note the tiny little chunks of gold that I have circled
Those are bigger than the nuggets I've ever panned!
Well, maybe not, but definitely in the same approx. 1-2mm diameter range.
That said, I was thinking of a contact meter that could detect trace amounts (of a few precious metals) from a freshly chipped stone face, not a tool for locating mineable veins or actual nuggets...
(The "mother lode" part was supposed to be a joke. There are interesting formations in Northern Finland, including completely eroded old volcanoes, with just the "magma pipe" (not a geologist, don't know the correct term) parts left, and the bedrock on the surface; not much soil. It'd be interesting to just go for a trek, chip some stones, and see what lies under the surface, mapping some out.)