Well, an historically popular fraud was to present potential investors with small but potentially very significant ore samples to persuade them to buy your worked-out mine for ginormous sums.
And/or to claim to have a wonderful new method of doing something or other so that the worked-out mines people already own can be made once again profitable.
I’m sure some of those people genuinely believed in their wonderful new thing - and of course some of them did have exciting new technology that actually worked.
But lots didn’t.
