Mineralization in auriferous ground

[WM6]

Not only "hot rocks",  red clay soil can sounds like gold too:

Mineralization in auriferous ground is one of the great annoyances in metal detecting. In highly mineralized areas, like Golden Gully, at Wedderburn and Queens Reef, Moliagal, I have dug up several "hot rocks", that sounded so much like gold. Only the most experienced of detector operators who have worked this ground for long periods develop the ear to discriminate between hot rocks and gold. For me, I dig all targets, regardless. I have also dug up quantities of red clay soil that sounds like gold. It can be quite disconcerting at times, and I am not alone in telling you that this is a regular occurrence, especially if you are not totally familiar with an area.
Sue "Goldie" Reynolds in Gold-Net_Au, Dec 2000

[Doug]

Not only "hot rocks",  red clay soil can sounds like gold too:

Mineralization in auriferous ground is one of the great annoyances in metal detecting. In highly mineralized areas, like Golden Gully, at Wedderburn and Queens Reef, Moliagal, I have dug up several "hot rocks", that sounded so much like gold. Only the most experienced of detector operators who have worked this ground for long periods develop the ear to discriminate between hot rocks and gold. For me, I dig all targets, regardless. I have also dug up quantities of red clay soil that sounds like gold. It can be quite disconcerting at times, and I am not alone in telling you that this is a regular occurrence, especially if you are not totally familiar with an area.
Sue "Goldie" Reynolds in Gold-Net_Au, Dec 2000

The red clays that give a signal that sounds like gold(particularly bigger, deeper gold ) here we call "clay domes"Despite "improvements" in technology they can still be an annoying problem in some places particularly if the ground is wet.
doug

[Huego]

I contend that these noisy red clay in goldfields are simply a finely broken down concentrates of iron oxides (in their various chemical / oxidsation & magnetic states) that HOT ROCKS are composed of. Perhaps in higher oxidation state, ie closer to haematite than magnetite.

Also I found such clay is sometimes associated with nuggets or fine gold (in many goldfield sites) because gold originally co-occurred with mineralised (mostly iron-bearlng sulphides and / or oxides) near the surface. (Near the surface the brittle sulphides, but not the malleable gold it was once associated with, over millions of years have been broken down and oxidised by air to oxides, some reduced by carbon in fires, often eroded in rounded forms, some ground & concentrated (with the gold) to smaller reddish (clay) particles. That is simplistically what I see has happend to such red clays & red clay domes.

Huego

[Doug]

I contend that these noisy red clay in goldfields are simply a finely broken down concentrates of iron oxides (in their various chemical / oxidsation & magnetic states) that HOT ROCKS are composed of. Perhaps in higher oxidation state, ie closer to haematite than magnbetite.

Also I found such clay is sometimes associated with nuggets or  fine gold (in many goldfield sites) because gold originally co-occurred with mineralised (mostly iron-bearlng sulphides and / or oxides) near the surface. (Near the surface the brittle sulphides, but not the malleable gold it was once associated with, over millions of years have been broken down and oxidised by air to oxides, some reduced by carbon in fires, often eroded in rounded forms, some ground & concentrated (with the gold) to smaller reddish (clay) particles. That is simplistically what I see has happend to such red clays & red clay domes.

Huego

Some clays have a very porous structure and when they take up water containing electrolytes they become mildly conductive and then may give  a good signal on a detector.
Some clay minerals I believe themselves are also mildly conductive when wet.
doug

[Huego]

Good points Doug.

During & after severe bush fires in woodlands (common in our Victorian goldfields) there is a lot that happens to change the ground.  Very high temperatures, contact with carbon on the surface and below ground and reduction by carbon monoxide etc - these can all change the nature & behaviour of ground minerals (especailly multivalent metals like IRON) comprising hot rocks in contact with them.

We often get good rains after periods of bush fires  These leach the soluble (conductive) salts / minerals from the wood ash into the soil and clays. The fine, but less soluble minerals (oxides), are also mobilised & wash into the in clays pores. In other dry (eg WA) areas after rain, ions of salt and gypsum are mobilised.

This is dynamic & changing environment has being recylcing & changing  like this over millions of years.

Huego 

[Doug]

Here is some info on soil conductivity which is relevant to Australia because we have saline(sodic) affected soils.
http://www.science.org.au/nova/035/quirk.htm
doug

[Doug]

Some more links.
A good one!Geonics technical note TN5
https://docs.google.com/viewer?a=v&q=cache:vGGJYAQ-ve8J:www.georentals.co.uk/tn5.pdf+conductive+soils&hl=en&gl=au&pid=bl&srcid=ADGEESjbTtGzYcqxNxNE8_m04jnWawHx16Y1njVWo192jIATeSuWa-5x4NiqjKGlFNhRzheIszlK2fE7QSphCwcpxyQ9sfk5xHwyDT07BKBJuIG3gEtgEkQEh6VtClauHmnN0Zk-jC7L&sig=AHIEtbSj9hTSiyL6HRrZoFHSEMhr-FNlow

https://docs.google.com/viewer?a=v&q=cache:X2KLjuK70RUJ:pubs.ext.vt.edu/442/442-508/442-508_pdf.pdf+conductive+soils&hl=en&gl=au&pid=bl&srcid=ADGEEShrr65qAelzHV33N96DnNnKw-XuvUwJ5fBZ5xDgHSnPq190Dc_WFnRbhQ28vyduaFv4piM92s-QnLkm4zP5i3kpXePeEFIvB2-u9FQerchyJEwhFpQTFfeXy3cSG38f_t_T-j8l&sig=AHIEtbRoRlOzPvmD-UzaSFRjIsAyh5BdUQ
doug

[Aziz]

Hi all,

"Clay minerals have a wide range of particle sizes from 10's of angstroms to millimeters. (An angstrom () is a unit of measure at the scale of atoms.) Thus, clays may be composed of mixtures of finer grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate, and metal oxides."
from
http://pubs.usgs.gov/info/clays/

The particle size can vary from 1 nm (10 Angstrom) to ....
In conjuction with the iron minerals (Limonite) some clay domes may have superparamagnetic (SP) features, hence single domain magnetic particles.

Well, Limonite is a composition of different kind of iron oxides and oxyhydroxides:
- Geothite (alpha-FeO(OH)) - Hämatite (not magnetic)
- Lepidocrocite (gamma-FeO(OH)) - Maghemite (magnetic, if particle size very small -> SP)
- and other (not dominating) minerals

Red clay may have more of the stable hämatite (reddish colored) but this can be transformed with organic matter and heat (bush fire) into maghemite (brown colored).

The false signals may come from the soil conducitivy or from magnetic viscosity effects or from other effects, which must be invastigated further.

But the soil conductivity effect can be tested easily:
Find a place where the detector does not give false signals. Pour saline solution (water+salt solution) over the place and make the same measurement (take more salt and water). This should give some more info.

Cheers,
Aziz

[Aziz]

BTW, I have managed to produce high concentrate maghemite with very fine particle size (going down into nm .. µm range).

Steel wool -> (oxygene depleted wet and slow corrosion) -> Green rust -> (oxidation) -> Lepidocrocite -> (dehydratation) -> Maghemite

When I have more time, I will post more on this on a seperate topic.

Cheers,
Aziz

[Doug]

Hi all,

"Clay minerals have a wide range of particle sizes from 10's of angstroms to millimeters. (An angstrom () is a unit of measure at the scale of atoms.) Thus, clays may be composed of mixtures of finer grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate, and metal oxides."
from
http://pubs.usgs.gov/info/clays/

The particle size can vary from 1 nm (10 Angstrom) to ....
In conjuction with the iron minerals (Limonite) some clay domes may have superparamagnetic (SP) features, hence single domain magnetic particles.

Well, Limonite is a composition of different kind of iron oxides and oxyhydroxides:
- Geothite (alpha-FeO(OH)) - Hämatite (not magnetic)
- Lepidocrocite (gamma-FeO(OH)) - Maghemite (magnetic, if particle size very small -> SP)
- and other (not dominating) minerals

Red clay may have more of the stable hämatite (reddish colored) but this can be transformed with organic matter and heat (bush fire) into maghemite (brown colored).

The false signals may come from the soil conducitivy or from magnetic viscosity effects or from other effects, which must be invastigated further.

But the soil conductivity effect can be tested easily:
Find a place where the detector does not give false signals. Pour saline solution (water+salt solution) over the place and make the same measurement (take more salt and water). This should give some more info.

Cheers,
Aziz

On some ground when its wet its noisy (on ML detectors salt gold  timing often solves the problem)and when dry its ok. So I assume that this is due to conductivity effects or earth currents (eg from SWER power lines or leakage currents from AC HV lines which always take the path of lowest resistance=highest conductivity) traveling closer to the surface(LF noise?) because the water table (path of lowest resistance) or clays that are conductive when wet are closer to the surface?
doug

[Aziz]

Hi all,

"Clay minerals have a wide range of particle sizes from 10's of angstroms to millimeters. (An angstrom () is a unit of measure at the scale of atoms.) Thus, clays may be composed of mixtures of finer grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate, and metal oxides."
from
http://pubs.usgs.gov/info/clays/

The particle size can vary from 1 nm (10 Angstrom) to ....
In conjuction with the iron minerals (Limonite) some clay domes may have superparamagnetic (SP) features, hence single domain magnetic particles.

Well, Limonite is a composition of different kind of iron oxides and oxyhydroxides:
- Geothite (alpha-FeO(OH)) - Hämatite (not magnetic)
- Lepidocrocite (gamma-FeO(OH)) - Maghemite (magnetic, if particle size very small -> SP)
- and other (not dominating) minerals

Red clay may have more of the stable hämatite (reddish colored) but this can be transformed with organic matter and heat (bush fire) into maghemite (brown colored).

The false signals may come from the soil conducitivy or from magnetic viscosity effects or from other effects, which must be invastigated further.

But the soil conductivity effect can be tested easily:
Find a place where the detector does not give false signals. Pour saline solution (water+salt solution) over the place and make the same measurement (take more salt and water). This should give some more info.

Cheers,
Aziz

On some ground when its wet its noisy (on ML detectors salt gold  timing often solves the problem)and when dry its ok. So I assume that this is due to conductivity effects or earth currents (eg from SWER power lines or leakage currents from AC HV lines which always take the path of lowest resistance=highest conductivity) traveling closer to the surface(LF noise?) because the water table (path of lowest resistance) or clays that are conductive when wet are closer to the surface?
doug

Earth currents (due to AC mains) and magnetic induced earth currents (due to earth magnetic field disturbations) could be one of the reasons.
The latter one would form a very big "coil" on large conductive ground regions, which could cause a current flow and hence cause the detector noise.

But generally, the ground has always conductive paths (ground water drains). A rain fall does not change it very much I think. So the question is, whether it's a local ground conductivity problem (conductivity near the surface and coil changes) or a global ground conductivity change problem.

So many questions.
Aziz