SEDIMENTARY CYCLES AND SELECTIVE DOLOMITIZATION IN LIMESTONES HOSTINGTHE GIANT NAVAN ZINC-LEAD ORE DEPOSIT, IRELAND

The Navan zinc-lead ore deposit (70 million tonnes) is hosted principa lly (97%) by the Meath Formation (Lower Carboniferous, Courceyan, Nava n Group), comprising a lower, mainly carbonate mudstone unit, the Stac kallan Member, and an upper grainstone-dominated unit. The Stackallan Member, about 60 m thick, comprises about 35 peritidal cycles includin g an oolitic grainstone interval. Grainstones forming the upper part o f the formation, about 150 m thick, comprise at least six shallowing-u pward ramp cycles. Emergence horizons are indicated by subaerial disso lution surfaces, in situ breccias, and a single palaeosol. Most dolomi tization at Navan is confined to the Meath Formation. The linear dolom ite body, trending NE-SW, has a flattened, laterally limited, tabular geometry in cross-section, suggesting that dolomitizing fluids initial ly rose vertically, cross-cutting stratigraphic and sedimentological b oundaries, although fluid flow was subsequently controlled by these fe atures. Three stages of dolomitization are indicated by textural relat ionships, cathodoluminescence (CL), oxygen isotopes, and fluid inclusi on data. Stage 1 crystals, with generally dully luminescing cores and up to six overlying subsidiary zones, are the main replacive phase. Cr ystal surfaces are commonly corroded and overlain by bright stage 2 rh ombs (which also form new nuclei lining molds and fractures). Three su bsets of zones are present in stage 2 crystals: bright, non-luminescen t, and dull; these are locally separated by additional corrosion surfa ces. Stage 3 comprises relatively large baroque crystals, generally du lly luminescent but with up to seven sub-zones. Isotopic and fluid inc lusion data suggest growth of successive stages from waters which beca me progressively hotter (60 degrees-160 degrees C). The distribution o f the ores follows the dolomite trend. Ores were precipitated between Stages 1 and 2 of dolomite growth, following Stage 2, and also during the nucleation and growth of Stage 3 crystals. Dolomitization and mine ralization appear to have been temporally and genetically related. The cyclic sedimentation of the host rocks, which accentuates lithologica l and petrographical contrasts between depositional units, controlled dolomitization. The relationship between dolomitization and mineraliza tion, which is closely associated with faults, provides powerful explo ration criteria. The recognition of these features in limestones elsew here in the Irish ore field may point toward unrecognized zinc-lead pr ospects. (C) 1997 Canadian Institute of Mining, Metallurgy and Petrole um.