IAP-25-107
Fluid transport of heat and metals: understanding the formation of highly permeable fracture systems in granite
The North Pennines massif is underlain and isostatically supported by the Weardale Granite, a component of the North Pennines Batholith (NPB) (Dunham et al 1961). Drilling at Rookhope cored over 400 m of the uppermost granite. This work proved that the intrusion was Late Devonian in age, was exposed at surface pre-Carboniferous and hosts parts of Permo-carboniferous base metal mineralisation of the North Pennines Ore Field (Fig. 1).
Recent interest in the NPB is driven by its potential for geothermal energy and critical mineral resources. Circulating fluids show significant lithium enrichment of lithium and are hosted in highly permeable structures associated with the mineral veins (Manning et al. 2007). Understanding the nature and evolution of these structurally controlled fluid pathways in granite is essential for assessing both the geothermal potential and the origin of mineralising fluids.
This PhD project will investigate the emplacement, deformation history and fluid transport mechanisms of the Weardale Granite using borehole core and analogue intrusion datasets. Emplacement style controls the cooling rate of the magma and the magmatic-hydrothermal fluids released during crystallisation. Structures preserved in the Rookhope cores show a protracted history of ductile shearing, faulting, fault reactivation and mineralisation. While exposed at surface – which happened to the Weardale Granite within the Carboniferous – fissuring can play an important role in creating long-lived, highly permeable structures in crystalline basement terrains (Holdsworth et al 2109; McCaffrey et al 2020). This research will provide an opportunity to refine our structural understanding of the Weardale granite offering new insights into its emplacement mechanism, tectonic evolution and fluid flow history.
A central focus will be on structural inheritance – how early emplacement and tectonic structures control deformation and fluid migration. The study will also explore the effects of fissuring in enhancing permeability and its implications for geothermal energy and mineralisation potential of the NPB.
Click on an image to expand
Image Captions
Figure 1. Contours show the outline of the North Pennines Batholith and loca-tion of Rookhope and Eastgate boreholes. Major faults (yellow) and mineral veins (red) also shown. (Figure from Kimbell et al 2010).
Methodology
The study objectives and methods are to:
1) Collect fracture datasets and samples of the Rookhope core held at Durham and BGS. Make a compilation of well data from Rookhope and Eastgate documenting lithology, size and spatial attributes, fracture type, vein fills, orientation (relative to core axis), alteration type and mineralisation. Investigate fracture and vein mineralisation histories and geochemistry using SEM to help verify interpretations of structural evolution.
2) Analyse the granite mineralogy through the Rookhope core and interpret any changes in geochemistry or mineralogy with depth or fracture properties. Evaluate structural models for emplacement of the granite. Analyse vein-fill geochemistry and geochemical alteration styles in the host granite.
3) Carry out a field studies of analogue granites to gain insights into fracture types, mineralisation, alteration and the 3D geometries of the fracture systems. Possible case studies include the Skiddaw Granite exposed at surface in the Lake District (Caldew and Grainsgill Rivers), the Loch Doon granite exposed in New Galloway Forest and coastal exposures of the Donegal granites (Fanad, Thorr intrusions).
4) Re-evaluate the NPB structures in light of recent Durham work highlighting the role of fissure systems in sub-unconformity basement terrains and their importance for fluid storage and transmissivity.
Project Timeline
Year 1
Collect fracture datasets and samples of the Rookhope core at BGS. Make a compilation of well data from Rookhope and Eastgate documenting lithology, size and spatial attributes, fracture type, vein fills, orientation (relative to core axis), alteration type and mineralisation. Carry out a field study of analogue granites to gain insights into fracture types, mineralisation, alteration and the 3D geometries of the fracture systems.
Year 2
Analyse fracture and vein-fill geochemistry and mineralisation histories to help verify interpretations of structural evolution. Analyse geochemical alteration styles in the host granite and use modelling to help interpret changes in fluid geochemistry through time. Paper 1
Year 3
Analyse the granite mineralogy through the Rookhope core and interpret any changes in geochemistry or mineralogy with depth or fracture properties. Paper 2. Evaluate structural models for emplacement of the granite. Re-evaluate the NPB structures in light of recent Durham work highlighting the role of fissure systems in sub-unconformity basement terrains and their importance for fluid storage and transmissivity.
Year 3.5
Write and submit thesis. Paper 3
Training
& Skills
Research experience highly relevant for traditional and net zero energy industry, including geothermal energy and geothermal lithium
Fieldwork and structural interpretation
Structural modelling skills
Petrographic observation and SEM analysis
Potential for in situ geochemistry work
Fault and fissure evolution and mineralisation
Geochemical modelling
Writing and oral communication skills
References & further reading
Dunham, K. C., Bott, M. H. P., Johnson, G. A. L. & Hodge, B. L. 1961. Granite beneath the Northern Pennines. Nature, London, 190, 899-900.
Holdsworth, R.E., McCaffrey, K.J.W., Dempsey, E., Roberts, N.M.W., Hardman, K., Morton, A., Feely, M., Hunt, J., Conway, A. and Robertson, A., 2019. Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs. Geology, 47(8), pp.700-704.
Kimbell, G.S., Young, B., Millward, D. and Crowley, Q.G., 2010. The North Pennine batholith (Weardale Granite) of northern England: new data on its age and form. Proceedings of the Yorkshire Geological Society, 58(2), pp.107-128
Manning, D. A. C., Younger, P. L., Smith, F. W., Jones, J. M., Dufton, D. J. & Diskin, S. 2007. A deep geothermal exploration well at Eastgate, Weardale, UK: a novel exploration concept for low-enthalpy resources. Journal of the Geological Society, London, 164, 371-382
McCaffrey, K.J.W., Holdsworth, R.E., Pless, J., Franklin, B.S.G., & Hardman, K. 2020. Basement reservoir plumbing: fracture aperture, length and topology analysis of the Lewisian Complex, NW Scotland. Journal of the Geological Society, 177, 1281-1293.
