The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H2O and CO2-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.
The ~500km-long mid-Cretaceous Semail nappe of the Sultanate of Oman and UAE (henceforth referred to as the Oman ophiolite) is the largest and best-preserved ophiolite complex known. It is of particular importance because it is generally believed to have an internal structure and composition closely comparable to that of crust formed at the present-day East Pacific Rise (EPR), making it our only known on-land analogue for ocean lithosphere formed at a fast spreading rate. On the basis of this assumption Oman has long played a pivotal role in guiding our conceptual understanding of fast-spreading ridge processes, as modern fast-spread ocean crust is largely inaccessible.
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.
An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
