On your way down to the middle section of the mine you'll see a guy guarding a chest, and a digger named Snipes watching him. Talk to Snipes and he'll offer to pay you 10 Ore if you lure Aaron away from the chest. Agree to his offer.
Talk to Aaron. Tell him Ian wants him but you don't know why. He'll head off. Now talk to Snipes again for your 10 Ore. He'll offer to sell you the key for 30 Ore; you can't pick this chest open, so you'll have to get the key. You can either pay 30 Ore for it, or simply beat him up and take it (alternately, you could pay him and then beat him up to get your Ore back). Use it on the chest and grab its contents (some Beer and about 40 Ore). Now look over the edge, and see where Aaron went to. It's a walled area with a barrier in front. Head on down and he'll accost you, then tell him about the key. He'll buy it back for 20 Ore.
Right beside him is Ian; if you're doing the Test of Faith for Diego ask him for the list now.