The geology of the South West region of England is linked more closely to the geology of Europe (the ‘Rhenohercynian zone’ across central northern Europe) than it is to the rest of the UK. The Variscan (Hercynian) orogeny juxtaposed SW England with the rest of England in the Late Carboniferous (~310 million years ago) along the Bristol Channel-Bray Fault, and as such the geology of this region is dominated by processes surrounding this collisional belt.
The Devonian and Carboniferous age sediments and volcanics present in Cornwall today were originally deposited in a series of E/W trending basins related to the Rhenohercynian passive margin. These units were later folded and fractured as a result of the compressional regime associated with the Variscan orogeny which occurred after the passive margin: this deformation is characterised by an ENE/WSW slaty cleavage and the presence of tight isoclinal folds. The units are known locally as ‘killas’. Crustal thickening led to some prograde metamorphic reactions, however any associated mineralisation (NNW/SSE trending) tends to be of a low grade (Shail & Leveridge, 2009).
Post-orogenic extension in the Early Permian led to the intrusion of the Cornubian batholith between 295-270Ma. This granite batholith is over 300km in length, ranging from Dartmoor in the east, to west of the Isles of Scilly; the granites today tend to form the topographic highs in the region (see Figure 1).
Figure 1. Schematic cross-section of the geology of the Southwest region from St Just to Crewkerne (from BGS). Granites are shown in red, Devonian sediments in purple, Carboniferous units in blue and Permo-Triassic in pink
Much of the metalliferous mineralisation in the region occurred a result of the intrusion through the metamorphosed sediments of various granite plutons which are linked at depth to this batholith (Chen et al. 1993). These granites acted as a source of both heat and mineralised fluids which metamorphosed the surrounding area, forming skarns and mineralised veins and lodes. These are often rich in Sn and Cu as well as Au, Ag, As and W and other metals of economic importance. The mineralised ‘lodes’ follow the dominant ENE-oriented structures in the region, which were present in the surrounding sediments before the intrusion of the granites. The granites are also notable for the presence of saline brines which were often encountered in workings as hot springs when the region was extensively mined. The granites have a high concentration of heat-producing radioactive elements, which suggests their potential as a reservoir for geothermal energy.
Following a hiatus of approximately 10 million years, renewed magmatism at 270 Ma produced a prolific swarm of ‘elvan’ dyke intrusions across the region. The dykes predominantly trend ENE/WSW – their orientation is also controlled by the pre-existing structures in the region – and can be up to 40m wide.
Figure 2. Overview geology of SW England. Redrawn from British Geological Survey (BGS) 1:25000 linework (Digimap 2008).
A final period of mineralisation occurred in the Triassic; evidence for this is seen in characteristic NNW/SSE trending ‘crosscourse’ structures which cut the earlier Sn-Cu veins related to the granite intrusions and are often nearly vertical. Tertiary inversion related to the opening of the Atlantic then uplifted the rocks in the region, causing erosion of younger units than those described above (Shail & Leveridge, 2009). A summary map of the geology of the region is seen in Figure 1.
Lithium was first discovered in Cornwall in 1864 by Miller, an academic at Queen’s College London who was intrigued by the hot springs encountered in the Cornish mines. These hot springs occur when geothermal brines circulate through fractures and faults within the granites, and discharge from the crosscourse structures and mineralised lodes due to the natural permeability of structures relative to the surrounding rock.
Miller performed the first geochemical analysis on a brine sample taken from the Hot Lode at United Downs. The brine was notable for its temperature of 1240F at 230 fathoms depth (equivalent to 510C at 420m depth). The hot springs were often 10 degrees higher in temperature than the rock temperature at the depth they were encountered, suggesting that they had travelled from a deeper source. When these geothermal waters were tested, they were found to be ‘extremely rich in lithia’ – of academic interest in 1864, but not commercial interest. Further geochemical analyses of these hot springs at South Crofty in the 1980s confirmed the presence of consistent levels of lithium enrichment in the brines, at concentrations which can now be economically extracted directly from the brines by use of new technologies which have been developed in the past few years.
Figure 3. Wheal Clifford abandoned mine plan – R103 transverse section showing the Hot Lode where ‘the Hot water issuing in great quantities at these depths is rich in Lithia’
Figure 4. Miller, W.A., 1864, Chemical examination of a hot spring containing caesium and lithium in Wheal Clifford, Cornwall: Chem. News, v. 10, p. 181-182; Mining and Smelting Mag., v. 6, p. 197-198
Recent advances in technology mean that it is now possible to economically extract lithium directly from brines, in the concentrations historically present in Cornwall’s hot springs. Various companies have developed their own proprietary extraction technologies; Cornish Lithium are in discussion with potential partners to work out which may be best suited to extract lithium from Cornish brines.