Tyllau Mwn
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Geology & landscape
Geological Period: Ordovician · Rock Type: Ironstone
Landscape processes: Volcanic / Igneous
Description
Description Located in remote moorland at an altitude of 600m on the north-western flank of the Aran range of mountains, Tyllau Mŵn is a key site for the study of bedded ironstones in North Wales. The mine exploited a subvertical, lenticular body of ironstone hosted by volcaniclastic rocks belonging to the Aran Volcanic Group of Ordovician (Arenig-Caradoc) age. Originally composed of chamosite ooids that accumulated upon shallow-water topographic highs on the former sea bed, the sediment was subsequently altered during regional low-grade metamorphism into a highly distinctive rock consisting of magnetite octahedra in a calcite-rich, chloritic matrix. Stilpnomelane is the principal accessory phase in associated late-stage calcite veins, whilst fluorapatite, rare ilmenite and pyrrhotite are also present within the assemblage. The mineralogical evolution of the deposit was probably influenced by thermal effects associated with the emplacement of adjacent igneous intrusions. Remarks Tyllau Mŵn 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, and is described in 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.