A Deep Geothermal Exploration Well at Eastgate, Weardale, UK, A Novel Exploration Concept for Low...

Journal of the Geological Society, London, Vol. 164, 2007, pp. 371–382. Printed in Great Britain.

371

A deep geothermal exploration well at Eastgate, Weardale, UK: a novel exploration

concept for low-enthalpy resources

D. A. C. MANNING 1,2, P. L. YOUNGER 2, F. W. SMITH 3, J. M. JONES 4, D. J. DUFTON 5 &

S. DISKIN 6

1School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, UK

(e-mail: [email protected])2Institute for Research on the Environment and Sustainability, Newcastle University,

Newcastle upon Tyne NE1 7RU, UK3FWS Consultants Ltd, Merrington House, PO Box 11, Merrington Lane Trading Estate, Spennymoor DL16 7UU, UK

4March House, Horsley NE15 0HZ, UK5PB Power, William Armstrong Drive, Newcastle upon Tyne NE4 7YQ, UK

6Foraco–Boniface SA, ZI des Fournels, BP 173, 34401 Lunel, France

Abstract: The first deep geothermal exploration borehole (995 m) to be drilled in the UK for over 20 years

was completed at Eastgate (Weardale, Co. Durham) in December 2004. It penetrated 4 m of sandy till

(Quaternary), 267.5 m of Lower Carboniferous strata (including the Whin Sill), and 723.5 m of the Weardale

Granite (Devonian), with vein mineralization occurring to 913 m. Unlike previous geothermal investigations

of UK radiothermal granites that focused on the hot dry rock concept, the Eastgate Borehole was designed to

intercept deep fracture-hosted brines associated with the major, geologically ancient, hydrothermal vein

systems. Abundant brine (<46 8C) was encountered within natural fracture networks of very high permeability

(transmissivity c. 2000 darcy m) within granite. Evidence for the thermal history of the Carboniferous rocks

from phytoclast reflectance measurements shows very high values (>3.3%) indicating maximum temperatures

of 130 8C prior to intrusion of the Whin Sill. Geochemical analysis of cuttings samples from the Eastgate

Borehole suggests radiothermal heat production rates for unaltered Weardale Granite averaging 4.1 �W m�3,

with a mean geothermal gradient of 38 8C km�1. The Eastgate Borehole has significant exploitation potential

for direct heat uses; it demonstrates the potential for seeking hydrothermal vein systems within radiothermal

granites as targets for geothermal resources.

Geothermal energy is used globally in applications ranging from

electricity generation (using steam to drive turbines) to space

heating using ground source heat pumps (GSHP), which extract

energy from shallow groundwater and/or soil atmospheres. In

Europe, geothermal energy use depends on the balance of

availability of differing energy sources. Thus Iceland, with read-

ily available volcanogenic geothermal resources, is one of the

world’s major users for space heating. The UK has been very

slow to develop its more modest geothermal resources, with only

one significant scheme (<1.4 MW; a single geothermal well

contributing to Southampton’s district heating scheme), and a

growing number of GSHP applications for individual buildings

(mainly new-build domestic premises). The slow uptake in the

UK partly reflects the ready availability of indigenous oil and

gas over the last three decades. Norway and Sweden show the

contrast in geothermal uptake between countries with and with-

out hydrocarbon resources: Norway makes almost no use of

geothermal energy, whereas Sweden is one of Europe’s greatest

users, entirely in the form of GSHP (Sanner et al. 2003).

As energy costs rise, especially for fossil fuels, interest in

alternative sources is increasing. Globally, nuclear energy and

coal (with varying degrees of ‘cleanliness’ in burning) will be

used increasingly for electricity generation during the 21st

century. Interest in renewable sources of energy, such as wind

power, is also increasing. But geothermal energy can play a

much greater role within a portfolio of energy supply options

than is currently considered. There are potentially many locations

where geothermal energy is insufficient to generate electricity,

but where it can contribute to space heating and other low-grade

uses currently met by distributed fossil fuel combustion. Dis-

placement of the domestic gas boiler from tens of millions of

homes would contribute more to the UK’s greenhouse gas

emission reduction obligations than almost any other techno-

logical change (cf. Caneta Research 1999).

In Weardale, County Durham, UK, the recent decommission-

ing of the Lafarge cement works has led to a major redevelop-

ment opportunity at a rural site. The Weardale Task Force

anticipates a mixed-use redevelopment, making use of indigen-

ous renewable energy sources, including wind and hydroelectric

power generation, and biomass combined heat and power.

Geothermal energy is being considered for space heating and for

use in a spa tourist attraction.

The concepts underlying the geothermal exploration activity

undertaken at the Eastgate site are novel, and represent a

significant development from those previously applied to the

inferred geothermal resources of the UK. Initiated in 1976 at

the time of the Middle East oil crisis, geothermal exploration in

the UK focused until its end in 1990 on the potential for

generating electricity. The cost-effective generation of electricity

from geothermal resources (DiPippo 2005) generally requires

that waters yielded by production wells have temperatures con-

siderably in excess of 100 8C, so that on exposure to atmospheric

(or near-atmospheric) pressure, they will ‘flash’, i.e. liberate

steam at rates high enough to drive conventional turbines.

Additionally, binary cycles (including the Kalina cycle) are used

for power generation in which geothermal waters at ,140 8C

vaporize other ‘working fluids’ to produce gas streams sufficient

to turn turbines (DiPippo 2005). Outside active volcanic areas,

there are few parts of the world in which economically accessible

geothermal waters are hot enough for flash or binary cycle

electricity generation. However, there are many direct uses that

can exploit the thermal energy in sub-100 8C geothermal waters

(e.g. space or district heating; greenhouse heating; aquaculture;

health spa tourism developments, etc.; Lund et al. 1998; Dickson

& Fanelli 2005). These options were not a priority during the

1976–1990 investigations in the UK. Rather, two principal

options for deriving supra-100 8C waters from the UK continen-

tal crust were examined (Barker et al. 2000).

(1) Hydrothermal aquifers: corresponding principally to Meso-

zoic basins, in which permeable formations are likely to be

present at depths that will ensure that they contain hot water.

(2) Hot dry rock (HDR): in which the heat generated by

radiothermal granites was to be exploited by drilling deep bore-

holes, between which fractures would be developed artificially (e.g.

hydraulic fracturing; explosives). Cool water would be pumped

down into the fractured granite, left to equilibrate thermally, and

then pumped out again at much higher temperatures.

In many ways, both of these exploration concepts were decades

ahead of their time. Although the hydrothermal aquifer investiga-

tions did not result in the identification of flash or binary grade

energy, they did identify resources suitable for direct-use applica-

tions, as used to the present day by the Southampton Geothermal

Heating Company. Many other similar resources await exploita-

tion. Although the HDR experiments undertaken in the Carnme-

nellis Granite (Cornwall), using boreholes sunk at Rosemanowes

Quarry to depths of 1700–2600 m, did not result in an operational

geothermal exploitation scheme (Richards et al. 1994), they did

yield a dataset used widely to test numerical simulation codes

(Kolditz & Clauser 1998). They provided much of the conceptual

basis upon which the European Union pilot project at Soultz-sous-

Forets (Rhine Graben) has successfully built (e.g. Bachler & Kohl

2005), using artificial fracturing to enhance natural structures in

granite to obtain an exploitable resource, which is now generating

electricity using a binary power plant.

Given their focus on resources suitable for electricity genera-

tion, the 1976–1990 investigations did not fully consider the

direct use for space heating of deep groundwater or GHSP

resources. They were also too early to investigate the possibilities

of man-made hydrothermal aquifers now associated with satu-

rated mine workings at great depth (.1000 m), which have

become flooded only since 1990 (see Younger & Adams 1999;

Banks et al. 2004).

In this paper, we present a further exploration possibility: the

concept that ancient hydrothermal vein structures associated with

the radiothermal granites of the UK still function as geothermal

plumbing systems, and may be economically viable. In testing this

concept, we have sunk the first deep geothermal exploration

borehole to be drilled in the UK for 20 years, and only the second

borehole ever to penetrate the Weardale Granite, a key component

of the classic ‘block-and-basin’ geology of the Carboniferous of

northern England (e.g. Fraser & Gawthorpe 2003).

Geological background

Weardale lies within the UK’s first Geopark, designated in

recognition of the importance of the North Pennines in shaping

views on the origin of Mississippi-Valley Type mineral deposits.

The North Pennines Orefield is famous (Dunham 1990) for its

zoned fluorite–sphalerite–galena–barite mineralization, which is

principally developed in Lower Carboniferous limestones. The

zoning of the orefield stimulated geophysical investigations that

demonstrated a gravity ‘low’ coinciding with the fluorite zone of

the mineralization (Bott & Masson-Smith 1953, 1957), which

was interpreted as being due to the presence of a buried granite.

Drilling at Rookhope in 1960 (Dunham et al. 1965) proved the

existence of the Weardale Granite, which unexpectedly proved to

be Early Devonian in age, hence older than the mineralized host

rocks, a finding that required a complete revision of ore deposit

models for the North Pennines (Dunham 1990).

Interest in the Weardale Granite as a ‘radiothermal’ granite is

based on its comparatively high concentrations of uranium,

thorium and potassium. It is one of a family of similar granites

in the UK with high heat production rates (Webb et al. 1985,

1987; Downing & Gray 1986), including the Carnmenellis

Granite (the former HDR prospect in Cornwall; Richards et al.

1994).

In the late 1980s, investigations were carried out on a tepid

saline water found at depth in Cambokeels Mine at Eastgate

(Manning & Strutt 1990), issuing from the eastern forehead of

the fluorite-bearing Slitt Vein where it cuts Dinantian limestones

and clastic sediments. Manning & Strutt (1990) compared the

Cambokeels mine water with saline waters, originating from

fractures in the Carnmenellis Granite, reported by Edmunds et

al. (1984). They suggested that the Eastgate mine water was

derived from deep within the Weardale Granite, and that it had

precipitated silica minerals during its ascent through the Slitt

Vein fracture system. Use of geochemical thermometers sug-

gested that the water had equilibrated with a granitic host at

temperatures up to 150 8C.

There is further evidence of a high geothermal gradient in the

Eastgate–Rookhope area. Downing & Gray (1986) reported a

temperature gradient of 30 8C km�1 for the Rookhope Borehole,

and Younger (2000) reported a gradient of over 60 8C km�1 from

the Frazer’s Grove Mine (Greencleugh Vein, similar in structure

and filling to the Slitt Vein), several kilometres west of the

Rookhope Borehole and directly above a mineralization spread-

ing centre (Dunham 1990).

On the basis of the geological information summarized above,

it was decided that the geothermal exploration well at Eastgate

should commence within the Slitt Vein as a suspected pathway

for deep water upflow, and attempt to follow the vein and

associated splays vertically downwards for up to 1 km (Fig. 1),

the total depth being limited by the available budget. A starting

position above the sub-crop of the Slitt Vein against the base of

the Quaternary deposits was identified by trial-pitting and drilling

five inclined boreholes (at 45–608) to depths of up to 60 m. The

final borehole site was precisely surveyed by global positioning

system (GPS) with reference to the National Grid, and lies at

393890.932 E 538200.147 N, commencing at a surface elevation

of 250.867 m above Ordnance Datum (AOD). In the interests of

speed and economy, the deep borehole was drilled ‘open-hole’

(by Foraco, Lunel, France), recovering cuttings at 1 m intervals

(within sedimentary rocks and granite) down to 615 m, and then

at 5 m intervals (granite). From surface to 93 m the well was

cased and grouted to 1338

inches, from 93 m to 403 m cased and

grouted to 958

inches (with this casing being continued to

surface). From 403 m to full depth at 995 m, the borehole was

completed without casing, at a drilled diameter of 812

inches.

Cuttings taken from the well were washed and their lithologies

logged on site. A complete suite of cutting samples was

D. A. C. MANNING ET AL.372

deposited with the British Geological Survey. Selected samples

of cuttings from the sedimentary sequence were taken for

vitrinite reflectance determination, and samples of cuttings from

the granite were taken at intervals of c. 50 m for analysis using

X-ray fluorescence (XRF; fused beads for major elements;

powder pellets for trace elements) at the University of Leicester,

Department of Geology. Water samples were taken for analysis

every 10–20 m down to 300 m, and then at 50 m intervals.

Specific electrical conductance (‘conductivity’) and temperature

of the water arising from the borehole were monitored continu-

ously at the cuttings separator at the wellhead.

Summary of findings: geology

The Eastgate Borehole penetrated 271.5 m of recent and Lower

Carboniferous cover rocks then nearly 723 m of basement

granite. Overall, the sequence closely resembled that penetrated

by the Rookhope Borehole (Dunham et al. 1965; Fig. 2).

Superficial deposits of Recent and Quaternary age were encoun-

tered down to rockhead at about 4 m (all depths in this section

are below drilling table, which was at 252.75 m AOD), and

consisted mostly of sand, gravel and boulders. The borehole wasFig. 1. Schematic cross-section to illustrate the design of the Eastgate

Borehole.

Fig. 2. Summary lithological log, comparing Rookhope and Eastgate boreholes. Lst, limestone; qtz, quartz; fluor, fluorite; gal, galena; py, pyrite; hm,

hematite.

GEOTHERMAL EXPLORATION WELL, WEARDALE, UK 373

open-holed to 10 m without casing, so that there was consider-

able cross-contamination of rock cuttings with caved drift

material over that interval.

The Scar Limestone was present from rockhead to about 12 m

below drilling table, and was karstified (the shallow inclined

boreholes penetrated caves that were entirely filled with damp

mud). This was underlain by the ‘Alternating Beds’, a sequence

of thinly bedded limestones, mudstones and sandstones. The

Tynebottom Limestone was recognized between 39.5 and 51.5 m.

As with the other horizons below the Scar Limestone, this

formation was heavily mineralized (by a combination of quartz

replacements and veinlets); it was also the source of a significant

increase in groundwater influx to the borehole. The Jew Lime-

stone was encountered between 76 and 83 m, again mineralized.

Immediately beneath it (83–87 m) was a vein of coarse green

fluorite, interpreted as a branch of the Slitt Vein. The Great Whin

Sill, extensively carbonated and altered to ‘white whin’, was

intercepted at 92 m, and continued to 158.5 m.

Rocks beneath the Great Whin Sill were heavily silicified and

veined by quartz down to about 175.5 m. Beneath this the

sequence was essentially devoid of mineralization for almost

50 m, and the Smiddy, Upper and Lower Peghorn, and Birkdale

Limestones were easily recognized. The Robinson Limestone

(223.5–234.5 m) was fractured and heavily replaced by quartz

and calcite. Similar mineralization continued into the Melmerby

Scar Limestone (239.5–249.5 m), below which the Orton Group

and Basement Beds (sandstones with occasional thin limestones)

occurred between 249.5 and 272.5 m, cut by veinlets of miner-

alization.

The granite surface at 272.5 m below table (271.5 m below

surface) was marked by the occurrence of cuttings of a sticky

white clay, presumably kaolinitic, over an interval of 0.5 m. The

first few metres of granite were relatively soft, then became

harder and more coherent. The granite resembles the cored

material from the Rookhope Borehole, and is fairly uniformly

coarse grained (2–6 mm), consisting of feldspars, quartz, musco-

vite and biotite. It has a greenish hue, probably the result of

hydrothermal alteration of the feldspars. Mineralization was

found throughout the granite, usually as quartz veinlets with

sporadic occurrences of pyrite, chalcopyrite or galena.

Cuttings from 415 to 615 m were uniform, with sparse

indications of mineralization. Below 615 m cuttings were taken

at intervals of 5 m. Fluorite mineralization was intersected

between 620 and 650 m (recognized in waste cuttings, rather

than in the samples collected at 5 m intervals). Minor quartz–

pyrite veinlets were encountered at 655.5 m, associated with

sticky white clay (presumably kaolinite). White quartz veins were

encountered at 690 m and 720–721 m. Deeper, three fine-grained

quartz veins with pyrite and hematite were encountered (740–

742 m, 888.5 m, 912–913.5 m).

Comparing the depths at which different marker horizons have

been encountered in other boreholes and at outcrop, it appears

that the pre-Carboniferous surface was very flat (8 m or so relief

over the 10 km distance between Rookhope and Eastgate). This

suggests that future boreholes can be planned with greater

confidence using existing deep borehole data.

In summary, typical North Pennine Orefield mineralization

was found down to about 720 m, with more complex mineraliza-

tion beneath that depth. These observations suggest that the

borehole followed the Slitt Vein structure down to 720 m. Below

this depth the Slitt Vein may have died out, or more probably

deviated too far from the borehole azimuth to be recognized.

In addition to the lithological description, phytoclast reflec-

tance (individual macerals were not distinguished because of the

high maturity) was determined on cuttings from the sedimentary

sequence and compared with those obtained for material from

the Rookhope core (Table 1). Reflectance data are similar for

both boreholes, and clearly show the contact metamorphic effects

of the Great Whin Sill, with a regular profile above the sill (Fig.

3; cf. Creaney 1980), perturbed by the Little Whin Sill in the

Table 1. Phytoclast and vitrinite reflectance data for sediments from theEastgate and Rookhope boreholes

Eastgate Borehole Rookhope Borehole

Depth (m) Phytoclastreflectance (%)

Depth (m) Vitrinitereflectance (%)

19 4.56 26 2.5623 4.50 63 3.1027 4.77 94 3.7530 4.85 112 3.0638 5.48 122 2.9452 5.80 146 3.4662 5.56 159 4.6975 7.36 183 6.70164 9.31 195 7.91167 8.05 284 6.04172 5.79 293 6.76179 4.84 337 4.53190 5.07 373 3.73203 5.25 381 3.54215 5.13223 4.75236 4.35239 3.76251 4.42256 4.47264 4.50268 3.52271 3.37

Fig. 3. Phytoclast reflectance profiles for Carboniferous sediments from

the Eastgate and Rookhope boreholes.


Rookhope Borehole (the Eastgate Borehole started lower in the

succession than this). Below the Great Whin Sill, Figure 3 shows

considerable scatter in the profile of reflectance measurements.

This may be due to caving from higher in the borehole prior to

the installation of casing. Immediately above and below the

Whin Sill, reflectance values reach 9% or more, consistent with

contact temperatures in excess of 400 8C (Karweil 1955).

Furthest below the Great Whin Sill, reflectance values for the

lowest two samples are remarkably similar: 3.52% and 3.37% for

Eastgate and 3.73% and 3.54% for Rookhope. The similarity

between these values and the profiles shown in Figure 3 suggests

that the heat flow from the granite to the overlying sediments

was very similar for the two locations, which, although separated

by 10 km, have identical intervals between the base of the Great

Whin Sill and the top of the Weardale Granite (114 m).

Although it is difficult to recast vitrinite reflectance data into

palaeotemperatures, it can be assumed that the very high

reflectance values reported here (not less than 3.3%) correspond

to maximum temperatures of the order of 130 8C close to the

contact between the overlying sediments and the granite. These

temperatures may have been reached prior to intrusion of the

Great Whin Sill (305 Ma; Fitch & Miller 1967; recalculated by

Cann & Banks 2001), which, according to the sill emplacement

model of Goulty (2005), was into sediments with an approximate

depth of 1.4 km (total sediment thickness on top of the granite of

1.5 km). This is consistent with the conclusion of Dunham

(1987) that the Weardale area had exceptionally high geothermal

gradients prior to the emplacement of the Whin Sills.

Geochemistry of the Weardale Granite: implicationsfor heat production

Limited geochemical analyses of selected samples from the

granite were made to evaluate its heat production potential, and

to provide additional geological information and indicate the

degree of uniformity of the rock. Sixteen samples of cuttings

were selected at c. 50 m intervals, and analysed for major and

trace elements using XRF (Table 2). Originating from a borehole

drilled in conditions that were aggressive towards the drilling

equipment, the cuttings were contaminated with tungsten, molyb-

denum and chromium (metals associated with hardened tools), as

well as iron. Despite this contamination, compositional data are

similar to those reported by Brown et al. (1987) for core samples

of the Weardale Granite from the Rookhope Borehole.

Downhole homogeneity is readily assessed from data for

Na2O, K2O and CaO (Fig. 4). For both the Rookhope and

Eastgate samples, the top 200 m of the granite show considerable

variation. Below this depth, these oxides are relatively constant.

Similarly, the trace elements Rb and Sr, and the naturally

radioactive elements Th and U show variable behaviour above,

and greatest values below, 200 m depth within the granite, in

both boreholes (Figs 5 and 6).

Variation within the top 200 m of the granite may partly be

due to the presence of fluorite mineralization (consistent with

observed F contents; Table 2) as well as weathering prior to the

deposition of the overlying sediments. Similarly Dunham et al.

(1965) showed considerable enrichment in Al2O3 (and K2O)

within the upper 25 m of the granite, consistent with the

development of an illitic or micaceous palaeosol.

The heat production capacity of the granite can be calculated

from the chemical analysis (Downing & Gray 1986), using the

equation (P. C. Webb, pers. comm.)

A ¼ 0:1326r(0:718U þ 0:193Th þ 0:262K)

where A is heat production in �W m�3, r is density in g cm�3, U

is uranium content in mg kg�1, Th is thorium content in mg kg�1

and K is potassium content in element percent.

For the purpose of calculation, the specific gravity of the

granite was assumed to be 2.63 (Dunham et al. 1965); estimation

of specific gravity for each sample by measuring the displace-

ment of 100 ml of water by 100 g of cuttings to obtain their

volume, agitating ultrasonically to remove occluded air, gave a

value of 2.58.

Figure 7 shows downhole variations in the calculated heat

production capability of the granite at Eastgate, compared with

recalculated values for the Rookhope Borehole using geochem-

ical data from Brown et al. (1987). For the Eastgate samples in

general, heat production values rise from 3 �W m�3 to an

average value of 4.1 �W m�3 below 200 m depth from the

granite surface. Observed reductions in heat production values at

depths .400 m within the granite may be due to quartz veining,

and have been excluded in the calculation of the average heat

production value.

Overall, the geochemical data from the granite yield the

following results (Table 3).

(1) The heat production value for unaltered granite is

estimated to be 4.1�W m�3, excluding samples at borehole

depths of 400 m and shallower, and excluding the 740 and 950 m

samples, which contain quartz vein material. This value exceeds

that reported by Downing & Gray (1986) for the Weardale

Granite (3.7 �W m�3).

(2) Heat production appears to increase with depth, overall,

with local perturbations related to the occurrence of veins of

quartz and other discontinuities that are unlikely to affect the

bulk heat production capacity.

No attempt has been made to measure the thermal conductiv-

ity of the granite from the Eastgate Borehole. Heat flow has been

estimated on a preliminary basis with the following assumptions.

For the entire vertical interval, a mean value for thermal

conductivity of 2.99 W m�1 K�1 has been estimated using values

from Downing & Gray (1986) for different rock types (Weardale

granite from the Rookhope core and Carboniferous lithologies),

weighted according to thickness. Heat production within the

granite has been neglected, and a ground surface temperature of

8 8C has been used. With these assumptions, the heat flow at

Eastgate is estimated to be 115 mW m�2, well in excess of

values reported by Downing and Gray (1986) for Rookhope.

Water strikes and hydrogeological conditions

Significant water strikes were encountered during drilling, with

unusually high rates of groundwater ingress from the Carbonifer-

ous sequence (requiring a tri-cone roller bit instead of using

hammer drilling). At times, the water yield from the Carbonifer-

ous strata of this one borehole exceeded the entire former

dewatering rates of a number of local mines. These high water

yields reflect the high permeability of fractures associated with

the Slitt Vein structure.

Installation of casing isolated shallow-sourced groundwaters

from the borehole. The first casing sealed the hole off from water

associated with limestones above the Whin Sill, on the assump-

tion that the Sill itself is rarely a prolific aquifer. However, two

major water feeders were encountered within the Whin Sill,

bringing the borehole water yield back to levels found in the

overlying sedimentary strata (c. 60 m3 h�1).

Once the borehole had penetrated 130 m into the granite, the

installation of the second casing was intended to eliminate all

shallow feeders. Given the generally low permeability of granite,


Table 2. Chemical composition of the Weardale Granite, Eastgate Borehole

Sample depth (m)

280 300 350 400 450 505 550 600 650 700 740 745 750 800 850 900 910 915 920 950 995

wt%

SiO2 78.89 76.66 70.14 79.46 70.56 74.15 75.87 74.29 69.99 73.62 89.79 75.44 74.39 72.74 71.11 72.66 74.23 72.47 71.81 76.08 72.33

TiO2 0.13 0.14 0.22 0.07 0.10 0.10 0.09 0.12 0.10 0.15 0.08 0.09 0.14 0.14 0.14 0.15 0.12 0.10 0.16 0.12 0.14

Al2O3 9.76 12.82 12.07 9.68 15.45 13.19 12.91 14.57 12.97 14.24 5.50 13.33 14.21 14.44 15.70 14.76 14.15 14.40 15.14 12.62 13.83

Fe2O3 0.83 0.67 2.50 0.62 0.80 0.79 1.02 0.85 5.66 0.95 0.56 1.01 1.03 1.26 1.11 1.20 1.00 1.11 1.24 1.15 1.18

MnO 0.04 0.02 0.08 0.02 0.03 0.02 0.01 0.03 0.06 0.03 0.01 0.04 0.03 0.04 0.04 0.04 0.03 0.04 0.05 0.04 0.04

MgO 0.74 0.61 0.94 0.32 0.28 0.30 0.29 0.33 0.31 0.28 0.22 0.25 0.29 0.33 0.30 0.38 0.27 0.28 0.33 0.49 0.37

CaO 1.21 0.89 2.87 2.13 0.92 0.82 0.56 0.67 0.51 0.53 0.41 0.63 0.60 0.81 0.77 0.65 0.52 0.59 0.66 0.52 0.65

Na2O 1.22 1.63 2.12 1.55 3.60 2.66 2.41 3.34 2.55 3.30 0.83 3.32 3.35 3.43 3.96 3.49 3.54 3.76 3.88 1.82 3.49

K2O 4.09 5.04 4.55 4.35 6.06 5.84 5.68 5.15 5.52 5.28 1.89 4.42 5.00 4.98 5.37 5.44 5.11 4.89 5.25 5.37 5.54

P2O5 0.13 0.14 0.22 0.18 0.27 0.22 0.21 0.26 0.24 0.28 0.12 0.24 0.26 0.26 0.29 0.29 0.25 0.27 0.32 0.24 0.26

SO3 0.33 0.12 0.29 0.09 0.06 0.18 0.15 0.02 0.10 0.02 0.05 0.05 0.03 0.02 0.01 0.01 0.03 0.01 0.02 0.07 0.01

LOI 1.95 1.44 3.49 1.21 1.09 0.97 0.71 0.85 1.29 0.81 0.63 1.04 0.87 0.92 0.87 0.93 0.95 1.01 0.99 1.14 0.91

Total 99.30 100.18 99.48 99.68 99.21 99.23 99.90 100.47 99.28 99.50 100.09 99.84 100.19 99.37 99.66 99.99 100.19 98.93 99.84 99.66 98.75

mg kg-1

As 19.9 24.3 23.4 18.2 9.9 31.8 14.3 13.9 21.5 4.8 ,2 ,2 2.1 5.1 6.6 1.0 3.9 2.2 4.5 9.9 3.1

Ba 395.6 303.8 271.2 256.8 245.5 299.3 371.2 223.7 342.7 190.0 83.5 186.5 200.0 186.0 196.6 194.0 173.5 172.3 186.9 174.3 176.0

Bi 3.1 ,1 2.1 ,1 3.3 3.4 1.4 1.7 1.9 ,1 30.7 2.7 1.3 ,1 1.3 2.0 ,2 2.1 3.0 2.3 1.4

Ce 23.1 40.9 36.8 30.4 37.5 31.3 22.1 30.6 25.3 32.5 22.6 34.3 37.2 32.8 34.7 34.4 32.8 40.4 37.6 30.3 35.4

Co ,1 ,1 3.5 ,1 ,1 ,1 ,1 ,1 15.3 ,1 ,2 ,2 ,1 ,1 ,1 ,1 ,2 ,2 ,2 ,1 ,1

Cr 198.5 214.2 171.5 205.8 152.6 173.3 225.3 149.4 584.7 147.3 157.8 113.2 171.5 171.0 197.5 145.2 117.3 137.9 121.3 217.2 153.8

Cs 31.1 32.9 38.8 26.8 29.7 23.6 31.1 33.8 45.8 33.5 31.1 37.3 37.6 31.4 37.8 44.6 43.0 38.9 40.9 61.9 54.2

Cu 23.9 15.6 23.7 7.5 19.9 1.8 2.3 1.2 27.8 ,1 ,2 ,2 ,1 3.7 1.4 2.5 ,2 ,2 ,2 5.6 ,1

Ga 13.7 15.6 17.3 15.3 22.5 17.5 18.4 20.4 21.6 22.5 10.0 21.7 20.7 21.6 22.3 23.7 23.2 23.0 24.3 20.0 23.1

Ge ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,1 ,2 ,2 ,1 ,1 ,1 ,1 ,2 ,2 ,2 ,1 ,1

Hf 3.2 4.1 4.2 2.8 3.7 2.2 2.5 3.5 1.9 3.8 2.4 2.5 3.4 2.3 2.6 4.2 3.3 3.4 4.4 2.3 2.9

La 19.1 25.6 25.1 17.5 21.7 23.0 18.2 20.0 20.2 23.7 12.5 18.6 20.2 20.8 20.7 20.6 17.9 20.7 21.8 22.5 23.2

Mo 2.7 2.3 3.4 2.7 ,1 1.3 7.1 ,1 68.3 1.9 3.3 2.6 2.8 2.7 1.5 1.5 ,2 2.0 ,2 3.5 2.3

Nb 8.5 9.4 11.5 10.6 9.6 11.5 10.2 12.4 10.7 15.2 9.0 12.1 13.7 13.0 15.5 16.0 14.6 15.8 17.3 14.2 16.4

Nd 11.8 17.6 17.4 15.3 19.3 16.6 14.1 15.1 13.0 19.4 12.0 13.3 18.4 13.8 17.3 18.1 15.6 17.7 15.5 15.8 18.1

Ni ,1 2.2 4.7 ,1 1.3 ,1 1.3 ,1 26.5 ,1 ,2 ,2 ,1 ,1 ,1 ,1 ,2 ,2 ,2 ,1 ,1

Pb 20.7 160.3 146.1 16.8 27.3 18.9 21.4 20.2 20.1 14.1 9.5 38.1 47.7 35.9 30.0 23.6 31.7 25.8 18.8 20.3 14.8

Rb 270.3 295.0 365.6 361.3 483.7 444.8 471.2 444.3 485.6 462.0 259.8 439.8 445.9 433.2 459.9 469.9 474.6 486.7 504.3 501.6 456.8

Sc 2.5 3.4 4.4 3.1 2.7 ,1 ,1 ,1 ,1 1.5 2.8 4.1 3.6 3.1 ,1 3.1 4.3 ,2 ,2 ,1 2.2

Sn 14.5 9.4 8.9 13.7 16.2 15.1 11.7 17.0 23.8 18.5 8.5 15.3 20.1 12.2 18.7 20.6 19.4 20.6 24.6 16.2 21.3

Sr 46.1 70.9 99.5 37.6 76.8 54.5 49.7 58.9 64.9 53.4 21.7 63.7 56.9 69.5 76.5 66.6 63.3 65.8 65.5 54.6 66.3

Ta ,1 ,1 6.3 4.8 ,1 ,1 ,1 2.7 ,1 ,1 3.6 2.7 1.4 6.3 6.4 ,1 4.3 4.7 4.6 5.3 3.2

Th 8.6 13.2 15.0 11.1 12.4 12.3 13.1 10.0 10.5 12.2 15.4 19.6 14.4 15.2 14.7 16.5 17.3 19.9 21.3 12.6 15.6

U 6.2 4.9 7.1 5.3 9.5 8.9 11.2 8.9 8.2 9.4 4.4 8.4 10.1 13.4 10.7 11.5 9.4 6.8 11.2 3.9 10.5

V 15.9 21.6 29.1 10.3 10.8 15.2 11.5 12.4 14.7 11.6 9.2 11.7 14.6 12.4 14.3 11.4 9.9 9.9 13.7 12.9 15.3

W ,1 1.6 ,1 ,1 ,1 ,1 2.5 1.9 46.2 1.9 34.4 4.6 3.7 2.7 1.1 ,1 2.7 5.4 3.3 1.5 2.3

Y 3.8 4.3 6.5 3.5 1.2 ,1 ,1 2.8 ,1 ,1 ,2 ,2 1.2 2.0 3.6 ,1 ,2 ,2 ,2 1.6 ,1

Zn 5.3 7.0 30.5 14.2 30.3 164.9 15.2 28.3 23.2 27.4 8.4 28.3 29.0 66.9 35.9 39.9 37.8 39.2 42.0 39.5 35.9

Zr 72.0 93.2 86.0 47.2 72.0 67.6 52.1 71.7 1.8 90.8 36.6 68.3 76.8 73.1 79.5 80.6 71.3 84.3 83.9 70.3 80.3

Cl 141.6 86.1 171.2 106.8 168.4 174.4 89.7 43.8 541.9 65.5 178.3 168.5 67.4 57.2 52.3 142.5 138.4 173.4 123.0 89.7 67.0

F 1147.6 2251.7 1591.7 11581.0 1464.8 2128.5 2826.3 1717.1 2029.4 2182.5 1083.4 1966.8 2366.7 2784.4 2453.7 2720.6 2091.3 2198.8 1706.3 1862.2 2147.9

S 1172.8 129.0 588.7 133.6 192.4 251.9 699.6 357.0 100.8 338.0 26.2 34.5 229.6 382.8 360.8 111.8 49.5 67.2 71.0 288.8 252.2

Heat

production

(�W m-3)

2.57 2.67 3.29 2.55 3.87 3.69 4.31 3.48 3.36 3.76 2.35 3.91 4.04 4.94 4.26 4.59 4.09 3.58 4.80 2.41 4.29

LOI, loss on ignition.

D.

A.

C.

MA

NN

ING

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76

subsequent feeders would be expected to be minimal unless there

was unusually intense fracturing. As expected, after the second

casing had been grouted into the borehole, water yield dropped

to zero, and this continued for a further 7 m. However, at around

410 m below ground, the drill stem pressure gauge jumped to

23 bar, and the drill bit suddenly dropped by 0.5 m. At this point

the pressure gauge went off-scale (.30 bar), and water surged

into the hole, rapidly rising to within 10 m of the ground surface.

It is clear that a major open fissure had been encountered at this

point. The electrical conductivity of this water greatly exceeded

that of the waters previously encountered in the Carboniferous,

and it was also warm to the touch (around 26 8C). Air-lifting at

rates of up to 60 m3 h�1 (maximum capacity of the equipment)

failed to lower the water level by more than 1 m, indicating a

transmissivity in excess of 2000 darcy m. This is believed to be

an unequalled value for granites worldwide (E. Sudicky, pers.

comm.), although it clearly reflects the influence of the Slitt

Vein, rather than the permeability of more typical extensional

joints and faults typical of most plutons. Other fractures were

intersected at depths of 436, 464, 492–496, 654, 720–721, 739.5

and 813–814.5 m. These fracture intersections were not accom-

panied by dramatic events such as occurred at 410 m, and the

quantities of water that they introduced to the borehole were

difficult to discern given that the 410 m feeder had already

exceeded the air-lift capacity of the rig. A gradual increase in the

temperature of the water arriving at the well head (.27 8C

towards the end of drilling) indicated that a significant amount of

warmer deeper water was mixing with the 410 m feeder water.

After the end of drilling geophysical logs for fluid tempera-

ture, conductivity and flow rate (by impeller) were run twice:

first through the static water column, and then with a 100 mm

electric submersible pump stimulating the borehole water column

by pumping from just below the water surface at a rate of around

1.4 m3 h�1. Comparison of the two suites of logs indicates

Fig. 4. Distribution with depth of CaO, Na2O and K2O within the

Weardale Granite for the Eastgate Borehole (this study) and the

Rookhope Borehole (Brown et al. 1987).

Fig. 5. Distribution with depth of Rb and Sr within the Weardale Granite

for the Eastgate Borehole (this study) and the Rookhope Borehole

(Brown et al. 1987).

Fig. 6. Distribution with depth of Th, U and K within the Weardale

Granite for the Eastgate Borehole (this study) and the Rookhope

Borehole (Brown et al. 1987).


significant water feeders associated with fractures at c. 730 and

756 m depth. (Other feeders were also indicated by the geophy-

sical logs at 420, 434, 447, 485, 497, 527, 540, 557, 670 and

686 m.) Although none of these were as prolific as the 410 m

feeder, they demonstrate the occurrence of permeable fractures at

depths approaching those that could be considered for a long-

term production borehole.

Groundwater quality

The electrical conductivity and temperature of water from the

borehole were measured continuously at the cuttings separator.

Water samples were taken initially every 10–20 m, and every

50 m for depths .300 m. Cations were determined using induc-

tively coupled plasma atomic emission spectroscopy, alkalinity

by titration, ammonium inductively coupled plasma atomic

emission Kjeldahl digestion and anions by ion chromatography.

Water samples up to and including 86.5 m were acidified before

filtration; this meant that dissolved suspended solids were

reported in the chemical analysis, giving misleading results that

overestimated dissolved cations (high positive charge balances).

From 135 to 995 m samples were filtered prior to acidification,

with charge balances predominantly less than �5%. Analyses are

given in full in Table 4.

With depth, conductivity and temperature rise until c. 400 m

(Fig. 8), at which point there is a sharp increase in both. This

corresponds to the point at which the hole intersected the large

open fracture at 410 m. Individual chemical species show con-

trasting behaviour with depth. The major cations (Na, K, Mg, Ca),

lithium, strontium and chloride increase in the same way as

conductivity, with a sudden increase once the 410 m feeder had

been intercepted (Fig. 9). Observed pH decreases, from 7.6–8.1 to

5.8–6.0, probably reflecting the absence of bicarbonate buffering

at depth: alkalinity (expressed as mg l�1 CaCO3) decreases with

depth to very low values, as does dissolved sulphate.

The constancy of the chemical composition of the water below

400 m reflects a combination of two possible causes: the water

column within the hole is well mixed as a consequence of the

drilling operation and/or the contribution of water from the

fissure at 410 m is sufficiently strong to dominate water chem-

istry during drilling. There is no evidence from the water

chemistry of any substantial flow of water that is more saline

(i.e. deeper sourced) or less saline (i.e. derived from nearer the

surface) into the hole at depths greater than 410 m.

The deep brine encountered by the borehole has as dominant

solutes 27 700 mg l�1 chloride, 10 030 mg l�1 sodium and

5320 mg l�1 calcium. Comparing its composition with water

recovered from Cambokeels mine (Manning & Strutt 1990; Table

5), there is little doubt that the Eastgate Borehole has intersected

the same water system (over 15 years after the brine was sampled

in the mine) where it is deeper, more saline and warmer.

Importantly, the Eastgate Borehole water is about 25% more

saline than seawater, but less than half (36%) the salinity of the

water currently produced at the Southampton geothermal plant. It

is more than twice the salinity of thermal waters reported from

South Crofty Mine (Edmunds et al. 1984). As in South Crofty,

molar Cl/Br ratios for the Eastgate water are below that typical

of seawater (650), which could be attributed to loss of Cl via

precipitation of halite through evaporation; in contrast, the high

value for Southampton suggests halite dissolution. Molar Na/Li

ratios are very low for the Eastgate and South Crofty waters,

indicating substantial interaction with basement rocks, and their

molar Na/Br ratios are again consistent with water–rock inter-

action (whereas the Southampton water’s high Na/Li and Na/Br

ratios are consistent with halite dissolution). The Eastgate water

is close in composition to that identified by Cann & Banks

(2001) as being associated with fluorite zone mineralization.

Although unlikely to be a mineralizing fluid in its own right

(although silica minerals are likely to have precipitated) the

Eastgate water seems to have shared a common fluid–rock

interaction history with the waters responsible for fluorite miner-

alization. Psyrillos et al. (2003) discussed the role of water from

adjacent basins as part of the kaolinization process in the SW

England granites. As the South Crofty and Eastgate waters are so

similar in their major solute proportions, it is likely that

analogous processes led to their formation, and the Eastgate

waters may be derived ultimately from adjacent deep aquifers.

Using the chemical data given in Table 5, temperatures at

Fig. 7. Variation with depth of calculated heat production for the

Weardale Granite from the Eastgate and Rookhope boreholes.

Table 3. Summary of thermal properties for granite from the EastgateBorehole

Average heat production 4.07 �W m�3

Thermal conductivity 2.99 W m�1 K�1

Surface heat flow at Eastgate 115 mW m�2


Table 4. Water compositional data (in mg l�1) for samples taken from the Eastgate Borehole (135 m and deeper)

Sample: E9 E10 E11 E12 E13 E14 E15 E16 E17 E18 E19 E20Date (2004): 11/9 11/9 11/9 12/9 15/9 15/9 16/9 16/9 16/9 6/10 6/10 7/10Depth (m): 135 144 152 166 181 192 214.5 234 236.5 266.5 275 300Temperature (8C): 14 14.1 14.1 14.3 14.6 15 14.9 16.4 17 17.2 17.9 18.9Cond. (field) (�S cm�1): 3750 3870 3900 4390 5150 5780 5680 11820 14940 27390 27130 26150pH: 7.6 7.8 7.8 7.8 7.9 8.1 8.1 7.7 7.8 6.4 6.8 6.9Cond. (lab) (�S cm�1): 3600 3794 3867 4215 5204 5502 5802 13290 14560 18740 16100 16030Alkalinity: 256 246 240 246 250 256 246 202 206 110 142 126

Nitrate ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5Chloride 559 648 826 761 1037 1123 1231 3895 4262 7449 5810 5906Sulphate 466 444 399 412 389 392 378 290 289 269 291 290BromideAmmoniumCalcium 222 228 231 244 293 316 339 807 969 1170 976 1066Magnesium 9.5 8.5 8.4 8.6 8.1 9.6 8.2 16.8 16.2 25 22.4 20.4Sodium 484 510 521 552 676 705 730 1984 2248 2697 2205 2243Potassium 40.1 42.9 44.1 42.3 51.2 52.4 52.5 103 111 159 138 140Iron ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 0.4Manganese 0.5 0.7 0.6 0.8 0.8 0.8 1.1 1.7 2 2.2 1.5 1.6Zinc ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 0.2 0.2 ,0.1 ,0.1 ,0.1Copper ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Lead ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2Lithium 3.4 3.7 3.8 4.3 5.5 5.9 6.2 16.1 17.4 25.1 20.1 20.5Silicon 6 6 6 6 6 6 6 6 6 5 5 4Strontium 8.1 8.5 8.7 10.0 12.5 13.7 14.3 39.6 45.65 76.8 60.5 61.8BariumCharge balance (%) 6.2 5.4 0.5 5.3 5.4 5.1 4.5 5.5 8.3 �7.5 �5.6 �4.3

Sample: E21 E22 E23 E24 E25 E26 E27 E28 E29 E29A E30 E31Date (2004): 8/10 19/10 27/10 28/10 29/10 2/11 6/11 9/11 25/11 30/11 1/12 2/12Depth (m): 335 411.5 485 561 590 674 725 770 847 910 951 995Temperature (8C): 19.2 26 24.5 26 25.5 26.2 26.6 26.5 26.6 26 27 –Cond. (field) (�S cm�1): 26010 181000 181200 188900 190100 279600 210903 221014 212000 215000 239000 –pH: 6.9 6.2 6.4 6.4 6.3 6.4 6.4 6.4 5.8 5.9 6 6Cond. (lab) (�S cm�1): 23210 65400 65700 66200 66500 66800 63200 64000 65800 66500 66200 65200Alkalinity: 130 60 56 56 58 54 56 50 54 58 47 56

Nitrate ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5 ,5Chloride 9486 28750 25840 26970 25660 28560 25730 25700 28400 30000 31200 27800Sulphate 255 48.5 52.2 44.7 46.9 47.8 68 69 41.2 44.3 43.4 45.3Bromide 140 140 150 160 140 140 130 160 150 150 140Ammonium 11 – – – – – – 11 12 11 11Calcium 1595 5285 5256 5424 5345 5250 5410 5620 5312 5375 5264 5009Magnesium 28.6 72.4 71.9 72.8 72.1 73.1 68.9 69.1 79.2 79.2 78.2 75.3Sodium 3333 9630 9580 9940 10000 9790 9930 9940 10100 10300 11000 10100Potassium 201 631 642 646 667 656 782 551 689 708 646 638Iron ,0.1 0.4 ,0.1 0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Manganese 4 20.3 19.1 18.3 19.8 17.6 21.5 22 19 18.7 19.4 19Zinc ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Copper ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1 ,0.1Lead ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2 ,0.2Lithium 30.7 90.6 91.7 93.2 93 93.5 92.8 94.8 94.3 91.3 91.3 89.5Silicon 5 6 4 4 3 3 6 4 5 4 5 6Strontium 103 343 344 352 350 353 304 313 311 315 313 305Barium 12.9 13.6 12 13 13.3 12.5 14.7 13.8 12.3 12.2 11.5Charge balance (%) �7.0 �5.6 �0.5 �1.0 1.5 �4.8 1.4 1.8 �3.4 �5.3 �5.8 �3.5

Alkalinity is measured as mg l�1 as CaCO3.

GE

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37

9

which the water may have equilibrated can be estimated

(Truesdell 1984; Table 6). The quartz geothermometer gives a

temperature of 38 8C (less than the observed bottom-hole tem-

perature of 46 8C), whereas the alkali geothermometers give

temperatures between 146 and 191 8C, similar to those calculated

by Manning & Strutt (1990). It is likely that the water has lost

silica by precipitation of quartz close to the surface and possibly

in the recent geological past (quartz precipitation is abundant

within fractures in the Slitt Vein). The alkali geothermometers

suggest that the water achieved equilibrium with respect to Na,

K and Ca at depths of 3–4 km (assuming a geothermal gradient

of 40 8C km�1). This suggests that the water in the borehole

forms part of a deep circulation system, which appears still to be

active given the similarity of the water to that encountered at

Cambokeels between 15 and 20 years ago.

Geothermal potential

Geophysical logging of the settled water column in the borehole,

3 days after the end of drilling, indicated the magnitude of the

geothermal resource potentially developable in this vicinity. The

bottom-hole temperature at 995 m was 46.2 8C, which yields a

mean geothermal gradient estimate for this borehole of

38 8C km�1. This compares very favourably with the UK average

(c. 21 8C km�1; Downing & Gray 1986), from which a bottom-

hole temperature of only 30–35 8C would be predicted at this

depth. As the geothermal gradient is likely to continue on the

same linear trend as logged from 411 m to 995 m, the implica-

tion of the measured bottom-hole temperature at 995 m is that a

borehole sunk to a typical ‘production’ depth of about 1800 m

would be expected to return a bottom-hole temperature in the

range 75–80 8C.

Given that the heat production capacity of the Weardale

Granite at Eastgate is similar to that previously calculated by

Downing & Gray (1986), the key issues relate to the availability

of natural groundwater to act as a transmission fluid for heat

produced in the granite at various distances from the borehole.

The extraordinary transmissivity of the major fracture at around

410 m depth provides unequivocal evidence of the association of

highly permeable fractures with the Slitt Vein structure. Although

the deeper fractures were not quite so permeable, geophysical

logging indicates that they are still significant water-bearing

structures. The occurrence of permeable fractures associated with

the Slitt Vein at depth means that there is no need to artificially

introduce water to the geological environment, in contrast to the

usual assumption that similar UK granites are (at best) HDR

prospects (Downing & Gray 1986; Barker et al. 2000). Instead of

an HDR prospect, therefore, the Eastgate resource may be

classified as a ‘low-enthalpy hydrogeothermal resource hosted in

vein-bearing granites’. This is the first time that this category of

geothermal resource has been described (see Downing & Gray

1986; R. A. Downing, pers. comm.). Similar veins occur within

the nearby (also radiothermal) granites of the Lake District and

Wensleydale. Elsewhere, the Lecht Mine has a similar structure

in the radiothermal granite of the Eastern Highlands, and the SW

England Batholith hosts many analogous lodes, some known to

Fig. 8. Variation of (a) water temperature and (b) electrical conductivity

with depth in samples taken for analysis.

Fig. 9. Variations with depth in (a) pH, (b) alkalinity (mg l�1 CaCO3)

and sulphate, (c) calcium, sodium and chloride and (d) lithium and

strontium.


have transmitted brines similar to that found at Eastgate

(Edmunds et al. 1984). It may be that further exploration in

geological settings similar to that at Eastgate would lead to the

identification of other geothermal resources of significant magni-

tude.

Table 7 compares aspects of the Eastgate Exploration Borehole

with the existing Southampton Borehole. Given that the amount

of water that may be produced from Eastgate is very similar to

that yielded by the Southampton Borehole, the latter is an

appropriate model for possible future development at Eastgate.

The Eastgate water has the advantage that it is much less saline

than the Southampton water (44 500 mg l�1 total dissolved solids

(TDS) compared with 124 440 mg l�1 TDS at Southampton).

Given the findings from the Eastgate exploration borehole, it

is reasonable to predict that a further borehole sunk to 1800 m

could provide a resource similar in magnitude to that at South-

ampton. However, because the Southampton borehole was drilled

to intersect a more or less horizontal aquifer unit, a production

borehole at Eastgate would need to intersect fractures associated

with the Slitt Vein at the target depth. From the experience of

drilling this exploration borehole, it is unlikely that a depth

of 1800 m could be reached without intersecting a number of

shallower fractures carrying cooler water. Further casing close to

the total depth would therefore be needed, so that shallower

feeders could be sealed off before drilling on in pursuit of

sufficient large fractures at depth to yield a viable low-enthalpy

resource. Alternatively, a new production borehole could be

deliberately drilled off-centre from the target vein structure, with

directional drilling techniques being used to deviate the lower-

most parts of the borehole until they contact the permeable

fractures along the vein azimuth. Implementation of either of

these options would be both the most significant engineering

Table 5. Water compositions (in mg l�1) for samples taken from the Eastgate Borehole (mean of 10 samples below 400 m), compared with analysesreported for water from Cambokeels Mine (Manning & Strutt 1990), from the Southampton geothermal project (Downing & Gray 1986), and for seawater

Eastgate deepbrine

Cambokeels minewater 1988

South Crofty 380level

Southamptongeothermal water

Seawater

Temperature (8C): 26.1 16 41.5 76Electrical conductivity (�S cm�1): 65591Discharge (m3 day�1): 1600 300 1700pH: 6.2 6.50 6Alkalinity (mg l�1 as CaCO3): 55 111 62

Sodium 10028 7345 4300 41300 10770Potassium 660 340 180 705 380Magnesium 73.8 121 73 752 1294Calcium 5323 5236 2470 4240 412Ammonium 11.2 36Iron ,0.1 4.75 4.1Manganese ,0.1 4.50 1.26Zinc ,0.1Copper ,0.1 0.023Lead ,0.2Lithium 92.4 70.5 125 31 0.18Strontium 328 134 7.9Barium 12.9 0.52Silica (SiO2) 9.6 34.2 32.1Chloride 27692 24600 11500 75900 19350Sulphate 50.1 145 1230 2712Nitrate ,5 ,0.2Bromide 146 43 91 67TDS 44482 37713 19002 124440 35000Molar Cl/Br 427 602 1877 650Molar Na/Li 32.7 10.4 402 18060Molar Na/Br 239 348 1577 558

Table 6. Temperatures derived from borehole water compositions usingchemical geothermometers (Truesdell 1984)

Geothermometer Eastgate Cambokeels

Quartz 38 43NaK (Fournier) 184 169NaK (Truesdell) 146 129NaKCa 191 159

Table 7. Comparison of aspects of the Eastgate Exploration Borehole with the existing production facility at Southampton

Eastgate (existing borehole only) Southampton

Temperature (8C) 27 (bulk, mixed water in late stages of drilling)46 (bottom-hole temperature)

76 (water)

Depth (km) 1 2Yield (m3 day�1) .1600 860 (originally 2330 in 1987)Salinity (mg l�1) 44500 124500Power (MW) c. 0.75 1.4 (originally 2 in 1987)


challenge and the most significant risk element in proceeding to

full-scale geothermal energy production at Eastgate.

We acknowledge funding by the Weardale Task Force, made up of One

NorthEast, Durham County Council, Wear Valley District Council and

Lafarge Cement. We are grateful to Rob Westaway for his help in

reviewing this paper. This project received funding from One NorthEast

through the County Durham Sub Regional Partnership. The project was

part-financed by the European Union, European Regional Development

Fund.

References

Bachler, D. & Kohl, T. 2005. Coupled thermal–hydraulic–chemical modelling

of enhanced geothermal systems. Geophysical Journal International, 161,

533–548.

Banks, D., Skarphagen, H., Wiltshire, R. & Jessop, C. 2004. Heat pumps as a

tool for energy recovery from mining wastes. In: Giere, R. & Stille, P.

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Received 13 February 2006; revised typescript accepted 13 July 2006.

Scientific editing by Mike Fowler


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