Mass Wasting / Downhill Creep


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Hi,

I've been researching residual and eluvial placers and got curious about the effects of slope and downhill creep on the formation of said placers. I found a great write-up on the subject HERE.

The key items to note is that for slopes of less than 15 degrees downhill creep generally does not occur. That means any gold eroding out of the rock will pretty much stay put and form a residual placer. Slopes over 15 degrees may have varying rates of downhill creep that will lead to the formation of an eluvial placer, and over time eventually move the gold all the way to the bottom of the slope.

The rates can be pretty small. At 0.15 inch per year, it will take that nugget 40,000 years to move 500 feet down the slope.

But it can happen much faster than that. I remember a place high in the mountains where we stripped a small area down to bedrock on the side of a steep sided little gully. That was back around 1972, and when I went to try and find that spot 25 years later it was gone. The blanket of soil and tundra on the gully walls had crept down all the way to the water line of the gully, completely removing any sign the spot had ever been mined.

Steve Herschbach

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Steve:

When I was at Moore Creek in 2006, I noticed the extreme amount of downhill sliding that must take place on slopes in AK. When the ground freezes and heaves like that, it really runs!

I bet in a lot of places in AK you can get 6 inches to a foot or more each year.

Chris

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Steve,

Your talking hardrock outcropping. That float gold does take a longtime to work it's way down a hill. Most of our gold in NorCal is old river gold. You can find these acient rivers on top of hills and mountains. The oldtimer's have some famous hydro pits with hundreds of feet of paying gravels in some of them. I like to detect that downhill creep below a hilltop placer, they do produce some color.

LuckyLundy

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Interesting post Steve. I've been working on a post called "Be The Nugget" which basically tries to get the reader to imagine being a piece of gold in a vein and, over time, weathering out of the vein and working it's way to the place where it is today. In other words, gold is where you find it but how did it get there? Where is the rest of it?

The problem in many areas is, up today wasn't necessarily up a million years ago. Down wasn't necessarily down. The earth has gone through some serious changes over time. Some rich veins have been found and mined only to reach a point where they were severed and the remainder of the vein was never found. It all adds to the mystique and allure of the yellow metal we call gold.

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Great post. This has been on my mind alot lately while looking for promising prospects. Does the material on the side of a mountain flow in a similar fashion that a glacier flows? Or is material mainly moved through the process of erosion. The way I picture it is much like a glacier flow in which the whole body of the soil is moving together.

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LL - I'd totally agree that most placer areas worked by detector operators are ancient river channels worked by hydraulic mining.

However, there are more residual placers in northern CA than most would think.

These angular nuggets came off the mesa like top of a mountain in Plumas County. No ancient channel - every rock is sharp and angular - none show any sign of wear. It's pretty flat, so no real soil mantle creep happens there. What makes these deposits hard to work in California is the organics layer capping the soil. The leaves, roots, pine needles, etc. build up and form a layer over the top which contains very little gold.

The nugget areas in northern Nevada are commonly less than 15% and probably loose more erosion to wind than to water washing.

Steve - another thing about that report: the soil creep at the top of the soil is normally faster than the layer that contacts the bedrock. It does not all more at the same speed as a uniform blanket. (Well, maybe in AK where it all freezes hard and heaves up from ice expansion, it might behave more like a blanket that all moves at the same speed)

Chris

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Nuggets can be "born" fairly smooth. A certain percentage of the nuggets at Rye patch in Nevada appear smooth and water worn, yet the majority are very angular and the rocks are all angular. Its not like a small percentage of Rye Patch nuggets rolled around in a stream, but were then separated from the stream and all of the round rocks in the stream, and then got mixed in with a whole bunch of angular ones. The rounded ones were formed in the ground in a rounded, blob like shape, just like the angular ones at Rye Patch were formed with sharp edges.

Chris

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Hi Chris,

Downhill creep can do some weird stuff. If you remember at Moore Creek over by the pond there is a large "beach" area of scraped bedrock backed by a small rise that jumps up 30 feet to the next bench level.

Summer before last we were digging into the base of the slope and got into what appeared to be placer gold locked in layers in the shale bedrock. It was puzzling for awhile until I figured out that that gold had been laid down and then later the shale on the slope above had slowly slumped down over it and then set back up. It was a very rich layer that produced quite a few ounces of chunky gold before it played out.

Steve Herschbach

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Hi,

I've been researching residual and eluvial placers and got curious about the effects of slope and downhill creep on the formation of said placers. I found a great write-up on the subject HERE.

Steve Herschbach

Great post Steve, thanks... check your PM inbox.

Jennifer

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Hi Jennifer,

I read the threads. Funny how the one guy came out swinging over the report claiming to represent conditions everywhere when it does nothing of the sort. Just food for thought is the way I see it, as nature is a bit too complicated for pat formulas. That is where the prospector's thinking cap comes in.

Hey LuckyLundy, we have lots of high ancient channels in Alaska and Canada. The Klondike placer itself comes from reconcentrated high river deposits - the White Channel gravels. Similar deposits are seen in Alaska's Fortymile district and Nome and elsewhere. They all represent places with great potential that have seen little or no metal detecting activity.

More on the White Channel here

Steve Herschbach

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Hi Chris,

Downhill creep can do some weird stuff. If you remember at Moore Creek over by the pond there is a large "beach" area of scraped bedrock backed by a small rise that jumps up 30 feet to the next bench level.

Summer before last we were digging into the base of the slope and got into what appeared to be placer gold locked in layers in the shale bedrock. It was puzzling for awhile until I figured out that that gold had been laid down and then later the shale on the slope above had slowly slumped down over it and then set back up. It was a very rich layer that produced quite a few ounces of chunky gold before it played out.

Steve Herschbach

Hi Steve,

I see it's getting all white again up North. Brrrrr As for that creeping phenomenon at the pond, that must have been the reason we found good gold the first year at the bottom of that same slope? Maybe more accumulations deeper into that slope? Great post!

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Steve. I've noticed that clay content of soils slows creep . Sometimes to the point that nuggets will cling to hillsides far in excess of 30 degrees.----Bob

Hmmm... Kinda reinforces what I posted in Azo's forum November 12,

(Quoting myself here in part)

Crystalline nuggets.......

"I suspect that I dug them off of a tailing pile that had a lot of clay in it. As delicate as they are

they would not have lasted long in that crystalline condition had they been exposed to rain and

snow and downhill movement."

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