If the gold is in the PPM range, single digits PPM, even a fire assay may miss it. And if the gold is nanoparticles and as widespread as implied in this thread, finding an area where it has been naturally concentrated may be nigh on impossible. If it doesn't settle, it isn't dropping out as placer unless the water dries up. So it can spread far and wide and be quite low in concentration.
Fortunately I'm spoiled, I have always worked with recycling gold from manufactured goods be it jewelry, machinery, or e-scrap. No nano gold in my beakers!