A review of the behaviour of radon in the geological environment is presented. The general geochemistry of the element is described and the factors controlling its emanation from minerals and rocks and into the disperse phases itemized. A brief summary of analytical procedures for the analysis of the radon isotopes in waters and soil gases is given. The emanation of radon depends upon the source term uranium concentration, the nature of the host mineralogy, the permeability of the host rock and soil and the characteristics of the transporting medium. Weather can have a profound effect upon the concentration of radon in soil gas but often the variation due to the geological substrate is greater. Radon is not a problem unless it collects in buildings and underground structures. Some guidelines are given for identifying areas of high radon emanation based upon existing data sets.

The Ordovician (Caradoc) volcanic rocks of NE Snowdonia constitute two major groups, the Llewelyn Volcanic Group and the Snowdon Volcanic Group, which accumulated predominantly in shallow-water marine conditions. The younger Snowdon Volcanic Group comprised a bimodal, basalt-rhyolite suite and included a major caldera-forming eruption of acidic ash-flow tuffs superseded by both Surtseyan and Strombolian basaltic volcanism. Rhyolite domes were intruded into the volcanic sequence in the vicinity of the caldera. The Snowdon Cu-Pb-Zn vein mineralization comprises five paragenetic mineral assemblages. The veins cut rocks deposited within and over the caldera and it is proposed that the dominant controls of mineralization were volcanogenic. Circulation in hydrothermal cells, involving both juvenile fluids and seawater, deposited the minerals at a late stage in the evolution of the caldera.

Galena has been collected from different styles of mineralization throughout Wales. Galenas from the Ordovician volcanogenic mineralization at Parys Mountain, north Wales, give highly variable lead isotope ratios, which could have been produced either by a two-stage history or by the mineralizing fluids interacting and mixing with radiogenic lead derived from uranium-rich upper crust. The variation in the lead isotopic ratios of galena from veins cutting lower Palaeozoic sediments is considered to be due to separate periods of mineralization during the early Devonian and early Carboniferous. The most likely source for this mineralization is the thick sequence of lower Palaeozoic basinal mudstones. Both the disseminated galena in Mesozoic sedimentary rocks and galena from veins within the underlying Carboniferous Limestone give Triassic and Jurassic model lead ages, which agree with mineralization ages deduced from geological considerations. It is suggested that the source for this lead mineralization is the underlying Carboniferous shales. These and other galenas from Wales have significantly different isotope ratios to those from the Irish carbonate-hosted ore deposits, which would indicate that the mineralizing fluids had either different sources or evolutionary histories.

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