Abstract In the intensive Irish exploration program which has followed the discovery of the Tynagh deposit (Northgate Exploration, Ltd.) in 1962, three base metal discoveries have been made to date. These include the lead-zinc-silver deposits at Silvermines (Consolidated Mogul Mines, Ltd.), which are now being readied for production, the copper-silver deposit at Gortdrum (Gortdrum Mines, Ltd.) and the lead-zinc deposits near Keel (Rio Tinto-Zinc Ltd . ) . Each of these discoveries is the result of a combined geological-geochemical-geophysical exploration sequence in which pulse-type induced polarization surveys defined the precise location and lateral extent of the near-surface metallic sulfide mineralization and guided the initial drilling program. Whereas the Silvermines mineralization is, in part, composed of massive sulfides, the other two deposits are characterized by generally less than 5 percent conducting sulfides and constitute an excellent demonstration of the unique merits of the pulsetype induced-polarization system.

Abstract Introduction Iron oxide minerals occur in both Irish zinc-lead deposits and, in southern Ireland, in regionally extensive zones within Waulsortian Limestone (Fig. 1). The zinc-lead deposits contain zones consisting of ironstone (iron oxide + silica) bands which are generally surrounded by red to pink, hematized limestone and green, bleached or chloritic limestone. These zones range in size from very large at Tynagh (2400 x900m area; up to 50m thick) to extremely minor occurrences of isolated ironstone nodules or veins. In southern Ireland, regional iron oxide alteration zones contain red to pink, hematized, and locally silicified, limestone. One zone extends over 30km along strike. Textural evidence suggests that both the ironstones associated with zinc-lead deposits and the regionally extensive iron oxide zones formed by replacement of carbonate sediments. The relationship, if any, between the two iron oxide-rich styles of alteration is unclear.