In the depths at 1574, -3117, -0646,

I haven't found the closest Lightroot yet, but I'll update this when I do.

It's guarded by 5 yellow and 2 blue Cave Trolls that are on the roof of what looks like a skeleton. If you're coming from the west it may be hard to see.

There's also a few more huge zonaite and non-zonaite sites at 1292, -2922, -0601. I'm guessing there's a bunch all around the Spirit Temple, which is below the Bronas Forest.