LA-ICP-MS analyses of minor and trace elements and bulk Ge isotopes in zoned Ge-rich sphalerites from the Noailhac – Saint-Salvy deposit (France): Insights into incorporation mechanisms and ore deposition processes

Abstract

The increasing worldwide demand in germanium (Ge) is driving renewed research for understanding its geological cycle and the factors controlling its concentration in minerals. The advent of accurate, high-resolution trace element analysis by LA-ICP-MS, as well as the advances in MC-ICP-MS technique for Ge isotopes in sulphides, has enhanced studies in this field. Ge isobaric interferences, standard calibration and data interpretation remain outstanding issues needing to be addressed for more precise and comprehensive LA-ICP-MS analyses.An integrated mineralogical and geochemical study was carried out on typical sphalerite (ZnS) samples from the main Ge deposit in western Europe: the vein-type Zn–Ge–Ag–(Pb–Cd) deposit of Noailhac – Saint-Salvy (Tarn, France). In situ coupled measurements of trace elements and S isotopes were performed using LA-ICP-MS and SIMS, respectively, together with bulk Ge isotopes by MC-ICP-MS. Principal component analyses revealed element clusters antithetically distributed within distinct zoning types in sphalerite: sector zonings are enriched in Cu, Ge, Ga, Sb and As, whereas rhythmic bandings (dark brown bands primarily) are enriched in Fe, Cd, In and Sn. This typical distribution points to crystallographic controls on trace element uptake during sphalerite growth, occurring with concomitant microscale variations in fluid compositions at the fluid–crystal interface. Regardless of the zoning type, in all spots, Cu contents approach the sum of tri- and tetravalent cations (Ge, Ga, In, etc.) so that Cu could provide charge-balance for the entire set of coupled substitution mechanisms responsible for the incorporation of the whole range of trace elements in this sphalerite. Strong binary correlations suggest direct substitutions as Zn2+↔(Fe2+, Cd2+) and coupled substitutions as 2Zn2+↔Cu++Sb3+, 3Zn2+↔Ge4++2Ag+, and 3Zn2+↔In3++Sn3++□ (vacancy) despite no clear evidence for the presence of Sn4+.δ74GeNIST3120a in bulk sphalerite varies from −2.07±0.37‰ to +0.91±0.16‰ (2σ SD) and positively correlates with bulk Ge content. This indicates considerable Ge isotopic fractionation within sphalerite during low-T hydrothermal deposition and zoning processes, associated with possible microscale open system fluid mixing. The trace element features in sphalerite from Saint-Salvy compared with those of other deposits confirm their use as discriminators among genetic types of ores (e.g., high In contents for magmatic-related deposits, and Ge for low-temperature deposits).