Desert wind errosion and placer gold

[Bejay]

I just thought I might share something worthy for those who prospect the desert. Have you ever noticed that the desert landscape is often cluttered with rocks/boulders that are rust stained on the bottom third or so? At one time the soil was as high as the upper rust portion...but over time the wind has blown away the fine sands/dust leaving the rock/boulder exposed on the bottom side. When one sees this occurrence, the miner should realize that the heavies are left (lead/gold/metal/nails/etc.) The wind did not move them but rather concentrated them in the surface area (not deep). Often the miner who is used to "water erosion deposition" will dig deep thinking the gold is concentrated deeper....when in fact it is not a reality. It makes things very confusing if one does not realize/consider the power of the wind. I used to try to locate gold bearing areas in the desert and simply go scrape off the upper 2 inches or less and process that material. Fine gold and pickers were often the reward. Digging deep was not productive unless I felt the source was lying below or that water had altered the deposition.

Bejay

 

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[TerryC]

"Fists Full of Gold" Chris Ralph explains this process well. These areas are called Bajadas or pediments. Other sources will explain this process but I think Chris does it best. ╦╦Ç

 

[Hoser John]

Back in the late 70s,early 80s you could find nuggets on sand pinnacles out at Rye Patch from the horrendous winds. Amazing really. John

 

[gold tramp]

Meteorites.
Gt.....

 

[Goldwasher]

a pediment is created more by erosion in general including water and uplift plus gravity . It is the flat to sloping plateau up against mountains. Like an alluvial fan but not at the mouth of a drainage. A pediment can have buried dead alluvial fans and also be from volcanic activity mud and ash flows etc.

The foothills of the Sierra Nevada are an eroded pediment for the most part.

Bejay is speaking more of a flat deposit.

A Bajada is a slope like Terry said.

There are places in the flat desert that are like large gold pans that are created by wind. Heavies in side them concentrating.

 

[Bejay]

With Placer Gold deposition we most commonly think of water erosion causing the deposition. That deposition is similar to our using the gold pan and water...concentrating the gold "Down". even using a pan dry we concentrate the gold "Down" by shaking it. Using a sluice we rely on water to concentrate the gold "Down". Using a dry washer the vibration puts the gold "Down". But the wind simply blows the light material away and leaves the heavier material lying on top of the surface. Often we see a flat desert surface atop a ridge or just an open flat area that has nothing but rock. The wind has blown the fines away. When free gold is present and the wind has blown the fine material away the gold is simply lying atop the surface.

If one goes to Quartzsite Az you see flat areas that look like they were graveled and graded: The wind has blown away the fines. If water erosion is not present, it can not move the gold and concentrate it in washes etc. If a outcropping containing gold decomposes, (long geologic time process), the gold becomes a surface deposit or nugget patch. But often the gold is not nuggets but rather fine gold/flakes/small pickers.

If one goes to Rich Hill Az one sees boulders and rocks atop the ridges. Since the soil/overburden is rich in iron one sees that the boulders and large rocks are stained with rust about 1/3 of the way up from where they sit on the surface. The wind blew away the fine material (long geologic time) and left the free gold lying on the surface (concentrated in the surface) as opposed to being put "Down".

One must remember that wind erosion is far different than water erosion, and that unlike water worn areas the wind leaves heavy things lying on top.

Someone define Bajida for me...the term escapes me. I have never heard that term applied to mining or geology.

Bejay

 

[arizau]

Probably bajada rather than bajida and termed as a coalescing of several alluvial fans.

 

[Goldwasher]

https://www.britannica.com/science/bajada

Would be considered an Eluvial deposit. Wind, rain some chemical break down.. freeze thaw So, Terry is right .

What Bejay is describing is an Eolian deposit. A residual deposit.

Wind and rain affect both not moving water.

 

[arizau]

He asked what a "bajida" was. "Someone define Bajida for me...the term escapes me. I have never heard that term applied to mining or geology." That was the way you originally spelled it thus his question. "Bejay is speaking more of a Bajida."

 

[Goldwasher]

yep I did spell it wrong... I renamed it a "little" Bajada on accident

 

[Bejay]

OK. Got it. Tks

Piedmont, in geology, landform created at the foot of a mountain (Italian: ai piede della montagne) or mountains by debris deposited by shifting streams. Such an alluvial region in a humid climate is known as a piedmont for the Piedmont district of Italy; in arid climates such a feature is called a bajada (q.v.)

Bejay

 

[TerryC]

Bejay has it. Other than the fact that a bajada is a Spanish (Mexican term), it is essentially the erosion of the mountain coming down off the slopes. The same as the slopes around the "Devil's Tower". As long as the mountain has height, the debris around it will slope from gravity, give or take a wash or two. ╦╦Ç