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 (Fournier and Rowe, 1966). 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. Rimstadt and Cole (1983) 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, as is illustrated in Fig. 3.1.

White et al . (1964), in a classic study of the Steamboat Springs thermal area, Nevada, devised the classification of sinters summarized in the next two sections.

Single-Stage or Primary Sinters

Thin-bedded opaline sinters are thought to have been formed by primary deposition of silica on broad discharge aprons (Fig. 3.2). 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"

Fig. 3.1
Amorphous silica solubility at 25°C as a function of the radius (r) of the particles.
For particles with a positive radius of <0.05 µm, the solubility is noticeably greater 
than the bulk solubility. If the negative radius of curvature in the embayment be-
tween two particles is <0.05 µm, the silica solubility is less than the bulk solubility 
and the rate of silica precipitation will be accelerated—cementing them together.
(Adapted from Rimstadt and Cole, 1983.)

habit. White et al . (1964) noted that this sinter type is particularly useful in geothermal exploration because it is always deposited close to former spring vents and fissures.

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.

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.

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

Fig. 3.2
Layered and knobby silica sinter terrace at San Ignacio, Honduras, deposited as overlapping low fans
from springs located along a fault. Springs are issuing from fractures developed in the hard, brittle sinter.

Beowawe, Nevada, Rimstadt and Cole (1983) 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.