Foel Ispri
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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Cambrian
Description
Description Foel Ispri Mine is located on the northern flank of the Mawddach Valley, approximately 4 km north-west of Dolgellau, and is one of numerous mines in the vicinity which lie within the Dolgellau Gold Belt. The site is of special scientific interest for the study of in situ mineralisation and associated wall-rock alteration as well as facilitating investigations into the mechanism of lode emplacement in various types of rock. The principal vein is hosted by sedimentary rocks of the Cambrian Maentwrog Formation and by younger, concordant greenstone instrusions. This vein has an arcuate outcrop in the vicinity of Foel Ispri, trending E-W at its western end, but trending NE-SW further to the east where it was only worked sporadically. Another large, quartz-filled vein, which trends NW-SE, has been worked in an impressive opencast running down the south-eastern flank of Foel Ispri. Where these two structures meet high up on Foel Ispri, the best surface mineralisation is seen. In the highest working, a large mass of intensely bleached and pyritised country rock, (a “horse” in mining terminology) occurs at the junction of two branches of the vein, the latter being rich in galena, sphalerite, arsenopyrite, pyrite and chalcopyrite. Cross fractures on the SW-trending vein are also mineralized and adits are accessible on both structures. Mineralogically the veins at Foel Ispri differ from those further west in the ore-field in that arsenopyrite, although still early in the sequence of mineral crystallisation (paragenesis), is associated with pyrite and is followed by chalcopyrite and sphalerite. Gold is associated with arsenopyrite/pyrite but electrum and galena may replace these minerals. Tellurides are absent, but mackinawite and cubanite are accessories. The mineralisation is therefore of the lower temperature Gwynfynydd type, rather than of the Cogau type.