An understanding of the tectono-stratigraphical setting of the basement to Carboniferous rocks is important when considering Irish carbonate-hosted base metal deposits. Firstly, the underlying structural grain was crucial in controlling basin development (Brown and Williams, 1985) and the geometry of the structural setting of the mineral deposits (Phillips et al., 1976; Johnston et al., in press). Secondly, there is a body of isotopic evidence (O’Keeffe, 1986; Mills et al., 1987; Le Huray et al., 1987) which suggests that the basement has played a major role in metal supply. Russell (1978 and 1986) has suggested that hydrothermal cells excavated metal from basement rocks directly beneath the deposits.
Much of the template upon which the Carboniferous rocks were deposited comprises Upper Proterozoic crystalline schists overlain by a veneer of Lower Paleozoic assemblages of sedimentary and igneous rocks formed, deformed and metamorphosed between about 550 and 400Ma. This latter assemblage, because of the deformational and metamorphic characters impressed upon it, forms a coherent entity and forms part of the Caledonian fold belt. These rocks represent the remnants of the early Paleozoic Iapetus ocean (Phillips et al., 1976). The surface trace of this ancient ocean lies beneath the Irish Midlands in a sigmoidal belt of east-to northeast-trending shear zones. Arguably, both Silvermines and Navan lie above one of the most major strands of the Iapetus shear zone (Phillips et al., 1976). As a result of this geometry, there is a fundamental difference between the basement geology in the southeast of Ireland from that
Figures & Tables
Irish Carbonate-Hosted Zn-Pb Deposits
This paper describes the stratigraphy, basin evolution, and structural geology of Ireland so that the hydrothermal processes which formed the deposits can be placed in a geological context. Informal regional nomenclature are used throughout this paper to provide a clearer overview, rather than the myriad of existing local formation names (see Jones and Earls, this volume). The stratigraphy, together with series and stage designation and available radiometric ages, is presented in Figure 1.