After defeating the ghost, you can pick up the key, which becomes the Crystal mine key. Not only does it allow passage deeper into the mine where you can mine the crystal that makes a salve amulet, it also allows full access to other doors in the mine and gives access to shortcuts inside the mine. This makes it much easier to get to the bottom, since you can go straight through the front door and all the way down the stairs. In addition, it allows access to the rich ore veins in the north-west of Mort Ridge Mine level 2.
This key can be placed on the steel key ring.
To re-obtain the key, one must first acquire a glowing fungus, via the carts used in the quest, to access the lowest level of the Haunted Mine. You will not be required to kill Treus Dayth again.