| Title | Geothermal fluids of the Carnmenellis Granite, Cornwall, UK |
|---|---|
| Authors | Chris Rochelle, Tom Olver, Chris Yeomans, Ali Salisbury, Keith Bateman, Alexander Hudson, Alicja |
| Year | 2026 |
| Conference | World Geothermal Congress |
| Keywords | |
| Abstract | Geothermal development in Cornwall has seen a major step forward recently, with both the UK’s first powerplant, and significant exploration for dissolved mineral resources – both targeting deep geothermal waters held within fractures in c. 290 Ma old thermogenic granite. Recent industry projects and scientific studies, coupled with information from the 1980s UK Hot Dry Rock geothermal project, provide much data on fluid chemistry within the upper 5 km of the northern boundary of the Carnmenellis granite. Geothermal drilling at United Downs, between Redruth and Falmouth, accesses water from a 4-5 km deep section of the Porthtowan Fracture Zone, one many similar NNW-SSE trending fracture zones in SW England (known locally as ‘crosscourses’). This confirms a NaCl-dominated water at about 185°C and with a TDS of c. 35 g/L. As well as heat, this water is enriched in elements such as lithium (c. 280-300 mg/L). Fractures allow thermally-driven advection of the deep geothermal waters to shallower levels, creating mixing zones with local surface groundwaters and intermediate water chemistries. These carry a deep geothermal signature, and can still have commercially attractive Li concentrations (c. 100 mg/L) at 1-2 km depth. We present a summary of current understanding of the geochemistry of these thermal waters, and how they interact with the host rocks. |