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

1Institute of Earth Science and Engineering, University of Auckland, New Zealand

e-mail: [email protected]

while their textural characteristics enable the ABSTRACT mapping of broad temperature gradients from high temperature vent to low-temperature distal-apron Discharging alkali chloride hot springs are surface areas. expressions of a deeper geothermal reservoir. As the discharging hot spring fluid cools, silica carried in Exploitable geothermal systems may exist at depth solution precipitates and accumulates to form rocks even when there is no evidence of thermal activity at referred to as siliceous sinter. Distinctive sinter the surface. For example, the Blundell power plant, architecture forms depending on the environmental Roosevelt, Utah, USA, commissioned in 1984, conditions such as water temperature or flow rate. generates 26 MW (gross) of electrical power These textures are preserved over time and (Blackett and Ross, 1992). Currently, at Roosevelt, throughout diagenesis. Geothermal reservoirs may there are no discharging hot springs. However, an remain long after hot spring discharge ceases. extensive sinter sheet dated 1630-1920 years old Therefore ancient sinters provide a direct link with a represents historic surface flow of alkali chloride deeper potentially exploitable geothermal resource in water (Lynne et al. 2005). Sinter textures are areas where there are no active surface preserved long after discharge ceases and their manifestations. Recognition of preserved sinter recognition allows the mapping of high temperature textures enables mapping of former high temperature to low temperature flow gradients across ancient vent to low temperature, distal-apron flow pathways. sinter deposits. This technique is useful in the Sinter dating enables the tracking of fluid flow to the reconnaissance stage of geothermal exploration. surface, providing a regional context of fluid movement. The combination of sinter textural SINTER ARCHITECTURE mapping and dating complements standard techniques used in assessing a geothermal resource Walter (1976) developed a general framework and provides new methodology for assisting in illustrating the relationship between hot spring water identifying hidden geothermal reservoirs. temperature, microbial communities and sinter textures. Cady and Farmer (1996) produced a model INTRODUCTION that summarizes major biofacies, lithofacies and taphofacies trends along a thermal gradient in modern As discharging alkali chloride hot springs cool to hot spring settings where each distinctive texture temperatures less than 100 °C, the silica carried in inferred a specific location on the vent to distal-apron solution deposits and accumulates to form rocks flow path. Their model was based on hot springs at known as siliceous sinters (Fournier and Rowe, 1966; Yellowstone National Park but similar trends have Weres and Apps, 1982; Fournier, 1985; Williams and been observed around the world (Walter, 1976; Lowe Crerar, 1985). As the silica accumulates, distinctive et al., 2001; Lynne and Campbell, 2003). sinter textures are formed depending on the environmental conditions of the hot spring such as Thermal settings are represented by a variety of sinter water temperature, pH, flow rate, or water depth. textures. For example, sinter architecture differs for These environmentally significant textures are high, mid- and low-temperature pools and discharge preserved long after hot spring discharge ceases, and channels. Splash zones, deep pool floors, shallow are also preserved during diagenesis (Lynne et al., terracettes, fast flowing channels, and turbulent high 2005, 2007, 2008). Therefore, sinters provide a temperature vents are examples of environmental record of alkali chloride hot spring paleo-flows, conditions that produce distinctive sinter textures. Sinters undergo a 5-step series of silica phase stacked, convex laminations within each transformations from opal-A to opal-A/CT to opal- column. (A) Cross-sectional view of opal- CT to opal-C and eventually to quartz (Lynne et al., A geyserite from extinct geyser. Geyser 2005, 2007, 2008). Over time, and during these silica Valley, Wairakei, New Zealand. (B) phase changes, environmentally-significant textures Cross-sectional view of quartzose within sinters are preserved (Walter et al., 1996; geyserite. Coromandel Penninsula, New Trewin, 1994; Rice et al., 1995; Lynne et al., 2005; Zealand, ~3 million year old. 2008). Therefore sinter architecture provides a tool Photograph: R. Martin. whereby former thermal fluid flow pathways can be identified from ancient sinter deposits.

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 up- flow 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): Cross- sectional 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.

Coniform Textures Microbial communities that form coniform sinter Figure 1: Columnar geyserite showing multiple 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, microbial mat surrounding the bubbles silicifies to they decay, leaving hollow tubes which become produce macro-scale sinter textures of multiple infilled with secondary silica (Figure 3). 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 Figure3: Quartzose coniform sinter from indicate former sites of gas accumulation Coromandel Penninsula, New Zealand, (arrows). (B) Flattened oval voids formed (~3 million year old). (A) Cross-sectional in a discharge channel (arrows). view of coniform texture with quartz infilling individual columns (arrows). (B) Void dimensions indicate where you are in the Plan view of coniform texture reveals discharge channel. Oval voids parallel to the bedding white colored silica around filamentous plane form on pool floors. Flattened oval voids form molds. Arrows indicate opaque silica in discharge channels and elongated voids indicate infill. fast flowing water.

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 LOW-TEMPERATURE TEXTURES (<35 °C) Plant-rich Textures Plant-rich sinters are common within geothermal Palisade Textures systems (Figure 6). 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, micro- pillar structures referred to as palisade texture (Figure 5).

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.

They represent distal-apron, low-temperature areas

Figure 5: Quartzose palisade sinter reveals near- where shifting channels have drowned reeds, grasses or small plants. The orientation of plant stems may be vertical, micro-pillar, filamentous random or if the flow of water is swift enough the structures. (A) 11,493 ± 70 year old sinter plant stems may be aligned with the channel flow from the High Terrace, Steamboat direction. Plant material can also be transported by Springs, Nevada, USA, showing palisade horizon (inside dotted lines). (B) 1920 ± wind and water before it becomes silicified. Silicified 160 year old sinter from Opal Mound, plant molds are distinctive in outcrop where they appear as circular or elongated tubes (Campbell et al., Roosevelt, Utah, USA, showing two 2001). distinct palisade horizons (arrows).

Environmental conditions favorable for palisade Streamer Textures textures are low-temperature, shallow fluid flowing In fast flowing channels microbial communities form long strands which are aligned in the flow direction over micro-terracettes of previously formed sinter. (Smith et al., 2003). When these microbes become Palisade textures are visible at the macro-scale with silicified the fabric formed is referred to as streamer lamination thicknesses controlled by water depth texture (Figure 7). (commonly < 5 mm thick).

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 9: Hand specimen of opal-A/CT breccia sinter shows previously-formed white and Figure 7: Sinter with streamer texture located on grey-colored sinter fragments cemented dry sinter terraces. Arrows indicate into a new, temporally younger (6283 ± paleo-flow direction. (A) Opal-A streamer 90 years), iron-rich (orange-colored) sinter. Orakei Korako, New Zealand. (B) sinter deposit. Steamboat Springs core 1630 ± 90 year old opal-A streamer sinter SNLG 87-29, Nevada, USA. from Opal Mound, Roosevelt, Utah, USA. Therefore, breccia sinters may record multiple pulses Oncoidal and Pisoidal Textures of hot spring discharge or sinter fragments of Sinter oncoids and pisoids are circular nodules that different ages (Lynne et al., 2008). rotate in alkali chloride fluid and grow by accreting silica to their exterior surface. Pisoids (generally <5 SINTER DATING mm diameter) form in turbulent shallow pools near 14 high and mid-temperature hot spring vents (Figure 8). 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 Figure 8: Hand specimen shows multiple pisoids in as the location of faults, the timing of fault opal-CT sinter from Steamboat Springs movements, topography or stratigraphy, enhances our core SNLG 87-29, Nevada, USA, 6283 ± 90 understanding of the deeper geothermal system. years old. CONCLUSION Breccia Sinter Textures Sinters form from discharging alkali chloride hot Several processes may form brecciated sinter springs and provide evidence at the surface of a deposits. Hydrothermal eruptions, freeze/thaw, deeper geothermal reservoir. Long after hot spring hydraulic fracturing and trampling by wildlife all discharge ceases, sinter textures are preserved and an Spring Systems,” Chemical Geology, 132, 237- exploitable geothermal system may remain at depth. 246. Therefore, sinters may be the only evidence at the Jones, B. and Renaut, R. W. (1997), "Formation of surface of a hidden geothermal resource. The Silica Oncoids Around Geysers and Hot Springs recognition and mapping of preserved at El Tatio, Northern Chile,” Sedimentology, 44, environmentally-significant textures in ancient sinters 287-304. reveals hot spring paleo-flow conditions and temperature gradient profiles from high temperature Lowe, D. R., Anderson, K. S. and Braustein, D. vents to low-temperature distal-apron slopes. Sinter (2001), "2001. 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