Pull the cart into a room with a miner, give an iron pickaxe to that miner and wait for it to fill the cart. Depending on south or north mine the cart will either have iron ore or a diamond. Once full pull the cart all east, then pull it again to empty it.
You will either be awarded a small amount of gold, or the mark.
For teaching a hill giant a smarter way to mine, the Powers have granted thee a Mark. You have been granted The Mark of Assistance! You gain 21 practices! You gain 75 quest points! You gain 50,000 experience points!