|
1 Schlumberger Data and Consulting Services, 1325 S. Dairy Ashford Road, Houston, TX 77077, USA znagy{at}houston.oilfield.slb.com
2 Department of Geology & Geophysics, University of Missouri-Rolla, Rolla, MO 65401, USA
3 Department of Geological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA
4 Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland
Shallow-marine, Lower Carboniferous carbonate sequences of the SE Irish Midlands, close to the Leinster Massif, are intensely dolomitized. Fine-crystalline (<50 µm), planar-s (subhedral) dolomite is associated with evidence for evaporites, typical of arid peritidal sequences. However, stable isotope data suggest a diagenetic overprint. Volumetrically more important medium-crystalline (50–200 µm), planar-s and minor planar-e (euhedral) dolomites were precipitated from slightly modified Lower Carboniferous seawater. These dolomites replace open-marine intraclastic and bioclastic packstones and grainstones.
Length-slow, fibrous quartz partially replaces crinoids and fills dissolution cavities beneath peritidal strata. Associated dolomites are gradually enriched in 18O downward through the underlying strata, suggesting vertical brine migration. The widespread occurrence of skeletal material replaced by chalcedony in open-marine wackestones and grainstones further to the west, within the Rathdowney Trend, suggests evaporite cementation in the Zn-Pb mineralized area.
Base-metal mineralization in the fractured Waulsortian ‘reservoir’ is associated with chloride-enriched brines (beyond that expected from seawater evaporation alone). The presence of evaporites in the Leinster Massif area suggests a possible source of the excess chloride. The dolomitizing brine may have contributed to the overall chemistry of the Zn-Pb mineralizing fluid and also to the distribution of porosity within the carbonate platform. An Arundian or younger age is suggested for the mineralization, based on the timing of evaporite cement emplacement, and this is compatible with numerical fluid-flow models of brine movement through the carbonate platform.
Dolomitization of Lower Carboniferous carbonate rocks of the Irish Midlands is comparable to diagenetic histories of several important dolomite petroleum reservoirs. This study provides an example that may be applied to petroleum exploration in similar geological settings.
