Geochemical characterization of geothermal systems in the Great Basin: Implications for exploration,

1
Geochemical characterization of geothermal
systems in the Great Basin Implications for
exploration, exploitation, and environmental
issues

• Greg B. Arehart, Mark F. Coolbaugh, Simon R.
  Poulson
• Department of Geological Sciences
• Great Basin Center for Geothermal Energy
• University of Nevada, Reno
• Ted DeRocher
• Caithness Operating Co., LLC.
• Reno, Nevada

2
Objectives
Development of a geochemical database for Great
Basin and other geothermal systems

• Representative (of all types of Great Basin
  systems)

Comprehensive (more than the typical suite of
data)
Grass Valley, NV
3
What makes Great Basin systems unique?
Great Basin

• Most geothermal (globally) is magmatic-driven
• Great Basin geothermal driving forces are
  multiple
• elevated geothermal gradients and/or
• magmatism associated with thinning crust

4
What makes Great Basin systems unique?
Wide variety of host rocks Most geothermal
systems hosted in volcanic or volcanic-derived
rocks Great Basin systems may be hosted in a
variety of other lithologies

• Wide variety of host rocks
• Most geothermal systems hosted in volcanic or
  volcanic-derived rocks
• Great Basin systems may be hosted in a variety of
  other lithologies
• Therefore, major and trace element geochemistry
  of Great Basin systems may be much more varied
  than typical geothermal systems

Grass Valley, NV
5
Some questions

• Are there magmatic-related systems in the
  interior of the Great Basin?

Are there extensional systems on the margins of
the Great Basin?
And how do we tell them apart?
(both academic and economic implications)
6
Preliminary results Cl

• Cl appears correlated with temperature at a
  regional scale (? playa salt input?)

7
Preliminary results Cl

• Cl appears correlated with temperature at a
  regional scale (? playa salt input?)
• No difference from global average salinity, just
  more interaction with reservoir rocks at higher T
  (?not playa related)

8
Comparison of Great Basin systems

• Magmatic systems are richer in incompatible
  elements (As, Li, Cs, B)

9
Preliminary results As

• High-T systems contain more As than low-T systems

Because HT systems form in rocks having more As
(?) or HT systems are more effective at
leaching As from rx (?)
In addition, slope of the As-T relationship is
different for magmatic vs. GB extensional systems
(but still positive slope) does this imply a
magmatic source for As?
10
Preliminary results As

• No relation between As content and reservoir rock
  type
• Would expect Great Basin (extensional) systems to
  be higher in As than observed because of the
  relative high background of As (GB rocks in
  general abundant As in paleogeothermal systems)

11
Preliminary results Li vs. Cl

• Li vs. Cl delineates differences between magmatic
  and extensional systems

12
Preliminary results Cs vs. Cl

• Cs vs. Cl delineates differences between magmatic
  and extensional systems

13
Preliminary results B vs. Cl

• B vs. Cl delineates differences between magmatic
  and extensional systems

14
Steamboat magmatic or extensional?

• Geology spatial association with 1.1 Ma rhyolite
  domes
• (too old to be present-day driving force
  possible younger intrusions below Steamboat?)
• Spatially associated with very active extensional
  margin of the Great Basin (Blewitt et al.)
  permissive of either type
• Trace chemistry supportive of magmatic

Steamboat main terrace
15
Great BasinNC gas data
Volcanic and geothermal base data from
Giggenbach, 1992
16
Shallow
Deep
Adapted from Flynn and Schochet, 2001
17
Ongoing analyses

• ICPMS went down while running analyses
• Very few to no data on trace elements in the
  literature

18
Future tasks

• Complete analytical data acquisition
• Acquire additional trace metal data from other
  localities (globally and locally)
• Upload data and provide access to database
  through website

19
Potential contributions from this research

• Understanding of relationships between fluid
  geochemistry and wallrocks

How does major element chemistry affect
solubility of trace elements and characteristics
of geothermal fluids in general?
Can trace chemistry provide insights into which
type of system underlies a surface manifestation?
20

• Development of predictive models for exploration
  and exploitation

Magmatic systems appear to be more potentially
productive at shallower levels, but both types
will have a similar surface temperature
expression Can we use trace element
geochemistry to tell the difference?
21
Potential contributions from this research
Understand development of geothermal systems
through time and tectonics
Abundant fossil geothermal systems allow us
insights into the development of these systems
over the past 40 million years
22
Potential contributions from this research
Assessment of duration and stage of life of
geothermal systems of various types
23
Fingerprinting contributions from multiple systems
Fingerprinting contributions from multiple
systems or ruling out contributions from
geothermal
Developing mitigation strategies
24
Credits

• Contributors (field crew, analysts, etc.)
• Greg Arehart Mack Kennedy (LBL)
• Mark Coolbaugh Bret Pecoraro
• Ben Delwiche Simon Poulson
• Mario Desilets Lisa Shevenell
• Ted DeRocher Chris Sladek
• Larry Garside Matthijs van Soest (LBL)
• Deborah Goetz

Funding The Great Basin Center for Geothermal
Energy, through the cooperative grant
DE-FG07-02ID14311 administered by DOE, Idaho
Operations Office and the Idaho National
Engineering and Environmental Laboratory.