Detecting bigger gold in NSW...."Alluvial" vs "Eluvial"

[Geepeezee7]

Which is why it is important to focus on the most likely areas (as WalnLiz have pointed out with their discussion of eluvial gold when chasing coarse and nuggety gold)

It is the same when chasing alluvial gold. I looked at this in detail, and while gold can transport tens of km down a creek, all but the first couple of km from the source is usually finer than you would want. In fact we looked at one large goldfield in particular where there were good records (Ballarat) and most nuggety gold was in gullies only 300 m or less from the source. Nuggets over 15 kg (of which Ballarat had a number) in a number of cases never even made it to a gully but were found on hillslopes below the source reefs - all only got as far as small gullies immediately below the reefs - they were not transported long distances along the lead streams (which had lots of gold, but finer).

I have sometimes had comments when I have posted that it was "academic" and that "gold is where you find it". Neither is so - some simple knowledge of geology increases your chances by 1000% or more, and coarse gold only occurs in specific locations, to some degree predictable.

(1) In rocks of specific type (although these will differ a bit on different goldfields locations).
(2) In structural zones, commonly narrow and linear within those rock types
(3) nuggety almost on top of the source reefs (and their structural projections) or downslope of them
(4) nuggety in gullies and streams within no more than 500 m in the main from the source reefs, and even in those cases where they are not covered by alluvial brought downstream from elsewhere

Here is the diagram that I prepared for nuggets over 15 kg (all mine shafts shown are those on quartz reefs, not alluvial, thus giving the position of source areas). I did another one for coarse gold and smaller nuggets in streams (and old buried leads), which gave the 300-500 m transport distance, which if you look at the scale of this map of the goldfield is a tiny proportion of the goldfield area. Imagine how much you reduce the area that is worth focussing on if you limited yourself to within those parameters.

View attachment 2713

What do you mean by "(2) In structural zones, commonly narrow and linear within those rock types"? Could you provide an example on a geological map of what you are referring to in this point?

Which is why it is important to focus on the most likely areas (as WalnLiz have pointed out with their discussion of eluvial gold when chasing coarse and nuggety gold)

It is the same when chasing alluvial gold. I looked at this in detail, and while gold can transport tens of km down a creek, all but the first couple of km from the source is usually finer than you would want. In fact we looked at one large goldfield in particular where there were good records (Ballarat) and most nuggety gold was in gullies only 300 m or less from the source. Nuggets over 15 kg (of which Ballarat had a number) in a number of cases never even made it to a gully but were found on hillslopes below the source reefs - all only got as far as small gullies immediately below the reefs - they were not transported long distances along the lead streams (which had lots of gold, but finer).

I have sometimes had comments when I have posted that it was "academic" and that "gold is where you find it". Neither is so - some simple knowledge of geology increases your chances by 1000% or more, and coarse gold only occurs in specific locations, to some degree predictable.

(1) In rocks of specific type (although these will differ a bit on different goldfields locations).
(2) In structural zones, commonly narrow and linear within those rock types
(3) nuggety almost on top of the source reefs (and their structural projections) or downslope of them
(4) nuggety in gullies and streams within no more than 500 m in the main from the source reefs, and even in those cases where they are not covered by alluvial brought downstream from elsewhere

Here is the diagram that I prepared for nuggets over 15 kg (all mine shafts shown are those on quartz reefs, not alluvial, thus giving the position of source areas). I did another one for coarse gold and smaller nuggets in streams (and old buried leads), which gave the 300-500 m transport distance, which if you look at the scale of this map of the goldfield is a tiny proportion of the goldfield area. Imagine how much you reduce the area that is worth focussing on if you limited yourself to within those parameters.

View attachment 2713

Could you explain what you mean by the following italics? Is it possible to show an example of what you mean by a structural zone, narrow and linear on a geological map?
(2) In structural zones, commonly narrow and linear within those rock types
(3) nuggety almost on top of the source reefs (and their structural projections) or downslope of them

 

[user 4386]

What do you mean by "(2) In structural zones, commonly narrow and linear within those rock types"? Could you provide an example on a geological map of what you are referring to in this point?


Could you explain what you mean by the following italics? Is it possible to show an example of what you mean by a structural zone, narrow and linear on a geological map?
(2) In structural zones, commonly narrow and linear within those rock types
(3) nuggety almost on top of the source reefs (and their structural projections) or downslope of them

Quartz reefs occur in fractures - the hot water containing gold pumped through the fractures. Often the zones of fractured rock are broad (e.g. tens to hundreds of metres - usually tens.), representing a structural zone fold zone or fracture zone (folds and fractures are called structures). So a number of parallel reefs can occur in such a fractured (or more tightly folded) zone. The tighter folding of such a zone can be recognized, and actual fractures without quartz can also be recognized, once you learn some geology and get your eye in. Detecting in this broader fractured and folded zone can turn up nuggets etc in unknown quartz veins, since even veins only cms wide can contain multi-ounce nuggets. Linear simply refers to the fact that these structural zones define a line by their intersection with the ground surface.

Sometimes the rocks are more intensely cleaved in such zones, because cleavage tends to be more intense in tightly folded zones. Here is a photo of both quartz veining and intense cleavage (without quartz) in a zone. The intensity of cleavage (or fracturing, or folding) will tend to die out perpendicular to such a zone, so it is best to focus on the zone itself.




In central Victoria, gold reefs often follow what is labelled here as the hinge zones of folds (especially anticlinal folds, not synclinal folds). These are also linear features where they intersect the ground surface.

You can see here how the auriferous quartz reefs form linear zones up to hundreds of km in length.


And you can see here in cross-section how quartz reefs (black) are confined to the vicinity of an anticlinal fold.
A structural projection is simply the zone containing quartz reefs but where you cant see quartz reefs in part of it. That is, on a single fold or fracture zone you might see a quartz reef, and another one a km north of it, but no obvious quartz between. But you can imagine a projection between the two quartz reefs and focus in that area between.

 

[Mackka]

Great knowledge, fantastic commentary as always. Thanks Goldie for sharing. Mackka

 

[Geepeezee7]

Quartz reefs occur in fractures - the hot water containing gold pumped through the fractures. Often the zones of fractured rock are broad (e.g. tens to hundreds of metres - usually tens.), representing a structural zone fold zone or fracture zone (folds and fractures are called structures). So a number of parallel reefs can occur in such a fractured (or more tightly folded) zone. The tighter folding of such a zone can be recognized, and actual fractures without quartz can also be recognized, once you learn some geology and get your eye in. Detecting in this broader fractured and folded zone can turn up nuggets etc in unknown quartz veins, since even veins only cms wide can contain multi-ounce nuggets. Linear simply refers to the fact that these structural zones define a line by their intersection with the ground surface.

Sometimes the rocks are more intensely cleaved in such zones, because cleavage tends to be more intense in tightly folded zones. Here is a photo of both quartz veining and intense cleavage (without quartz) in a zone. The intensity of cleavage (or fracturing, or folding) will tend to die out perpendicular to such a zone, so it is best to focus on the zone itself.


View attachment 7784 View attachment 7785 View attachment 7786

In central Victoria, gold reefs often follow what is labelled here as the hinge zones of folds (especially anticlinal folds, not synclinal folds). These are also linear features where they intersect the ground surface.View attachment 7787

View attachment 7788 You can see here how the auriferous quartz reefs form linear zones up to hundreds of km in length.

View attachment 7789
And you can see here in cross-section how quartz reefs (black) are confined to the vicinity of an anticlinal fold. View attachment 7790
A structural projection is simply the zone containing quartz reefs but where you cant see quartz reefs in part of it. That is, on a single fold or fracture zone you might see a quartz reef, and another one a km north of it, but no obvious quartz between. But you can imagine a projection between the two quartz reefs and focus in that area between.

I somehow missed this reply until now.

Absolutely fascinating. As always goldierocks, your replies are a lucid exposition of concepts I have read in the literature but have had trouble grasping.

…The tighter folding of such a zone can be recognized, and actual fractures without quartz can also be recognized, once you learn some geology and get your eye in…

In the field and/or from a geological map? How can one recognise? In the field would it be a matter of that more intense cleavage you suggested?

Interesting about detecting the structural projection vs downhill. I just watched an interesting video about just that today on this forum here funnily enough in which Goldtalk Leonora discusses detecting the structural projection.

 

[user 4386]

It is more a case of "hey, these old workings seem to be in rocks with stronger fracturing/cleavage/folding" - then following these features into areas without old workings, or focusing in areas where more old but shallow workings occur within such a zone. A bit suck it and see, but overall giving better chance of success. But simple logic as well - if all gold-bearing veins in an area strike northeast, perhaps don't waste time on structural zones that strike northwest (it may simply be an irrelevant structural zone cutting across a line of old workings).

However look at this relationship - it is fairly obvious that the fractures and the quartz veins are part of the same zone - so follow the fractures.

 

[Calico]

So can you please recommend any basic gold related geology books to improve success in the field ? I’m very interested to learn more. Any knowledge to improve my chances would be most appreciated. I follow these type of posts with interest but need to start with the basics! New terminology and concepts do present an older brain with a few challenges, but I’m a sponge when it comes to learning new things. Thanks for you and Walnliz’ excellent informative posts. Cheers

 

[user 4386]

So can you please recommend any basic gold related geology books to improve success in the field ? I’m very interested to learn more. Any knowledge to improve my chances would be most appreciated. I follow these type of posts with interest but need to start with the basics! New terminology and concepts do present an older brain with a few challenges, but I’m a sponge when it comes to learning new things. Thanks for you and Walnliz’ excellent informative posts. Cheers

Sorry but I cannot specifically recommend anything on basic gold geology (such books either are not basic, or are not written by geologists and tend to be flawed). You really need to start with basic geology. A friend started writing one, but he found it too difficult to make it basic, because something so specialized assumes a lot of prior geological knowledge. So it ended up as specialist....

If I come across anything I will post the reference here.

 

[user 4386]

So can you please recommend any basic gold related geology books to improve success in the field ? I’m very interested to learn more. Any knowledge to improve my chances would be most appreciated. I follow these type of posts with interest but need to start with the basics! New terminology and concepts do present an older brain with a few challenges, but I’m a sponge when it comes to learning new things. Thanks for you and Walnliz’ excellent informative posts. Cheers

Calico, why not do a search here on things written by Walnliz and goldierocks.
What state are you in - I have a paper that helps a bit with central Victoria (unless you got a copy when I offered it to everyone in genera recently).

 

[davent]

We hear about the big nuggets found in Vic and WA, but what about the other states?
Have any significant nuggets turned up in NSW or Qld in recent years? I know some good ones have been found in the NT, Tanami and other areas.

 

[Moneybox]

About three years back we dropped in and met a guy working a mining lease at the old ghost town of Mainland. I'd heard about his 50oz chunk that he pulled out. We didn't get to see the nugget but watched him gnawing away at the bedrock 4m down. He had a single tine on a 50T excavator and was scratching small grooves in the rock about 150mm apart and then doing the same at 90° to eventually chip the rock away. I'd have given up long before that and hit it with a drill and explosives but that's what 50oz does to you. He was sure there'd be another one down there but I've never heard about it and I'll bet he's spent all that he got out of that one looking for the next

 

[Jaros]

Sad isn't it----

 

[mbasko]

Have heard of a few double figure (oz's) nuggets around Central West NSW but they are few & far between.
Even 1 to 10oz bits are rarely heard of but probably found more than you hear?
There was a fairly big specimen found around here a few years back.
Can't remember the exact amount of gold that came out of it but from memory it was a few kilos worth.

 

[Calico]

Calico, why not do a search here on things written by Walnliz and goldierocks.
What state are you in - I have a paper that helps a bit with central Victoria (unless you got a copy when I offered it to everyone in genera recently).

Hi guys , yes I do search for any posts by you two they are extremely helpful, thanks to you both. I find it fascinating and try to look for suggestions or examples in the field. So many rock types to learn apart from the obvious. I’m in SE QLD by the way. Lots of new terms to Learn but that’s half the fun. I’m studying a Bsc at the mo and if I was not completely retarded at maths I would have chosen Geology for sure but way too much calculus and algebra for me, I haven’t been to school for over 30 years and struggle big time. Cheers

 

[user 4386]

Hi guys , yes I do search for any posts by you two they are extremely helpful, thanks to you both. I find it fascinating and try to look for suggestions or examples in the field. So many rock types to learn apart from the obvious. I’m in SE QLD by the way. Lots of new terms to Learn but that’s half the fun. I’m studying a Bsc at the mo and if I was not completely retarded at maths I would have chosen Geology for sure but way too much calculus and algebra for me, I haven’t been to school for over 30 years and struggle big time. Cheers

Stick at it - it all happens then.

Most Queensland gold deposits are rather different to central Victoria. Better to just learn some basic geology, and there are plenty of good books for that. Then one can plug in understanding of the local gold geology of any area that interests you. using Geological Survey or AGSO publications. A good reference that uses Australian geology is (the latest edition):

https://www.booktopia.com.au/the-geology-of-australia-robert-henderson/book/9781107432413.html
Once you understand some geology basics, this gives a review of many of Australia's gold province (one chapter of a free book)

https://press-files.anu.edu.au/downloads/press/p194981/pdf/chapter-8.pdf
There are some excellent books specifically on the geology of gold deposits - this one is aimed at senior undergraduate level, so one needs to know some basic geology first.

https://link.springer.com/book/10.1007/978-981-16-3081-1#about-this-book

 

[Moneybox]

If you're after alluvial gold don't take too much from what the geologists tell you. Alluvial gold has travelled and therefore the deposits can be on or in any type of mineralised soil.

We spent too much money on a course at a Perth prospecting shop that was run by a geologist. He emphasised the untrue information that gold is not found in granite country so if you are on granite you are wasting your time.

We live and mine in granite because this is granite country. The gold isn't in the granite but the reefs of quartz and ironstone that run through it however granite is certainly the predominate rock in the area and it cost me a couple of thousand dollars to find out I was finding gold in the wrong place

 

[Hawkear]

His comments would probably be true for the younger granites over here in Victoria where there has been insufficient time, metamorphic or tectonic forces to allow for gold emplacement. Similar comments could apply to basalts here in Victoria. most of which are geologically recent in origin and devoid of gold.
Over west where the granites and basaltic rocks are much more ancient I would expect them to have had the huge amount more time to be subjected to those conditions so see no reason why they should not have gold emplacement.
In his defense, perhaps he was differentiating between the younger and older granites. His view may have been to define older granites and basalts that have been subject to a large degree of metamorphic or tectonic activity, as other kinds of rocks such as gneisses, schists and greenstones.
I can personally vouch for places in Victoria where gold is found in granite areas, but then I think that the gold had been transported there rather than originating there.

 

[user 4386]

His comments would probably be true for the younger granites over here in Victoria where there has been insufficient time, metamorphic or tectonic forces to allow for gold emplacement. Similar comments could apply to basalts here in Victoria. most of which are geologically recent in origin and devoid of gold.
Over west where the granites and basaltic rocks are much more ancient I would expect them to have had the huge amount more time to be subjected to those conditions so see no reason why they should not have gold emplacement.
In his defense, perhaps he was differentiating between the younger and older granites. His view may have been to define older granites and basalts that have been subject to a large degree of metamorphic or tectonic activity, as other kinds of rocks such as gneisses, schists and greenstones.
I can personally vouch for places in Victoria where gold is found in granite areas, but then I think that the gold had been transported there rather than originating there.

It is simple - in Victoria almost all granites and basalts are younger than the gold deposits, so naturally there is no gold in the granite. There is some gold in granite but 99.99% of gold is not (an example is Glen Wills where the granite is old (Silurian) and the gold younger (Devonian) - in fact most of these exceptions to the rule are in eastern Victoria, another example is Granite Flat (one exception is in the west- Mafeking in the Grampians).

It varies from state to state - quite a bit of gold in NSW (eg Cadia) and Queensland (e.g. Charters Towers) is in granite. However his comment re WA is fairly much correct - 99.9% of gold in WA is not in granite, which is a bit better than the 99.99% in Victoria. The fact that a tiny amount is, does not really negate the observation that the best chance of success for gold in WA is NOT in granite. If I was going across to WA on a detecting trip without prior knowledge of the few areas where gold occurs in granite, I would not spend a minute detecting in granite areas, but would focus on greenstones in which the overwhelming majority of gold occurs (and I have spent years full-time prospecting in WA in the past). However obviously if you are in one of the few areas in WA where gold does occur in granite - go for it!

A couple of other points. While gold in granite is negligible in the Eastern Goldfields (I can only think of about 3 places, e.g. Paynes Find, although there might be some more) WA is a big place - there are some examples in granite in the Pilbara and Kimberley region etc. So local knowledge is important if you don't want a lot of wasted effort. However in general those mines in granite were historically real squibs for companies, and as a rule of thumb you best chance when detecting is in areas that had mega-mines not squibs. But there can be exceptions - the largest nugget ever found, the Welcome Stranger, came out of an insignificant and fairly solitary quartz vein). However I work on probabilities of success, not "might finds".

One last factor - I find that a lot of prospectors refer to rock as "granite" which is not granite but gneiss. Completely different rocks (gneiss is commonly cooked up sandstone or volcanics, and is a metamorphic rock that has never melted). Granite is a rock that crystallizes from a magma so has been molten - an igneous rock. They can often look similar although few geologists would confuse the two. The top one is granite, the bottom one gneiss. It is more common to find gold in gneiss in WA than in granite (e.g. around Cue from memory, an important field?). But the highest probability of success for the recreational prospector in the Eastern Goldfields who does not have good local knowledge is the greenstone areas (also a metamorphic rock, often cooked-up basalt).

 

[Geepeezee7]

It is simple - in Victoria almost all granites and basalts are younger than the gold deposits, so naturally there is no gold in the granite. There is some gold in granite but 99.99% of gold is not (an example is Glen Wills where the granite is old (Silurian) and the gold younger (Devonian) - in fact most of these exceptions to the rule are in eastern Victoria, another example is Granite Flat (one exception is in the west- Mafeking in the Grampians).

It varies from state to state - quite a bit of gold in NSW (eg Cadia) and Queensland (e.g. Charters Towers) is in granite. However his comment re WA is fairly much correct - 99.9% of gold in WA is not in granite, which is a bit better than the 99.99% in Victoria. The fact that a tiny amount is, does not really negate the observation that the best chance of success for gold in WA is NOT in granite. If I was going across to WA on a detecting trip without prior knowledge of the few areas where gold occurs in granite, I would not spend a minute detecting in granite areas, but would focus on greenstones in which the overwhelming majority of gold occurs (and I have spent years full-time prospecting in WA in the past). However obviously if you are in one of the few areas in WA where gold does occur in granite - go for it!

A couple of other points. While gold in granite is negligible in the Eastern Goldfields (I can only think of about 3 places, e.g. Paynes Find, although there might be some more) WA is a big place - there are some examples in granite in the Pilbara and Kimberley region etc. So local knowledge is important if you don't want a lot of wasted effort. However in general those mines in granite were historically real squibs for companies, and as a rule of thumb you best chance when detecting is in areas that had mega-mines not squibs. But there can be exceptions - the largest nugget ever found, the Welcome Stranger, came out of an insignificant and fairly solitary quartz vein). However I work on probabilities of success, not "might finds".

One last factor - I find that a lot of prospectors refer to rock as "granite" which is not granite but gneiss. Completely different rocks (gneiss is commonly cooked up sandstone or volcanics, and is a metamorphic rock that has never melted). Granite is a rock that crystallizes from a magma so has been molten - an igneous rock. They can often look similar although few geologists would confuse the two. The top one is granite, the bottom one gneiss. It is more common to find gold in gneiss in WA than in granite (e.g. around Cue from memory, an important field?). But the highest probability of success for the recreational prospector in the Eastern Goldfields who does not have good local knowledge is the greenstone areas (also a metamorphic rock, often cooked-up basalt).

View attachment 7984
View attachment 7983

Goldierocks, I would (and I imagine everyone else would) be very interested if you were to post a geological map of a goldfield in Victoria in which you markup the map and talk about the particular areas on the map you would and would not (just as important) focus on detecting for alluvial and eluvial gold, and why. I think there would be a whole lot learned for a lot of us here. Is that something you would consider doing for us ?

 

[davent]

If you're after alluvial gold don't take too much from what the geologists tell you. Alluvial gold has travelled and therefore the deposits can be on or in any type of mineralised soil.

We spent too much money on a course at a Perth prospecting shop that was run by a geologist. He emphasised the untrue information that gold is not found in granite country so if you are on granite you are wasting your time.

We live and mine in granite because this is granite country. The gold isn't in the granite but the reefs of quartz and ironstone that run through it however granite is certainly the predominate rock in the area and it cost me a couple of thousand dollars to find out I was finding gold in the wrong place

Funny that.
An old prospector, well known for finding deposits, whacking an EL on it, then selling the EL to big companies for good $, told me that ALL the gold in the GROVE HILL area, was within 1km of the granite......he even has a mine named after him, Langleys.

 

[user 4386]

Funny that.
An old prospector, well known for finding deposits, whacking an EL on it, then selling the EL to big companies for good $, told me that ALL the gold in the GROVE HILL area, was within 1km of the granite......he even has a mine named after him, Langleys.

A miss is as good as a mile...(or a kilometre). How much would you find if you detected IN the granite?
I worked around there for a couple of years (Iron Blow, Mt Bonnie, Cosmo, Union Reef, Pine Creek) - and the deposits I am familiar with there are unrelated to the granite - also, I don't have a geological map to hand but from memory that observation would be incorrect.