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Localizing Desert Hillside Detectable Gold


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I believe it's backwards - TrinityAU, aka Ray Mills, should be able to find his stuff by one of those two names.

Tboykin has illustrated (masterfully) what are known as bench or terrace deposits. I agree it's a likely reason for what you've found. Depending what part of AZ you are in, the situation can be a bit more complex as many of the pediments in which these gold bearing washes are located have been incised, then re-incised many times over, cutting through multiple layers of old wash channels and bench deposits. 

So, often you will find that one particular wash has cut down into, and intersected an older, buried, gold bearing wash channel, and the small current-day wash is now re concentrating that gold with no apparant hardrock source. No other wash in the area will have gold because the modern day wash has not yet incised the old gold bearing wash channels. It also depends where a newer wash cuts into an older wash, some parts of the older fossil wash channel may not have many (or any) nuggets while some parts may be quite rich. Also many of these old washes are now basically just caliche and some caliche is soft (basically still gravel) while some is hard (false bedrock) and more resistant to secondary erosion, so some erodes into modern washes easily while some is not as easy a few hundred feet away.

This is very common in Western Arizona where heavy monsoon rains and a once much wetter climate have created complex layers of washes in the pediment gravels.

The situation can be further complicated by the fact that in AZ many of the rich nugget areas were originally fed by lenses/pods  (aka pockets), largely surface occurences of very rich ore and not long, continuous veins. Many of these are now entirely eroded away, or buried under the pediment gravels leaving no trace, making the source a "mystery". They can form in swarms, so that you will have one nearby shedding prickly gold and another further up the pediment which is providing more worn gold since it has travelled. 

So, often you will find both slightly water worn gold along with pocket gold in the same spot, though the gold might have multiple sources. Especially if a modern wash has incised an older drainage. Also, a piece of speci gold that has travelled and then broken up can often have both water worn looking gold crumbling apart on the outside, and more prickly gold on the inside, which can lead to what you have in your hand as that speci erodes downhill and breaks up.

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9 hours ago, Goldseeker5000 said:

Remember, that regolith, including gold has a tendency to move down a slope or Hill in a fan shape, spreading out towards the lower sections.This will apply whether it is coming from a lode or or an old placer gold pocket eroding out of a hillside that was once part of a former drainage.

Do you think the breadth of a fanned deposit is more dependent on distance traveled or grade of incline?  For instance, this locale has hills no more than 10-30 feet high.  Would you expect that translate into a tighter line because it hasn't far to travel, or would that more likely mean a broader fan, because the slope is so shallow?

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2 hours ago, jasong said:

So, often you will find both slightly water worn gold along with pocket gold in the same spot, though the gold might have multiple sources. Especially if a modern wash has incised an older drainage. Also, a piece of speci gold that has travelled and then broken up can often have both water worn looking gold crumbling apart on the outside, and more prickly gold on the inside, which can lead to what you have in your hand as that speci erodes downhill and breaks up.

This may actually be occurring, which I hadn't considered.  I found some of the water worn pieces at the higher point in the wash and some of the larger, rougher pieces in the mid portion.  The other things you mentioned lay out a dizzying mystery of geologic layers.  I suppose the one good thing here may be that the bedrock is superficial from the wash to the hilltops.  I hope that bodes well for having less layers for less complication.

Tied into your explanation may be an answer to one of my other questions.  It sounds like hillside gold can truly be either deep or shallow without easy generalization.  I keep thinking about that Jack London short story, All Gold Canyon, where the pocket was deeper towards the source.  Thanks for the in depth insights.

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In the watershed there is a big flat area of concentrated, undisturbed, coarse quartz with dark, black and red, vuggy inclusions.  It looked so good, but there just wasn't much to find detecting up that way past a certain point in the wash.  I did multiple test pans feeding from those areas and found some flour gold, but nothing bigger.  Running right through the wash perpendicularly is a contact zone in the bedrock.  The only way I know there is a contact zone is from the Macrostrat bedrock map.  This seems to transect the area with more nuggets, but the contact zone isn't really obvious to my eye.  It doesn't help much that there are quartz veins everywhere. 

Do you find that these major contact zones on maps are of any help in locating sources?  In this case the part that looks the most alluring with all of the vuggy quartz is in the middle of the area without any contact zone on the map.  The part that doesn't look the that great in person is nearest to the contact zone and closer to the finds.

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A contact zone isn't necessarily a definite, easy to trace line between two types of rock. It can be, but not always. For instance, if you have shearing between two rock types along the contact where structural weakness occurred then you might have quite a large zone (10+ meters) of gouge, and it may look undefinite or difficult to delineate. The gouge is also structurally weakened compared to surrounding rock so it can provide channels for fluids, so it might appear as a large swarm/zone of quartz veining, or at least that may be the easiest way to see it. Calcite is another common one to see here. Veins don't necessarily mean contact zone, faulting or shearing though, conversely. 

Geology is complex because so much time can pass in which things can change, and one small area can experience a massive variety of different mineralization sequences, erosion, and structural alteration because our planet is dynamic. There is often no simple answer, and each location kinda needs to unwrapped and solved individually.

You are asking the right questions though. I don't have the answers to all of them because I am still learning too. Going beyond "gold is where you find it" and thinking about stuff like this is, IMO, what makes the difference between a prospector and detectorist.

I think Tboykin also mentioned something else I agree with, and you have spoken about too - a pan is sometimes more useful then a detector looking for pockets/hard rock sources. As most pockets I've ever found or known about have all had a lot of flour gold in the dirt leading to the pocket. If you aren't finding flour in that hillside patch, then it might indicate that wash/stream reworking is occuring as the desert washes during monsoon can just concentrate heavier gold and wash away flour. 

If you are finding flour but no nuggets in that area with all the interesting looking potential ore, then you may want to spend a few hundred bucks to assay some samples. One good hard rock project, if it has scale, can be worth 20+ years of nugget shooting.

Similarly, the depth at which you find the flour can go a long ways pointing you where to look for a source. If it's only deeper than it may mean your source eroded away already, or has been reburied. Finding it on the surface is a good indicator that there may yet be something to discover in situ still.

I spend less and less time with a detector these days and more time with an ATV, GPS, and a ton of sample bags. Take them back to camp and pan them, then concentrate on places showing color with the detector. But I am not really looking for nuggets any more, I'm concentrating on larger scale stuff to develop and sell to companies, as the challenge and puzzles never deplete so I can do this for the rest of my life, while nuggets are getting really not very profitable to find these days unless one lives right in the goldfields.

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There are no "simple" answers to Skookum's excellent questions,  but Tom and Jason have given some really masterful info. Several more thoughts need to be mentioned or repeated. The current geological formations we detect are often far deeper (lower in elevation) than the original auriferous deposits of times past. Thus, erosion at euvial locations, over time, have repeatedly both revealed and re-covered, shifted, dispersed or concentrated, auriferous deposits, making it difficult to precisely locate the original lode gold deposits of times past. The original lode deposits may no longer exist. My favored method of locating eluvial gold: at sites known to produce alluvial gold, walk along the hillside above the wash, looking for the typical "markers," quartz, greenstone, and red dirt. Maybe even manzanita or desert trumpet. Detect uphill, especially concentrating on low spots, wash outs, and small tributaries. One thing to remember is that success at electronic prospecting is more of a "boots on the ground, detector in hand" scenario rather than anything else.

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Skookum, the shorter the distance it has to travel and gentler slopes the more it will keep a tighter path. The gold will have less likelyhood to hit obstructions and change course, thus widening the fan. Something else to consider as Jim has mentioned,the geological formations in the crust, various types of hydro-thermal gold deposits do happen at deeper depths. However with metasomatism, the replacement of one rock type with that of another through hot mineral Rich magmatic fluids, gases, and other fluids such as highly mineralized hot waters cause the deposit to become brittle speeding up the erosion process around the contact zones. If there is no visible sign of the contact zones anymore then look for all the right rock samples lying on the surface. When this happens and the gold bearing contact has completely eroded away, detect the slopes around where the various rock samples are lying on the ground. The gold should be within detectable depth. There are however more rock types that you should pay attention too as well, such as, limestone, dolomite, marble, pyrite, galena, bornite,scheelite,garnet, traces of arsenopyrite in the quartz. Knowing the rocks associated with gold deposition in your area is important and it is important to know what the gangue rocks are as well because it will help you map out your boundary of the rich area of the contact zones and where it transitions to barren gangue absent of gold.  Those surveyor flags can help you with this as well. It will help form a visual picture of the contact zones.  Circular or a tight zig-zag around and up the slope will give you the best coverage.

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Skookum, Back in Jan this year I did a post on several google shots showing the spread of nuggets from a few of my patches in West Aust they are not 3D but a lot of terrain and different spreads can be seen.      .....Link.....

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3 hours ago, geof_junk said:

Skookum, Back in Jan this year I did a post on several google shots showing the spread of nuggets from a few of my patches in West Aust they are not 3D but a lot of terrain and different spreads can be seen.      .....Link.....

That was a very interesting and well done project of recording your finds!

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