1. Manifestations of High-Temperature Systems
The characteristic active and nonactive surface manifes- tations of high-temperature systems are
discussed with respect to the topography of surrounding volcanic rocks
and their inferred heat sources. This allows recognition of three groups:
1. Manifestations of hydrothermal systems hosted by high standing volcanic centers (with an inferred lo-cal
cooling pluton as their heat source) 2. Manifestations of hydrothermal systems in rather flat terrain
transferring heat from inferred exten- sive hot crust or plutons 3. Manifestations associated with high-temperature
systems over extensive hot crustal rocks in a plate collision regime
A. Manifestations of Hydrothermal Systems Associated with High Standing Volcanic Centers
Many well-known high-temperature systems have this setting. Depending on the overall permeability of
the reservoir rocks and their surroundings and the extent of recharge (infiltration of groundwater), three
distinct types of reservoirs can be recognized. Here we use the terms ‘‘low,’’ ‘‘moderate,’’ and ‘‘high’’
permeability for rocks with average permeabilities, k, of the orders of 1 to 3, 3 to 10,
and 10 millidarcy (1 millidarcy
1 10 15 m2), respectively. In each case almost all the heat reaching the surface is carried by deeply
circulating meteoric waters that sweep heat from a source (usually a cooling pluton) and ascend under
free convection. If the k value of the reservoir rocks is high but that of rocks in the recharge area is
moderate, then this results in the formation of a liquid dominated system (liq- uid saturation of the
reservoir rocks, Sl, is between 1 and 0.7). If the k values of both the reservoir rocks and those in the
recharge area are moderate, than a two- phasemixturecandevelopinpartsofthereservoir(natu- ral two-phase
system) with 0.7 Sl 0.4. Where k in the surrounding area is low (i.e., there is little recharge) but
the k value of the reservoir rocks is high, then the dominant fluid in the reservoir will be vapor (0.4 Sl
0), i.e., a vapor dominated system. Inallthreecases,however,mosthydrothermalminer- als that form by
replacement in the reservoir do so from interaction between the host rocks and a liquid phase.
Further,rocksattheKawahKamojangandtheDarajat fields (both in Java) contain vein calcsilicate minerals
such as epidote, wairakite, and prehnite that clearly de- posited directly from liquid even though
boreholes in both fields discharge steam. This is because the perme- ability of the host rocks determines
whether a field will supply steam only or a two-phase mixture of steam and water to producing
boreholes.
1. Manifestations of Liquid Dominated Systems
A schematic diagram of a liquid dominated system be- neath an eroded volcanic complex (Fig. 3) shows
that all manifestations over the central part of the reservoir derive from ascending steam that is
discharged by fuma- roles and from minor steaming (hot) ground. Condensed
steam,withoxidisedH2Sgas,feedsminorhotacidsprings. Steam and CO2 can discharge together through
hot mud pools,orseparatedCO2 (rarelywithH2S)candischargeat the surface producing characteristic gas
discharge features that have local names: for example, kaipohan in the Phil- ippines, putizza in Italy.

2. Downslope, the ascending CO2 dissolves in perched groundwaters to produce warm springs that
discharge bicarbonate (HCO3) waters. Many liquid dominated systems with the hydrological setting
shown in Fig. 3 have concealed (subsurface) out- flows of neutral pH chloride waters originating from
the upper part of the reservoir. Silica deposition can partly seal the top of the outflows to descending
surface water. Where these outflows discharge in valleys or at lower elevations, hot springs and hot
pools occur (often boiling and occupying hydrothermal eruption or dissolution cra- ters). Some
manifestations here discharge a mixture of
hot water and steam (spouting spring or geyser). Further downstream, mixed chloride–bicarbonate
waters may discharge as warm springs and seepages, some with traver- tine (mainly CaCO3) deposits.
This lateral zonation of discharge features is a characteristic of these systems and was first described for
the Hakone system (Japan). Liquid dominated systems with the manifestations just described occur at
Palinpinon and Tongonan (Philippines). At Tongonan, another characteristic dis- charge feature occurs
near the toe of an outflow, a hot ebullient pool, whose ebullition is caused by ascending CO2 gas.
Manifestations over major outflows from a liquid dominated system occur also at Berlin (El Salva- dor)
and Momotombo (Nicaragua). Elsewhere, the lo- cationsofreservoirsbelowsteepandsometimesinacces-sible
terrain are not well known, and the existence of a liquid dominated system can only be inferred
from discharge features along an outflow. This occurs, for example, at El Tatio (Chile), the ‘‘type system’’
for this hydrological setting. Other prospects, known only from manifestations at the toe of subsurface
outflows, are Cisolok and Cisukarame (Java), whose outflows, based on geophysical evidence, extend for
more than 10 km. The prospects at Sipoholon (Sumatra) and Songwe (Tanzania) are only known from
their travertine depos- iting springs. Spouting springs (spouters) occur over an outflow of the Ulebulu
system (Sumatra) and at El Tatio. A seasonal geyser occurs at the toe of
small concealed outflow from the Rajabasa system (Sumatra).
2. Manifestations of Natural Two-Phase Systems
The fact that water in many liquid dominated systems beneath high standing volcanic complexes boils,
thus creating a two-phase zone (Tongonan, for example), does not mean these are themselves two-phase
systems; these can only be recognized where wells intersect a deep, coherent two-phase zone.
The Olkaria prospect (Kenya) was such a two-phase system prior toits exploitation. Extensive areasof
steam- ing ground with minor fumarolic activity occur there. Practically all heat from this huge reservoir
transfers to thesurfacebyascendingsteamthatcondensesatshallow depths, maintaining dominantly
conductive heat trans- fer to the surface with only feeble fumaroles. There are no significant liquid
discharges except for some minor warmspringsthatdischargesmallamountsofcondensate (Fig. 4).
Extensive steaming ground is also the dominant type of manifestation at the nearby Eburru prospect
FIGURE 4 Simplified model of a high-temperature steaming ground system with a natural two-phase
(coexisting liquid and va- por) reservoir beneath a broad volcanic center in a semiarid environment

3. showing the restricted variety of surface manifestations in this setting. The model has some affinity to
the Olkaria system (Kenya) and many other similar systems in the East African Rift Valley.
and over many other high-temperature systems associ- ated with young volcanic centers in the Kenya
Rift Val- ley. None discharge appreciable amounts of hot water. In part, this is due to the semiarid
conditions and the regionally deep water table. Fossil sinter occurs in this setting (shown in Fig. 4), at
Namarumu (N. Kenya), for example, indicating that when infiltration rates were higher in the past some
reservoirs were liquid domi- nated. Further north, in the Ethiopian Rift, another natural two-phase
system, Aluto, discharges not only heat from steaming ground and fumaroles, but also neu- tral pH
chloride water from hot springs and seepages that are located above an outflow at the foot of the young
volcanic dome that hosts the reservoir.
3. Manifestations of Vapor-Dominated Systems
A characteristic spectrum of manifestations occurs over these rare systems, for example, on the broad
volcanic massifs at Kawah Kamojang and Darajat (Java). Heat transfer is dominantly by steam ascending
from the top of a thick concealed layer with condensates (condensat
layer) of almost neutral pH bicarbonate waters (Fig. 5) that, together with intense alteration, may act as
a confining cover. Steaming ground and fumaroles are com-mon;
inaddition,minoracidcondensatesform‘‘muddy’’
hotpoolsandsmallacidlakeswithverylowmassdischarge. The low permeability of the rocks surrounding
these reservoirs prevents any significant mass outflow, al - though shallow, minor bicarbonate–sulfate
springs oc- cur about 15 km outside Darajat. There are no neutral pH chloride springs on the lower
flanks, and their ab- sence is probably the most characteristic feature of these systems. The same types
of surface manifestations also occur over Ketetahi (NZ), hosted by a young andesite volcano (Mt.
Tongariro), although no drillholes have yet tested this tentative classification.
Surficialandshallowhydrothermalalterationisexten- sive above vapor dominated systems but does not, in
itself, reveal whether or not the underlying reservoirs are occupied by vapor or a two-phase fluid: kaolin
clays dominate (kaolinite, halloysite, and more rarely dickite), but sulfur and sulfates are also common
(alunite, natro- alunite, gypsum, and a variety of hydrous phases, many ephemeral). Hematite and
hydrous iron oxides are also
typically present, as is silica residue, although fine - grained, black pyrite may persist near some thermal
features.Thealterationisusuallypervasiveandtheover- all process is dominantly destructive of the host
rocks rather than depositional. The Matsukawa field (Hon- shu, Japan) has extensive alteration (7 1.5
km) com- prising pyrophyllite and diaspore, alunite, kaolinite, and smectite zonally distributed around
the main structural feature of the field. This alteration records former ther- mal activity as the present-day
thermal manifestations consist of only a few areas of warm ground.
B. Manifestations of High-Temperature Systems in Moderate Terrain

4. The heat sources for these systems appear to be exten- sive, hot crustal rocks whose thermal energy is
main- tained by the following:
1. Partial melting within the ductile upper crust (set- ting for the NZ systems hosted by young rhyolitic
rocks and also for some at Yellowstone, Wyoming)
2. Deep, laterally aligned crustal and dyke intrusions in rift environments beneath systems hosted in ba-salts
(e.g., systems over spreading centers such as Iceland) or sedimentary rocks (Baja California) 3. Deep
cooling plutons (relics of an older subduc- tion cycle?) now distant from an active subduction zone
In all these settings there are systems with one of the three reservoir types mentioned in the previ ous
para- graph. An additional reservoir type with the second set- ting listed is the nonconvecting brine
system.
1. Manifestations of High-Temperature Systems in Moderately Steep Terrain Underlain by Extensive Hot
Crustal Rocks
The terrain surrounding these systems is not steep, and young volcanic cones are usually peripheral to
the geo- thermal reservoirs. Because of their high recharge rate and the terrain, the hot fluids can
ascend close to or reach the surface. Any zonation of discharge features, if it occurs, is not controlled by
lateral pressure gradients created by differences in relief. Manifestations of dis-chargingsteam(
fumarolesandsteamingground),conden-
FIGURE 6 Conceptual model of a liquid dominated system standing in rather flat terrain; the heat source
is an extensive layer of hot crustal rocks that contains some partial melts and host intrusions. The model
has some similarity to the Wairakei system (NZ). (Modified from Hochstein, 1990.)
sates and noncondensable gases (minor acid springs and mudpools) can, therefore, occur close to others
that dis- charge neutral pH chloride waters (clear hot pools, hot springs). This thermal regime favors
hydrothermal erup- tions, if the liquid is very close to boiling in the shallow subsurface. The setting also
favors the discharge of two- phase boiling fluids as large geysers. Outflows are rare because the
horizontal pressure gradient is very small (flat terrain). Systems with the largest natural heat dis - charges
(up to 500 MW at Waiotapu, NZ; see Fig. 1) occur in this setting with many prospects discharging
300 MW. An idealized section through a liquid domi- nated reservoir
with its spectrum of manifestations is shown in Fig. 6. Thesilicasinterthatdepositsfromdischargingwaters
showsawidevarietyofforms.Spouters(spoutingsprings)
orgeysersdepositnodularsilicacalledgeyserite.Terrac- ing is a common feature, with individual steps
having heights ranging from a few millimeters to 2 meters (as
attheRotomahanasystemnearRotorua,NewZealand, destroyed by a volcanic eruption in 1886). The steps
typically have lips to them and are usually closest to- gether on the steepest slopes. The flow paths of
the cooling waters change constantly through deposition of

5. ilica. Other silica varieties include banding, palisade structures, and wave forms. The last have the form
of barchan sand dunes, but their crests grow toward the flow direction of thermal water. Silica sinter is
usually hard and white, but it also may be porous, pale yellow, and friable. Microbiological activity plays
a major part in silica deposition in some areas and bacteria and plants may be preserved as fossils. Sinter
covers several acres at Norris Geyser Basin (Yellowstone) and Waiotapu (New Zealand). When first
deposited, silica sinter is opaline, but it transforms with time, first to cristobalite and finally to quartz,
pro- gressively losing water as it does so. Metal rich deposits precipitate with the silica from some
springs: for exam- ple, ore grade gold and silver plus appreciable arsenic, antimony, and thallium, as are
now precipitating at the Champagne Pool, Waiotapu. Sinter with locally 3 wt% tungsten is depositing at
Waimangu (New Zealand). The heat output (Qs) of all (20) high-temperature hydrothermal systems in
the Taupo Volcanic Zone (TVZ), over an active arc segment about 200 km long, is probably three times
greater than the extrapolated cumulative heat discharged (Qv) from all its volcanic centers. The
Wairakei system was liquid dominated prior to its exploitation and its manifestations included all those
listed earlier. Impressive manifestations also occur over other liquid dominated systems nearby, such as
at Waiotapu, which exhibits an apparent reversed hy- drological zonation whereby the deep reservoir
fluid dis- charges from a slightly higher large hot pool (an old hy- drothermal eruption crater) that is
surrounded by acid springs at lower elevations. Acid leaching of thick pumice has produced a number of
steep sided or over- hanging walled dissolution pits, some with acid conden-satesinthem.
Araremanifestationwhichoccursoverthe Rotokawa system is a cold acid lake into which a
vigorous flux of H2S dissolves. The Whakarewarewa system is well known for the occurrence of large
geysers, clear hot (near boiling) pools, andwidespread sinter deposits. Natu- ral two-phase systems also
occur (e.g., Broadlands- Ohaaki)buthavefewornomajordischargefeatures.The rate of natural heat
discharge (Qs) from Broadlands (be- foreexploitation)wasonly75–100MWcomparedwith that from the
other three prospects just cited (each with Qs values between 300 and 500 MW). A large number (up to
eight) of probably liquid domi- nated high-temperature systems occur in the Yel- lowstone National Park
(Wyoming, USA). The heat source here is a mantle plume extending into the base of the crust, which it
has heated and partially melted. The total thermal output from all the systems at Yel - lowstone is
therefore large, its magnitude being proba-bly
half that of the Taupo Zone systems. Many geother- mal systems in Yellowstone show the
characteristic thermal manifestations that occur over the liquid domi- nated systems, including large
geysers, clear boiling pools, and extensive sinter deposits at the Norris Geyser Ba- sin, for example.
Deposits of calcium carbonate occur in the outflow areasofhigh-temperaturesystemsbutarealsoassociated
withlowertemperaturesystems.Thecarbonatesdeposit as a
consequence of loss of CO2 from the discharging water in which it was formerly dissolved. Calcite is the
dominant carbonate, but where loss of CO2 is very fast, aragonite occurs instead. The reason for this is
not known. The calcium carbonate forms deposits that re- semble those of silica sinter, e.g., bedded or
layered, terraced or forming ridges and even columns up to 3 m high. Extensive and beauti ful deposits
of calcium car- bonate occur at Mammoth (Yellowstone), but traver- tine very commonly surrounds
many small springs or pools.

6. 2. Manifestations of High-Temperature Systems in Crustal Spreading Environments
Many geothermal systems are hosted by young basaltic rocksinactiverifts,suchasIceland.Thesehigh-temper-
ature systems derive their heat from a set of dykes or sills. Generally, their manifestations are
less vigorous than those listed in the previous paragraph. Acid alter- ation and sinter deposits are not
extensive; the salinity of thermal water in prospects away from the ocean is low, generally 1 g/kg of
total dissolved solids (TDS). A few liquid dominated systems have geysers, as at Hau- kadalur, including
Great Geyser itself which gave its name to all similar intermittently discharging features; the term comes
from the Viking verb ‘‘gjose’’ (to gush). Liquid and two-phase geothermal reservoirs occur to- gether at
Krafla and Namafjall, as indicated by the initial enthalpy of fluids discharged from wells there. Near the
coast, infiltration of sea water is shown by the high mineral concentration (up to 20 g/kg) in hot water
from the Svartsengi and the Reykjanes high-T reser- voirs (both liquid dominated). Several wells at Krafla
discharged fluid with a magmatic signature as a result of an intrusion and eruption that occured there in
1975. The term ‘‘geothermal brine’’ has been used for liq- uids with high total dissolved solids (TDS).
This term applies, in general, to liquids with TDS 20 g/kg (i.e., more
saline than seawater); hot brines with TDS 100 g/kg are called
‘‘hypersaline’’ brines. Elongate, deep ( 6 km) intrusions likely heat a
num- ber of the geothermal systems in Baja California (Mex-ico)
andtheImperialValley(California);theseareliquid dominated and mainly high-temperature systems.
The fluviatile sediments host reservoirs that contain a brine or ‘‘hypersaline’’ brine that arguably derives
its high salinity by dissolving surrounding evaporites. Because of their high density, the brines have little
surface dis- charge, so the dominant mode of heat transfer is by conduction. An example is Cerro Prieto
(Mexico) where, prior to exploitation, some heat reached the sur- face via small patches of hot,
steaming ground and conduc- tive losses were about 30 MW. Other brine systems in the Imperial Valley
(USA), such as Brawley, have no surface manifestations whatsoever. Minor steaming ground and small
mud pots occur over the ‘‘hypersaline,’’ stagnant high-temperature reservoir of the Salton Sea (also
Imperial Valley). This is similar to the ‘‘hypersa- line’’ system with a similar geological setting, the Ces-ano
prospect, in Central Italy. Hypersaline brines derived from the lateral infil - tration of seawater occur
in the Lake Assal system (Djibouti), hosted by basalts, over an incipient rift with extensive evaporites at
its surface. At Dallol (Danakil Depression, N. Ethiopia) meteoric water enters a salt dome by advection
and dissolves salt almost to the limit of NaCl solubility. The hypersaline brine is heated by
conductionfromacooling intrusion.Thisproduceshot, hypersaline brine pools (T
110C); the TDS of the brine can be up to 420 g/kg, high in Na, K, Mg, and Cl. Here an overflow of hot
brine causes the formation of salt mounds.
3. Manifestations of High-Temperature Reservoirs Hosted by Sedimentary Rocks
Another group of high-temperature systems occur in
sedimentarytolowgrademetamorphicrocksinasetting with deep cooling plutons (perhaps the product of

7. an older subduction cycle). A hot water–CO2 gas domi- nated reservoir occurs, for example, at Ngawha
(NZ), which is several hundred kilometers distant from the presently active subduction zone, but lies
above a sub- duction zone that was active more than 10 Myr ago. Quaternary basalts occur nearby but
are not part of the high-temperature reservoir, which comprises Mesozoic greywackes covered by about
600 m of almost imperme- able (to water) sediments. Vigorous upflow of CO2 with mercury occurs
throughout the entire area and through several cold lakes, but most heat (50 MW) transfers to the
surface by conduction. The two largest systems known have similar settings and are both vapor-dominated,
namely Larderello (It- aly)andTheGeysers(California).Thereiscircumstan-tial
evidence that steam, which ascended to the surface prior to the exploitation of both reservoirs,
came from an extensive condensate carapace similar to that shown in Fig. 5. At Larderello, large
amounts of boron were mobilized by vapor from marine sediments and depos- ited at the surface in
borax ponds, the laguni that were once mined. Fumaroles discharging B-rich steam have been described
as soffioni. Themagnitude of natural heat transferandthatbysteamwasunfortunatelynotassessed at
either place before exploitation began.
C. Manifestations of Systems over extensive Hot Crustal Rocks in a Plate Collision Environment
The heat sources for all the high-temperature systems mentioned so far involve mobilization of upper
mantle meltsandfluids.Acharacteristictracecomponentwhich reveals the involvement of subcrustal melts
is the 3He isotope.Itsrolecanbeassessedfromthe 3He/4Heratios, R, of gases, normalized with respect to
its atmospheric ratio. Geothermal gases from all high-temperature sys- tems described so far have R
values that are 1 to 2 orders ofmagnitudegreaterthantypicalvalues(0.15)ofgases discharged by low-temperature
systems far distant from activemargins,e.g.,overa‘‘cold’’continentalcrust(with no volcanic
history). However, a number of high-temperature systems, hosted by metamorphic or sedimentary
rocks, occur in Tibet and Kashmir that discharge steam and gases with anomalously low 3He/4He ratios
(R 0.15). These sys- tems transfer heat derived from young granites, proba- bly generated by shear-heating
from plate collision. At one, Yangbajing (Tibet), temperatures
250C have been measured in drillholes. Hot geothermal fluids here
ascend beneath the flanks of a high mountain range (Inner Himalayas) and discharge as a concealed
outflow within a wide valley. The zonation of manifestations is again controlled by the relief of the
terrain. Traces of acid condensates and extensive acid steam alteration (alunite, residual silica) occur
over the flanks near the inferred upflow. There is no significant discharge of liquid over the outflow,
whose top is sealed by extensive deposits of silica and carbonates. Only at the toe of the outflow is there
a spectrum of manifestations that discharge neutral pH chloride waters, namely; boiling pools, hot
springs, spouting (two-phase) boiling springs, and some steaming ground. A number of large
hydrothermal eruption craters occur further down the valley, where there are also massive deposits of
travertine. A similar,

8. although less obvious, zonation of manifestations occurs at the nearby Yangyi prospect, another system
with temperatures 200C in 500-m deep wells. From de- scriptions of
their discharge features and chemical geothermometry, it is inferred that at least another dozen such
high-temperature systems occur in Tibet. These appear to be associated with 30 to 50 km wide,
elongatebandsofhot,uppercrustalrocks(‘‘heatbands’’) produced by shear heating resulting from lateral
move- ments of large crustal blocks. Also impressive are the intermediate-temperature systems within
the same heat bands that occur at more than 100 places (see later dis- cussion).
IV. Manifestations of Intermediate- and Low-Temperature Systems
Intermediate- and low-temperature systems occur in many different geological and hydrological
settings, both along and outside active plate margins. It is often difficult to dis tinguish them from high-temperature
sys- tems, since standard chemical and isotopic geothermo- meters, based on slow
equilibration processes (i.e., Na/K and most gas geothermometers), give tempera- tures reflecting
conditions much deeper than, say, 1 km depth. However, faster equilibrating fluid/rock interac- tions
allow application of the silica (assuming equilibra- tion with chalcedony) and the Mg/K
geothermometers. These,andthelackofsignificantshiftsinthe 18Oisotope values, can be used to predict
likely temperatures in the upper fewkilometers of suchreservoirs. Becauseof their lower temperatures
and less buoyant fluids, their natural heatoutputsarealsolower.Theythushavefewersurface
manifestations. All intermediate temperature prospects have liquid dominated reservoirs that can
extend to great depths ( 5 km). Fumaroles and steaming ground are ab-sent,
although boiling springs occur in some; however, the maximum fluid discharge temperatures are
usually below boiling. Although afew intermediate-temperaturesystems are located in active and
inactive volcanic arcs, where they areheatedbyconvectingplumesofhotwater,themajor-ityderivetheirenergyfromdeeplypenetratingmeteoric
water that ‘‘sweeps’’ heat from the hot but brittle
upper crust into a discharge area (‘‘sink’’), often via fractures (‘‘fracture zone’’ systems). According to
their geological
and hydrological settings, most intermediate-tempera- ture systems can be grouped as follows:
1. Systems over active and inactive volcanic arcs, i.e., hosted by volcanic rocks 2. ‘‘Heat-sweep’’ systems
in active rifts and at plate collision boundaries 3. Fracture zone systems hosted by sedimentary or
metamorphic rocks
A. Intermediate-Temperature Systems over Volcanic Arcs
A few intermediate-temperature systems have this set- ting but they are everywhere outnumbered by
the high- temperature systems (1:10 in New Zealand and Su- matra). Some intermediate-temperature
systems are probably decaying high-temperature systems over a waning heat source. A few dead
systems host epithermal mineral deposits, but most of these were likely once high-temperaturesystems.
Awell-studieddeadreservoir is at Ohakuri (NZ), which was probably a high-temper-aturesystemabout100kyrago.
Stillactiveintermediate- temperature systems occur at Horohoro and
Atiamuri (NZ). Their surface manifestations are not impressive. Minor silica sinter and a boiling spring
occurs at Atia- muri, where drilling shows that the temperature at a depth of about 1 km is only 175C. At

9. Horohoro, there is minor hot spring activity and cold altered ground. The chemical composition of these
thermal waters points to their being appreciably diluted with ground waters. Hot crustal rocks beneath
an extinct arc segment can take several million years to cool conductively. In such a setting, a fe w
intermediate-temperature systems may survive, for example, at Te Aroha (Coromandel, NZ), which is
hosted by andesites along an arc segment that became extinct 5 million years ago. Here little heat (1
MW; see Fig. 1) is transferred by a few hot springs and CO2-rich fluids that deposit aragonite. Over the
sameextinctarcsegmentoccurstheKaitokehotsprings (Great Barrier Island, NZ), which are similar to those
at Te Aroha.
B. ‘‘Heat-Sweep’’ Systems
The reservoirs of ‘‘heat-sweep’’ systems may be of vol- canic or sedimentary rocks. Fracture zone
systems can develop in a part of the crust with anomalously high
heat flow but not associated with volcanism, and in con- tinental rifts. Intermediate-temperature
systems have not been described in detail, but some, however, have been explored by drillholes in the
hope that they were high-temperature systems. The number of examples that follow is therefore small
and restricted to a few of the better known prospects.
1. Heat-Sweep Systems in Active Rifts
The East African Rift Valleyis underlain along its entire length by hot crustal rocks heated mainly by
intrusions. Rain infiltrating over its higher standing rift shoulders favors the development of large heat -
sweep systems which discharge hot fluids along the axis of the arid rift valley (see Fig. 7). The large
hydraulic head sets up its own convection pattern, that is, a pattern of ‘‘forced
convection.’’Ifhotfluidsascendthroughevaporitesthey discharge hot saline water in springs at 40 to 80C,
for example,alongthemarginsofLakeNatron(Tanzania), Lake Magadi (Kenya), Lake Afrera, and Lake Asale
(bothinnorthernEthiopia).Evaporationproduceslarge surface deposits of crystalline carbonates of sodium
(trona) at the first two lakes. The compositions of brines ofshalloworiginisnotgovernedbytemperature-depen-
dent equilibria; however, silica and isotope data indicate
thatthedeepfluidtemperaturesaremostlikely170C. The area affected by such heat sweeps is large
( 100 km2), which explains the high heat outputs (of the order of 100
MW) of the first two examples cited; the anoma- lous position of the Lake Natron heat output is shown
FIGURE7 Conceptualmodelofaheat-sweepsystem(forced convection) producing intermediate-T
reservoirs within an active continental rift. The model is based on lake systems such as those in northern
Tanzania, Kenya, and Ethiopia.
in Fig. 1. Theyare the largest intermediate-temperature systems known. Elsewhere along the East
African Rift, where evapo- rites are thin or absent, less saline hot water discharges into lakes and sinks.
Lake Bogoria (Kenya) has a heat output from several boiling springs and ebullient pools of the order of

10. 100 MW. Cation geothermometers clearly point to the mean reservoir temperature of this sweep
system as being 180C. The surface discharge features could be mistakenly interpreted as being
manifestations of a high-temperature system. Manifestations of inter- mediate-temperaturesweepsystemsoccuralsoinnorth-
ern Kenya and the Southern Lakes District of Ethiopia.
Several systems in the Basin and Range Province of the United States are probably heat-sweep systems
(Soda Lake, Beowawe, and Stillwater in Nevada, for example). Their manifestations are mostly minor.
2. Heat-Sweep Systems in a Plate Collision Setting
In Tibet, Kashmir, and west Yunnan, there are several intermediate-temperature systems, as indicated
by the chemical geothermometry of their discharge fluids. The topography, high infiltration (some from
snow melt), and large hydraulic heads over recharge areas favor the development of a heat-sweep
hydrology over crustal strips heated by shear deformation (‘‘heat bands’’). A good example is the Naqu
prospect in Central Tibet characterized by hot springs (T max
60C), which de- posit travertine from waters that the K/Mg geothermo- meter indicates to be 130C at
depth. Laduogang (near Yangbajing) is another intermediate-temperature system explored by drilling.
Ebulliant pools discharging bicarbonate waters here locally deposit carbonate nod- ules (pseudo-geyserite).
In the foothills of the Himala- yas, similar systems occur (Manikaran in northern In- dia, for
example). Subsurface temperatures as hot as 150C are indicated by the K/Mg geothermometer for most
systems in Tibet depositing travertine. This is also a characteristic product of many extinct systems
there.
C. Fracture Zone Systems
Deep-reaching heat-sweep systems can also develop in terrain with rather flat topography if fluids
ascend via a deep, highly permeable ( 100 millidarcy) frac- ture zone in a
brittle crust of high heat flux ( 70
mW/m2).Suchhighfluxesoftenoccurwherethickgran-
FIGURE 8 Conceptual model of a heat sweep system (free convection) discharging hot fluids through a
deep reaching frac- ture zone (fracture zone system). The heat source gives a higher than normal
terrestrial heat flow; this setting can occur far away from active margins and volcanism. The model is
based on the Fuzhou system in South China.
ites provide radiogenic heat (see Fig. 8). Fracture zones near the surface may be ‘‘narrow’’ (100 m) or
‘‘wide’’ ( 200 m). A good example of the former is the Fuzhou prospect in
southern China; the San Kamphaeng pros- pect in northern Thailand is an example of a wide frac- ture
system. Another dozen or so fracture zone-sweep systems occur in northern Thailand (e.g., Fang) and a
few within the coastal strip of southern China (e.g., Zhangzhou). The dominant manifestations of all

11. these systems are hot springs, and occasional hot pools, both with minor encrustations of sinter and
travertine; alteration of the surrounding rocks is rare. Conduction contributes to
theheattransfer,whichcommonlyliesbetween3and10 MW. Prospects with indicated high Na/K
equilibrium temperatures (some greater than 225C) can also be misinterpretedasbeing‘‘high -
temperaturesystems,’’al- though their low heat outputs and isotopic signatures (no significant 18O shift)
show them to have intermedi- ate-temperature reservoirs.
D. Manifestations of Low-Temperature Systems
A large number of convective low-temperature systems occur in geological settings that favor the
development of structurally controlled, smaller heat sweep systems.
Mostdischargewarm(i.e.,40C)waterfromanetwork of fractures that constitute their reservoir.
Stratigraph- icallycontrolledsystemsinsedimentaryrocks,discharg-ing
over an anticline, for example, are rare. Since tem- peratures in their upper reservoirs are low (i.e.,
125C), buoyancy forces, and hence the heat output of these systems, are also low (typically between 0.1
and 3 MW). Rock/fluid interactions occur at a much slower rate and mineral –fluid equilibrium is seldom
attained; 18O shifts do not occur. The ‘‘true’’ low-temperature systems are rare in volcanic arc settings,
whereas they are common where topography and tectonics allow small heat-sweep systems to develop,
for example, along themarginoflargeriftvalleylakes,suchasLakeMalawi (East Africa). Low-temperature
systems can form even in brittle crust with an average terrestrial heat flux (60 mW/m2), corresponding
to temperature gradients of only 25 to 30C/km, although they are more com- mon where fluxes are
higher. In the Basin and Range Province and Colorado Plateau in the western United States, there are at
least 900 low-temperature systems, which thus outnumber the intermediate and high-tem- perature
systems by 20:3 and 20:1, respectively. In the Himalayan area there are 500 low-temperature systems
that discharge fluids hotter than 40C; they outnumber the intermediate and high -temperature sys- tems
by 20:6 and 20:1, respectively. Despite the worldwide occurrence of low-tempera- ture systems, their
surface manifestations differ little, consisting usually of warm (T 40C) and sometimes hot springs (T
40C) without any surface alteration or deposits other than travertine.
The compositions of the discharge fluids reflect the sweep depth and mixing con - tribution from saline
pore fluids in the sedimentary host rocks. Numerical modeling shows that these systems are longer-lived
than all others. The development of free convection in a fracture network of a low-temperature system,
for example, can take a million years, whereas full convection within permeable high-temperature res-ervoirs
may develop in only 10,000 years. Because of their low temperatures, mineral deposition is so
slow that it does not block the fluid-flow channels, and they are likely to be long-lived systems.
Manifestations of a few low-temperature systems dif- fer, but include warm, often tepid springs that
deposit travertine, for example at Acque Albule near Rome (Lacus Albulus), which was the main quarry
for ancient Rome, and distant from active volcanoes. In western Turkey, Bursa and Pamukkale have a
similar setting. However,CO2 gasalsodischargesclosetolow-tempera- ture systems depositing travertine,
and these could be described as ‘‘moffete.’’ Because of the high solubility of CO2 in cold water, carbon
dioxide rising from the mantle,asrecognizedbyitstypicallyhigh 3He/4Heratio, can dissolve at the bottom
of cold lakes, such as at Lake

12. Nyos (Cameroon) and the Laacher See (Germany), which thus act as ‘‘gas’’ traps.