Strange, what are your game settings, I usually play uncommon or common but just did a rare and i still had durantium  on my maps. Do you have MODs ?, just for kicks try turning mods off

I have asteroids turned off.  are the two connected?

Ryat is correct.  Durantium is spawned in asteroid fields.  No asteroids, no durantium.

Is there a way to change the asteroid strategic map icon? I guess there's not ingame, but an easy way for someone who's bad at modding?
it's very "clutter'd" as compared to the other icons and the fact that there are always several asteroids right next to eachother makes it hard to see what's going on that their vicinity?

I like the gameplay where they act as a sort of soft barrier creating "choke points", but the clutter is really annoying. Not to mention the fact that you lose durantium.

Fast and dirty:

1. Go to  C:\Steam\steamapps\common\Galactic Civilizations III\Gfx\StrategicIconsGC3_Strategic_Icons.dds
2. Edit that file (I use GIMP with a DDS plugin)
3. Copy it to My Games\GalCiv3\Mods\ExampleMod\Game\GFX\StrategicIcons
4. Make sure that you have mods enabled in the Game options screen (requires a restart)
5. Start a new game!