Geothermal manifestations

The surface manifestations of a geothermal system in a volcanic area are generally the features that first stimulate exploration. Consequently, the recognition, mapping, and evaluation of these features are important in the second stage or prefeasibility study, during which the geothermal potential is evaluated. The prefeasibility stage also involves sampling fluids and gases to be studied by hydrogeochemical techniques that help estimate the temperatures and compositions of hydrothermal reservoir fluids.

The most obvious expression of a geothermal reservoir occurs when fluids leak to the surface along faults and fissures or through permeable rock units. Depending on the reservoir temperatures and discharge rates, these surface manifestations take the form of seeps, fumaroles, hot springs, boiling springs, geysers, phreatic explosion craters, and zones of acid alteration. In addition, there are deposits of silica sinter, travertine, and/or the bedded breccias that surround phreatic craters. In this chapter, we describe the most common geothermal features seen in hot-water systems and vapor-dominated systems; Appendix A presents in detail the methods for mapping these features.

Hot springs are the most visible manifestation of hot-water geothermal systems that transfer heat to the ground surface Some spring groups directly overlie a geothermal system and therefore may be used to locate drilling sites. However, springs may also discharge at the surface after flowing many kilometers down gradient from a hydrothermal reservoir; such outflow plumes can be misleading—they have a finite thickness, perhaps 0.5 to 1.0 km, and they overlie colder groundwater. Drillholes that penetrate these hot-water plumes show an increasing temperature with depth in the upper part and then a rapid temperature decline at the bottom.

Chemical analyses of spring waters, considered along with the volcanic structure and hydrologic regime, will provide the data needed to interpret the degree of mixing between cold groundwater and an outflow hot water plume from a geothermal reservoir. Hot springs can act as an excellent guide for geothermal drilling if (a) the water analyses indicate there is minimal mixing with cold groundwater and (b) the geologic structures (for example, a crater) imply that the hot springs overlie a thermal source and maximum reservoir temperatures can eventually be reached through drilling.

Reservoir temperatures for hot-water systems show a considerable range: <90°C (low temperature), 90 to 150°C (intermediate temperature), and 150 to 240°C (high temperature) (White and Williams, 1975). The temperature of a hot spring will not exceed that of the boiling temperature of water at the altitude of that spring. The salinity of hot water systems can range from 0.1 to 3%.

Vapor-dominated reservoirs—generally more than 85% steam—are ideal geothermal resources but, unfortunately, they are less numerous than hot-water systems. Although such systems have been developed in many parts of the world (for example, the Geysers geothermal area of California), little is known about what lies under them; one known possibility is high-chloride brine. Usually these reservoirs occur where there is very high heat flow but low water recharge.

Near-surface gases from vapor-dominated reservoirs condense to form acids, which leach rocks in the spring area. These areas are characterized by bleached rock, acid-sulfate springs, and no chloride waters; acid springs may be accompanied by mudpots, geysers, and fumaroles .

1                    Hot Springs and Geysers

Very few hot springs on this planet have not been developed in some way. As spas, hot springs have offered comfort to mankind throughout the millennia; as an alternate energy source, hot springs are increasingly being considered for more practical uses. Hot springs range in size from seeps that produce barely enough hot water for bathing a few individuals to the awesome thermal areas of Yellowstone and the North Island of New Zealand, where hot water and steam are used for heating domestic buildings, heating greenhouses, and generating electricity.

2                    Siliceous Sinter Deposits

Deposits of siliceous sinter are common to many high-temperature hydrothermal areas. The mound-like or terraced deposits are associated with boiling hot springs and serve as excellent indicators of the presence of hydrothermal reservoirs with temperatures of >175°C.To form siliceous sinter deposits, fluids from alkaline hot springs must have enough silica in solution to become saturated with amorphous silica as they cool from 100 to 50°C.described three steps in the formation of siliceous sinter:

(1) quartz-saturated hydrothermal fluids in the reservoir rise to the surface where they cool and become supersaturated with amorphous silica;

(2) amorphous silica particles nucleate to produce a colloidal suspension; and

(3) amorphous silica particles are agglomerated and cemented as amorphous silica precipitates between particles.

3                    Single-Stage or Primary Sinters

Thin-bedded opaline sinters are thought to have been formed by primary deposition of silica on broad discharge aprons The fluids have a high content of dissolved silica, were discharged at near-boiling temperatures, and evaporated quickly.

Geyserite or banded opaline sinters , most abundant on sinter cones, are deposited either by geysers or by vigorously spouting springs. Water with a high silica content at or above boiling temperatures is ejected; it cools and evaporates quickly, precipitating silica at the moment the water reaches the ground surface. These deposits are characterized by fine banding and a botryoidal or "knobby"

Bedded opaline sinter with plant casts that lie parallel to bedding indicate that the plants were dead when incorporated into the sinter. In some situations, the casts are perpendicular to the bedding planes, implying that there was cooler water in the pools and plants continued to grow during silica deposition.

Cellular opaline sinter is deposited on the algae-covered discharge aprons of active hot springs. The rounded or oval cells are formed when gas is released from algae and other organisms. When a spring stops discharging, algal growth dries up and the deposit disintegrates into dust, and therefore cellular opaline sinter is rarely preserved in older deposits. Other types of cellular sinter are associated with filamentous bacteria that survive at temperatures of 70 to 90°C.

Flocculated silica deposits are soft and usually poorly preserved.

4                    Multiple-Stage Sinters

Fragmental sinter , the most common opaline sinter, breaks easily into fragments when deposits dry out and are exposed to weathering and frost action. This fragmental debris may remain in place or be transported by wind and water. If younger, sinter-depositing springs flow over or through these deposits, they may become a cemented sinter breccia.

Opaline sinter is formed when opal is deposited by percolating thermal water. All of the previously described sinter types decrease in porosity after they are buried by younger deposits through this deposition. In some sinters, the process produces massive, glassy opal. On a microscopic scale, the cavities are filled with banded opal, which leaves geopetal structures.

Chalcedonic sinter is the most common within older deposits. During late-stage solution and deposition, chalcedony and quartz are deposited and earlier opal phases are at least partly recrystallized.

Sinter cement is an intermediate stage between clastic sediments and sinter deposits. Because hot springs often occur along rivers, sinter-cemented alluvial gravels are fairly common.

5                    Form and Extent of Siliceous Sinter Deposits

Where hot springs issue from point sources, sinter deposits are cone-like or mounded. If water issues from a line of springs—most likely along a fault trace—nearly flat-lying, terrace-like deposits are formed downslope, becoming thinner with distance from the springs, as is depicted in Fig. 3.3. The terraces are topped by scattered sinter cones or ridges; ridges mark hot spring locations and are commonly associated with open fissures that break the terrace surface. Grey, white, or tan sinters that are bedded to massive and friable to dense and hard make up these terraces. By mapping layered sinters at

found that each sinter terrace is composed of overlapping delta-shaped deposits and that each delta begins at a spring. These beds are nearly flat (dipping <10°). As each spring becomes choked with sinter, water begins to flow laterally through the sinter terrace to discharge at its flanks. The flank deposits dip more steeply (10 to 20°).

Siliceous sinter deposits range in magnitude from small mounds that cover a few square meters to terraces that comprise many square kilometers; thicknesses range from a few centimeters to tens of meters.
6                    Travertine

Meteoric water, heated either around magma bodies or during deep circulation along faults, reacts with carbonate rocks and liberates CO2 . The hot waters are subsequently cooled as they mix with cooler groundwater and reach chemical equilibrium with the aquifer rocks at ~70°C .If the water reaches the ground surface through fractures, CO2 escapes and the water becomes supersaturated with CaCO3 ; precipitation of the carbonate forms travertine near or above the ground surface. Distinctive mounded travertine deposits form around these springs, which have temperatures ranging from ~30 to 100°C.

Travertine deposits are indicators of geothermal reservoir temperatures that may be too low to generate electricity but may have direct-use applications such as for greenhouses or hot-water heating for nearby communities.

Although the hydrothermal reservoir immediately below travertine deposits might not have impressive temperatures, it could point toward a much hotter reservoir nearby. For example, Goff and Shevenell (1987) found that waters of the carbonate-depositing springs at Soda Dam, New Mexico, originate as outflow from a hotter intracaldera hydrothermal system located 10 to 12 km northeast of the dam. The outflow plume is diluted by groundwater, and therefore cooled, before it surfaces at Soda Dam.

Travertine ranges from a dense, banded rock, in which the banding is parallel to the fissure/spring orifice, to a porous, layered carbonate that dips away from the orifice. These outward-dipping layers may decrease in thickness with distance: from banded layers several meters thick near the spring orifice to laminae less than a millimeter thick hundreds of meters downslope from the spring. On close examination, travertines consist of mostly fine-grained (2- to 20-┬Ám) sparry calcite, but they may also contain micrite (muddy calcite), chert, and clays. Ooids are common. These deposits vary from white to dirty gray but can also have a yellowish or reddish hue as a result of limonite or hematite staining. Travertine deposits sometimes preserve records of other geologic events in forms such as interbeds of clastic sediment or volcanic ash.

Most travertine structures fall into one of the three categories described here.

· Hot-spring cones or towers are formed through deposition by a spring flowing from a single point;

· Fissure ridges are elongate deposits that occur along springs that issue from faults or joints These ridges can cross drainages and act as dams behind which flat travertine terraces may be deposited

· Terraces build up through the accumulation of travertine and clastic sediment behind fissure ridges or as "terracettes" where hot springs flow down steep slopes.

The size and thickness of travertine structures will depend on the length of the spring orifice, flow volume, rate of deposition, and time that the spring has been active. These deposits range from thin coatings to fissure ridges nearly 100 m thick; fissure ridges can be as much as several kilometers long.

Travertine deposition is a geologically fast process.found that the travertines of Mammoth Hot Springs at Yellowstone Park are deposited at an average rate of 21 cm/yr. used uranium-thorium disequilibrium age determinations to interpret the history of the travertine deposit of Soda Dam, New Mexico, they found that the depositional rate has been variable over the last 1 Ma; most of the deposition occurred during three pulses of increased hydrothermal activity in the nearby Valles caldera.

7                    Older Spring Deposits

Useful information about the history of the geothermal system is provided by the extent of these deposits, the relationship of older sinter and travertine deposits to faults or fissures, and the relationship between these deposits and presently active deposits. In fact, the relationship of old spring and fumarole deposits to new ones at Steamboat Springs, Nevada, has provided much of he background for our understanding of the evolution of hydrothermal systems.

8                    Hydrothermal (Phreatic) Craters and Deposits

Steam eruptions that involve little or no juvenile tephra are termed hydrothermal eruptions,phreatic eruptions , or mud volcanoes they are characteristic of the periodic behavior of many fumarolic areas. These eruptions form small craters, usually less than several hundred meters in diameter, which are surrounded by breccia, surge, and fallout deposits composed of lithic debris.

Hydrothermal eruptions occur in areas where the vapor pressure of geothermal fluids exceeds the hydrostatic boiling pressure for a given temperature. These eruptions take place at a point where the convective rise of geothermal fluids is impeded by a relatively impermeable layer termed caprock.Caprocks are commonly formed when rock permeability is sealed by the precipitation of solids from geothermal fluids. Where a hydrothermal reservoir has a significant vapor-dominated region, additional over-pressure might be transmitted from the deep, cold-water hydrostatic head.The vapor pressure of a geothermal fluid receives significant partial contributions from CO2 and H2 S, as well as H2 O. Because the former two gases have lower sublimation temperatures than water does, they can increase the vapor pressure by up to several tens of bars more than that of pure water for any given temperature.When expansion of the overpressured fluid is initiated by a trigger such as the chemical breakdown and failure of the caprock or seismic or hydraulic fracturing, the eruption proceeds as a vaporization wave propagates down through the fluid and accelerates the vapor and fluid out through the conduit in a manner similar to that proposed for geyser eruptions.

Two examples of hydrothermal eruption models,The first model is for a shallow hydrothermal reservoir with a temperature of 195°C and a depth of 200 m. The second is for a reservoir with a temperature of 230°C and a depth of 400 m. In both models, a sealed caprock has developed at 100-m depth through deposition of silica—and possibly carbonate where lower temperatures have allowed exsolution of CO2 . Because the overlying rock is sealed, fluid flow may be diverted at some greater depth and may follow another pathway to the surface. As vapor continues to accumulate below the sealed rock, the liquid water surface falls, and vapor pressure from the greater reservoir depths is transmitted to the seal (especially if significant noncondensible vapors are present).

In the case of eruption from the shallow hydrothermal reservoir, the transmitted vapor pressure is just above the lithostatic pressure. When the eruption occurs, hydrodynamic flow through the fracture conduit becomes hydrodynamically choked [u = usonic , as in Eq. (2-5)], and a pressure-balanced eruption occurs.found that water-steam systems have greatly reduced sound speeds, ranging from 1 m to several hundred meters per second. In such an eruption, ejecta will follow ballistic trajectories to form fallout deposits.

For eruption from the deep hydrothermal reservoir, vapor pressure transmitted to the caprock can greatly exceed that required to lift the overburden. Overpressure builds because of the strength of the caprock. When the caprock fails, choked flow in the conduit is at a pressure above the lithostatic pressure and vent erosion occurs. The eruption is overpressured and supersonic at the surface; this circumstance produces blast conditions that form a crater and pyroclastic surges dominate ejecta dispersal.

In both shallow and deep reservoirs—but especially in shallow ones—a triggering mechanism is required to initiate caprock failure. The gradual breakdown of caprock strength through rate-limited chemical dissolution, sudden jarring by a seismic event, rapid heating by magma intrusion, the sudden influx of noncondensible gas, or unloading of overlying material through an avalanche or draining of a lake might trigger failure. An additional mechanism, hydraulic fracturing, contributes to the failure of caprock.

Hydrothermal eruption phenomena are strong indicators of active hydrothermal reservoirs. Phreatic craters and their deposits range from pits 1 m across to lake-filled depressions up to 1 km in diameter, as is seen in the example of the Eastern Kawerau Geothermal Field in New Zealand. Ejecta deposits from these eruptions can extend >1 km from the crater center. Studies of lithic clasts within phreatic explosion breccias indicate that the foci for hydrothermal eruptions occur at many depths and have been observed as deep as 350 m.

Most phreatic breccia deposits are massive, but they may also be bedded and may include graded bedding and pyroclastic surge dunes features that are indicative of multiple steam blasts. The massive deposits consist of poorly sorted angular tephra from sub-millimeter size to blocks several meters in diameter in a muddy matrix. Nearly all the lapilli and blocks are hydrothermally altered and/or silicified; the glass has been altered to clay or hydrothermal quartz, and lithic and crystal components are replaced by clays, pyrite, chlorite, and other hydrothermal minerals. Many of the lithic clasts retain their relict textures and may contain several generations of fractures, filled with hydrothermal minerals, that formed in-situ during earlier hydrothermal

explosions (Nairn and Solia, 1980). Bedding-plane sags are common; they resulted when blocks ejected during steam blasts impacted the muddy, fine-grained beds deposited earlier.

Phreatic craters and their deposits are undisputed indicators of the presence of a high-temperature hydrothermal system and therefore are excellent prospecting tools. If there is a sequence of phreatic breccia deposits, dating each deposit may provide information on thermal pulses that occurred during the history of the geothermal field. Because phreatic eruptions can be initiated accidentally by drilling or failure of a casing in a geothermal well (Bixley and Browne, 1988), these events must be considered potential hazards during the drilling and production processes.

Hydrothermal Alteration

Hydrothermal alteration is a general term embracing the mineralogical, textural, and chemical response of rocks to a changing thermal and chemical environment in the presence of hot water, steam, or gas.By mapping alteration mineral assemblages at the surface (but more commonly within drillholes), it is possible to locate the zones with highest temperatures, pressures, or permeabilities—all of which are important in geothermal exploration. The same techniques are used to map fossil hydrothermal systems associated with epithermal ore bodies.

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