Vertical fluid flow

It is clear that some upward component of fluid flow must have occurred to create

mineralization in the carbonate sediments above the metal source in the basement (Everett

et al., 2003, LeHuray et al., 1987, O’Keeffe, 1986, Walshaw et al., 2006). The driving force for the hydrothermal fluids is debated; possible mechanisms include continental heat (Russell et al., 1981), mantle heat (Davidheiser-Kroll et al., 2014), volcanic (Wilkinson and Hitzman, in press) and topographic flow (Hitzman and Beaty, 1996).

The presence of vertically similar metal distribution patterns, as demonstrated for Navan below, requires rapid vertical fluid flow, resulting in small changes in metal precipitation activities. Rapid vertical flow can be caused either by over-pressuring and hydraulic jacking (Andrew and Ashton, 1985), or by pervasive fracture-induced faulting (Ashton et al., 1992). Conceptually, the hydraulic jacking and creation of vertical pathways for fluid migration would be assisted by unloading due to the removal of the overlying stratigraphy by the erosional event, and several authors show that mineralization was ongoing at this time (Andrew and Ashton, 1982, Boyce et al., 1983a, Ashton et al., 1992, Ford, 1996, Anderson et al., 1998, Everett et al., 2001, Fallick et al., 2001, Blakeman et al., 2002, Ashton et al., 2003, Ashton et al., in press). Given that the ultimate cause of the erosional event was faulting (Boyce et al., 1983a), there is an obvious potential connection between this faulting, and the fracturing that allowed for the vertical flow: vertical flow may thus have occurred as part of a feedback process, where hydraulic jacking and faulting caused rapid vertical fluid flow. Further, metal precipitation activities must have been relatively constant to prevent vertical variation in metal concentration values. If one considers hydraulic pressure as the main variant during the evolution of rapid vertical flow, and that relative activities (Reed and Palandri, 2006, see figure. 1) of Zn and Pb in chloride

complexes (Wilkinson et al., 2009) vary little over the 100 m thickness of ore lenses at Navan, it is clear that changes in pressure would not have been sufficient to induce differential precipitation between Pb and Zn (Susak and Crerar, 1982).

There is abundant evidence for vertical flow at Navan in the form of textures, metal emplacement patterns, and metal ratios. Textural evidence includes ramifying veins that cross-cut bedding, anastomosing veins, collapse breccias, and hydraulic breccias, all of

which have been reported previously (Anderson et al., 1998, Andrew and Ashton, 1982, Andrew and Ashton, 1985, Ashton et al., 1992, Ashton et al., 2003, Blakeman et al., 2002, Ford, 1996). These observations are generally made on the drift scale (ca. 3m vertical), with vertical mineralization seen in places terminating at areas of bedding-parallel mineralization below aquitards (Anderson et al., 1998). On a larger scale, metal

distribution patterns across stratigraphic lenses faithfully track the passage of the rising fluids (Figures 2.4 and 2.6). The observed similarities between metal ratios in vertically adjacent sections, even in areas where textural observations demand multiple episodes of ore emplacement (Andrew and Ashton, 1982, Gagnevin et al., 2012), support the notion that vertical fluid incursion was persistent and focused in particular areas (Figures 2.4 and 2.6). Below we provide detailed evidence for vertical flow from two areas at Navan: the SWEX and the areas in the main mine with the highest ore grades.

Ashton et al. (2003), showed that in an area of the SWEX bound geographically by the E Fault and the Y Fault, and vertically by the structurally complex 5 Lens and the Boulder Conglomerate, mineralization is dominated by vertical ore texture relationships, with stratabound mineralization found locally below marker horizons. This is supported by large-scale mapping of metal concentration patterns (Figure 2.6), where the vertical

continuity of high-grade areas from the lower-most 5 Lens up to the Boulder Conglomerate (see Boulder Conglomerate and Conglomerate Group Ore section) implies dominantly vertical flow. That Pb/Zn ratio values are homogenous on a km scale indicates that no systematic horizontal flow preferentially enriched one geographic area of the SWEX with one element over the other, as in the main mine 1 Zone (Figure 2.5). The small-scale heterogeneities that do exist in the SWEX 1 Lens Pb/Zn values are not present within the overlying homogenous and slightly lower Pb/Zn U Lens (Figure 2.6 a, e and i).

The highest ore grades at Navan are found in the northeastern area of the main mine (1 and 2 Zones), with grades of up to >30% Zn+Pb over 25m (Andrew and Ashton, 1982). In these areas of intense mineralization it is possible to trace focused high concentrations and

homogenous Pb/Zn from the lowest and most pervasively mineralized lens through all of the overlying lenses (compare Figure 2.4 e, I and m with Figure 2.4 f, g, j, k, n and o).

These vertical zonations are generally bound by the local large listric faults (B and T) but are believed to be causally related to the smaller faults and fractures in this region (Andrew and Ashton, 1982, Blakeman et al., 2002). Although these are relatively small faults, meshes with numerous small extensional and related faults are thought to be able to accommodate large scale fluid flow (Sibson, 1996). These vertically mineralized areas do not show any significant variation in Pb/Zn ratios as seen at other deposits, such as

Silvermines, Rammelsberg, Mount Isa, McArthur River, Sullivan, and Tom (Large, 1980, Taylor, 1984, Andrew and Ashton, 1985). A similar lack of Pb/Zn ratio anomalies, coupled with intense areas of element concentration, is seen in the SWEX, where vertical fluid flow also dominates (compare Figure 2.6e and j with Figure 2.6 f, g j and k).

A situation involving primarily vertical flow still requires sufficient mixing with the sulfur-rich surface fluid to account for the dominance of bacteriogenic S in the deposit (Fallick et al., 2001). Furthermore, the complexity of texture in many zones within the main mine indicates repeated influx of metals with attendant overprinting (Andrew and Ashton, 1982), as might be expected in Irish ore environments that created economic ore deposits such as Navan (Barrie et al., 2009). These processes must have, to a greater or lesser extent, smoothed out some of the perhaps originally more coherent trends in the mine. That distinct, identifiable trends remain is a testament to the persistence and intensity of the metal discharge from the hydrothermal fluid.