Geophysical and geochemical investigations of the manganese deposits of Rhiw, western Llyn, North Wales

Detailed magnetic surveys have been carried out near Rhiw, in western Llyn, North Wales. These surveys covered ground south-west from the old Benallt manganese mine as far as the old Nant mine, and northwards from Benallt towards Sarn. Rocks within the area are of Arenig and Llanvirn age (Lower Ordovician) and consist of mudstones, siltstones and sandstones with interbedded basic lavas and sills. The manganese deposits occur in a structurally complex setting within sediments of Arenig age. These sediments are confined between a basic sill and a dolerite or basalt lava. That part of the manganese mineralisation which is of ore grade is unique within the British Isles because of its strong magnetisation, caused by the presence of the mineral jacobsite, an iron-manganese oxide analogous to magnetite. The geophysical survey was supported by the collection of soil samples from across-strike traverses. The soils, which were analysed for a wide range of elements, proved of limited use for exploration purposes, mainly due to the variable depth of drift cover. Both Ti and V proved useful in areas of limited drift cover as an aid to mapping the sub-crop of the basic igneous rocks. Several very localised magnetic anomalies were identified, and three of these were investigated by drilling. These anomalies were thought likely to be due to discrete bodies of manganese ore. Other more extensive anomalies mark the sub-crop of a basic sill (the Footwall Sill) which occurs below the sediments which host the manganese mineralisation. Two of the anomalies drilled were found to be due to stratabound ironstones of very limited lateral extent, and of particularly high magnetic susceptibility. These ironstones contain up to -70% FesOs and show a marked depletion in manganese when compared to the enclosing sediments and basic igneous rocks.