You need at least 30 lemmings from each tribe along with a full gold talisman. You will get the golden piece for a tribe if you achieve a gold medal on all ten levels.
Most levels require to save every lemming in order to get gold. If the intended solution loses a few lemmings, then you'll get gold for losing at most that many. :-)
As you already have 30 lemmings from each tribe in the ark, you'll only have to replay levels without a gold medal, not the levels after them. If you save 50 on Sports level 1, finish the tribe and later get 58 (= gold medal) on Sports 1, you don't have to replay Sports 2 with 58 instead of 50 if you already have gold there.
-- Simon