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Economic gold deposits result from a 100- to 10,000-fold enrichment in gold relative to crustal background. In hydrothermal systems, this enrichment is achieved through the transport and accumulation of metals via deeply sourced fluids to a site of deposition. However, the generally low metal solubility of Au in aqueous solutions in orogenic systems requires additional processes in order to explain high-grade gold formation. Reports of Au nanoparticles in high-grade gold veins infer that their formation is linked to mineralisation. However, processes leading to nanoparticle nucleation and deposition remain poorly understood. Here we show that formation of metal nanoparticles (Au, AuAg, Cu, Ag 2 O) is one of the essential contributors to efficient and focused gold deposition. We report systematic and previously unrecognized metal nanoparticles preserved in amorphous silica and/or carbonic phases in five high-grade deposits. The association of metal, silica and carbonic phases helps to constrain the multiple reactive processes involved in Au, Cu and Ag metallogenesis and formation of high-grade gold mineralisation. The authors present novel observations providing insights into the formation of extraordinary gold-rich veins. We discovered metal nanoparticles associated with amorphous silica and carbon indicating their essential contribution to efficient gold deposition.

Complex microstructures of pyrite induced by superimposed deformation and fluid infiltration in lode gold deposits can record time-integrated physicochemical changes of ore-forming fluids. This study used a combination of techniques (EBSD, μ-XRF, EMPA, APT, and TEM) to characterize and decode the microstructures of pyrite in an optically homogeneous polymetallic sulfide veinlet of the Wulong lode gold deposit. The EBSD data show that the sulfide veinlet is composed of microstructurally heterogeneous pyrite grains in three generations, i.e., Py1, Py2, and Py3. The pyrite grains display diverse deformation-induced microstructures, i.e., kinked low-angle (0.5 to 5°) boundaries within Py1, curvilinear low-angle boundaries penetrating both Py1 and Py2, and foam-like textured Py3. These microstructures may offer pathways to transport exotic fluids, leading to superimposed microstructures in early-formed pyrite. Integrated μ-XRF, EMPA, APT, and HRTEM data sets present grain- to nanometer-scale elemental distributions of Au, As, Bi, and Te, and reveal that the Bi-(Te)-rich mineral associations, rather than arsenian pyrite, are major gold carriers in the polymetallic sulfide veinlet. The superimposed microstructures in pyrite reveal that the major Au mineralization stage likely comprises two episodes of ore-forming fluids. The early-episode fluids had T-fO2 condition overlapping the stable field of pyrrhotite, which dissolved Py1 and developed Bi-(Te)-rich melts as major Au scavengers. The late-episode fluids crystallized arsenian Py2 containing 20–60 ppm of invisible gold. This study highlights the effectiveness of multiple techniques to reveal complex microstructures of pyrite, which can serve as critical fingerprints to establish a temporal framework of fluid evolution and Au deposition in lode gold deposits.

Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes. Chromian-spinel from mafic-ultramafic rocks is used as a reliable geotectonic and mantle melting indicator. Here, the authors argue that this only works partially – it can be used to assess information on mantle metasomatic processes but not petrogenesis.

Understanding elements uptake and release from minerals in source rocks is crucial for comprehending critical metals accumulation, yet the mechanisms and kinetics of element mobilization at the atomic scale remain mostly unknown. Here, we analyzed the distribution of cobalt (Co) in natural pyrite from a Cu-Co ore deposit and found that metals distribution is best described by steady-state diffusion with constant flux and concentration-dependent diffusivities, rather than transient-state diffusion with time-evolving concentrations. First-principles calculations and diffusion modelling further demonstrate that this diffusion is accelerated by vacancy pathways and is far more efficient than traditional vacancy-mediated lattice diffusion, with element transfer rates higher by almost two orders of magnitude. We conclude that steady-state lattice diffusion induced by vacancies in the presence of fluid can be an efficient mechanism promoting the preferential release of metals into ore fluids and the accumulation of metals during ore formation. Vacancies in sulfides facilitate fluid-induced solid-state diffusion at rates nearly two orders of magnitude faster than traditional lattice diffusion, promoting critical metal accumulation during ore formation.

Rubble piles asteroids consist of reassembled fragments from shattered monolithic asteroids and are much more abundant than previously thought in the solar system. Although monolithic asteroids that are a kilometer in diameter have been predicted to have a lifespan of few 100 million years, it is currently not known how durable rubble pile asteroids are. Here, we show that rubble pile asteroids can survive ambient solar system bombardment processes for extremely long periods and potentially 10 times longer than their monolith counterparts. We studied three regolith dust particles recovered by the Hayabusa space probe from the rubble pile asteroid 25143 Itokawa using electron backscatter diffraction, time-of-flight secondary ion mass spectrometry, atom probe tomography, and 40Ar/39Ar dating techniques. Our results show that the particles have only been affected by shock pressure of ca. 5 to 15 GPa. Two particles have 40Ar/39Ar ages of 4,219 ± 35 and 4,149 ± 41 My and when combined with thermal and diffusion models; these results constrain the formation age of the rubble pile structure to ≥4.2 billion years ago. Such a long survival time for an asteroid is attributed to the shock-absorbent nature of rubble pile material and suggests that rubble piles are hard to destroy once they are created. Our results suggest that rubble piles are probably more abundant in the asteroid belt than previously thought and provide constrain to help develop mitigation strategies to prevent asteroid collisions with Earth.

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° show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at larger disorientation angles (> 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.

Vertebrate fossils embedded in amber represent a particularly valuable paleobiological record as amber is supposed to be a barrier to the environment, precluding significant alteration of the animals' body over geological time. The mode and processes of amber preservation are still under debate, and it is questionable to what extent original material may be preserved. Due to their high value, vertebrates in amber have never been examined with analytical methods, which means that the composition of bone tissue in amber is unknown. Here, we report our results of a study on a left forelimb from a fossil Anolis sp. indet. (Squamata) that was fully embedded in Miocene Dominican amber. Our results show a transformation of the bioapatite to fluorapatite associated with a severe alteration of the collagen phase and the formation of an unidentified carbonate. These findings argue for a poor survival potential of macromolecules in Dominican amber fossils.

The southern part of the West African Craton includes the Baoulé-Mossi Domain, the world’s premier Paleoproterozoic gold province (~10,000 metric ton gold endowment). Structural, metamorphic, and geochronological data suggest gold mineralisation occurred during three episodes that span much of the Eoeburnean and Eburnean orogenic cycles. Eoeburnean orogenic and rare skarn-hosted gold deposits formed between ca. 2200 and 2135 Ma during repeated episodes of volcanism, plutonism, and shortening, which thickened the Paleoproterozoic crust. Early Eburnean orogenic and placer gold deposits formed between ca. 2110 and 2095 Ma during inversion, metamorphism, and subsequent oblique shortening of intra-orogenic basins filled after ca. 2135 Ma. This episode of mineralisation terminated when the Baoulé-Mossi Domain docked with the Archean Kénéma-Man Domain at ca. 2095 Ma. Late Eburnean orogenic and less common intrusion-related gold deposits formed between ca. 2095 and 2060 Ma during strike-slip to oblique-slip tectonics, post-collisional high-K plutonism and crustal reworking across the western and southern Baoulé-Mossi Domain. Eoeburnean gold deposits include ca. 10 % of the gold endowment of the Baoulé-Mossi Domain, whereas the Early Eburnean and Late Eburnean deposits include ca. 50–70% and 20–40%, respectively. Here, we highlight the favourable confluence of accretion-collision tectonics, involving juvenile crust formation as well as protracted magmatic, metamorphic, and deformation histories that resulted in diachronous gold events spread over at least 100 myr throughout the Baoulé-Mossi Domain.

Cristobalite is a low-pressure, high-temperature SiO 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 [AlO /Na —coupled substitution of Al 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 SiO 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 SiO 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 SiO polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies.