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| Titel |
Geothermal potential of the buried Drogheda and Kentstown granites, Co. Meath, Ireland |
| VerfasserIn |
T. Fritschle, J. S. Daly, B. J. McConnell |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067405
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| Zusammenfassung |
| Granites generally have abundant radioactive heat producing elements such as U, Th and K.
Hence, granites buried beneath thermally insulating rocks may serve as targets for geothermal
exploitation.
The Drogheda and Kentstown granites are situated in the Iapetus Suture Zone about 35 km
north of Dublin in Co. Meath, Ireland. They were drilled in 1997/98 and are overlain
unconformably by several hundred metres of Carboniferous sediments. As buried granites,
they are potential targets for geothermal exploitation and in this context, their major and trace
element geochemistry is being evaluated. The likely thermal insulating role of the
overlying sediments and the structural setting of the Drogheda-Kentstown is also being
investigated.
Based on available geochemical analyses[1], the Drogheda and Kentstown granites have
average heat production rates of 4.5 μW/m3 and 2.5 μW/m3, respectively. These differ
mainly due to variations in thorium contents. Both granites have uranium concentrations up
to 9 ppm, yet the Drogheda granite has a thorium content up to 40 ppm, almost ten
times higher than the Kentstown granite and it also has slightly higher potassium
concentrations[1].
The two granites also appear to be distinct from each other in their major and trace element
geochemistry and in terms of mineralogical features. The Drogheda granite is petrologically
relatively ‘primitive’, exhibiting a high Mg# of ~58, SiO2 concentrations around 64 wt% and
comparatively high overall trace element contents[1]. The Drogheda body is a quartz
monzonite, containing K-feldspar, plagioclase, biotite and amphibole as well as abundant
accessory zircon and apatite. All of the major minerals are generally subhedral and show little
indication of hydrothermal alteration.
In contrast, the Kentstown body (quartz monzonite to granite) is more leucocratic and
exhibits extensive hydrothermal alteration. Plagioclase is generally intensely sericitised;
biotite is almost entirely altered to chlorite ± opaque and oxides ± muscovite ± calcite, yet
perthitic K-feldspar appears to be less altered[2]. It is hoped that further geochemical studies
will reveal the extent to which alteration has affected the composition of the Kentstown
granite (Mg# around 48; SiO2 ~72 wt% [1]) and possibly reduced its heat production
potential.
The sedimentary sequence covering the granites consists predominantly of Carboniferous
limestones (grain- and packstones), interlayered with thin shale and quartz-sandstone and is
considered likely to serve as a thermal insulator. However, the possibility that the cover rocks
(particularly the shales) may also be significant radiogenic heat producers is also being
evaluated.
[1] McConnell & Kennan 2002: Irish Journal of Earth Sciences. Vol. 20, 53-60
[2] O’Reilly et al. 1997: Transactions of the Institution of Mining and Metallurgy. Section B.
Applied Earth Science. Vol. 106, B31-7 |
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