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Glaciers & Gold


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Steve please continue to post info such as this because the geological changes on the earth are indisputiable ---the rub with a lot of folks is that the pols try to take natural occurances and place a tax on them.The REASON I read your site is for real and scientific information.

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On 2/4/2015 at 11:58 AM, Steve Herschbach said:

I have to admit I have a heck of a time deciding what is old glacial material and what might be old stream material or even ancient beach deposits at times. They can grade imperceptibly from one into the other. Good news with a detector is, well, just detect the darn stuff and see what happens.

Just a quick way to check (by no means always the case) is to look for any sort of bedding within the gravels.  Most stream conglomerates show a high level of stratification, fining upwards as is the case with inside bends (point bars). A FANTASTIC place to see this is at the Gold Run rest stop on the westbound side of highway 80.  You can get up and close with the big tertiary channel and stick your nose right on it from a safe location.  Its incredible the amount of scouring, bedding, and grading that you can see.  

Now if you are working in a intervolcanic channel like are common in the motherlode, then things get a little more challenging.  (Picture 1) This shows the base of the channel where there is no bedding at all.  Look at the chaotic nature of the boulders and smaller cobbles.  The matrix of the conglomerate is also clay produced from the weathering of the volcanic material.  Compare this chaos to nice bedding (Picture 2).

Where it gets really tricky is comparing intervolcanic conglomerates with glacial deposits.  Since the defining characteristic of both is that there is almost no sorting of the stream load you need to inspect the rocks a little more closely.  The first thing I look for is a change in boulder type.  In the Sierras this is very easy to see. The rocks that dominate the tertiary channels are largely metamorphic and in my area lots of blue lead (serpentine). The rocks that make up the glacial deposits are dominated by granites eroded from the batholith to the east.  Picture 3 and 4 show what some recent Sierra glacial deposits look like.  Picture three is till, while picture four is deposition of glacial outwash sediments above modern day river deposits.  

Now a lot of the generalities that I have mentioned apply to the Sierra, but the biggest thing that is a dead giveaway for glacial sediments is a change in rock type (provenance) and the utter chaotic nature to the material.  

If you have examples of glacial material that is bedded I would love to see them just out of curiosity.  

Hope this helps a little, at least in the general sense.    

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Great thread. Here in the UAE there are a lot of ancient sand dunes buried under new ones, along comes a road cut and the cross bedded layers are revealed. No gold of course, excep on the Bedouin ladies' wrists.

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  • The title was changed to Glaciers & Gold
  • 3 weeks later...

I have been up close with some glaciers and am familiar with the moraines from the last Ice Age. I think there could be quite the difference as a mile thick ice sheet does quite a number on terrain compared to a receding glacier, which I thought could expose pre glacier placer or a new lode and its associated placer.

Glacial till I see leaves stream rocks exposed far away and above current drainages and hundreds of feet to bedrock, without a visible clay layer I wouldn't bother looking for gold even in a rich area under that condition.

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On 4/19/2021 at 8:25 PM, wwace said:

without a visible clay layer I wouldn't bother looking for gold even in a rich area under that condition.

Spent most of my life where ice was a mile thick, and nearly all the gold I’ve ever found was related to glacial action. Most glacial action is more recent than the massive ice sheets, with most small Alaskan glaciers being Pleistocene age, for example. Nobody not interested should waste their time in glacial terrain. I’m just the opposite. I’ll run a detector over anything just to see. I’ll never forget a patch I walked past for a decade because “gold can’t be there”, until a novice who was not as smart as I dug there and found out there was. Glacial terrain can be exciting as gold can literally be anywhere or nowhere.

Glaciers also bury and preserve pre-glacial placer that can be very rich. Crow Creek Mine near Anchorage is a perfect example.

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