Bismuth: economic geology and value chains

Abstract

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 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 co-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.