Desert Alluvial Gold- What Form And What Distribution

[LeighB]

Hi all

I'm seeking input from the experts to answer an archaeological question about ancient gold recovery. As a geologist, I know the questions I'm asking don't have specific answers but I would value your collective experiences.

I'm looking at a situation where gold is recovered from ephemeral streams and where mostly it will be the result of flash floods (and of course before metal detectors made life a bit easier). For those who have worked in these environments, I know the gold distribution can be a lot less predictable than in regular streams. My first question is how effectively gold is separated towards bedrock in such a setting. Can one strip off less prospective sediment and focus on specific bands the way one might in other settings?

Secondly, what proportion of the gold will be in particulate form (fine flour and discrete nuggets) versus within composite cobbles with quartz and iron etc (I know primary geology will be a factor here, so assume the sources are numerous quartz veins with gold and associated sulphides).

Last, whether anyone has found by detecting a quartz cobble or fragment in a defined stream bed (i.e. not at the base of slope or in gullies) containing gold that is not visible from the outside, and how rare this might be.

Any thoughts, observations or questions for clarification will be most welcome. Thanks in advance for your time.

[Couch Potato]

Matt Matson has many videos on youtube about prospecting.

[jasong]

The answer to most of your questions is "it depends where you are at" and there is no real exact way to answer them.

Dependent on the amount of precipitation and severity of seasonal flooding, most nuggety gold is still found towards bedrock in these sorts of washes in US desert areas, or at least deep enough to be within the undisturbed hardpack that escapes churning during flooding. The finer gold can be commonly dispersed through the entire depth of the alluvium though. In some places you definitely can strip the first 75% or so of alluvium from a wash without too many nugget losses, but in other places you cannot.

If you get out of the washes then things get more complex. Landslides can leave nuggets dispersed randomly. Eluvial processes can leave nuggets dispersed towards the surface of hillslopes, or towards the surfaces of flats due to soil deflation. This is a very common occurence on the benches/terraces of these washes in Arizona in some places.

It's impossible to give a percentage of flour/nugget gold vs gold bearing cobbles. It's highly dependent on how the gold mineralized to begin with, the distance it traveled, the rock which makes up the gravel, the ore bearing rock and it's resistance to erosion, the topography, the amount of precipitation, and probably a lot of other factors. It changes place to place - some places have almost only just nuggety gold and some places have almost only flour gold. Some places specimen type (quartz bearing) gold is quite common, other places it doesn't exist at all.

I've never personally detected a quartz cobble in a stream bed that had literally no visible gold, because the detection depth on such pieces is quite low and stream beds host detectable size gold usually deeper towards bedrock. I've found it very commonly in dry eluvial placers though where it's closer to surface, or eroding directly out of the vein itself.  I have detected pieces of quartz in washes that had so little gold visible that I had to scrub it down with a toothbrush to see some small shiny bits. 

I've seen such pieces posted before by others who detected them, that were later crushed and shown to have quite a lot of gold inside. 

[GotAU?]

Geomorphology also affects the size and distribution of gold and alluvium in a stream channel, especially by the size of its watershed and the degree of its slope.  Wineglass valleys form by cutting into very steep slopes, often along steep fault scarps. They have narrow and very steep alluvial fans at their base, and a steep funnel shaped valley above it at the head, and are subject to high velocity flows with poorly sorted materials when it rains. Wineglass valleys may possibly be good places to prospect if access is good as the alluvium is poorly sorted and any gold and other heavy materials  is more evenly mixed in, whereas those large  alluvial fans that spread out almost flat across a valley from large washes are formed by lower velocity flows of well sorted alluvial materials with larger heavier materials, including gold nuggets becoming more deeply deposited first, and the fines deposited further downslope.

[geof_junk]

LeighB  Below is a post from a topic that I did 2½ years ago that might be what you are after, if so just click on this link                                          .......LINK......

There is a few Google Earth images that shows some more nugget spread found with a detector in some areas that show the draining paths when the desert gets flooded as the ground is fairly flat and are mainly dry.

Copy of the post

This spread was south of a large reef area and digging hole sloping down to the north. The area in the circle is sloping southwards. About 40 nuggets were found and about half were sitting in the sun having a tan. Most were about 2 or 3 grams in weight. At the bottom of the slope about 400 metres south I got a quarter ounce (8 gm) fairly deep. If I get back that way I know that there is a good prospect for a week or two in untouched ground. I guess everyone want to keep their runs of nuggets to themselves.  ­By the way I will be unable to post anything till the end of February.

I forgot to say the circle is 300 meter across.

[Gold Catcher]

An interesting aspect of alluvial fans is the often observed "reverse grading". This means that the heavier rocks (and gold) can often be found on the surface of the debris flow, not at the bottom like in water-only flows, such as rivers. This has to do with the density and strength of the flow material (matrix) which creates pressure gradients by which larger (and heavier) material are moved upwards, not downwards. This is why you don't have to look for bedrock in alluvial fans (good luck...), instead the heavy materials (including coarse gold) can be on the surface and within detector reach.🤠🤑

GC

"Reverse grading often forms due to sieving and density differentials during transport, which forces large clasts to the top"

 

https://sites.google.com/site/wvugeol616advancedsed/home/alluvial-fans

 

https://en.wikipedia.org/wiki/Graded_bedding

 

 

 

[Jim_Alaska]

I agree; you can also see this effect in a pan of unclassified material. The larger rocks move to the top when you first start panning.

[jasong]

Don't confuse size sorting with density sorting though. For example: the larger rocks may go to the top of the pan, but the denser nuggets stay at the bottom. Unless you really shake the pan hard, and oscillate it up and down to bring the nuggets to the top. It requires both a high energy input plus a turbulent impulse to do such a thing. If the energy isn't both high plus chaotic, it's unlikely to overcome the natural tendency for gravity to sort things by density.

But even if that succeeds to overcome gravity and bring nuggets to the top and then suddenly stop the agitation, if you leave that pan sitting in the open for a long time - geologically speaking like a million years - those nuggets will still end up sinking right back down to the bottom of the pan again because gravity never stops working and every tiny vibration will move the nugget slightly further down until it hits the bottom of the pan (bedrock) and stops. At least, assuming the wind doesn't blow all the lighter gravel off the top of the pan and expose the nuggets (soil deflation). 

That's why gravity and wind are the principal erosional forces that create nugget concentrations in dry, alluvial fans over time (ignoring for now the wash bottoms). They act upon the gravel daily. That isn't to say there aren't exceptions, especially in cases where landslides and floods have happened relatively recently geologically speaking and are still the dominant sorting factor involved. Rich Hill is one such place that seems to have both a massive landslide as well as potentially massive flooding. But even there, it's already clearly visible that bedrock concentration is starting to occur in some areas, and many of the surficial nugget patches appear to be the result of soil deflation. This is because over enough time, gravity and wind become the dominant erosional forces ahead of intermittent chaotic, rare events like landslides.

The amount of density sorting by the gentler, but ever present erosional forces can be used to gauge the age of catastrophic events like landslides and floods, for this reason.

 

[GotAU?]

4 hours ago, Gold Catcher said:

An interesting aspect of alluvial fans is the often observed "reverse grading". This means that the heavier rocks (and gold) can often be found on the surface of the debris flow, not at the bottom like in water-only flows, such as rivers. This has to do with the density and strength of the flow material (matrix) which creates pressure gradients by which larger (and heavier) material are moved upwards, not downwards. This is why you don't have to look for bedrock in alluvial fans (good luck...), instead the heavy materials (including coarse gold) can be on the surface and within detector reach.🤠🤑

GC

"Reverse grading often forms due to sieving and density differentials during transport, which forces large clasts to the top"

 

https://sites.google.com/site/wvugeol616advancedsed/home/alluvial-fans

 

https://en.wikipedia.org/wiki/Graded_bedding

 

 

 

That’s a great reference, Good photos too.  Death Valley has so many great examples of this stuff to see, every geology class I’ve taken have always included field trips there and I recognize some of the stops there.  If one really wants to delve into this, there’s a great Geomorphology book available on the web in PDF version. New editions of the book are available to buy  but the 2nd and 3rd editions are available from several links online. Just search for “Fundamentals of Geomorphology by Richard John Huggett PDF”