Volcanology and Geothermal Energy "d0e7172"

Characterization and Interpretation

The two basic types of alteration associated with volcanic geothermal systems, acid-sulfate and adularia-sericite are modeled in Fig. 3.9. Acid-sulfate alteration occurs within the uppermost parts of a volcano or along caldera ring fractures where there is abundant, cool groundwater; acid-sulfate water is formed where the groundwater mixes with rising magmatic gases. Adularia-sericite alteration occurs within a flow regime high above or adjacent to a deep heat source and is characterized by neutral pH and alkalichloride waters (Heald et al ., 1987). Alteration rank , used as an empirical indication of temperature and permeability within a volcanic field, is determined through studies of secondary minerals; for example, epidote is an indicator of high temperature and adularia is characteristic of high temperature and high permeability within a hydrothermal system (Browne, 1977).

Many terms used to describe alteration assemblages have evolved in the literature of both geothermal and ore-deposit exploration. Heald et al . (1987) evaluated and correlated these terms and their uses in the two fields to generate the material presented in Table 3.2.

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Fig. 3.7
Phreatic explosion breccia on the east wall of
South Crater, Inyo Craters, California. South
Crater is a 100-m-diameter phreatic (and
possibly partly phreatomagmatic) crater that
overlies both rhyolitic and basaltic dikes.
Deposits here are 25 m thick and consist of a
poorly bedded, cross-bedded lithic ash that
contains blocks up to 0.5 m in diameter (lower
half of the deposit) and massive, block-
bearing coarse ash (upper half of the deposit).

Browne (1977) described 51 hydrothermal minerals found in active geothermal systems; some of these minerals also occur in low-grade metamorphic rocks. The water/rock interactions in the system result in alteration of, first, volcanic glass and then a sequence of mineral phases—replacing them, leaching them, or depositing new minerals in available pore space (Browne, 1982). Typical alteration replacement products are listed in Table 3.3. The mineral assemblage depends on temperature, pressure, fluid composition, and permeability,

Fig. 3.8
The hydrothermal cycle. This diagram
demonstrates the interplay of water/rock
interaction, deposition of hydrothermal
minerals, and fracturing in a constantly
evolving hydrothermal reservoir.
(Adapted from Elders, 1981.)

and the sequence of mineral alteration and replacement varies from system to system, as is shown in Table 3.4 for several hydrothermal systems.

There is a general relationship between temperature and mineralogy for aluminosilicate alteration minerals (see Fig. 2.47), and mineral suites can be used to interpret temperatures within a geothermal system (Fig. 3.10; Henley and Ellis, 1983). For example, the minerals epidote and wairakite do not appear until 200°C.

During his examination of the geothermal field at Broadlands, New Zealand, Browne (1970) determined that many hydrothermal minerals are of little use in estimating subsurface temperatures and permeability; among these are chlorite, pyrite, calcite, and quartz, which are stable over a wide temperature range. Calcite is strongly affected by underground CO₂ pressure. Mordenite, siderite, and cristobalite, which form at low temperatures, and epidote, which forms at high temperatures, are not greatly affected by permeability. Clays, which are excellent indicators of temperature, are not good

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Fig. 3.9
Models of two types of fossil hydrothermal systems that are responsible for epithermal ore deposits.
(a) In the system characterized by acid-sulfate alteration, wiggly arrows represent rising sulfur-rich
magmatic gases; these gases condense and oxidize to form the acid fluids responsible for leaching
and argillic alteration of rocks within the volcano and at the surface. (b) In the system characterized
by adularia-sericite alteration, alkali-chloride waters have a neutral pH.
(Adapted from Henley and Ellis, 1983, and Heald et al ., 1987.)

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Table 3.2. Terminology for Alteration Assemblages^a
*Terminology*	Alteration Characteristics	Common Synonyms	*Notes*
Silicic	Characterized by introduced silica	Silicification	Wallrock is silicified; amethyst or chalcedony is present in veins
Potassium (K)-Feldspar	Introduced K-feldspar present as veins	Adularia, potassic selvages	Typically adularia
Potassium (K)-Feldspar-Sericitic	K-feldspar and white mica-type minerals ± pyrite	Sericitic, potassic, K-silicate	Structurally controlled; disseminated near veins
Sericitic	Mica-type mineral (for example, illite + quartz + pyrite; includes mixed-layer illite in which illite layers are dominant	Phyllic quartz-sericite, illitic
Sericitic-Argillic	Both white mica-type and kaolin-smectite-group minerals	Argillic, intermediate argillic, sericitic, phyllic
Argillic	Kaolin- and smectite-group minerals (for example, montmorillonite); does not typically include mica-type minerals	Intermediate argillic	Often zoned, with kaolinite nearer veins and montmorillonite farther from veins.
Advanced Argillic	Minerals representing extreme base leaching (for example, kaolinite) and sulfate or halogen fixation (for example, alunite, zunyite)	Argillic, alunitic, quartz + alunite
Chloritic	Introduction of a chlorite component (usually Fe-rich) into the vein; may occur alone or with hematite, quartz, and pyrite or other sulfides; occurs sparsely as a replacement of phenocrysts or pumice fragments in wallrock	Chloritic	Vein mineral or selvage; rarely disseminated in wall rock
Propylitic	Characterized by chlorite, albite, epidote, carbonate ± pyrite, Fe-oxides, and minor sericite	Quartz-chlorite-pyrite	Typically a regional alteration
Potassium Metasomatism	Introduced potassium, resulting in recrystallization of wall rocks to K-feldspar- and biotite-rich assemblages	Potassium silicate	Regional alteration
^a From Heald et al . (1987).

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Table 3.3. Typical Hydrothermal Alteration Replacement Products^a
*Original Mineral or Phase*	*Replacement Products*
Volcanic Glass	Zeolites (for example, mordenite, laumontite), cristobalite, quartz, calcite, clays (for example, montmorillonite)
Magnetite Ilmenite Titanomagnetite	Pyrite, leucoxene, sphene, pyrrhotite, hematite
Pyroxene Amphibole Olivine Biotite	Chlorite, illite, quartz, pyrite, calcite, anhydrite
Calcic plagioclase	Calcite, albite, adularia, wairakite, quartz, anhydrite, chlorite, illite, kaolin, montmorillonite, epidote
Anorthoclase Sanidine Orthoclase	Adularia
^a From Browne (1982).

Fig. 3.10
Summary of temperature ranges for common
aluminosilicate minerals. Solid lines indicate the
most common temperature ranges for these
occurrences. The three ranges shown for chlorite
are related to the transition, with rising
temperature, from swelling chlorite through
mixed swelling and nonswelling
chlorite to nonswelling chlorite.
(Adapted from Henley and Ellis, 1983.)

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Table 3.4. The Relative Order of Replacement During Alteration for Primary Minerals in Several Hydrothermal Systems^a
Primary Mineral	*New Zealand Fields (Taupo Volcanic Zone)*	*Ngawha*	*Olkaria*	*Philippines and Indonesia*
Volcanic glass	First	Absent	First	First
Magnetite Titanomagnetite Ilmenite	Second	Second	Fifth	Second
Pyroxene Amphibole Olivine	Third	Absent	Second	Third
Biotite	Fourth-fifth	Second	Absent	Rare?
Calcic-plagioclase	Fourth-fifth	Albite present	Third	Absent
Microcline Sanidine Orthoclase	Absent	Third	Fourth	Absent
Quartz	Not affected	Not affected	Not affected	Absent
^a From Browne (1982).

guides to permeability. Browne (1970) found that the most important minerals in hydrothermal surveys are the feldspars, which are sensitive to temperature and permeability. Andesine (the most common feldspar at Broadlands) is altered at temperatures between 70 and 290°C, depending on permeability, to quartz, clay, calcite, albite, or adularia. Albite replaces andesine above 230°C. Adularia may replace andesine completely or may be mixed with albite. Good production zones contain abundant secondary quartz, adularia, and calcite (Table 3.5). At Broadlands, there is a correlation between high steam and water production and the presence of adularia as the dominant feldspar in the reservoir rocks. Browne and Ellis (1970) noted that pyrrhotite occurs above 180°C but is limited to impermeable zones.

In near-surface steam-heated zones of acidsulfate geothermal systems, underground boiling adds dissolved magmatic gases to the steam phase, and oxidation creates an acid condensate above the boiling zone (Henley and Ellis, 1983). The mineral assemblages that are characteristic of acid alteration include kaolinite, alunite, gypsum, opal, and hydrated iron oxides (Steiner, 1977); this assemblage is referred to as advanced argillic alteration.

Temperatures decrease and pH increases outward from the central portion of an acid hydrothermal system, producing a systematic variation in stable mineral assemblages that can be mapped horizontally—or vertically if there are coreholes. These trends are shown in Table 3.6 (Hayashi (1973) and in Fig. 3.11 (Heald et al ., 1987).