"..You can also bet your bottom dollar that in a market for zinc...

"..You can also bet your bottom dollar that in a market for zinc concentrates like this if the material is 0.001% outside of the contractual spec the smelter/customer will insist to buy these volumes under the spot contract instead of the long term contract..."

Well that depends on the demand for zinc at the time as the spot price may actually be higher than the long term offtake agreement, that's one reason why offtake agreements are signed, so both parties can have some certainty around pricing