Ironstone within the Nant Ffrancon Formation has been mined near Betws Garmon (SH544 579), Llandegai (SH590 696), Aber (SH670 724) and in the Nant Ffrancon Pass (SH643 622).
I have strong suspicions that Cwm Ceunant (SH 625 637) was for the same purpose, as were other trials in the same rock formation. See,
Geology of the country around Bangor.
Howells, Reedman and Leveridge.
British Geological Survey. H.M.S.O. 1985.
ISBN 0 11 884417 2