Preparation of Samples with Both Hard and Soft Phases for Electron Backscatter Diffraction: Examples from Gold Mineralization
Abstract Preparation of high-quality polished sample surfaces is an essential step in the collection of microanalytical data on the microstructures of minerals and alloys. Poorly prepared samples can yield insufficient or inconsistent results and, in the case of gold, potentially no data due to the “beilby” layer. Currently, preparation of ore samples is difficult as they commonly contain both hard and soft mineral phases. The aim of our research is to produce suitably polished sample surfaces, on all phases, for electron backscatter diffraction analysis. A combination of chemical–mechanical polishing (CMP) and broad ion-beam polishing (BIBP) was used to tackle the problem. Our results show that it is critical to perform CMP first, as it produces a suitable polish on the hard mineral phases but tends to introduce more damage to the soft mineral surfaces. BIBP is essential to produce a high-quality polish to the soft phases (gold). This is a highly efficient method of sample preparation and is important as it allows the complete quantification of ore textures and all constituent mineral phases, including soft alloys.
Pyrite deformation textures in the massive sulfide ore deposits of the Norwegian Caledonides
Abstract The Norwegian Caledonides represent a belt of late Precambrian to early Palaeozoic rocks, emplaced as a series of nappes onto the Fennoscandian basement. Massive sulfide orebodies occur along virtually the whole length of the orogenic belt making it possible to study deposits of a similar origin, chemistry and mineralogy but variably deformed at different metamorphic conditions. Samples from seven pyrite-rich ore deposits deformed at temperatures from ∼ 320 °C to ∼ 610 °C have been investigated using reflected light petrographic observation, orientation contrast (OC) imaging and electron backscatter diffraction (EBSD). While a range of brittle, annealment and growth textures are preserved in the deposits, all preserve evidence for lattice misorientation and low-angle (∼ 2°) sub-grain boundary development indicative of dislocation creep. These results are at odds with those implied from the published pyrite deformation mechanism map, which suggest that pyrite does not deform by plastic mechanisms at natural geological strain-rates and that the brittle–ductile transition is at ∼ 425 °C. Our results imply that pyrite is more ductile than generally inferred and the current deformation mechanism map needs to be revised with plastic deformation mechanisms operating at a much wider range of temperature and strain-rate conditions.
Paleozoic and Mesozoic silica-rich seawater: Evidence from hematitic chert (jasper) deposits
Laterally extensive beds of highly siliceous, hematitic chert (jasper) are associated with many volcanogenic massive sulfide (VMS) deposits of Late Cambrian to Early Cretaceous age, yet are unknown in analogous younger (including modern) settings. Textural studies suggest that VMS-related jaspers in the Ordovician Lokken ophiolite of Norway were originally deposited as Si- and Fe-rich gels that precipitated from hydrothermal plumes as colloidal silica and iron-oxyhydroxide particles. Rare earth element patterns and Ge/Si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 10 3 to 10 4 , similar to modern examples. We propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids-as suggested by previous workers-but instead was deposited from silica-rich seawater. Flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume. A model involving amorphous silica (opal-A) precursors to the jaspers suggests that silica contents of Cambrian-Early Cretaceous oceans were at least 110 mg/L SiO 2 , compared to values of 40-60 mg/L SiO 2 estimated in other studies. The evolution of ancient silica-rich to modern Fe-rich precipitates in submarine-hydrothermal plumes reflects a changeover from silica-saturated to silica-depleted seawater through Phanerozoic time, due mainly to ocean-wide emergence of diatoms in the Cretaceous.