At a recent luncheon of the Denver Mining Club, a Denver-based global mining-exploration consultant chatted with table mates about his recent visit to a gold-producing area in Latin America. The gold, it seems, was entirely confined to the upper soil layer, known as the "A" horizon, or what laymen would call the topsoil. This is not at all where gold would be expected because it is almost entirely decomposing organic matter. Furthermore, the gold was in discrete crystals, not the rounded amorphous nuggets familiar to placer miners. That is compelling evidence that the crystals formed in place. Gold crystals have so far been found weighing as much as (gasp!) 4 kilos! The area, as expected, has many people working methodically on hands and knees, carefully sifting the one or two-foot thick gold-bearing layer. Also, as expected, is the ever present gentleman with the large bankroll in hand to buy gold for cash on the spot. The report is that the diggers do very well indeed.
The obvious question, quickly put to consultant, was "What is the source of the gold?" With something of a shrug, the consultant's reply was, "It must be plant activity." He theorizes that deep-rooted plants selectively pick up gold from decomposing bedrock, which is not itself economic, and carry it in chemical solution to the stem and leaves of the plants. After the plants die and decompose, the gold is liberated and is free to grow unobstructed into crystals as it seldom can in solid bedrock. Finally the topsoil acts as a packing medium, like styrofoam chips, to preserve the shape and delicateness of the crystals.
With modern analytical techniques, geobotany is a fast expanding field. For example, in Nevada pine cones are assayed for gold as a clue to ore location, and it has long been known that certain plants prefer to grow above copper-rich bedrock. But in this case the plants are not merely clues to location, but the actual mineralizing agent. The consultant's area sounds like a place for someone's PhD thesis to me!
In practicality, keep in mind that gold is elusive and can sometimes be found in "the durndest places." And it doesn't always require expensive equipment and advanced education to find it!