Genuine Gold Bugs
The explanation of why so much of interior Alaska's gold comes in fine particles may have been the first bit of northern science I learned. Somewhere there's a lode, the old-timers explained, a rich vein of metal. Then time passes, wind and weather get at the vein, and it washes away a grain at a time. But even tiny grains of gold are heavy---just check out your gold pan, greenhorn---and so they fall to the bottom of the streams. There they stay, even after the stream that carried them has long since dried up. The old stream bed forms the new lode of weathered flecks of gold. It's called a "placer," pronounced plass-ur, from the Spanish word for an underwater plain.
The old-timers gave me a fine explanation for what causes placer gold deposits; it's logical and workable. The only trouble is, it may be wrong.
That, at any rate, is the view of John Watterson, a chemist with the U.S. Geological Survey in Denver. He looked at placer gold particles from nine Alaska sites, and he looked at thousands of them very closely indeed---by means of scanning electron micrographs.
If you could observe the image of a fleck of gold at the tremendous magnifications possible with a scanning electron microscope, you might expect to see perhaps sharp angles where the fleck broke away from a larger piece of metal, akin to what you would find on a bit of stone chipped off a larger rock. Then again, even the tiniest piece might show rounded edges, as a water-worn rock get its corners knocked off in passage downstream. But neither of those configurations was what John Watterson found on his micrographs. He saw instead what could have been a gilded coral reef, a complex array of delicate cylinders and globes, all hollow but all gold.
I'd have been surprised to see such structures in a fleck of gold magnified thousands of times, but Watterson wasn't surprised at all. He'd suspected some such effect would appear, and he knew just what he was looking at: the residue of a long-dead colony of soil-dwelling bacteria.
Bacterial biomineralization, as it's known among the experts, has been observed in other places and for other minerals. In fact, bacterial abilities to precipitate metals from solution have been used in some very high-tech contemporary methods of treating polluted water. It's even been appreciated that some bacteria can precipitate gold. Watterson himself had found that the spore coats of another bacterial breed serve as nuclei for luring gold out of solution in broths of gold chloride.
Exactly what happened to cause Alaska's placer-building bugs to build up a gold molecule at a time isn't certain. Grossly oversimplified---and I certainly hope no chemist reads this---the metabolic products exuded by the bacteria interact with compounds in the environment virtually an electron at a time. So to speak, the bugs sweat solid gold. Others think the process may have had another purpose. British chemist Steven Mann speculates that the bacteria could be using "gold complexes...as terminal electron acceptors. If so, then this would be a novel form of energy transduction in anaerobic respiration"---that is, the gold buildup was an important part of the bacteria's life processes, not just a waste product like the crust of salt on an athlete's drying skin.
For whatever purpose, the cell membranes of each bacterium in the colony eventually were either coated with or replaced by high-purity gold. After the cell laid down a single layer of gold, that gold would attract more gold ions, which would react with and bind to the first layer. The process can continue for thousands of years, long after the bacteria colony has died---long enough, in fact, for fairsize nuggets to accumulate.
