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 ofte n dislocated by faulting and truncated by a submarine erosional slide structure. Above this surface (unconformity) lies the economically min or, but genetically important Conglomerate Group ore hosted by a compl ex debris flow termed the Boulder Conglomerate. Timing constraints on the mineralization can be defined by the relationship to the erosion s urface and style of mineralization, suggesting a late Chadian or early Arundian age (similar to 345 Ma). The bulk of the ore formed below th e 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 pre sent in subordinate amounts in the majority of the Pale Beds ore but d ominate the Conglomerate Group ore and the Pale Beds ore immediately b elow the erosion surface, particularly lens 2-1. Cangue minerals are d ominated by calcite, dolomite, and barite. The morphology of the miner alization reflects subsea-floor emplacement. The orebody occurs as num erous 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 growt h 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 de lta(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 considere d to be the result of bacteriogenic reduction of lower Carboniferous s eawater sulfate (similar to 20 parts per thousand), with a characteris tic fractionation around 35 to 45 per mil away from sulfate; this is t he 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 f rom the isotopic data and trends in the mineral paragenesis. Barite ha s a mean of delta(34)S = 21 + 2 per mil, which falls within the range generally recognized for lower Carboniferous seawater sulfate. We cons ider that contemporaneous seawater sulfate was the dominant source of sulfur in the barite at Navan, implying that seawater accessed the Pal e Beds at the time of mineralization.