Ore prospecting and the low-hanging fruit effect

I’m trying to acquire an understanding of how the difficulty of prospecting for useful ore has increased over historical timescales.

Intuitively, I would expect that centuries or millennia ago, you could find deposits, at least of common things like iron and copper, just by walking around looking at rock formations, whereas nowadays those have all been discovered, and to find anything new would take a team of experts carefully analyzing subtle clues from geological maps, perhaps with the aid of exotic instruments like gamma-ray spectrometers.

Is this general intuition correct? If so, are there any available numbers, even plausible estimates, for how much the difficulty has increased in quantitative terms? Anything like ‘in preindustrial times, the average prospecting effort to find an iron ore deposit was X months; by 1800 it had increased to Y months’?