Abstract

We explore three- and two-dimensional, regional- and local-scale numerical models for the development of Irish Zn-Pb-Ag mineralization. The calculations examined one of the proposed genetic models for the deposits in which mineralization was associated with mixing of two fluids of different salinities in the hanging walls of faults undergoing active deformation during mineralization. The numerical models involve coupling between volume changes arising from elastic-plastic deformation, single-phase fluid flow, and mixing of fluids with different salinities together with thermal transport both by conduction and by advection. We evaluated critical parameters that could optimize the rate of mineralization in three dimensions at the regional scale and in two dimensions at the local scale. Topographic relief is proposed as one driver of fluid flow. The models indicate a competing process, through convection of high-salinity fluids, mainly within faults and originating as partially evaporated seawater, which mix with lower salinity fluids driven by topographic relief. At the regional scale, although there was substantial fluid flow through the basinal sediments, considerable flow of basin-derived fluids also occurred through the basement, with subsequent focusing through faults. The effect was enhanced if the basement permeability was increased by fracturing. Basinal fluids (fluid 1, initially in the basin or introduced from meteoric sources in the adjacent highlands) were extensively exposed to the basement and became focused into both northward- and southward-dipping faults. A second, more saline fluid (fluid 2) was driven down into the rock column by convection; it had most exposure to the basinal sediments and, in some situations, to the basement before mixing with fluid 1.

The positions of the fluid mixing sites involved a balance between topographically driven flow and convection in three dimensions at a regional scale. Changes in porosity and permeability induced by plastic deformation influenced the rates of mineralization at the local scale and local Darcy flow rates at mineralizing sites. Mineralization at the local scale resulted from the interplay of fault dip direction, displacement, deformation-induced dilation, seal thickness, and integrity. The modeling results are compatible with empirical data from the Irish ore field, suggesting that north-facing faults, dipping away from the inferred position of topographic highs, preferentially host economic deposits. There is nothing intrinsic in the modeling that constrains the timing of the mineralization relative to compressional or extensional events, although a body of research favors mineralization overlapping with extension, as at the Navan, Silvermines, and Tynagh deposits. In contrast, at the Lisheen deposit, mineralization postdates the main extensional fault offsets. We suggest that a switch to shortening is a possible driver for this mineralizing system.

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