Reconnaissance drainage survey for base-metal mineralisation in the Lleyn peninsula, North Wales

A geochemical drainage survey has been carried out over the Precambrian and Lower Palaeozoic sedimentary and volcanic rocks of the Lleyn peninsula using a combination of minus 100 BSI mesh stream sediment samples and panned concentrates collected from the same site. There is widespread contamination of drainage sediment with metallic material of domestic and, locally, industrial origin. Accordingly, 39 concentrates containing high levels of base metals were examined mineralogically to help distinguish anomalous samples containing natural minerals from those containing metallic contaminants. Chalcopyrite, sphalerite, galena, secondary lead minerals and baryte were the main minerals of hydrothermal origin detected. In some samples weathering of contaminants was seen to have produced secondary copper and lead minerals similar to those of natural origin. Samples containing high levels of base-metals are widespread throughout the area and those with the largest contents contain contaminants. Anomalies largely of natural origin which probably reflect mineralisation upstream, are concentrated within the outcrop of Lower Ordovician shales towards the western end of the peninsula, mostly between Llanbedrog and Botwnnog. From the spatial distribution of anomalies in this area both structurally-controlled mineralisation and stratabound sulphide enrichment are possible types of source. In addition, there are several Cu and Ba anomalies along the outcrop of the Precambrian Gwna Group, an olistostome containing a variety of igneous and sedimentary rock fragments, which probably reflect minor vein mineralisation.