Geochemical and geophysical investigations north-west of Llanrwst, North Wales

Reconnaissance stream sediment surveys, carried out both by IGS and by Noranda Exploration (UK) Limited, detected zinc anomalies in the Afon Dulyn. It seemed probable either that they reflected a hidden extension of the Llanrwst lead-zinc mining field, or that they were concentrations related in the first instance to hydrous oxide precipitates produced by changes in the secondary environment. To test which of these possibilities was the most likely, a more detailed drainage survey of the Afon Dulyn and Llyn Eigiau areas was carried out. Stream sediment , panned concentrate and stream water samples were collected from each of 28 sites and analysed for a range of elements which included copper, lead and zinc in all three sample types, and iron, manganese and barium in sediments and concentrates. Readings were taken of the pH in stream water and soil at many points in the area. In addition , ground magnetic traverses were surveyed in the Foe1 Fras and Drosgl areas where there are known to be positive aeromagnetic anomalies. The analytical results were examined in some detail by various statistical methods, from which it was concluded that most of the high zinc in stream sediment results were related to hydrous oxide precipitates. The distribution of elements in a few samples showed some indications of mineralisation, but mineralogical examination of the panned concentrates from these sites found no mineral phases which could be related directly to mineralisation or contamination, zinc occurring mainly in iron oxide coatings. It is thus apparent that a source of zinc exists in the general area, perhaps on the wide interfluves, but it is not known whether this source is dispersed or concentrated (i.e. mineralisation).