Controls on the geothermal potential

1. Controls on the geothermal potential of the buried Kentstown and Glenamaddy plutons, Ireland – implications from hydrothermal alteration Tobias Fritschle1*, J. Stephen Daly1, Martin J. Whitehouse2, Stephan Buhre3, Brian McConnell4 and the IRETHERM Team 1 UCD School of Earth Sciences, University College Dublin, Dublin 4, Ireland *(Tobi.Fritschle@gmx.de) 2 Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Sweden 3 Institut für Geowissenschaften, Johannes Gutenberg-Universität, Mainz, Germany 4 Geological Survey of Ireland, Beggars Bush, Dublin 4, Ireland ??

2. Geochemical evaluation of the available specimens in • terms of their heat production (uranium, thorium, potassium and density) Aims of the study / Outline of the talk • HPR [µW/m³]= 0.1326*ρ[g/cm³]*(0.718*U[ppm]+0.193*Th[ppm]+0.262*K[wt%]) • (Webb in Manning et al. 2007) • Comparison of the buried granites with exposed analogues (contemporaneous granites in the Iapetus Suture Zone) Implications for the geothermal exploitation potential of the buried Kentstown and Glenamaddy plutons

3. High heat producing rocks are ‘stimulated’ at depth, using borehole hydro-fracturing techniques, to create a permeable reservoir in which water can circulate. Cool water is injected down the injection well, and hot water, having heated up during passage through the artificial reservoir, is pumped back up at the production well. • Enhanced geothermal systems (EGS) / • Hot dry rock (HDR): Granite as a possible target for geothermal exploitation • Currently five EGS power plants are fully operational. • About a dozen EGS projects are under construction (e.g. Australia, England, Germany, Netherlands, USA,…) • Soultz-sous-Forêts in France was the first to be commissioned in July 2011, reaching a depth of 5,000m, and yielding a water temperature of 200 °C and power of 1.5 MWe (Ove Arup and Partners Ltd. 2011)

4. Buried high heat production (?) granites High heat production (?) granites Gravity anomaly map Geological map I-Type S-Type Glenamaddy Kentstown (Map courtesy of Geological Survey of Ireland and Geological Survey of Northern Ireland) (Map courtesy of Dublin Institute for Advanced Studies)

5. Timing of Irish & IoM Late Cal. granites Modified after Brown (2008) and Holdsworth (2009) • Late Caledonian granites likely intruded in multiple phases between 425 – 405 Ma • Very few drill cores accessible therefore exposed analogues rock are used as proxies • Majority of the Late Caledonian granites have their latest intrusive phase around 410 Ma • No correlations of the heat production with age, geographical distribution or isotopic signature?

6. Heat production rates (Recalculated after: 1Genter et al. 1997, Alexandrov et al. 2001, Stussi et al. 2002 and Greksch et al. 2003 ; 2Manning et al. 2007)

7. Geochemistry reflecting the geothermal potential • Rb and Nb correlate with the heat production rate • Both Rb and Nb are enriched in the upper continental crust. Naturally, contribution of such material may enrich the abundance of heat producing elements • I-type and S-types exhibit distinct slopes in Rb v HPR for Rb >150ppm • Glenamaddy Rhyolite enriched in Nb compared to granites • Unsurprisingly, Rb and Nb generally correlate with each of the heat producing elements • Drogheda and Soultz granites exhibit thorium enrichment • Foxdale Granite exhibits uranium enrichment

8. Geochemistry reflecting the geothermal potential • Th/U ratio suggests different underlying causes for the elevated heat production rates in the Drogheda and Foxdale granites (red and blue stars) • I-type granites are enriched in thorium; S-type granites are enriched in uranium • S-type granites have a lower Th/U than the crustal average (Th/U = 3.8) – except for very altered samples • The most altered samples correspond with the highest Th/U ratios • Sub-parallel trends of decreasing HPR correlate with increasing Th/U We suggest this trend is produced by hydrothermal redistribution of uranium and that the latter may be a major mechanism controlling the heat production in a granite.

9. Hydrothermal alteration in Glenamaddy Granite Calcite and quartz veinlets in the Glenamaddy Granite showing U-enrichment in a calcite vein, and uranium precipitation around pyrite interpreted as the result of a redox reaction of the sulphide with a U-bearing fluid

10. Glenamaddy pluton • Thermal conductivity values for the sandstone ranges between 1.7 – 3.1 W/mK, whereas those of the granite and rhyolite are around 2.5 W/mK • An unconformable contact between the overlying Carboniferous sandstones and the rhyolite was intersected at 154 m • The overlying sandstone comprises up to 90% angular quartz grains and subordinate microcline feldspar (both up to 100 µm)

11. Glenamaddy pluton • A single drill-core comprises 160 m of alternating rhyolitic and granitic rock • Four sections of each granitic and rhyolitic rock were drilled • The contact between the granite and the rhyolite is intrusive • Large parts of the pluton are strongly hydrothermally altered and mineralized

12. Kentstown Granite • Two boreholes intersected the unconformable contacts of the overlying Carboniferous limestone with the granite at 492 m and 662 m, respectively Gravity anomaly map I-Type S-Type • Each of the cores only comprises 15 m of granite • Granite is strongly hydrothermally altered, depleted in U, presumably linked to the Carboniferous-hosted Zn-Pb orefield • Stratigraphic differences between the west and east parts of the pluton Kentstown (Map courtesy of Dublin Institute for Advanced Studies)

13. Drilling of Kentstown granite • GSI drilled Kentstown Granite based on Tom Farrell’s preliminary MT modelling, that predicted granite at 370 ± 30 m • Drilling had to be abandoned in soft Namurian shales at a depth of c. 300m (much thicker than expected)

14. Cover rocks on top of the Kentstown Granite • The Kentstown granite is overlain by Visean limestonesin the west, and by thick Namurian shales (presumably underlain by limestone) in the east • Unfortunately, Irish Carboniferous rocks are generally poor thermal insulators due to diagenesis (occlusion of pore-space) 200m 1000μm • Values for the thermal conductivity of both granite samples and cover rocks range between 2.3 – 2.7 • The thermal conductivity for the Visean limestone cover appears to have been increased due to diagenetic compaction and cementation of the pore spaces. 500μm (after Pickard et al. 1992)

15. Drilling of Kentstown granite • GSI drilled Kentstown Granite based on Tom Farrell’s preliminary MT modelling, that predicted granite at 370 ± 30 m • Drilling had to be abandoned in soft Namurian shales at a depth of c. 300m (much thicker than expected)

16. Conclusions • Trends of increasing HPR with decreasing Th/U are suggested to relate to the hydrothermal redistribution of uranium. Gravity anomaly map • Geothermal potential of the buried Kentstown and Glenamaddy granites requires further investigation Glenamaddy • HPR for Glenamaddy is relatively high (3.3 µW/m³) and Kentstown (2.3 µW/m³) only moderate, but only 15 m of altered granite drilled at Kentstown Kentstown • Possibility for unaltered granite in the eastern part of the Kentstown pluton due to the N-S trending fault system Midlands Killarney • Deep drilling and further petrophysical research is required for assessing the dimensions of the plutons and for characterising the associated fault systems • Apart from that, the Midlands and Killarney gravity lows which are likely related to subsurface granites invite for geothermal research (Map courtesy of Dublin Institute for Advanced Studies) Thank you for your attention!!