PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, February 11-13, 2013
SGP-TR-198
SILICEOUS SINTER: AN EARLY EXPLORATION TOOL AND DIRECT LINK TO A
GEOTHERMAL RESERVOIR
Bridget Y. Lynne1
1
Institute of Earth Science and Engineering, University of Auckland, New Zealand
e-mail: [email protected]
ABSTRACT
Discharging alkali chloride hot springs are surface
expressions of a deeper geothermal reservoir. As the
discharging hot spring fluid cools, silica carried in
solution precipitates and accumulates to form rocks
referred to as siliceous sinter. Distinctive sinter
architecture forms depending on the environmental
conditions such as water temperature or flow rate.
These textures are preserved over time and
throughout diagenesis. Geothermal reservoirs may
remain long after hot spring discharge ceases.
Therefore ancient sinters provide a direct link with a
deeper potentially exploitable geothermal resource in
areas where there are no active surface
manifestations. Recognition of preserved sinter
textures enables mapping of former high temperature
vent to low temperature, distal-apron flow pathways.
Sinter dating enables the tracking of fluid flow to the
surface, providing a regional context of fluid
movement. The combination of sinter textural
mapping and dating complements standard
techniques used in assessing a geothermal resource
and provides new methodology for assisting in
identifying hidden geothermal reservoirs.
INTRODUCTION
As discharging alkali chloride hot springs cool to
temperatures less than 100 °C, the silica carried in
solution deposits and accumulates to form rocks
known as siliceous sinters (Fournier and Rowe, 1966;
Weres and Apps, 1982; Fournier, 1985; Williams and
Crerar, 1985). As the silica accumulates, distinctive
sinter textures are formed depending on the
environmental conditions of the hot spring such as
water temperature, pH, flow rate, or water depth.
These environmentally significant textures are
preserved long after hot spring discharge ceases, and
are also preserved during diagenesis (Lynne et al.,
2005, 2007, 2008). Therefore, sinters provide a
record of alkali chloride hot spring paleo-flows,
while their textural characteristics enable the
mapping of broad temperature gradients from high
temperature vent to low-temperature distal-apron
areas.
Exploitable geothermal systems may exist at depth
even when there is no evidence of thermal activity at
the surface. For example, the Blundell power plant,
Roosevelt, Utah, USA, commissioned in 1984,
generates 26 MW (gross) of electrical power
(Blackett and Ross, 1992). Currently, at Roosevelt,
there are no discharging hot springs. However, an
extensive sinter sheet dated 1630-1920 years old
represents historic surface flow of alkali chloride
water (Lynne et al. 2005). Sinter textures are
preserved long after discharge ceases and their
recognition allows the mapping of high temperature
to low temperature flow gradients across ancient
sinter deposits. This technique is useful in the
reconnaissance stage of geothermal exploration.
SINTER ARCHITECTURE
Walter (1976) developed a general framework
illustrating the relationship between hot spring water
temperature, microbial communities and sinter
textures. Cady and Farmer (1996) produced a model
that summarizes major biofacies, lithofacies and
taphofacies trends along a thermal gradient in modern
hot spring settings where each distinctive texture
inferred a specific location on the vent to distal-apron
flow path. Their model was based on hot springs at
Yellowstone National Park but similar trends have
been observed around the world (Walter, 1976; Lowe
et al., 2001; Lynne and Campbell, 2003).
Thermal settings are represented by a variety of sinter
textures. For example, sinter architecture differs for
high, mid- and low-temperature pools and discharge
channels. Splash zones, deep pool floors, shallow
terracettes, fast flowing channels, and turbulent high
temperature vents are examples of environmental
conditions that produce distinctive sinter textures.
Sinters undergo a 5-step series of silica phase
transformations from opal-A to opal-A/CT to opalCT to opal-C and eventually to quartz (Lynne et al.,
2005, 2007, 2008). Over time, and during these silica
phase changes, environmentally-significant textures
within sinters are preserved (Walter et al., 1996;
Trewin, 1994; Rice et al., 1995; Lynne et al., 2005;
2008). Therefore sinter architecture provides a tool
whereby former thermal fluid flow pathways can be
identified from ancient sinter deposits.
stacked, convex laminations within each
column. (A) Cross-sectional view of opalA geyserite from extinct geyser. Geyser
Valley, Wairakei, New Zealand. (B)
Cross-sectional view of quartzose
geyserite. Coromandel Penninsula, New
Zealand,
~3
million
year
old.
Photograph: R. Martin.
This paper documents some commonly found
environmentally-significant sinter textures. The
recognition of these textures enables the mapping of
hot spring paleo-flow conditions (i.e., temperature,
flow rate) and the identification of former hot upflow zones (i.e., high temperature vents) in areas
where hot springs no longer discharge.
HIGH TEMPERATURE TEXTURES (>90 °C)
Spicular, nodular and columnar textures commonly
form in high temperature (>90 °C), near-vent
environments and are referred to as geyserite. These
textures form in the splash zone of eruptive hot
springs, pools and geysers, where the sinter surface is
intermittently wet and dry (White et al., 1964; Renaut
and Jones, 2000). Geyserite architecture consists of
stacked, convex laminations within each column,
nodule or spicule (Figures 1 and 2).
Figure 2: Quartzose geyserite sinter texture from
Coromandel Penninsula, New Zealand
(~3 million year old). (A-B): Crosssectional views of columnar geyserite
showing
multiple
stacked
convex
laminations within each column. (C) Plan
view of columnar geyserite showing
concentric rings of silica (photographs by
R. Martin).
The characteristic convex laminations distinguish it
from mid-temperature coniform and low-temperature
palisade textures.
MID-TEMPERATURE TEXTURES (35-59 °C)
There are several types of environmentally
significant mid-temperature sinter textures. Two such
textures are referred to as coniform and bubblemat.
Both result from the silicification of microbial
communities that thrive in mid-temperature water.
Figure 1: Columnar geyserite showing multiple
Coniform Textures
Microbial communities that form coniform sinter
textures consist of long filaments that grow on the
base of deep, quiet pools. These pools are often
adjacent to high temperature vents. Once the
filamentous microbes become entombed in silica,
they decay, leaving hollow tubes which become
infilled with secondary silica (Figure 3).
Figure3: Quartzose
coniform
sinter
from
Coromandel Penninsula, New Zealand,
(~3 million year old). (A) Cross-sectional
view of coniform texture with quartz
infilling individual columns (arrows). (B)
Plan view of coniform texture reveals
white colored silica around filamentous
molds. Arrows indicate opaque silica
infill.
The lack of laminations within the silica infill
distinguishes coniform texture from geyserite sinter.
Bubblemat Textures
Mid-temperature hot springs and discharge channels
flowing over mid-slope settings are inhabited by thin,
sheathed filamentous cyanobacteria. The filamentous
cyanobacterium Leptolyngbya (previously called
Phormidium) thrives in mid-temperature alkali
chloride waters (cf. Walter, 1976; Lowe et al., 2001)
and consists of microbes with finely filamentous, < 5
μm exterior diameters (Hinman and Lindstrom, 1996;
Campbell et al., 2001; Lynne and Campbell, 2003).
As these microbes photosynthesize, released gas
accumulates within the mat and forms bubbles. The
microbial mat surrounding the bubbles silicifies to
produce macro-scale sinter textures of multiple
curved laminations with oval or lenticular voids
(Figure 4).
Figure4: Quartzose bubblemat sinter from the High
Terrace, Steamboat Springs, Nevada,
USA. 11,493 ± 70 year old. (A) Oval voids
indicate former sites of gas accumulation
(arrows). (B) Flattened oval voids formed
in a discharge channel (arrows).
Void dimensions indicate where you are in the
discharge channel. Oval voids parallel to the bedding
plane form on pool floors. Flattened oval voids form
in discharge channels and elongated voids indicate
fast flowing water.
LOW-TEMPERATURE TEXTURES (<35 °C)
Palisade Textures
Low-temperature
alkali
chloride
hot-springs
commonly support the cyanobacterium Calothrix and
form sinters that contain coarsely filamentous,
sheathed cyanobacteria (Cassie, 1989; Cady and
Farmer, 1996). These microbes consist of an inner
tubular filament mold or trichome of porous cellular
material, and thick outer sheaths, with a total exterior
diameter of >8 μm. Often the trichome decays and
secondary silica infills the void. Silica infill consists
of silica that is not laminated. Sinter architecture
consists of closely-packed, vertically-stacked, micropillar structures referred to as palisade texture (Figure
5).
Plant-rich Textures
Plant-rich sinters are common within geothermal
systems (Figure 6).
Figure 6: Plant-rich sinter from ancient outcrops in
New Zealand. (A) Opal-A plant-rich sinter
reveals hollow circular molds (arrows).
Pukemoremore, New Zealand. (B)
Quartzose plant-rich sinter shows
elongated molds of former plant stems
and hollow tubular molds (arrows).
Umukuri, New Zealand.
Figure 5: Quartzose palisade sinter reveals nearvertical,
micro-pillar,
filamentous
structures. (A) 11,493 ± 70 year old sinter
from the High Terrace, Steamboat
Springs, Nevada, USA, showing palisade
horizon (inside dotted lines). (B) 1920 ±
160 year old sinter from Opal Mound,
Roosevelt, Utah, USA, showing two
distinct palisade horizons (arrows).
Environmental conditions favorable for palisade
textures are low-temperature, shallow fluid flowing
over micro-terracettes of previously formed sinter.
Palisade textures are visible at the macro-scale with
lamination thicknesses controlled by water depth
(commonly < 5 mm thick).
They represent distal-apron, low-temperature areas
where shifting channels have drowned reeds, grasses
or small plants. The orientation of plant stems may be
random or if the flow of water is swift enough the
plant stems may be aligned with the channel flow
direction. Plant material can also be transported by
wind and water before it becomes silicified. Silicified
plant molds are distinctive in outcrop where they
appear as circular or elongated tubes (Campbell et al.,
2001).
Streamer Textures
In fast flowing channels microbial communities form
long strands which are aligned in the flow direction
(Smith et al., 2003). When these microbes become
silicified the fabric formed is referred to as streamer
texture (Figure 7).
brecciate sinter sheets. Hot spring discharge may be
intermittent allowing sinter surfaces to dry, crack and
brecciate. Upon further discharge of thermal water
the brecciated sinter fragments become cemented into
a newly-formed sinter (Figure 9).
Figure 7: Sinter with streamer texture located on
dry sinter terraces. Arrows indicate
paleo-flow direction. (A) Opal-A streamer
sinter. Orakei Korako, New Zealand. (B)
1630 ± 90 year old opal-A streamer sinter
from Opal Mound, Roosevelt, Utah, USA.
Oncoidal and Pisoidal Textures
Sinter oncoids and pisoids are circular nodules that
rotate in alkali chloride fluid and grow by accreting
silica to their exterior surface. Pisoids (generally <5
mm diameter) form in turbulent shallow pools near
high and mid-temperature hot spring vents (Figure 8).
Figure 8: Hand specimen shows multiple pisoids in
opal-CT sinter from Steamboat Springs
core SNLG 87-29, Nevada, USA, 6283 ± 90
years old.
Figure 9: Hand specimen of opal-A/CT breccia
sinter shows previously-formed white and
grey-colored sinter fragments cemented
into a new, temporally younger (6283 ±
90 years), iron-rich (orange-colored)
sinter deposit. Steamboat Springs core
SNLG 87-29, Nevada, USA.
Therefore, breccia sinters may record multiple pulses
of hot spring discharge or sinter fragments of
different ages (Lynne et al., 2008).
SINTER DATING
14
C Accelerator Mass Spectroscopy (AMS) was used
to date entombed plant material in sinter samples
from New Zealand and the US. Sinter dating provides
a temporal context for the spatial distribution of
sinters and hot spring paleo-flow paths. Dating also
distinguishes those sinters that may be sufficiently
young (i.e., ~<50,000 years) to be related to a
potentially exploitable geothermal resource at depth,
from those that are less likely to be associated with a
useable resource based on age.
Sinter dating enables fluid flow migration trends to
be identified on both a local and regional scale. The
timing of discharging fluids can be related to the
geology and structure of the area. The identification
of geologic and structural controls to fluid flow such
as the location of faults, the timing of fault
movements, topography or stratigraphy, enhances our
understanding of the deeper geothermal system.
CONCLUSION
Breccia Sinter Textures
Several processes may form brecciated sinter
deposits. Hydrothermal eruptions, freeze/thaw,
hydraulic fracturing and trampling by wildlife all
Sinters form from discharging alkali chloride hot
springs and provide evidence at the surface of a
deeper geothermal reservoir. Long after hot spring
discharge ceases, sinter textures are preserved and an
exploitable geothermal system may remain at depth.
Therefore, sinters may be the only evidence at the
surface of a hidden geothermal resource. The
recognition
and
mapping
of
preserved
environmentally-significant textures in ancient sinters
reveals hot spring paleo-flow conditions and
temperature gradient profiles from high temperature
vents to low-temperature distal-apron slopes. Sinter
dating reveals the timing of discharging hot springs,
enables the tracking of discharging fluids on a
regional scale and identifies sinters that are most
likely to be related to a blind geothermal resource.
Sinter textural mapping combined with sinter dating,
provides a simple tool that assists existing
exploration techniques used in the search for hidden
geothermal resources.
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