Nuggets are formed by earthquakes!

[XLOOX]

https://www.livescience.com/planet-...z-into-forming-giant-gold-nuggets-study-finds

Well we already knew it was mainly from hydrothermal fluid precipitation but never really knew why it was localised so much in some spots in the vein to make nuggets.

I guess now we do !

 

[grubstake]

Interesting, but it's a pity that the article is so sloppily written. Contrary to the statements therein, orogenic gold deposits aren't formed during earthquakes and 75% of all the gold ever mined didn't come from (large) nuggets:





What the researchers actually found was that the natural piezo-electric mechanism within quartz crystals that are subject to heat, pressure and repeated minor seismicity, can explain the deposition and growth of substantial gold structures within hydrothermal fluids.

The ore samples used by the researchers came from the Fosterville mine, so it's not too surprising that large gold was a focus of their experimental work!

 

[Hawkear]

I think It is important to realise that this latest theory is not a complete new theory but a refinement adding an additional component to the earthquake/pressure/temperature/chemical and now electric model of orogenic gold deposits.
The difficulty most of us have is in visualising how small particles of gold, already embedded in a hard and impervious matrix of quartz, can grow to larger masses by episodic influx of more fluids if those fluids cannot penetrate that existing impervious quartz.
Maybe it is suggested that gold particles already sealed off and surrounded by quartz cannot grow and it is only gold particles at the rock margins, which would be exposed to the fluid ingress, that could grow to any significant size.
On many occasions, the larger nuggets/specimens that I have detected seemed to be associated with not just quartz but rock or ironstone indicating that they came from the edges of the reef. Crushing of the rest of the quartz mass often revealed very fine gold that may have been isolated from later fluid flows.
So maybe the theory needs to include not only the hows but the where.

Here is a link to a geology film which sums up earlier understanding as to orogenic gold deposits and their relationship to earthquakes.

 

[XLOOX]

To deposit enough gold to make a decent nugget many cubic metres of gold bearing solution must have flowed by each point.

I could never figure out why the hydrothermal fluid passing a point in a crack should keep depositing gold on gold on gold at that very very localised point and not just as small particles in the fluid if the deposition was pressure /temp drop initiated, or just as a thin layer all along the wall rock if it was chemically initiated.

Piezoelectrical deposition for (some) nuggets sound like a theory with a chance.

Besides local piezo electricity, pointy broken quartz crystals at the edge of a fracture would be good charge concentrators from the bulk of the host rock and the hydro thermal fluid a good conductive "earth".

Underground lightning anyone ?

Actually, more like electroplating..

 

[Hawkear]

Studies of large nuggets in WA, which involved slicing them open (Ouch) and etching the cut surface have revealed that they all have an internal crystalline structure. Having that structure would be indicative that they grew in the same way that other crystals grow ie by atom by atom attachment. I remember my days in school growing copper sulphate crystals by suspending seed crystals in a super concentrated copper sulphate solution.
Most gold masses would not show their true internal crystalline structure at their surface because the surface shape would be restrained and determined by the shape of the space in which the gold crystal has room to grow. Many specimens of "crystalline gold" are actually what are known as pseudomorphs where the gold takes the reverse shape of crystals against which it forms.
Here is a beautiful example of an unrestrained gold crystal growth probably formed in an open void within a reef matrix (source Wikipedia).

 

[19.3]

For a long time now I have had the theory that gold forms in molten quartz in the same way as zone refining of Silicon works. This article and some of the earlier comments here convinces me even more.
So the way this would work is that as the quartz cooled, the impurities in the quartz (gold) would be pushed together, since the 2 materials (quartz and gold) have different temperatures at which they crystallise leaving the gold fluid while the quartz crystallised, pushing all the separate atoms of gold into clusters.

 

[Flash Jack]

For a long time now I have had the theory that gold forms in molten quartz in the same way as zone refining of Silicon works. This article and some of the earlier comments here convinces me even more.
So the way this would work is that as the quartz cooled, the impurities in the quartz (gold) would be pushed together, since the 2 materials (quartz and gold) have different temperatures at which they crystallise leaving the gold fluid while the quartz crystallised, pushing all the separate atoms of gold into clusters.

Don't know if I am correct or not but I was thinking that as the hydrothermal fluid is pushed up through the cracks and fissures, it will begin precipitating out quartz on to the faces of the fissures due to contact cooling. As this happens the ratio of gold to quartz in the remaining fluid is going to increase the further up the fissure the fluid is going. Towards the end of its travel I imagine the fluid would be a slurry of growing quartz crystals, still-liquid fluid and molten gold. I think then, that the liquid gold would tend to come together to form nuggets of varying sizes. Anyway, just my thoughts.

 

[19.3]

Don't know if I am correct or not but I was thinking that as the hydrothermal fluid is pushed up through the cracks and fissures, it will begin precipitating out quartz on to the faces of the fissures due to contact cooling. As this happens the ratio of gold to quartz in the remaining fluid is going to increase the further up the fissure the fluid is going. Towards the end of its travel I imagine the fluid would be a slurry of growing quartz crystals, still-liquid fluid and molten gold. I think then, that the liquid gold would tend to come together to form nuggets of varying sizes. Anyway, just my thoughts.

To my understanding basically that is how zone refining works. Further, given that the quartz is in relatively thin sheets it will cool gradually at various levels allowing the formation of 'horizontal' gold veins at different levels in the quartz. I have seen many, many gold mines at Hill End with horizontal drives at different levels off near vertical shafts which follow the quartz sheet downwards. This would account for these.

 

[Hawkear]

Because of the very high melting point of pure quartz 1677 degrees Celsius, far higher than normal magmas 700 to 1200 degrees, a theoretical vein of molten quartz would not rise very far into cooler rocks before solidifying and blocking its own passage.
Indeed it is hard to envisage how a channel of pure or nearly pure molten quartz could originate as temperatures at the mantle/crust interface zone are only in the order of about 1000 degrees and molten magmas formed by those rocks are a mixture of many different minerals. Some plausible method of differentiating those minerals whilst in a molten state to provide a source of nearly pure molten quartz would have to proposed.
Evidence against a molten quartz origin for quartz veins also comes from
1. Observations that show lack of sidewall melting as one would expect if a rock of lower melting point had come into contact with molten quartz of very high temperature.
2. Lack of signs of contact heat metamorphism of the country rock.
3. Blocks of unmelted country rock detached from a sidewall now fully enclosed by the reef's quartz matrix.
Most common types of gold deposition such as volcanically related relatively shallow epithermal deposits or deep seated orogenic deposits, can be better explained by the water transport model where superheated water can scavenge elements like quartz, iron, sulphur and other metals from deep seated rocks. These are things now commonly found in quartz vein deposits
I did however recently see a you tube video by Oz Geology which described a unique situation at Camperdown in South West Victoria where an abnormally high gold content was found in relatively recent scoria which originates from molten volcanic rock.
The high gold content was attributed to the passages of magmas through large tracts of much older underlying gold rich rocks, probably melting many auriferous reef lines and leads before explosively erupting to the surface. This type of situation is not unknown in other parts of the world but in all the cases the volcanic eruption of gold relies heavily on preexisting gold deposits underlying the area of the later volcanics.
The same sequence of rocks that are highly auriferous in the Golden Triangle continue South and underlie the balsaltic plains around Camperdown. Just because they do not outcrop at the surface there is no reason not to believe that they too would be highly auriferous.
Perhaps someday it may be possible to prospect below the vast basaltic plains of that area.
Oz Geology link

 

[19.3]

Because of the very high melting point of pure quartz 1677 degrees Celsius, far higher than normal magmas 700 to 1200 degrees, a theoretical vein of molten quartz would not rise very far into cooler rocks before solidifying and blocking its own passage.
Indeed it is hard to envisage how a channel of pure or nearly pure molten quartz could originate as temperatures at the mantle/crust interface zone are only in the order of about 1000 degrees and molten magmas formed by those rocks are a mixture of many different minerals. Some plausible method of differentiating those minerals whilst in a molten state to provide a source of nearly pure molten quartz would have to proposed.
Evidence against a molten quartz origin for quartz veins also comes from
1. Observations that show lack of sidewall melting as one would expect if a rock of lower melting point had come into contact with molten quartz of very high temperature.
2. Lack of signs of contact heat metamorphism of the country rock.
3. Blocks of unmelted country rock detached from a sidewall now fully enclosed by the reef's quartz matrix.
Most common types of gold deposition such as volcanically related relatively shallow epithermal deposits or deep seated orogenic deposits, can be better explained by the water transport model where superheated water can scavenge elements like quartz, iron, sulphur and other metals from deep seated rocks. These are things now commonly found in quartz vein deposits
I did however recently see a you tube video by Oz Geology which described a unique situation at Camperdown in South West Victoria where an abnormally high gold content was found in relatively recent scoria which originates from molten volcanic rock.
The high gold content was attributed to the passages of magmas through large tracts of much older underlying gold rich rocks, probably melting many auriferous reef lines and leads before explosively erupting to the surface. This type of situation is not unknown in other parts of the world but in all the cases the volcanic eruption of gold relies heavily on preexisting gold deposits underlying the area of the later volcanics.
The same sequence of rocks that are highly auriferous in the Golden Triangle continue South and underlie the balsaltic plains around Camperdown. Just because they do not outcrop at the surface there is no reason not to believe that they too would be highly auriferous.
Perhaps someday it may be possible to prospect below the vast basaltic plains of that area.
Oz Geology link

All makes good sense - thank you!
I'm still having trouble envisaging how hydro thermal fluids can lead to the formation of the quartz veins - in the shape they form. Also, how water can take up and carry Silicon and gold (and other minerals/ elements) - then find its way thru crevices - under pressure - and deposit in the form factor of quartz veins - without blocking the cracks and fissures before the massive sheets are formed.

 

[Hawkear]

The movement of fluids up reef lines is one of the hardest things to get ones head around and for years I tried to visualise an open crack or fissure being filled with reef forming material.
More recently the development of the orogenic model which involves the repeated opening and sealing of those cracks and fissures due to seismic activity over eons of time has made me more comfortable.
Under this model rock derived from seabed sediments containing raised levels of heavy elements such as gold is subducted under continental crusts by tectonic forces. At a great depth of around 15 - 20 Kms these rocks are "cooked" by the heat at that depth to mobilise large quantities of their water content which was previously contained either as free water in voids or bound chemically in minerals.
Due to the heat and pressure at that depth, this water, superheated to around 400 degrees is able to "scavenge" or dissolve and retain many elements from the heated plastic but not molten rocks it saturates.
This reservoir of hot fluid is normally kept in place by the overlaying layers of hardened, impervious and cooler rocks above.
During earthquakes, however, these rocks can be cracked which allows a conduit for the hot mineral rich water to escape the depths and under immense pressure be forced up the fault zones and cracks so formed.
As this fluid ascends its temperature, and pressure falls and it becomes unable to hold all of its dissolved minerals and some will deposit out. As the fluids ascend even higher progressive temperature and pressure drops enable more deposition over great lengths of the crack until the fluid is depleted of much of its mineral content. Current thinking puts this deposition zone for quartz and gold between 15Km to 3Km.
It is tempting to think of these cracks as open channels into which fluids flow upwards for long periods, but it is more probable that they are quite narrow and easily blocked by deposited quartz with closure further facilitated when the pressure of the escaping fluid drops and it is unable to counteract inward sidewall pressure.
A single crack/seal event is not likely to provide a great deal of reef material, but unstable tectonic periods can be long lasting, perhaps many millions of years in extent, during which hundreds if not thousands of earthquakes will occur. Each earthquake can reopen previously quartz sealed cracks and allow a fresh pulse of mineral and quartz rich fluid often along one wall of the crack allowing more deposition before resealing. Over many such crack/seal events reefs of considerable extent can build up.
The best evidence we have for these is the laminated quartz common in Victorian reefs where each lamination would appear to be evidence of a separate crack/seal event. Some samples show evidence at the microscopic scale of thousands of such laminations.
The subject is a complex one involving many variables but at least with a coherent model of orogenic reef formations, details such as the role of piezo electrostatic effects can start to be put in place.

 

[Moneybox]

I think It is important to realise that this latest theory is not a complete new theory but a refinement adding an additional component to the earthquake/pressure/temperature/chemical and now electric model of orogenic gold deposits.
The difficulty most of us have is in visualising how small particles of gold, already embedded in a hard and impervious matrix of quartz, can grow to larger masses by episodic influx of more fluids if those fluids cannot penetrate that existing impervious quartz.
Maybe it is suggested that gold particles already sealed off and surrounded by quartz cannot grow and it is only gold particles at the rock margins, which would be exposed to the fluid ingress, that could grow to any significant size.
On many occasions, the larger nuggets/specimens that I have detected seemed to be associated with not just quartz but rock or ironstone indicating that they came from the edges of the reef. Crushing of the rest of the quartz mass often revealed very fine gold that may have been isolated from later fluid flows.
So maybe the theory needs to include not only the hows but the where.

Here is a link to a geology film which sums up earlier understanding as to orogenic gold deposits and their relationship to earthquakes.

This video explains to me the most believable theory if seen so far. Perhaps because geologists seem come up with too many unbelievable theories

 

[Hawkear]

This video explains to me the most believable theory if seen so far. Perhaps because geologists seem come up with too many unbelievable theories

Yes, when dealing with things we cannot witness first hand, theories and models are our best guesses. Yet it is also true that many a beautiful theory has had to be abandoned because of an ugly fact.
The orogenic model has so far explained many of the observable features of Victorian deposits, but a true test is for it to make predictions that can be tested or observed.
In the case of the orogenic model in which mobile fluids are theorised to "scavenge" gold from deep subducted sea floor rocks, the prediction was that those rocks should now show a depletion of gold concentration.
A researcher subsequently found upturned strata in New Zealand where those subducted "source" rocks, originally at depths of around 20Km, had been upturned and exposed at the surface. His measurements confirmed that gold values were indeed depleted in those rocks.
Here is a link to an interview with the geology researcher who confirmed the gold depletion prediction. I enjoy watching these geology films and recommend them to all prospectors to round out their general knowledge.

 

[19.3]

The movement of fluids up reef lines is one of the hardest things to get ones head around and for years I tried to visualise an open crack or fissure being filled with reef forming material.
More recently the development of the orogenic model which involves the repeated opening and sealing of those cracks and fissures due to seismic activity over eons of time has made me more comfortable.
Under this model rock derived from seabed sediments containing raised levels of heavy elements such as gold is subducted under continental crusts by tectonic forces. At a great depth of around 15 - 20 Kms these rocks are "cooked" by the heat at that depth to mobilise large quantities of their water content which was previously contained either as free water in voids or bound chemically in minerals.
Due to the heat and pressure at that depth, this water, superheated to around 400 degrees is able to "scavenge" or dissolve and retain many elements from the heated plastic but not molten rocks it saturates.
This reservoir of hot fluid is normally kept in place by the overlaying layers of hardened, impervious and cooler rocks above.
During earthquakes, however, these rocks can be cracked which allows a conduit for the hot mineral rich water to escape the depths and under immense pressure be forced up the fault zones and cracks so formed.
As this fluid ascends its temperature, and pressure falls and it becomes unable to hold all of its dissolved minerals and some will deposit out. As the fluids ascend even higher progressive temperature and pressure drops enable more deposition over great lengths of the crack until the fluid is depleted of much of its mineral content. Current thinking puts this deposition zone for quartz and gold between 15Km to 3Km.
It is tempting to think of these cracks as open channels into which fluids flow upwards for long periods, but it is more probable that they are quite narrow and easily blocked by deposited quartz with closure further facilitated when the pressure of the escaping fluid drops and it is unable to counteract inward sidewall pressure.
A single crack/seal event is not likely to provide a great deal of reef material, but unstable tectonic periods can be long lasting, perhaps many millions of years in extent, during which hundreds if not thousands of earthquakes will occur. Each earthquake can reopen previously quartz sealed cracks and allow a fresh pulse of mineral and quartz rich fluid often along one wall of the crack allowing more deposition before resealing. Over many such crack/seal events reefs of considerable extent can build up.
The best evidence we have for these is the laminated quartz common in Victorian reefs where each lamination would appear to be evidence of a separate crack/seal event. Some samples show evidence at the microscopic scale of thousands of such laminations.
The subject is a complex one involving many variables but at least with a coherent model of orogenic reef formations, details such as the role of piezo electrostatic effects can start to be put in place.

That is just mighty!
All makes sense - thank you for taking the time to address my uncertainties and explain the theory so clearly!
I'll have to follow up some of the videos mentioned here to help consolidate my understanding!

 

[FindSumStuff]

The research seems pretty sound to me. Only thing comes to mind is that not all volcanoes seem to produce gold. Perhaps that is because concentrations are so low from these events as opposed to cooling with quartz in processes that don't need volcanoes?

 

[Hawkear]

Hi Findsumstuff. Probably need to clarify that the type of gold deposits being talked about in this topic are orogenic deposits.
That is the type of gold deposit that prevails throughout the world, most of Australia and especially Victoria. Although there are some recently extinct volcanoes in the golden triangle the gold reefs pre- date the volcanics by millions of years and there is no connection between them.
There are other types of gold deposits related to volcanic activity. These are known as epithermal deposits and caused by ground water interaction with the rising magma chambers that can also give rise to eruptive vulcanism.
Whilst both Epithermal and Orogenic deposits share a common hydrothermal aspect, Epithermal deposits involve cooler fluids and occur at shallower depths or even as surface hot spring deposits in volcanic or extinct volcanic regions.
Orogenic deposits on the other hand are deep seated hydrothermal deposits associated with deep rock metamorphism and seismic activity without magmatic or volcanic activity. Deep erosion is required to expose them at the surface, and they are therefore commonly associated with widespread alluvial deposits.
It is probably only in relatively recent years that geologists have drawn a distinction between these two types of hydrothermal processes. There are other types of gold deposits but Orogenic (75%) and Epithermal (8%) account for most of the world's gold production.