The project, which would become the European Union's only open-cast uranium mine if given the go-ahead, has faced local opposition since it was proposed.
Hydro, which is not interested in acquiring aluminium plants that use coal-fired power, was hoping the deal would be part of efforts to make its output as green as possible.
The proposed tax, to be applied on the nominal value of extracted metals, would affect copper miners that produce more than 12,000 tonnes of the metal and those extracting 50,000 tonnes of lithium a year.