I'm certainly no Geo, just some random with a progressively...

I'm certainly no Geo, just some random with a progressively worsening back. But I got reasonably good at finding the yellow shiney stuff because i like to read and given enough time can figure most stuff out.
lithium (sg .534) will behave in the exact opposite manner to gold (sg 19.3) because it floats. But like gold it is readily held by clays, hence why there is little lithium in sea water, the deep ocean clays have grabbed it all.
I'd speculate that the high water flood mark along creeks where the water slows or sheds laterally, like a boil, would be the place to look, with particular attention to mosses which often line creeks at that level.
I'd love to know what the industry standard is, I suspect simple randomly selected stream sediment is the go.
"Lithium reacts intensely with water, forming lithium hydroxide and hydrogen. The colourless solution is highly alkalic. The exothermal reactions lasts longer than the reaction of sodium and water, which is directly below lithium in the periodic chart.

2 Li(s) + 2 H2O -> 2 LiOH (aq) + H2(g)"

We definitely need a resident Geo.