As is my wont on Fridays after work, I go for a bit of a cross country run. Today was a run around Mynydd Mawr, starting and finishing at Rhyd Ddu. If you are local, then you'll know that the South West of Llyn Cwellyn is a no no. You can follow a public footpath up to the northern end of the lake and that's it.. Signs saying 'This is a private nature reserve, no right to roam access' - fair enough - you can go round the edge of this via particularly rough ground and eventually join the public footpath going up the elephant's nose (Mynydd Mawr being known colloquially as Elephant Mountain from it's aspect from Caernarfon).

Anyway.. at approximately SH 5472 5580, I came across a hole, about 10' deep on initial inspection. Having a closer look, it may be an adit of some description - difficult to say, as you'd have to desend 10' or so first. So, any ideas ? The Ystrad mine is nearby, so possibly iron trials, slate trials? Or some exotic mineral consequential to the nearby Mynydd Mawr micro-granite mass?

Robin

The grid ref suggests this may be the small trial working that I investigated 10 years or so ago. The tip shows massive green to purple fluorite with some quartz, carbonate, pyrite and chalcopyrite. Petrological investigation revealed the presence of magnetite, pyrrhotite and microscopic grains of Pb, Bi and Ag tellurides.

An unusual mineral assemblage but practically worthless!

Mentioned in passing in the following references:

Colman, T.B. 1990. Sediment-hosted base-metal vein mineralisation at Drws y Coed and Cwm Pennant, Snowdonia, North Wales. British Mining, no. 41, 41-59.

Colman, T.B, and Appleby, A. K, 1991. Volcanogenic quartz-magnetite-hematite veins, Snowdon, North Wales. Mineralogical Magazine, 55, 257-262.

Hope that helps.

Cheers - John

Interesting. Associated with the microgranite intrusion then possibly.

The rare-earth content of the fluorite and the adjacent Mynydd Mawr microgranite are very similar according to the papers, so a late magmatic vein looks the most likely explanation, yes.

Cheers - John