Supernormal enrichment of cadmium in sphalerite via coupled hydrothermal dissolution-reprecipitation process

Abstract

Abstract The supernormal enrichment of cadmium, as well as other critical elements are frequently observed in the sphalerite of sedimentary-hosted zinc deposits. However, the physico-chemical processes that are responsible for their mineralization is still poorly understood. Here we display a coupled dissolution-reprecipitation model that could explain the occurrence of cadmium from solid solutions up to several wt.%, sulfide nanoparticles, and independent mineral inclusions in sphalerite. The selective dissolution by oxidative acidic fluids causes depletion of zinc and concentration of cadmium in the lattice of primarily formed sphalerite, especially at interface of structural defects, such as low-angle tilt boundaries. Further dissolution triggers reprecipitation of cadmium-poor sphalerite nanocrystals as relics, followed by deposition of amorphous cadmium-rich sulfide nanoparticles as fillings in dissolution pores by reaction between cadmium ions and released reduced sulfur from sphalerite lattice. The cadmium is derived from both dissolution of primary Cd-rich sphalerite and re-supply by highly-evolved oxidative acidic fluid rich in cadmium. Growth and/or passivation of these amorphous nanoparticles finally forms greenockite blebs and/or nanocrystals in sphalerite. This model provides insights for critical-element redistribution and mineralization via hydrothermal processes. Similar mechanisms may also have broad implications in synthesis of sulfide nanomaterials used in green energy technologies.