Special tax rate for copper exporters ("Optional Register of Copper Exports"). Reference values:.(i) base value: USD 7,700/tn.(ii) reference value: USD 11,500/tn.(iii) international price: on the last business day of each month, the Mining Secretariat will publish the listing price of grade A copper ton, cathodes, on the London Metal Exchange..Cases:.1. If the international price is equal to or less than the base value: 0% (export tax rate).2. If the international price is equal to or higher than the reference value: 8% (export tax rate).3. A special formula is applied to define the export tax rate if the international price is higher than the