Non-formula Elements Research Articles

Alteration and Non-Formula Elements Uptake of Zircon from Um Ara Granite, South Eastern Desert, Egypt

The Um Ara granites are a suite of granitoid rocks located in the southern part of the Eastern Desert of Egypt. The integration of various electron probe micro analyzer (EPMA) techniques, such as backscattered electron (BSE) imaging, X-ray compositional mapping, and wavelength dispersive spectrometry (WDS), has provided valuable insights into the alteration process of zircon in the Um Ara granite. The zircon exhibits high concentrations of non-formula elements such as P, Al, Ca, Fe, Ti, and REEs, suggesting that the alteration involved coupled dissolution-reprecipitation processes influenced by aqueous fluids. The negative correlations between Zr and the non-formula elements indicate that these elements were incorporated into zircon at the expense of Zr and Si, significantly affecting the distribution and fractionation of REEs in the original zircon. Based on the presented data and literature knowledge, the sequence of alteration events is proposed as follows: (1) initial zircon crystallization around 603 Ma accompanied by the formation of other U- and Th-bearing minerals like xenotime, thorite, monazite, and apatite; (2) long-term metamictization leading to fractures and cracks that facilitated fluid circulation and chemical changes; (3) a major hydrothermal event around 20 Ma that released a suite of non-formula elements from the metamicted zircon and associated minerals, with the enriched hydrothermal fluids subsequently incorporating these elements into the modified zircon structure; and (4) further low-temperature alteration during subsequent pluvial periods (around 50,000–159,000 years ago), facilitated by the shear zones in the Um Ara granites, may have allowed further uptake of non-formula elements. The interplay between hydrothermal fluids, meteoric water, and the shear zone environments appears to have been a key driver for the uptake of non-formula elements into the altered zircon.

Short communication: Inverse correlation between radiation damage and fission-track etching time on monazite

Abstract. In this study, we explored the impacts of radiation damage and chemical composition on the etching time of fission tracks in monazite. Despite the potential of monazite fission-track (MFT) dating as an ultralow-temperature thermochronology, the comprehensive effects of radiation damage and non-formula elements, especially on the etching rate of MFTs, remain unexplored, and established analytical procedures are lacking. We quantified the degree of radiation damage (ΔFHWM) in Cretaceous to Quaternary monazites distributed in the Japan arc through Raman spectroscopy and chemical composition analyses. Subsequently, MFT etching was performed to examine the correlation between these parameters and the etching time. Estimation of the degree of radiation damage showed an increase in radiation damage corresponding to the cooling age of each geological unit. For example, the Toya ignimbrite (ca. 0.1 Ma) and the Kurobegawa granodiorite (< 0.8 Ma), both of which are types of monazite from Quaternary geological units, have ΔFHWM values of 0.27 and 0.55 cm−1, respectively. In contrast, the Muro ignimbrite (ca. 15 Ma) has a ΔFHWM value of 4.01 cm−1, while the Kibe granite and the Sagawa granite, both of which are Cretaceous granitoids, yielded 7.35 and 6.31 cm−1, respectively. MFT etching of these samples according to the existing recipe (6 M HCl at 90 °C for 60–90 min) was completed at 1200, 860, 210, 120, and 90 min for the Toya ignimbrite, Kurobegawa granodiorite, Muro ignimbrite, Sagawa granite, and Kibe granite, respectively. These outcomes highlight an inverse relationship between MFT etching time and the degree of radiation damage in monazite, while the correlation between MFT etching time and chemical composition was unclear. The results affirm earlier considerations that the etching rate of MFTs is strongly influenced by radiation damage. Conversely, young samples with lower levels of radiation damage exhibit higher chemical resistance, suggesting that existing etching recipes may not adequately etch MFTs.

On the Age-Old Problem of Dating a Granite: Combined Zircon, Apatite, and Titanite Petrochronology in an I-Type Granite from Mt Stirling, Australia

Abstract To address the limitations of current dating methods, it is crucial to not only enhance existing techniques like U–Pb zircon dating but also explore alternative tools. This study focuses on three common mineral phases—zircon, apatite, and titanite—in an I-type granite. The goal is to assess their reliability as dating tools and propose improved methods for dating granitic rocks. In the case study of the Mt Stirling pluton within the Mt Buller igneous suite in Southeastern Australia, significant variability in laser ablation U–Pb zircon ages (around 100 million years) was observed. To improve the reliability of zircon age data and reduce non-magmatic-related variabilities, a data filtering protocol is applied. This protocol involves several steps such as trimming zircons with excessive K and Ca, excluding zircons with unusual core–rim age relationships, removing zircons with excessive non-formula elements (Al, Fe, and Mn), identifying hydrothermally altered zircons, and applying a 10% discordance threshold. The filtered Concordia Age (406 ± 1 Ma; mean square weighted deviation (MSWD) = 0.7, n = 80) of the host rock exhibits improved precision and reduced error compared to the unfiltered data (399 ± 2 Ma; MSWD = 9.3, n = 240). The filtered individual dates show less scatter and a mean that is different (i.e. outside statistical uncertainty), noting that their total still spans a considerable time range of ~50 million years, exceeding the individual zircon analytical reproducibility of 2 standard errors (~15 million years of 2 SE). Caution is advised when using the proposed error for the pooled analyses as a definitive precision. Similarly, trace element filtering approaches were applied to apatite and titanite samples from Mt Stirling, two phases that arguably cannot be inherited. For apatite, monitoring Ca and P as well as Zr/Y and Th/U ratios, along with identifying age groupings based on Sr concentrations, was effective in eliminating outliers and enhancing dating precision. In the case of titanite, monitoring Ca and Ti, Sr/Zr and Sr/Th ratios, and Sr/Ca and Zr/Ti ratios successfully enhanced dating precision. Notably, apatite and titanite grains were grouped in distinct Sr concentrations (high-, mid-, and low-Sr), with these groups corresponding to different date groups: high-Sr apatite and high- and low-Sr titanite returned c. 403 Ma, while low-Sr apatite and mid-Sr titanite returned c. 420 and 393 Ma, respectively. The spuriously younger or older dates may indicate an open system and influences from various common-Pb sources. The 403 Ma date coincides with the filtered zircon data, placing further confidence in the coupled approach, and is interpreted here as the igneous intrusion age. Notable is that this age is 25 Myr older than previously reported K–Ar age data, thus far considered to be the age of the intrusion. This study underscores the potential for erroneous zircon dates due to cryptic chemical influences. To enhance the reliability of age interpretation using laser ablation analyses, employing a petrochronological approach using split-stream combined age and trace element data is recommended in addition to the combination of multiple geochronometers. In the case of Mt Buller, it has proven crucial to carefully verify chemical closure of all applied geochronometers by monitoring concomitant trace element concentrations. Applied to other intrusions, petrochronology can play a critical role in obtaining reliable age information, even for igneous rocks that appear pristine. With this, we emphasise the importance of a careful approach towards individual age data interpretation, which can be produced fast and in abundance with modern analytical approaches.

Occurrence and geochemistry of altered radioactive accessory minerals as sources of radionuclide in Mesozoic granite aquifers (Korea)

Accessory minerals in granitic rocks are unlikely significant radionuclide contributions to groundwater due to their remarkable durability. However, accessory minerals incorporating U and Th may suffer structural damages due to the radioactivity and become highly susceptible to alteration. This study investigates geochemistry coupled with textural analysis of the U–Th bearing accessory minerals using a field emission scanning electron microscope and an electron probe micro-analyzer. Altered zircons with numerous open structures related to the radioactive decay show higher contents of U and Th and low analytical totals. Some thorites show high contents of U and non-formula elements due to the hydrothermal alteration in the metamicted thorite. The cerianite including U occurs as micro-veinlet in fracture with trace of Fe and Mn oxides, which indicates secondary phase formation from the decomposed accessory minerals in an oxidizing environment. Some accessory minerals with the high content of U and Th have been found in Mesozoic granite terrain in South Korea, where high concentration levels of radionuclide in groundwater were also reported. The leaching of U may be more likely when the accessory minerals are highly metamicted or altered as found in our samples. The altered zircon and thorite of the study area could be major carriers of radioelement in Mesozoic granitic aquifers where the occurrence of soluble U-minerals has not been reported.

Magmatic−hydrothermal evolutionary processes in highly evolved granitic systems: Insights from zircons of the Baishitouquan pluton, NW China

As a robust accessory mineral in igneous rocks, the mineralogical and geochemical characteristics of zircon can record the lithological differentiation and magmatic−hydrothermal evolution of highly evolved granitic systems. The F-Rb−rich, highly evolved Baishitouquan pluton of NW China exhibits gradual lithological changes from leucogranite, amazonite-bearing granite, and amazonite granite in the lower levels of the pluton to topaz-bearing amazonite granite, topaz albite granite, and pegmatite in the upper levels. In this study, three types of zircon grains were identified in five lithological zones based on textural and chemical characteristics. Type I zircon, which mostly occurs in leucogranite and amazonite-bearing granite, exhibits oscillatory zoning in cathodoluminescence images and experienced low degrees of radiation damage (0.21−0.68 × 1015 α-decay events/mg), which is indicative of its magmatic origin. Type II zircon, which mostly occurs in amazonite granite and amazonite pegmatite, exhibits textures that are indicative of hydrothermal alteration (e.g., spongy texture, porosity, and microcracks), and has elevated concentrations of some cations, such as Ca and Al. Type II zircon contains a higher concentration of non-formula elements, including rare earth elements (REEs), and Hf, Th, and U, than Type I and III zircons. Additionally, Type II zircon exhibits a significant M-type lanthanide tetrad effect and experienced varying levels of radiation damage (3.75−11.72 × 1015 α-decay events/mg). These characteristics suggest that Type II zircon has a hydrothermally altered origin. Type III zircon, which is restricted to the topaz-albite granite, has the smallest crystal size among all types of zircon grains, shows a euhedral to anhedral mottled appearance, and is characterized by patchy, cloudy, or irregular zoning, with numerous fluid inclusions. This type of zircon contains higher concentrations of Ti (110−1030 μg/g) than other types of zircon grains. Additionally, this type of zircon experienced limited radiation damage (2.18−3.69 × 1015 α-decay events/mg), and has a smooth surface and homogeneous internal textures. These characteristics suggest that Type III zircon is the product of fluid interaction with hydrothermally altered Type II zircon. Accordingly, this type of zircon crystallized directly from a Zr-saturated hydrothermal fluid during the later stages of magmatic−hydrothermal evolution. These contrasting textural and compositional features of the three types of zircon grains are indicative of three stages of magmatic−hydrothermal evolution of the Baishitouquan pluton: magmatic, magmatic−hydrothermal transition, and hydrothermal. These magmatic and hydrothermal processes were involved in the enrichment, transport, and precipitation of rare metals, such as Rb. Accordingly, this contribution demonstrates that the textures and chemistry of zircon grains can serve as petrogenetic indicators for assessing magmatic−hydrothermal evolution and rare-metal mineralization in highly evolved granitic systems. Furthermore, this study presents a model of the magmatic−hydrothermal evolution of F-rich, highly evolved granitic systems through the lens of zircon.

Tetrad Effect of Rare Earth Element Fractionation in Zircon from the Pegmatite of the Adui Massif, Middle Urals

The zircon from the pegmatite of the Adui granitic massif displays the unique the rare earth element (REE) distribution spectrum with the tetrad effect in REE fractionation. The tetrad effect often occurs in granitoid rocks, but it is rarely encountered in minerals, e.g., zircon. Fluid saturated with volatiles, water and trace elements is a factor responsible for the tetrad effect in the zircon. The detailed isotopic-geochemical study of the zircon has revealed several zones differing in internal structure (in the back-scattered electron (BSE) image), composition and REE distribution. The zones indicate changes in the crystallization environment provoked by the evolution of the pegmatite-forming melt. They occur as the gradually growing changes in composition from the unaltered zones that are light-colored in BSE to the altered zones that are dark-colored in BSE. The unaltered zones are consistent in composition and geochemical features with magmatic zircons. The high content of trace (U, Th, REE) and volatile elements (F, Cl) in water suggests its crystallization from the fluid-saturated magmatic melt. The altered zircon zones occur as recrystallized zones with high content of non-formula elements (Y, Ca, Sr, Nb, P), a non-differentiated REE distribution spectrum and an absent Ce anomaly. These features are consistent with those of hydrothermal-metasomatic zircon.

The role of zircon in hydrothermal heavy REE mineralisation: The case for unconformity-related ore deposits of north-west Australia

Zircon is hailed for its chemical and physical durability, but can undergo extensive chemical and structural modification due to radiation damage via the emission of alpha (α) particles, and subsequent low-temperature hydrothermal alteration. Here, we investigate Archean zircons from arkosic metasediments of the Browns Range Metamorphics (BRM) to evaluate their role in the formation of local unconformity-related heavy rare earth element (REE) ore deposits, within the Browns Range Dome, Western Australia. We determine that the heavy REE inventory of the BRM are primarily hosted in zircon, and that these zircons have a wide range of major element totals (77 to ~100 wt%, including low SiO2 and ZrO2 contents), and high and variable ‘non-formula’ components (U, Th, Y, REE, Nb, P, Al, Ca, Fe, Ti, F and OH− or H2O). Concentrations of Y + REE in some cases exceed 8 wt%. Extensive radiation damage (metamictisation) is confirmed by structural features including porous and amorphous domains, cavities, and voids. The lack of regional thermal events over an extended period likely prevented thermal annealing of these radiation-damaged zircons. Uptake of non-formula elements in metamict zircon, most likely during sedimentation in the late Archean, promoted further radiation damage, such that these grains remained highly susceptible to alteration by subsequent hydrothermal fluid circulation. We propose that the unconformity-related REE mineralisation was formed by saline fluids leaching Y + REE (and possibly P) from metamict zircon in the BRM, followed by ore mineral precipitation in fault zones near, and along the regional unconformity. More broadly, this model of ore formation may be relevant to other basin-hosted mineral systems, and could be used to guide exploration for unconformity-related REE deposits in Australia, and globally.

Reliability of petrogenetic information obtained from the distribution of trace elements in zircon in light of the complex infusion by foreign-element-bearing low temperature solutions

The increasing application of minor elements such as REE, Ti, Fe in zircon as petrogenetic indicators depends on zircon remaining a closed chemical system over a history that can include metamorphic events, erosion, transport, and sedimentation. Whereas examples have been described of UPb loss and element exchange during metamorphic reactions, for over 50 years there have been reports of the accumulation of a range of non-formula elements such as Ca, Al, Fe, in zircons with no metamorphic history that have been tentatively related to hydrothermal action or some other process such as weathering. This open system behaviour of zircons needs further explanation and in this contribution we present data from three zircons from a late Archaean granite from Western Australia that confirm previous observations on the distribution of non-formula elements in radiation damaged zircon and support our earlier conclusion that elements are added from percolating low temperature weathering solutions and provide new insights into the mechanisms and processes controlling access of weathering fluids into the zircon. These include radiation damage accumulation over time from the decay of U and Th to produce domains of porous amorphous material that vary in porosity and the presence of weaknesses along the boundaries of oscillatory zones which allow the penetration of low temperature groundwater fluids carrying solute elements. Also important is the size of particles of the solute elements such as Al and Ca, which depends on Eh - pH of the groundwater and porous domains within the zircon structure. This low temperature fluid interaction is distinguished from hydrothermal activity by a lack of any reaction fronts or annealing of the radiation damage and a lack of any modification of existing zircon structures. Capillary action is proposed as the mechanism driving the penetration of weathering solutions into radiation damage expansion fractures and the radiation damaged zonal structure. The susceptibility of radiation damaged zircon to infusion by low temperature fluids and deposition of accompanying solute elements has implications for the application of zircon trace element chemistry as a petrogenetic and temperature indicator as well as a geochronometer and host for nuclear waste.

First Age and Geochemical Data on Zircon from Riebeckite Granites of the Verkhnee Espe Rare Earth\u2013Rare Metal Deposit, East Kazakhstan

This paper is dedicated to the isotope-geochemical study of zircon from riebeckite granites of the Verkhnee Espe rare earth-rare metal deposit and the specification of its U–Pb age. Zircon from the Verkhnee Espe massif is peculiar in the high content of non-formula elements (up to 43000 ppm REE, up to 22000 Y, and others) and demonstrates a clearly expressed heterogeneous structure. The central and rim zones of the zircon show a “magmatic” rare-earth element (REE) distribution. The intermediate zones are characterized by a flattening of the REE patterns and an anomalous enrichment in REE, Y, Nb, and Ca. This compositional feature of the zircon may be caused by impact of fluid-saturated granite melts enriched in incompatible trace elements. The δ18О values in the zircon are 5.83–7.16‰, which generally corresponds to zircon formed from granitoid melts. The age of zircon from the Verkhnee Espe rare earth–rare metal deposit is 283 ± 3 Ma, which indicates that there is no significant age gap between granite crystallization, on the one hand, and metasomatic processes and ore generation, on the other.

Metamictization and fluid-driven alteration triggering massive HFSE and REE mobilization from zircon and titanite: Direct evidence from EMPA imaging and LA-ICP-MS analyses

The major and trace element composition of the main HFSE-bearing phases from highly evolved miaskitic syenites has been investigated to assess their stability, an important control on HFSE retention and mobility during post-magmatic hydrothermal activity. The major HFSE carrier, abundant zircon (up to 5 vol%), has undergone intensive metamictization, caused by high initial U and Th along with the presence of early magmatic U-thorite inclusions. Subsequent to metamictization, zircon experienced pervasive fluid-driven alteration, as evidenced by strong depletion in Zr and Si, enrichment in non-formula elements (Ca, Fe, Al, P), deficient EMPA analytical totals, high porosity, and low BSE-intensities of strongly altered crystal interiors. Element maps suggest that extensive HFSE remobilization from altered zircon was controlled by interaction with oxidized aqueous solutions circulating through abundant microfractures that formed by volume expansion due to zircon metamictization. Enhanced HFSE mobility in hydrothermal fluids is reflected by the presence of a hydrated amorphous Zr-Th-U-Si-phase, filling microveinlets intimately associated with metamict zircons. In addition to HFSE remobilization from altered zircon, titanite released a significant amount of REE during interaction with the hydrothermal fluids that concurrently caused a fractionation of Eu in titanite, producing increased EuN/EuN* (4.16–22.2) in altered crystals. Extensively altered titanite grains with elevated EuN/EuN* are considerably depleted in ∑REE + Y compared to unaffected crystals, showing that degree of positive Eu anomalies, relating to post-magmatic metasomatic overprint, appears to be a sensitive indicator for an intensity of hydrothermal alteration and even the total amount of released incompatible elements. Pronounced LREE enrichment accompanied by HREE depletion in the strongly altered crystals indicates greater LREE retention in titanite and higher HREE mobility in the hydrothermal system probably caused by complexing with fluoride ligands. Fluorine was most likely supplied through amphibole and biotite chloritization, as suggested by notably low F in the mafic silicates along with the formation of secondary F-rich titanite at the expense of altered biotite. Moreover, the dissolution of zircon and titanite and resistance of apatite to pervasive alteration indicates that remobilization of HFSE and REE from these phases occurred under alkaline conditions (pH 8–12), presumably controlled by potassium-dominated complexes transporting HFSE along with REE over long distances, as indicated by a close spatial linkage of syenite outcrops with U-mineralization. Our results clearly demonstrate: (1) extensive chemical dissolution of zircon controlled by its self-induced radiation damage; (2) high solubility of titanite and metamict zircon under alkaline conditions; (3) enhanced mobility and long-distance transport of HFSE and REE in alkali- and F-rich hydrothermal systems.

LA-ICP-MS and TEM constraints on the magmatic and post-magmatic processes recorded by the zircon-xenotime intergrowth in pegmatite (Piława Górna, Góry Sowie Block, SW Poland)

A strongly altered zircon-xenotime intergrowth in pegmatite from the Piława Górna (the Góry Sowie Block, SW Poland) was studied with transmission electron microscopy (TEM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Distinct domains demonstrate features related to fluid-induced coupled dissolution-reprecipitation processes resulting in microscale to nanoscale porosity in the zircon and in xenotime, and diffusion-reaction processes in the metamict core of the zircon. The inclusions of secondary phases include fluorapatite, unknown layered Fe-silicate, fibrous Fe-oxides, solid solutions of coffinite-thorite and uraninite-thorianite, uraninite and secondary U-rich zircon. The fluid had high concentrations of Ca and Fe, transporting these nonformula elements through the zircon and xenotime structure while mobilizing U, Th and Pb even in robust xenotime, thus forming small inclusions in nanopores. Nanoinclusions of galena in the zircon represent major Pb mobilization that may cause disturbance during UPb geochronological analysis. The trace element composition of zircon matches patterns typical of altered zircon. The differentiation of geochemical characteristics between the metamict core and the altered rim of zircon implies different alteration mechanisms, with a stronger role of fluids in the alteration of the rim. Nanoscale observations complement trace element microanalysis and serve as an important and necessary tool to further understand the alterations of geochronometers in the presence of alkaline-rich fluid, especially in the complex environments of pegmatites.

Alteration and elements migration of detrital zircons from the Daying uranium deposit in the Ordos Basin, China

To study the alteration and elements migration characteristics of detrital zircon in sandstone-hosted uranium deposits, we analyzed three groups of zircon samples from the Jurassic Zhiluo Formation of the Daying uranium deposit in the northern Ordos Basin, China. One group of zircons was selected from the uranium metallogenic belt, while the other two groups were obtained from the non-metallogenic belt. According to the back-scattered electron (BSE) images and energy dispersive spectrometer (EDS) elemental maps, zircon can be divided into altered and unaltered zircon, and alteration types include oscillatory zoning alteration and rim alteration. The rare earth element (REE) pattern of unaltered zircon is characterized by positive Ce anomalies and negative Eu anomalies, similar with those of the Early Precambrian metamorphic rocks of the Khondalite Belt in the North China Craton. The results of the trace element analysis showed that REEs, uranium, and non-formula elements (e.g., Al, Fe, and Ca) are abundant in the altered area, specifically observable in the BSE images, and along with the EDS elemental maps of altered zircon in the uranium metallogenic belt, exhibited higher uranium concentrations and more developed cracks. Interaction of fluids with the radiation-damaged zircon lattice can provide a favorable conditions for fluid infiltration and element migration. The action of uranium-bearing fluids and the radiation of uranium minerals can induce zircon alteration and crack formation, which is instructive in uranium prospecting.

ПЕРВЫЕ ДАННЫЕ О U-Pb ВОЗРАСТЕ И СОСТАВЕ ЦИРКОНА ИЗ РУДОНОСНЫХ СИЕНИТОВ ГОРЫ PУДНАЯ (ЮЖНАЯ ЯКУТИЯ)

The isotope-geochemical study (SHRIMP-II, SIMS) of zircons from syenites of Gora Rudnaya (South Yakutia) was carried out. Gora Rudnaya is a syenite massif in the form of a laccolith. It is located within the Central Aldan ore district and includes the recently discovered Morozkinskoye gold deposit. Vein and vein-disseminated gold mineralization occurs in low-temperature acid metasomatites – beresites (Qz-Ser-Ank-Py). Mineralization is restricted to steeply dipping submeridional crush zones within the intrusion. According to the zircon dating data, the age of ore-bearing syenites is about 130 Ma. The obtained age corresponds to the main stage of magmatism and the associated hydrothermal-metasomatic activity within the Central Aldan ore district. Two groups of zircons have been distinguished in syenites. The first group is characterized by features of magmatic genesis. For the second group of zircons, there is evidence of the influence of fluids on zircons: increased content of U, Th, and some non-formula elements (LREE, Ca, Ti, Sr). The presence of two varieties of zircons that are contrasting in composition and appearance, but of the same age, indicates that the magmatic crystallization of syenites from Gora Rudnaya and their fluid processing occurred simultaneously.

First Data on the Age (U–Pb, SHRIMP-II) and Composition of Zircon from the Unique Yarega Oil–Titanium Deposit, South Timan

Isotope–geochemical study of zircon from leucoxene–quartz sandstone of the Yarega deposit in South Timan was conducted for the first time using the Sensitive High-Resolution Ion Microprobe II (SHRIMP), Secondary Ion Mass Spectrometer (SIMS) technique. Two groups of zircons were established. The first group has signs of hydrothermal processing with a high content of Light Rare Earth Elements (LREEs), Y, Ca, Sr, and other nonformula elements. Zircon that had been hydrothermal altered from titanium deposits of Middle Timan (Pizhma and Ichet’yu) is characterized by similar geochemical features. The second group of zircons has signs of magmatic genesis, including enrichment in Heavy Rare Earth Elements (HREEs), a positive Ce-anomaly, and a negative Eu-anomaly. The concordant ages of magmatic zircon grains lie in the range of 1100–2100 Ma with maxima around 1230 and 1540 Ma. The age of the magmatic zircon grains indicates the predominance of detrital material in sandstones, which was received during erosion of the ancient Mesoproterozoic basement, and excludes the participation of clastic material from the Timan–Ural folded region.

Micron- to nanoscale characterisation and U-Pb geochronology of zircon from granites of the Samphire Pluton, South Australia

Zircons from three distinct granites within the Samphire Pluton, South Australia, are characterised at the micron- to nanoscale. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating generated 207Pb/206Pb weighted average ages of 1586 ± 9.3 Ma, 1583.2 ± 8.5 Ma and 1578 ± 9.5 Ma, respectively. Although zircons from all three granites display evidence of hydrothermal overprinting, this is most strongly expressed in a distinct, red-colored granite referred to here as Granite C. Alteration is expressed as variation in the concentrations of HREE + Y, Th and Pb, whereby grain margins are relatively enriched in these elements and both Zr and Si are depleted. Altered zircon cores have high U contents relative to grain margins, which are appreciably poorer in this element, whereas Th is strongly enriched within crosscutting microfractures. Coupled with the relative enrichment in non-formula elements and marked disturbance of U-Th-Pb isotope ratios, the nanoscale observations of Pb-bearing nanoparticles (galena?) implying mobility of Pb and other elements, nanofractures and structural defects demonstrate that zircon in Granite C has undergone multi-stage alteration impacting upon accurate dating. Importantly, and with implications for analogous systems elsewhere, our results confirm nanoscale open system U-Pb behaviour in Hiltaba Suite zircon. SHRIMP U-Pb zircon geochronology cannot confidently resolve any statistical differences in the age of the three granites, despite their distinct appearance suggesting they might represent temporally distinct phases of a larger magmatic system. Zircons within the most altered Granite (C), directly underlying the Blackbush uranium prospect contain convincing micron- to nanoscale evidence for an alteration event that triggered a remobilisation of uranium from granite into the cover sequence.

Zircon U-Pb Geochronology Recorded Late Cretaceous Fluid Activation in the Central Aldan Gold Ore District, Aldan Shield, Russia: First Data

The gold mineralization in the Central Aldan ore district is genetically related to potassic calc-alkaline and alkaline magmatism dated at 115–150 Ma. The objective of this study is to establish the age of hydrothermal processes that accompanied the formation of Au-Te mineralization at the Samolazovsky Deposit. Based on the isotope-geochemical study of zircons from quartz-feldspar metasomatic rocks of the deposit, the granitoids and charnokites of the Nimnyr Complex (1 900–1 960 Ma) at the contact with the Yukhta monzonite-syenite massif (∼127 Ma) were studied. Zircon U-Pb dating was performed on a SHRIMP-II ion microprobe, and rare-earth and trace elements analyses of zircon were carried out in the same craters by secondary-ion mass spectrometry on a Cameca IMS-4f ion microprobe. It is revealed that the studied zircons have heterogeneous structures: dark core and lighter rim, which differ greatly in isotope-geochemical parameters. Zircon rims are cut by a network of fractures, extending into the central part of zircon grains. The rims yield a subconcordant age of 1 937±24 Ma, with an average total REE content of 550 ppm, which corresponds to the formation age of the Nimnyr Complex. All zircon cores yield a discordant age of 83±11 Ma and are characterized by a higher total REE content (∼1 812 ppm), as well as higher contents of U and non-formula elements (Ca, Sr, and Y) with respect to rims, due to the effect of fluid on zircons. Despite the limited number of zircon grains, the additional geochronological study of zircons from syenites of the ore-bearing Ryabinovy Massif has revealed the presence of two distinct age clusters: ∼125–138 and 76–83 Ma. The older ages of zircons from syenites are typical for the Central Aldan ore district. Until now, there is still no explanation for an age range (76–83 Ma) of single zircon grains from ore-bearing syenites of the Ryabinovy Massif. The obtained data suggest that the processes of activation (the effect of fluid) within the Central Aldan ore district continued until Late Mesozoic. With regards to the equivocal geotectonic position of the Mesozoic potassic magmatism in the study area and its high metallogenic potential, there exists an absolute necessity to determine the geochronological age of the rock formations. Therefore this study presents the Late Cretaceous geochronological data for the first time which can constrain the time-frame for the formation of gold-bearing magmatic and metasomatic rocks of the Aldan ore district.

Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity

This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

The accumulation of non-formula elements in zircons during weathering: Ancient zircons from the Jack Hills, Western Australia

In this contribution we describe the influx of non-formula elements (Fe, Ca, Al, Y, U and Th) into fractures and selected zone lamellae in zircons from Jack Hills during recent weathering and discuss the effects of this on overlapping SIMS U-Th-Pb and oxygen isotope analyses. Previous research has recognised the importance of fractures in the generation of anomalous U-Th-Pb and oxygen isotope systems. In this report we show that besides fractures specific zones in euhedrally zoned zircon can act as pathways for the influx of weathering solutions and contain a similar range of trace element materials as do the fractures. Whereas zero-age discordant U-Pb systems of Jack Hills zircons have been explained by many authors in terms of Pb loss, present results confirm conclusions of our previous study that the main discordance mechanism of Jack Hills zircons is U-Th gain, due to overlap of SIMS analyses with mineralized fractures and zone lamellae with excess weathering-fluid-deposited U and Th. We explain the anomalously light and heavy oxygen isotopes and significant OH in SIMS analyses that overlap fractures and mineralized zones as due to the presence in the fractures of Ca, Fe, Al oxides and hydroxides with complexly fractionated oxygen isotopic systems. There is a suggestion in some of the elemental maps that there has been minor dispersion of trace elements away from fractures. But SIMS U-Th-Pb and oxygen isotope analyses on parts of the zircon away from fractures and mineralized zones show no evidence of interaction with weathering-fluid, indicating that penetration of weathering fluids into the body of the zircon at the location of the SIMS spots has not occurred. Results of this study have implications for other SIMS U-Th-Pb and oxygen isotope studies of zircons from rocks that have been subjected to weathering and also for early TIMS U-Pb measurements of bulk zircon samples that show zero Ma U-Pb discordance.

Zircon at the Nanoscale Records Metasomatic Processes Leading to Large Magmatic–Hydrothermal Ore Systems

The petrography and geochemistry of zircon offers an exciting opportunity to better understand the genesis of, as well as identify pathfinders for, large magmatic–hydrothermal ore systems. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, and energy-dispersive X-ray spectrometry STEM mapping/spot analysis were combined to characterize Proterozoic granitic zircon in the eastern Gawler Craton, South Australia. Granites from the ~1.85 Ga Donington Suite and ~1.6 Ga Hiltaba Suite were selected from locations that are either mineralized or not, with the same style of iron-oxide copper gold (IOCG) mineralization. Although Donington Suite granites are host to mineralization in several prospects, only Hiltaba Suite granites are considered “fertile” in that their emplacement at ~1.6 Ga is associated with generation of one of the best metal-endowed IOCG provinces on Earth. Crystal oscillatory zoning with respect to non-formula elements, notably Fe and Cl, are textural and chemical features preserved in zircon, with no evidence for U or Pb accumulation relating to amorphization effects. Bands with Fe and Ca show mottling with respect to chloro–hydroxy–zircon nanoprecipitates. Lattice defects occur along fractures crosscutting such nanoprecipitates indicating fluid infiltration post-mottling. Lattice stretching and screw dislocations leading to expansion of the zircon structure are the only nanoscale structures attributable to self-induced irradiation damage. These features increase in abundance in zircons from granites hosting IOCG mineralization, including from the world-class Olympic Dam Cu–U–Au–Ag deposit. The nano- to micron-scale features documented reflect interaction between magmatic zircon and corrosive Fe–Cl-bearing fluids in an initial metasomatic event that follows magmatic crystallization and immediately precedes deposition of IOCG mineralization. Quantification of α-decay damage that could relate zircon alteration to the first percolation point in zircon gives ~100 Ma, a time interval that cannot be reconciled with the 2–4 Ma period between magmatic crystallization and onset of hydrothermal fluid flow. Crystal oscillatory zoning and nanoprecipitate mottling in zircon intensify with proximity to mineralization and represent a potential pathfinder to locate fertile granites associated with Cu–Au mineralization.

Zircon as a metamorphic timekeeper: A case study from the Caledonides of central Norway

During metamorphism, inherited or detrital zircon grains commonly serve as a template onto which new generations of metamorphic zircon precipitates, charting successive episodes in the thermal history of rocks. In addition, chemical modification of pre-metamorphic zircon can leave an imprint in the U–Pb systematics. We present a case study from a sample of migmatitic amphibole–biotite gneiss from central Norway that contains zircon grains that chart much of the history of an entire orogenic episode, not only through generation of new zircon, but also resetting of the U–Pb system via radiogenic-Pb loss. The investigated gneiss, from the Helgeland Nappe Complex, within the Uppermost Allochthon of the Scandinavian Caledonides records a prolonged metamorphic evolution. Phase equilibria modelling implies peak metamorphic conditions of 7.5–8.0 kbar and 650–675 °C, consistent with low-degree partial melting and the presence of small volumes of leucosome. Laser ablation split stream analysis that simultaneously measures U–Pb, REE and Lu–Hf in the same volume of zircon, yields a range of analyses that spread along concordia. A zircon population between 485 and 443 Ma is characterized by faded and blurred oscillatory-zoned domains that have gained nonformula elements. These analyses are interpreted to indicate the presence of reactive solutions during this time period that led to leaching and mobility of Pb. Another texturally distinct zircon component yields an age of 478 ± 4 Ma, has typical igneous REE zircon profiles, and records low apparent alpha doses, interpreted to reflect new zircon growth during migmitization. A further population of zircon rims and discrete grains yields an age of 453 ± 2 Ma, interpreted as a later episode of zircon growth during high-grade metamorphism. A final set of zircon overgrowths yield an age of 439 ± 3 Ma, also interpreted to reflect new metamorphic growth. Many zircon growth phases in this rock have similar REE patterns, implying broadly isochemical metamorphism, consistent with the evidence for only limited partial melting. Furthermore, the Hf isotopic signature in the analyzed zircon grains suggests that there was no release of radiogenic Hf during metamorphism. The metamorphic zircon in the studied rock records several episodes of growth and/or alteration over a period of >40 Ma that was related to protracted destruction of the Iapetus Ocean. In the context of regional data, the new results suggest that the Helgeland Nappe Complex, which is generally considered to be exotic to Baltica, underwent geographically widespread, long-lived Ordovician high-temperature metamorphism. This record of metamorphism is distinct from that recorded by tectonostratigraphically lower units, which are conventionally regarded to have been derived from Baltica. The results shed light on the assembly of the Scandinavian Caledonides and show that zircon from individual samples may be used to unravel complex orogenic histories.