to 25g/t
Ni - 6% - 20% for sulphide ores, 1% to 3% for laterite ores
Zn - 5% - 20% pt
Grade isn't the only consideration re whether its an economic resource. Proximity to infrastructure ie: transport, electricity, water etc are major factors.
