Gold & Its Association With Ironstone

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Ironstone is something that still fascinates me every time I come across it. I dont just mean the occasional little pea-sized piece you see kicking around on the surface at times, but the stuff that exists in more serious quantities. As a relative newbie to the hobby (Ive been swinging the GPX 5000 for just over a year now), it still quite often stops me in my tracks in wonder when I come across it.

I should have taken some photos of examples that have fascinated me from time to time, but as Im already loaded up with a pick, pinpointer, scoop, sP-01 enhancer, drink bottle, Snake Bandages & Camelbak as well as my detector with harness, theres really no room for a decent camera, and once Ive put my phone away safely in a back pocket to keep it away from my 25 coil, Ironstone seems hardly worth the effort of stopping and getting it out for a snap or two.

In some places as Im sure many members have seen, broken pieces of Ironstone can be found in huge chunks, and in one particular detecting session at Kingower some months ago I came across a grapefruit sized chunk that had a side where it hung down in festoons. I had assumed at the time that this was the result of some molten upsurge from within the earth, until only recently when learning that Ironstone is in fact sedimentary. Now in the absence of any Geological training, this revelation completely turned my understanding of things upside down. Upon further reading I came across something that member Goldierocks had written..
In Post # 57 under the Thread heading Gold Beyond The Workings in the Metal Detecting for Gold forum https://www.prospectingaustralia.com/forum/viewtopic.php?pid=518855#p518855

Goldierocks states the following-

(i) the nuggets found in the laterites are usually secondary, not primary. Primary nuggets (that were deposited with quartz veins etc) usually have at least 3% silver (up to more than 50% silver). Secondary nuggets that grow in laterites usually have no detectable silver (e.g. they are 99.7% gold, the 0.3% commonly being copper).
(ii) any nuggets found within the laterite are usually above or only offset horizontally at most tens of metres from the source at depth (but that can be as much as 30 m below, often with white clay lacking gold in the intervening interval). The nuggets actually grow in the iron-rich ferricrete at the top of the laterite (the gold gets dissolved from the primary ore at depth and re-deposited where there is iron near surface - they actually require the iron for the gold to be re-deposited). For those chemically inclined the gold is dissolved in cold, oxygen-rich, highly salty water at depth as a gold chloride ion, and has to be reduced again to metallic gold by encountering iron (oxidation state +2) that reduces it to metallic gold. Nuggets of this type sometimes have ironstone inside them, completely enclosed by gold. Since we know the ironstone formed by weathering, the gold must also have deposited in the weathering zone because it includes ironstone inside it. When the gold dissolves from silver-rich gold at depth, the silver and gold separate from each other and re-deposit separately (sometimes the silver simply stays in the groundwater and washes away), giving the high purity of secondary gold nuggets.


So my question is this- Im assuming that here in Victoria we dont have the laterite soils of Western Australia? But is it possible that a similar process still takes place in some Victorian goldfields whereby primary gold, or particles of gold in the soil in general, gets dissolved by surface water mixing with the high salt content in the soil of areas affected by naturally occurring Dryland Salinity, and the readily available oxygen of the atmosphere at the surface? Im intrigued by the whole thing because a number of times now I have seen photos of, or read about a decent sized gold nugget found here in Victoria that is associated with Ironstone that is nowhere near a reef or other obvious primary gold origin. And my other question is this- When such nuggets are found, if formed by a similar process as mentioned above by Goldierocks, would they exhibit a different grain/crystalline structure inside when cut in half compared to gold that is primary gold? Or, wouldnt their purity also be much higher than any reef gold nearby that might be suspected of being the primary source?

I love reading the contributions of Goldierocks, Swright and other Geologists on the forum and Id appreciate if someone could enlighten me further please on gold when it is associated with Ironstone.

Thanks in advance
D.S.
 
Goldierocks can help (good questions).

Im assuming that here in Victoria we dont have the laterite soils of Western Australia

Firstly, your assumption that Victoria does not have lateritic soils is incorrect. Victoria had soils much like Western Australia, but Victoria had more topographic relief (hilly) so more of this iron-rich and kaolinitic white clay of the lateritic profile has washed away (WA is flatter so more of this weathering veneer remains). However the white clay is still widespread in Victoria, and in places so is the ironstone. I have a technical reference with papers that you could PM me for, but if you don't feel your geology is up to it you might find it overly technical. Up to you.

But is it possible that a similar process still takes place in some Victorian goldfields
Not on any significant scale - the lateritic soils in Victoria and WA occurred long ago (in the Neogene, say 30 million years). When I drill I often find them preserved under great thicknesses of younger stream alluvium. However it might have occurred on a small scale again about 1 million years ago, when the ironstone "Karoonda Surface" formed in the Murray Basin and adjacent areas (eg around Bendigo) - but I can't find any good evidence for this and suspect that ironstone may be a red herring and not very useful to prospect around. That is, we had wet conditions 30 million years ago that formed the clays, and a lot of ironstone probably formed over the clays at the end of that interval as things dried out and were more saline. We then had another wet burst around 1-2 million years ago when mostly ironstone was deposited as that dried out. Otherwise your reasoning is fine.

a number of times now I have seen photos of, or read about a decent sized gold nugget found here in Victoria that is associated with Ironstone that is nowhere near a reef or other obvious primary gold origin.
If you re-read what I wrote, you will see that lateritic gold nuggets occur in close proximity to an underlying source - usually within 50 m. So the source is not known in the cases you mention (you have to have the gold originating somewhere), Some nuggets (Boort, Watchem) have been found associated with aboriginal campfire charcoal, so were probably brought there.

When such nuggets are found, if formed by a similar process as mentioned above by Goldierocks, would they exhibit a different grain/crystalline structure inside when cut in half compared to gold that is primary gold? Or, wouldnt their purity also be much higher than any reef gold nearby that might be suspected of being the primary source?
Yes, they have a different internal structure (primary gold can have "foam" structure, that shows up under a microscope after etching lightly with aqua regia vapour). Yes they often have higher purity if lateritic. Most primary Victorian gold has 3% or more silver alloyed with it (up to 50%, but commonly less than 10%). Secondary lateritic gold commonly has no significant silver (way under 1%) - if anything else is present it can be 0.3% copper for example.

1572244364_norval_regolith.jpg


1572244395_karoonda.jpg
 
In this typical cross-section, bedrock is unweathered, saprock is bedrock weathered to white clay (but you can still recognise the texture of the bedrock in it - bedding, cleavage etc). In saprolite it is so totally weathered to white clay that you cannot recognise any features of the original bedrock). The mottled zone is where ironstone nodules start to appear (see fig 13.46 above). Lateritic duricrust is commonly massive ironstone above the mottled zone (also in that figure) - "ironstone". Lateritic gravels above that form by break-down during more recent weathering of the underlying duricrust (all those rounded black pebbles you see mixed in with soil - the soil itself being a thin, modern layer on top of it all.

1572245128_idealized-weathering-profile-evolving-toward-a-mature-laterite-modified-from-mcqueen.jpg
 
Wow! That is awesome information thanks Goldierocks, and thank you so much for your kind offer of a technical reference with papers. I may take you up on it if the offer is still available in the future, but I think for now I've kind of bitten off more than I can chew and I'm chewing like crazy to digest it all. For example, it's only recently that I've actually discovered that many of the areas I have been detecting in are just too deep for a metal detector. "Depth of Regolith" meant absolutely nothing to me this time last year, and yet now with the little knowledge I have gathered on the subject I can understand why most of us would be kidding ourselves if we thought that given a chance to detect somewhere like Catto's Paddock in Rheola we had some chance of pulling a big one out of the ground with any type of hand-held metal detector and a pick. Most of the Nuggets I have seen records of from around there came from between 9 and 12 feet, usually within only a few feet of bedrock. I just put it down to learning and move on.

As for the Ironstone revelation though, one other thing I am wondering about it is would it show up in any way on say an aerial magnetic survey? Hypothetically speaking, could one for example draw inferences say from Radiometric readings for potassium (inferring hydrothermal activity in a known gold producing area), and then say follow up with data from a magnetic survey which may show the effects in a specific area close by, on readings caused by Ironstone formations? i.e., indicating the required conditions for the type of nuggets mentioned earlier that "grow" in situ within a roughly 50 meter radius? Sorry if it sounds like pie-in-the sky stuff, but I'm just trying to find some type of scientific indication of where to look (a Goldielocks zone for want of a better term), whereby the depth is right for a 5000 with a big coil, the geological conditions are favorable for such gold, and where one can apply some science to the situation rather than simply go where other people have been before in the hope of stumbling over something that they have missed.

Thanks again for all your wonderful help,
D.S
 
I am fascinated by laterite :)
Try and download and read "GOLD AND ASSOCIATED ELEMENTS IN THE REGOLITH - DISPERSION PROCESSES AND IMPLICATIONS FOR EXPLORATION P24 I Final Report"
by Butt et al.
 
Great post DS, and great info Goldierocks.
Im with you DS as far as trying to learn more about Geology to give me better chance of identifying more likely places to find shallow nuggety gold with a detector. Reading with great interest.
Thanks guys. :Y:
 
would it show up in any way on say an aerial magnetic survey?

The ironstone will sometimes show up, but in Victoria it is limonite (goethite) in most areas, which is not magnetic. Any magnetic material is mostly a very thin (and not very responsive) layer of pebbles in the soil which have been converted from limonite to maghemite during bushfires (magnhemite is magnetic, but not as magnetic as magnetite). Too thin to give much response.

Hypothetically speaking, could one for example draw inferences say from Radiometric readings for potassium (inferring hydrothermal activity in a known gold producing area), and then say follow up with data from a magnetic survey which may show the effects in a specific area close by, on readings caused by Ironstone formations? i.e., indicating the required conditions for the type of nuggets mentioned earlier that "grow" in situ within a roughly 50 meter radius?

The problem is that the weathering commonly changes illite (potassium clay) and white mica (also with potassium) to kaolinite (which has no potassium). So we do use that approach in areas where nearly all the weathered material has been weathered away, and we have relatively unweathered bedrock at surface (so that potassium rich alteration minerals such as white micas are present at surface). However in most lateritic areas the potassium has already been weathered away at surface.

illite, white mica = hydrated potassium aluminium silicate
kaolinite = hydrated aluminium silicate (the potassium has weathered out and washed away)

However a further complication is that only some types of gold deposits have ENOUGH potassium-rich alteration of the rock around the gold veins to be detectable. Not common in say Victoria, but important in central western NSW (eg Cadia). So you have to consider the nature of the gold deposits in your area. It also depends on whether you are using airborne measurements or measurements with a hand-held instrument (since the gamma radiation from the potassium rapidly decreases with height above the ground, the hand-held instrument is more sensitive to lesser and thinner alteration zones).

A ground magnetic survey (handheld) might show up the ironstone - but so will looking in the soil for ironstone pebbles (more effective).

I have sometimes used airborne thorium as an indicator of ironstone - for some reason it seems to lock up with the iron oxides. Biut that just tells me where ironstone is on the scale of km - if I am actually walking in that area I can see the ironstone anyway.

Another thing is that more of the gold in a weathered area is not now in the ironstone (even if it originally formed there) - so if you ONLY detected where you see ironstone you might miss most of it. The ironstone might be largely stripped away, but the heavy gold will stay around.

Recognising what depth gold occurs at is critical - if miners were mining gold-rich gravels at 10 m depth, there is no point in systematically detecting the area (but often their dumps, which contain material that they brought up from depth, can still contain good gold in those areas - but not between their dumps). I discussed this on this site somewhere...
 
kingswood said:
I am fascinated by laterite :)
Try and download and read "GOLD AND ASSOCIATED ELEMENTS IN THE REGOLITH - DISPERSION PROCESSES AND IMPLICATIONS FOR EXPLORATION P24 I Final Report"
by Butt et al.
People like Butt are good value - did important work.
 
Well, thanks to you Goldierocks, & Kingswood, my thirst for further geological education has well and truly been whetted.

I've ploughed my way through "GOLD AND ASSOCIATED ELEMENTS IN THE REGOLITH - DISPERSION PROCESSES AND IMPLICATIONS FOR EXPLORATION P24 I Final Report" by Butt et al, and also I have read further regarding the various zones below the topsoil, all the way to bedrock. Mottled Zone, Saprolite, etc., I find this stuff amazing, and it goes some way to explaining why for instance a large nugget may be sitting above bedrock and not actually on it (although, I suppose given enough time and the right geology and climate it may/will end up sitting on bedrock one day.

But just as I was ready to accept that many gold nuggets were in fact Supergene in origin, helped along by the chemical process of gold ions finding their way in saline environments to areas of ironstone and building a nugget (if in fact I have understood the process correctly), I came across this by Butt:-

Only thing is, it seems to contradict what he said in the report above. There's no date on this page that I have just found, so I don't know if it was published before the above report or after. It's one of the problems I have with trying to understand whether gold is Supergene or Hypogene in origin, or indeed both?

http://crcleme.org.au/Pubs/Articles and papers/Goldsmidt_butt01004.pdf
 
Yes, many workers have diverged from Butt in more recent times. It is correct that many large nuggets are hypogene as he says - we always knew that (deposited in the primary quartz veins when the quartz veins formed etc). However many are supergene (there are good photos of gold completely enclosed inside nuggets, and they are chemically distinct in the same places). He may well be correct about the very large nuggets, I have little doubt about that (they are rarely more than a couple of hundred metres downhill of a quartz vein, and often have internal textures inconsistent with supergene origins). But most of us are satisfied with an ounce or two size....
 
We even find gold precipitating as platings on tree roots (I assume because of the carbon), and in the veinlets within leaves. It moves around.
 
goldierocks said:
We even find gold precipitating as platings on tree roots (I assume because of the carbon), and in the veinlets within leaves. It moves around.

Yes, I have read a number of times now online where people have mentioned tree roots being found encased in gold, a bullet shell found in a creek in Tasmania that has become gold-plated, and even a coin found in a stream in Alaska that has become plated with gold as well. As yet though, no matter how hard I have searched I haven't found a single photo or paper in relation to such phenomenon. Surely, without any scientific evidence of such claims they simply remain mythical, B.S or "anecdotal"?

I have read of research regarding gold found in leaves of trees, and that does make sense if it is being pumped up from the ground by the tree. Although surely that is a completely different mechanism to some type of naturally occurring electrolysis?
 
But that's my point goldtrapper. Everybody has heard of these pictures or stories, or they recount them as though they are a scientific fact. Even the Prospecting and Miners Association of Victoria states on one of its pages regarding the golden triangle's large gold nuggets at http://www.pmav.org.au/stories-a-reports/triangle-gold

Such evidence includes a brass cartridge lying in mine water being completely replaced by gold (Tasmania), a $20 gold coin being recovered from a stream after several decades coated in gold crystals (Alaska)

But just where are these pics or "evidence" that never seem to be produced? It would be nice if they do exist to have them shared with the rest of us, and some scientific investigation undertaken into how these events may have occurred. Until then they are no more than claimed religious relics, giving succour and comfort to the faithful.
 
some of what i teach relates to iron/gold and it's association.

keyword from above that Goldierocks mentions above is 'precipitate'.

how? gold compounds. elements that it can bind with at different conditions.under the right conditions,iron precipitates gold.

how many gold compounds do you know?
 
Deepseeker said:
goldierocks said:
We even find gold precipitating as platings on tree roots (I assume because of the carbon), and in the veinlets within leaves. It moves around.

Yes, I have read a number of times now online where people have mentioned tree roots being found encased in gold, a bullet shell found in a creek in Tasmania that has become gold-plated, and even a coin found in a stream in Alaska that has become plated with gold as well. As yet though, no matter how hard I have searched I haven't found a single photo or paper in relation to such phenomenon. Surely, without any scientific evidence of such claims they simply remain mythical, B.S or "anecdotal"?

I have read of research regarding gold found in leaves of trees, and that does make sense if it is being pumped up from the ground by the tree. Although surely that is a completely different mechanism to some type of naturally occurring electrolysis?
There are photos of gold in leaves on-line. The gold on tree roots is reliable, specimens from the 1800s are in the British Museum.
 
Deepseeker said:
But that's my point goldtrapper. Everybody has heard of these pictures or stories, or they recount them as though they are a scientific fact. Even the Prospecting and Miners Association of Victoria states on one of its pages regarding the golden triangle's large gold nuggets at http://www.pmav.org.au/stories-a-reports/triangle-gold

Such evidence includes a brass cartridge lying in mine water being completely replaced by gold (Tasmania), a $20 gold coin being recovered from a stream after several decades coated in gold crystals (Alaska)

But just where are these pics or "evidence" that never seem to be produced? It would be nice if they do exist to have them shared with the rest of us, and some scientific investigation undertaken into how these events may have occurred. Until then they are no more than claimed religious relics, giving succour and comfort to the faithful.
Not at all, some can be viewed on-line. Not sure why you think this unusual (don't know about the nail and coin example though).
 

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