The Tainted Ore is a cut quest from Baldur's Gate that is included in the Unfinished Business mod. It allows for completion of the Investigating the Nashkel Mines quest. In BG:EE this quest does not appear and instead Investigating the Nashkel Mines simply disappears after completing Baldur's Gate Chapter 2.
After clearing the Nashkel mine, speak to Berrun Ghastkill and tell him about the poisoning of the ore. He will then ask you to bring a sample of the Contaminated Iron to Taerom Fuiruim, and the "mineral poison" (Vial of Mysterious Liquid) you found to Thalantyr the Conjurer so that they might figure out a way to counteract its effects.