I've been searching around my third base for all the things one would need to survive. Everything has been going great (even nickel, titanium, and chromite were close and abundant), but couldn't locate a source of black coal.
Prospected all over a small island chain just west of the base, and had a low reading, but thought, why not. Started making my stairway down. 1st I came across borax. LOTS of borax. Cleaned that out, and brown coal was just a little ways under that. I'm pretty full up on brown coal, though, so I continued downward. More borax. Sure, I'll take that. Then quartz, just LOADED with silver and gold. Then readings for lead and kernite. Then limonite (oddly, just one lone block of bountiful. Found the rest shortly after fairly far away). Then tin, flourite, phosphorite, more borax, & BLACK COAL OUT THE WAZOO! (Also, oddly, no fire clay under it, but I have a bunch of that already). Made two trips on the boat so far, and headed out again because I'm still getting readings for pretty much everything! There were also readings for zinc, but have yet to come across it.
Who knows what else I'll find, but so far this has been the single greatest dig I've had in 800 hours of gameplay!