It said Rio had spent C$14 million on exploration to date, more than the $10 million required to earn a 51% stake in Janice Lake, adding Rio had to make a $100,000 cash payment by May to complete its 51% earn-in obligation.
Rio can earn up to 80% of the project under a $30 million 2019 agreement.
Forum president and CEO Rick Mazur said Rio's drilling had "added tremendous value" to Janice Lake.
The final assay results from summer drilling at the Rafuse target announced on Friday included 42.25m at 0.15% copper, 1.63g/t silver and 0.41% lead from 221.25m.
Higher grades were reported from Rafuse in June.
Mazur said Forum would focus on its uranium portfolio in the first quarter of 2022.
It had $4.8 million in working capital at August 31.
Its shares closed down 11% to C24c - double the price of 12 months ago but below a September peak of 56c - on what was a downbeat day on markets.
