Geological structure The giant zone of zinc-lead mineralisation at Navan, Co. Meath, was discovered in November 1970. Subsequent exploration has identified a wedge-shaped orebody, dipping from the thinner north-eastern end, where it reaches the surface, towards the south-east, where it lies at a depth of at least 600 metres (Fig. 1). The deposit is 1800 metres long, 400 metres wide, and, on average, 100 metres thick. The geological nature of the orebody is similar to other base-metal deposits found in Ireland's carboniferous limestone in that mineralisation has occurred along an E.N.E. to W.S.W. faultline and then encroached into the surrounding dolomitic limestone. The deposit therefore represents a combination of classic veined and contact metasomatic (replacement) elements, usually ascribed to deposition from mineralised hydrothermal solutions originating at depth within the earth's crust and making their way to the surface via natural rock fissures such as faultlines. The orebody at Navan is for the most part divided into distinct lenses, indicating selective replacement along the bedding planes in the host limestone.