Controls on the geothermal potential of the buried Kentstown and Glenamaddy plutons, Ireland – implications from hydrothermal alteration Controls on the geothermal potential of the buried Kentstown and Glenamaddy plutons, Ireland – implications from hydrothermal alteration Tobias Fritschle 1*, J. Stephen Daly 1, Martin J. Whitehouse 2, Stephan Buhre 3, Brian McConnell 4 and the IRETHERM Team 1 UCD School of Earth Sciences, University College Dublin, Dublin 4, Ireland 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 ??

Comparison of the buried granites with exposed analogues (contemporaneous granites in the Iapetus Suture Zone) 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] = *ρ [g/cm³] *(0.718*U [ppm] *Th [ppm] *K [wt%] ) (Webb in Manning et al. 2007) Implications for the geothermal exploitation potential of the buried Kentstown and Glenamaddy plutons

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.  Currently five EGS power plants are fully operational.  Enhanced geothermal systems (EGS) / Hot dry rock (HDR):  About a dozen EGS projects are under construction (e.g. Australia, England, Germany, Netherlands, USA,…) (Ove Arup and Partners Ltd. 2011) Granite as a possible target for geothermal exploitation  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 MW e

Buried high heat production (?) granites Gravity anomaly map (Map courtesy of Geological Survey of Ireland and Geological Survey of Northern Ireland) Geological map High heat production (?) granites (Map courtesy of Dublin Institute for Advanced Studies)

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

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

 Drogheda and Soultz granites exhibit thorium enrichment  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 Geochemistry reflecting the geothermal potential  Glenamaddy Rhyolite enriched in Nb compared to granites  Foxdale Granite exhibits uranium enrichment  Unsurprisingly, Rb and Nb generally correlate with each of the heat producing elements

 Sub-parallel trends of decreasing HPR correlate with increasing Th/U  I-type granites are enriched in thorium; S-type granites are enriched in uranium 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)  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 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.

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

Glenamaddy pluton  The overlying sandstone comprises up to 90% angular quartz grains and subordinate microcline feldspar (both up to 100 µm)  An unconformable contact between the overlying Carboniferous sandstones and the rhyolite was intersected at 154 m  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

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  Large parts of the pluton are strongly hydrothermally altered and mineralized  The contact between the granite and the rhyolite is intrusive

Gravity anomaly map (Map courtesy of Dublin Institute for Advanced Studies)  Each of the cores only comprises 15 m of granite Kentstown Granite  Two boreholes intersected the unconformable contacts of the overlying Carboniferous limestone with the granite at 492 m and 662 m, respectively  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

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)

 The Kentstown granite is overlain by Visean limestones in 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) 500μm 1000μm  The thermal conductivity for the Visean limestone cover appears to have been increased due to diagenetic compaction and cementation of the pore spaces. 200m Cover rocks on top of the Kentstown Granite (after Pickard et al. 1992)  Values for the thermal conductivity of both granite samples and cover rocks range between 2.3 – 2.7

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)

Gravity anomaly map (Map courtesy of Dublin Institute for Advanced Studies)  Apart from that, the Midlands and Killarney gravity lows which are likely related to subsurface granites invite for geothermal research  Geothermal potential of the buried Kentstown and Glenamaddy granites requires further investigation Conclusions  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  Deep drilling and further petrophysical research is required for assessing the dimensions of the plutons and for characterising the associated fault systems  Trends of increasing HPR with decreasing Th/U are suggested to relate to the hydrothermal redistribution of uranium.  Possibility for unaltered granite in the eastern part of the Kentstown pluton due to the N-S trending fault system