Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Boogie on 16/08/2012 21:37:55
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I found this following quote on a wikipedia article on gold.
"Such free gold is always richer at the surface of gold-bearing veins[clarification needed] owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets."
Wikipedia article on gold : http://en.wikipedia.org/wiki/Gold
My question is, Really? Water action can weld gold fragments into nuggets? I find this a little hard to believe. If it was possible, I'm picturing the so called "nuggets" to be very crumbly with a grainy look to them. Has such a thing been found or proven to be true?
I feel like keeping this piece of information in the same place I keep other nonsense such as "diamonds were formed from coal".
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That is most interesting.
I'm seeing additional notes about natural "Cold Welding" of gold.
http://en.wikipedia.org/wiki/Cold_welding#Applications
http://www.ehow.com/info_10063213_gold-nuggets-form-river.html
I had always thought of the nuggets as having formed in the veins.
I am seeing some notes that gold leaf may "cold weld". Also, gold tooth fillings may be cold welded.
So, if a dentist can cold weld a gold tooth filling, perhaps the same thing can happen with nuggets.
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Sadly, cold weld has never occured while I was panning, or left evidence of it in areas that I've worked.
I'm still having a very hard time accepting this information. In fact, I've been trained that just the opposite occurs to a nugget in a stream. I was taught that when lode gold becomes placer, it will get beat on, banged around, ground up and eventually turned into flour gold the more it travels downstream. The headwaters of the Arkansas River in Colorado has nuggets. By the time they get to Kansas, there is barely anything left. Now, I would think that if nuggets were formed by being welded by water action, we would be finding nuggets in Kansas.
There must be some very special conditions to allow nuggets to form this way. Perhaps it's the job of nymphs and midges to assemble these nuggets.
The Alaskan cold welded nugget is the most amazing of them all. Maybe it was really "cold" welded by a glacier?
You are correct. Though nuggets are called lode gold when in the vein. Only when they become placer, do we call them nuggets.
How was it discovered? Is there any proof? What conditions are require? Wiki and the other couple of sites make it sound like this is a common thing that happens. It sounds like an old wives tale to me.
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Most metals in our normal gas-laden environment are covered with a very thin layer of adsorbed gases, particularly water, carbon dioxide, and/or oxygen. When two pieces of metal are placed together, this adsorbed layer cushions them, and minimizes metal to metal contact. In the absence of these adsorbed gases, the local frictional heating when metal surfaces rub together will weld them. This has been a major problem for docking in the space programmes. An adsorbed layer may be weakly held in place by physical forces like ion/induced dipole attraction (physisorbed) or it may be strongly and irreversibly held in place by forces akin to chemical bonding (chemisorbed).
Gold is a very unreactive metal, and so on the whole it is only a candidate for physisorption of environmental gases or liquids. Given the right conditions there would be a good prospect of extensive metal-metal contact, and "cold welding". That is the theory and a summary of the observations in this sort of interaction. I have no knowledge about this particular system of gold-on-gold friction. I would think that the action of ice, where there would be air gaps between the two gold surfaces, would be more productive of cold welds than running water, where there would be a liquid film between the surfaces.
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I want to add my observation. First off, I want to say that I have no scientific education that I base this on. I have worked an area that has free gold and ore close to each other. I have found the ore to have very fine gold and no nuggets. I have also found sandstone close by with very small nuggets if you could call it nuggets. The sandstone has a silicate crust. I believe that the weather plays a part in forming nuggets. The summer and winter environment will squeeze these sandstone rocks naturally. Since this happens I believe that the squeeze will mold these grains together to form nuggets. To picture what I'm saying, imagine that a very close clump of very fine gold grains inside a rock contracts in cold months. If you squeeze fine gold grains together they will weld together due to the malleability of gold. This is just an opinion based on my layman observations.
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I want to add this to what I said. Sandstone is a hardening of sand wherever it happens to be. The sandstone I'm referring to could very well have placer grains touching each other and then the "winter weld" would make a nugget. I do admit that this is just my non educated guess.
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A published study (I'll try to find the citation) in a Canadian journal years ago looked at fineness from the interior to exterior of gold nuggets. The authors found that fineness increased outward and concluded that some nuggets may grow by removing gold in solution from the stream waters. They did test on gold solubility in natural conditions and suggested that certain acids, especially humic acid, may dissolve gold.
Of course, the other reason for increased fineness outward in gold nuggets may be silver dissolution from the nugget.
I suspect that the reason you don't find large nuggets downstream has more to do with gradient than it does with mechanical breakdown of gold particles. Since gold is so dense, larger particles will drop out of the stream lode early, leaving only the smaller particles to be washed further downstream with lower gradients and energy. Same reason that you don't get boulders in Kansas streams.