Alluvial gold sources?

[Scottish888]

Hi folks,

I am interested in the potential geochemical origins of alluvial gold and interested to find out if anyone has ever traced any alluvial gold directly to a source. ie: vein or large deposit.
I don't ask were, just if it has happened.
I don't think that prospecting here in the UK has yet produced a source for any alluvial gold, there is some hard rock gold mining (small particle size) but I am mostly interested in original alluvial sources.
If any of you folks have worked alluvial gold, how high above the current rivers have you found gold and is this always in old river bed sediments? Or would you say you are working glacial till that has carried gold from another source?

Regards

 

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[BillA]

Hi Scott, there are many many books, papers, etc. on this topic
time can mix up the crust such that a geologist is the one to sort it out
all areas are different, with some constants:
mountains erode over time and gold, being heavy, remains; to being eventually transported to the rivers existant at that time
these rivers can be folded, compressed again into rock, eroded over and over

I am in Costa Rica where there is gold all over -> ALL placer; the original mountains with veins are long gone
in California there have been many placers traced to their hard-rock sources
depends on the area

there is no "original alluvial source", alluvial gold is placer, secondary in nature

 

[Scottish888]

Thanks,

Just trying to make some sense of what I find and why it occurs in particular areas, in the Scottish Lowlands, sometimes a richer steam with bigger grain size find is only along 10 meters or so, upstream and downstream being just fine gold. yet no direct source. Scotland has lots of mountains and had glaciation though gold is still quite rare here it can be found.
I had not heard of any placer gold being traced directly to a source, I am glad that is the case.

 

[BillA]

the variation you're seeing could be to the somewhat random way glaciers stop and retreat
washed then eventually to your creek as randomly dumped, w/o rhyme or reason

 

[63bkpkr]

Scottish888,
I would have to believe there is a significant variation between sites and countries.

In Northern California where I play there is one particular stream that goes all the way out to the main river. I'm certain the "Old Timers" { 1849, 1920/1930} found color in the lower portion of the stream and followed it up till the color quit. They then backed up testing both sides of the stream for color, found it, followed it uphill and Bingo the Vein. How do I know this? There is an 8' diameter hole bored into the hard rock with narrow gauge tracks and an ore cart in the mine.

I would also Guess that the 'Color' they found was fairly rough/good size as the gold did not travel more than about a mile in the stream till it reached the main river.

We are looking at the distance traveled, the gradient of the stream moving the Au, the amount of water during winter through Summer available to move the Au, the type of soil along the stream, "catch basins" along the stream, how much Ice in the actual Vein to help break the ore loose from the mother rock and probably a few hundred or more active events that cause the ore to come loose from the vein and things that break it apart on its travel down stream and deposit it where it is findable. One of those Active Events is Time, how long did "the elements" work on the vein before the Au was discoverable?

But at least you have an actual stream with placer in it that came directly from the Vein of gold in a rock wall. One fork of the stream ran right over the the rock that held the Au and eventually the abrasion, weather, pH of the soil, etc. broke through the rock to the vein and started pulling the Au into the stream.

Rugged country to get into but the Old Timers with crude equipment/clothing/boots etc. did it, got it first and kept working the mine till the vein ended some 100' into the mountain. That had to be a disappointment!! There was no "easy come, easy go" in this area as it was all hard work just hiking the mountains and even harder sampling and boring that 8' diameter hole! Quite a puzzle to work out especially where Glaciation is thrown in as well as active Volcanoes, Major Earthquakes/Mountain Formations,. TIME and the like..........63bkpkr

(note: In this area I find Ocean Animal Fossils at 6000', so big time earth fractures/movement. Also in the area there has been Major Hydraulic Mining up to 8000'[buried ancient rivers with gold in them]). Heavy Duty Stuff so think Big changes all around gold bearing veins!

Ok, one more. If there are Ocean Animal Fossils at 6000' elevation {and there are} then was the Gold injected into fissures in bedrock that was underneath the ocean? Or, for Caliofornia, it has been speculated that as the mantle along the Coast folded up along the coast line forming vertical fissures that were later filled with gold bearing molten rock and then the land mass continued to change as the young earth did all sorts of cataclysmic jittering and jiving? Yeah, too many things to think about. Hope this helps!

 

[Scottish888]

Thank you for the information.
Gold quantity and distribution here in Scotland is on a much much smaller scale, also find that god does not travel very much in the streams here, only the smallest flakes seem mobile.
Flows are not really high velocity and very small stream widths so areas with more coarse gold are quite specific and in a very small area but there are many areas I have not yet visited to say it is nationwide here in Scotland.
Scotland was covered during the ice ages so glacial till is almost everywhere, strangely the area I am looking at the moment has a gold bearing gravel layer (max 10 cm) right above bedrock (as you may expect) but in the alluvium directly above, zero. I have not sampled a large area as I only dig where gravel has been exposed but it is difficult to consider that the alluvium directly above has nothing. this could suggest it was never there or 100% gold transportation to bedrock, highly unlikely.