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GRC Transactions, Vol. 35, 2011
Investigation of Geothermal Resource Potential
in the Northern Rio Grande Rift, Colorado and New Mexico
Elisabeth Easley, Laura Garchar, Mitchell Bennett,
Banks Beasley, Rachel Woolf, and Joyce Hoopes
Colorado School of Mines, Golden, Colorado
Introduction
Keywords
Rio Grande Rift, Poncha Springs, Embudo Fault Zone, Taos
Plateau, noble gas isotopes, geochemistry, geochemical modeling, geophysics, geothermal exploration
The Rio Grande Rift stretches over 1,000 km from Chihuahua,
Mexico to northern Colorado and exists in a setting that experienced nearly continuous deformation in the Cenozoic (Keller et
al., 1990). Deformation during the Late Cretaceous Laramide
Orogeny was characterized by NE-SW compression, and the
formation of foreland basins and uplifted fault blocks (Corbitt and
Woodward, 1973; Drewes, 1978; Seager and Mack, 1986). Around
32 to 30 Ma, a shift to backarc extension onset rift inception, and
was followed by alkali rhyolite and basaltic andesite magmatism
(29 to 30 Ma) that produced large shield volcanoes, fissures,
thick flood-basalts, cinder cones, and tuffaceous ash layers scattered about the San Luis Basin and the Rio Grande Rift (Aldrich
et al., 1986). The rift is composed of a series of north-trending
extensional basins that consist of asymmetric grabens (Olsen et
al., 1987) bounded on one side by north-striking normal rangefront faults.
The San Luis Basin extends from approximately 10 km
south of Salida, CO to over 80 km south of the Colorado-New
Mexico border near Taos (Kellogg, 1999). The basin comprises
two east-tilting half grabens separated by the north-trending
Alamosa Horst (Brister and Gries, 1994), which divides the
basin into the Monte Vista graben to the west and the Baca
graben to the east. Near the Colorado-New Mexico state line
the basin is divided into the San Luis Valley to the north and
Taos Plateau to the south. The San Juan Volcanic Field marks
the western boundary of the basin, while the east side is bounded
by the Sangre de Cristo normal fault system, marking the front
fault system of the Sangre de Cristo Range (Kellogg, 1999).
Strata in the San Luis Basin are tilted predominantly to the east,
but adjacent basins within the rift are separated by complexly
faulted zones, across which basin tilting is reversed (Bauer et
al., 2004), coinciding with zones of structural weakness associated with Laramide and pre-Laramide tectonism. In several
areas, these complexly faulted zones localize magmatic activity
(Russell and Snelson, 1994).
ABSTRACT
Geothermal anomalies within the Rio Grande Rift are associated with transfer and scissor faults between successive basins,
intrabasin rift faults, where range-front faults are offset in accommodation zones, volcanic complexes, and possibly where blind
intrusions of magma have risen to mid-crustal depths. This study
is part of the National Geothermal Student Competition sponsored
by the National Renewable Energy Laboratory and aims to assess the potential for geothermal resources in the San Luis Basin
of southern Colorado and northern New Mexico. A preliminary
analysis of existing data was used to identify two areas of interest
for field investigations located near zones of structural complexity
at Poncha Pass, Colorado and near Taos, New Mexico, representing the northern and southern ends of the basin.
A suite of techniques including geological, geophysical, hydrologic, and geochemical applications was used. Geochemical
modeling of existing data for the Taos Valley and Valles Caldera
in New Mexico simulate the mixing of thermal and groundwater
end-members to represent geochemical processes potentially
occurring at depth. Noble gas isotope data suggest the presence
of mixing in a three end-member model system composed of atmospheric, crustal, and mantle components. Helium isotope data
from Poncha Springs, Colorado yields a R/RA value indicative
of a mantle-sourced gas signature that may be associated with
crustal penetrating faults, ongoing rift activity, and/or the Aspen
anomaly. The use of noble gas isotopes for this research implies
that there is a direct and unique relationship between noble gas
compositions and structural, magmatic, and tectonic geological
phenomena. Geophysical data indicate faults that serve as conduits
and barriers to upwelling thermal water near Poncha Springs.
761
Easley, et al.
Background
Results
Geophysics
Poncha Pass is located in south-central Colorado between
the Upper Arkansas Basin and the San Luis Basin. It represents
a structurally complex transition between the two basins, and
lies in an intersection of two sets of crossing fault patterns (Coe
et al., 1982; Grauch and Drenth, 2009). Poncha Hot Springs are
located well above the valley floor on the northern side of the pass
along the intersection of an east-west to southwest trending fault
and a north to northwest-trending fault. Quaternary, Tertiary, and
Precambrian rocks are exposed in the Poncha Pass area.
Gravity measurements support the interpretation of easttrending faults at Poncha Hot Springs, and estimate about 1,000
ft of displacement along the fault just north of the hot springs. It
has been hypothesized that the geothermal resource is found in
Precambrian rocks and confined to faults and fractures (Coe et
al., 1982). Aeromagnetics has identified faults that correspond
to geologic mapping, and supports the idea of a faulted basement
block buried underneath sediments between the town of Poncha
Springs, to the north, and the hot springs (Grauch and Drenth,
2009). Barrett and Pearl (1976; 1978) sampled the Poncha
Springs area and interpreted reservoir temperatures ranging from
96-145 °C based on silica and alkali chemical geothermometers.
Mixing is expected to occur between the deep geothermal reservoir and local groundwater, but the degree of mixing is not well
constrained and thus reservoir equilibration temperatures should
be used with extreme caution. Four temperature gradient holes
have been drilled and showed anomalous temperature gradients
ranging from 56.4 to 65.6 °C/km. Coe et al. (1982) proposed the
heat source is a rift-related elevated geothermal gradient, however
helium isotope data from Karlstrom et al. (2011) and this research
provide evidence of mantle-sourced gases in water samples collected from Poncha hot springs.
The geology of the southern San Luis basin is distinctly different from Poncha Springs, with massive Neogene basalt flows
incised by the Rio Grande, and the adjacent Valles Caldera on its
southwestern margin. This study focuses on the northeastern extension of the Jemez lineament, the Embudo fault transfer zone
that separates the San Luis and Española basins of north-central
New Mexico, which underwent significant Laramide shortening,
Miocene to Holocene extension, and episodic volcanism since
the early Oligocene (Bauer and Kelson, 2004). Three major
fault zones are reported in this area and are of interest for geothermal energy development because they may serve as conduits
or barriers for hydrothermal fluid flow and storage. These fault
zones include the strike-slip Embudo transfer fault zone, the 8
km-wide north trending strike-slip Picuris-Pecos fault system,
and the dip-slip Cañon section of the Sangre de Cristo Range
front fault. Seismic interpretations by Reiter and Chamberlin
(2011) suggest that upwelling of the mantle-lithosphere occurs
near the Rio Grande Rift and the Great Plains craton boundary.
Previous reports on the springs and hydrology of Taos County
are numerous (Bauer, 2007; Summers, 1976; White and Kues,
1992; Garrabrant, 1993; Johnson, 1998; TetraTech, 2003; and
Benson, 2004). The report of Bauer et al. (2007) was used for
this research, and includes geochemical data in the Rio Grande
Gorge near Taos and the Embudo Fault Zone.
The Colorado School of Mines geophysics department collected several sets of data along Hot Springs Rd (CO Road 115)
near Poncha Springs, CO as part of summer field camp (Colorado
School of Mines, 2010). The Sand Gulch area was investigated
as part of the NREL National Student Geothermal Competition
in April of 2011. At the Poncha Pass field area, the Hot Springs
Road direct current (DC) resistivity and self-potential surveys
identified two faults that coincide with evidence of upwelling
water. The southernmost fault apparent in the resistivity profile
appears to be a splay off of a main fault when compared with the
aeromagnetic results. An additional smaller fault was inferred on
the north end of the line and is considered to be a fault leg that
continues eastward. The fault identified at the southern end of the
Sand Gulch line appears to be the same fault, though the aeromagnetic data suggests that this eastern fault expression is also a
splay. Although the two survey lines crossed the same fault, the
eastern survey did not reveal any evidence for upwelling water
along either of the faults found there.
Geochemistry
Geochemical data were collected at locations (Figure 1) near
Taos (red points) and Poncha Springs (green points). The samples
were collected from thermal springs, cold springs, and surface
waters. Low precipitation for 2011 rendered sampling of springs
difficult because many documented springs were not flowing in
April. Geochemical data for the Rio Grande Gorge springs from
Bauer et al. (2007) and for the Valles Caldera springs from Goff
and Grigsby (1982) were used to model mixing of groundwater
and thermal end-members. The dataset of Bauer et al. (2007) has
not been extensively analyzed in the literature. Existing datasets
(Bauer et al., 2007; Goff and Grigsby, 1982) were used to construct
preliminary simulations of fluid mixing chemical reaction path
models using The Geochemist’s Workbench (Bethke, 2008) that
define a reaction pathway for mixing of the Valles Caldera water
end-member with an end-member representative of groundwater
in Taos County. The data collected by Bauer et al. (2007) are plotted on a Piper diagram, a basic ternary plot of primary anions and
cations, to emphasize the position of the end-members presented
on this diagram, as shown in Figure 2.
Table 1. Valles Mixing Model Parameters.
Valles Mixing Model Parameters
Basis
H2O
1 free kg
Na+
2100 ppm
Ca++
0.01 ppm
K+
777 ppm
Mg++
0.01 ppm
H+
9 (pH)
HCO3323 ppm
Cl3618 ppm
SO4-66 ppm
SiO2(aq)
820 ppm
Temp
126 C
Data from Bauer et al., 2007
762
Reactants
H2O
Na+
Ca++
K+
Mg++
H+
HCO3ClSO4-SiO2(aq)
Temp
mole
55.51
0.001653
0.00078
0.000537
0.000494
0.000237
0.0042
0.00026
0.00027
0.00011
13 C
Easley, et al.
Figure 1. Geochemistry sample locations from National Student Geothermal Competition.
I
J
K
L
M
O
P
A
B
C
D
E
G
H
I
J
K
L
M
O
P
A
B
C
D
reactions of the defined system became
buffered by some mineral or gas phase.
This may be due to the lack of dissolved
CO2 gas partial pressure or fugacity data
for the Valles caldera, as Goff and Janik
(2002) report that the carbon dioxide gas
component of dry gas samples from this
hydrothermal system ranges from 96.9 to
99.0 mol %. The data of Goff and Janik
(2002) does not include the ratio of dry
gas to water, or dissolved gas, and the geochemical modeling approach to represent
this system allows for input parameters of
dissolved gases only. Future work to measure a suite of dissolved gases, including
CO2 and H2S is underway.
The Rio Grand Gorge-Valles Caldera
reaction path model also reveals information regarding the scaling potential
of thermal waters. Mineral saturation
is shown in Figure 4, and is defined as
(Q/K), where Q is the reaction quotient
and K is the solubility product. A Q/K
value greater than one indicates that a
Cisnerso
Sunshine Trail
Desagua Trail
S of Sheep
Gaging Station S
Bear Crossing spring zone
Feisenmeere Middle
North Big Arsenic 2
Little Arsenic
Rael (Stark)
Black Rock Hot
Godol North 1 spring zone
Godol North 2 spring zone
Godol South spring zone 1
Godol South Spring zone 2
Manby Hot Spring south pool
Taos Junction South
Rio Grande (Klauer)
Acequia de los Ojos
Souse Hole cienega
Valles Caldera (Baca 4)
Valles Caldera
Soda Dam Springs
Main Jemez Springs
Travertine mound spring
Figure 2. Piper diagram of Rio Grande Gorge and Valles Caldera Waters.
Figure 3. Piper diagram of reaction pathway between meteoric and magmatic end-members.
The REACT module in GWB was used to construct a fluid
mixing reaction pathway of a chemical system defined in Table 1.
This reaction path model simulates an equilibrated hydrothermal reservoir fluid mixing with a fluid representative of a local
equilibrated groundwater. The results are presented on a Piper
diagram in Figure 3, and simulate a chemical pathway as thermal
reservoir equilibrated water mixes with cold local groundwater to
produce a water chemistry similar to water chemistry measured
at the Godoi South Spring and Rio Grande by Bauer et al. (2007,
Figure 2). Both reaction paths plotted on the ternary diagrams
(Figure 3) indicate linear mixing trends and future work to calculate the mixing fraction of hot and warm springs in the Taos
Valley is in progress. The reaction pathway after 300 simulated
mineral phase is oversaturated in solution and will precipitate.
Calcite (red curve) and quartz (dashed black line) mineral
saturation is shown in Figure 4 and provides evidence for
quartz saturation with increasing reaction progress. In Figure
4, quartz remains slightly oversaturated for the entire reaction
progress, which may be an artifact of the model or a result of
the slow kinetics of quartz precipitation. The model predicted
the mineral with the highest potential for scaling is quartz, and
precipitation as thermal fluids ascend may be controlled by pH.
The calcite mineral saturation curve drops quickly during the
reaction progress and remains undersaturated for the majority
of the reaction path. This is most likely due to insufficient dissolved gas data for model parameters because calcite scaling
763
Easley, et al.
tribution to the total mass balance of noble gases in the crust is
negligible (Ballentine and Burnard, 2002). The goal of using
noble gases for this research is to define sources of geothermal
heat and understand the relationship between noble gas content
and geological phenomena.
Noble gas geochemistry studies were performed, and reported
in Table 2 as the ratio of 3He/4He in the sample to the ratio of
3
He/4He in the atmosphere, or R/Ra values for helium, and may
indicate different mantle signatures in springs distal and proximal to major tectonic structures. The use of a three end-member
system is the basis for the conceptual model defined in Figure 6,
which plots R/RA versus the ratio 4He/Ne*air, or the 4He/Ne in
the sample normalized to the He/Ne in the atmosphere. Samples
attributed to the atmospheric end-member plot close to an R/RA
value of 1.0 shown by the blue horizontal trend (Figure 6), but may
also vertically deviate from 1.0 if they are enriched by tritiogenic
3
He. Conventionally speaking, samples derived from the mantle
(mid-ocean ridge basalt, MORB) end-member have a high R/
RA value around 8.0, whereas crustal end-member varieties are
indicated by a R/RA value of 0.02.
These three end-members illustrated in Figure 6 are responsible for the production of noble gas isotopes, however only the
magmatic and crustal end-members may be responsible for the
Quartz
Mineral Saturation (Q/K)
1
Calcite
0
0.1
0.2
Reaction progress
Figure 4. Mineral saturation along the reaction path.
is probable for this system, as indicated by travertine mounds
near the caldera.
Noble gas isotopes in the crust are useful tracers in hydrothermal systems, due to their relative abundances, chemical inertness,
and unique isotopic characters. They can therefore be used to
determine the source of fluids, the environment of fluid origin,
manner of physical transport, and phase changes associated with
chemical interactions in the crustal fluid system (Ballentine and
Burnard, 2002; Ballentine et al., 2002). Figure 5 (not to scale)
is a conceptual model that illustrates three sources of noble gas
isotopes attributed to atmospheric, mantle, and crustal radiogenic/
nucleogenic processes, which are defined as end-member components for the purpose of this research. The release of 3He/4He in
the mantle has a constant and anomalous value of approximately
1.2 x 10-5 compared to that of tectonically “old” and stable crust,
which may have a 3He/4He production ratio on the order of 10-8
(Ballentine and Burnard, 2002). Additional sources of noble
gases include interplanetary dust particles (IDP), cosmogenic
reactions, and anthropogenic production, however their con-
Table 2. Noble Gas Isotope Results.
Noble Gas Isotope Results
4
He
Sample Location
[cc STP/g]
R/RA
RC/RA
Manby Spring
2.024E-6
0.32
0.31
Big Arsenic Spring
4.885E-8
0.99
3.46
Joyful Journey
3.733E-7
0.73
0.70
Poncha Spring
1.131E-7
1.90
2.16
Waunita
3.725E-7
0.17
0.10
9.300E-6
0.58
0.58
Cottonwood1
Mt Princeton1
3.540E-7
0.57
0.64
1.040E-6
2.16
2.24
Poncha1
Waunita1
6.390E-6
0.18
0.18
1
Data from Karlstrom et al., 2011 (in review)
Atmospheric Component
3, 4
20, 21, 22
He R/RA = 1.0
Ne
36, 38, 40Ar Q
ATM
MORB
ASW
Salinity, T, P
R/RA = 0.98
Tritiogenic
3He
Atmospheric
Radiogenic/Nucleogenic Component
235, 238 U Æα4He*
Magmatic Component
He‐dominated 3
He
R/RA = 8 (MORB)
QMAGMATIC
Crustal
232ThÆα4He*
40 K Æec40Ar* (Also β‐ 40Ca)
24Mg Æ
(n,α) 21Ne*
6Li Æ (n, α) 3H (β‐) 3He*
QCRUSTAL
R/RA = 0.02
Figure 6. R/RA vs. 4He/Ne * Air.
Figure 5. R/Ra Conceptual Model of Noble Gas Isotope Reservoirs.
764
Ne
[cc STP/g]
1.257E-7
1.686E-7
1.057E-7
8.758E-8
1.074E-7
na
na
na
na
Easley, et al.
production of heat associated with a geothermal anomaly. Further,
the MORB end-member for this conceptual model is hypothetical and would not be reached in this system because an upper
crustal pluton does not have the same 3He/4He ratio as a MORB.
Radiogenic and nucleogenic reactions in the crust account for
approximately 50 to 75% of the total crustal heat budget (Ballentine and Burnard, 2002; O’Nions and Oxburgh, 1983; Turcotte
and Schubert, 1982), and the remainder may be attributed to
localized phenomena, for example, plutons at depth, volcanism,
extensional basins, etc. It is important to again note in Figure 6
that neither the crustal or magmatic end-member is reached, thus
all fluid samples have some degree of noble gas mixing with the
magmatic and atmospheric end-member. It is possible that noble
gas isotopes may be slowly leaking from the mantle along crustal
penetrating structures, and traveling to the surface by advection
without further isotopic fractionation (Ballentine and Burnard,
2002; Bickle and McKenzie, 1987). Understanding the different
sources of geothermal heat and identifying mantle signatures,
which often are masked by crustal processes, are important aspects
of geothermal exploration because it is essential to discern high
radiogenic crustal heat flow from heat flow of magmatic origin
from intrusions into the crust or lithospheric thinning from extension (Ballentine et al., 2002).
Mixing trends are also observed in Figure 7, which plots RC/
RA, or the R/RA corrected for neon, versus excess helium in cc
STP/g. Excess helium was calculated by subtracting the atmospheric helium contribution, separating the crustal and mantle
components. Figure 7 suggests mixing between samples that
plot in a linear trend between the crustal and mantle (MORB)
end-members. Data represented by purple points was collected
as part of the National Student Geothermal Competition and
orange points represent data collected by Karlstrom et al. (2011).
Figure 8 shows a ternary plot of a helium mass balance broken
down into percentage of the three model parameters. The model
of the helium system parameters was generated from results by
defining the measured helium isotopic compositions as a percent
between the atmospheric, crustal, and magmatic end-members.
The highest atmospheric component is observed in the Big Arsenic Spring near Questa, New Mexico and a crustal component
ranging from 57 to 97 % is present in all other samples. The
sample collected from Poncha Springs during this research, and
data from Karlstrom et al. (2011) contain a high percentage (21-26
%, respectively) of the magmatic end-member. Finally, Figure
Helium Budget
0
100
CSM data, 2011
Karlstrom et al., 2011
20
80
r
ate
dW
ate
tur
Sa
Air
60
%
Ma
ntl
e
%
40
60
40
80
20
100
0
0
20
40
% Crustal
60
80
100
Figure 8. Ternary model of Helium Budget.
1 displays a map of sample locations in the San Luis basin and
a ternary plot of each sample locations to show a correlation between tectonic and rift-related structures. It is important to note
that from this preliminary suite of data, that there appears to be
a direct and unique relationship between noble gas compositions
and geologic phenomena, such as young faults, tectonic occurrences like the Aspen anomaly, or volcanic activity such as the
Valles caldera.
Conclusions
At the Poncha Pass field area, the Hot Springs Road geophysical surveys identified two faults that coincide with evidence of
upwelling water. Water chemistry collected at each site is consistent with previously collected data, and the noble gas data indicate
a direct and unique relationship between geological phenomena
and noble gas composition. Elevated mantle noble gas signatures
are found near areas of extensive structural deformation, such
as the Villa Grove transfer fault zone, and proximal to tectonic
or volcanic occurrences, such as the Aspen anomaly and Valles
caldera, respectively. Thus, noble gases may potentially serve as
a useful tool for geothermal exploration because they indicate the
presence of deeply penetrating geologic structures and, in some
cases, sources of mantle-derived heat.
Further work to continue noble gas isotope and bulk gas
sampling in the San Luis basin is scheduled. Future bulk gas
results will allow for more a realistic modeling approach to
simulate geothermal reservoir mixing processes, and additional
noble gas isotope data will enhance the current conceptual model
of the relationship between noble gas isotope compositions and
geological occurrences.
!"#$%&'()*+*,-(
Figure 7. RC/RA vs Excess Helium.
765
Easley, et al.
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