A reconnaissance geochemical survey of Anglesey

A reconnaissance geochemical survey of Anglesey was based on a drainage survey (at one sample per km2 ) supplemented by low-density soil sampling. A field survey of known mineralisation was carried out to aid data interpretation and assessment of mineral potential. The reconnaissance field survey indicated that the non-ferrous mineralisation of Anglesey may be divided into three groups: (a) copper, (b) copper (lead, zinc) and (c) baryte (lead). Economically, group (b) is the most important and its occurrence is virtually confined to the Lower Palaeozoic rocks. Groups (a) and (c) are small vein occurrences, within the Mona Complex and adjacent to the basal Carboniferous unconformity respectively. The drainage survey encountered major difficulties from the lack of surface drainage, contamination , subdued topography, variable background geology and extensive drift deposits. At least 35% of the island was not effectively covered by the drainage sampling but this figure was reduced by the collection of soil samples from three areas of poor drainage and thin drift cover. Problems were further countered by collecting water, stream sediment and panned concentrate samples at all drainage sites, mineralogically examining anomalous concentrates, and resampling streams with sites found to be contaminated. 440 drainage sites were sampled. Cu, Pb, Zn, Ba, Fe, Mn, Co, Ni and MO were determined in sediment samples and Cu, Pb, Zn, Ba, Fe,Mn, Ti, Ni, Sn, Sb and Ca in panned concentrates. Cu, Pb and Zn were determined in soil and water samples. Large-scale regional variation of the drainage results was examined using computer-generated greyscale maps. Variation was related to bedrock geology, mineralisation and contamination. The comparison of statistical analyses and mineralogical observations indicated that all high Sn and Sb levels were related to contamination and that, in this area, factor analysis was an effective means of discriminating between anomalies caused by contamination and those due to mineralisation. Inter-element relationships also indicated the presence of two chemically distinct types of mineralisation : a Cu-Pb-Zn-Fe sulphide type and a Ba type, which correspond to groups (a + b) and (c) of those defined by the field survey. Threshold levels were established using cumulative frequency plots, and eighteen anomalous areas related to sulphide or baryte mineralisation were delineated. Four of these, at Carmel Head, Llandyfrydog, City Dulas and Llanbadrig were the subject of further study. Other areas considered worthy of investigation are the basal Carboniferous between Dulas and Malltreath, the Gwna rocks around Cerrigceinwen, and the area east of Parys Mountain; the latter area has been investigated by mining companies without success. Further areas not adequately covered by the survey, for example the basic rocks around Rhoscolyn, may also be worthy of further consideration.