I have uploaded some photos of rocks other than slate, that I have found on and around the Cwmorthin tips in recent years

Any help with identification would be most welcome


First is a rather nice large slab which has been photographed dry (even better when wet)

🔗120258[linkphoto]120258[/linkphoto][/link]


Inclusions caught within a volcanic ash flow is a possibility


below are a couple of closeups of this material

🔗120260[linkphoto]120260[/linkphoto][/link]
🔗120261[linkphoto]120261[/linkphoto][/link]

below is possibly a fine ashfall tuff about an inch across (sorry forgot to put reference for scale)

🔗120262[linkphoto]120262[/linkphoto][/link]

photo below of same stone shows a well defined inclusion

🔗120263[linkphoto]120263[/linkphoto][/link]

next is a fragment of what was a rather nondescript looking lump of rock which I think was from the tips. It had a thin yellowish/fawn coloured skin on it, and looked like concrete but I later took a hammer, to it as it was annoying me (poor rock!) and it broke into sharp fragments. The fresh material inside has a nice sparkle which is not so obvious here - I'm not certain what this rock is

🔗120264[linkphoto]120264[/linkphoto][/link]

below shows outer cortex of the above with small flake removed

🔗120265[linkphoto]120265[/linkphoto][/link]

below - The following yellowish fragment was found on Cwmorthin back vein incline around floor B

Unsure what it is but in the 2nd photo you can see the junction of this material with something more like a mudstone

🔗120266[linkphoto]120266[/linkphoto][/link]

🔗120267[linkphoto]120267[/linkphoto][/link]


Like the following material - but don't really know what it is. I have named it "tapestry band granite" Found on track up to Cwmorthin, but since it only seems to come in agregate size lumps it may have been imported from elsewhere. Has lovely green crystals most noticable when wet

🔗120268[linkphoto]120268[/linkphoto][/link]

🔗120269[linkphoto]120269[/linkphoto][/link]


The same material as above and found on the track as before -.A dolerite or a Gabbro?

🔗120270[linkphoto]120270[/linkphoto][/link

Below - Lovely volcanic tuff with layers of mudstone found in the Cwmorthin opencast - have mentioned these before

🔗120271[linkphoto]120271[/linkphoto][/link]

below is an attractive material in layers with specks and blebs in it. There is a lot of this below Cwmorthin lower down the track, and I think it's peeled of the large oucrops of rock next to the track - love to put a name to this, but some sources describe the outcrops in this vicinity as rhyolitic tuff

This is a reasonable size. First photo shows speckles though a layer

🔗120272[linkphoto]120272[/linkphoto][/link]

next photos show the layer cake effect. All layers contain speckles Sample is about 1 foot approx


🔗120274[linkphoto]120274[/linkphoto][/link]

🔗120275[linkphoto]120275[/linkphoto][/link]

🔗120273[linkphoto]120273[/linkphoto][/link]

next is a fine grained light coloured very compact rock from the tip. Don't know if it's some kind of rhyolitic material. (size is about 1 foot)

🔗120276[linkphoto]120276[/linkphoto][/link]


closeup showing texture

🔗120277[linkphoto]120277[/linkphoto][/link]


Lastly a rock found on the track up to Rhosydd.

🔗120278[linkphoto]120278[/linkphoto][/link]

lots of variations of this material up there, but curiously not that common in vicinity of Cwmorthin


(Sorry must use a scale in future but it slipped my mind)

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Hi one simple way to help in you is to carry out a S.G test on your rocks
This then gives you some idea how likely it is you have found any potentially ores!

I think any ores worth extracting in Cwmorthin would have been found long ago. However there are many trace minerals in Cwmorthin and Oakeley the names of which mostly escape me.

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The green crystals may well be olivine, although it's microgabbro that's in the area, so I wouldn't expect visible crystals or clasts that big.

Is the light one a rhyolite or a felsite?

The green crystals seem to be in bands running across the material and associated with patches of lighter minerals to give the"tapestry band" like effect,although in other samples, these lighter patches are often salmon pink. the areas between these bands tend to have smaller grains more equal in size. An interesting material, and I would love to see what a large slab looks like. The big question: is rock actually from the area and if not, where was it sourced?

as for the compact light rock (I assume you refer to my 2nd to last example) rhyoite is very much a possibility, as there is lots of it in the area in various guises. For example the trap rock under the Oakeley new vein is described in some sources as rhyolite, whether an extrusive lava, or an ash fall, I don't know. Some sources mention intrusive rhyolite in snowdonia as well. I assume this means it can also form dykes and sills.

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Chlorite is a fairly common metamorphic alteration, and has a greenish tinge. The layered samples would indicate waterlayed ashes / tuffs with other compositions mixed in. Some of the more acidic lavas could be ignimbrites, a super heated pyroclastic flow that sets into a lava like deposit, rather than true rhyolite or andesite flows.
This area on N Wales would have been volcanoes above and below water, with many deep basins and seas collecting the fine sediments that became slate. Life on dry land would be very rare and primitive at this time, and marine life mostly excluded due to toxic waters.
Definitive identification would need thin sections, XRD and XRF analysis

Come to think of it, does anyone know if the dolerite which intrudes the slate in Cwmorthin quarry ,is a quartz dolerite or an olivine dolerite?

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dolerite, or maybe ultramafic

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C
Definitive identification would need thin sections, XRD and XRF analysis

Thin sections, definitely, electron microprobe, yes, X-ray powder diffraction on a multiphase mixture with alteration products, only for the hardcore masochist :lol:

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http://earth2geologists.net/Microscopes/documents/How_to_make_rock_thin_sections.pdf 

Also it seems you can get thin sections made for you by at least these two sources:

https://www.ed.ac.uk/geosciences/facilities/sampleprep 
http://www.thinsectionlab.com/service.html 

I love a good polarising microscope. I had one for a couple of years when I did an OU earth science degree back in the 90s.

And I'm now on the hunt for a second hand one!

The banded tuffs on the tips at Cwmorthin host thin veins that occasionally have pockets of fine crystals. Quartz, albite and anatase all occur, but especially good are the rare-earth minerals, synchysite and xenotime. Specimens are very time-consuming and exhausting to collect, though! Eye-protection is essential due to flying splinters - in fact a full facial visor would not be a bad idea. Not a bad activity for a nice afternoon, when they let us out again...

Here is another rock I found around Cwmorthin in the vicinity of the tips. It has some attractive colours and an area which almost looks a bit like flow banding. It is heavily eroded on one side. I wonder if it's some kind of extrusive lava?
It seems quite a brittle material


🔗120711[linkphoto]120711[/linkphoto][/link]

🔗120713[linkphoto]120713[/linkphoto][/link]

🔗120712[linkphoto]120712[/linkphoto][/link]


Secondly some interesting mineralisation I found in what looks like a volcanic material, possibly an ash - but I'm only making a guess on this one. Some time ago I mentioned in another post that I found a tiny deposit of pink material in a wall on lake level in Cwmorthin. I have no idea how I came to notice it and wondered if I had imagined it as it was so small an amount.

However, it later became evident that it wasn't my imagination as this rock contains the exact mineral that I previously saw underground - an attractive pink, merging into a more orange tinted area, indicative more of iron oxides maybe? Next to it are cavities containing crystals which sparkle when they catch the light

And I've just also noticed this pink almost matches the pink in my Welsh blanket :)


I was wondering if this mineralisation is in rhyolitic ash?

🔗120715[linkphoto]120715[/linkphoto][/link]

🔗120714[linkphoto]120714[/linkphoto][/link]

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Hi I would check your pink samples, with a couple of spots of dilute Hydrochloric acid, since it looks like a pink flowstone, which essentially is a calcite!
If I am correct then this material will bubble as the acid reacts with iit! If it does not then at least you have eliminated one distinct possibility!
Regards, John Lawso

Hi I would check your pink samples, with a couple of spots of dilute Hydrochloric acid, since it looks like a pink flowstone, which essentially is a calcite!
If I am correct then this material will bubble as the acid reacts with iit! If it does not then at least you have eliminated one distinct possibility!
Regards, John Lawso

One possibility for the iridescent coloured bit would be chalcopyrite - to find out you could leave it in dilute acid overnight and see if the acid takes on a greenish tinge.

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Interesting feed back, worth a try, but I won't risk damaging the sample for now as I sadly won't be back to find any more on the tips any time soon in the present situation 😞

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Interesting feed back, worth a try, but I won't risk damaging the sample for now as I sadly won't be back to find any more on the tips any time soon in the present situation :(

We could post you one? 😉

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yes, I've managed to get post from Wales 🙂

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Cobalt bloom?

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