Individual Mineral Grains Research Articles

High‐Angular Resolution Electron Backscatter Diffraction as a New Tool for Mapping Lattice Distortion in Geological Minerals

AbstractAnalysis of distortions of the crystal lattice within individual mineral grains is central to the investigation of microscale processes that control and record tectonic events. These distortions are generally combinations of lattice rotations and elastic strains, but a lack of suitable observational techniques has prevented these components being mapped simultaneously and routinely in earth science laboratories. However, the technique of high‐angular resolution electron backscatter diffraction (HR‐EBSD) provides the opportunity to simultaneously map lattice rotation and elastic strain gradients with exceptional precision, on the order of 0.01° for rotations and 10−4 in strain, using a scanning electron microscope. Importantly, these rotations and lattice strains relate to densities of geometrically necessary dislocations and residual stresses. Recent works have begun to apply and adapt HR‐EBSD to geological minerals, highlighting the potential of the technique to provide new insights into the microphysics of rock deformation. Therefore, the purpose of this review is to provide a summary of the technique, to identify caveats and targets for further development, and to suggest areas where it offers potential for major advances. In particular, HR‐EBSD is well suited to characterizing the roles of different dislocation types during crystal plastic deformation and to mapping heterogeneous internal stress fields associated with specific deformation mechanisms/microstructures or changes in temperature, confining pressure, or macroscopic deviatoric stress. These capabilities make HR‐EBSD a particularly powerful new technique for analyzing the microstructures of deformed geological materials.

Mineral grains recognition using computer vision and machine learning

Identifying and counting individual mineral grains composing sand is an important component of many studies in environment, engineering, mineral exploration, ore processing and the foundation of geometallurgy. Typically, silt (32-128μm) and sand (128-1000μm) sized grains will be characterized under an optical microscope or a scanning electron microscope. In both cases, it is a tedious and costly process. Therefore, in this paper, we introduce an original computational approach in order to automate mineral grains recognition from numerical images obtained with a simple optical microscope. To the best of our knowledge, it is the first time that the current computer vision based on machine learning algorithms is tested for the automated recognition of such mineral grains. In more details, this work uses the simple linear iterative clustering segmentation to generate superpixels and many of them allow isolating sand grains, which is not possible with classical segmentation methods. Also, the approach has been tested using convolutional neural networks (CNNs). However, CNNs did not give as good results as the superpixels method. The superpixels are also exploited to extract features related to a sand grain. These image characteristics form the raw dataset. Prior to proceed with the classification, a data cleaning stage is necessary to get a usable dataset for machine learning algorithms. In addition, we present a comparison of performances of several algorithms. The overall obtained results are approximately 90% and demonstrate the concept of mineral recognition from a sample of sand grains provided by a numerical image.

Nitrogen diffusion in silicate minerals, with implications for nitrogen transport and cycling in the lithosphere

Diffusion laws for nitrogen in orthoclase, quartz, olivine and clinopyroxene were determined using a combination of experimental strategies that included: in-diffusion from C-H-O-N vapor; in-diffusion from molten NH4Cl; and ion implantation followed by heating to mobilize N. Most experiments were conducted at pressures near 1 bar, but seven experiments were also run in a piston-cylinder device at Ptotal = 1 GPa. The C-H-O-N-source experiments were “wet” (H2O present); those on ion-implanted samples were dry. All N sources were highly enriched in 15N to avoid spurious 14N signals resulting from surface contamination. Use of 15N also enabled depth profiling by nuclear reaction analysis (NRA) using the 15N(p,αγ)12C reaction, which has a detection limit of ~10 ppm. Nitrogen profiles in three samples were characterized by both NRA and SIMS, with similar results. A total of 17 to 28 diffusion coefficient measurements were made on each of the four minerals (83 in all), yielding results that conform for each mineral to an Arrhenius relation of the form D = D0exp(−Ea/RT). The following values for the pre-exponential constant (D0) and activation energy (Ea) were obtained:Unlabelled TableMinerallog(D0, m2/s)Ea (kJ/mol)T range (°C)Orthoclase−13.9 ± 0.495.4 ± 6.6500–900Quartz−12.4 ± 0.3147.3 ± 5.7550–1150Olivine−14.2 ± 0.3135.2 ± 6.6650–1400Clinopyroxene−14.1 ± 0.3135.6 ± 7.4750–1300The various experimental strategies yield generally indistinguishable results at a given temperature, and no significant effects of total pressure or H2O are discernible in the overall diffusion data set. Experiments that involved in-diffusion from external N sources provide qualitative insight into the compatibility of N in the structures of the four minerals. Nitrogen concentrations attain the highest levels in orthoclase (~100–8000 ppm atomic), with quartz, olivine and clinopyroxene falling in the tens to hundreds of ppm range. The new diffusion laws enable us to model the diffusive release of N acquired at depth in the crust during fluid-absent exhumation and cooling. In general, orthoclase is the least retentive of the minerals investigated, and will lose most of its N during cooling from 600 °C at tectonically typical rates. Olivine and clinopyroxene are the most retentive of N. Quartz is intermediate in retentivity between the mafic minerals and feldspar, but in all cases the extent of N exchange with the surroundings depends critically on the specific t-T path of the mineral of interest. In subduction settings where N-bearing fluid is released to the mantle wedge, diffusion is sufficiently fast to homogenize individual mafic mineral grains with respect to N over geologically plausible time scales, but equilibration on the outcrop or regional scale via volume diffusion is precluded by our data.

Characterizing mineral wettabilities on a microscale by colloidal probe atomic force microscopy

For finely intergrown ores the characterization of reagent-mineral interactions in flotation systems holds difficult challenges for the applicability of standard techniques like Hallimond tube tests or contact angle experiments or renders them impossible, while other techniques might not work in an aqueous environment. We present the utilization of an atomic force microscope with a hydrophobic colloidal probe to characterize the wettabilities of individual mineral grains on a microscale. A sulfidic ore sample containing chalcopyrite, pyrite and quartz is investigated in an aqueous environment. The mineralogy of the sample is characterized by SEM + EDX and its wettability by contact angle measurements. Force mappings on the respective minerals are performed and allow for a distinction between quartz, chalcopyrite and pyrite with the resulting force distributions. An additional focus of this paper lies on the heterogeneities within one mineral surface domain and the applicability for grain mappings.

Analysis of rare earth elements in coal fly ash using laser ablation inductively coupled plasma mass spectrometry and scanning electron microscopy

Reference standard NIST SRM 1633b and FA 345, a fly ash sample from an eastern U.S. coal power plant, were analyzed to determine and quantify the mineralogical association of rare earth elements (REE). These analyses were completed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a scanning electron microscope, equipped with an energy-dispersive X-ray spectrometer (SEM-EDS). Internal standardization was avoided by quantifying elemental concentrations by normalizing to 100% oxides. Mineral grains containing elevated REE concentrations were found in diverse chemical environments, but were most commonly found in regions where Al and Si were predominant. Dividing the spot analyses into time segments yielded plots that showed the REE content changing over time as individual mineral grains were being ablated. SEM-EDS images of FA 345 confirmed the trends that were found in the LA-ICP-MS results. Small grains of apatite, monazite, or zircon were frequently observed as free mineral grains or embedded in amorphous aluminosilicate glass and were not associated with ferrous particles. This finding is consistent with previous reports that magnetic enrichment may be an effective way of concentrating non-magnetic REE phases. Furthermore, aggressive mechanical and chemical-based separation schemes will be required to separate and recover REE from aluminosilicate glass.

Sequential mineral transformation from kaolinite to Fe-illite in two Brazilian mangrove soils

Mangrove soils have peculiar chemical and physical conditions generated by the variable water salinity, cyclic tidal changes and intense biological activity that produce complex clay suites. In this study, the crystal-chemical characteristics of clay minerals from two mangrove soils were investigated in detail to further our understanding of the processes taking place. The two Brazilian mangroves are from zones of contrasting climatic type and continental sediment character: (1) tropical wet climate area receiving highly weathered sediments (Bragança, Pará State) and (2) mild semiarid climate area supplied with a mixture of highly and moderately weathered sediments (Acaraú, Ceará State). The investigation of the 2–0.2μm and <0.2μm size fractions of the soils using XRD, Mössbauer spectroscopy, chemical analysis of individual mineral grains (TEM-EDS) and bulk samples (XRF) showed mineral assemblages composed by endmember and mixed-layer clay minerals. Three to four mineral phases were detected by XRD in each sample such as kaolinite-smectite minerals (K-S), illite-smectite (I-S) and Fe-rich illite, suggesting a progressive transformation via mixed-layering from detrital kaolinite (unstable in mangrove soil conditions) to K-S and I-S minerals rich in smectite, and then to Fe-rich illite. The chemical characterization corroborates this gradual transformation: there is an enrichment in K, Fe and Mg at the expense of Al in all samples. A solid-state reaction explains the sequential transformation, where crystal-chemical changes take place without complete disruption of the lattice. There is evidence of octahedral Fe reduction during the illitization stage, showing the influence of a reducing environment, from decaying organic matter and bacterial activity, on the clay minerals. The increase in layer charge is balanced by K and NH4 ions, abundant in the interstitial water. Superior Fe-rich illite contents in Acaraú mangrove, compared to Bragança mangrove, are explained by high inputs of 2:1 clays from sediment source areas inland. The above clay mineral reactions shed light on the role of clays and the interaction between minerals and biotic components in mangrove soils. In addition, these reactions may influence the cycle of major elements in coastal ecosystem surrounding mangroves.

A geochemical approach to constraining the formation of glassy fallout debris from nuclear tests

Glassy nuclear fallout debris from near-surface nuclear tests is fundamentally reprocessed earth material. A geochemical approach to analysis of glassy fallout is uniquely suited to determine the means of reprocessing and shed light on the mechanisms of fallout formation. An improved understanding of fallout formation is of interest both for its potential to guide post-detonation nuclear forensic investigations and in the context of possible affinities between glassy debris and other glasses generated by high-energy natural events, such as meteorite impacts and lightning strikes. This study presents a large major-element compositional dataset for glasses within aerodynamic fallout from the Trinity nuclear test (“trinitite”) and a geochemically based analysis of the glass compositional trends. Silica-rich and alkali-rich trinitite glasses show compositions and textures consistent with formation through melting of individual mineral grains—quartz and alkali feldspar, respectively—from the test-site sediment. The volumetrically dominant glass phase—called the CaMgFe glass—shows extreme major-element compositional variability. Compositional trends in the CaMgFe glass are most consistent with formation through volatility-controlled condensation from compositionally heterogeneous plasma. Radioactivity occurs only in CaMgFe glass, indicating that co-condensation of evaporated bulk ground material and trace device material was the main mechanism of radioisotope incorporation into trinitite. CaMgFe trinitite glasses overlap compositionally with basalts, rhyolites, fulgurites, tektites, and microtektites but display greater compositional diversity than all of these naturally formed glasses. Indeed, the most refractory CaMgFe glasses compositionally resemble early solar system condensates—specifically, CAIs.

Non-Destructive Micro-Chemical and Micro-Luminescence Characterization of Jadeite.

Jadeite was greatly appreciated by pre-Hispanic cultures in Mesoamerica. Despite its importance, knowledge of its mining sources was lost after the Spanish conquest. In the 1950s the only confirmed jadeite deposits in Mesoamerica were found in the Motagua River Fault (MRF), Guatemala. The aim of this study is to present a methodology that is appropriate for the study of archeological jadeite objects using non-destructive spectroscopic and micro-ion beam analysis techniques. This methodology has been applied to perform mineral, elemental, and luminescence characterization of five jadeite samples from the MRF, with white, lilac, and green colors. Fourier-transformed infrared spectroscopy and X-ray diffraction analysis confirmed the presence of jadeite, albite, and omphacite as the main mineral phases in the samples. Elemental maps using particle-induced X-ray emission (PIXE) with a nuclear microprobe and elemental concentration analysis from individual mineral grains using micro-PIXE coupled with micro-ionoluminescence (IL) allowed the detection of minor feldspar, titanite, and grossular mineral contents. Distinctive features from the mineral, elemental, and luminescence characterization have been found that allow the identification of these five jadeite samples.

Multi-scale quantification of leaching performance using X-ray tomography

The performance of heap leaching is dictated by a large number of processes acting at a wide range of length scales. One important scale is that of the individual particles, where the interaction between the rate kinetics at the surfaces of the individual mineral grains and the mass transport through the particle combine to give the overall apparent particle scale kinetics. It has been recognised for a long time that variability in the mineralogy, size and spatial distribution of the mineral grains within the particle are likely to have a large effect on the leach performance and its variability and thus, ultimately, the performance of the heap. In this paper a new method for quantifying this behaviour and its variability at scales from the particle through to the grain and down to the surface kinetics is presented. This method is based on the use of a series of XMT (also called micro-CT) images of a column taken at regular intervals over 168days of leaching. The key development in the analysis of this data is an algorithm that has allowed every single one of the hundreds of thousands of mineral grains within the column to be individually tracked across all the time points as they undergo dissolution. This has allowed the dependency of the mineral grain leach rate on its size and position in the particle to be decoupled from one another. It also meant that the variability in the surface kinetics of the grains could be assessed, with mineralogical variability being the key source of this variability. We demonstrate that understanding and quantifying this underlying kinetic variability is important as it has a major impact on the time evolution of the average kinetics of the leaching.

Modelling particle scale leach kinetics based on X-ray computed micro-tomography images

The apparent leach kinetics for an ore particle within a heap leaching system depend on the chemical conditions in the fluids around the particle, the mass transport within the particle and the reaction kinetics at the surface of each mineral grain. The apparent rate kinetics thus depend upon the distribution of the mineral grains, in terms of both size and position, within the individual ore particles, as well as the evolution of this distribution. Traditionally this behaviour has been modelled using simplified relationships such as the shrinking core model. In this paper a method for simulating this evolution and the resultant kinetics based directly on 3D XMT images of the internal structure of the particles is presented. The model includes mass transport through the gangue matrix, surface reaction kinetics and the dissolution and subsequent evolution of the individual mineral grains within the ore particle. Different minerals and mineral associations will result in different surface reaction kinetics. One of the key inputs into this model is thus the distribution of the surface rate kinetics. A method for experimentally determining this distribution is presented. The simulation results are compared to the evolution of real particles as they undergo leaching as measured using a time sequence of 3D XMT images of a leaching column. It was found that these simulations are able to accurately predict both the overall leaching trends, as well as the leaching behaviour of mineral grains in classes based on their size and distance to the particle surface. The leaching behaviour did not follow that of a simple shrinking core approximation, with the actual spatial and size distribution of the grains, as well as the distribution of their surface rate kinetics, all impacting the apparent leach kinetics. For the copper ore particles used in this work the best fit to the experiments was achieved at an intermediate value of the dimensionless group that characterises the relative importance of surface kinetics to diffusion indicating that both need to be considered for accurate modelling. This paper thus demonstrates that using 3D XMT to provide both structural and kinetic data and incorporating this information into a particle scale simulator provides an improved basis for predicting particle scale leach performance.

Grain size measurement from two-dimensional micro-X-ray diffraction: Laboratory application of a radial integration technique

Two-dimensional X-ray diffraction data contain information about not only the type of mineral phases present in an assemblage, but also the textural or grain size relationships between minerals in a sample. For minerals within a certain grain size range, ~0.1 to 100 μm, the appearance and characteristics of a Debye ring can reveal the mean grain size of a sample. In this contribution, using mineral and rock samples of known grain size ranges, we investigate the applicability of calculating the grain size of a material using a two-dimensional X-ray diffraction crystallite size analysis method for micrometer-sized materials. A radial integration technique was used to derive the number of grains contributing to produce diffraction spots in the Debye ring. Monomineralic pyroxene and magnetite samples of known grain size ranges were analyzed, and the calculated grain size was observed to broadly correlate with the sample size except at the upper and lower extremes. To evaluate the technique on broader geological materials, polymineralic basalt samples with known grain size ranges were analyzed, and the calculated grain sizes did not correlate with the size of the rock fragments, but did correlate closely with the size of the individual mineral grains. Using a Bruker D8 Discover X-ray diffractometer with a 300 μm nominal incident beam diameter, the effectiveness of the applied method appeared limited to the grain size range of ~15–63 μm for monomineralic samples. The method is further limited by several complicating factors and assumptions, including the requirement for the crystallite size to correlate with the sample grain size. The effective range of this method will vary with different instrumental and experimental conditions. When applying this method to calculate the grain size of geological materials, the calculated result should be interpreted as a minimum estimate of the grain size.

Pristine stratospheric collection of interplanetary dust on an oil‐free polyurethane foam substrate

AbstractWe performed chemical, mineralogical, and isotopic studies of the first interplanetary dust particles (IDPs) collected in the stratosphere without the use of silicone oil. The collection substrate, polyurethane foam, effectively traps impacting particles, but the lack of an embedding medium results in significant particle fragmentation. Two dust particles found on the collector exhibit the typical compositional and mineralogical properties of chondritic porous interplanetary dust particles (CP‐IDPs). Hydrogen and nitrogen isotopic imaging revealed isotopic anomalies of typical magnitude and spatial variability observed in previous CP‐IDP studies. Oxygen isotopic imaging shows that individual mineral grains and glass with embedded metal and sulfide (GEMS) grains are dominated by solar system materials. No systematic differences are observed in element abundance patterns of GEMS grains from the dry collection versus silicone oil‐collected IDPs. This initial study establishes the validity of a new IDP collection substrate that avoids the use of silicone oil as a collection medium, removing the need for this problematic contaminant and the organic solvents necessary to remove it. Additional silicone oil‐free collections of this type are needed to determine more accurate bulk element abundances of IDPs and to examine the indigenous soluble organic components of IDPs.

Recovery of copper sulphides mineral grains at coarse rock fragments size

Assuming the random distribution of sulphide mineral grains and random rock breakage, a relatively small percentage of sulphide grains will be exposed on the rock surface. Early liberation of sulphide grains needs to be considered in terms of the mechanical properties of such grains relative to the properties of the host rock matrix.Clustering of sulphide mineral grains, will make early liberation possible. Depending on the nature of mineral associations, crushing of such rocks will result in different outcomes. Where clustering is manly of very soft copper minerals, with the host rock being moderately strong feldspars or quartzite’s, the copper rich parts of rock are likely to fragment first, resulting in relatively small size being rich in copper minerals. However, in the case of moderately strong chalcopyrite, the difference in elastic properties between chalcopyrite and feldspar or quartz, will not be significant enough to cause a propensity for early liberation.Where clustering of copper minerals occurs with grains of pyrite (or magnetite), the stronger part of the rock fragment will be one rich in valuable minerals. During crushing of such rock, the sulphides rich zones will fragment in a different way than gangue. Stress concentration within pyrite (or magnetite) will result in failure of the relatively soft surrounding matrix, thus promoting liberation of chalcopyrite or chalcocite grains. Therefore, textural information about the associations of sulphide minerals (copper sulphides vs. pyrite/magnetite/garnet) will be of critical significance in the evaluation of the propensity for coarse liberation of copper sulphide minerals. An absence of close spatial associations will significantly reduce the possibility of early liberation of copper sulphides.During blasting ore is exposed to sufficiently intense, high-strain rate loading to be able to induce micro-fracturing originating from individual sulphides mineral grains as well as their clusters. Due to the high rate of loading, a substantial amount of energy can be dissipated with embryonic rock fragment, before macro-failure of rock, which will relieve rock of blast induced stress. Created micro-cracks will play a significant role in subsequent comminution, where rock fragments with enhanced density of micro-cracks will be crushed more easily. Extensive micro-cracking is also likely to play a significant role during heap or dump leaching, stimulating infiltration/diffusion of leaching fluids into the interiors of rock fragments.

Forensic provenance investigations of soil and sediment samples

Abstract In recent decades, the polarizing light microscope has been slowly phased out of the geology curricula of many universities in the United States. This is unfortunate, as the polarizing light microscope is an instrument that is particularly well suited to overcoming problems typically encountered in forensic scenarios. Its versatility makes it uniquely powerful in provenance investigations in which sample size is limited and sample history is unknown. The polarizing light microscope can be used to determine the source rock type(s) of a sample of unconsolidated grains, either based on the mineral assemblage present in the sample or on properties observed for individual mineral grains or lithic fragments. Biological particles including pollen, spores, diatoms and botanical macerals can be identified and used to constrain the possible source of a sample. In addition, individual particles observed in the sample (geological, biological or anthropogenic) can be isolated and re-mounted for subsequent analyses that are useful for provenance determination. The inferences made can be used to produce maps illustrating potential source regions by means of geographical information systems (GIS). Several examples are provided to illustrate the various capabilities of the polarizing light microscope and its contributions to forensic soil analysis, specifically in forensic provenance investigations.

Melt inclusions in olivine: Reliable witnesses to Earth’s interior?

959 Our knowledge of the composition and chemical state of the interior of Earth is largely based on a combination of geochemical measurements of samples from depth, geophysical observations, and experimental studies of phase equilibrium and material behavior. While plate tectonic processes do bring deep subsurface rocks to Earth’s surface, such as in ophiolites, these are signifi cantly altered during ascent, and extraction of appropriate chemical and volatile information is unlikely to be highly robust. By far, the most reliable measures of chemical conditions in Earth’s interior come from samples brought to the surface with minimal opportunity for resetting the key chemical markers (e.g., ferric-ferrous iron ratios, volatile contents). The most common of these samples are xenolithic materials transported in basaltic or kimberlitic magmas and cooled rapidly. A critical question for such samples involves the extent to which they have been reset in terms of the signature of the source region during entrainment and ascent to the surface. Such signatures may be retained in the individual xenolith mineral grains or within inclusions trapped in the interior of these minerals. In this issue of Geology, Gaetani et al. (2012, p. 915) provide the results of an experimental study that give fresh insight into the effectiveness of melt inclusions within xenolithic olivine grains in retaining the water content and oxygen fugacity (f O2 ) at the time of entrapment. Hydroxyl signatures in minerals contained in xenoliths have been

Modeling the evolutionary rise of ectomycorrhiza on sub-surface weathering environments and the geochemical carbon cycle

For the past two decades, the spread of angiosperm plants in the Cretaceous and Paleogene has been thought to have enhanced silicate weathering fluxes of Ca and Mg to the oceans, thereby drawing down atmospheric CO~2~ and ultimately sequestering it in marine carbonate sediments. However, the rise of angiosperm trees in the Cretaceous was coincident with the evolution of ectomycorrhizal fungal associations in angiosperm and gymnosperm trees that have increasingly supplanted trees with the ancestral arbuscular-mycorrhizal associations. This represents the most profound alteration in root functioning to occur in plant evolutionary history, with far-reaching implications for weathering and soil biogeochemistry because the fine roots are enveloped with a fungal sheath. Ectomycorrhizal fungi provide the main nutrient and water-absorbing interface with soil, and the pathway through which organic acids and protons are actively secreted at the scale of individual mineral grains. Here, we test the hypothesis that the rise of ectomycorrhizal trees was a major contributor to the drawdown of atmospheric CO~2~ over the past 120 Ma through enhanced silicate weathering. We developed a process-based soil chemistry model incorporating the effects of plants with ancestral arbuscular mycorrhizas, and more recently evolved ectomycorrhizas on soil chemistry via its effects on the biological proton cycle, and integrated it into a leading model of the long-term carbon cycle (GEOCARBSULF). Our mechanistic, process-based modeling reveals that the rise of ectomycorrhizal trees can explain the CO~2~ drawdown previously attributed empirically to the spread of angiosperms. We suggest, therefore, that the evolutionary rise of ectomycorrhizas represents an important driving force of the long-term carbon cycle by enhancing chemical weathering and draw-down of atmospheric CO~2~ into marine carbonates.

Investigation of multi-layered silicate ceramics using laser ablation optical emission spectrometry, laser ablation inductively coupled plasma mass spectrometry, and electron microprobe analysis

AbstractThe applicability of laser ablation (LA) inductively coupled plasma (ICP) spectrometry for assessing elemental distributions in layered ceramics was investigated and compared with electron probe microanalysis (EPMA). Ordinary glazed wall tiles were employed as model specimens due to their defined structure and composition. They were used for calibration in the analysis of ancient pottery. A qualitative depth profile was acquired by single-spot laser drilling perpendicular to coatings with a Nd:YAG (1064 nm) laser coupled with an ICP optical emission spectrometer (OES). The lower lateral resolution associated with the laser spot diameter of 1.0 mm led to smoothing of the depth profile due to the averaging of local irregularities. In addition, transverse line scans by ablation across the tile section using an ArF* (193 nm) laser coupled with an ICP mass spectrometer (MS) were performed. LA-ICP-OES depth profiles and LA-ICP-MS transverse scans were validated by EPMA section scans and 2D back-scattered electrons images. The LA-ICP-OES acquisition was less dependent on sample surface and layer irregularities, whereas the transverse line scan over the tile section with the small-spot beam offered insight into the micromorphology of the individual layer. The combined approach revealed the occurrence of individual mineral grains, micro-heterogeneities and the character of interfaces between layers.

GOLD IN KAROO SUPERGROUP HEAVY-MINERAL DEPOSITS, KWAZULU-NATAL, SOUTH AFRICA

To date, all sites where gold has been recorded in KwaZulu-Natal have been in rocks of Archaean and Proterozoic Age. The site described is a heavy-mineral-rich sandstone bed of the Permian Vryheid Formation (Ecca Group) near Muden in the KwaZulu-Natal midlands. Sedimentary structures in the sandstone units of the Vryheid Formation indicate that these deltaic deposits were produced by rivers flowing from the northeast, with minor longshore drift towards the southeast. The deltaic deposits consist of three units: a basal unit (Unit A) of shale and mudstone, which coarsen upward into siltstones with lenses of fine-grained sandstone; a middle unit (Unit B) of ferruginous sandstone and immature greywacke (argillaceous sandstone) beds that coarsen upwards; and an upper unit (Unit C) of medium- to coarse-grained lithic sandstones with lenses of grit and sparse small pebbles of white vein quartz. The sand grains are physically and chemically very immature, suggesting the possibility that most of the sand grains were reworked from the then unlithified glaciogenic Dwyka Group sediments; the primary source of which was the Transvaal Supergroup and the granite - greenstone belts of the provenance area presently located in northern and eastern South Africa. The identification of gold-bearing grains in sandstones of the Vryheid Formation (Karoo Supergroup) from near Muden lends strong support to the concept that the gold-bearing horizons of similar Permian age in the Beaufort Group near Prince Albert, Western Cape Province were formed as natural placer deposits. The heavy-minerals take the form of both individual mineral grains and inclusions in quartz and feldspar with gold occurring only as inclusions. The majority of heavy-minerals occur as thin lenses and stringers in sandstone of the upper part of Unit B, where sorting was poor and flow rates were normally insufficient to winnow out the lighter minerals. Less abundant, but moderately well sorted, heavy-mineral lenses occur on the upper surface of Unit C in the troughs of megaripples, where deposition was rapid prior to subsequent preservation by an overlying organic-rich mud of Unit A.

Ontogenetic analysis of individual olivine grains in ultramafic rocks

The morphology and internal structure of individual olivine grains from ultramafic rocks in the Guli and Gal’moenan dunite massifs differing in origin are considered. To restore the ontogeny of mineral aggregates, traces of elastic deformation retained in mineral grains have been used. Comparison of anatomy of olivine grains from these two massifs showed that the mechanism of accommodation of rocks to changing geological settings is expressed as the response of the mineral aggregate structure and variation in the anatomy of individual mineral grains. At the level of individual grains, this is annihilation of older defects and origination of younger dislocations; refinement of the crystal lattice; exsolution; formation and transformation of new mineral phases; and creep and migration of subboundaries within grains. At the aggregate level, this is rotation and migration creep of the internal boundaries of rock; formation of new boundaries of mineral intergrowths; reorientation of boundaries; and variation in their extent, density, and grain dimensions. The prehistory of massifs controls the manifestation and abundance of various elastic deformations and related types of recrystallization of olivine grain boundaries and subboundaries in aggregates. New conditions and accommodation of mineral aggregates to these conditions have instigated specific schemes of recrystallization, which bear information on the history of rocks and their massifs.

Formation of gold deposits: a metamorphic devolatilization model

AbstractA metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many goldfields such as those found in Archean greenstone belts, slate‐belts and other gold‐only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist–amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H2O and CO2 but also S and, in turn, Au. Elevated gold in solution is achieved by complexing with reduced S, and by H2CO3 weak acid buffering near the optimal fluid pH for gold solubility (the buffering is more important than being at the point of maximum gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre‐scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold. Deposition of gold from solution necessitates breakdown of the gold–thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon‐bearing host rocks and/or by sulphidation of wallrocks to generate iron‐bearing sulphide and precipitated gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite‐bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major gold deposits with rock type, even when the gold is primarily in veins, argues for rock‐dominated depositional systems, not fluid‐dominated ones. As a consequence, a general role in gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at gold‐only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher‐grade metamorphism and deformation, and/or (palaeo)‐weathering may provide many of the most‐obvious features of goldfields including their mineralogy, geochemistry, geometry, small‐scale timing features, geophysical response and even mesoscopic gold distribution.