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.We present 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.