Patrick R.L. Browne

MIXED-LAYER CLAY GEOTHERMOMETRY IN THE WAIRAKEI GEOTHERMAL FIELD, NEW ZEALAND

Mixed-layer clays of variable composition and structure occur in core samples from two drillholes (WK207 and WK210) drilled into the Te Mihi sector of the Wairakei geothermal field. These were identified by X-ray diffraction analysis of glycolated and oriented sample fractions at less than 2 um and less than 0.2 ¼m.Low permeability lacustrine sediments encountered by drillhole WK207 contain a well-developed sequence of mixed-layer clays. The shallowest downhole appearance of mixed-layered illite/smectite (I0.6/Sm) occurs at 146 m depth where temperature is only 100°C. Discrete illite is present only below 297 m (200°C) in the finer size fraction (less than 0.2 ¼m). Chlorite first appears downhole, in association with illite-smectite, at 177 m depth (110°C).Drillhole WK210 encountered predominantly ignimbrites and rhyolites, and fluid flow here is mainly in channels. Within these rocks, a sequence of interlayered clays is poorly developed. Discrete illite and chlorite are present in core from only 244 m (180°), but the measured temperatures where interlayer clays occur ranges from 140 to 209°C.Differences in the identity of clay minerals present in the Wairakei reservoir, where conditions are otherwise the same, demonstrate the strong control that the type of fluid flow has on their formation. In poorly-permeable sediments, where diffuse fluid flow prevails, a clearly-defined sequence of mixed-layer clays occurs. These are absent where channel flow dominates, the discrete chlorite and illite deposit directly from solution.

The evolution of the Waiotapu geothermal system, New Zealand, based on the chemical and isotopic composition of its fluids, minerals and rocks

Abstract The Waiotapu geothermal system is hosted by silicic rocks of the Taupo Volcanic Zone, New Zealand. Exploration drilling in the late 1950s down to 1100 m provided physical information on the system. Measured temperatures show a boiling profile to 295 °C, with shallow inversions, particularly in the north. Total discharge fluid samples were collected; the geothermometry and measured temperatures show that fluids derive mainly from a shallow (~400 m deep) reservoir at about 225°C. Petrologic study of drillcore samples recovered from seven wells reveals an alteration assemblage of quartz and albite + adularia, with a variable distribution of chlorite, pyrite, calcite, zeolites, epidote, pyrrhotite, sphene, leucoxene, apatite and minor base metal sulfides; white mica is a late overprint, particularly well developed at shallow depths. Surficial alteration of kaolin, cristobalite, alunite and smectite clays reflect alteration by acid sulfate, steam-heated waters. The activities of components in minerals (determined from microprobe analyses and composition-activity relations) and fluids (speciated to reservoir conditions) indicate equilibrium now exists between the fluids and white mica; the Na/K ratio of the fluid is being controlled by dissolution of albite and adularia, while its H 2 H 2 S ratio is buffered by pyrite replacing pyrrhotite. The fluids are now slightly undersaturated with respect to calcite. The present deep fluids boil adiabatically from at least 300°C to 230°C; at depths of ≤500 m, this ascending chloride fluid is variably diluted by a steam-heated water (of zero chloride) that lies over, and occurs on the margin of, the system like a discontinuous umbrella; the steam-heated water is relatively CO2-rich (≤0.1 m). The cooling at shallow levels by this mixing has shifted the alteration from albite-adularia stability to white mica stability; this shift is enhanced by the CO2-rich nature of the diluent. Dilution of ascending chloride fluids by shallow waters is also supported by the oxygen and hydrogen isotopic composition of the mixture, with the deep fluid being enriched in δ18O and δD from local meteoric by ~7 and 10%., respectively. The patterns in whole rock δ18O indicate that they were largely shifted in isotopic composition prior to incursion of steam-heated waters (possibly induced by a series of hydrothermal eruptions ~900 years ago). In contrast, the δ18O composition of late vug calcite indicates its formation is related to the initial incursion of steam-heated groundwater and subsequent cooling; this is supported by fluid inclusion evidence. The δD shift from local groundwater composition, and the δ13C composition of CO2 determined from calcite (−4 to −6%.), may be evidence for a magmatic input to the meteoric convection cell. The shallow portion of the Waiotapu geothermal system has recently evolved, both chemically and physically, by incursion of fluids from a steam-heated carapace. Continued refluxing of these relatively cool, hybrid fluids progressively deeper (with their ‘recycled’ CO2 content) will hasten hydrolytic leaching (in contrast to a single pass of adiabatically cooling deep fluids). This action, accompanied by argillic alteration, may eventually seal the deeper portions of the system, hastening its demise. There is evidence for similar events occurring in the fossil environment at epithermal depths

Sedimentary Facies and Mineralogy of the Late Pleistocene Umukuri Silica Sinter, Taupo Volcanic Zone, New Zealand

ABSTRACT The Umukuri silica sinter is a large, late Pleistocene hot-spring deposit exposed along the eastern upthrown block of the Umukuri Fault, one kilometer southwest of the active Orakei Korako geothermal area, Taupo Volcanic Zone, New Zealand. Uplift along the fault has frozen sinter maturation to produce a characteristic silica-phase stratigraphy revealed by X-ray diffraction. Paracrystalline opal-CT dominates upper layers; opal-C occurs throughout middle and lower horizons; and recrystallized fabrics of microcrystalline quartz constitute lowermost exposed layers. Original and secondary matrix fabrics in the sinter include: fine-grained, porous, friable; dense, vitreous; and massive-mottled, diffusely layered. Original fabrics combine with silicified plant matter, peloids, pisoids, sinter fragments, detrital grains, diatoms, ostracodes, and filamentous to tufted microbial remains, to form nine depositional microfacies. Thinly laminated, plant-rich, and palisade types dominate. Sinter breccia and wavy-laminated varieties also are common, whereas curved laminae with lenticular voids ( = bubble mats), clotted, peloidal, and pisolitic microfacies are minor. Umukuri microfacies represent silicification in mid to low temperature waters on sinter apron terrace and slope areas, and in distal, marshy settings. Closely spaced, lateral and vertical intercalation of various microfacies in outcrop implies changing local flow and temperature conditions. No facies typical of high-temperature, proximal vent areas have been identified. Comparison with modern thermal spring analogs suggests that original Umukuri sinter fabrics recorded varying degrees of polymerization vs. monomerization of juvenile opal-A. Mineralogical and textural modification of matrix fabrics reflects a microscale, incremental continuum, following granular or fibrous habits along solution-precipitation pathways. Late-stage quartz rims and infills pores throughout the sinter.

Subsurface andesite lavas and plutonic rocks in the Rotokawa and Ngatamariki geothermal systems, Taupo Volcanic Zone, New Zealand

Abstract Deep (> 2 km) drillholes into the Rotokawa and Ngatamariki geothermal systems, Taupo Volcanic Zone (TVZ), New Zealand, penetrate a sequence of silicic volcanic rocks and andesite lavas, the latter being locally more than 1100 m thick. One drillhole at Ngatamariki bottoms in diorite (2200 m), the first plutonic rocks reached by any well drilled into the Taupo Volcanic Zone. The Rotokawa Andesite lavas, which occur below 1400 m depth, are older than 330 ka, and locally rest upon Mesozoic metasedimentary basement rocks of the Torlesse terrane. They are typically dense, jointed, and, where fresh, contain phenocrysts of andesine-labradorite, augite, hypersthene, titanomagnetite and, in places, hornblende. Geochemically, these rocks are medium-K, calc-alkaline, orogenic andesites. Their major- and trace-element contents and Sr isotopic ratios show them to be chemically coherent, with little lateral or vertical variation. Hydrothermal alteration has typically added volatiles but has been largely isochemical on a hand specimen scale. Mineralogical, geochemical and stratigraphic evidence indicates that the Rotokawa Andesites are not genetically or temporally related to andesite lavas exposed at Rolles Peak, 5 km east of Rotokawa, nor to andesite or diorite penetrated at Ngatamariki, but they have closer chemical affinities with the plagioclase-pyroxene phyric lavas of the Tongariro Volcanic Centre at the south end of the TVZ. The Ngatamariki Andesites have distinctive trace-element compositions (particularly Ti and Zr) and are chemically and petrographically different from most other TVZ lavas. The occurrence of thick, subsurface andesite sequences in the Rotokawa-Ngatamariki area suggests that early eruptives here were of intermediate composition rather than the more silicic volcanics now so widespread both at the surface and in the shallow levels of several nearby geothermal systems.

Routine instrumental procedures to characterise the mineralogy of modern and ancient silica sinters

Tightly constrained determinative methods can be used to characterise the silica minerals (opal-A, opal-CT, opal-C, quartz, moganite) and physical properties of silica sinters. Optimal X-ray powder diffraction operating parameters indicate silica lattice order/disorder using untreated, dry, <106 μm powders scanned at 0.6° 2θ/min with a step size of 0.01° from 10–40° 2θ and an internal Si standard. Simultaneous differential thermal and thermogravimetric analysis of 15.0±0.1 mg sinter samples of <106 μm grain size, at a heating rate of 20°C/min in dry air, identify thermal events associated with dehydration, organic combustion, and changes of state. Where abundant organic matter is present, nitrogen is the preferred atmosphere for thermal analysis. Thermogravimetric-determined water contents of sinters differ from Penfield determinations reflecting the differing nature of the two techniques. Laser Raman microprobe techniques can be used to explore the mineralogy of particular sinter morphologies and habits down to 10 μm diameter. The nature of the silica species present can assist in characterising individual sinter deposits and, combined with textural, density and/or porosity determinations, can lead to a better understanding of the hydrology and paleohydrology of a geothermal prospect.

RELATIONSHIP BETWEEN ILLITE CRYSTALLINITY AND TEMPERATURE IN ACTIVE GEOTHERMAL SYSTEMS OF NEW ZEALAND

Illite crystallinity (IC), widely used to evaluate variations in metamorphic conditions, primarily depends on temperature ( e.g., Kisch, 1983; Frey, 1987). IC is determined by measuring the half-peak-width of the 10-A illite peak on oriented mineral aggregate preparations of the <2-μm size fractions (Kubler, 1967, 1968), and is expressed in °Δ2𝛉. Over the past three decades, major advances were made in the study and understanding of illite crystallinity. The effects of sample preparation (Krumm and Buggisch, 1991), instrumental conditions and interlaboratory standardization and calibration (Kisch, 1990, 1991), and measured precision and reproducibility (Robinson et al., 1990; Warr and Rice, 1994), represent some of these advances in technique. Although methods more advanced than the IC method for sample characterization have been developed recently, such as the integral peak width method (Drits et al., 1997) and the Warren-Averbach method ( e.g., Eberl et al., 1998), the IC method still remains a most suitable monitor of very low-grade metamorphism of clastic sedimentary rocks. However, the factors that precisely control crystallinity are not known. Active geothermal systems are especially fruitful places to study the genesis of clay minerals since these environments are natural laboratories where fluid/rock interactions occur under measurable conditions. Illite is abundant and occurs at temperatures > 160°C in active geothermal fields of New Zealand. Host rocks are mainly andesitic to rhyolitic lavas and pyroclastics. In this study, we correlate crystallinity values of illites in cores recovered from two wells, drilled into two active geothermal systems, with the measured drillhole temperatures. Hence, we can evaluate factors that may influence the crystallinity of illites. The Taupo Volcanic Zone is one of the most active volcanic regions on earth; the area has experienced volcanic and tectonic activity for at least one million years (Wilson, 1993). Because of this activity, geothermal systems occur …

Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo volcanic zone, New Zealand

Abstract Erosion and excavations at Ohakuri in the Taupo Volcanic zone have exposed the upper portion (100–150 m) of a hydrothermal system that was active sometime between 700,000 and 160,000 years ago. Extensive hydrothermal alteration occurred within a host sequence of young, relatively undeformed, chemically and lithologically similar unwelded rhyolitic ignimbrite and air-fall tuffs. Mapping and petrologic work have identified six distinct alteration types. An early event formed a concentrically zoned suite of alteration through the pervasive movement of alkaline chloride type water. In the innermost zone, primary rock components were almost entirely converted to quartz + adularia ± illite ± hematite ± leucoxene. Mineralized veins and breccias of quartz ± pyrite ± adularia ± chlorite formed here in response to episodic hydraulic fracturing. This zone grades outward and upward into a zone of less intense, lower rank alteration with a mordenite + clinoptilolite + smectite + opal ± hematite assemblage, then a zone of weak clay alteration and into fresh rock. Calcite is conspicuously absent from the entire suite. Acid-sulphate type water, formed from steam-condensate, dominated the shallow activity in a second stage of alteration that followed local erosion. Widespread but discontinuous alteration converted the ignimbrite to kaolinite + opal ± hematite, with alunite occurring in the more intense zones. This alteration locally overprints the early alkali-chloride produced suite, but the focus of the second-stage activity was north of the focus of the older event. Scattered opaline sinters and silicified surficial deposits are products of either still later activity or the waning part of the second stage. Chemical analysis shows that the various alteration types have characteristic patterns of major element addition and removal; these reflect the key hydrothermal mineral reactions that formed the new assemblages. Quartz-adularia alteration involved mainly silicification, dehydration and cation exchange (K+ for Na2+, H+, Ca2+, Mg2+), whereas alteration in the mordenite zone was mostly a moderate hydration process. Kaolinite alteration involved strong hydration, hydrolysis and redistribution of silica. Trace elements show varying degrees of mobility and correlation with major elements. Alteration features identify the important upflow zones, zones of mixing between hydrothermal and shallow groundwater, and changes in alkali chloride water level. They also reflect a transition from diffuse to channel flow as sealing eliminated original rock porosity, and led to hydraulic fracturing which maintained fracture permeability in the system. Mineralogy and fluid inclusion studies indicate that the primary fluid at now-exposed levels was a high-pH (7–8), low-CO2 and low-H2S water cooler than 200°C, probably modified by boiling at depth.

The mineralogy, texture and significance of silica derived from alteration by steam condensate in three New Zealand geothermal fields

Abstract Opaline silica residue accumulates on the surface and in the near surface of the Te Kopia, Tikitere and Rotokawa geothermal fields, where rhyolitic tuffs are attacked by steam condensate, made acid (pH 2-3) by sulphuric acid produced by oxidation of H2S that accompanies steam discharge. Silica residue is one product of this alteration process that also yields kaolinite, sulphur, sulphide and aluminous sulphates, including alunite and alunogen, as pH, Eh and available moisture fluctuate across the field surface. Coagulation of colloidal polymeric silica or, possibly, direct deposition of monomeric silica can occur from the acid solutions of the digested country rock, depending on pH, concentration, temperature and the presence and concentration of other species. As with silica sinter, the first-formed silica phase consists of disordered opal-A microspheroids, commonly 0.1-5 μm in diameter. These coalesce and become overgrown by further opaline silica to yield a mass resembling gelatinous ‘frog spawn’ that lines cavities and envelops surfaces. This mass is the principle component of botryoidal, transparent to translucent hyalite that comprises much residue. Following deposition, this juvenile residue may crystallize to opal-CT lepispheres, 1-3 μm across and, subsequently, to chalcedonic quartz. Both the opal-A and opal-CT of the New Zealand residues are more disordered than those occurring in typical moderate- to low-temperature sinters. The opaline silica of silica residues enjoys a reaction relationship with both kaolinite and aluminium sulphates, including alunite and alunogen. These phases and the silica precipitate continuously and undergo dissolution at the surface of all three localities. The precise pathway followed depends upon the prevailing surface conditions, including humidity, pH, Eh, and Al and K activities. As Al is flushed from the system, the ultimate stage of alteration that may result is the dissolution of the silica itself in acidified rainwater, fogdrip or further steam condensate.

Characterising the hydrothermal alteration of the Broadlands–Ohaaki geothermal system, New Zealand, using short-wave infrared spectroscopy

Abstract Hydrothermal clay minerals present in the Broadlands–Ohaaki geothermal field were characterised by field portable short-wave infrared spectroscopy. Three major alteration zones, an upper smectite, a middle illite and a lower illite–chlorite, are spectrally separable. The zoning pattern is generally consistent with the thermal structure of the geothermal field, although occasionally zone boundaries cut present-day isotherms. The data indicate that temperature is the major control on clay zoning and permeability plays a subordinate role. Both beidellite and montmorillonite are common in the upper, low-temperature smectite zone. Kaolinite, mainly of low crystallinity, marks the margin of the field where cool acidic ground waters inflow. In the middle alteration zone, illite, dominantly K-rich, shows a narrow compositional variability. Some highly permeable zones are characterised by illite with low octahedral Al contents. Ammonium-bearing illite and buddingtonite are present locally in permeable horizons within the illite zone, where temperatures are above 200°C. Chlorite is most abundant in the lower alteration zone (temperature >250°C), although it also occurs unevenly in the upper and middle alteration zones. Chlorite varies from Mg- to Fe-rich varieties (but mostly with Mg# values

Tracking crystallinity in siliceous hot-spring deposits

Siliceous hot spring deposits (sinters) entrap paleoenvironmentally significant components and are used as extreme-environment analogs in the search for early Earth and extraterrestrial life. However, sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals. For ancient hydrothermal settings including those close to the dawn of life, these transformations have long since occurred, so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change. Three young sinters preserve the entire diagenetic sequence of silica phases, from opal-A to quartz. The 6000 to ∼ 11,500 years BP ± 70 years sinter at Steamboat Springs, Nevada, the ∼ 1600 - 1900 ± 160 years BP Opal Mound sinter at Roosevelt Hot Springs, Utah, and the ∼ 456 ± 35 years BP deposit at Sinter Island, Taupo Volcanic Zone, New Zealand, provide an opportunity to track crystallographic, mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches. Worldwide, sinter forms from cooling, alkali chloride waters as noncrystalline opal-A, transforming first into noncrystalline opal-A/CT, then paracrystalline opal-CT ± moganite, paracrystalline opal-C, and eventually to microcrystalline quartz. In this study, these changes were identified by the novel and combined application of electron backscatter diffraction, X-ray powder diffraction, and scanning electron and optical microscopy techniques. We show that mineralogical changes precede morphological and accompanied crystallographic transformations. During this modification, silica particles grow and shrink several times from the micron- to nano-meter scales via dissolution, reprecipitation and recrystallization, and diagenesis follows the Ostwald Step rule. All deposits followed nearly identical diagenetic pathways, with time as the only variable in the march toward physicochemically stable quartz crystals. Diagenesis alters original environmental signatures trapped within sinters. After five silica phase changes, filamentous microfossils are modified but still remain recognizable within sinter from the Opal Mound and Steamboat Springs deposits, and during the opal-A to opal-CT silica phase transformations at Sinter Island. Therefore, delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot-spring deposits.