It depends what the objective is. C1 is good for benchmarking...

It depends what the objective is. C1 is good for benchmarking operating performance over time.
i.e. it is good to compare C1 in current period vs a previous period to understand how whether they are keeping operating costs under control.

If you are trying to understand project costs over the life of a project C3 costs are a better measure as they include sustaining CAPEX (i.e CAPEX you need to spend over the life of the mine to get all the zinc out and processed) and royalties.
If you didn’t spend money on sustaining CAPEX or royalties you wouldn’t be able to mine in future years and so need to be included in any investors assessment. As an investor you may also need to factor in financing costs & corporate costs as they are sometimes not included in C3 costs.