SUPERGENE MANGANESE MINERALISATION AT GYEN DDU AND BWLCH MAWR, CLYNNOG-FAWE, CAERNARFON, GWYNEBD, WALES

摘要

Nineteenth century trial workings for manganese occur in Ordovician sediments at Gyrn Ddu and Bwlcfa Mawrin the parishes of Lianaelhaearn and Clynnog-Fawr respectively. Scant historical information relating to theseworkings exists, but what little there is suggests that manganite and psilomelane are the main manganese phases.Recent fieldwork and analyses have shown the earlier reports to be erroneous with the mineralisation consisting ofcryptomelane, pyrolusite and lithiophorite. The manganese assemblage occurs in Joints and fractures, rather than trueveins, and has formed through supergene processes. The source of the manganese is uncertain, but we postulate thatweathering of the granodiorite porphyry, significant intrusions of which form the summits of Gyrn Ddu andBwlch Mawr, may have played a part in the mobilisation of manganese and in the formation of these deposits.