unknown find from slatey

[shivan]

Lefty, here is a bit of petrified wood found recently by a mate. This is not glossy and looks porous, in fact when he first gave it to me i was expecting it to be rotten drift wood. It is quiet solid but does not show any sign of chalcedony that normally fills the cavities or the black. While i have not seem much like that from the Valley, it could be easily overlooked if broken or weathered i guess.

 

[fossickeract]

Hi Shivan,
Nethercote Pyropholite is the precise name for the silicified rhyolite not to be confused with pyrophillite which is also found in the Pambula area, ribbon jasper is a reasonable name to be less precise, I could give you directions in a pm to avoid half of Aus paying a visit.
Kangaroo Valley yep visit occasionally, I see you have found the Pet. wood there ask away I will try to answer any questions.I have some nice agates and jasper from KV but that was 40 years ago. I will mainly chase sapphires at Grabben Gullen this fossicking season, already have couple buckets of KV Pet wood.Hope to visit Sapphire up Lefty's way for winter.

 

[shivan]

Cheers for the info. I have a few old lapidary books that talked about ribbon jasper in Pambula and have thought about checking it out, no one at the lapidary club seemed to know what i was on about though. Now i see the name, i have read about the stuff i believe in a mining report on the area, but had no idea that is what they were talking about.

Will shoot you a PM about the Valley, thanks again.

 

[user 4386]

Are you and your mates on gas exploration? Nope, mainly gold - the gemstones are more an interest for me personally, opals to a greater degree, gold is bread and butter.

I didn't think we were likely to see the formation of banded agate with quartz crystal centres in the middle of a piece of pet wood without related thermal activity. Perhaps the wood was already fossilized and the hollows and cracks were later filled by hot silica-saturated water at the time of the eruptions. Is there such a thing as "cold" agate formation? Yes, cold is perhaps the most common, and quartz crystals can form from cold water. Keep in mind too that ordinary groundwater gets warmer underground without any need for volcanic activity (eg increases around 30 degrees per km, but in western Queensland reaches boiling temperatures at one km). The heat is from radioactive decay, not from volcanic activity.

 

[user 4386]

Just outside Bilo is an interesting place called Mount Scoria. My guess is that is lava flow not volcanic neck - colums like that ("collonade") are a feature of cooling of the top of flows.

 

[user 4386]

I have to agree with Lefty regarding the chalcedinized petrified wood he finds in Queensland, and that from Kangaroo Valley (NSW South Coast) being associated with and caused by volcanic intrusions. It is quite different to some I collected from the Moon Plains in SA and Deep leads of the Nundle area, neither of which contain chalcedony but resemble Silcrete. To my understanding Silcrete and opal do precipitate at low temperatures up to 140c but chalcedony, jasper/agate form from 140c to 300c bit above normal ground temperatures. Have to disagree - silcrete is a rock that commonly consists of the mineral chalcedony (which is more strictly microquartz) and like jasper/agate they commonly form at near surface temperatures. Jasper and agate are really just gem names for microquarttz (chalcedony). Opal is not the same as chalcedony (Microquartz) - it has a completely different structure consisting of packed spheres with water between - it also commonly forms at near-surface temperatures. Chalcedony (microquartz) CAN form at temperatures up to 300 degrees or so, but most does not - usually that is in things like gold deposits that are termed epithermal veins (eg some Nevada gold veins, Martha gold mine near Coromandel NZ, even seen some at Pambula)

In fact in specimens from Kangaroo Valley the initial petrification was by jasper/chalcedony often preserving the original wood grain and later cracks were infilled by common opal bearing no relationship to the original wood grain, different precipitation temperatures. Again, have to disagree - commonly all much the same temperature, the difference simply being some silica replaces the wood, but where there are open cracks it fills them as vein fill - looks different but all one process as a rule. If you look at a modern tree trunk that is drying out you will see radial and concentric cracks - the cracks are later than the tree ring wood, but both pre-date the silica coming in


As to theory fitting the facts I recall the story of a geo student given a mapping project in SA, 10 sq km of horizontally bedded Sandstone with occasional silcrete, after a fortnight map was done but he walked up the highest hill to take in a last view, avoided because of the heat and flies until then and found sitting there on the silcrete a 40 ton boulder of granite. How did he explain that Its not there. He He and now he is a geo professor in Vic. Sure, lots of ways that can happen - I've seen similar things in South Australia (granite "dropstones" - boulders rafted on floating glaciers which then drop into the bottom muds when the ice melts). At Heathcote Vic granite boulders sit on top of Permian coal measures - dumped there by glaciers (called "erratics"). Faulting can do similar things, I have a photo of Ordovician granite overlying Tertuiary granite in a Kiewa hydroelectric tunnel (I've seen similar in NZ along the Alpine Fault just north of the Fox township - metamorphic rocks on top of young gravels)

 

[user 4386]

Hi Shivan,
Nethercote Pyropholite is the precise name for the silicified rhyolite not to be confused with pyrophillite which is also found in the Pambula area, ribbon jasper is a reasonable name to be less precise, I could give you directions in a pm to avoid half of Aus paying a visit.
Kangaroo Valley yep visit occasionally, I see you have found the Pet. wood there ask away I will try to answer any questions.I have some nice agates and jasper from KV but that was 40 years ago. I will mainly chase sapphires at Grabben Gullen this fossicking season, already have couple buckets of KV Pet wood.Hope to visit Sapphire up Lefty's way for winter.

You guys make it hard! All these local names that are not true mineral names that are invented for the same mineral with a different appearance! As I pointed out with agate=chalcedony=jasper= the mineral microquartz. Was working for a few years at Pambula-Nethercote until 5 years ago. Relying on memory but the pyropholite (not a mineral species, a name I have otherwise only heard in Namibia) is not pyrophyllite as occurs nearby (as you say) - from memory it is an altered rhyolite and really consists of fine-grained quartz and white mica or illite finely intergrowth with each other

 

[user 4386]

Cheers for the info. I have a few old lapidary books that talked about ribbon jasper in Pambula and have thought about checking it out, no one at the lapidary club seemed to know what i was on about though. Now i see the name, i have read about the stuff i believe in a mining report on the area, but had no idea that is what they were talking about.

Will shoot you a PM about the Valley, thanks again.

I didn't see anything I would call a true jasper when working at Pambula - I wonder if they are referring to the flow-banded rhyolite that occurs in the quarry at the north end of the goldfield?

 

[user 4386]

Here is a photo of the granite gneiss overlying young alluvium (gravel).

 

[Lefty]

Lefty, here is a bit of petrified wood found recently by a mate. This is not glossy and looks porous, in fact when he first gave it to me i was expecting it to be rotten drift wood. It is quiet solid but does not show any sign of chalcedony that normally fills the cavities or the black. While i have not seem much like that from the Valley, it could be easily overlooked if broken or weathered i guess.

Yep, that looks more like the couple of bits I've found in areas that did not have obvious volcanics nearby. They have that porous quality and do not "clink like glass marbles" when struck together as the Riverslea stuff does.

 

[Lefty]

Hope to visit Sapphire up Lefty's way for winter. wink

Ted, if you have time on your way, drop into Riverslea crossing, have a look and tell me what you think. It's only a short distance off the Capricorn highway after you head west from Rockhampton/Gracemere. Turn off is at Gogango and it's about 15km down to the river. If you have room grab some of the wood, it's plentiful on the gravel banks and won't be there too much longer, the Rookwood weir will put paid to this fossicking area.

Cheers

 

[Lefty]

Goldirocks - some of the things you are saying seem completely at odds with everything I have ever experienced first-hand and heard from fossickers throughout our area, albeit I have no scientific background. With respect I will say that casting doubt on the origins of the Riverslea wood for starters is pushing credibility for one who has been there many times and if you were to spend some time there I think you would need to be very attached to a particular theory to not reach the same conclusion as to the greatest likelyhood of how it formed.

This vast volcanic region seems missing from your resume' and may be totally different to your experiences around other parts of Australia. Until demonstrated otherwise, I tend to think that fossilized wood from this part of Australia may have a rather higher than 0.1% volcanic hot silica solution origin.

Cheers

 

[Lefty]

To clarify my position so as for me to not be responsible for sparking arguments.....

It's ok. I understand that you are defending a theory that you subscribe to. That's good, that's how things advance. It may well be that you are correct.

What I'm implying is that the theory that less than 1 in every 1000 occurances of fossil wood - including that which is clearly agate-filled - bear any direct relationship to hot solutions from volcanic intrusions, may hold true in some geological regions but not in others. In areas heavily replete with extinct volcanics, the ratio may be higher - perhaps much higher.

Here's the important bit for me - if the information does not hold true in the areas that I am searching, then the information is of little use to me as a fossicker or prospector. It would lead me to disregard vital clues, brush them off as mere coincidence, unworthy of attention.

My contention is that if you live in many of the eastern regions of the eastern states - and definitely in eastern QLD - fossil wood of the nature described and photographed here is probably more likely than not to have a direct link to nearby volcanics - which in turn can often be the source of other gemmy materials as well.

Same goes for agates and other chalcedony-type materials. Same for quartz crystals - I have collected probably tens of thousands of quartz crystals up and down eastern QLD but every single one has been in areas not necessarily of eruptive volcanics but definitely igneous. Never found a one in areas without this - though admittedly igneous geology is so prominant here, so maybe elsewhere who knows.........

So I'm getting the feeling that my region - and it's a very big region - seems quite different to the ones you describe, and knowledge designed to describe processes elsewhere may be less helpful to me here as I roam the hills and gullies is search of interesting rocks.

 

[shivan]

Just trying to get my head around some of this. Great info though guys

Have to disagree - silcrete is a rock that commonly consists of the mineral chalcedony (which is more strictly microquartz) and like jasper/agate they commonly form at near surface temperatures. Jasper and agate are really just gem names for microquarttz (chalcedony)

It was my understanding that quartz and chalcedony have the same makeup, but quartz is microcrystalline whereas chalcedony is cryptocrystalline, and that agate was a banded form of chalcedony and jasper was opaque form of chalcedony or a sedimentary rock if formed like chert.
Again from my understanding of this, i would think quartz would form in a warmer environment than chalcedony because of the different type of crystallization.
Many things would come into play here, the depth/temp gradient, proximity to volcanic activity and the like . But i would still think that magma heating the water to be able to dissolve the silica in the first place would still need to be a factor? I thought that cooler waters were no good at dissolving silica except amorphous silica, whereas waters heated from magma would often become supersaturated with silica.
Waters of 100 C and extreme pressure would be able to start dissolving the silica, but again my understanding is extreme pressure is needed which would indicate this is happening at depths not near the surface?

You guys make it hard! All these local names that are not true mineral names that are invented for the same mineral with a different appearance!

Its funny you say that, the more into the lapidary side of thing i get, the further away from geological terms and mineral names i seem to get which at times can be frustrating. Hence the trouble i was having with Pambula i guess

But again you got to love this thread, thanks guys.

 

[Lefty]

Just trying to get my head around some of this. Great info though guys

Same and agreed. Really informative stuff from Ted and Goldirocks. Huge amounts of info from Goldirocks, taking some time to get my head around.

My experiences on the ground in a place he has not been lead me to disagree with some of his conclusions but I'm quite happy to agree to disagree with a few things while still taking on board the rest of it as potentially valuable information.

If we assumed the Riverslea wood to be derived from a very rare type of petrification process then the argument would logically follow that at least the next 999 pet wood occurences in all directions would be of a completely different sort with no volcanically-supplied hydrothermals involved - I would harbour very strong doubts about this being the case. This region is pretty much "dead volcano city". An on-the-ground count of just how many there are would be interesting. Not that every volcano would have produced the right conditions for hot solution chalcedonization of wood tissue I'm sure.

The 1 in 1000 occurrence argument sounds to me as though it would logically imply a very small window of opportunity for the process to occur. I didn't think the heat generated by volcanics would be the same as just flicking the switch of a stove on and off. I thought volcanoes over a current hot spot area could continue baking the ground around them for some time even as other natural process like heavy floods introduced water into the still-hot ground. Further, I thought at least some volcanoes saw multiple eruptions over spans of time, perhaps hundreds or thousands of years. As I recall geologists generally believe that some of the different wash layers on the Anakie field - 3-4 through a depth of up to 50 feet in some spots - were the result of a sequence of eruptions by the same volcanoes over a significant time period, not merely erosion moving dirt around.

I'm not really sure that conditions suitable for the hot silica solution petrification of wood were either exceptionally rare or exceptionally brief, not in this part of the country at least.

 

[Lefty]

Just outside Bilo is an interesting place called Mount Scoria. My guess is that is lava flow not volcanic neck - colums like that ("collonade") are a feature of cooling of the top of flows.

Not sure, might well be the case. I've only been there once - one climb was enough - but from memory it is a conical protrusion sticking straight up out of relatively flat ground with nothing similar looking in very close proximity. I thought the name "Mount Scoria" might have implied something about it but I guess just because the stuff is found on and around the base of the formation it might not mean it's a volcanic neck. It does have the same colums as the top of Mount Hoy at Tomahawk creek on the Anakie field, but again Mount Hoy might be part of a flow rather than a core. I've never seen Mount Hoy first hand, I only know about the colums from photographs taken by members of my club who climbed it.

An interesting thing about Mount Scoria is that whenever there is a thunderstorm about, people living close by can hear the mountain echo thunderclaps in the key of C. Supposedly anyhow.

 

[fossickeract]

To clarify my position so as for me to not be responsible for sparking arguments.....

Goldierocks, Perhaps I should do the same and no offense is intended to anyone.
Firstly I am familiar with glacial erratics and drop stones they are common in the lower Permian beds of the Sydney basin. The point of my mapping story was that being a geologist even budding uni Prof, in that case, does not make ones observations any more valid than Lefty's whose observations seemed to be dismissed a with a certain hubris. This prompted me to respond which I rarely do, as despite my geology degree I would not dismiss Lefty's observations.Perhaps my point was a bit obscure.
Yes chalcedony, agate, jasper, and may be all the same thing, micro quartz however their optical properties vary with their structure and impurities. Again silcrete may be mico quartz, may form from cool solutions but it certainly has a different appearance to chalcedony, agate and jasper.Why?
The scientific paper I referred, listed in a previous post,expressed the opinion based on laboratory work that chalcedony was a transitional form of micro quartz with amorphous silica or cristobalite (opal) transforming to chalcedony transforming to crystalline quartz. They also claimed it was temperature dependent and no quartz crystal formed below 100 c. Now you have claimed they all form at the same temperature, I would be most interested to see a scientific reference to the same as my reference may be dated or shown to be wrong.
Yes we know ground water can be hot in outback Queensland, I spent 6 months mapping temperatures recorded in bore holes and that included those capped because of high radon levels so heated by radioactive deposits. But whats the relevance if agates,and lumps of chalcedony form at low temperatures bit both ways? Besides lefty isn't finding agates down bore holes or around uranium deposits but associated with volcanic intrusions
Nethercote Pyropholite is a published name in NSW Mines Dept reports, yes it is an altered rhyolite my samples were described by an AGSO petrologist as more of a silicified rhyolite, found where volcanics from Mt Dromadary Complex intrude the Devonian Rhyolites near Pambula.I don't know about Namibia but Pyroholite is also referred to in some North American reports.

"You guys make it hard! All these local names that are not true mineral names"
Ha any high silica content, opaque coloured rock which takes a polish if you don't know what it is, is called jasper by lapidaries opps will lightning strike. Its a bit more polite than the term" passion stone"ie"" F in Rocks" given by my structural geo lecturer to over enthusiastic students asking what samples were on excursion.
In conclusion I still agree with Lefty's observations, willing to reconsider with scientific references to low temperature formation of agates and quartz. I have seen agate studded vesicular lava showing chalcedony on the rapid cooling margins of the vesicules grading into quartz crystal centres which may have a different explanation than temperature variation but it seems reasonable.I think this will end my contribution to this topic I think I'll retire to faceting.

 

[Kingsolomon]

kangaroo valley petrified ( waxy lustre ) and Illawarra tiger and hawks eye.

 

[Lefty]

Nice big chunk there Barney - maybe there's a big eye of agate in the middle?

 

[Kingsolomon]

It weighs a ton , you or someone asked about the lustre from that material , not best pic , but it's a waxy - greasy lustre . The tiger eye and such I'm thinking about doing faceted pendants. Trilliants or such with checkerboard squares or triangles , honeycomb hex and such . Should look good now I'm finnished school I've allowed myself to facet again . A month or so off can drive one crazy!