Silician Magnetite: Si–Fe-Nanoprecipitates and Other Mineral Inclusions in Magnetite from the Olympic Dam Deposit, South Australia

Ciobanu, Cristiana L.; Verdugo-Ihl, Max R.; Slattery, Ashley; Cook, Nigel J.; Ehrig, Kathy; Courtney‐Davies, Liam; Wade, Benjamin P.

doi:10.3390/min9050311

Cited by 28 publications

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“…Changes in mineral signatures, phase associations, and zoning patterns are interpreted in terms of partitioning and cooling paths in magmatic systems [24,28]. The presence of critical metals, invisible gold in sulphides, and fluid-rock interactions in magmatic-hydrothermal systems are addressed for single deposits or ore provinces [23,25,27,[29][30][31][32]. The mapping of trace elements in ore minerals using nanoSIMS is revolutionary in terms of its detection limits and spatial resolution, but also permits the visualization of transient radionuclides at ultra-trace concentrations [33].…”

Section: Discussionmentioning

confidence: 99%

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Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

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Section: Discussionmentioning

confidence: 99%

“…Ciobanu et al [30] address the question "what is silician magnetite? ', describing the results of a comprehensive nanoscale study on magnetite in samples from the outer, weakly mineralized shell at Olympic Dam, supported by STEM simulations.…”

Section: The Special Issuementioning

confidence: 99%

Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

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“…Comparable phenomena in which a mineral can host nanoscale inclusions of the same mineral but with subtle yet distinct chemical differences, are known from Fe oxides, notably Si-Fe nanoprecipitates within silician magnetite. These are documented both from banded iron formation deposits [53] and the Olympic Dam deposit [54].…”

Section: Zircon Alteration Model and Magma "Fertility"mentioning

confidence: 99%

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

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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.Minerals 2019, 9, 364 3 of 34 followed by partial to complete recrystallization. The amorphous domains begin to interconnect due to the radiation damage (α-decay), over a time period determined by U/Th concentrations and annealing rates [16]. The time dependency of structural damage is highly relevant for geochronology, as it can result in discordance of the U-Pb system, most commonly through Pb-loss. Disturbances to zircon U-Pb systematics via metamictization can, however, be selectively eliminated by chemical abrasion of damaged zones, prior to ID-TIMS, permitting high-precisio...

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“…In comparison with sulphides, relatively little is known about the presence of NPs in Fe-oxides. Nanoscale studies of NPs in magnetite (e.g., [12][13][14]) have been used to identify conditions of ore-formation. In contrast, the potential constraints that could be obtained from NPs in hematite remain largely unexplored.…”

Section: Introductionmentioning

confidence: 99%

Copper-Arsenic Nanoparticles in Hematite: Fingerprinting Fluid-Mineral Interaction

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Metal nanoparticles (NP) in minerals are an emerging field of research. Development of advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, the technique provides a link between micron-scale textures and chemical patterns if the sample is extracted in situ from a location of petrogenetic interest. Here we use HAADF STEM imaging and energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned and weave-twinned hematite from the Olympic Dam deposit, South Australia. We aim to determine the role of solid-solution versus the presence of discrete included NPs in the observed zoning and to understand Cu-As-enrichment processes. Relative to the grain surface, the Cu-As bands extend in depth as (sub)vertical trails of opposite orientation, with Si-bearing hematite NP inclusions on one side and coarser cavities (up to hundreds of nm) on the other. The latter host Cu and Cu-As NPs, contain mappable K, Cl, and C, and display internal voids with rounded morphologies. Aside from STEM-EDX mapping, the agglomeration of native copper NPs was also assessed by high-resolution imaging. Collectively, such characteristics, corroborated with the geometrical outlines and negative crystal shapes of the cavities, infer that these are opened fluid inclusions with NPs attached to inclusion walls. Hematite along the trails features distinct nanoscale domains with lattice defects (twins, 2-fold superstructuring) relative to hematite outside the trails, indicating this is a nanoprecipitate formed during replacement processes, i.e., coupled dissolution and reprecipitation reactions (CDRR). Transient porosity intrinsically developed during CDRR can trap fluids and metals. Needle-shaped and platelet Cu-As NPs are also observed along (sub)horizontal bands along which Si, Al and K is traceable along the margins. The same signature is depicted along nm-wide planes crosscutting at 60° and offsetting (012)-twins in weave-twinned hematite. High-resolution imaging shows linear and planar defects, kink deformation along the twin planes, misorientation and lattice dilation around duplexes of Si-Al-K-planes. Such defects are evidence of strain, induced during fluid percolation along channels that become wider and host sericite platelets, as well as Cl-K-bearing inclusions, comparable with those from the Cu-As-zoned hematite, although without metal NPs. The Cu-As-bands mapped in hematite correspond to discrete NPs formed during interaction with fluids that changed in composition from alkali-silicic to Cl- and metal-bearing brines, and to fluid rates that evolved from slow infiltration to erratic inflow controlled by fault-valve mechanism pumping. This explains the presence of Cu-As NPs hosted either along Si-Al-K-planes (fluid supersaturation), or in fluid inclusions (phase separation during depressurisation) as well as the common signatures observed in hematite with variable degrees of fluid-mineral interaction. The invoked fluids are typical of hydrolytic alteration and the fluid pumping mechanism is feasible via fault (re)activation. Using a nanoscale approach, we show that fluid-mineral interaction can be fingerprinted at the (atomic) scale at which element exchange occurs.

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Silician Magnetite: Si–Fe-Nanoprecipitates and Other Mineral Inclusions in Magnetite from the Olympic Dam Deposit, South Australia

Cited by 28 publications

References 50 publications

Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

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

Copper-Arsenic Nanoparticles in Hematite: Fingerprinting Fluid-Mineral Interaction

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