A hydrogeologic analysis of the Irish carbonate-hosted lead-zinc ore deposits

The Carboniferous carbonate rocks covering most of Ireland are heavily faulted and ore mineralization within all of the presently known lead-zinc deposits is associated with these faults. In particular, the faults appear to have controlled the flow of mineralizing fluids with each of the deposits occurring predominantly in the hanging wall of a normal fault. Several genetic models have been proposed for hydrothermal ore formation in Ireland. The models are based primarily on fluid inclusion studies indicating homogenization temperatures ranging from 100{dollar}spcirc{dollar}C to 300{dollar}spcirc{dollar}C and Pb isotope studies indicating a local basement source for the basinal fluid. In all of the models, the actual mineralizing event occurs near the early Carboniferous seafloor where hydrothermal waters vented along fracture zones.; The first part of the study is a quantitative hydrogeologic analysis of basin-scale flow systems thought to be responsible for the formation of the Irish Pb-Zn deposits. A numerical analysis, using two-dimensional finite elements, evaluates topography-driven flow and density-driven free convection as two competing mechanisms. Results suggest that topography-driven flow could have provided suitable volumes of fluid to the Irish Midlands for the formation of the deposits. Fault plane free convection produced significantly higher temperatures than topography-driven flow, but failed to produce volumetrically significant fluid flow in a reasonable time.; In the second part of this study, the deposit-scale dual-porosity hydrogeology of the Irish Pb-Zn deposits is investigated in the context of a topographically-driven flow system. Fluid flow and solute transport are modeled using finite element techniques. It is demonstrated how faults affect the distribution of basinal brines and simulations of varying geologic complexity separate stratigraphic effects from dual-porosity effects on brine distribution. Faults are shown to control both the distribution of brine and overall flow patterns in both map view and cross-sectional models producing patterns similar to those observed in the field.