| Title | Geothermal Potential of Caledonian Granites Astride the Iapetus Suture Zone in Ireland and the Isle of Man – Implications for EGS Prospectivity |
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| Authors | Tobias FRITSCHLE, J. Stephen DALY, Martin J. WHITEHOUSE, Stephan BUHRE, Brian MCCONNELL and The IRETHERM Team |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Buried granites, heat production rate, heat-producing elements, radioactivity, Hot Dry Rock (HDR), Th/U ratio, hydrothermal alteration |
| Abstract | Ordovician to Devonian (Caledonian) granites are common in the Iapetus Suture Zone in Ireland and Britain. Some of these are situated beneath Upper Palaeozoic sedimentary basins at various tectonic levels. The buried Kentstown and Glenamaddy granites in Ireland are targets for Enhanced Geothermal System (EGS) / Hot Dry Rock (HDR) exploration. Several other subsurface granites likely exist based on geophysical and petrological considerations. In order to test regional geothermal potential, drill-core samples of the buried granites and samples from equivalent exposed intrusions are being investigated geochemically. The whole-rock geochemistry of the granites varies significantly, but with no obvious geographical control. Average heat production rates range from 1.4 μW/m³ for the Leinster Granite to 4.9 μW/m³ for the Drogheda Granite. The heat-producing elements uranium (U), thorium (Th) and potassium (K) and calculated heat production rates generally correlate with the abundances of niobium and rubidium. Given the enrichment of these two trace elements in the upper continental crust, contributions of the latter to granite genesis may increase the abundance of the heat-producing elements in a granite. In spite of a positive correlation of each of the heat-producing elements with heat production it is demonstrated that elevated heat production rates in a pluton can be related to enrichment in one of the heat-producing elements alone, e.g., uranium-enrichment in the Foxdale Granite or thorium-enrichment in the Drogheda Granite. This is mirrored in the Th/U ratio. Furthermore, the Th/U ratio correlates with heat production, exhibiting a trend towards a high heat production rate with a low Th/U ratio. In this paper, hydrothermal alteration is suggested to cause this correlation between the Th/U ratio and the heat production rate. Hydrothermal alteration is further suggested to be a major mechanism for redistributing mobile elements such as uranium, and therefore as a mechanism controlling the granite’s heat production. In view of that, the distribution of the heat-producing elements in whole-rock samples, and calcite and quartz veinlets are being investigated. |