Acid Sulfate Waters Research Articles

Boiling and dilution in the shallow portion of the Waiotapu geothermal system, New Zealand

The Waiotapu geothermal system is the largest in terms of surface extent and heat flow (17 km 2 and 600 MW, respectively) of the 20 major geothermal systems in the Taupo Volcanic Zone, New Zealand. Two extinct dacite cones and a rhyolite dome rise about 500 m above the Waiotapu valley. The majority of thermal features occurs in the valley over a relief of only 100 m and lies within coincident apparent resistivity and 30 m depth thermal anomalies. Thermal features comprise near-neutral pH chloride springs (often boiling), acid sulfate springs and mud pools, and mixed waters, the latter including both bicarbonate-chloride and acid sulfate-chloride springs. The majority of the sulfate discharges to the surface in the northern, higher altitude area, while most chloride that reaches the surface does so in the southern portion of the system. Comparison of measured and calculated surface chloride flux (the latter from measured heat flow and reservoir chloride concentrations) indicates that at least half of the deep chloride does not discharge at the surface, but flows laterally to the south at shallow depths, mixing with groundwater. The composition of well discharges indicates a shallow (400 m) reservoir of chloride fluid (about 1350 mg Cl/kg at 230°C) is diluted by a CO 2-rich steam-heated water (nil chloride at about 160°C), the latter located on the margins of the system. This same dilution trend is apparent from the compositional variation and K-Na and K-Mg geothermometry of hot spring waters. However, there is chemical and stable isotope evidence from hot springs that cold groundwater also acts as a diluent at shallow levels, and very near the surface steam-heated acid sulfate waters mix with the upflow. Boiling of the deep chloride fluids prior to dilution results in the stabilization of K-feldspar. In contrast, dilution at shallower depths by the steam-heated waters (both CO 2-rich and acid sulfate) leads to the water compositions initially shifting to K-mica (and interstratified clay) stability, and to kaolinite stability with further dilution and temperature decrease. The composition and distribution of these fluids are consistent with alteration mineralogy and zonation observed in drill core. Although boiling occurs at deeper levels and up to the surface in the upflow zone at Waiotapu, mixing is also common and is the dominant process on the margin and in the near-surface portion of the system. Boiling of deep chloride fluid is very common in the upper 1–2 km of the upflow zone of most geothermal systems, though it may be quenched by mixing with marginal waters, either cold groundwater in high relief terrane, or steam-heated waters in areas of lower relief. CO 2-rich steam-heated waters are common on the margins of systems and will generally underlie the near-surface, acid sulfate steam-heated waters. Interaction with CO 2-rich waters will form K-mica and interstratified clays, while acid sulfate waters cause kaolinite and even alunite to form. The former assemblage is commonly associated with typical low sulfidation epithermal gold mineralization as wallrock alteration, where gold-bearing veins comprise adularia and illite. In contrast, kaolinite is rare, and alunite absent, from a direct spatial and genetic association with gold mineralization in this environment. This implies that mixing of the deep, neutral pH chloride fluid with surficial acid sulfate waters does not commonly lead to ore-grade gold deposition. Mixing with (and cooling by) CO 2-rich, steam-heated waters leads to clay stability, accounting for the large halos of clay alteration and low-grade metal anomalies which often surround veins of high gold grade. In contrast, mixing with surficial acid sulfate waters is unlikely to cause precious metal mineralization because the chloride fluids have already lost the majority of their gold through deeper boiling and dilution.

Geothermal prospecting in the Semporna Peninsula with emphasis on the Tawau area

Thermal manifestations in the Semporna Peninsula include hot springs, seepages, mud pools and old steaming ground. Mapping of these thermal features showed that they are structurally controlled. The chemistry of the thermal waters indicate the presence of two types of waters - near neutral sodium chloride and sulphate waters. Thermal waters from the Apas Kiri area are near-neutral sodium chloride waters and the C11B and C11F ratios suggest they have a common source of geothermal fluid. The thermal water from upper Tawau is acid sulphate type and its chemistry suggests it is steam-heated water or ground water mixed with gas (H2S). The chemistry of thermal waters from Balung and Sungai Jepun areas suggests they are acid sulphate waters which may have come from a far away source and come into contact with and neutralised by cold ground water. The upper Tawau and Balung springs may indicate the perimeter of a field centered on the Maria Volcanic Complex. The Apas Kiri area is underlain mainly by Quaternary dacitic lava, tuff and agglomerate. The surface thermal manifestations cover an area of 3 km'. The volcanic rocks are calcitised, chloritised, argillised and pyritised. Around the thermal features, the rocks are strongly argillised and calcitised. Several characteristics of the Apas Kiri thermal waters e.g. chloride contents, ClIB ratio and geothermometer suggest high subsurface temperature. The ClIB ratio also suggests the reservoir rocks are andesitic. Total heat loss from the Apas Kiri 2, Apas 5 and Sungai Jepun areas have been tentatively estimated as 28.65, 0.75 and 15.05 MW-Thermal respectively.

Chemical and stable isotopic models for boundary creek warm springs, southwestern Yellowstone National Park, Wyoming

Abstract Thermal springs of the Boundary Creek hydrothermal system in the southwestern part of Yellowstone Park outside the caldera boundary vary in chemical and isotopic composition, and temperature. The diversity may be accounted for by a combination of processes including boiling of a deep thermal water, mixing of the deep thermal water with cool meteoric water and/or with condensed steam or steam-heated meteoric water, and chemical reactions with surrounding rocks. Dissolved-silica, Na + , K + and Ca 2+ contents of the thermal springs could result from a thermal fluid with a temperature of 200 ± 20°C. Chloride-enthalpy and silica-enthalpy mixing models suggest mixing of 230°C, 220 mg/l Cl − thermal water with cool, low-Cl − components. A 350 to 390°C component with Cl − ≥ 300 mg/l is possibly present in thermal springs inside the caldera but is not required to fit observed spring chemical and isotopic compositions. Irreversible mass transfer models in which a low-temperature water reacts with volcanic glass as it percolates downward and warms, can account for observed pH and dissolved-silica, K + , Na + , Ca 2+ and Mg 2+ concentrations, but produces insufficient Cl − or F − for measured concentrations in the warm springs. The ratio of a Na / a H , and Cl − are best accounted for in mixing models. The water-rock interaction model fits compositions of acid-sulfate waters observed at Summit Lake and of low-Cl − waters involved in mixing. The cold waters collected from southwestern Yellowstone Park have δD values ranging from −118 to −145 per mil and δ 18 O values of −15.9 to −19.4 per mil. Two samples from nearby Island Park have δD values of −112 and −114 per mil and δ 18 O values of −15.1 and −15.3 per mil. All samples of thermal water plot significantly to the right of the meteoric water line. The low Cl − and variable δD values of the thermal waters indicate isotopic compositions are derived by extensive dilution with cold meteoric water and by steam separation on ascent to the surface. Many of the hot springs with higher δD values may contain in addition a significant amount of high-D, low-Cl − , acid-sulfate or steam-heated meteoric water. Mixing models, Cl − content and isotopic compositions of thermal springs suggest that 30% or less of a deep thermal component is present. For example, the highest-temperature springs from Three Rivers, Silver Scarf and Upper Boundary Creek thermal areas contain up to 70% cool meteoric water and 30% hot water components, springs at Summit Lake and Middle Boundary Creek spring 57 are acid-sulfate or steam-heated meteoric water; springs 27 and 48 from Middle Boundary Creek and 49 from Mountain Ash contain in excess of 50% acid-sulfate water; and Three Rivers spring 46 and Phillips could result from mixing hot water with 55% cool meteoric water followed by mixing of acid-sulfate water. Extensive dilution by cool meteoric water increases the uncertainties in quantity and nature of the deep meteoric, thermal component.

Rare-earth element and heavy-metal behaviour associated with the epithermal gold deposit on Lihir Island, Papua New Guinea

A thick sequence of alkaline intrusions and volcanic rocks underlies a Quaternary caldera on Lihir Island. The sequence is host to a still-active subaerial hydrothermal system and associated epithermal gold mineralization. Chondrite-normalized (La/Lu) cn and (La/Sm) cn ratios progressively increase up the alteration sequence from the potassic alteration zone, to the argillic zone to the advanced argillic zone. (Tb/Lu) cn ratios only significantly increase in argillic and advanced argillic assemblages. Surface oxide alteration lithologies and acid sulphate water precipitates possess distinctly lower (La/Lu) cn and (Tb/Lu) cn ratios than the underlying subsurface alteration units. The changes in the REE, LREE and HREE fractionation trends from subsurface to surface alteration zones reflect the transition from a magmatic-hydrothermal, neutral chloride fluid regime at depth to acid sulphate meteoric waters in the upper portion of the alteration profile. Boiling of the LREE-Eu-enriched magmatic fluids occurred at a depth of at least 750 m. It is proposed that pronounced differential flow rates of the vapour and liquid phases and solution chemistry changes approximately above 300–350 m caused the incorporation of LREE and Eu into anhydrite-calcite veins and the deposition of LREE and HREE into wallrocks of upper parts of the potassic alteration unit. Condensation of the vapour phases into meteoric waters gave rise to low-temperature acid fluids that deposited large amounts of LREE within argillic and advanced argillic alteration units. This was also accompanied by HREE mobility due to large fluid volumes, acid fluid conditions and abundant sulphate complexes within the solutions. The HREE were either lost from the hydrothermal system or deposited in oxide assemblages and acid sulphate water precipitates.

Evolution of a broadlands-type epithermal ore fluid along alternative P-T paths; implications for the transport and deposition of base, precious, and volatile metals

Boiling and acidification of hydrothermal solutions are important mechanisms that lead to the deposition of base, precious, and volatile metals in epithermal systems. Computer models of these processes with a Broadlands-type geothermal water are presented in a three-part study: boiling, gas phase metal transport, and acid reactions with metal-bearing waters. Revised and partly rederived thermodynamic data necessary for such a study are presented as an appendix.Boiling of a Broadlands-type water along alternative P-T-enthalpy paths induces the precipitation of base metal sulfides, sulfosalts, and electrum. The precipitation of base metals results from the pH increase due to CO 2 degassing, and the precipitation of electrum results from the pH increase as well as sulfide loss to the gas phase. Heat transfer to or from wall rocks controls the amount of boiling; thus, because the extent of boiling controls pH, heat transfer controls the precipitated mineral assemblage.Oxyacids and hydroxides are generally the dominant aqueous species of arsenic and antimony, respectively. At low temperatures, elevated sulfide concentrations, and intermediate pH, arsenic and antimony thiosulfides become dominant. Upon boiling, the precipitation of copper, lead, and silver sulfosalts instead of sulfides is most likely at low temperatures and low pH values. Calculated low-temperature sulfosalt precipitation is consistent with shallow arsenic and antimony mineralization in epithermal systems and also with the commonly observed late precipitation of these metals as temperature decreases with time. Calculated electrum compositions do not exceed 56 mole percent gold. These compositions vary with temperature but depend mainly on the amount of other silver-bearing minerals competing with electrum.Upon boiling, all mercury fractionates into the gas phase from 300 degrees to 101 degrees C. Numerical condensation by cooling the gas phase below 101 degrees C at a constant pressure induces the precipitation of cinnabar between 80 degrees and 100 degrees C, depending on the mercury concentrations (0.8-800 ppb). Arsenic and antimony, however, do not fractionate significantly into the gas phase as long as an aqueous phase is present. Upon 100 percent boiling of the solution, antimony stays in the dry gas phase down to 220 degrees C where stibnite precipitates, and arsenic down to 130 degrees C where realgar precipitates. The transport of base metals in the dry gas phase requires temperatures above 400 degrees C.The acidification of the boiled Broadlands-type water by acid sulfate waters or by H 2 SO 4 produced from magmatic SO 2 leads to the precipitation of gold-rich electrum or gold with copper sulfides (at 200 degrees , 150 degrees , and 101 degrees C), sulfosalts (at 150 degrees and 101 degrees C), and enargite (at 101 degrees C). Acidification in a hot spring environment at 90 degrees C causes orpiment and stibnite to precipitate. Comparative results between H 2 SO 4 and HCl acidification reactions indicate that the replacement of copper, lead, and silver sulfides by sulfosalts is driven by pH decrease and is favored by higher sulfide and sulfate activities. The enargite-covellite-tetrahedrite-chalcopyrite downward zoning of acid sulfate-type epithermal deposits can be explained by acidification at constant sulfide activity. Also, the formation of shallow epithermal deposits rich in gold, relative to silver, can be explained by the reaction at shallow depths between ascending boiling waters and downward percolating acid sulfate waters.

The hydrothermal system of Nevado del Ruiz volcano, Colombia

Hot springs and steam vents on the slopes of Nevado del Ruiz volcano provide evidence regarding the nature of hydrothermal activity within the summit and flanks of the volcano. At elevations below 3000 m, alkali-chloride water is discharged from two groups of boiling springs and several isolated warm springs on the western slope of Nevado del Ruiz. Chemical and isotopic geothermometers suggest that the boiling springs are fed by an aquifer having a subsurface equilibration temperature of at least 175°C, and the sampled warm spring is fed by an aquifer having a subsurface equilibration temperature near 150°C. Similarities in conservative solute ratios (e.g., B/Cl) indicate that the alkali-chloride waters may be related to a single reservoir at depth. Isotopic ratios of hydrogen and oxygen indicate that recharge for the alkali-chloride aquifers comes mostly from higher elevations on the volcano. Steam vents and steam-heated bicarbonate-sulfate springs at higher elevations, along a linear structural trend with the alkali-chloride springs, may be derived partly from the alkali-chloride water at depth by boiling. Steam from the vents (84°C) yields a gas geothermometer temperature of 209°C. Acid-sulfate-chloride and acid-sulfate waters are discharged widely from warm springs above 3000 m on the northern and eastern slopes of Nevado del Ruiz. Similarities in B/Cl and SO4/Cl ratios suggest that the acid waters are mixtures of water from an acid-sulfate-chloride reservoir with various proportions of shallow, dilute groundwater. The major source of sulfate, halogens, and acidity for the acid waters may be high-temperature magmatic gases. Available data on hot spring temperatures and compositions indicate that they have remained fairly stable since 1968. However, the eruption of November 13, 1985 apparently caused an increase in sulfate concentration in some of the acid springs that peaked about a year after the eruption. Long-term monitoring of hot spring compositions over many years will be required to better define the effects of volcanic activity on the Nevado del Ruiz hydrothermal system.

An interpretation of the alteration assemblages at Sulphur Springs, Valles Caldera, New Mexico

Sulphur Springs, located near the western edge of the Valles caldera, north central New Mexico, is the only vapor‐dominated surface manifestation of any significance found in the Valles. The percolating acid sulfate waters and fumarolic gases have leached and oxidized the country rock near the hot springs to a bleached wasteland. The near‐surface mineral assemblages at 90°C are restricted to one and two phases occurring in contact with a variety of solutions and gases. The conditions of local equilibrium at 90°G are approximately halotrichite + alunogen, log fO2 −45, log fS2−12, pH 0.3; szomolnokite + pyrite, log fO2 −46, log fS2−16, pH 0.8; alunite, log fO2−50, log fS2−10, pH 1–2; kaolinite + sulphur, log fO2 −53, log fS2 −10, pH 1–2, log aK+ = 0.0012 m, log aSiO2 = 0.0033 m; and pyrite, log fO2 −60, log fS2 −19, pH 4.3. This study has demonstrated that local equilibrium is closely approached even under the low‐temperature and ‐pressure conditions found near the surface.

Chemistry of thermal and nonthermal springs in the vicinity of Lassen Volcanic National Park

Meaningful applications of water geothermometry to thermal springs in and around Lassen Volcanic National Park (LVNP) are limited to Growler Hot Spring and Morgan Hot Springs. Most hot springs located within LVNP are low-chloride, acid-sulfate waters associated with nearby steam vents. This type of hot-spring activity is characteristically found above vapor-dominated hydrothermal systems. These acid-sulfate waters are not generally useful for liquid chemical geothermometry, however, because their chemical compositions result from water-rock interaction at relatively shallow depths. Thermal waters at Drakesbad and in Little Hot Springs Valley have neutral-pH, low-Cl concentrations and have estimated Na-K-Ca and Na-Li geothermometer temperatures close to measured spring temperatures of 65 to 95°C. Hot-spring waters located south of LVNP at Growler Hot Spring, Morgan Hot Springs, and in the south-central part of LVNP in the Walker “O” No. 1 well at Terminal Geyser are rich in chloride and yield calculated geothermometer temperatures between 220 and 230°C. These thermal waters probably originate within a zone of upflow of high-enthalpy fluid inside LVNP and cool conductively during lateral flow to the south and southeast.

Isotopic geochemistry of acid thermal waters and volcanic gases from Zaō volcano in Japan

The chemical composition and D/H, 18O 16O and 34S 32S ratios have been determined for the acid hot waters and volcanic gases discharging from Zaō volcano in Japan. The thermal springs in Zaō volcano issue acid sulfate-chloride type waters (Zaō) and acid sulfate type waters (Kamoshika). Gases emitted at Kamoshika fumaroles are rich in CO 2, SO 2 and N 2, exclusive of H 2O. Chloride concentrations and oxygen isotope data indicate that the Zaō thermal waters issue a fluid mixture from an acid thermal reservoir and meteoric waters from shallow aquifers. The waters in the Zaō volcanic system have slight isotopic shifts from the respective local meteoric values. The isotopic evidence indicates that most of the water in the system is meteoric in origin. Sulfates in Zaō acid sulfate-chloride waters with δ 34S values of around +15‰, are enriched in 34S compared to Zaō H 2S, while the acid sulfate waters at Kamoshika contain supergene light sulfate ( δ 34 S = ∼ + 4‰ ) derived from volcanic sulfur dioxide from the volcanic exhalations. The sulfur species in Zaō acid waters are lighter in δ 34S than those of other volcanic areas, reflecting the difference in total pressure.

Geochemistry of hydrothermal alteration at the Roosevelt hot springs thermal area, Utah

Hot spring deposits in the Roosevelt thermal area consist of opaline sinter and sintercemented alluvium. Alluvium, plutonic rocks, and amphibolite-facies gneiss have been altered by acidsulfate water to alunite and opal at the surface, and alunite, kaolinite, montmorillonite, and muscovite to a depth of 70 m. Marcasite, pyrite, chlorite, and calcite occur below the water table at about 30 m. The thermal water is dilute (ionic strength 0.1–0.2) sodium-chloride brine. The spring water now contains 10 times as much Ca, 100 times as much Mg, and up to 2.5 times as much SO 4 as the deep water. Although the present day spring temperature is 25°C, the temperature was 85°C in 1950. A model for development of the observed alteration is supported by observation and irreversible mass transfer calculations. Hydrothermal fluid convectively rises along major fractures. Water cools by conduction and steam separation, and the pH rises due to carbon dioxide escape. At the surface, hydrogen and sulfate ions are produced by oxidation of H 2S. The low pH water percolates downward and reacts with feldspar in the rocks to produce alunite, kaolinite, montmorillonite, and muscovite as hydrogen ion is consumed.

The geothermal system of the Hakone volcano

The isothermal map of the Hakone volcano at sea level shows that the geothermal area now 12 by 13 km in diameter is clearly related with the present volcanic activity in central cones. Four zones are recognized in the distribution of thermal waters. Zone I is characterized by acid-sulfate water associated with solfataric gases, and is found at the highest elevation of the volcano. Zone II is characterized by bicarbonate-sulfate water issuing at the middle elevation. Zone III is high-temperature streams rich in sodium chloride, poor in bicarbonate and sulfate, flowing from the depth of active fumaroles across the zone of bicarbonate-sulfate water (zone II). Zone IV is characterized by sodium chloride-bicarbonate-sulfate water discharged at the lowest elevation. Chemistry and occurrence of thermal waters suggest the following mechanism for the genesis of thermal waters. Groundwater which infiltrates through the western side of the caldera seems to be flowing eastward passing through the basal part of central cones, where high-temperature dense steam is coming up through volcanic vents. Subsurface streams of high-temperature waters thus start from the vent area, run through the permeable zone mixing with groundwater and appear as hot springs on the steep slopes of the Hayakawa gorge at the east. The average contribution of volcanic dense steam rich in sodium chloride is estimated to be 25 ∼ 35% to the sodium chloride water of zone III. Thermal discharge evaluated by the geochemistry of sodium chloride water is 2.1 ∼ 3.0 × 10 7 cal/sec, which in turn suggests that the energy economy of the Hakone geothermal system is essentially controlled by the activity of dense steam carrying a considerable amount of sodium chloride. The bicarbonate waters are widely distributed over the area remote from the fumarole. Hot waters and volcanic gases directly coming from the vent area are relatively poor in carbonate matter. All these facts suggest that most of the carbonate materials are supplied by decomposition of fossil plants intercalated in volcanic formations. It is also suggested that acid volcanic gases rich in hydrogen sulfide are secondary products by repeated processes of vaporization and condensation of volcanic steam in the hot shallow depth of central cones. The relation between the earthquake activity and the thermal water of the volcano is briefly discussed.

The hot springs and hydrothermal eruptions of Waiotapu

Summary On the basis of chemical composition, the waters of hot springs of the Waio-tapu thermal area have been divided into four classes: acid sulphate waters, sulphate-chloride waters, chloride waters, and bicarbonate-chloride waters. The origin of the waters and the relationship of the hot springs to geological features is discussed. The natural heat flow of the Waiotapu thermal area has been estimated as 302,000 Kcal/sec above 15°C. About 900 years ago hydrothermal eruptions took place throughout the Waiotapu district, and these are discussed especially in relation to the close association of present thermal activity with craters formed, during these outbursts, and the effects of the volcanicity on the hydrothermal system. A possible mechanism for triggering these eruptions is advanced.