Contaminated Ore and Mysterious Green Liquid

Carrying the contaminated ore in your inventory makes non-magic metal weapons more likely to break. The Mysterious Green Liquid, when consumed, causes 1 point of constitution loss. In the standard version of the game, they have no other purpose. In the Enhanced Edition, "Investigating Nashkel Mines simply disappears after completing Chapter 2."

If you haven't installed the mod "Unfinished Business" the items are indeed useless. With the mod installed you are supposed to take a sample of the ore to Taerom Fuiruim in Beregost and the mysterious liquid to Thalantyr in High Hedge. But this will only give you minor XP, you won't be able to stop the iron shortage this way.