How do I get to the center of the Mines of Avarice?

There's a big black hole in the middle of my Mines of Avarice map and I can't seem to uncover it. I've tried entering from the gated side in the canyon and from the dwarf fortress to no avail. I've tried hopping on the wall decorations, using the blue rune for extra melee power, solving the cube puzzle, and doing the other quests in the area. How do I uncover this last part?

Missing portion of map is near orange triangle in following map:

I just accomplished this myself. Assuming you have already finished the main story mission for this area, return to the eastern portion of the map and kill the Gold Golem. Afterward you will notice the surrounding molten river will turn a greenish hue. Backtrack west to a small dwarven complex (which you had to pass through to reach the Gold Golem). You will notice a gate leading to the blocked area is now opened.

Hope this helps.

It's just a shortcut back to the entrance if you go back and kill the gold golem a second time. Nothing interesting in there at all.