Journal of Conference Abstracts

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

Jay M. Gregg (greggjay@umr.edu)¹, Kevin L Shelton (sheltonkl@missouri.edu)², Ian D. Somerville (idsomer@macollamh.ucd.ie)³ & Wayne R. Wright (wayne.wright@ucd.ie)³

¹ Department of Geology & Geophysics, University of Missouri-Rolla, Rolla, MO 65401, USA

² Department of Geological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA

³ Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland

The Waulsortian facies (Lower Carboniferous) of the southern Irish Midlands is the primary host of that region's large base metal sulfide deposits. These deposits occur along southwest to northeast trending fault and fracture systems. Ores are associated with large volumes of open-space-filling epigenetic dolomite and calcite. The Waulsortian is dolomitized pervasively south of a line extending southwest from Dublin to Mallow, although local, heavy dolomitization of the Waulsortian has been observed northwest of this line. Hitzman et al. (1998) suggested that the replacement dolomite is related to large-scale flow of warm basinal fluids northward from the South Munster Basin.

Waulsortian samples were collected from 8 drill core and 6 outcrop sections throughout the southern Midlands. Partially dolomitized and several completely dolomitized sections are characterized by fine crystalline (0.01 to 0.3 mm), replacive, planar dolomite. Planar dolomite replaces preferentially micritic limestone in partially dolomitized samples and is largely nonferroan, except for patches of ferroan dolomite replacing argillaceous limestone. Planar, replacive dolomite occasionally is associated with coarse (>= 1 mm), white to pink, saddle-shaped, void-filling dolomite cement. This association is more common in mineralized areas. The coarse dolomite cement is followed paragenetically by coarse, void-filling calcite cement.

Adjacent to and near dolomite-cement-filled voids, replacive dolomite is characterized by medium to coarse crystalline (0.1 mm to 1 mm), nonplanar texture. Locally, the Waulsortian facies contains fabrics that mining geologists call "white matrix breccia", in which breccia clasts are replaced by fine planar and coarser, nonplanar dolomite, with the matrix composed of very coarse, white saddle dolomite and calcite cement.

Planar replacive dolomite displays ¹⁸O values (‰ PDB) ranging from -6.3 to -3.6 (mean = -5.2, n = 16) and ¹³C values ranging from 3.2 to 4.8 (mean = 3.9, n = 16). Planar replacive dolomite displays slightly lower oxygen and higher carbon isotope values than marine limestones in the Waulsortian facies (¹⁸O = -4.1 to 0.1, mean = -2.2, n = 21; ¹³C = 2.6 to 4.5, mean = 3.7, n = 21). Planar replacive dolomite values are distinct from nonplanar and planar/nonplanar (transitional) replacive dolomites (¹⁸O = -9.2 to 4.3, mean = -6.6, n = 11; ¹³C = 2.3 to 3.9, mean = 3.3, n = 11) and dolomite cements (¹⁸O = -8.1 to -3.8, mean = -6.5, n = 9; ¹³C = 1.8 to 3.4, mean = 2.6, n = 9). Our nonplanar, transitional and cement values fall within the range found for replacive dolomites and coarse white dolomite cements by Hitzman et al. (1998).

The petrography and stable isotope geochemistry of the fine-crystalline, planar, replacive dolomite are consistent with an early diagenetic, near-surface origin, possibly in equilibrium with Carboniferous seawater. Where the Waulsortian was exposed to hydrothermal fluids moving upward along faults and fractures and other available porosity, planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, nonplanar dolomite texture characterized by lower oxygen isotope values more similar to those of Hitzman et al., 1998 for replacive and coarse white dolomite. The hydrothermal fluids also precipitated void-filling dolomite cement that is similar isotopically to the nonplanar, replacive dolomite.

We conclude that there is compelling evidence for at least 2 episodes of replacive dolomitization in the Waulsortian facies (early and later neomorphic) with the possibility that much of the dolomite is early and related to seawater diagenesis.

Hitzman MW, Allan JR & Beaty DW, Geology, 26, 547-550, (1998).

178