Publication - Research and analysis

Hill of Banchory geothermal energy project: feasibility report

Published: 23 Mar 2016
Directorate:
Energy and Climate Change Directorate
Part of:
Environment and climate change
ISBN:
9781786520753

Report of a study which explored the potential for a deep geothermal heat project at the Hill of Banchory, Aberdeenshire.

Hill of Banchory geothermal energy project: feasibility report
4. Geological Background

4. Geological Background

Introduction

Any study of the feasibility of a potential geothermal energy project needs to address three key questions regarding the geological suitability of the selected site.

1. How much heat is in the target rock mass (and is there enough to sustain a viable energy distribution scheme)? The size of the heat resource at any given point in Earth's crust depends on heat flow at that point, which in turn depends on the amount of heat emanating from deep in the Earth ( background heat) and any contribution made by heat generated in the rock by the radioactive decay of naturally occurring elements ( radiogenic heat). Heat flow can only be measured accurately in a deep borehole, but the amount of radiogenic heat created by any rock (i.e. its Heat Production capacity) can be measured in an outcrop or a hand sample.

2. Is the heat readily extractable? Currently, all tested technologies for accessing deep geothermal energy use water to transport heat from depth to the surface (via one or more boreholes), so this really is a question about how much water can be abstracted sustainably from the rock mass at a given depth. This in turn depends on rock permeability and the amount of water that is accessible within a network of permeable features.

3. Are there geological factors that might complicate or constrain the process of accessing and extracting heat? Such factors might include: geological structures and rock properties that have the potential to cause drilling problems; groundwater that is naturally corrosive or polluting if brought to the surface; and physico-chemical conditions in the network of permeable fractures that might cause new minerals to precipitate, thereby reducing the permeability of the system over time.

The Banchory area was identified as a good target for a geothermal energy feasibility study because of two key factors: (i) the presence at Hill of Banchory of an existing biomass- (and gas-) fuelled district heating scheme that could be adapted to incorporate geothermal energy; and (ii) the relative proximity to the district heating scheme of a geological unit with the potential to provide above-background levels of geothermal energy. This geological unit - the Hill of Fare Granite Pluton - is reported to have elevated concentrations of radiogenic elements at outcrop, raising the possibility that it is, or is close to being, a body of High Heat Production granite (Gillespie et al., 2013). The pluton (which simply means large intrusion) crops out about 5 km to the north of Banchory, so a successful project would require the district heating scheme and a geothermal borehole to be connected by a pipeline.

This section of the report presents a review of the geology of the Hill of Fare pluton and, as far as the available information allows, addresses the questions set out above.

In common with other large granite intrusions in the region, the Hill of Fare pluton is a three-dimensional body of rock that may extend 5, 10 or even 15 km into the subsurface. However, no boreholes have been drilled into the pluton, so all of the available published information about it comes from observations at the ground surface. For this reason this section of the report includes some information from other nearby intrusions of granite that are believed to be related to the Hill of Fare pluton and could be considered analogues of it; shallow boreholes have been drilled into some of these intrusions.

A general introduction to the geology of the Banchory district is also presented to provide geological context.

Geology of the Banchory district

Bedrock

Most of north-east Scotland, including the town of Banchory, is underlain by the Dalradian Supergroup, a thick sequence of metamorphosed sedimentary strata consisting mainly of interlayered beds of sandstone and mudstone, with occasional beds of limestone and other rock types. The original sedimentary rocks were altered by high heat and pressure (metamorphism) between 500 and 400 million years during a major tectonic event, the Caledonian Orogeny, which accompanied a collision of tectonic plates. During metamorphism sandstone became metasandstone (also known as psammite) and mudstone became metamudstone (also known as pelite). Another effect of the Caledonian Orogeny was to cause widespread melting of rocks deep in Earth's crust, with the result that numerous large and small bodies of magma were emplaced into the Dalradian strata where they cooled to form intrusions of crystalline igneous rock. These intrusions collectively are known as the Caledonian Supersuite. Rapid uplift and erosion of the crust during the Caledonian Orogeny brought to the ground surface rocks that previously had been up to 10 km deep. These rocks form the bedrock of north-east Scotland today.

The bedrock throughout the Banchory district is formed entirely of geological units assigned to the Dalradian Supergroup and the Caledonian Supersuite (Figure 8). The town of Banchory is underlain by two units of the Dalradian Supergroup; the Queen's Hill Formation, which consists of interbedded and partly melted psammite and pelite, and the Tarfside Psammite Formation, which consists mainly of psammite. Several large and many small intrusions of the Caledonian Supersuite - including the Hill of Fare Granite Pluton - crop out in the ground around Banchory (Figure 10); part of the Crathes Granodiorite Pluton underlies the eastern-most edge of the town.

The Dee Fault is a large geological fault that extends from the coast at Aberdeen into the ground south of Banchory, and truncates the southern edge of the Crathes Granodiorite Pluton (Figure 8). Other than this structure, very few geological faults are recognised in the Banchory district. Poor exposure means that smaller faults may simply be concealed, but the lack of obvious large offsets in any of the mapped geological boundaries in the district suggests there are few large faults near to Banchory and the Hill of Fare pluton.

Superficial deposits

For the past 2.6 million years Earth has been in an Ice Age, with higher latitudes experiencing alternating periods of cool (glacial) and warm (interglacial) climate. The landscape of north-east Scotland has been modified in two ways by the Ice Age. During periods of ice formation and advance the bedrock surface locally has been scoured and sculpted by moving ice (this effect is most pronounced at higher elevations where ice streams have carved out glacial valleys and localised accumulations of ice on slopes have produced corries) and in places has been 'plastered' in a layer of poorly sorted, clay-rich sediment ( till) that formed beneath ice sheets. In flatter, lowland areas (such as Banchory) the ice caused little focussed erosion but instead produced a widespread general denudation of the land surface. During periods of ice melting and glacier retreat meltwater channels formed at the margins of glaciers and beneath glaciers, scouring out v-shaped channels, and glacial rivers deposited vast quantities of sand and gravel in mounds and fans.

Bedrock exposure around the town of Banchory is generally very poor: nearly all of the bedrock in the local area is concealed by vegetation, soil and glacial deposits, or by alluvium and river terrace deposits on the banks of the River Dee.

Figure 8: Generalised bedrock geology of the ground west of Aberdeen. The dark grey polygon represents the approximate extent of Banchory. Coloured polygons representing generalised bedrock units from the BGS Digital Geological Map of Great Britain 1:625,000 scale model are superimposed on the OS 1:250,000 scale topographic map. Key to colours: green = Dalradian Supergroup; red, purple and orange = Caledonian Supersuite; brown = Old Red Sandstone Supergroup. Dashed black lines are geological faults.

Figure 8: Generalised bedrock geology of the ground west of Aberdeen.
Contains British Geological Survey materials © NERC 2016

Figure 9: Bedrock geology of the Banchory district. The dark grey polygon represents the approximate extent of Banchory. Coloured polygons representing bedrock units are from the BGS Digital Geological Map of Great Britain 1:50,000 scale model.

Figure 9: Bedrock geology of the Banchory district.
Contains British Geological Survey materials © NERC 2016

Hill of Fare Granite Pluton

The Hill of Fare pluton has been mapped in detail by British Geological Survey but has not been the subject of detailed geological research. The information presented in this section is based on a review of the published BGS 1:50,000 scale geological map sheet encompassing the Hill of Fare and surrounding country (Sheet 76E Inverurie; BGS 1992), the unpublished BGS 1:10,000 scale field geological maps of Hill of Fare, and a review of rock and thin section [3] samples held by BGS.

Extent and topography

The Hill of Fare pluton has a roughly pear-shaped outcrop and is relatively small compared to many plutons, measuring c. 8 km on its longest (E-W) dimension, 6.5 km on its shortest (N-S) dimension, and with a surface area of around 37 km 2. The pluton underlies a topographically upstanding massif, the Hill of Fare; the ground overlying the pluton rises gently on all sides from a base at ~ 100-200 m OD to an extensive area of undulating upland with a high point of 471 m OD. The close spatial coincidence between the Hill of Fare massif and the Hill of Fare pluton leaves little doubt that the massif has formed because the underlying granite has in general eroded at a slower rate than the surrounding rocks.

Exposure

The outcrop of the Hill of Fare pluton is very largely concealed beneath heather moorland and blanket forestry. Natural bedrock exposure in general is sparse and limited to scattered patches of ice-scoured pavement and low crag; however, in places loose boulders provide an indication of the character of the underlying bedrock. The best exposures of fresh bedrock are presented in a dozen or so disused quarries (many of which are flooded) on the south flank of the massif, particularly around Corfeidly, Raemoir and Craigton.

Geology

The contact between the Hill of Fare pluton and the surrounding rocks is not exposed, but is inferred on BGS geological maps to be the original, intrusive contact with the exception of one short section on the south-east margin of the pluton where a faulted contact is inferred (Figure 10). The simple, smooth outline of the contact, and the fact that in places it cuts directly across undulating ground, suggests it is everywhere very steep or sub-vertical, though the contact may not remain as steep throughout its depth.

The Hill of Fare pluton is emplaced into, and is in contact with, three older geological units (Figure 10).

  • All of the south-east margin and most of the west margin of the pluton are in contact with the Crathes Granodiorite Pluton
  • The entire north margin is in contact with the Balblair Granodiorite Pluton
  • Most of the south margin is in contact with strongly metamorphosed (and in places partly melted) sedimentary rocks of the Dalradian Supergroup (assigned to the Craigievar Formation and Queen's Hill Formation)

A 'metamorphic aureole' approximately 400 metres wide is recorded in the Dalradian rocks bordering the south margin of the Hill of Fare pluton; the aureole represents the zone in which the mineral and textural character of the Dalradian rocks has been changed as a result of the intense heat the rock was subjected to when the hot magma of the Hill of Fare pluton was emplaced.

The geological character of exposed rock in the Hill of Fare pluton can be summarised as follows -

  • All of the rock is of 'granite' composition (i.e. there is no other type of igneous rock, such as granodiorite or diorite)
  • The main mineral constituents are quartz, alkali feldspar and plagioclase feldspar, which occur in roughly equal proportions and together occupy more than 95% of the rock volume (Figure 11). The remainder is occupied mainly by mica minerals (2-3%; including dark biotite mica and silvery muscovite mica), iron-titanium oxide minerals (~1%), and tiny amounts (<<1%) of the minerals apatite, zircon and monazite; the latter two minerals are likely to be the main repositories of the radiogenic elements uranium and thorium
  • Chemical analysis of rock samples from many parts of the Hill of Fare pluton reveal that between 73% and 79% of the granite is silica (SiO 2); this is towards the high end of the range for granite intrusions in general, and the Hill of Fare granite can be said to be compositionally highly evolved
  • The granite is texturally variable. In the commonest variant (referred to hereafter as 'main granite') the rock is evenly textured and individual crystals typically are 3-5 mm in size. In many parts of the outcrop the granite is slightly or significantly finer-grained than this, and in some places the bodies of finer-grained rock are of significant size: three discrete, km-scale bodies of relatively fine-grained granite ( microgranite) are mapped within the Hill of Fare pluton, one roughly in the centre, one close to the west margin and one abutting the south-east margin (Figure 10). In many respects the rock forming these bodies is similar to the main granite, but the rock commonly consists of scattered, relatively large crystals of feldspar and quartz set in a mass of smaller crystals (this association is commonly referred to as porphyritic microgranite or porphyritic aplogranite). A small body of very fine-grained granitic-rock ( aplite) is mapped close to the west contact of the pluton (Figure 10). Microgranite and aplite have been recorded in many localities on Hill of Fare outwith these mapped bodies, suggesting there is significant textural variability in some (perhaps most) parts of the pluton (reflecting a fairly complex history of magma emplacement and cooling). Granite that is coarser-grained than typical main granite ('very coarse-grained granite') has been recorded in a few places
  • The boundaries between the main granite and the textural variants are not well exposed, but rare observations suggest that both abrupt and gradational boundaries occur. Veins of microgranite cutting the main granite are reported in places, indicating that the finer-grained variants in general crystallized later. The three mapped bodies of microgranite crop out along a curved, roughly east-west-trending line (Figure 10), but there is otherwise no evidence of any geometric regularity (such as a concentric zoning pattern) in the distribution of textural variants at any scale; it therefore is not possible to predict how they might be distributed in the subsurface
  • Fresh samples of the main granite typically are orangeish pink (Figure 11). This colour comes mainly from crystals of alkali feldspar, which suffered mild chemical alteration as the pluton cooled; the alteration produced numerous tiny crystals of secondary minerals, such as clay and iron oxide, within the alkali feldspar crystals, which create the colour
  • No fragments of country rock ( xenoliths) or entrained igneous rock ( enclaves) have been reported, and there is no evidence that the present erosion level is within, or close to, either the roof zone or the base of the pluton
  • The weathering state of exposed granite varies considerably: reasonably fresh granite forms ice-scoured pavements and quarry faces, while completely decomposed granite (essentially sand) underlies slopes and hollows that have not been ice-scoured. Sections of strongly to completely decomposed granite ('gruss') can be up to 2 metres thick
  • No geological faults have been reported within the Hill of Fare pluton, and none appear to offset the contact; however, a faulted contact is inferred along one short section of the south-east margin of the pluton (Figure 10)
  • No substantial zones of hydrothermally altered granite (such as those that occur in the Cairngorm pluton and the Bennachie pluton; see below) have been reported in the Hill of Fare pluton. However, the paucity of exposure means that this should be viewed more as an absence of evidence rather than as definitive evidence of absence
  • There is no topographic evidence (such as steep-sided valleys, prominent cols and deep gullies) that might point to the presence of zones of weak rock. This suggests the pluton does not contain significant zones of chemically altered or strongly fractured rock, at least at the level of present outcrop
  • Mineralized joints (veins) appear to be very rare at outcrop; a vein of quartz 50 mm wide on Hill of Corfeidly is recorded on a BGS field slip
  • There has been no systematic survey of unmineralized joints in the Hill of Fare pluton (or any part of it), but sparse notes on BGS field slips and observations in quarries on the south flank of the Hill of Fare massif (see Chapter 7) suggest the distribution and character of unmineralized joints is essentially normal/typical compared with similar granite plutons in north-east Scotland. Typically, three sets of unmineralized joints are developed at a single locality, each with a distinct orientation. In any one set, joint spacing typically is on the metre-scale, but this varies locally (creating zones of higher and lower joint density)

Figure 10: Bedrock geology of the Hill of Fare Granite Pluton

Figure 10: Bedrock geology of the Hill of Fare Granite Pluton
Contains British Geological Survey materials © NERC 2016

Coloured polygons representing bedrock units from the BGS Digital Geological Map of Great Britain 1:50,000 scale model are superimposed on the OS 1:50,000 scale topographic map (1 km grid squares). The Hill of Fare pluton is the large pink polygon. The three small orange polygons represent discrete bodies of microgranite mapped within the generally coarser-grained main granite. The very small pale polygon close to the west margin of the Hill of Fare pluton is a discrete body of aplite (very fine-grained granitic-rock). Part of the south-east margin of the pluton is inferred to be faulted where the eastern-most body of microgranite terminates abruptly against it. (See Figure 10 for the names of adjacent bedrock units).

Figure 11: Polished surface of a block of Hill of Fare 'main granite'

Figure 11: Polished surface of a block of Hill of Fare 'main granite'
Contains British Geological Survey materials © NERC 2016

The overall orangeish pink colour of this sample is typical of fresh samples of the main granite. Grey crystals are quartz, orangeish pink crystals are alkali feldspar, white crystals are plagioclase feldspar, and black crystals are biotite mica. The sample is from the BGS rock collection (National Building Stone Collection).

Insights from other intrusions in north-east Scotland

The Hill of Fare pluton is generally considered to be a member of the Cairngorm Suite, a discrete 'family' of intrusions within the Caledonian Supersuite that includes a dozen large plutons as well as many smaller intrusions (Figure 12). The individual intrusions of the Cairngorm Suite are inferred to be related genetically because of similarities in their mineral and textural character, chemical composition, age, and topographic expression.

Figure 12: Location of plutons assigned to the Cairngorm Suite

Figure 12: Location of plutons assigned to the Cairngorm Suite
Contains British Geological Survey materials © NERC 2016

Coloured polygons representing generalised bedrock units from the BGS Digital Geological Map of Great Britain 1:625,000 scale model are superimposed on the OS 1:250,000 scale topographic map.

Plutons assigned to the Cairngorm Suite have the following characteristics -

  • Most underlie topographically upstanding massifs; the Cairngorm Mountains and the Lochnagar, Ben Rinnes, Bennachie and Hill of Fare massifs are good examples
  • They consist entirely of granite sensu stricto
  • The granite forming several Cairngorm Suite plutons (Cairngorm, Ballater, Bennachie and Mount Battock) has heat production ( HP) values above 4 μW/m-3 and therefore is considered to be High Heat Production granite (Gillespie et al., 2013)
  • They show considerable textural variability, particularly in grain-size and in the degree to which phenocrysts are developed, and several distinct units of km- to 10-km scale can usually be mapped within individual plutons on the basis of variations in textural character. Typically, the coarser-grained units are the earliest and least compositionally evolved (smaller silica content), and the finer-grained units are the latest and most compositionally evolved (higher silica content). This suggests the magma was emplaced in discrete batches over a period of time (instead of one large, single batch)
  • The mappable units within individual plutons usually are arranged irregularly, displaying no organised geometric pattern; the Lochnagar and Ben Rinnes plutons, both of which are concentrically zoned, are exceptions
  • The granite commonly is coloured in orangeish to pink tones
  • Zones of hydrothermally altered rock are prominent in some intrusions, notably in the Cairngorm and Bennachie intrusions, and to a lesser extent elsewhere

The Hill of Fare pluton is in most respects a typical member of the Cairngorm Suite, though it is relatively small (around one tenth of the area at outcrop of the Cairngorm pluton, and perhaps only one fiftieth of its volume), and apparently lacks prominent zones of hydrothermally altered rock.

Four plutons in the Cairngorm Suite - the Ballater, Bennachie, Cairngorm and Mt Battock plutons - were the focus of detailed investigations during a wide-ranging programme to investigate the geothermal potential of the UK in the late 1970s and early 1980s (details of this programme, and additional references, are in Gillespie et al., 2013). These plutons were selected because of their exceptionally high Heat Production values, and a vertical borehole 300 metres deep was drilled into each one. Unfortunately, the boreholes were not cored continuously: only three short (<7 metre) sections of core were obtained from each borehole, at approximately 100, 200 and 300 metres depth. However, geophysical logs recording a range of rock properties (including rock density, electrical resistivity and natural radioactivity) were obtained over the full length of each borehole. These logs, and the short cored sections, still provide the best information available about the character of Cairngorm Suite plutons in the (relatively shallow) subsurface. The following information, which may be relevant to the present Hill of Fare investigations, comes from a brief review of published details of the geophysical logs and core descriptions (full references are in Gillespie et al., 2013).

  • The rock in each borehole is texturally variable to some degree (i.e. none of the boreholes encountered texturally homogeneous granite throughout their length)
    • In the Cairngorm borehole around one third of the rock (all of it in the bottom third of the borehole) is described as "pale red granite, fresh"; around one quarter is described as "possible aplite/pegmatite sheets" (very little of which is in the fresh granite); and the remainder (around half of the borehole) is described as "undifferentiated granite" because "the imprint of alteration makes it difficult to identify rock type"
    • In the Mount Battock borehole around three quarters of the rock is described as "microgranite" and the remainder as "undifferentiated granite or microgranite"
    • In the Ballater borehole around four fifths of the rock is described as "coarse, pinkish-grey granite, fresh" and the remainder as "granite undifferentiated"
    • In the Bennachie borehole around one third of the rock is described as "pinkish-grey granite, mostly fresh"; around one tenth is a single body described as "aplite/pegmatite"; and the remainder was not described because the "imprint of jointing and alteration obscures variations due to changes in lithology"
  • The rock in each borehole displayed considerable variability in the degree to which it was chemically altered and fractured
    • In the Cairngorm borehole less than half the rock is described as "mainly fresh granite"; more than half is described as "slightly altered and/or jointed granite"; and roughly 10% is described as "mildly to severely haematized and jointed"
    • In the Mount Battock borehole less than half the rock is described as "mainly fresh but strongly jointed" and the remainder is described as "severely jointed and possibly partially altered"; in around half of the borehole these two variants occur in alternating bands of 1-metre- to 10-metre scale
    • In the Ballater borehole around two thirds of the rock is described as "fresh granite, some joints", and most of the remainder as "mildly altered and/or jointed granite"; an interval at least twenty metres long at the bottom of the borehole is described as "severely jointed and/or haematized granite"
    • In the Bennachie borehole around 20% of the rock is described as "mainly fresh rock with some haematization and jointing" and nearly all the remainder is described as "mainly altered, haematized and/or jointed rock". The top 20 to 30 metres of the borehole is described as "badly caved, especially over the most altered and jointed sections"
  • A crude log of the fractures in each cored section revealed the following details; unfortunately the width of joints and veins, and whether or not they contain pore space, is not recorded
    • In the Cairngorm borehole a joint density of approximately 2 joints per metre is recorded in the upper and middle cored sections, and a single haematite-quartz vein is recorded
    • In the Mount Battock borehole a joint density of around three joints per metre is recorded in the upper and lower cores, while the middle core is described as "severely jointed throughout". Several veins of quartz and hematite are recorded in the upper core, and "intense alteration near joints" is recorded in the lower core. Occurrences of calcite-mineralised veins and calcite-cemented breccia are recorded in both the middle and lower cores, and disseminated pyrite is recorded at one location in the middle core
    • In the Ballater borehole a joint density of around one joint per metre is recorded in the upper core, and only one joint is recorded in the 5.5-metre length of the middle core
    • In the Bennachie borehole a joint density of around three joints per metre is recorded in the upper core. No joints are recorded in the 5.5-metre length of the middle core, but several veins and patches of pegmatite and one "thin jasperoid vein" are recorded

Low heat flow values calculated in all the boreholes led the investigators to infer that the Heat Production capacity of the granite in each pluton must diminish rapidly with depth, but the project found no direct evidence to support this. It is now generally accepted that atmospheric warming since the last glaciation has perturbed the shallowest part of the geothermal gradient and is the main cause of suppressed heat flow values (Westaway and Younger 2013, Busby et al. 2015). It therefore remains unclear to what extent the concentrations of radiogenic elements encountered at outcrop in Cairngorm Suite plutons changes with depth.

Summary of key points

  • The Hill of Fare Granite Pluton is a relatively small intrusion of granite that is roughly pear-shaped at outcrop and underlies the Hill of Fare massif, around 5 km north of Banchory. The granite is reported to have moderately elevated concentrations of radiogenic elements at outcrop, raising the possibility that it is, or is close to being, a body of High Heat Production granite and therefore has the potential to provide above-background levels of geothermal energy. The pluton is the main subject of the geological component of this feasibility study because of this geological character and its relative proximity to an established district heating scheme in Banchory.
  • The Hill of Fare pluton is considered to be a member of the Cairngorm Suite, a 'family' of granite intrusions that crop out across north-east Scotland and are inferred to be related geologically because they display similarities in their mineral and textural character, chemical composition, age, and topographic expression. The other Cairngorm Suite intrusions, some of which have been characterised in significantly more detail than the Hill of Fare pluton, can to some extent be considered analogues of the Hill of Fare pluton.
  • The contact between the Hill of Fare pluton and enclosing rocks is not exposed, but the outcrop pattern of the pluton suggests the contact is very steep to subvertical (at least at, and near to, the outcrop level).
  • The depth to which the pluton extends into the subsurface has not been determined but is likely to be at least several kilometres.
  • At outcrop the pluton consists entirely of granite (made up very largely of the minerals quartz, alkali feldspar and plagioclase feldspar).
  • The granite has a silica content of between 73% and 79%, and is therefore compositionally highly evolved. Moderately elevated concentrations of the radiogenic elements potassium, uranium and thorium (more details in Chapter 8) are consistent with this highly evolved character. The concentrations of silica and radiogenic elements are likely to decrease with depth, but currently there is no evidence (in the Hill of Fare pluton or other plutons of the Cairngorm Suite) to indicate how rapidly such changes might occur. There is also no evidence to suggest that igneous rocks other than granite would be encountered within the depth range of interest to a geothermal project (i.e. down to ~5 km).
  • At outcrop the granite is texturally variable at the 10-metre to km-scale. Three discrete, km-scale bodies of microgranite have been mapped within the pluton; these crop out along a curved, roughly east-west-trending line. There is otherwise no evidence of any geometric regularity in the distribution of textural variants at any scale; it therefore is not possible to predict how they might be distributed in the subsurface.
  • Based on the evidence described above it is reasonable to assume that a borehole drilled vertically to a depth of up to several kilometres anywhere in the Hill of Fare pluton would encounter only granite, but the granite is likely to exhibit textural variability on a range of scales and may exhibit a measurable decrease in the concentration of radiogenic elements (and therefore Heat Production capacity) with depth.
  • The success of a geothermal energy scheme in the Hill of Fare pluton (and in any other body of granite) depends on the borehole(s) intersecting one or more fractures at depth that form part of a network of connected, permeable fractures. There are two ways in which a network of connected, permeable fractures can form naturally in a body of granite: (i) as a direct result of rock rupturing in response to Earth movements (e.g. displacement on a geological fault); (ii) as a result of the dissolution of soluble minerals in veins due to a change in groundwater conditions. These two processes can operate together, for example when Earth movements create new cracks in pre-existing calcite veins, allowing groundwater to come into contact with, and perhaps dissolve, the calcite.
  • No geological faults have been recorded within the Hill of Fare pluton, and few have been observed or inferred in the ground surrounding the pluton or in other intrusions of the Cairngorm Suite. It therefore seems unlikely that a borehole through the Hill of Fare pluton would intersect a sizeable fault or fault-zone, though this remains a possibility.
  • No major zones of strongly altered rock have been recorded within the Hill of Fare pluton, and there are no topographic features that might point to the presence of zones of weak rock. This suggests the pluton does not contain significant zones of chemically altered or strongly fractured rock, at least at the level of present outcrop. It therefore seems unlikely that a borehole through the Hill of Fare pluton would intersect a major fracture zone, or a major zone of altered rock, though this remains a possibility.
  • The distribution and character of unmineralized joints on the outcrop of the Hill of Fare pluton is essentially normal / typical when compared with similar granite plutons in north-east Scotland. However, unmineralized joints develop in the near-surface zone of all granite intrusions as they are uplifted in Earth's crust (because the brittle rock expands and cracks as the overlying rock is removed), and they provide little direct evidence of the character of the fracture network at depth.
  • Very few mineralized joints (veins) have been recorded at outcrop in the Hill of Fare pluton; those that have been recorded have fillings of quartz and, locally, hematite (an iron oxide mineral). Calcite, a common fracture-filling mineral, has not been recorded in joints on the outcrop of the Hill of Fare pluton; however, calcite is soluble in near-surface groundwater and may be present in fractures below the near-surface zone.
  • The evidence from 300 metre-deep boreholes drilled into several other Cairngorm Suite plutons suggests: a 'background' joint density of up to 3 joints per metre may be typical in Cairngorm Suite plutons, at least in the shallowest few hundred metres; zones of hydrothermally altered (and locally densely fractured) rock can be encountered to a depth of at least 300 metres; calcite-filled fractures and calcite-cemented breccia are present locally. The latter point raises the possibility that a network of fractures made permeable by calcite dissolution may exist at depth in the Hill of Fare pluton.
  • A test borehole will be needed to provide a more robust understanding of the geology at Hill of Fare and thereby reduce the risk attached to some of the geological uncertainties of a geothermal energy project here. The borehole, ideally drilled to a depth of around five hundred metres (which should be well below the influence of weathering), could be used to: measure heat flow in the borehole and changes in heat production capacity with depth; record changes in rock texture, rock weathering/alteration condition, and thermal conductivity with depth; determine the character and distribution of permeable fractures and other discontinuities; measure the temperature, composition and flow-rate of water in permeable features.
  • Boreholes for a geothermal energy project probably should be drilled at least 2-300 metres from the edge of the pluton due to uncertainty over the position of the contact at depth, and the possibility that there is a 'contact zone' within which the character of the rock mass changes and/or is unpredictable.

Contact