CRITICAL METALS IN THE IRISH CARBONATE-HOSTED ZN-PB DEPOSITS: A GEOCHEMICAL OVERVIEW

The Irish Midlands host a world-class Zn-Pb district with deposits occurring within Lower Carboniferous marine carbonate rocks. Critical metals have not been traditionally targeted by exploration, although analyses have shown that Ge can be utilised for trace-element vectoring. Nevertheless, sphalerite, the chief sulphide mineral in the Irish Midlands, incorporates a broad range of trace elements, including Ga, Ge and In all of which could potentially be recovered as smelting by-products. However, there is little publicly available data concerning the content of these metals in Irish ores. The growing usage of these metals in high-tech applications such as smartphones (Ga, In), fibre-optic cables (Ge) and photovoltaic cells (Ga, Ge, In) has seen a rapid increase in their economic importance as well as supply security-related concerns which are reflected in their critical raw material designation internationally.

The current study investigated the deportment of Ga, Ge, In in the Irish Zn-Pb deposits using a combination of in-situmicro-analytical techniques, i.e. SEM and LA-ICPMS. Sphalerite ore samples from ten Irish Zn-Pb deposits and occurrences were analysed for the trace element composition and distribution. The results of 1441 spot analyses show that Irish sphalerites contain a moderate amount of Ge (mean=65.89 ppm, n=1434) and very minor Ga (mean= 8.98 ppm, n=1426) but are relatively depleted in In (mean=3.57 ppm, n=371). In order to understand the possible enrichment mechanism of Ge, a statistical approach was applied to analyse the dataset and evaluate the element correlation. A notable positive Pearson correlation (r) was observed for Ge with Fe (r=0.356), Ag (r=0.207), Sb (r=0.207), As (r=0.169), Cu (r=0.139) and Tl (r=0.055), while Cd (r=-0.184) is negatively correlated with Ge. This likely suggests that the incorporation of Ge into the Irish sphalerite crystal structure occurs through coupled substitutions and is collectively controlled by the availability of those cations in the hydrothermal fluids. Individual deposits and sphalerite grains display variations of Ge content that likely reflect control by the source and physiochemical condition of the mineralising fluids as well as the occurrence of co-crystallised mineral phases.