Factors Controlling Natural Hg Release to the Atmosphere

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Regional Controls on the Distribution of
Geothermal Systems in Nevada
Mark Coolbaugh, UNR Gary Raines, USGS Lisa
Shevenell, NBMG
Using a Geothermal GIS started by Mark Mihalasky
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SUMMARY Mark Coolbaugh, doctoral
candidate in the UNR Department of Geological
Sciences, discusses Regional Controls on the
Distribution of Geothermal Systems in Nevada.
His work is in collaboration with Gary Raines,
remote sensing expert with the USGS, Mark
Mihalasky (geographic information system GIS
expert formerly with the USGS), and Lisa
Shevenell of NBMG. Two general types of
geothermal systems are magmatic (typically
associated with rhyolitic rather than basaltic
volcanoes) and extensional geothermal systems.
There are no clearly magmatic geothermal systems
in Nevada (such as at Coso, Mammoth, the Geysers,
and Salton Sea in California or Roosevelt Hot
Springs in Utah). A map of maximum inferred
temperatures of geothermal reservoirs in Nevada
illuminates some interesting relations,
particularly when compared with various GIS data
sets.  Mark sees some unexpected
correlations with nearness to young magmatic
activity in Nevada (most of which is basaltic,
rather than the rhyolitic suggesting a possible
link between heat flow and basaltic underplating
of the relatively thin crust in the Great Basin)
and with nearness to northeast-trending young
(Pleistocene and Holocene, less than 1.5 million
years old) faults, suggesting linkages to
regional northwest-oriented extension. He also
sees correlations between geothermal systems and
higher than normal concentrations of boron and
lithium in groundwater. There is also a
correlation with depth to groundwater (better
correlation with shallow groundwater) this might
open areas for exploration where groundwater is
deeper (because the geothermal waters didnt make
it to the surface to be seen easily as hot
springs). As previously recognized, there is a
negative correlation with the area of the deep
carbonate aquifer in east-central and southern
Nevada. -- summary by Jon Price, Nevada Bureau
of Mines and Geology -- paper and dissertation
in progress
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Two types of Geothermal Systems (Koenig and
McNitt, 1983 Wisian, Blackwell, Richards, 1999)
1 Magmatic Geothermal Systems - associated
with very youthful silicic volcanism 2
Extensional Geothermal Systems - associated
with regions of high heat flow and recent
faulting
Koenig, J.B. and McNitt, J.R., 1983, Controls on
the location and intensity of magmatic and
non-magmatic geothermal systems in the Basin
and Range province Geothermal Resources Council
Special Report No. 13, May 1983, p. 93. Wisian,
K.W., Blackwell, D.D., and Richards, M., 1999,
Heat flow in the western United States and
extensional geothermal systems Proceedings,
24th Workshop on Geothermal Reservoir
Engineering, Stanford, CA., p. 219-226.
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Boundary of Great Basin
Locations of Magmatic systems are relatively
easy to predict
Roosevelt
Mammoth
With the possible exception of Steamboat
Springs, all geothermal systems in Nevada are
not of the Magmatic type
Coso
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Nevada is well-endowed with geothermal systems
and hot springs
Fig. 1 Geothermal Training Sites used for
Statistical Analysis
Sources NBMG open file report 94-2 SMU
Geothermal database Mariner et. al. (1983)
NOAA thermal springs (1980) Trexler et. al.
(1979)
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Trend Surface of Maximum Temperatures of Known
Geothermal Systems From paper in progress
Regional Controls On the distribution of
Geothermal Systems in Nevada Mark Coolbaugh,
UNR, Gary Raines, USGS, and Lisa Shevenell, NBMG.
Maximum temperature is the greater of 1) the
highest measured temperature, or 2) the
geothermometer-based temperature (ave. of silica
Na-K-Ca-Mg). Locations of geothermal systems
used for contouring are shown with circles. Only
geothermal systems with accurate water
geochemical analyses were used. A minimum
distance of 10 km between geothermal systems was
required. Method of surface interpolation was
Inverse Distance Weighting, with power of 1,
using nearest 5 neighbors. Deep intersections of
hot water in some oil wells were excluded (where
the regional thermal gradient was not exceeded).
In terms of temperatures, geothermal resources
are unequally distributed in the state
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Geothermal Training Sites
Age Dates/Volcanism lt 1.5 Ma
High-temperature extensional geothermal systems
in Nevada are associated with young volcanics
Volcanic age lt 1.5 Ma
(from USGS DDS 41, Mihalasky)
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Cumulative Descending Contrast Geothermal Systems
vs. Volcanics lt 1.5 Ma
Geothermal systems, regardless of temperature,
are more likely to be found within 5-10 kms of
young volcanic vents, as indicated by higher
contrast values. This correl- ation might be
caused by structural conduits or, in rare cases
of youngest volcanic activity, residual heat
from intrusions.
Higher-temperature geothermal systems are
spatially associated with young volcanics even
at greater distances. Perhaps the distribution
of young vents roughly define a broader area of
basaltic underplating of the crust.
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Late Pleistocene and Younger Faults
Faults Compiled from State Maps
(from USGS DDS 41, J. Dohrenwend)
(from USGS DDS 41, D. Sawatsky)
gt160 100-160 60-100 37- 60 deg C
The degree of spatial association between
geothermal systems and faults is difficult to
assess visually because of the detail. Weights
of Evidence can be used for a more quantitative
assessment.
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Cumulative Descending Contrast Geothermal Systems
with Faults
High-temperature geothermal systems correlate
best with northeast-trending young faults.
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On this graph, high positive numbers indicate a
preference of geothermal systems for NE
structures, high negative numbers indicate a
preference for NW structures, and values near
zero indicate no preference. Directional
anisotropy occurs where distances to young faults
are less than roughly 5 kilometers.
If northeast-trending structures remain open at
greater depths, the surrounding rock can heat
circulating fluids to higher temperatures.
Northeast-structures are oriented perpendicular
to the current direction of crustal extension, as
documented in GPS studies (Bennett et. al.,
1999). Higher strain rates may be the causative
mechanism keeping northeast-trending structures
open at greater depths.
REF Bennett R.A. Wernicke B.P. Davis J.L.,
1998, Continuous GPS measurements of contemporary
deformation across the northern Basin and Range
Province Geophysical Research Letters 25, no.4,
p. 563-566.
GPS stations B. Wernicke, W. Thatcher
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A spatial correlation between high-temperature
geothermal systems (purple circles) and boron
and lithium concentrations in groundwater is easy
to see visually.
Lithium in Groundwater
Boron in Groundwater
No Data 0 10 10 31.6 31.6 100 100 316 316
1,000 1,000 3,160 gt 3,160 ug/l
No Data 0 10 10 100 100 1,000 1,000
10,000 gt 10,000 ug/l
(NWIS database)
(NWIS database)
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Cumulative Contrast Boron in Groundwater
Error bars indicate /- one standard deviation
Higher temperature groupings of geothermal
systems show progressively better correlation
with boron in groundwater.
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Boron vs. Groundwater Temperature USGS NWIS
Database
Known Geothermal Systems
Fluids from known high-temperature geothermal
systems have high boron concentrations relative
to groundwater from the NWIS database.
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Lithium vs. Groundwater Temperature USGS NWIS
Database
Known Geothermal Systems
Similar to boron, lithium concentrations are also
high relative to most groundwaters. R2 between B
and Li 0.62
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Depth to Groundwater (from NWIS database) High
temperature geothermal systems preferentially
occur in areas of shallow groundwater.
Depth in feet
Depth in feet

• lt 5
• 5 - 20
• 20 - 50
• 50 - 100
• 100 - 200
• gt 200 m
• No Data

• lt 5
• 5 - 20
• 20 - 50
• 50 - 100
• 100 - 200
• gt 200 m

b) Water depth in mountain ranges assumed
equal to 200 feet.
a) Mountain ranges no data
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Cumulative Ascending Contrast Depth to
Groundwater vs. High Temperature Systems (gt160
deg. C)
Because of differences in fluid density, buoyant
geothermal fluids may rise to the top of the
groundwater table but will have difficulty
rising above it. Surface manifestations of
geothermal systems (hot springs and geysers) are
less likely to be found if the groundwater table
is deep. Undiscovered geothermal systems may
occur in areas with deeper groundwater levels.
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Trend Surface of Geothermal Systems
Pz carbonates (NBMG open file rpt 96-2)
High-temperature geothermal fluids may become
entrained in the deep carbonate aquifer in the
south and eastern portions of the state.
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Young Faults (NBMG/USGS)
Young Volcanics (USGS)
Boron, Groundwater (NWIS)
Depth to Groundwater (NWIS)



Quakes (NV Seismo Lab)
PZ Carbonates (NBMG)
7 Evidence Layers Used to Build
Logistic Regression Model
Heat flux (SMU)
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The logistic regression model (on left) is
broadly similar to the trend surface of
known geothermal system temperatures (on right).
Statistical tools can be used to predict
areas favorable for geothermal exploration,
including areas where the groundwater table is
deep.
Increasing Favorability
Logistic Regression Favorability Map 7
Evidence Layers
Trend Surface of Known Geothermal Systems
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CONCLUSIONS Geothermal GIS study

• The preference of high-temperature geothermal
  systems in Nevada for northeast-trending
  structures may be related to current
  northwest-oriented crustal strain, as measured by
  GPS stations.
• Pleistocene basalts are a favorable indicator for
  extensional-type geothermal systems in NV.
  Related to basaltic underplating?
• Trace element analysis of boron and lithium may
  constitute a potential exploration tool for
  geothermal systems.
• A deep groundwater table may conceal geothermal
  systems in some areas of the state.