I am wondering about the other rocks at Cwmorthin/Oakeley which occur associated with, or in the vicinity of the slate veins and have been looking at a number of diagrams and plans with this in mind. For example, the veins are cut by a number of whinstone dykes, and I believe these are volcanic in origin. I am interested in what this whinstone actually is in geological terms, and if it is roughly of the same composition in all the individual dykes, or differs significantly from one to the other. Also the Glan y Pwll trap rock which underlies the new vein in Oakeley. I think this is again of igneous origin, but as before, I lack a geological term for it.

I have also noted the presence of diorite marked on the Oakeley plans on the deep floors P and Q for example. One of the deep cabans (marked as the “old Caban” on P) actually looks built into this rock, and I gather from this that it makes for a nice well supported little chamber in rock at the bottom of the workings It may have stated life as a level which was subsequently abandoned. I think diorite is an intrusive igneous rock, and was wondering if it also occurs in Cwmorthin as well, or if it is found only at these greater depths.

I am familiar with the rather lovely cream coloured tuff turbidites which I have seen in the open pit above Cwmorthin, but haven't noticed these underground. There is a mention in Caban magazine about miners encountering a resilient and hard "marble like" rock, but no other details are given. Are these tuff turbidite bands actually rare in the formation?

On a related note, I am interested in what rock the long number 9 adit at Rhosydd passes though on its way to the slate veins. What ever it is, it seems to support that long, impressive tunnel very well. Again this rock looks igneous to me, but beyond that I don't have a geological name for it, although a quick online search seems to suggest similar looking rock in the general area is extrusive igneous Rhyolite.

This also brings me to the question of what the massive blocks that have come down at the top of Wrysgan slate mine actually are as they don't look like slate to me. They are among the largest single blocks that I have ever seen!

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There's a good guide to what's going on in "Snowdonia Rocky Rambles" by Bryan Lynas (Sigma Press ISBN 1-85058-469-9).
Pages 62 to 91 deals with Cwmorthin and Moelwyn.

There are numerous igneous intrusions in the area especially in the Dolgellau gold belt but also within the area of Ffestiniog.

Taken from HMSO Institute of Geological sciences North Wales;

“The mineralisation of the lower Palaeozoic rocks is for the most part to be attributed to the deep-seated solutions associations of the Ordovician igneous intrusions”

It further talks of quartz veins cutting all the strata from the Barmouth Grits to the Ffestiniog Beds and being impregnated with sulphides of copper, iron, zinc and lead. The minerals doubtless precipitated when the ascending solutions came into contact with the strata containing pyrites of which there are considerable amounts.

Certainly within Cwmorthin there is plenty of evidence of precisely these mechanics – you can see sulphides in many chambers and there is a notable quartz/pyrites intrusion on the lake level.

:)

Ian

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There were huge (to me) cubic crystals of iron pyrite up on level 5 or 6 when they were un-topping in the 1990s.

There is alot of pyrite in the back vein, and I have seen masses of tiny yellow dusty deposits which sparkle, on the underside of slabs which have fallen away from the wall. There seems to be less in the parts of the old vien that I have seen, but perhaps i'm just imagining this. Will remember to check this next time I'm there. Also seen a photo of a lovely quartz vein on floor A, back vein but never found out how to get there.

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At Cwmorthin, best seen in the open workings, there are beds of tuff-turbidite interbedded with the slaty mudstones: the miners call them "cherts". Brutally hard and splintery due to the high silica content, they are strikingly banded so that blocks of the stuff can look like rocky liquorice allsorts. There are basic dykes that cross-cut the slate and the other interbedded strata too. Both are brittle lithologies, so that in the compression that formed the slaty cleavage, they deformed in a brittle manner. The cracks thus formed are often open and lined with crystals of a variety of minerals, which is where it gets interesting, as some of the species present offer the possibility of radiometric dating. This may in turn be a way to develop a method of dating cleavage-forming deformation episodes - the implications in helping to understand crustal evolution in the area and in other slate-belts around the world are potentially very good. Getting the necessary analyses done is the difficult bit! I collected a lot of samples from there over ten years ago, when there was a shedload of boulders of the stuff dumped above the opencast. Used to get a key from Oakley and drive up there (had a S3 Landy at the time). I must see if I can try and get this idea resurrected. No idea if similar stuff is present underground but would guess so as I also collected material from Oakley and saw very similar occurrences in Cwt-y-Bugail in 2013.

Many thanks, everyone for the interesting info :)

I've never come across the point where these tuff turbidite bands pass though the underground workings, will make a point of looking when I'm in Cwmorthin. They could well be present in an area I haven't explored or in an area now cut off from access. Haven't actually ventured above lake level underground yet. Generally tend to head down the inclines instead.

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They do seem to be most frequent in the area in and above the Cwmorthin opencast: likewise at Cwt y Bugail they are commonest in the higher levels.