Many zinc concentrates carry commercially significant silver grades, and US customs requires that the value attributable to silver be declared separately under 2608000040 rather than aggregated into the zinc-content line. The payable silver value is calculated from the final assay, the applicable silver price participation factor in the offtake contract, and any deductions for treatment charges. Failure to separately declare co-product metal values can result in undervaluation findings.

Zinc sulfide ores commonly contain lead as a co-occurring mineral, and the value of payable lead in the concentrate must be declared under 2608000020 separately from the zinc content line (2608000030). If lead content is below the smelter's payable threshold, it may have no commercial value and need not be separately declared, but importers should document this with the purchase contract terms to support the entry in the event of a CBP inquiry.