PROCEEDINGS, Twenty-First Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 27-29, 1997
SGP-TR-155

GEOCHEMICAL STUDY OF THE ALID HYDROTHERMAL SYSTEM,DANAKIL DEPRESSION, ERITREA

Jacob B. Lowenstern1, Cathy J. Janik1, Theoderos Tesfai2, and Robert O. Fournier1

1 U.S. Geological Survey, MS-910, 345 Middlefield Road, Menlo Park, CA, USA
2 Ministry of Energy, Mines and Water Resources, P.O. Box 5285 Asmara, Eritrea
ABSTRACT

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INTRODUCTION
There is an urgent need to develop new domestic
sources of energy to foster expansion of the Eritrean
economy while minimizing the importation of costly
fossil fuels. To help address this need, the USGS
was funded by USAID to undertake the first phase ofa study of the geothermal energy potential in theeastern lowland region of Eritrea. The Alid volcanic
center was selected for detailed study because (1) it is
the focus of geologically young rhyolitic volcanism
within a background of spreading-related basaltic
volcanism and (2) it is the site of many fumaroles(U.N.D.P., 1973; Beyth, 1994). The collection ofwaters and gases for chemical and isotopic analyses
was carried out in February 1996. Standard sampling
and analytical procedures were used for water andfumarolic gas samples (Trujillo et al., 1987;
Giggenbach and Goguel, 1989; Fahlquist and Janik,1992).

R

43˚

Da

Detailed geological studies indicate that a relatively
young, large, shallow, and still hot magmatic heatsource is probably present beneath the Alid volcanic
center in the northern Danakil Depression of Eritrea.
Fumaroles and boiling pools are distributed widely on
the north half of Alid, suggesting that an activehydrothermal system underlies much of that part ofthe mountain. The area of high convective heat flow
covers at least 10 square kilometers.
Gas
geothermometers indicate likely reservoir
temperatures in the range 250°C - 325°C. The
isotopic composition of condensed fumarolic steam
is consistent with 220-300°C boiling of groundwatersthat may have come from various sources, including
local lowland rain, fossil Red Sea water, orconceivably, highland rain water that evaporated
significantly before percolating underground. Somegases in the reservoir fluid, particularly CO2, H2, andH 2 S may be derived, directly or indirectly, from asilicic intrusion that very likely exists beneath Alid.

40˚E

Somalian Plate

Mainly Cenozoic Volcanicand Sedimentary Rocks
Mainly Precambrianbasement

0

25

50 km

Axis of spreading

Figure 1. Simplified plate-tectonic map of the Afar
Triangle region. Modified from figures in Barberi and
Varet (1977).

GEOLOGIC SETTING
The Alid silicic volcanic center is located along theaxis of the Danakil Depression, the graben trace of acrustal spreading center that radiates NNW from aplate-tectonic triple junction situated within acomplexly rifted and faulted basaltic lowland called
the Afar Triangle (Figure 1). The Danakil Depression
is an off-set subaerial segment of the spreading
system that is opening to form the Red Sea (Barberi
and Varet, 1977). The northern Danakil Depression
lies near or below sea level for much of its extent. It
is a graben bounded by the Danakil Alps to the east,
which in this area rise to elevations of 400 to 650
masl and the Eritrean plateau or highland to the west,
which rises to elevations of 2000-3000 masl. Alid
volcanic center is an elliptical structural dome locatedat the center of the graben. The summit of Alid sits
roughly 700 meters above a field of Quaternary
basaltic lava that laps unconformably against thenorth and south flanks of the mountain. The major
axis of the mountain is 7 kilometers, elongate in an
ENE-WSW direction, perpendicular to the trend of thegraben (Figure 2). The minor axis is about 5
kilometers long, parallel to the graben. The dome
apparently formed as a result of shallow intrusion ofrhyolitic magma, some of which was erupted (Clynne
et al. 1996a, 1996b). The doming uplifted
Precambrian mica and kyanite schists and deformed an
overlying sequence of initially flat-lying Pliocene orPleistocene sediments capped by lava flows of basalt,
basaltic andesite, and rhyolite composition. These
sediments and lavas now dip steeply and radially awayfrom the center of uplift (Clynne et al., 1996a,
1996b). Subsequent to structural doming, there were
eruptions of pyroxene rhyolite lavas on the flanks ofthe mountain. Still later, rhyolite was erupted as
pumice and a pyroclastic flow from a graben-likesummit depression.
REGIONAL HYDROLOGY
The meteorological characteristics of northeastern
Africa, including Eritrea and the Danakil region, havebeen described in various publications and reports,
including Food and Agricultural Organization (1983),
Michael (1986), Eklundh and Pilesjî (1990) andBeltrando and Camberlin (1993). The annual rainfall
on the central highlands is generally 500-700 mm andcomes from storms propelled by monsoonal winds
blowing from the southwest toward the northeast.
This rain occurs mostly in July and August. Little,
if any, rain falls on the central highlands from
December through February. In contrast, the eastern
lowlands receive most of their rain in December andJanuary from storms propelled by monsoonal winds
blowing across the Red Sea from the northeast toward
the southwest. Here less than 300 mm per year ofrain generally falls, but it may be very spotty with norain falling for a year or more on parts of the lowland

region while other parts are subjected to brief,
torrential down-pours. Rivers and streams flowing to
the eastern lowlands from the high plateau disappear
into alluvial fan deposits, or pond in closed-basinlakes where the water evaporates to form playas.
There is also the potential for lenses of relatively
fresh and non-evaporated water to pond in thesubsurface over deeper bodies of denser brine.
GEOTHERMAL MANIFESTATIONS
Fumaroles and boiling pools are distributed widely on
the north half of Alid, suggesting that a hydrothermal
system underlies much of the volcanic center (Figure2). The presently identified area of this high heatflow is at least 10 square kilometers. We visited nine
fumarole areas and selected six as being suitable forsampling of gas and water emanations (Table 1). Allvent areas contain fumaroles at temperatures near theboiling point for their elevation. The physical natureand geochemical environment of the hot springs andpools at these sites strongly suggest that the waters
consist of mixtures of condensate of fumarolic steam
mixed with cooler, shallow groundwater.
Table 1. Selected geothermal features ofAlid volcanic center
Sample Name
E l e v Temp δ D#
m °C‰
Thermal Pools
ELW96-5 Ilegedi #1
515 5050
ELW96-6 Ilegedi #2
515 3630
ELW96-7 As'ela #1
480 5433
ELW96-8 As'ela #2
480 5735
ELW96-9 Ilegedi #3
515 6624
ELW96-10 Humbebet
480 <60
12Fumaroles
ELG96-1 Hulma
ELG96-2 Darere
ELG96-3 Ilegedi #1
ELG96-4 As'ela
ELG96-5 Ilegedi #3
ELG96-6 Abakri
n.a. not analyzed

225
580
515
480
515
485

77
95
95
95
84
94

n.a.
10
5
5
n.a.
-1

δ 18 O
‰
9.69
3.85
3.18
3.61
4.44
-0.93

n.a.
-3.98
-0.88
-1.69
n.a.
-2.81

Fumaroles vent through rhyolite breccia (Abakri,
As'ela, Darere), siltstones (Humbebet), andPrecambrian basement rocks (Ilegedi). Therefore,
location of the thermal features does not appear to becontrolled by lithologic type or contacts of different
lithologic units. Most of the thermal manifestations
are located at elevations between about 460 and 600
meters, though hot rock and steaming vents also arepresent as low as 225 meters (Hulma) and slightly
higher than 760 meters (Airole). Some fumaroles are

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yb

Alluvium (al)Strike and dip of beds and lava flows

Young basalt (yb)Fault, bar and ball on downthrown side

Pyroxene rhyolite: lava (frhy)
and pumice (pf)
Lava shell (ls): includes basalt, andesite
and amphibole rhyolite

Fault, covered

Sedimentary sequence (ss): includes basalt
lava flows and sills, fine-grained clastic
sediments, rhyolite breccia and evaporites

Vent for other rhyolite,,,, Stratoid basalt (str),,,,Precambrian basement (pc)

Fault, approximately locatedVent for post- dome rhyolite pumice

Mafic ventFumarole
Contact

Figure 2. Generalized geologic map of Alid volcanic center. Because map was traced from lines on air photograph,the scale is approximate and varies somewhat across the area.

aligned, suggestive of fault control. For example,
several areas of alteration, including the one at
Hulma, lie in a N50E zone parallel to the north baseof Alid. Abakri, As'ela and Humbebet are present inadjacent drainages on the north flank of Alid along aroughly N45E trend.

fumaroles are listed in Table 2. The samples
generally contained more than 95% steam, with the
exception of the bubbling pool of Ilegedi #3 wheresome of the steam condensed in the pool andconsequently could not be collected. However,
because the fumaroles were not superheated withrespect to boiling temperatures at their ventelevations (all about 97°C or less), some steam may
have condensed as it rose through the mountain,
causing the remaining steam to become enriched in
non-condensable gas. Gas/steam ratios shown in
Table 2 therefore do not necessarily reflect thecompositions of deeper, hotter parent fluids.
With the exception of Hulma, most of the Alid
fumaroles have roughly similar gas compositions(Table 2) and insignificant air contamination, shownby oxygen concentrations generally being near or
below the detection limit. In contrast, Hulma's highAr, O2 and N2 indicated that 97% of the sample wasmoist air discharged from a relatively cool (77°C)
vent. As'ela was air-free, but was also slightly
unusual in its low NH3 and CH4 concentrations.

Chemical Compositions of Thermal Waters
The thermal pools at Alid all are acid-sulfate incomposition and contain little chloride (Clynne et al.,1996a). They are highly evaporated and haverelatively high total dissolved solids ranging fromabout 1500 to 2500 mg/kg. Their compositions aremainly controlled by dissolution of the surrounding
host rock by sulfuric acid, resulting in the formationof clay minerals after feldspars, and release of cations
to solution. These pools provide relatively little
information about deep fluids and cannot be used toinfer conditions in any deep reservoir beneath Alid.
Chemical Compositions of Alid Fumaroles
Complete analyses of gas samples from Alid

Table 2. Gas Geochemistry of Alid Fumaroles
Sample Num.
ELG96-1
ELG96-2
Location

ELG96-3

ELG96-4

ELG96-5

ELG96-6

Ilegedi#195

As'elaIlegedi #3

Abakri

95

94
98.86

Hulma

DarereTemp. (°° C)*

77

95

CO 2 (mol%) #
H 2S

2.50

97.93

95.53

98.20

84(gas frombubbling
pool)
95.89

0.112

0.219

0.876

0.749

0.662

0.143

H2

0.057

1.093

2.498

0.503

2.624

0.605

CH 4

0.031

0.225

0.132

0.061

0.144

0.085

NH 3

0.050

0.128

0.389

0.004

0.005

0.095

N2

76.1

0.412

0.598

0.473

0.653

0.209

O2

20.3

0.0023nd

nd

nd

0.0005Ar

0.908

0.00540.01260.01160.01400.0047He

nd0.001510.000470.000460.000730.00018

76.3

47.5

40.8

46.6

44.5

N 2 /Ar

83.8

Gas/Steam–
0.04480.01990.02591.701(mol/mol)
–
-3.4
-3.4
-4.9
-3.3
δ 13 C (‰VPDB) †Gases collected in bottles according to methods of Fahlquist & Janik (1992)*ELG96-2,3,4,6 were boiling-temperature fumaroles with no superheat. Temperature readings
may be ~2°C lower than actual conditions.
#Gas concentrations in mol% of dry gas.
nd not detected; – not determined† δ13C in CO2 from fumarole gas.

0.0565-3.3

The thermal pools at As'ela also contained
considerably less NH3 than those at Ilegedi (Clynne etal., 1996a). This is puzzling because As'ela islocated within the shell unit that contains sedimentaryrocks, whereas Ilegedi #1, with higher methane andammonia concentrations, vents directly from thebasement schists. Darere was unique in its higher He
concentrations. But, in spite of these variations inminor gas constituent abundances, similarities among
the Alid fumaroles were more noticeable thandifferences. The non-condensable gas compositionsof all samples range from 95.5-98.9 mol% CO2, andH 2 is generally the next most abundant component(0.5-2.6 mol%).
The N2 /Ar/He relations in the fumarolic gases arestrongly indicative of a parent geothermal reservoir
water that initially was air-saturated groundwater. Alldata fall on a trend from air-saturated water to acrustal/mantle basalt endmember (Figure 3). The
Darere fumarole is enriched in He, which may comefrom radioactive breakdown of crustal components or
could be provided by mantle input in the form ofbasaltic magmas underlying Alid. As would be
expected, there is little evidence for an arc/andesitic
input of nitrogen to the system through crustal
recycling at subduction zones (Giggenbach, 1995).

20

N2/100esitic
nd
40

Arc
/A

80

Air

60

60

40

20

40

60

80

10
He

Darere

Cr

ltic
20
sa
ba
/
l
ta
us

ASW 10°C

80

ASW max

Ar

Figure 3. Triangular diagram (N2/100 vs.
10He vs. Ar) for gas samples fromfumaroles on Alid volcanic center.
Within the geothermal system, the O2 in air-saturated
groundwater is removed by reaction with rocks athigh temperature, whereas N 2 and Ar remain
relatively unchanged. The ratio N2/Ar for fumarole

gases ranges from 41-48, similar to a value of 38 forair-saturated water (Figure 3). This "reacted"
groundwater has been enriched in CO2, H2 and He ofcrustal or magmatic origin, yielding the present"parent" geothermal reservoir water.
Gas Geothermometry
We employed nine gas geothermometers (Table 3).
In our experience, the D'Amore and Panichi (1980)
method, which utilizes the relative abundances of
CH 4 , CO2 , H2 S and H2 , generally gives the mostreliable results. The D'Amore and Panichi (1980)
geothermometer indicates reservoir temperatures of
265°C for Ilegedi, the largest and most active of the
Alid geothermal manifestations. The abundance of
methane is negatively correlated with the calculated
geothermometer temperature, so that addition oforganically derived methane in the shallow subsurface
would cause the geothermometer to give temperaturesthat are lower than the actual reservoir temperature.
Another commonly used geothermometer is the
H 2/Ar method outlined by Giggenbach and Goguel(1989).
As temperature increases, water is
progressively dissociated, creating higher partialpressures of hydrogen. Argon is used to normalize
the abundance of hydrogen, as it has a low solubility
in water, similar to that of hydrogen. The H2 /Ar
ratio therefore does not change significantly due tosecondary processes such as condensation, mixing andboiling. In general, there are no significant
extraneous sources of Ar to geothermal systems,
making Ar a reliable normalizing component. Air
contamination of a gas sample, if it were to occur,would result in high Ar concentrations and lower
temperatures calculated by the H2/Ar geothermometer.
Instead, at Alid the H2/Ar method yields very hightemperatures, ranging from 290-336°C.
The CH 4 / C O 2 , and CO2 /Ar geothermometers(Giggenbach and Goguel, 1989) also yield hightemperatures, up to 340°C for Ilegedi (Table 3).
Gases from Darere, As’ela and Abakri also hinted atconsistent and high temperatures of at least 210°C,
and generally much higher. Of the nine gas
geothermometers used, only one yielded temperaturesless than 200°C for any Alid fumarole (Table 3). As
discussed above, contamination of the samples byshallow groundwater and organically derived materials
would tend to force underestimations of the originalreservoir temperatures. The clear implication of thegas geothermometers is that the subsurface reservoir
feeding the fumaroles is quite hot, likely over 225°C
and possibly over 300°C.

Table 3. Results of gas geothermometry in °C
T-DP T-HA T-MC T-CA T-FT T-C

T-HC T - S C

T-CN

Min

Max

ELG96-2

218°

336°

323°

296°

293°

243°

248°

176°

297°

176°

336°

ELG96-3

266°

336°

340°

278°

324°

241°

268°

210°

286°

210°

340°

ELG96-4

225°

290°

370°

280°

280°

243°

229°

206°

293°

206°

370°

ELG96-5

262°

334°

338°

276°

325°

241°

269°

203°

284°

203°

338°

ELG96-6

206°

323°

358°

299°

283°

243°

234°

166°

315°

166°

358°

DP: D'Amore & Panichi (1980)
C: Carbon Dioxide; Arnórsson & Gunnlaugsson (1985)
HA: Hydrogen-Argon; Giggenbach & Goguel (1989)
HC: H2+CO2; Nehring & D'Amore (1984)
MC: Methane-Carbon Dioxide;
SC: H2S+CO2; Nehring & D'Amore (1984)
Giggenbach & Goguel (1989)
CN: CO2/N2; Arnórsson (1987)
CA: CO2-Argon; Giggenbach & Goguel (1989)
Min = lowest temp. of all geothermometers
FT: Fischer-Tropsch; Arnórsson & Gunnlaugsson (1985) Max = maximum temp. of all geothermometers
Isotopic Compositions of Thermal, Surface
and Ground Waters
Isotopic compositions of Alid hydrothermal featuresand of groundwaters from the highland and lowland
regions surrounding Alid are plotted in Figure 4,
which also shows the trend line for meteoric waters
throughout the world (Craig, 1961). In this figure,most of the Eritrean meteoric waters lie along a"local" trend slightly above the world average
meteoric line. An important observation is that all of
the meteoric waters collected in the central highlands
are isotopically lighter (more negative values) thanthe meteoric waters collected in the eastern lowlands.
These results were expected, given the regional
rainfall patterns and topography. Lowland rains areproduced from clouds coming from the northeast, off
the Red Sea. Because the Red Sea itself is partially
evaporated, and contains δD values greater than
VSMOW and up to +9 ‰ (Craig, 1966), rains
derived from the Red Sea should also be high in δD.
In contrast, summer rains in the Eritrean highlands
are not sourced from the Red Sea, and the highelevation of this area results in rains that are slightly
depleted in D.
The stable isotopic compositions of thermal poolsand steam condensates from Alid are listed in Table 1.
The waters collected from thermal pools all areisotopically very heavy (δD up to +51 ‰) and plot
below the world meteoric line (Figure 4). Theirisotopic compositions result from <100°C
evaporation of relatively stagnant bodies of water thatare mixtures of local groundwater and steam
condensate. Reaction of the relatively stagnant, acidic
pool water with wallrock will shift the δ 18O of thewater to heavier values relative to the meteoric-water
line. This might be a contributing factor in theattainment of the extremely heavy δ 18O values forsamples 5 and 9 (Figure 4).

The sample collected at Humbebet (ELW96-10, Table
1) plots above the average world meteoric line
(labeled H in Figure 4) and contains the lowest TDSof any of the pools. Its odd isotopic composition isnearly identical to that of rain water that fell on Alid
during the early morning of Feb. 5 (δD = +12‰,
δ 18 O = -1.4‰) which is typical of isotopicallyenriched precipitation derived from the Red Sea. This
is not surprising given that ELW96-10 was collectedfrom a tepid seep rather than a hot evaporating pool,and may have relatively less thermal component.
The isotopic compositions of the condensed fumarolicsteam samples all lie above the average world
meteoric line (Figure 4). This is expected because
boiling solutions always produce steam that has aδ 18 O value less than that of the coexisting liquidwater. In contrast, the δD of steam is less than thatof the coexisting liquid water only at temperatures<220°C. Above 220°C the δD of steam is greaterthan that of the coexisting boiling liquid. In Figure 4
arrows radiating from the dot ELG96-2 (Table 1)
point in the direction of the composition that a
coexisting boiling liquid would have if steam ofisotopic composition given by point ELG96-2
physically separated from that liquid at the indicatedtemperature. Steam separation from a boiling liquid
at a temperature lower than about 200°C appears to beunlikely as this would require the reservoir water tobe more enriched in deuterium than any groundwater
identified in this study.
The relations shown in Figure 4 indicate that thefumarolic steam could have been derived by hightemperature boiling of water that is isotopicallysimilar to meteoric water that falls in the lowland
region. The isotopic composition of the steam
condensate also is compatible with a deep reservoir

60
Lowland meteoric water
Fumarolic condensates
Highland meteoric water
Thermal pools
Boiling Trends (arrow toward lq)50δ D (‰)

40
30

ELW96-5

World average meteoric line
ELW96-9

20

100°

200°
H

10

ELG96-2

250°
300°

0
-10

150°

1

Winter rains from NE (Red Sea)
Local meteoric line

Summer rains from SW (Atlantic)

-20
-10

-5

0

5

10

18

δ O (‰)
Figure 4. Stable isotope compositions of Eritrean meteoric waters and hydrothermal fluids on Alid. See
text for discussion. Full analyses and locations are listed in Clynne et al. (1996a).
fluid that is fossil Red Sea water, perhaps left overfrom a time when the Red Sea filled the Danakil
Depression. In contrast, the isotopic data indicatethat the fumarolic steam could not have been derived
from a boiling reservoir water that is isotopicallysimilar to the highland meteoric water. However, it
is possible (though not very likely) that the parent
reservoir fluid is highland water that was previously
evaporated at a low temperature (atmosphericconditions) before percolating underground where it
eventually entered the Alid hydrothermal reservoir.
Finally, there is little indication of mixing withconnate, metamorphic or mantle-derived water which
would be relatively lower in δD and higher in δ18O.
The δ 13C of CO2 in the Alid fumaroles varies from
-3.3 to -4.9 ‰ VPDB, consistent with a magmatic
source of CO2 , possibly mixed with carbon frommarine carbonate. Much of the H2S, and He emitted
from Alid fumaroles may also be derived, directly or
indirectly, by magmatic input from the subsurface
granitic intrusion that likely exists beneath Alid.

RESOURCE POTENTIAL
The Alid volcanic center contains about a dozen
identified areas with steaming ground or active
fumaroles. All of these geothermal emanations havetemperatures at or below the boiling temperature ofwater, but gas geothermometers indicate highsubsurface temperatures of 250°C or more. The
formation of Alid, a structural dome, by forcefulintrusion of silicic magma is likely to have createdabundant fractured zones beneath the volcanic center.
Young basaltic volcanism on the flanks of Alid
indicates that heat flow continues to be high, though
it is unknown whether the silicic intrusion stillcontains any molten material. All of these factors
can be interpreted as favorable for the potential of
Alid as a viable field for geothermal power
production. Future work should focus on identifying
any zones of high heat flow and permeability thatwould serve as ideal sites for geothermal wells.
ACKNOWLEDGMENTS
M. Clynne, W. Duffield, G. Kahsai, J. Smith, L.
Woldegiorgis and K. Woldemariam helped with fieldwork. We thank C. Kendall and L. D. White of the

USGS for analyzing the stable isotopes, and D.
Counce and F. Goff of Los Alamos National Lab forwater chemistry analyses. P. Lipman, F. Simon and
M. Nathanson made the initial contacts that started
this collaboration. T. Debretsion, H. Zerai and S.
Baire of the Ministry of Energy, Mines and Water
Resources gave their continuing help andencouragement. We appreciate the inspired logisticalefforts of A. Gartner. W. Evans and R. Mariner
carefully reviewed the manuscript. This project was
funded, in large part, by the U.S. Agency forInternational Development.
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