Ore Deposits

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Geochemistry in geothermal exploration and
management
Greg B. Arehart Department of Geological
Sciences University of Nevada, Reno
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Sampling natural hot springs
Webre steam separator
Production well rig

• Geochemistry is an integral part of any
  geothermal project from grass-roots exploration
  through development and production to
  environmental issues

Proper sampling is critical to interpretation!
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Fluid geochemistry
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Subsurface geothermometry

• There are a number of geothermometers that can
  yield information about the temperature and
  processes of the deep reservoir, without having
  to drill.

Natural geyser, Main Terrace, Steamboat, NV 1986
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Chemical geothermometers
Surface measurements reflect subsurface processes
IF the water is brought to the surface relatively
rapidly Based on solubility affected by
boiling and dilution Empirical (based on
exploited geothermal areas) For T lt
250?C Quartz, no steam loss Na-K-Ca
Also, several other chemical geothermometers
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Gas geothermometers

• The relative proportions of gas in a geothermal
  system are a function of temperature
• logCO2 4logH2 - logCH4
• -5.922 - 13178/T 0.01959T

Wairakei, NZ flash plant
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Isotopic geothermometers

• Less dependent on water-rock interaction
• May be affected by mixing/boiling problems

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Soil and soil gas geochemistry for exploration

• Vapor-borne and water-borne species may interact
  with near-surface horizons to produce distinct
  geochemical anomalies over permeable and upflow
  zones (e.g. ammonia, boron)

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Soil and soil gas geochemistry

• Soil gases (CO2, H2S, He, organic compounds,
  etc.) are more mobile than liquids

Mokai geothermal field, New Zealand
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Origin and age of fluids

• Radiogenic and stable isotopes can give an
  indication of the age and origin of waters

Such data are important for understanding
recharge issues and permeability/ porosity of the
aquifer
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Gas geochemistry
Total gas content, and changes in content, can be
useful in assessing overpressuring (blowouts,
hydrothermal eruption)

• Inert noble gases (in fluid inclusions and in
  fluids)
• He or N indicative of magmatic sources
• Ar indicative of crustal or atmospheric sources

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Fluid sources
White Island, New Zealand

• Important in understanding the source of heat
  (magmatic vs crustal circulation) and has
  implications for the duration of systems

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Geochemical tracer tests
Helps develop reservoir model in terms of fluid
flow paths and timing Natural tracers vs.
synthetic tracers
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Reservoir modeling

• Computer modeling based on spatial and sequential
  sampling will identify

• changes in flow regimes (also sources, pathways)

• changes in fluid/gas geochemistry that will have
  an effect on operating systems (corrosive
  components, non-condensible gases) or the
  environment

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Scaling issues in production
? calcite or silica may clog pipes
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Baseline environmental studies
Champagne pool, NZ
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(No Transcript)
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Geothermal system development in the broader
context of the evolution of the Great Basin
Little recent volcanism in the Great Basin makes
our geothermal systems somewhat unique
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Regional origins of geothermal systems

• Noble gas, Sr, and Nd isotopic compositions of
  fluids and rocks may indicate relative
  contributions from crustal versus mantle sources

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Regional geology, geochemistry, and geophysics
will lead to a better understanding of the
relationship between extension rates, magmas, and
heat flow
Regional origins of geothermal systems in the
Great Basin
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Frontier research areas
Use of REE geochemistry for exploration modeling
New thermometers (e.g. N isotopes)
Projects related to the duration stage of life
(expanding vs contracting) of geothermal systems
Africa pool, Orakeikorako, NZ
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Geochemistry in geothermal exploration and
management
Greg B. Arehart Department of Geological
Sciences University of Nevada, Reno