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To test the potential of deformation twins to record the age of impact events, micrometre-scale size mechanical twins in shocked monazite grains from three impact structures were analyzed by atom probe tomography (APT). Shocked monazite from Vredefort (South Africa; ∼300 km crater diameter), Araguainha (Brazil; ∼40 km diameter), and Woodleigh (Australia; 60 to 120 km diameter) were studied, all from rocks which experienced pressures of ∼30 GPa or higher, but each with a different post-impact thermal history. The Vredefort sample is a thermally recrystallised foliated felsic gneiss and the Araguainha sample is an impact melt-bearing bedrock. Both Vredefort and Araguainha samples record temperatures > 900 °C, whereas the Woodleigh sample is a paragneiss that experienced lower temperature conditions (350–500 °C). A combined 208Pb/232Th age for common {1 2 ¯ 2 ¯ } twins and shock-specific ( 1 ¯ 01) twins in Vredefort monazite was defined at 1979 ± 150 Ma, consistent with the accepted impact age of ∼2020 Ma. Irrational η1 [ 1 ¯ 1 ¯ 0] shock-specific twins in Araguainha monazite yielded a 260 ± 48 Ma age, also consistent with the accepted 250–260 Ma impact age. However, the age of a common (001) twin in Araguainha monazite is 510 ± 87 Ma, the pre-impact age of igneous crystallisation. These results are explained by the occurrence of common (001) twins in tectonic deformation settings, in contrast to the ( 1 ¯ 01) and irrational η1 [ 1 ¯ 1 ¯ 0] twins, which have only been documented in shock-deformed rocks. In Woodleigh monazite, APT age data for all monazite twins [(001), ( 1 ¯ 01), newly identified 102°/ 4 ¯ 23> twin], and host monazite are within uncertainty at 1048 ± 91 Ma, which is interpreted as a pre-impact age of regional metamorphism. We therefore are able to further constrain the poorly known age of the Woodleigh impact to

To evaluate the mechanisms driving nanoscale trace element mobility in radiation-damaged zircon, we analyzed two well-characterized Archean zircons from the Kaapvaal Craton (southern Africa): one zircon remained untreated and the other was experimentally heated in the laboratory at 1450 °C for 24 h. Atom probe tomography (APT) of the untreated zircon reveals homogeneously distributed trace elements. In contrast, APT of the experimentally heated zircon shows that Y, Mg, Al, and Pb+Yb segregate to a set of two morphologically and crystallographically distinct cluster populations that range from 5 nm tori to 25 nm toroidal polyhedra, which are confirmed to be dislocation loops by transmission electron microscopy (TEM). The dislocation loops lie in {100} and {001} planes; the edges are aligned with , , and . The largest loops (up to 25 nm diameter) are located in {100} and characterized by high concentrations of Mg and Al, which are aligned with . The 207Pb/206Pb measured from Pb atoms located within all of the loops (0.264 ± 0.025; 1σ) is consistent with present-day segregation and confirms that the dislocation loops formed during our experimental treatment. These experimentally induced loops are similar to clusters observed in zircon affected by natural geologic processes. We interpret that differences in cluster distribution, density, and composition between experimentally heated and geologically affected zircon are a function of the radiation dose, the pressure-temperature-time history, and the original composition of the zircon. These findings provide a framework for interpreting the significance of clustered trace elements and their isotopic characteristics in zircon. Our findings also suggest that the processes driving cluster formation in zircon can be replicated under laboratory conditions over human timescales, which may have practical implications for the mineralogical entrapment of significant nuclear elements.

Carbonaceous matter (CM) plays a critical role in the formation of sediment-hosted ore deposits, but metal partitioning between CM and associated sulfides remains unclear. Here we use synchrotron X-ray fluorescence microscopy (SXRF), laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), and nanoscale secondary ion mass spectrometry (NanoSIMS) to characterize the distribution and deportment of trace elements between CM and sulfides from the Daqiao orogenic gold deposit, China. Four types of CM co-existing with different generations of sulfides are recognized: sedimentary CM1 with pyrite polyframboids in black shale host; indigenous CM2 with early- to main-ore pyrite and marcasite; CM3 with main-ore pyrite; CM4 with pyrite porphyroblasts in underlying graphitic schists. The SXRF results reveal that As, Se, Ni, Pb, and Cu are enriched in pyrite polyframboids relative to CM1 in black shales, although a small proportion of As, Pb, Bi, and U is accommodated in the latter. NanoSIMS images show that Au and As are associated with the CM1 matrix within pyrite framboids that are overgrown by recrystallized pyrite rims. Most trace elements are enriched in the newly-formed aggregates of pyrite and marcasite relative to the CM2 rims that formed via the interactions between ore fluids and pre-existing CM2 cores. Gold, As, Tl, Se, and Hg are highly enriched in the main-ore pyrite relative to co-precipitated CM3 veinlets, and this deportment might reflect changes in fluid redox conditions in response to hydraulic fracturing that is the main mechanism of gold deposition. The pyrite porphyroblasts in the schists are enriched in As, Se, Ni, and Cu relative to CM4, but have lower Au, As, Co, Ni, and Pb than sedimentary pyrite framboids. Taken together, we suggest that biological activity causes accumulation of As and Au within CM interstitial to the framboidal pyrite, and that these elements are released during metamorphic recrystallization and hydrothermal replacement. Trace element deportment in hydrothermal CM and sulfides is controlled by a variety of factors such as the fluid chemistry and interaction between indigenous CM and fluids. Our quantitative results provide significant new insights into the complex processes that formed carbonaceous sediment-hosted ore systems worldwide.

Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0—coupled substitution of Al3+ charge-balanced by interstitial Na+—which are typical of cristobalite. However, new electron probe microanalysis (EPMA) element maps show individual crystals to have impurity concentrations that systematically decline from crystal cores-to-rims, and atom probe tomography reveals localized segregation of impurities to dislocations. Impurity concentrations are inversely correlated with degrees of crystallinity [observed by electron backscatter diffraction (EBSD), hyperspectral cathodoluminescence, laser Raman, and transmission electron microscopy (TEM)], such that crystal cores are poorly crystalline and rims are highly ordered tetragonal α-cristobalite. The VPC-plagioclase interfaces show evidence that dissolution-reprecipitation reactions between acidic gases and plagioclase crystals yield precursory amorphous SiO2 coatings that are suitable substrates for initial deposition of impure cristobalite. Successive layers of cubic β-cristobalite are deposited with impurity concentrations that decline as Al-bearing gases rapidly become unstable in the vapor cooling within pores. Final cooling to ambient temperature causes a displacive transformation from β→α cristobalite, but with locally expanded unit cells where impurities are abundant. We interpret this mechanism of VPC deposition to be a natural proxy for dopant-modulated Chemical Vapor Deposition, where halogen-rich acidic gases uptake silica, react with plagioclase surfaces to form suitable substrates and then deposit SiO2 as impure cristobalite. Our results have implications for volcanic hazards, as it has been established that the toxicity of crystalline silica is positively correlated with its purity. Furthermore, we note that VPC commonly goes unreported, but has been observed in silicic lavas of virtually all compositions and eruptive settings. We therefore suggest that despite being metastable at Earth's surface, cristobalite may be the most widely occurring SiO2 polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies.

Glassy melt inclusions are unique geological repositories that preserve evidence of the formation and evolution of mantle and crustal-derived magmas. However, the mechanisms responsible for their preservation in slowly cooled crustal rocks remain contentious, in some part due to their small size (commonly

The geometry and composition of deformation-related low-angle boundaries in naturally deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed by subgrain rotation recrystallisation associated with the (100)[001] slip system during fluid-catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Fe, Na and Co) to the low-angle boundaries. Boundaries with 2°), the interfaces become more ordered and linear enrichment of trace elements coincides with the orientation of dislocations inferred from the EBSD data. These boundaries show a systematic increase of trace element concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle boundaries are interpreted to be captured and travel with dislocations as they migrate to the subgrain boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-present deformation. The observed compositional segregation of trace elements has significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. The observation that similar features are widely recognised in manufactured materials, indicates that the segregation of trace elements to mineral interfaces is likely to be widespread.

Hydrous phyllosilicate minerals, including the serpentine subgroup, are likely to be major constituents of material that will be bought back to Earth by missions to Mars and to primitive asteroids Ryugu and Bennu. Small quantities (< 60 g) of micrometre sized, internally heterogeneous material will be available for study, requiring minimally destructive techniques. Many conventional methods are unsuitable for phyllosilicates as they are typically finely crystalline and electron beam sensitive resulting in amorphisation and dehydration. New tools will be required for nanoscale characterisation of these precious extra‐terrestrial samples. Here we test the effectiveness of atom probe tomography (APT) for this purpose. Using lizardite from the Ronda peridotite, Spain, as a terrestrial analogue, we outline an effective analytical protocol to extract nanoscale chemical and structural measurements of phyllosilicates. The potential of APT is demonstrated by the unexpected finding that the Ronda lizardite contains SiO‐rich nanophases, consistent with opaline silica that formed as a by‐product of the serpentinisation of olivine. Our new APT approach unlocks previously unobservable nanominerals and nanostructures within phyllosilicates owing to resolution limitations of more established imaging techniques. APT will provide unique insights into the processes and products of water/rock interaction on Earth, Mars and primitive asteroids.

In our study we explore the applicability of rutile as a pathfinder for orogenic gold deposits, which are an important source of this metal worldwide. We analysed rutile associated with orogenic Au deposits from three different Precambrian terranes, the Capricorn Orogen, the Barberton Greenstone Belt and the Ashanti Belt, all of which formed under greenschist conditions and share similarities in the style of mineralisation. Microtextural evidence from scanning electron microscopy and electron back-scatter diffraction indicates that rutile formed during the main deformation and alteration stage in these rocks, and is therefore related to mineralisation. We used electron microprobe and laser ablation ICP-MS to investigate the trace element compositions of rutile and we compared our results to other gold deposits. We find that hydrothermal rutile from gold deposits contains certain trace element characteristics, in particular high Sb concentrations (up to ∼1500 ppm in Au deposits of the Capricorn Orogen), that are distinct from rutile from non-mineralised rocks of various petrogenetic origin. Other elements, such as W and Sn, are found to be more enriched in rutile from other rock types, namely felsic magmatic rocks and hydrothermal veins, and are therefore not diagnostic of Au mineralisation in this type of deposits. We also find that the presence of sub-µm-scale inclusions – in particular Zr-(Si, Th)-bearing phases, sulfide minerals and native Au – can severely affect analyses of this type of rutile and compromise the applicability of Zr-in-rutile geothermometry.

In a hypervelocity impact event, the temperatures and pressures generated by the shock waves far exceed the values produced by endogenic processes. The shock-induced processes can modify the distribution of trace elements in zircon grains located in target rocks, potentially affecting the geochemical reliability of zircon, but also providing an opportunity to better understand the mechanisms of shock deformation. The formation of reidite lamellae by the shock-induced phase change of zircon has previously been proposed to be a diffusionless, martensitic transformation, with no associated atomic mobility over nanometre length scales. However, nanoscale characterization of the zircon–reidite interface and a low-angle boundary within the reidite by atom probe tomography, transmission electron microscopy and correlative analytical techniques, shows localised enrichment of particular trace elements (Y, Al, Ca, Be, Mg, Mn, and Ti). These observations indicate the presence of additional short-range diffusional components to explain the local compositional variations observed at the nanoscale for the high-pressure transformation of zircon to reidite lamellae. A new model for this transformation is proposed that consists of two stages: 1) the early stage of the impact event where the shock waves induce defects in the zircon grain and trigger a phase transformation, resulting in trace element segregation by interface migration; and 2) the recovery stage where the trace elements and shock induced defects migrate to areas of lower energy.

Mineral-scale episodic replacement of auriferous pyrite by texturally-complex pyrite, marcasite and minor arsenopyrite occurred in breccia ores from the Daqiao epizonal orogenic gold deposit, West Qinling Orogen, China. This study uses a novel combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), Nanoscale secondary ion mass spectrometry (NanoSIMS), and secondary ion mass spectrometry (SIMS) to investigate the remobilization and re-concentration of gold and other trace elements during this complex replacement process and the probable mechanism. Several lines of evidence including some degree of preservation of external morphology, sharp contacts and compositional differences between the parent pyrite and product pyrite and marcasite, and reaction-induced porosity suggest that the replacement of parent pyrite proceeds via a two-step replacement via a dissolution and reprecipitation mechanism, plus an additional marcasite overgrowth. During the replacement of euhedral pyrite, depletion of gold and other trace elements (Te, Se, Zn, Co, Tl, Ni, W, and As) in porous product pyrite relative to its precursor indicate exsolution and remobilization of these metals from crystal lattice of the original pyrite. In the subsequent replacement of porous pyrite by two types of marcasite and minor arsenopyrite, euhedral product marcasite contains low contents of trace elements, possibly due to high metal solubility in the acidic fluids favorable for marcasite precipitation. The complex-zoned marcasite significantly enriched in gold and other metals relative to porous pyrite (W, Tl, As, Sb, Ag, Se, and Zn) is thought to have formed via precipitation triggered by further oxidation and/or immediate reduction in threshold supersaturation. Dissolution of the impurity-rich pyrite and precipitation of new pyrite and marcasite generations could have occurred at low pH plus high concentrations of dissolved Fe2+ condition caused by partial oxidation of aqueous H2S and/or S2− in ore fluids. The fluid oxidation is evidenced by a general decreasing trend of δ34S values from the parent euhedral pyrite, to product porous pyrite, euhedral marcasite, and complex-zoned marcasite. The isotopic results are consistent with ore fluid oxidation controlled by pressure fluctuations during multistage hydraulic fracturing in a fault-valve regime at Daqiao deposit. This quantitative study emphasizes that the pressure-driven hydrothermal process plays a key role in the micron- to nano-scale redistribution and re-enrichment of gold and other trace metals during episodic replacement of auriferous pyrite in brittle rheological zones from epizonal orogenic gold systems.

International audience; 10 Monazite U-Th-Pb geochronology is widely used for dating geological processes, but current 11 analytical techniques are limited to grains greater than 5 μm in diameter. This limitation 12 precludes the analysis of both micrometre-scale discrete monazite grains and fine textures 13 within monazite crystals that are commonly found. Here, we analyse reference materials by 14 atom probe tomography and develop a protocol for 208 Pb/ 232 Th dating of nanoscale domains 15 of monazite (0.0007 μm 3 analytical volume). The results indicates that the 208 Pb ++ / 232 ThO ++ are 16 higher than the true values. Such fractionation can be corrected using a linear regression 17 between 208 Pb ++ / 232 ThO ++ and the M/ΔM10 peak shape parameter, where M is the position of the 18 O2 + peak and ΔM10 the full-width-tenth-maximum for the same peak. This correction results 19 in 15 to 20 % analytical uncertainty on the corrected 208 Pb/ 232 Th age. Nonetheless, this approach 20 opens the possibility of obtaining 208 Pb/ 232 Th ages with sufficient precision to address 21 geological questions on an unprecedented small scale. To illustrate the approach, atom probe 22 geochronology of a small monazite grain from the contact aureole of the Fanad pluton 23 (Ireland) yielded a 208 Pb/ 232 Th atom probe age of 420 ± 60 Ma (2σ) and is consistent with the 24 known metamorphism in the region. 25 26 27 28

In recent years, atom probe tomography (APT) has been increasingly used to study minerals, and in particular the mineral zircon. Zircon (ZrSiO4) is ideally suited for geochronology by utilising the U-Th-Pb isotope systems, and trace element compositions are also widely used to constrain petrogenetic processes. However, while standard geoanalytical techniques provide information at micrometer scale lengths, the unique combination of chemical/isotopic sensitivity and spatial resolution of APT allows compositional and textural measurements at the nanoscale. This interlaboratory study aims to define the reproducibility of APT data across research facilities and assess the role of different aspects of the atom probe workflow on reproducibility. This is essential to allow correct evaluation of APT results and full utilization of this emerging technique within the geoscience community. In this study, nine samples from the same homogeneous, GJ-1/87 zircon reference grain were sent to nine APT institutes in Germany, the UK, USA, Canada and Australia. After preparing the sample out of a selectioned slab, each institute conducted three different rounds of APT analyses: using (i) unconstrained analysis parameters, (ii) pre-defined analysis parameters, and (iii) interpreting and quantifying a provided dataset. Data such as the measured elemental composition, acquisition parameters, or mass spectrum peak identifications, were recorded and analyzed. We observe a significant variation in the measured composition across this interlaboratory study as well as the number of trace elements identified. These differences are thought to directly result from the user's choice of atom probe data analysis parameters. The type of instrument does not seem to be a critical factor. Consequently, comparison of absolute trace element concentrations on zircon using APT between laboratories is only valid if the same workflow has been ensured.

The West Qinling Orogen is endowed with more than 100 sediment-hosted gold deposits with an estimated resource of > 2000 t Au. Previous radiometric dating results have shown that most deposits formed during a Late Triassic to Early Jurassic period of contractional deformation over the orogen. However, here we show that the newly discovered Daqiao gold deposit (> 105 t at 3–4 g/t) in the southern belt of the West Qinling Orogen formed in latest Jurassic to Early Cretaceous under a different tectonic regime. The Daqiao gold deposit is hosted in weakly metamorphosed Triassic turbidites and is spatially associated with hydrothermally altered granodiorite and diorite porphyry dykes. Six granodiorite dykes have similar zircon U–Pb ages ranging from 215.0 ± 1.1 to 211.5 ± 1.5 Ma (1σ), whereas one diorite porphyry dyke has a zircon U–Pb age of 187.5 ± 2.1 Ma (1σ). The age of gold mineralization is constrained by two types of sericite: sericite aggregates coexisting with disseminated auriferous pyrite in relatively high-grade breccia ores and sericite coexisting with auriferous pyrite in weakly mineralized granodiorite dykes. Sericite aggregates from the breccia ores have 40Ar/39Ar plateau ages ranging from 150.7 ± 3.1 to 142.3 ± 2.5 Ma (2σ), whereas grains from the altered granodiorite dykes and low-grade breccia ores have 40Ar/39Ar plateau ages of 130.8 ± 3.1 to 127.2 ± 0.6 Ma (2σ). The 40Ar/39Ar ages thus suggest two periods of gold mineralization in the latest Jurassic and Early Jurassic that are likely related to repeated brecciation at Daqiao. These Jurassic-Cretaceous mineralization ages coincide with discounted ages from several other gold deposits in the region and suggest that there is an underappreciated gold event in the West Qinling Orogen that may not have been associated with the orogenic deformation but is genetically related to the far-field effects of plate reorganization during Paleo-Pacific subduction beneath the eastern Eurasian continent.

Atom probe microscopy (APM) is a relatively new in situ tool for measuring isotope fractions from nanoscale volumes (< 0.01 μm3). We calculate the theoretical detectable difference of an isotope ratio measurement result from APM using counting statistics of a hypothetical dataset to be ± 4δ or 0.4% (2s). However, challenges associated with APM measurements (e.g., peak ranging, hydride formation and isobaric interferences), result in larger uncertainties if not properly accounted for. We evaluate these factors for Re‐Os isotope ratio measurements by comparing APM and negative thermal ionisation mass spectrometry (N‐TIMS) measurement results of pure Os, pure Re, and two synthetic Re‐Os‐bearing alloys from Schwander et al. (2015) (the original metal alloy (HSE) and alloys produced by heating HSE within silicate liquid (SYN)). From this, we propose a current best practice for APM Re‐Os isotope ratio measurements. Using this refined approach, mean APM and N‐TIMS 187Os/189Os measurement results agree within 0.05% and 2s (pure Os), 0.6–2% and 2s (SYN) and 5–10% (HSE). The good agreement of N‐TIMS and APM 187Os/189Os measurements confirm that APM can extract robust isotope ratios. Therefore, this approach permits nanoscale isotope measurements of Os‐bearing alloys using the Re‐Os geochronometer that could not be measured by conventional measurement principles.

Common Pb, the portion of non-radiogenic Pb within a U bearing mineral, needs to be accurately accounted for in order to subtract its effect on U-Pb isotopic ratios so that meaningful ages can be calculated. The propensity to accommodate common Pb during crystallization, or later, is different across the range of U bearing minerals used for geochronology. Titanite frequently accommodates significant amounts of common Pb. However, the most appropriate method to correct for this requires knowledge on the mechanism and timing of common Pb incorporation; information that is commonly difficult to extract. In this study, the spatial and compositional distribution of trace elements (including Pb) in metamorphic titanites from a Greenland amphibolite is investigated on the grain- to nano-scale. Titanites have an isotopically similar signature for both common and radiogenic-Pb in all grains but significantly different quantities of the non-radiogenic component. Microstructural and compositional examination of these grains reveals undeformed, but high common Pb (F207%) titanites have homogeneous element distributions on the atomic scale suggesting common Pb is incorporated into titanite during its growth and not during later processes. In contrast, deformed titanite comprising low-angle boundaries, formed by subgrain rotation recrystallization, comprise networks of dislocations that are enriched in Mg, Al, K and Fe. Smaller cations may migrate due to elastic strain in the vicinity of the dislocation network, yet the larger K cations more likely reflect the mobility of externally-derived K along the orientation interface. The absence of Pb enrichment along the boundary indicates that either Pb was too large to fit into migrating lattice dislocations or static low-angle boundaries and/or that there was no external Pb available to diffuse along the grain boundary. As the common Pb composition is distinctly different to regional Pb models, the metamorphic titanite grew in a homogeneous Pb reservoir dominated by the break-down of precursor U-bearing phases. The different quantity of common Pb in the titanite grains indicates a mineral-driven element partitioning in an isotopically homogeneous metamorphic reservoir, consistent with low U, low total REE and flat LREE signatures in high F207% analyses. These results have implications for the selection of appropriate common Pb corrections in titanite and other accessory phases.

The widespread use of monazite (LREEPO4) in U-Pb geochronology is underpinned by the assumption that it incorporates negligible amounts of Pb during initial growth, and that radiogenic Pb remains immobile after formation. We have investigated the nanoscale distribution of Pb in monazite from granulite facies rocks of the Sandmata Metamorphic Complex (Rajasthan, India) by atom probe microscopy to further understand the utility of monazite as a geochronometer. The studied monazite contains distinct 10 nm clusters, enriched in Ca and with a bulk composition consistent with them being apatite (Ca5(PO4)3(OH)), that are also enriched in Si and Pb relative to the monazite host. The 208Pb/232Th ratios of the clusters ranged from 1.1 ± 0.1 to 1.4 ± 0.2 (2σ), indicating that the clusters hold unsupported Pb. The 208Pb/232Th ratios of the whole specimen (including clusters) and the matrix alone are similar (

The supergiant Obuasi gold deposit is the largest deposit in the Paleoproterozoic Birimian terranes of West Africa with 62 Moz of gold (past production + resources). The deposit is hosted in the Paleoproterozoic Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dikes and granites. A three-stage deformation history is defined for the district. The D1Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at Obuasi. The D2Ob is the main deformation stage affecting the Obuasi district and corresponds to a NW-SE shortening. Tight to isoclinal folding, as well as intense subhorizontal stretching, occurred during D2Ob, parallel with the plane of a pervasive NE-striking subvertical foliation (S2Ob). Finally, a N-S shortening event (D3Ob) refolded previously formed structures and formed a distinct ENE-striking, variably dipping S3Ob cleavage that is domainal in nature throughout the deposit. Two economic styles of mineralization occur at Obuasi and contribute equally to the gold budget. These are (1) gold-bearing sulfides, dominantly arsenopyrite, mainly disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins that are as much as 25 m wide. Microstructural evidence, such as strain shadows surrounding gold-bearing arsenopyrite parallel with S2Ob, but folded by S3Ob, indicates that the sulfides were formed during D2Ob. Concentrations of as much as 700 ppm Au are present in the epitaxial growth zones of the arsenopyrite grains. Although the large mineralized quartz veins are boudinaged and refolded (indicating their formation during D2Ob), field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by D3Ob, where the S3Ob cleavage crosscuts the quartz veins in the main ore zones. Thus, these observations constitute the first evidence for multiple stages of gold deposition at the Obuasi deposit. Futhermore, three-dimensional modeling of stratigraphy, structure, and gold orebodies highlights three major controls on oreshoot location, which are (1) contacts between volcaniclastic units and pre-D1 felsic dikes, (2) fault intersections, and (3) F3Ob fold hinges. The maximum age for the older disseminated gold event is given by the age of the granites at 2105 ± 2 Ma, which is within error of hydrothermal rutile in the granites of 2098 ± 7 Ma; the absolute age of the younger gold event is not known.

The facility is being developed under the auspices of the National Resource Sciences Precinct (NRSP) – a collaboration between CSIRO, Curtin University and The University of Western Australia – and is supported by the Science and Industry Endowment Fund (SIEF RI13-01). SMR acknowledges support from the ARC Core to Crust Fluid System COE (CE11E0070). The widespread use of zircon in geochemical and geochronological studies of crustal rocks is underpinned by an understanding of the processes that may modify its composition. Deformation during tectonic and impact related strain is known to modify zircon trace element compositions, but the mechanisms by which this occurs remain unresolved. Here we combine electron backscatter diffraction, transmission Kikuchi diffraction and atom probe microscopy to investigate trace element migration associated with a ∼20 nm wide, 2° low-angle subgrain boundary formed in zircon during a single, high-strain rate, deformation associated with a bolide impact. The low-angle boundary shows elevated concentrations of both substitutional (Y) and interstitial (Al, Mg & Be) ions. The observed compositional variations reflect a dynamic process associated with the recovery of shock-induced vacancies and dislocations into lower energy low-angle boundaries. Y segregation is linked to the migration and localization of oxygen vacancies, whilst the interstitial ions migrate in association with dislocations. These data represent the direct nanoscale observation of geologically-instantaneous, trace element migration associated with crystal plasticity of zircon and provide a framework for further understanding mass transfer processes in zircon. Postprint

Several examples of zircon grains from high- to ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) metapelites exhibit a characteristic, yet atypical, core–rim interface domain

International audience; The Inata gold deposit is hosted in the Bouroum greenstone belt of northern Burkina Faso and contains ca. 5 Moz of gold resource. The greenstone belt is divided into 4 distinct domains: The Pali West, Pali-Minfo and Fété Kolé domains comprised of variable proportions of mafic to intermediated volcanic, volcaniclastic and sedimentary rocks, and the Sona Basin comprised of feldspathic sandstones and turbidites. Potential Tarkwaian-like conglomerates are rarely observed on the eastern margin of the basin. The stratigraphy is crosscut by a series of intrusions between 2172 ± 15 Ma and 2122 ± 4 Ma. A complex deformation sequence is recorded in the rocks and has been interpreted in a five stage scheme: early syn-depositional basin margin faults reactivated through time and partitioning all subsequent regional deformation (DeB); N–S compression (D1B > 2172 Ma); E-W compression (D2B, < ca 2122 Ma); NW–SE compression (D3B), and a late N–S compression (D4B). D2B-D4B overprint all rocks, including those of the Sona Basin and Tarkwaian-like conglomerates. Peak metamorphism is mid- to upper-greenschist facies.Mineralisation at Inata is hosted in black shales and volcaniclastic rocks of the Pali-Minfo domain and comprises shear-zone hosted quartz-tourmaline-ankerite veins with associated sulphides dominated by pyrite and arsenopyrite. Three generations of pyrite (py1, py2, py3) and one generation of arsenopyrite (apy2) have been identified. Py1 is parallel to bedding and early D1B foliation and not associated with gold. Py2 and apy2 are coeval, contain up to 1 ppm gold and are spatially associated with auriferous quartz veins. Py3 locally overprints previous assemblages and is also associated with Au. Fluid inclusions in quartz indicate H2O to H2O–CO2–NaCl fluids in auriferous quartz veins.Microscopic to macroscopic observation of fabric-mineral-vein crosscutting relationships indicate that mineralisation is syn-D2B, disrupted and remobilised during D3B. All observations and data are consistent with Inata representing an orogenic style of gold mineralisation formed relatively late in the evolution of the host terrane.

Auriferous sulfides, most notably pyrite (FeS 2 ) and arsenopyrite (FeAsS), are among the most important economic minerals on Earth because they can host large quantities of gold in many of the world’s major gold deposits. Here we present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. In contrast to models that link gold distribution to gold concentration, gold incorporation in arsenopyrite is shown to be controlled by the rate of crystal growth, with slow growth rate promoting the formation of gold clusters and rapid growth rate leading to homogeneous gold distribution. This study yields new information on the controls of gold distribution and incorporation in sulfides that has important implications for ore deposit formation. More broadly this study reveals new information about crystal-fluid interface dynamics that determine trace element incorporation into growing mineral phases.

Both gold-rich sulphides and ultra-high grade native gold oreshoots are common but poorly understood phenomenon in orogenic-type mineral systems, partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations. The world-class Obuasi gold deposit, Ghana, has gold-rich arsenopyrite spatially associated with quartz veins, which have extremely high, localised concentrations of native gold, contained in microcrack networks within the quartz veins where they are folded. Here, we examine selected samples from Obuasi using a novel combination of quantitative electron backscatter diffraction analysis, ion microprobe imaging, synchrotron XFM mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations. The auriferous arsenopyrites are shown to have undergone partial replacement (∼15%) by Au-poor, nickeliferous arsenopyrite, during localised crystal-plastic deformation, intragranular microfracture and metamorphism (340–460 °C, 2 kbars). Our results show the dominant replacement mechanism was pseudomorphic dissolution-reprecipitation, driven by small volumes of an infiltrating fluid that had relatively low ƒ S 2 and carried aqueous NiCl 2 . We find that arsenopyrite replacement produced strong chemical gradients at crystal-fluid interfaces due to an increase in ƒ S 2 during reaction, which enabled efficient removal of gold to the fluid phase and development of anomalously gold-rich fluid (potentially 10 ppm or more depending on sulphur concentration). This process was facilitated by precipitation of ankerite, which removed CO 2 from the fluid, increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation. Gold re-precipitation occurred over distances of 10 μm to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides. We suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic/ultramafic rocks, which act as a source of Ni and Co scavenged by chloride-bearing fluids. Both the preference of the arsenopyrite crystal structure for Ni and Co, rather than gold, and the release of sulphur during reaction, can drive gold remobilisation in many deposits across broad regions.

Unravelling complex microbial activity in modern hydrothermal vents can provide crucial insights into the evolution of ancient life on Earth. It is well established that microorganisms in hydrothermal vents have a significant impact on the cycling of metals and mineral formation. However, the detailed roles played by microorganisms in driving sulfide deposition and cycling of toxic metals, like arsenic (As) and lead (Pb), in high-temperature deep-sea hydrothermal vents remain unknown. The understanding of these mechanisms in extreme environments is of particular importance as As has been postulated as a driver of microbial activities on the early Earth. Here, we present the first geologic evidence of Pb As rich microbial filamentous clusters observed in a modern high-temperature black smoker from the Manus back-arc basin, Papua New Guinea. The clusters occur as net-like structures on the surface of barite and sulfides and are composed of multiple filaments and fine-grained Pb As sulfosalt. Each of the filaments includes an As-Pb-rich sulfosalt core and organic-rich shell structure with elevated carbon, nitrogen and phosphorus. Further synchrotron X-ray absorption near edge structure analysis shows that the clusters contain a mixture of As (II) and As (III). Additionally, those filaments show a close association with realgar (As4S4), by penetrating and dissolving this As-sulfide mineral. We interpret the filaments to be a result of As-related microbial activity in As- and Pb-enriched hydrothermal environments. The findings show possible processes through which extremophiles live in Pb and As-rich environments during chimney growth. In addition, as hydrothermal vents are regarded as modern analogs of ancient volcanogenic massive sulfide deposits, the observed biominerals present the potential to be used as proxies to trace the signatures of early life in ancient geological systems.

Transmission Kikuchi diffraction (TKD) is a relatively new technique that is currently being developed for geological sample analysis. This technique utilises the transmission capabilities of a scanning electron microscope (SEM) to rapidly and accurately map the crystallographic and geochemical features of an electron transparent sample. TKD uses a similar methodology to traditional electron backscatter diffraction (EBSD), but is capable of achieving a much higher spatial resolution (5–10 nm) (Trimby, 2012; Trimby et al., 2014). Here we apply TKD to refractory metal nuggets (RMNs) which are micrometre to sub-micrometre metal alloys composed of highly siderophile elements (HSEs) found in primitive carbonaceous chondrite meteorites. TKD allows us to analyse RMNs in situ, enabling the characterisation of nanometre-scale variations in chemistry and crystallography, whilst preserving their spatial and crystallographic context. This provides a complete representation of each RMN, permitting detailed interpretation of their formation history.\ud \ud We present TKD analysis of five transmission electron microscopy (TEM) lamellae containing RMNs coupled with EBSD and TEM analyses. These analyses revealed textures and relationships not previously observed in RMNs. These textures indicate some RMNs experienced annealing, forming twins. Some RMNs also acted as nucleation centres, and formed immiscible metal-silicate fluids. In fact, each RMN analysed in this study had different crystallographic textures. These RMNs also had heterogeneous compositions, even between RMNs contained within the same inclusion, host phase and even separated by only a few nanometres. Some RMNs are also affected by secondary processes at low temperature causing exsolution of molybdenite. However, most RMNs had crystallographic textures indicating that the RMN formed prior to their host inclusion. TKD analyses reveal most RMNs have been affected by processing in the protoplanetary disk. Despite this alteration, RMNs still preserve primary crystallographic textures and heterogeneous chemical signatures. This heterogeneity in crystallographic relationships, which mostly suggest that RMNs pre-date their host, is consistent with the idea that there is not a dominant RMN forming process. Each RMN has experienced a complex history, supporting the suggestion of Daly et al. (this issue), that RMNs may preserve a diverse pre-solar chemical signature inherited from the Giant Molecular Cloud.

The Maia large solid-angle detector array and imaging system is capable of collecting high-resolution images of up to ∼100 M pixels in size with dwell times of less than 0.2 ms per pixel and thus it is possible to document variation in textures associated with trace element chemistry by collecting quantified elemental maps of geological samples on the scale of entire thin sections in a short time frame (6–8 hr). The analysis is nondestructive and allows variation to be recognised on a centimetre scale while also recognising zonations at the micron scale. Studies of ore systems require microanalysis of samples to collect information on mineral chemistry in order to understand physiochemical conditions during ore genesis and alteration. Such studies contribute to the debate on whether precious metals are remobilised or introduced in multiple hydrothermal events. In this study we demonstrate the microanalytical capabilities of the Maia large solid-angle detector array and imaging system on the X-ray fluorescence microscopy beamline at the Australian Synchrotron to provide data for these studies. We present a series of case studies from orogenic gold deposits that illustrate the power of the Maia detector for constraining chemical zonations in sulphides and associated alteration minerals, which can be used to decipher ore-forming processes associated with gold deposition. A series of large-area (