Bryn Coch a Capel Hermon
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Description
Description Bryn Coch a Capel Hermon consists of two discrete localities situated in Coed-y-Brenin, approximately 7km north of Dolgellau. The section at Bryn Coch is of special scientific interest because it provides the best, unweathered exposure of porphyry copper mineralization developed within the Afon Wen intrusive complex. The locality lies in an area that is marginal to the main Coed-y-Brenin orebody, but exhibits ore of a similar grade. Dated as Tremadoc in age, this is one of the oldest porphyry copper deposits known and is certainly the finest example of such a deposit in great Britain. Pyrite and chalcopyrite mineralization, which typically occurs both in veinlets and in disseminated form, is well exposed in a bluff of ‘greenstone’, the presence of copper being denoted by common malachite stains. The section at Capel Hermon is complementary to that at Bryn Coch and lies within the ore-zone of the porphyry copper deposit. The major difference is that it shows to advantage the supergene oxidation of porphyry copper mineralization. This very accessible exposure reveals heavily decomposed and altered microtonalite of the Afon Wen intrusive complex, thickly impregnated with malachite resulting from the decomposition of chalcopyrite through interaction with groundwaters. Remarks Bryn Coch a Capel Hermon was selected following the completion of a joint research initiative undertaken on behalf of CCW by the National Museums and Galleries of Wales. The site is a revision to the former Nature Conservancy Council’s Geological Conservation Review, a national survey and evaluation of sites of geological and geomorphological interest. The site is described in the forthcoming GCR volume entitled Mineralization in great Britain (Bevins, R.E., Young, B., Mason, J.S., Manning, D.A.C. & Symes, R.F. 2010. Mineralization of England and Wales. Geological Conservation Review Series, No. 36, JNCC, Peterborough). The site lies within Snowdonia National Park.