Ore depositional processes in the Navan Zn-Pb deposit, Ireland

The Navan Zn-Pb deposit in Ireland is hosted by a lower Carboniferous, carbonate-dominated sedimentary sequence. In excess of 97 percent of the ore is hosted by a Courceyan, shallow-water succession termed the Pale Beds. The ore occurs as complex strata-bound, tabular lenses often dislocated by faulting and truncated by a submarine erosional slide structure. Above this surface (unconformity) lies the economically minor, but genetically important Conglomerate Group ore hosted by a complex debris flow termed the Boulder Conglomerate. Timing constraints on the mineralization can be defined by the relationship to the erosion surface and style of mineralization, suggesting a late Chadian or early Arundian age (similar to 345 Ma). The bulk of the ore formed below the erosion surface in the semilithified to lithified Pale Beds ore. The mineralogy of the economic ores is relatively simple, being dominated by sphalerite and galena in a 5/1 ratio. Pyrite and marcasite are present in subordinate amounts in the majority of the Pale Beds ore but dominate the Conglomerate Group ore and the Pale Beds ore immediately below the erosion surface, particularly lens 2-1. Cangue minerals are dominated by calcite, dolomite, and barite. The morphology of the mineralization reflects subsea-floor emplacement. The orebody occurs as numerous strata-bound horizons, ranging from intense, high-grade massive sulfides to lower grade sulfide layers separated by barren limestone. In all cases, there is strong evidence for tectonic disruption during ore deposition. Sulfides were dominantly deposited as open-space growth and replacement of host rock. Open-space textures include dendritic-skeletal, stalactitic, internal sediment, geopetal, and coarse-bladed forms. The general absence of coarse, euhedral, cavity lining textures or substantial collapse brecciation suggests that large cavities were not present prior to mineralization. Replacement textures range from delicate pseudomorphs of bioclasts to more destructive granular styles. Sulfide sulfur isotope analyses exhibit two dominant groupings of delta(34)S from around -23 to -5 and 0 to 15 per mil, respectively, and a third minor grouping around -32 to -28 per mil. At least two sources of sulfur are invoked. Values around -23 to -15 per mil are considered to be the result of bacteriogenic reduction of lower Carboniferous seawater sulfate (similar to 20 parts per thousand), with a characteristic fractionation around 35 to 45 per mil away from sulfate; this is the most common range of delta(34)S in the deposit. Values around 8 to 15 per mil are interpreted to be hydrothermal sulfide transported with the metal-bearing fluid. Mixing of these two components is inferred from the isotopic data and trends in the mineral paragenesis. Barite has a mean of delta(34)S = 21 + 2 per mil, which falls within the range generally recognized for lower Carboniferous seawater sulfate. We consider that contemporaneous seawater sulfate was the dominant source of sulfur in the barite at Navan, implying that seawater accessed the Pale Beds at the time of mineralization.