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Abstract

Sphalerite and arsenopyrite can host trace elements (e.g., Ge, Cd and Au) that are important for high technology applications. These trace elements are incorporated into the structure of the host mineral (e.g., by substitution) if inclusions are apparently absent. Focused ion beam technique and transmission electron microscopy combined with electron microprobe analysis allow for the correlation from micrometre to the nanometre scale in order to investigate the locations of trace elements in situ within the host minerals. Therefore, Ge-rich sphalerite from the Tres Marias Zn–Pb–Ge deposit in Mexico and auriferous arsenopyrite from the Ashanti Au mine in Ghana are investigated. Sphalerite from Tres Marias contains Ge concentrations of up to 1430 μg/g and is characterised by elevated Fe contents. Germanium is homogeneously distributed within the sphalerite lattice without the presence of distinct Ge-bearing phases in the form of inclusions or exsolutions. In arsenopyrite from the Ashanti mine, Au occurs as oriented sub-microscopic discrete phases showing at least two distinct orientations, which cause the observed trace-element concentrations visible at larger scale. The formation of nanometre-sized gold particles in arsenopyrite is interpreted as enrichment of Au at the interface of crystallising arsenopyrite with the hydrothermal fluid and subsequent crystallisation of nanometre-sized gold grains at the surface of arsenopyrite after the critical concentration for nucleation was overstepped.