Application of synchrotron-based techniques to examine Ge speciation in sphalerite

Many critical elements are recovered as byproducts of other commodities, which fuels concerns regarding future supply elasticity. Understanding the mineral hosts and speciation of critical elements in ore-forming environments provides insight into the conditions controlling their enrichment in ore deposits, which can aid in exploration, quantification of future resources, and development of extraction methods. We applied a suite of traditional and synchrotron-based techniques to examine the speciation of Ge in sphalerite (ZnS) from four mineral districts where Ge is or has been recovered within the United States. Examination of Ge in ZnS by traditional methods is complicated by spatially variable and low Ge concentrations in ores (typically 1-10 ppm) and the inability of traditional techniques to examine oxidation state, which is critical in determining substitution mechanisms. We examined speciation of Ge and co-substituents, such as Cu, in a sphalerite matrix using micro-focused X-ray fluorescence (XRF) mapping, bulk and micro-focused X-ray absorption spectroscopy (XAS), and micro-focused X-ray diffraction (XRD). A new high energy resolution fluorescence detector (HERFD) micro-XAS was used to reduce the contribution from the Zn K to the measured Ge K X-ray fluorescence signal, which significantly improved signal to noise and spectral resolution. Multiple energy XRF maps enabled us to map the distribution of Ge between sphalerite and oxidized weathering products in weathered mine wastes. Observed mixtures of Ge oxidation states (Ge2+ and Ge4+) in sphalerite depend on the presence and abundance of co-substituent elements (especially Cu, Ga, Sb, and Ag, depending on the deposit), and, in some cases, point to specific substitution mechanisms. Standard laboratory-based techniques failed to distinguish isotropic sphalerite from anisotropic domains observed by optical microscopy, whereas synchrotron-based XRD mapping revealed a distinctly different diffraction pattern in areas of anisotropy where hexagonal wurtzite (ZnS) may exist. The analytical tools employed here can be extended to other byproduct critical elements in complex matrices, in order to more fully understand ore enrichment processes, which have important applications in exploration, resource quantification, and extraction.