Black sand, gold and precious metals

Jack wrote this in November, 2005:

Afternoon blogsters:

JAP69 pointed out to me in a comment on the Lightning Strike entry that I didn’t explain myself in a way I could expect anyone to understand, which I appreciate.  Sometimes I take too much for granted.

Gold, silver, platinum and several other precious metals are the heaviest elements around.  When the rocks weather away from around them and erosion carries them into channels it drops them at the first opportunity.  It takes a lot of energy (velocity) for the water to carry any particulate, but it takes more, the greater the density of the material being moved.  So the heaviest sediment load drops to the bottom first.

Prospectors rely on this.  It’s how placer gold is found.  Nature has spent millions of years concentrating these precious metals, shaking the ligher overburdens with water movement, sinking the heavier particles to the bottom of the sediment layers.

Lighter than the precious metals, but heavier than the silicone sands the water also carries, is the black sand.  It’s mainly an iron product.

So a prospector studies the channel, examines it for places where the water loses energy, particularly during flood stage.  Where there’s a curve in the channel there’ll be a place just on the downstream side of the curve where the stream drops silt.  That’s one of the places a prospector looks for gold.  Similarly, backwaters behind rocks and other obstructions, ancient beaver dams, anywhere the stream flow was interrupted.

The prospector digs into these spots and finds a hardpan or bedrock, or maybe a crack.  He takes the material and places it into a pan, sluicebox, dry washer, and agitates it to stratify the material, just as nature has done to concentrate the metals in this spot.

Gradually the prospector washes away the sand until there’s only black sand in the lower rim of his pan, and hopefully a few specks of gold, sometimes a nugget.

What happened to me in No Name Canyon was that there was a LOT of material that was heavier than black sand.  So much of it that the black sand was floating above it in a barely noticeable layer.  Such things don’t happen, or happen so rarely that when a person sees them he tends not to consider it a possibility.

There’s a thing that happens to gold sometimes.  It’s called telluride.  Prospectors have to know about it to recognize it.  Throughout history prospectors who didn’t recognize telluride gold because it doesn’t look like gold, have thrown it aside with a few curses because it’s screwing up their panning.


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