Not sure what you mean by agreed prices but the TCs for spot...

Not sure what you mean by agreed prices but the TCs for spot sales would be negotiated on a spot basis.
They have long term offtake agreements that they are yet to deliver into however that TC is typically would be relative to an a industry benchmark sale which is agreed annually between the world’s largest smelters and miners (usually Korea Zinc and Teck). However this means nothing unless they deliver within contractual specifications.

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...