Ore Minerals Down to Nanoscale: Petrogenetic Implications

Abstract

Like most minerals, sulphides and oxides are compositionally heterogeneous at the smallest scale. The application of a variety of different techniques is necessary to fully understand their crystal-chemistry, each with its own inherent advantages and dis-advantages, and each with limited ranges of obser-vational scale. Focussed Ion Beam Scanning Electron Microscopy (FIB-SEM) opens up new avenues for in-situ sampling of small volumes of material which have been compositionally characterised by other techniques, including SEM, EPMA and LA-ICPMS. Slices lifted from the surface of a polished section, typically 40 x 20 µm in size and a few µm thick, can be imaged and prepared for Transmission Electron Microscope (TEM) analysis. This procedure allows for bridging micro- to nanoscale observations on a site which is of petrogenetic interest (e.g., Ciobanu et al., 2011). One of the main themes of inquiry is the distribution of trace elements in ores relative to subtle changes in mineral speciation, nanoparticle nucleation or crystal-structural modifications. Here we show examples of the application of FIB-SEM and TEM studies to such topics from across ore geology. 2 Homology in Mixed-Layer Chalcogenides Bismuth minerals are common accessory minerals in gold deposits, and include Pb-Bi-sulphosalts and – tellurides. They form homologous or polysomatic series where small chemical variation from one species to another is linked to modifications of crystal-structural layers (modules or blocks) in a predictive manner. These minerals can thus record subtle variations in the chemistry of an evolving ore system via crystallisation processes at the smallest scale. One example is the tetradymite group, where the crystalchemical formula nBi2·mBi2X3 (X = Te, Se, S; e.g., Cook et al., 2007) indicates that all the phases can be derived from archetypal layers of fixed width, but of variable numbers (n, m) in the stacking sequence. Based on TEM studies that show monotonic decrease of crystal modulation vectors with increase in Bi, the homology in the group has been