Confused about sudden change in deposition layers in creek

[Telling Ghosts]

Hi everyone,

I'm new to prospecting trying to understand what's going on in a section of creek that I was working near Daylesford.

I found a large section of exposed bed rock on the edge of the creek which ran back up in to the side of the gully, a smaller piece of it is jutted out in to the water and sat above the water line overgrown with black berry.
But I figured it was a likely spot of low pressure when the river was in flood and started working it.

Digging down brought up a bunch of iron stone, small zircons typical of the area and some fine gold, nothing unusual for this creek.
This was all sitting on top of a layer of grey really sticky clay about an inch thick.

Beyond the clay layer I started to find some more iron stone so kept digging it out but no gold, fewer zircon, mostly just iron stone, albeit much smaller than what was sitting on top.

Again I hit another clay layer, this one thicker and this was now beginning to move further in under the bedrock slab and away from the creeks current water flow.

Having not found any gold in this second layer I thought it might not be worth digging much more so just decided to fill up a bucket and process just to see if anything was there.

This next layer was really different to the rest, I've taken photos of the stones I found as I'm not sure what they are, but they were larger and more water worn apart from some pieces of chunky mineralized quartz.
Among it I also started find these clear quartz crystals, some clear bluish flecks of crystal and a quite a nice little gold picker, although there was no fine gold in this layer.

I'm hoping someone could explain what would cause a sudden difference between these clay layers and is it possible that I was starting to dig through an older part of a river system or is it still just deposits from the modern one in place?

The plan is to move further down stream following the slab of rock (it's metres long) and try and identify similar material to what I found in the 3rd layer and hopefully find some more of those crystals and maybe some more bits of gold like the first one.

Thanks in advance.

Realistic depiction of the layers. layful:

Quartz crystals found in 3rd layer, zircon was in the top layer, find lots of these here

little picker

I haven't seen any rocks like this in the creek before - some sort of conglomerate I presume.

Not sure what these ones are, quite nice and blue

These are the sorts of rock I was finding in the 3rd layer, very different to the stuff before

I'm not sure what this one is, has a bluish tinge to it though.

 

[Sushi]

First picture is quartz with most of the crystal structure intact so you can assume these have not travelled far within the river otherwise they would be a lot more rounded.

Photo 3 is a conglomerate. Spot on.

Photo 4. Congratulations, you've found sapphire.

In terms of the deposits in the river, you have to remember that the process of deposition is happening in what is referred to as 'deep or geological time'. This process of deposition has been happening for thousands and potentially millions of years. The layers of mud/clay/silt are periods of when the river has been exceedingly flooded but without a lot of water movement to allow sedimentation on the river bottom. So you've got lots of water but not enough flow to be causing all the silts on the bottom to be stirred up. At some point in time before the high water deposited silt, Sapphires have formed in metamorphic schists or igneous basalts, and have been weathered out through erosion, likely water in this case, and have been deposited along the bottom of the river due to being more dense. During this point in time you could assume there has been more water flow to push the Sapphires down the stream (assuming they're found in the whole system). As time goes in you've got different periods of flooding and drought within the river causing deposition of different materials. If you found a really good assortment of water worn pebbles in a wash layer, you could even conclude how fast the water was travelling during that point in time due to their orientation.

Long story short, you've discovered a number of geological processes in how deposits form in river beds over thousands of years.

Also, would love to see a photo of the last specimen with light coming through it. Could be sapphire or zircon.

 

[RM Outback]

Nice finds just one question though have you checked the exempt rivers /tributaries list on Earth Resources website and confirmed your working in a legal waterway. I don't want to burst your bubble it sounds and looks great but if I didn't ask I'm sure someone would have. Whether it's a go zone or not do your best to rehab the area.

 

[user 4386]

First picture is quartz with most of the crystal structure intact so you can assume these have not travelled far within the river otherwise they would be a lot more rounded.

Photo 3 is a conglomerate. Spot on.

Photo 4. Congratulations, you've found sapphire.

In terms of the deposits in the river, you have to remember that the process of deposition is happening in what is referred to as 'deep or geological time'. This process of deposition has been happening for thousands and potentially millions of years. The layers of mud/clay/silt are periods of when the river has been exceedingly flooded but without a lot of water movement to allow sedimentation on the river bottom. So you've got lots of water but not enough flow to be causing all the silts on the bottom to be stirred up. At some point in time before the high water deposited silt, Sapphires have formed in metamorphic schists or igneous basalts, and have been weathered out through erosion, likely water in this case, and have been deposited along the bottom of the river due to being more dense. During this point in time you could assume there has been more water flow to push the Sapphires down the stream (assuming they're found in the whole system). As time goes in you've got different periods of flooding and drought within the river causing deposition of different materials. If you found a really good assortment of water worn pebbles in a wash layer, you could even conclude how fast the water was travelling during that point in time due to their orientation.

Long story short, you've discovered a number of geological processes in how deposits form in river beds over thousands of years.

Also, would love to see a photo of the last specimen with light coming through it. Could be sapphire or zircon.

While that is one possibility I suspect another. These are not pristine drainages, they have been worked already multiple times in the past, and each time a large amount of clay is stirred up and travelled downstream. Mines discharged their tailings into the streams. So you could be looking at deposits 150 years old or less. For the coarse stuff, go to bedrock. The modern deposits are so deep in some areas (e.g. Bendigo) that we map them as a separate geological unit. You can have tens of millions of cubic metres of sludge wash down a single stream.

 

[user 4386]

First picture is quartz with most of the crystal structure intact so you can assume these have not travelled far within the river otherwise they would be a lot more rounded.

Photo 3 is a conglomerate. Spot on.

Photo 4. Congratulations, you've found sapphire.

In terms of the deposits in the river, you have to remember that the process of deposition is happening in what is referred to as 'deep or geological time'. This process of deposition has been happening for thousands and potentially millions of years. The layers of mud/clay/silt are periods of when the river has been exceedingly flooded but without a lot of water movement to allow sedimentation on the river bottom. So you've got lots of water but not enough flow to be causing all the silts on the bottom to be stirred up. At some point in time before the high water deposited silt, Sapphires have formed in metamorphic schists or igneous basalts, and have been weathered out through erosion, likely water in this case, and have been deposited along the bottom of the river due to being more dense. During this point in time you could assume there has been more water flow to push the Sapphires down the stream (assuming they're found in the whole system). As time goes in you've got different periods of flooding and drought within the river causing deposition of different materials. If you found a really good assortment of water worn pebbles in a wash layer, you could even conclude how fast the water was travelling during that point in time due to their orientation.

Long story short, you've discovered a number of geological processes in how deposits form in river beds over thousands of years.

Also, would love to see a photo of the last specimen with light coming through it. Could be sapphire or zircon.

While that is one possibility I suspect another. These are not pristine drainages, they have been worked already multiple times in the past, and each time a large amount of clay is stirred up and travelled downstream. Mines discharged their tailings into the streams. So you could be looking at deposits 150 years old or less. For the coarse stuff, go to bedrock. The modern deposits are so deep in some areas (e.g. Bendigo) that we map them as a separate geological unit. You can have tens of millions of cubic metres of sludge wash down a single stream.

I suspect that you are going to be good at this - if you can understand the geology, you can increase your success rate one hundredfold.....

 

[Telling Ghosts]

Nice finds just one question though have you checked the exempt rivers /tributaries list on Earth Resources website and confirmed your working in a legal waterway. I don't want to burst your bubble it sounds and looks great but if I didn't ask I'm sure someone would have. Whether it's a go zone or not do your best to rehab the area.

Thanks for looking out RM, it is a legal waterway and I will definitely leave it as close to how I found it as possible :Y:

 

[Telling Ghosts]

First picture is quartz with most of the crystal structure intact so you can assume these have not travelled far within the river otherwise they would be a lot more rounded.

Photo 3 is a conglomerate. Spot on.

Photo 4. Congratulations, you've found sapphire.

In terms of the deposits in the river, you have to remember that the process of deposition is happening in what is referred to as 'deep or geological time'. This process of deposition has been happening for thousands and potentially millions of years. The layers of mud/clay/silt are periods of when the river has been exceedingly flooded but without a lot of water movement to allow sedimentation on the river bottom. So you've got lots of water but not enough flow to be causing all the silts on the bottom to be stirred up. At some point in time before the high water deposited silt, Sapphires have formed in metamorphic schists or igneous basalts, and have been weathered out through erosion, likely water in this case, and have been deposited along the bottom of the river due to being more dense. During this point in time you could assume there has been more water flow to push the Sapphires down the stream (assuming they're found in the whole system). As time goes in you've got different periods of flooding and drought within the river causing deposition of different materials. If you found a really good assortment of water worn pebbles in a wash layer, you could even conclude how fast the water was travelling during that point in time due to their orientation.

Long story short, you've discovered a number of geological processes in how deposits form in river beds over thousands of years.

Also, would love to see a photo of the last specimen with light coming through it. Could be sapphire or zircon.

Thanks for the great info Sushi, it's answered a lot of questions and raised many more!

So basically if I'm understanding correctly, all the stuff that I found has ended up there from the same thing, a flood event at some point in time, some of it has traveled further than the rest to get there and there's thousands of years worth of stuff going on in each shovel I'm taking out?

When you say the quartz hasn't traveled far due to how little erosion it has experienced, is there any way of quantifying that?
Are we talking, it washed down from the gully and sat there, traveled hundreds of meters down river, or are there too many variables to know?

Very cool to have found some sapphire, I've looked a geological map of the area and there is isolated areas of basalt not far from where I was digging, does it stand to reason that if I was to look closer to these I could increase the likelihood of finding more sapphires?

I put the light up to that last rock and it's opaque I think I burnt out my retinas double checking though.

 

[Telling Ghosts]

While that is one possibility I suspect another. These are not pristine drainages, they have been worked already multiple times in the past, and each time a large amount of clay is stirred up and travelled downstream. Mines discharged their tailings into the streams. So you could be looking at deposits 150 years old or less. For the coarse stuff, go to bedrock. The modern deposits are so deep in some areas (e.g. Bendigo) that we map them as a separate geological unit. You can have tens of millions of cubic metres of sludge wash down a single stream.

Thanks Goldierocks, that's really interesting that so much impact was had just from run off that it's been assigned it's own geological unit.

So I can't really infer much from what I've found as it may simply just be a bunch of discharge from all the quarries and mines that are upstream from the area?

I have a question from something I saw marked in that river system on the geological map, there is a bunch of spots marked with an X as BE & BECH, the legend refers to them as "Graptolite fossil locality with biostratigraphic zone or zonal range - Bendigonian and Chewtonian"

What does that mean?

Much appreciated, T.G.

 

[user 4386]

While that is one possibility I suspect another. These are not pristine drainages, they have been worked already multiple times in the past, and each time a large amount of clay is stirred up and travelled downstream. Mines discharged their tailings into the streams. So you could be looking at deposits 150 years old or less. For the coarse stuff, go to bedrock. The modern deposits are so deep in some areas (e.g. Bendigo) that we map them as a separate geological unit. You can have tens of millions of cubic metres of sludge wash down a single stream.

Thanks Goldierocks, that's really interesting that so much impact was had just from run off that it's been assigned it's own geological unit.

So I can't really infer much from what I've found as it may simply just be a bunch of discharge from all the quarries and mines that are upstream from the area?

I have a question from something I saw marked in that river system on the geological map, there is a bunch of spots marked with an X as BE & BECH, the legend refers to them as "Graptolite fossil locality with biostratigraphic zone or zonal range - Bendigonian and Chewtonian"

What does that mean?

Much appreciated, T.G.

I suspect that BE means a Bendigonian age graptolite locality, and that BECH is a locality that straddles the Bendigonian and Chewtonian age boundary (or possibly that the fossil cannot be uniquely assigned to one or the other, but could be either). Rocks of Ordovician age in Victoria are subdivided into zones according to their graptolite species - we don't know exact ages, but we do know that a certain graptolite fossil occurs before (or after) another species, In a general way, the graptolites evolved to be simpler in form at younger ages. They are named after areas where they are abundant - so from oldest to youngest in rocks of Ordovician age they are the Lancefieldian, Bendigonian, Chewtonian, Castlemainian, Yapeenian, Darriwilian, Gisbornian, Eastonian and Bolindian. So in your area they are fairly old Ordovian (we would actually say "low in the Ordovician sequence" since it really represents their position in the stratigraphy rather than age, although the position correlates with age). The graptolites did not stop at the top of the Ordovician (Bolindian) but continued into overlying (and younger) Silurian and Devonian rocks - by the Devonian a single stipe ("arm") was common (Monograptus) whereas low in the Ordovician forms with 8, 4 and 2 stipes were common.

 

[Sushi]

So basically if I'm understanding correctly, all the stuff that I found has ended up there from the same thing, a flood event at some point in time, some of it has traveled further than the rest to get there and there's thousands of years worth of stuff going on in each shovel I'm taking out?

When you say the quartz hasn't traveled far due to how little erosion it has experienced, is there any way of quantifying that?
Are we talking, it washed down from the gully and sat there, traveled hundreds of meters down river, or are there too many variables to know?

Very cool to have found some sapphire, I've looked a geological map of the area and there is isolated areas of basalt not far from where I was digging, does it stand to reason that if I was to look closer to these I could increase the likelihood of finding more sapphires?

I put the light up to that last rock and it's opaque I think I burnt out my retinas double checking though.

In terms of depositing of material, this is generally the case, however if there's large scale mining going on in the area obviously this is going to change. I'm not knowledgeable on the area.

Rocks and minerals that have stayed within a river system for a long period of time get worn into rough circular shapes, its the same principle as tumbling rocks. Because one of those quartz crystals still has a nice termination (where the 6 crystal faces meet into a point), you can assume it hasn't traveled very far from the source or hasn't been moved rigorously enough to round the mineral.

You would need to research the area more on sapphires, which in Australia, are usually deposited in rivers. I think the website Mindat and the DIGS geological surveys would be good places to start. They usually have drillings from areas stating what they've found. If you could find the alluvial gravel sitting above or inside pockets of bedrock, you'd have a really good chance of sapphires, zircons, spinel and gold.

 

[user 4386]

When you say the quartz hasn't traveled far due to how little erosion it has experienced, is there any way of quantifying that?

Other than the fact that gravel pebbles tend to become more rounded with distance - no. It also depends on whether the rock is hard or soft, the turbulence of the river, the amount of flow, the water velocity, the gradient (slope) of the river downstream (since this affects velocity). Also, new rivers often erode the gravels left by old rivers (common in Victoria). So pebbles might have become rounded in an early large and vigorous river, perhaps 100 km long, but be eroded into a small and not very spectacular stream only a few km long. The shape of alluvial gold tends to be a more sensitive indicator, although the same applies (angular grains are usually close to the source, as are unrounded quartz crystals as someone else said). Gold tends to be coarser close to source, and really nuggety gold is usually only hundreds of metres from its source - medium-size is within kilometres, fine gold could be 30 km from the source.

Very cool to have found some sapphire, I've looked a geological map of the area and there is isolated areas of basalt not far from where I was digging, does it stand to reason that if I was to look closer to these I could increase the likelihood of finding more sapphires?

Possibly - I would give it a try. It is not a bad area for both blue sapphire and zircon (big zircon crystals can be seen in basalt in a quarry near Daylesford, and a few attempts have been made to mine blue sapphires near Trentham (Blue Hill or Mount?).

 

[RM Outback]

Exciting finds :Y: all the best when you get back to it.

 

[Telling Ghosts]

Thanks for much for so much great knowledge and taking the time to type it all up!
I will definitely keep an eye out for some fossils next time I'm there and look in to Trentham for potentially more little sapphires.

Goldie, are the large Zircon crystals accessible for public viewing? it would be great to see them at some point when I'm in the area.

 

[user 4386]

Thanks for much for so much great knowledge and taking the time to type it all up!
I will definitely keep an eye out for some fossils next time I'm there and look in to Trentham for potentially more little sapphires.

Goldie, are the large Zircon crystals accessible for public viewing? it would be great to see them at some point when I'm in the area.

So long ago that I cannot remember the exact locality - some were red I remember, and embedded I the basalt. Send me a PM and I will send you a general paper and map.

 

[deepblue]

Hi TG,
You will find the Trentham area very hard going as it is very overgrown and the track in was blocked the last time I looked.
cheers db