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

Abstract

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ18O and δ13C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ18O=–2·2, δ13C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct 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 involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ18O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.

Formation of saddle dolomites in Upper Cambrian carbonates, western Tarim Basin (northwest China): Implications for fault-related fluid flow

“…First, for the matrix dolomites, 87 Sr/ 86 Sr ratio decreases from the RD1 (n ¼ 4, mean ¼ 0.70986, s ¼ 0.000404) (0.709307-0.710297) to RD2 (n ¼ 2, mean ¼ 0.709235, s ¼ 0.000066) (0.709188-0.709281) dolomites ( (Burke et al, 1982;Nicholas, 1996;Montañ ez et al, 2000;Sial et al, 2008). In contrast, the values of Well Tong 1 are greater, suggesting that these dolomites may be affected by burial diagenesis or secondary alteration, because the 87 Sr/ 86 Sr ratio can increase with time and burial depth (e.g., Mountjoy et al, 1992;Qing, 1998;Gregg et al, 2001;Duggan et al, 2001;Chen et al, 2004). The 87 Sr/ 86 Sr ratios of the RD2 dolomites are lower than those of the RD1 dolomites, indicating recrystallization of the RD2 dolomites by fluids with lower 87 Sr/ 86 Sr ratios (Durocher and Al-Aasm, 1997;Machel, 1997) in equilibrium with the underlying Middle Cambrian evaporites because Middle Cambrian water has low 87 Sr/ 86 Sr ratios (Burke et al, 1982;Nicholas, 1996;Montañ ez et al, 2000;Sial et al, 2008) (Fig.…”

“…In general, the 87 Sr/ 86 Sr ratios and d 13 C values of the RD1 dolomites indicate that the dolomitization fluid is Late Cambrian seawater. Their depleted d 18 O values and slightly high 87 Sr/ 86 Sr ratios (greater than that of the Late Cambrian seawater) (e.g., in Well Tong 1) may be result of high temperatures due to deep burial setting (e.g., Mountjoy et al, 1992;Qing, 1998;Gregg et al, 2001;Duggan et al, 2001;Chen et al, 2004).…”

Polyphase dolomitization of the Wuchiapingian Zechstein Limestone (Ca1) isolated reefs (Wolsztyn Palaeo-Ridge, Fore-Sudetic Monocline, SW Poland)

“…The iso to pic com po si tion of dolomites may ac tu ally re flect a gross rep re sen ta tion of the chem is try of recrystallising flu ids, rather than the com po si tion of the orig i nal flu ids (e.g., Land, 1985;Hardie, 1987), and there fore diagenetic sta bi li za tion af ter mul ti ple pe ri ods of dolomitisation pro duces com plex do lo mite tex tures and chem i cal com po si tions (Gao and Land, 1991;Gregg et al, 2001;El-Tabakh et al, 2004).…”

The impact of eustasy, tectonics, and paleoclimate on the dolomitization of the syn-rift Neogene carbonate platforms in the Red Sea coastal area, Egypt

“…Clegg et al (1998) mentioned that the very low δ 18 O value is characteristic of dolostones formed under high temperatures. Ferroan spheroidal and saddle dolomites (E5) are assumed to have been formed later from hydrothermal waters that moved to higher stratigraphic levels, similar to void-filling saddle dolomite cement of the southern Irish Midlands described by Gregg et al (2001). This is corroborated by the opinion of Warren (2000) and Davies and Smith (2007), who mentioned that dolomites of hydrothermal fluids tend to be ferroan and consist of saddle-shaped crystals, and occur as replacive and voidfilling fabrics.…”