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Rare earth elements (REE) are essential raw materials used in modern technology. Current production of REE is dominated by hard-rock mining, particularly in China, which typically requires high energy input. In order to expand the resource base of the REE, it is important to determine what alternative sources exist. REE placers have been known for many years, and require less energy than mining of hard rock, but the REE ore minerals are typically derived from eroded granitic rocks and are commonly radioactive. Other types of REE placers, such as those derived from volcanic activity, are rare. The Aksu Diamas heavy mineral placer in Turkey has been assessed for potential REE extraction as a by-product of magnetite production, but its genesis was not previously well understood. REE at Aksu Diamas are hosted in an array of mineral phases, including apatite, chevkinite group minerals (CGM), monazite, allanite and britholite, which are concentrated in lenses and channels in unconsolidated Quaternary sands. Fingerprinting of pyroxene, CGM, magnetite and zircon have identified the source of the placer as the nearby Gölcük alkaline volcanic complex, which has a history of eruption throughout the Plio-Quaternary. Heavy minerals were eroded from tephra and reworked into basinal sediments. This type of deposit may represent a potential resource of REE in other areas of alkaline volcanism.
Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
Abstract Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid–rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g −1 , HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid – country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.
Bismuth occurs in a wide range of mineral deposit types and is usually regarded as a deleterious by-product. Its classification as a critical raw material by the European Commission in 2017 and a critical mineral by the USA in 2018 has, however, reawakened interest in Bi production and its security of supply. Demand for Bi is increasing, mostly as a substitute for Pb and for use in chemicals. Bismuth is mainly chalcophile in behaviour, although it has some lithophile characteristics. The element is strongly concentrated in felsic crustal lithologies, particularly fractionated granites, where it can substitute for Zr in zircon. It occurs within a diverse range of minerals; the most important hydrothermal minerals are native bismuth and bismuthinite. Bismuth can substitute for Pb in galena and Bi-rich galena is a major Bi ore. Bismuth alloys with gold to form maldonite at temperatures < 373 °C, thereby acting as a Au collector in felsic melts, particularly under reduced conditions. In the weathering environment Bi is generally immobile: it forms Bi oxide or hydroxide ochres or co-precipitates with Fe. Bismuth is found in a range of mineralised systems, sometimes in sufficient quantities to be economically extracted as a by-product. The most common sources of Bi are W-, Pb-, and, occasionally, Au-rich skarns, while five element (Co-Ni-Bi-Ag-As ± U) vein deposits were historically a major source of native Bi. Bismuth also occurs in large magmatic systems such in Sn- and W-rich greisens and associated veins as native bismuth and bismuthinite. Bismuth is present in trace concentrations in porphyry-hosted Mo-W-mineralisation and in some reduced intrusion-related Au, as well as some orogenic Au, deposits. VMS deposits can host minor Bi mineralisation, typically associated with the Au-rich parts of the mineralised system. Bismuth supply is strongly reliant on Asian production; notably the skarns deposits Núi Pháo in Vietnam and Shizhuyuan in China. Alternative supplies of Bi could be unlocked by greater consideration of bismuth by-production at the evaluation stage of polymetallic prospects elsewhere, and if more sustainable recovery techniques are developed for retrieval of Bi from conventional mineral processing circuits. The knowledge base for bismuth can be improved upon through interventions at the exploration, resource and reserve reporting and mineral processing planning stages. This in turn would provide a greater understanding of the deportment of Bi-bearing minerals, impacting on the design of mineral processing flow sheets and reducing waste, and thereby improving the sustainability and environmental footprint of mineral deposits.
Abstract The major share of raw materials needed to sustain our present lifestyle and even more importantly, required for the crucial green transition, are sourced outside Europe. The European Commission aims to enhance Europe's resilience and strengthen domestic sourcing. Although Europe has a long tradition of mining and extractive activities, it is acknowledged that there are several challenges to achieve European sourcing of certain raw materials such as the critical raw materials. A basic prerequisite to enable access to domestic raw materials is information on raw material occurrences, current and past mining activities, resources and reserves. The Geological Survey organizations (GSOs) of Europe play a key role in generating, compiling, gathering and storing the most up-to-date information as well as long-term data series on raw materials at national and regional levels. Over the last decade, the GSOs have joined forces and taken essential steps to harmonize and share data on raw materials. The results of this co-operation are illustrated as interactive maps on the European Geological Data Infrastructure (EGDI). This paper describes the data compiled in co-operation between the GSOs, and analyses the strengths and weaknesses of, as well as opportunities for and threats towards, the data.
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.
Rare earth element (REE) resources are commonly found associated with alkaline igneous complexes or carbonatites, or as secondary deposits derived from igneous rocks. Globally, many REE deposits occur around the margins of Archaean cratons, most in continental rift zones. Europe contains many such rift zones, which are generally younger in the south. Many of these rifts are intracontinental, whereas others are associated with the opening of oceans such as the Atlantic. All these rift systems have the potential to host REE resources, but whereas the older provinces of northern Europe are deeply exposed, exposures in southern Europe are largely at the supracrustal level. This paper considers how an understanding of the tectonic setting of Europe’s REE resources is vital to guide future exploration.
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