I've found a really good site for myself already around a volcano south of the island in the inland sea, adjacent to a tower. I fully intend to make a horrifying deathtrap rewarded with useless gold trinkets, and some steel gear
There's a massive load of ores on the map, the first 200k of which being gold, followed by hematite, the two most common forms of copper, galena, sphalerite, and a little aluminum. But the big plus is the massive depth of the map, which means lots of space to engineer my death traps and impressive structures. The first cavern lies ~57 z-levels below the main mountain valley, which is ~87 z-levels from the peak of the mountains. The first cavern layer has no groundcover besides mud, but the second, at -100 z's, has ground cover and much more vegetation. The magma sea starts at -123 from the main valley floor. The only drawbacks for me is that my FPS will likely plummet, there's no water above the caverns, which is a really, really long ways down, and there's not a lot of marble to accompany the hematite, nor much adamantine.
Time to start drawing up blueprints. And buying up picks. It takes YEARS to create a defensible outdoor fortress in the mountains, but goblins show up within roughly twelve-thirteen months. I'm going to need stout labor and a good defensive strategy that distracts bowmen.
I did find more hematite and marble immediately to the south-east of that volcano, in the forests immediately north of a tiny mountain range. The marble exists under a locale with clay in the soil, and the hematite under the adjacent area without clay. Not as good for building a massive deathtrap, but parts of it are very flat and shallow relative to these mountains, and it has surface water in the form of pools. No magma pipes, though.