K. A. Rodgers

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.

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.

The occurrence, detection and significance of moganite (SiO2) among some silica sinters

Abstract Moganite, monoclinic SiO2, is a component of microcrystalline, quartz-bearing, sinters of New Zealand derived from crystallization of non-crystalline and paracrystalline opaline silicas. It occurs at levels of <13 vol.% of the SiO2phases present in sinters between 20,000 and 200,000 y old but is generally either absent or below the level of detection in Tertiary sinters. Unambiguous identification of moganite is most readily accomplished by laser Raman spectroscopy; the technique allows individual microtextural elements of a sinter’s fabric to be analysed. Conventional scanning X-ray powder diffraction procedures are limited in their ability to discern the characteristic moganite diffraction lines from the very similar quartz pattern, especially in those samples where moganite is at low concentration and/or unanticipated. However, powder diffraction, using a position-sensitive detector system, allows not only the identification of the moganite pattern in the presence of a large proportion of quartz, but also semiquantitative estimates of the different silica phases present in bulk sinter samples of ~450 mg. Moganite is part of the sinter maturation sequence. It occurs as a metastable phase that will ultimately transform to quartz, given sufficient time or a change in ambient conditions.

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.

The taxonomic status, and isotopic evidence for paleoenvironments, of giant oysters from the Oligocene Te Kuiti Group, South Auckland, New Zealand

Abstract Close inspection of the morphologic characteristics of giant oysters from the Oligocene Te Kuiti Group indicates that they do not belong in any of the several genera so far assigned them in the literature, including the estuarine genus Crassostrea. They are most typical of the tribe Flemingostreini Stenzel, but without more extensive and specialist study it is premature to give them generic status, which might only perpetrate further taxonomic confusion. Carbon and oxygen isotopic analyses of the oyster shells, as well as independent evidence, suggest that oysters in: (1) the Mangakotuku Siltstone formed in a protected, shallow marine embayment, open only to the north, having locally restricted circulation, but mainly normal to only slightly reduced salinites, the light carbon in shells being derived more from the reaction with seawater of locally produced C02 from decomposition of abundant organic debris in the bottom sediments than from widespread dilution of seawater by freshwater; (2) the Wha...

The steam condensate alteration mineralogy of Ruatapu cave, Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand

Abstract Ruatapu cave has developed beneath a block of hydrothermally altered Quaternary vitric tuff in the active Orakei Korako geothermal field. The cave extends ~45 m, with a vertical drop of 23 m, to a shallow pool of clear, sulfate-rich (~450 μg/g), warm (T = 43-48°C), acid (pH = 3.0) water. Steam, accompanied by H2S, rises from the pool surface, from a second pool nearby, and from fumaroles and joints in the ignimbrite, to condense on surfaces within the cave. Oxidation of the H2S to H2SO4 produces acid sulfate fluids which react with the surficial rocks to generate three principal and distinct assemblages of secondary minerals. Kaolinite ± opal-A ± cristobalite ± alunite ± alunogen dominate the assemblage at the cave mouth; the essential Al, K and Si are derived from the tuffs and Na, Ca, Fe and Mg removed. In the main body of the cave the highly limited throughflow of water results in the more soluble of the leached constituents, notably Na and K, being retained in surface moisture and becoming available to form tamarugite and potash alum as efflorescences, in part at the expense of kaolin, along with lesser amounts of alunogen, meta-alunogen, mirabilite, halotrichite, kalinite, gypsum and, possibly, tschermigite; the particular species being determined by the prevailing physico-chemical conditions. Heat and moisture assist in moving Fe out of the rock to the air-water interface but, unlike typical surficial acid alteration systems elsewhere in the TVZ, there is an insufficient flow of water, of appropriate Eh-pH, to continue to move Fe out of the cave system. Much becomes locally immobilized as Fe oxides and oxyhydroxides that mottle the sides and roof of the cave. Jarosite crusts have developed where acid sulfate pool waters have had protracted contact with ignimbrite wallrock coated with once-living microbial mats. Subsequent lowering of the waters has caused the porous jarositic crusts to alter to potash alum ± akaganéite or schwertmannite. Meteoric water, with chloride concentrations of up to 10,000 μg/g, seeping through the roof produces a white, semi-thixotropic slurry which when dried yields 5.7 wt.% chloride and consisted of tamarugite plus halite. Some of this chloride (and sulfate) eventually enters the pool waters which have Cl- concentrations of 200 μg/g. This implies that the pools are not necessarily fed by a neutral pH alkali chloride fluid ascending from the geothermal reservoir, but are perched waters heated by ascending steam and fed largely by steam condensate.

The age and significance of in-situ sinter at the Te Kopia thermal area, Taupo Volcanic Zone, New Zealand

One hundred pollen grains and spores, recovered from a single sample of in situ silica sinter from the Te Kopia geothermal field, include some from a podocarp forest that grew in a temperate, frost-free climate, unlike that of today, as indicated by the presence of Ascarina. Also present are pollen from taxa introduced within the last 100 years. Ascarina has been absent from the area since at least 1800 B.P. and its presence in the sinter indicates that alkali chloride waters discharged at the surface of Te Kopia before 1800 B.P., and possibly before 3500 B.P. Although palynology is a powerful tool to place age limits on fluctuations in the shallow hydrology of geothermal fields, interpretation must be moderated by considering the present flora and also changes in both local and regional flora, habitat and climate.

Iron-rich and iron-poor prehnites from the Way Linggo epithermal Au-Ag deposit, southwest Sumatra, and the Heber geothermal field, California

Abstract Electron microprobe analyses of prehnites from the Way Linggo low-sulphidation epithermal Au-Ag deposit of southern Sumatra, show that the mole fraction of octahedral Fe3+, expressed as Fe3+/ (Fe3++AlVI), ranges from 0.0 to ~0.6, the higher values being among the most iron-rich reported for prehnite in a hydrothermal environment. Prehnites from a diabase sill in the Heber geothermal field of California have mole fractions of octahedral Fe3+ ranging from 0.03 to 0.3. The Way Linggo prehnites formed below 220°C, some 20-30°C lower than those at the Heber field; the lower crystallization temperatures perhaps enhanced the opportunity for Fe3+ to substitute in octahedral sites. In both occurrences, prehnite predates late-stage calcite, consistent with the need for waters depositing prehnite to have aCO2 <0.01 moles. At higher CO2 activities the stability field of calcite would swamp the range of aCa2+/aH+ values appropriate for crystallizing prehnite. Consequently, the presence of prehnite in a hydrothermal environment primarily indicates that degassing of the hydrothermal fluid in CO2 occurred prior to deposition.

A modern, evaporitic occurrence of teruggite, Ca4MgB12As2O28.18H2O, and nobleite, CaB6O10.4H2O, from the El Tatio geothermal field, antofagasta province, chile

Abstract Teruggite is the dominant phase in a soft, off-white, poorly-layered and weakly-cemented surface crust, 10−15 mm thick, occurring in the high-temperature El Tatio geothermal field of Chile. Other minerals present include halite, which is present throughout but also forms a thin (<0.5 mm), brittle, cratered surface to the deposit, nobleite, ulexite and opal-A, with possible traces of illite-smectite and at least one unidentified phase. With the exception of ulexite, none of the minerals associated with teruggite at El Tatio has been reported from other occurrences of this mineral, nor do they occur with nobleite in its sole other known occurrence in Death Valley. EDS and XPS analyses of the main mass of the deposit show the presence of Ca, As, B, Na, and Cl, consistent with the identified mineral assemblage, but with elevated concentrations in Ca and Cl that are presumably associated with a further phase. Little Mg is present and the El Tatio teruggite appears deficient in this element, with Ca presumably replacing Mg in the structure. Unlike earlier documented occurrences of teruggite, that at El Tatio is evaporitic, modern and surficial. It is located some 50 m from the nearest hot (~50°C) pool and there is no evidence of association with fluid discharge. As such, the deposit has presumably derived from a fluid moving in the uppermost levels of the El Tatio field; perhaps a heavily modified version of the brines found in the deep wells.

Stratigraphy and sedimentology of the Coalgate area, Canterbury, New Zealand

Abstract Stratigraphic and sedimentologic studies of the Cretaceo-Tertiary sequence overlying Torlesse rocks in the Coalgate area permit a review of the lithostratigraphy of the region. Much new information has come from borehole logs and cores. Coal measures conformably overlain by shallow marine strata and associated igneous rocks, including View Hill Basalt, are assigned to the Eyre Group (modified name) of upper Cretaceous and possibly Paleocene age. Burnt Hill Group (new name) is erected to accommodate some 200 m of Oligocene-Miocene non-calcareous marine, estuarine, and associated igneous rocks of central and northern Canterbury. Formations include: Homebush Standstone (new name for a massive estuarine deltaic deposit) (oldest unit); Thongcaster Formation of Duntroonian age (new name for a fossiliferous clayey fine sandstone); Wairiri Volcaniclastite (new name for a tuff-breccia deposit); Chalk Quarry Sandstone of Waiauan age (modified name); Chalk Hill Clay; Sandpit Tuff (name now formalised) consi...