Platinum-group element mineralisation in the Unst ophiolite, Shetland

The ophiolitic basic and ultrabasic rocks of the island of Unst, Shetland comprise a sequence of harzburgites, dunites, clinopyroxene-rich cumulates, and gabbro, within tectonic blocks that have been thrust over a migmatite complex during the Laxer Palaeozoic. Concentrations of chromite are found in the harzburgite and dunite, and to a small extent in the pyroxene cumulate rocks. They occur as disseminations, sometimes forming millimetre scale layers, and as more massive schlieren and pods of chromitite. Five alteration or hydrothermal events have been recognised in the ultrabasic rocks. These comprise early pervasive serpentinisation, later fracture controlled serpentinisation, veining and pervasive carbonation, minor late serpentine veining and talc-carbonate alteration controlled by fault zones. Exploration for platinum group element (PGE) mineralisation uas carried out using a combination of drainage, overburden and rock sampling. Analyses of PGE were obtained by fire assay followed by either neutron activation analysis or flameless atomic absorption spectrometry, and up to 20 other elements Here determined by X-ray fluoresence analysis. Panned concentrate samples were taken from 73 drainage sites distributed throughout the complex. Ir, the only PGE determined in all samples, showed a greater concentration in samples derived from the harzburgite unit than those from other units. Lox amplitude anomalies are present in three discrete areas in the harzburgite but the maximum level of 210 ppb Ir is associated with a sample derived from a prominent N-S zone of faulting and hydrothermal activity markedly discordant to the regional trend of layering in the harzburgite and dunite. This discordant zone, which extends for at least 7 km, is also marked by samples containing enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are associated with an area in the north of the harzburgite with no previous history of chromite working but where many locally derived pieces of chromitite float have been discovered. Relatively high Cr levels are also associated nith the area of dunite containing the greatest concentration of visible chromite and old norkings. A technique of collecting panned heavy mineral concentrates from overburden samples was adopted as a reconnaissance exploration technique after orientation sampling in the harzburgite unit at Cliff, an area with high PGE levels in chromitite and associated dunite. Systematic sampling in the Cliff area outlined a zone of coincident Pd, Pt and Rh enrichment near to but separate from the chromite workings knorrn to be enriched in PGE. In contrast the distribution of Ru was entirely different with scattered lon amplitude anomalous zones and a maximum anomaly 300m from the chromite-rich zone. Reconnaissance lines were sampled at other locations within the harzburgite, dunite and cumulate units. Lore amplitude Pd and Pt anomalies were detected xithin the dunite unit, especially in 'a traverse across the trace of the prominent N-S fault zone at Helliers Uater, adjacent to the outcrop of the cumulate unit. In general the overburden data suggest some association between PGE enrichment and enhanced levels of Ni relative to typical silicate levels apparent when expressed as the ratio Ni/MgC. Rock samples Here collected from all parts of the complex, including most of the main chromitite workings. Very high levels of all PGE occur in samples of chromitite, chromite-rich dunite and dunite from the Cliff area, with a strong positive intercorrelation between all PGE. The proportions of the various PGE are very similar to those present in deposits in major layered basic/ultrabasic complexes like Bushveld and Stillwater, irith strong relative enrichment in Pd and Pt. These PGE proportions are completely different from the Ru-Ir-0s dominant assemblage typical of ophiolitic rocks. Associated with high levels of PGE are enrichments in Ni, Cu, As, Sb and Te. There is no correlation rrith Cr and some samples of chromitite from the Cliff area contain only background levels of PGE. High to moderate levels of PGE with the same proportions of elements as the Cliff samples also occur in samples of chromitite and serpentinised dunite from the dunite unit and in samples of pyroxenite from the cumulate unit, In contrast PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave have entirely different proportions of PGE with Ru and Ir in greatest abundance. This PGE distribution is similar to that in some background samples of harzburgite and closely resembles the pattern found in typical ophiolites. The PGE in the Harold's Grave samples do not exhibit the Ni enhancement noted in the Cliff PGE mineralisation. In samples from the Cliff area the platinum-group minerals (PGM) sperrylite, stibiopalladinite, hollingnorthite, laurite and possibly irarsite have been identified, mostly as grains less than 10 microns in size. In chromite-rich rocks these minerals occur Rithin chlorite haloes around chromite, in the blackened altered rims of chromite grains and in interstitial Ni-rich serpentine/carbonate intergroxths in association with pentlandite, orcellite and other Ni sulphides and arsenides, sometimes spatially related to chlorite-carbonate-magnetite veins. They also occur as fine grains Rithin magnetite rims around chromite and in magnetite or carbonate veins in dunite. The Ni sulphide/arsenide assemblage associated Rith the PGH is characteristic of serpentinisation at temperatures less than 500'C, Rell belox the range of magmatic conditions. A hydrothermal origin for the PGE mineralisation is proposed, probably related to the second phase of serpentinisation. This involved the redistribution of Ni accompanied by the introduction of As, Sb and Te probably with a StrUCtUral Control. Pre-existing concentrations of chromite may have acted as a precipitation barrier causing rich PGH deposition in the alteration haloes around chromite grains. Continuous borehole or trench sections through mineralised zones are required to assess the economic significance of the PGE mineralisation. Nevertheless the high levels of PGE attained and the evidence of xidespread occurrence of the Cliff-type PGE enrichment are favourable indications. The PGE enrichments found in the cumulate complex are of potential interest as they may originally have been of magmatic origin. Larger tonnage targets may therefore be present in this unit compared Rith the likely size of structurally-controlled mineralisation elswhere in the complex

Potential for stratiform base-metal sulphides in SW England

The British Geological Survey (BGS) has recently re-evaluated the potential for stratiform base-metal sulphide mineralisation in Devon and east Cornwall (Rollin et al., 2001) (Figure 1). On the basis of their stratigraphy and tectonic setting the Lower Carboniferous strata between, and to the north of, Bodmin Moor and Dartmoor are favourable targets for Iberian Pyrite Belt (IPB)-type deposits (Figure 2). Similarly, the geological setting of the Middle to Upper Devonian sediments of Exmoor, north Devon, are comparable with those of the Harz basin in Germany which hosts the major polymetallic Sedex deposits at Rammelsburg and Meggen

Challenges to global mineral resource security and options for future supply

Minerals are vital to support economic growth and the functioning of modern society. Demand for minerals is increasing as global population expands and minerals are used in a greater range of applications, particularly associated with the deployment of new technologies. While concerns about future mineral scarcity have been expressed, these are generally unfounded and based on over-simplistic analysis. This paper considers recent debate around security of mineral supply and technical, geosciences-based options to improve utilization of the resource base and contribute to replenishing reserves. History suggests that increasing demand for minerals and higher prices will generally lead to technological and scientific innovation that results in new or alternative sources of supply. Recent assessments of global mineral endowment suggest that society should be optimistic about its ability to meet future demand for minerals, provided that there is continued innovation and investment in science and technology. Reducing energy consumption and breaking the current link between metal production and greenhouse gas emissions are among the greatest challenges to secure a sustainable mineral supply. However, widespread adoption of low-carbon mineral extraction technologies, underpinned by multidisciplinary research, and increased global utilization of low-carbon energy sources will allow these challenges to be met

Geology of the Huntly and Turriff districts : sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland)

This report summarises the results of detailed geological remapping and related studies under-taken by the Geological Survey in the Huntly and Turriff districts of North-east Scotland. It pro-vides an account of the geology to accompany the published 1:50 000 geological maps sheets 86W (Huntly) and 86E (Turriff). The districts occupy a tract of generally rolling country, which ranges from about 50 to 200 m above sea level in its eastern part, to more elevated hilly country, commonly reaching over 300 m and 400 m above OD, in its western and southern parts. The drainage is dominated by the sinuous incised valley of the River Deveron and its main tributar-ies, the rivers Isla and Bogie, except for the southern part of the Turriff district, which is drained by the headwaters of the River Ythan. Agriculture and forestry are the dominant economic ac-tivities, with tourism and whisky distilling also significant. The Huntly and Turriff districts are underlain mainly by Dalradian metasedimentary rocks of Neoproterozoic age that have been strongly deformed and metamorphosed during the Grampian event of the Caledonian Orogeny. The Dalradian rocks have been intruded by igneous intrusions that range from large plutons to small pods and dykes. Granite sheets and pods were emplaced at about 600 million years (Ma), followed later by small ultramafic and mafic bodies. However, the main intrusive igneous event occurred in the Early to Mid Ordovician at 474 to 470 Ma, coeval with the Grampian event. It resulted in the emplacement of mafic and ultramafic plutons, com-monly zoned; the Insch Pluton includes monzonites and syenites in its upper zone. Dioritic and granitic bodies are associated with the plutons. The major structural feature is the north-north-east-trending Portsoy Shear Zone, which traverses the Huntly district. This steeply easterly dipping zone is coincident with a major regional linea-ment, which separates stratigraphically discrete packages of Dalradian rocks with different tec-tonometamorphic histories. The shear zone has also facilitated and in part controlled the emplacement of the Huntly and Knock mafic-ultramafic plutons. The Dalradian metasedimen-tary rocks contain both Buchan and Barrovian metamorphic zonal assemblages. Folding and shearing have caused local repetition of lithological units, but overall the Dalradian sequence be-comes younger eastwards until the broad hinge of the regional Turriff Syncline is reached in the central part of that district. Here, the youngest Dalradian rocks are exposed and metamorphic grade is low (biotite grade). Farther east older Dalradian rocks again crop out and the Buchan metamorphic isograds are re-crossed such that the rocks contain andalusite and sillimanite. Following the Grampian orogenic event the area was uplifted, and during the Early Devonian fluvial and lacustrine Old Red Sandstone rocks were deposited in the northerly trending fault-bounded basins, namely the Turriff and Rhynie basins. In Strath Bogie a basaltic andesite lava unit linked to the Rhynie chert occurs within the Rhynie Basin succession. Subsequently, a Mid-dle Devonian conglomerate-dominated sequence, linked to the Orcadian Lake farther north, was deposited unconformably on the older succession in the Turriff Basin. In the Turriff district deep Tertiary weathering profiles and local fluvial sands and gravels are preserved, testifying to only limited erosion by the later Quaternary glaciations. The Devensian glacial and postglacial history of the districts was dominated by an ice sheet derived from the highland areas farther to the south-west. The related glacial and postglacial superficial deposits form a pervasive if generally thin cover over much of the bedrock. Till derived from the Moray Firth is present in the north-eastern part of the Turriff district. Eastward migration of glacial meltwater gave rise to channels that in places have significantly influenced development of the Holocene fluvial drainage pattern. The Huntly district has been the focus of several periods of exploration since 1970 for platinum group elements, copper and nickel linked mainly to the mafic-ultramafic plutons and their meta-morphic aureoles. The limited drilling and more extensive ground magnetic surveys, in combina-tion with geochemical studies and gravity modelling, have provided detailed information as to the nature, distribution and origin of prospective areas. No economic deposits have been deline-ated to date. However, this BGS report and the geological maps provide a sound foundation for existing and any future commercial and/or conservation-related developments

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

Many rare metals used today are derived from granitic pegmatites, but debate continues about the origin of these rocks. It is clear that some pegmatites represent the most highly fractionated products of a parental granite body, whilst others have formed by anatexis of local crust. However, the importance of these two processes in the formation of rare-metal pegmatites is not always evident. The Lewisian Gneiss Complex of NW Scotland comprises Archaean meta-igneous gneisses which were highly reworked during accretional and collisional events in the Palaeoproterozoic (Laxfordian orogeny). Crustal thickening and subsequent decompression led to melting and the formation of abundant granitic and pegmatitic sheets in many parts of the Lewisian Gneiss Complex. This paper presents new petrological, geochemical and age data for those pegmatites and shows that, whilst the majority are barren biotite-magnetite granitic pegmatites, a few muscovite-garnet (rare-metal) pegmatites are present. These are mainly intruded into a belt of Palaeoproterozoic metasedimentary and meta-igneous rocks known as the Harris Granulite Belt. The rare-metal pegmatites are distinct in their mineralogy, containing garnet and muscovite, with local tourmaline and a range of accessory minerals including columbite and tantalite. In contrast, the biotite-magnetite pegmatites have biotite and magnetite as their main mafic components. The rare-metal pegmatites are also distinguished by their bulk-rock and mineral chemistry, including a more peraluminous character and enrichments in Rb, Li, Cs, Be, Nb and Ta. New U-Pb ages (c. 1690–1710 Ma) suggest that these rare-metal pegmatites are within the age range of nearby biotite-magnetite pegmatites, indicating that similar genetic processes could have been responsible for their formation. The peraluminous nature of the rare-metal pegmatites strongly points towards a metasedimentary source. Notably, within the Lewisian Gneiss Complex, such pegmatites are only found in areas where a metasedimentary source is available. The evidence thus points towards all the Laxfordian pegmatites being formed by a process of crustal anatexis, with the formation of rare-metal pegmatites being largely controlled by source composition rather than solely by genetic process. This is in keeping with previous studies that have also challenged the widely accepted model that all rare-metal pegmatites are formed by fractionation from a parental granite, and raises questions about the origin of other mineralised pegmatites worldwide

Fluids and mineralisation in the Scottish Dalradian

Fluid inclusion studies of orogenic vein-type gold deposits show a strong genetic association with high-temperature, low-salinity, and volatile-rich fluids. Although fluid models for this type of mineralisation are well established, they commonly ignore several important facts that indicate a role for low temperature fluids. Visible gold is invariably fracture-controlled and is hence related to fluid processes represented by later (secondary) fluid inclusions. In the limited number of cases where these have been studied, a multiple fluid history involving low-temperature brines (<250ºC) is commonly observed. Gold is often associated with a suite of minerals characterised by sulphosalts, tellurides, sulpho-tellurides and rare gold alloys. Some of these are only stable below 275ºC, indicating an association with low-temperature processes. This study collates fluid inclusion and mineralogical data representing the major styles of mineralisation (e.g. Cononish and Calliachar Burn [Au–quartz]; Lagalochan [porphyry–Cu±Au]; Tomnadashan [igneous related Cu]; Tyndrum, Castleton and Inverneil [Pb–Zn]; Stronchullin and Corrie Buie [Pb–Zn±Au]) and regional fluids (e.g. Loch Lomond) in the Scottish Dalradian. Six fluid types are identified: 1. High-salinity (halite-bearing: NaCl>35 wt %) and high-temperature (>300ºC) fluid inclusions. These are typical of porphyry copper deposits world-wide and were recorded from samples at Lagalochan, Tomnadashan and Comrie. 2. High-temperature (250–400ºC), volatile-rich (major CO2+CH4+N2: 15–25 wt % NaCl eq) and moderate salinity (7–10 wt % NaCl eq) fluids inclusions. In the Dalradian, this fluid is ubiquitous. It occurs in veins and breccias, associated with igneous intrusions. It is also one of the major fluid-types in regional metamorphic quartz-veins and is recorded at nearly all mineralised localities. Elsewhere, this fluid-type is associated with orogenic gold mineralisation. However, in the Scottish Dalradian, its presence is not indicative of gold mineralisation. 3. Moderate to high-temperature (200–350ºC) and moderate salinity (7–10 wt % NaCl eq) fluids containing volatiles (minor CO2+CH4+N2: 10–15 wt %% NaCl eq). These have the same distribution and associations as Fluid 2. 4. Low to moderate-temperature (150–250ºC) low-salinity brines (<10 wt % NaCl eq) with little or no volatile component. This is analogous to fluids associated with epithermal gold mineralisation. It occurs in both igneous and metasedimentary rock-hosted mineralisation, and is present in a number of metamorphic quartz veins. Its presence as primary fluid inclusions in sphalerite, at Stronchullin, shows it plays a significant role in mineralisation. 5. Low-temperature (<150ºC) high-salinity (c. 20 wt % NaCl eq) brines. This fluid is typical of Mississippi Valley Type Pb–Zn deposits world-wide. It is present in most gold mineralised localities, but inclusions are low in abundance. Its role in Au metallogenesis is unclear, but a similar type of fluid is associated with gold-free base metal mineralisation (e.g. Tyndrum Pb–Zn). 6. A low temperature (monophase) aqueous fluid. This could be a low temperature equivalent of either Fluid 4 or Fluid 5. This fluid is sparsely distributed over a wide area. Types 1 and 2 and possibly 3 represent prograde fluids, which have a deep crustal (magmatic and metamorphic) origin, and are probably responsible for introduction of metals into the system. Then initiation of extensional tectonics permitted a major ingress of meteoric–basinal fluids (Types 4, 5 and 6). Fluid 4 and/or 5 remobilises earlier Fe–Cu–(Mo)–As–Au–S mineralisation and results in a base metal–gold overprint at many of the localities. Late-stage (Devonian-Carboniferous?) basin development is a possible source for the high-salinity low-temperature brines (Type 5 fluid). Fluid 6 could be responsible for the localised dickite–kaolinite mineralisation in the Highland Boundary Fault Zone and supergene alteration seen in mineralised localities (e.g. Calliachar Burn). Although the volatile-rich fluids play a major role in metallogenesis in the Scottish Dalradian, it is clear that low-temperature brines played a significant role in gold mineralisation. In terms of understanding the deposit-scale distribution of gold, it is of prime importance to know how these late-stage fluids interacted with pre-existing mineralised structures. Also, for exploration, there is a need to develop new technologies to predict where and how these fluids have acted, as they are probably responsible for the erratic distribution of gold-grades that characterise many orogenic gold deposits

Development of decision-making tools to create a harmonised UK national mineral resource inventory using the United Nations Framework Classification

The need to better understand how we source and consume the raw materials required for decarbonisation is driving a growing demand for data on mineral resources. A key application of these data is to understand resource potential, by evaluating known ‘geological stocks’ of raw materials based on estimates of mineral resources and reserves. However, the available resource data are often incomplete, totally lacking or compiled in different ways (i.e. industry reported data, which has significantly different user requirements to that of national level policy makers), making comparisons and aggregation near impossible. This study demonstrates the use of the United Nations Framework Classification (UNFC) to harmonise resource data for the UK. It highlights the benefits of this approach for improving the understanding of resource issues. Simple decision-making tools have been created, and are used to assist with classifying existing resource data using the three axes of UNFC, degree of confidence, technical feasibility and environmental socio-economic viability. These are designed to be applicable to a wide range of heterogenous datasets managed by national data providers. Their application to the UK, which has no system or national standard for collecting resource data, has served to highlight various issues relating to future mineral supply. These include variable data for deposits that may include multiple commodities such as co- and by-products, lack of data for minerals required for newly developing technologies and the variations in approaches for different commodity types. The compilation of standardised datasets can benefit national resource management, providing a ‘snapshot’ of the state of the UK minerals industry. For example, the results of this study facilitates inter-regional and international comparison and aggregations. In addition the consideration of the unique combination of geological, social and environmental factors by UNFC well as highlights where interventions may be needed if new projects to contribute to the green transition are to be developed. The use of the UNFC to classify mineral resource data, in a consistent way, by using the decision tools presented here, supports the creation and adoption of evidence-based raw material strategies. However, it is important to understand the limitations related to data gaps, consistency of approach and harmonisation of datasets from diverse sources

Location-specific comparison between a 3D in-stent restenosis model and micro-CT and histology data from porcine in vivo experiments

Background Coronary artery restenosis is an important side effect of percutaneous coronary intervention. Computational models can be used to better understand this process. We report on an approach for validation of an in silico 3D model of in-stent restenosis in porcine coronary arteries and illustrate this approach by comparing the modelling results to in vivo data for 14 and 28 days post-stenting. Methods This multiscale model includes single-scale models for stent deployment, blood flow and tissue growth in the stented vessel, including smooth muscle cell (SMC) proliferation and extracellular matrix (ECM) production. The validation procedure uses data from porcine in vivo experiments, by simulating stent deployment using stent geometry obtained from micro computed tomography (micro-CT) of the stented vessel and directly comparing the simulation results of neointimal growth to histological sections taken at the same locations. Results Metrics for comparison are per-strut neointimal thickness and per-section neointimal area. The neointimal area predicted by the model demonstrates a good agreement with the detailed experimental data. For 14 days post-stenting the relative neointimal area, averaged over all vessel sections considered, was 20 ± 3% in vivo and 22 ± 4% in silico. For 28 days, the area was 42 ± 3% in vivo and 41 ± 3% in silico. Conclusions The approach presented here provides a very detailed, location-specific, validation methodology for in silico restenosis models. The model was able to closely match both histology datasets with a single set of parameters. Good agreement was obtained for both the overall amount of neointima produced and the local distribution. It should be noted that including vessel curvature and ECM production in the model was paramount to obtain a good agreement with the experimental data