After you get the aluminium key: on the left wall is a block obscured by vines. Pull it out, climb on top then climb up the cliff and over to the right to get into a cave. Push the big butting in there to open the doors below. Go down, enter doors and get treasure. After that, go all the way back to the elevator, push button and go down. Where the lava flows into the room walk up the steps on the right side.