Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midla nds is dolomitized pervasively over a much larger region than previous stud ies have documented. This study indicates a complex, multistage, multiple f luid history for regional dolomitization. Partially and completely dolomiti zed sections of Waulsortian Limestones are characterized by finely crystall ine (0.01-0.3 mm) planar dolomite. Planar replacive dolomite is commonly fo llowed by coarse ( greater than or equal to0.5 mm) nonplanar replacive dolo mite, and pervasive void-filling saddle dolomite cement is frequently assoc iated with Zn-Pb mineralization. Planar dolomite has average delta O-18 and delta C-13 values (parts per thousand PDB) of -4.8 and 3.9 respectively. T hese are lower oxygen and slightly higher carbon isotope values than averag es for marine limestones in the Waulsortian (delta O-18 = -2.2, delta C-13 = 3.7). Mean C and O isotope values of planar replacive dolomite are also d istinct from those of nonplanar and saddle dolomite cement (-7.0 and 3.3; - 7.4 and 2.4 respectively). Fluid inclusions indicate a complex history invo lving at least three chemically and thermally distinct fluids during dolomi te cementation. The petrography and geochemistry of planar dolomites are co nsistent with an early diagenetic origin, possibly in equilibrium with modi fied Carboniferous sea water. Where the Waulsortian was exposed to hydrothe rmal fluids (70-280 degreesC), planar dolomite underwent a neomorphic recry stallization. to a coarser crystalline, planar and nonplanar dolomite chara cterized by lower delta O-18 values. Void-filling dolomite cement is isotop ically similar to nonplanar, replacive dolomite and reflects a similar orig in from hydrothermal fluids. This history of multiple stages of dolomitizat ion. is significantly more complex than earlier models proposed for the Iri sh Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.