The Lower Carboniferous carbonate rocks of the Irish Midlands host one of the great orefields of the world. Since 1960, a number of significant zinc-lead deposits have been discovered, including the giant orebody at Navan. Industry has ranked Ireland first in terms of zinc discovered per kilometre squared, and second with respect to lead. High metal grades, shallow occurrences and the clean metallurgy of orebodies, all result in a relatively low cost of mining for the 'Irish-type' zinc-lead deposits.