A key element in optimal exploration and utilisation of geothermal systems is an accurate picture of their geological and structural history. Siting of wells for extraction or reinjection, and strategies for exploration and reservoir management depend on understanding the location and duration of magmatic and tectonic events through geochronology. At several New Zealand geothermal fields, unique constraints on these events have been achieved by U-Pb dating of magmatic zircon in rocks penetrated by geothermal drillholes using secondary ion mass spectrometry (SIMS) techniques on the SHRIMP-RG instruments at Stanford University and The Australian National University. As well as underpinning magmatic and structural chronologies, the data have constrained the likely timing of heat and fluid contributions, the creation and disruption of permeability, and provided, verified or disproved stratigraphic correlations. New Zealand's high-temperature (>250 °C fluid) geothermal systems lie within the Taupo Volcanic Zone (TVZ), a rifting arc system active since ~2 Ma. Most systems are located in the central TVZ, and are hosted by the products of prolific rhyolitic and subordinate andesitic volcanism which unconformably overlie a faulted Mesozoic metasedimentary (greywacke) basement. Tectonic controls on geothermal system location and hydrology linked to caldera boundaries and fault systems of TVZ are understood on a regional scale. Many system-scale studies have provided insights through integration of geological and geophysical datasets, but until recently these have lacked a quantitative time scale. We highlight here four case studies where age data has proved to be critical in understanding the geology at New Zealand geothermal fields. At Mangakino, age data served to demonstrate that the rocks in the lowest two-thirds (~1.8 km) of the drilled stratigraphy were the intracaldera products of a linked pair of eruptions at ~1.0 Ma, indicating that the basement greywacke was at unreachable depths. At Kawerau, age data have led to a complete revision of the geological and structural history, leading to creation of a new geological model for field management. At Ngatamariki, the age data have demonstrated the minimum age for the earliest andesites (>1.9 Ma) and the onset of TVZ rifting and silicic volcanism in its present-day position, and provided reliable intrusion ages of the Ngatamariki intrusive complex. At Wairakei/Tauhara, the age data have demonstrated a major hiatus in activity from ~0.95 to ~0.35 Ma, and the extraordinarily rapid subsidence and infilling rates (~1 cm/yr) associated with rocks in the production area.
Abstract The transport and degassing pathways of volatiles through large silicic magmatic systems are central to understanding geothermal fluid compositions, ore deposit genesis, and volcanic eruption dynamics and impacts. Here, we document sulfur (S), chlorine (Cl), and fluorine (F) concentrations in a range of host materials in eruptive deposits from Taupō volcano (New Zealand). Materials analysed are groundmass glass, silicic melt inclusions, and microphenocrystic apatite that equilibrated in shallow melt-dominant magma bodies; silicic melt and apatite inclusions within crystal cores inferred to be sourced from deeper crystal mush; and olivine-hosted basaltic melt inclusions from mafic enclaves that represent the most primitive feedstock magmas. Sulfur and halogen concentrations each follow distinct concentration pathways during magma differentiation in response to changing pressures, temperatures, oxygen fugacities, crystallising mineral phases, the effects of volatile saturation, and the presence of an aqueous fluid phase. Sulfur contents in the basaltic melt inclusions (~ 2000 ppm) are typical for arc-type magmas, but drop to near detection limits by dacitic compositions, reflecting pyrrhotite crystallisation at ~ 60 wt. % SiO 2 during the onset of magnetite crystallisation. In contrast, Cl increases from ~ 500 ppm in basalts to ~ 2500 ppm in dacitic compositions, due to incompatibility in the crystallising phases. Fluorine contents are similar between mafic and silicic compositions (< 1200 ppm) and are primarily controlled by the onset of apatite and/or amphibole crystallisation and then destabilisation. Sulfur and Cl partition strongly into an aqueous fluid and/or vapour phase in the shallow silicic system. Sulfur contents in the rhyolite melts are low, yet the Oruanui supereruption is associated with a major sulfate peak in ice core records in Antarctica and Greenland, implying that excess S was derived from a pre-eruptive gas phase, mafic magma recharge, and/or disintegration of a hydrothermal system. We estimate that the 25.5 ka Oruanui eruption ejected > 130 Tg of S (390 Tg sulfate) and up to ~ 1800 Tg of Cl, with potentially global impacts on climate and stratospheric ozone.
The Taupō Volcanic Zone, New Zealand, is one of the most voluminous volcanic regions on Earth. Over the last 1.8 Myr, bimodal volcanism in this rifted-arc setting has been dominated by voluminous rhyolite ignimbrite eruptions (>6000 km3 cumulative) and minor arc-type andesite and dacite, underpinned by basaltic intrusions. The combination of the magmatism and extensionally thinned arc crust with high heat flow has resulted in more than 23 active geothermal fields in the region. The fluids sampled in the geothermal systems are dominated by dilute meteoric fluids that are low in components such as acids, alkalis, and some trace metals commonly ascribed to magmatic sources but can contain large amounts of deep sourced magmatic volatiles such as CO2. This study was designed to assess the long-term (>20,000yr) fluid and metal input into two Taupō geothermal systems, and the amount and proportion of magmatic-derived components in these fluids. We collected major and trace element compositions of hydrothermal altered whole rocks and clay minerals from two active geothermal systems and compared them with hydrothermally altered rocks of the fossil (0.6 Ma) Ngatamariki magmatic-hydrothermal system, which has a demonstrated large magmatic contribution. The Ohaaki and Rotokawa geothermal systems have both been drilled to a depth of 3000 m, with sampled reservoir fluids containing low concentrations of chloride (∼1000 mg/kg) but high levels of gas (CO2, N2) that have previously been interpreted to reflect subduction components derived from arc-type magmas. Whole-rock samples lack enrichment in chloride-transported metals such as Cu, Pb or Zn but have minor anomalies of bisulfide-complexed Au, Sb, and As in the uppermost 1500 m of the geothermal reservoirs. Hydrothermal water-rock interactions in the deep reservoirs of both geothermal systems at temperatures between 220 and 300 °C produced assemblages of illite + albite + adularia + calcite + pyrite that are in equilibrium with the observed neutral to slightly acidic pH (6 ± 1) of the present-day fluids. In the lower temperature end of this range, Mg and Fe increasingly enter the illite crystal lattice via Tschermak-type (phengite) substitution, whereas a few high-temperature (>300 °C) samples contain scarce muscovite compositions. The illites commonly have low contents of most trace metals, although minor amounts of Cs, Li, Cu, Sb and Sn occur in near-surface samples. Based on these data, over the >20,000-year lifetime of the Ohaaki and Rotokawa geothermal systems, fluids were dominated by chloride-poor meteoric water and contained little magmatic contributions other than conducted heat, some gases (CO2 - N2 ± H2S), and a small fraction of the total H2O of geothermal waters. Therefore, the inferred magma bodies of intermediate to silicic composition that lie at shallow depth beneath these geothermal systems currently are not, and likely have not been for >20,000 years, degassing significant water and chloride despite the high water and chloride contents of the magmas. By inference, the intermediate to silicic magmas at depth have not transferred large amounts of volatiles to the geothermal systems over this period, and rather are storing them in the magmatic bodies to be released in volcanic eruptions that are commonly explosive and pyroclastic in nature and highly hazardous.
Adularia and sanidine are polymorphs of potassium feldspar commonly present in felsic, hydrothermally altered volcanic deposits. Sanidine is a high-temperature volcanic mineral, whereas adularia forms post deposition by hydrothermal processes. Petrographically differentiating between these polymorphs in hydrothermally altered volcanic rocks may be utilised to distinguish geological units as well as provide insights into fluid–rock interactions. However, petrographic identification may be difficult or not possible in fine-grained drill cuttings. Here, polymorphic-sensitive, Raman spectroscopy and electron microprobe analyses are utilised to characterise adularia and potential sanidine in drill cuttings from the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand. Differences in Raman spectra are capable of distinguishing between adularia and sanidine whether using peak positions or principal component analysis. All the Ngatamariki Geothermal Field potassium feldspars analysed by Raman spectroscopy were found to be adularia, as expected, with typical high K, low Ca compositions between Or94 and Or99 confirmed with electron microprobe analyses. This applied approach demonstrates that Raman spectroscopy is a fast and effective method for lending confidence to adularia and sanidine identification, which can be utilised in geothermal fields worldwide.
Research Article| September 01, 2008 Anhydrite-bearing andesite and dacite as a source for sulfur in magmatic-hydrothermal mineral deposits Isabelle Chambefort; Isabelle Chambefort * 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA *E-mails: Isabelle.Chambefort@utas.edu.au; dillesj@geo.oregonstate.edu. Search for other works by this author on: GSW Google Scholar John H. Dilles; John H. Dilles * 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA *E-mails: Isabelle.Chambefort@utas.edu.au; dillesj@geo.oregonstate.edu. Search for other works by this author on: GSW Google Scholar Adam J.R. Kent Adam J.R. Kent 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (9): 719–722. https://doi.org/10.1130/G24920A.1 Article history received: 06 Mar 2008 rev-recd: 29 May 2008 accepted: 30 May 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Isabelle Chambefort, John H. Dilles, Adam J.R. Kent; Anhydrite-bearing andesite and dacite as a source for sulfur in magmatic-hydrothermal mineral deposits. Geology 2008;; 36 (9): 719–722. doi: https://doi.org/10.1130/G24920A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Magmaticanhydritefromandesitesanddacitesoccursasinclusionsinhigh-andlow-aluminum amphibole and pyroxene and indicates that sulfate-saturated magmas spanned a period of six million years at Yanacocha, Peru. Magmatic anhydrite from Yanacocha and other sites is characterized by light rare earth element–enriched patterns and elevated strontium contents distinct from magmatic-hydrothermal anhydrite. Petrologic arguments suggest that the hydrous and oxidized Yanacocha magmas contained more than ~1000 ppm sulfur both dissolved in the melt and as a separate sulfate phase, which is sufficient to provide all the sulfur for the genetically related giant sulfur-rich Yanacocha epithermal gold deposits. High-aluminum amphiboles contain unusual anhydrite with wormy and amoeboidal textures, which are tentatively interpreted to represent trapping of an immiscible CaSO4-water melt together with sulfur-rich apatite at a temperature of ~950 °C and a water pressure >3 kbar. Such unusually sulfate-rich magmas may be required to produce sulfur-rich magmatic-hydrothermal mineral deposits. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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