I did reveal the map afterwards and actually there's no spoilerite at all. It's a custom world with only one cavern layer and all the distances reduced to minimum. There's 9z of soil and rock, one cavern (with some water, as planned), some more rock, the magma sea, some semi-molten rock and then there's the circus, but no way to get there. No spoilerite tubes or veins whatsoever.
Now that I think about it, that is strange and probably a bugged map gen. I read that on every embark of more than 2x2 (I'm on 3x3) there should be at least one tube to the circus.
The prospector works alright... the numbers sound very plausible from what I saw in the revealed map
I just tried another embark nearby in the same world. It's pretty much the same, except it has more trees and is a little less hilly. Again, I ran the prospector (right after embark) and got this:
Vein materials:
LIMONITE : 2051
TETRAHEDRITE : 1985
RAW_ADAMANTINE : 401
...plus all the usual gems. Again, the embark screen showed almost the same as with the first site (two biomes, both with metals), but this time everything's actually there. I guess, it was just that one site that got broken.
I think, I'll just run prospector at every embark from now on, just to see if everythings actually there. It's really annoying to notice all your ores missing after already having played for a few hours. As it doesn't reveal anything, it's one of the most useful and non-cheaty tools available.