The Hydrothermal Framework of Geothermal Systems

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The Hydrothermal Framework of Geothermal Systems
Joe Moore Energy Geoscience Institute
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Hydrothermal Alteration(WHY BOTHER?)

• As the fluids circulate, they react with the
  rocks. The hydrothermal minerals they produce
• Influence the geophysical signatures of the rocks
  through changes in their densities, porosities,
  permeabilities, and electrical properties
• Provide information needed during drilling
  operations (casing points)
• Can be utilized to guide exploration and
  development by providing information on
  temperature distributions, thermal gradients,
  fluid compositions, permeable zones
• Provide spatial information with respect to the
  location of the cap rock and zones of discharge
  and recharge

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Factors Influencing Hydrothermal Alteration
(after Browne, 1978)

• Temperature
• Pressure
• Rock Type
• Permeability
• Fluid Composition
• Duration of Activity

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Useful Tools

• Binocular microscope and chipboards
• Thin section petrography
• X-ray diffraction analyses (whole rock and clay
  (lt5 micrometers) fractions)
• Scanning electron microscopy (textural
  relationships and mineral compositions)
• Electron microprobe analyses (mineral
  compositions)
• Dating techniques (14C, 40Ar/39Ar, K-Ar, fission
  track, U-Pb)
• Fluid inclusions measurements (temperature, fluid
  composition, evidence of boiling)

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• All geothermal systems are characterized by
• A heat source (magmatic or non-magmatic)
• Convective upflow
• Recharge by meteoric waters
• Deep mixing with meteoric waters
• Boiling and steam migration
• Outflow of the deep fluids to the surface or
  other hydraulic base level

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Conceptual Model of an Andesitic Volcano
(Henley and Ellis, 1985)
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THE UPFLOW ZONE FROM THE HEAT SOURCE TO THE
SURFACE
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Granodiorite
KRH 2-1 9850 ft.
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DIORITE
Field of view is 2.4 mm
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Duration of Geothermal Activity

• Life spans of geothermal systems are poorly
  known
• Few geothermal systems have been dated directly
• Hochstein and Browne suggest that
• Ohaaki-Broadlands (NZ) has been active for at
  least 300,000 y
• Kawerau (NZ) for at least 280,000 y
• Icelandic geothermal systems for lt 250,000 y
• Individual pulses in volcanic systems may be very
  short lived, with long periods of quiescence
• Dating of hot spring deposits and altered rocks
  at Karaha-Telaga Bodas, Tiwi and Coso suggest
  these pulses may last less than several tens of
  thousands of years

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Biotite Zone
Tourmaline (gt400oC)
Garnet/Pyroxene (gt325-350oC)
Pyroxene
Garnet
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Biotite Zone (gt300-325oC)
Hornfels
Pyroxene
Pyroxene
Biotite
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Significance of Biotite Zone Assemblages

• Tourmaline, diopside, biotite, actinolite, garnet
• Accessory epidote, quartz, illite, magnetite,
  pyrite, Cu, Zn, Pb sulfides, chlorite, adularia
• Indicative of high temperature alteration within
  the contact zone of a cooling magma
• Neutral pH fluids
• Boron (in tourmaline) provides evidence of the
  influx of magmatic volatiles
• First appearance of tourmaline expected within
  600 m or less of intrusive contact

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Inner Propylitic Zone
Actinolite, garnet, epidote and pyrite gt280oC
py
ep
cal
gar
act
act
act actinolite cal calcite ep epidote
gar garnet py pyrite
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Outer Propylitic Zone (gt250oC)
Epidote
ill
ep
py
vug
ep epidote ill illite py pyrite
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Significance of Propylitic Zone Assemblages

• Epidote, actinolite, garnet, chlorite, illite,
  calcite
• Accessory pyrite, Cu, Zn, Pb sulfides, adularia,
  albite, prehnite, wairakite
• Indicative of temperatures gt250oC
• Neutral pH fluids
• Commonly marks the top of the reservoir and base
  of the cap rock

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The Effects of Boiling

• Boiling is common in the propylitic zone,
  although it can occur anywhere. Zones of enhanced
  permeability are often sites of boiling. It
  effects
• Gas contents of the fluid
• Mineral precipitation
• The formation of steam-heated waters

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Mineral Deposition Resulting from Boiling
Bladed calcite
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Chalcedony Intergrown with Propylitic Assemblages
ep
chal
act actinolite chal chalcedony ep
epidote py pyrite
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Phyllic (illite (gt220 to 250oC) and Argillic
(lt220oC smectite and interlayered
illite-smectite and chlorite smectite) Alteration
ill
qtz
py
ill illite py pyrite qtz quartz
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East-West Cross Section Bulalo, Philippines
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First Appearances of Key Minerals Karaha-Telaga
Bodas
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Significance of Argillic Assemblages

• Smectite and interlayered illite-smectite and
  chlorite- smectite,
• Accessory zeolites, calcite, pyrite
• Argillic alteration is indicative of temperatures
  lt220oC
• Argillically altered rocks typically form low
  permeability cap rocks over geothermal reservoirs
• Argillically altered rocks are electrically
  conductive and are characterized by low
  electrical resistivities

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RECHARGE ZONES
(Henley and Ellis, 1985)
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Advanced Argillic Alteration
Photos by J. LaFleur and D. Foley
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Evidence of Acid-Sulfate Waters
cal
anhy anhydrite cal calcite fluor
fluorite py pyrite qtz quartz tour
tourmaline Alunite, kaolinite (lt180oC)
pyrophyllite, diaspore (gt250oC) are diagnostic
of acid sulfate fluids
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Anhydrite After Actinolite
T-21001.3 m
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Calcite Vein
Coso 64-16
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Wairakite After Anhydrite and Calcite
K-21 1546.9 m
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Relationship of Wairakite to Calcite and Anhydrite
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Significance of Descending Waters

• Anhydrite and calcite form at shallow depths
• Accessory smectite and mixed layer clays, pyrite
  contemporaneous silicate minerals (quartz)
  notably absent
• Wairakite typically forms within the propylitic
  zone
• Accessory chlorite, pyrite, calcite, anhydrite,
  illite
• Alteration spans a broad range of temperatures to
  300oC.
• Descending waters progressively seal fractures
  downward, increasing the thickness of the cap
  rock
• Anhydrite and calcite caps are frequently
  umbrella shaped over the central upflow zone

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Rock Types

• Volcano Hosted Systems
• Tuffaceous deposits (pyroclastic and epiclastic
  deposits, lahars)
• Lava flows
• Sediments (minor, in local basins)
• Intrusions (commonly form thin dikes and sills,
  rarely large stocks)
• Continental Interior
• Regionally metamorphosed sedimentary sequences
  and intrusive complexes)

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Rock Types
Tuffaceous deposits Alteration to clays can
begin shortly after deposition at very low
temperatures resulting in rocks with low
permeabilities At low to moderate
temperatures (below propylitic zone) rocks
commonly behave in a ductile fashion Poor
reservoir rocks until they become brittle when
affected by high temperature alteration Lava
flows Can behave in a brittle fashion even at
relatively low temperatures Rubble flow tops
and bottoms may have high porosities but be of
limited extent and be poorly connected to major
through going fractures
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Relationship of Rock Type to Fracturing
Fractured Lava Flows
Unfractured Pyroclastics
Mineralized fracture
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Conclusions

• Most geothermal systems can be described using
  relatively few simple conceptual models.
• These models can provide the basis for
  interpreting geophysical and geochemical data.
• The primary factors controlling hydrothermal
  alteration are temperature, rock type and
  permeability.
• High permeability channels (past and present) are
  frequently associated with hydrothermal
  breccias boiling repetitive fracturing and
  multiple vein sets.
• These zones may be reactivated and serve as fluid
  conduits.
• Cap rocks are characterized by smectite, mixed
  layer illite-smectite, anhydrite and calcite.

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Conclusions

• Cap rocks are characterized by smectite, mixed
  layer illite-smectite, anhydrite and calcite
  formed by a combination of low temperature
  upflowing fluids and later down flowing steam
  heated waters.
• These argillically altered caps are electrically
  conductive (low electrical resistivities) in
  contrast to the resistive propylitically altered
  rocks of the reservoir.

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Thermal Stabilities of Common Geothermal Minerals
Henley and Ellis, 1983
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THE END