Focussed ion beam–transmission electron microscopy applications in ore mineralogy: Bridging micro- and nanoscale observations

Focussed ion beam–scanning electron microscopy (FIB–SEM) is a relatively new analytical tool that has been little applied to problems of ore genesis. The technique enables high-resolution (cross-section) imaging and can be used to prepare thinned foils for study by transmission electron microscopy (TEM). FIB–SEM methods applied to sulphides and related compounds represent an in-situ approach for sample characterisation and thus provides for crystal–chemical data that can be placed into the geological context of a given ore deposit. sent four study cases: these deal with minor element incorporation and release in ZnS; intergrowths and replacement among Cu–(Fe)-sulphides; fabrics in Au-bearing, As-free pyrite; and symplectites of Bi-sulphosalts within galena. The data is discussed in the context of polytypism and planar defects for minor element incorporation and release, superstructure ordering and formation of fine particles (100–2500 nm) or nanoparticles (< 100 nm) during replacement processes. Several analytical difficulties encountered when preparing FIB–TEM samples from sulphides are discussed, in particular mechanical and chemical damage to the surface. The FIB–TEM foils are difficult to thin for direct high-resolution TEM imaging but are usable for Scanning Transmission Electron Microscopy (STEM) and High-Angle Annular Dark Field (HAADF)-STEM imaging.