• Craw, Dave
• Prior, David
• Stewart, James
• Hough, Rob
• Halfpenny, Angela
• Mackenzie, Doug
• Pearce, Mark

Abstract

Large flakes of gold in alluvial (river) deposits result from processes that include high - temperature annealing in the rocks in which the gold is first concentrated and low - temperature processes during subsequent weathering, erosion and transport and in the final site of deposition. The nature of these processes and their relative importance remains unclear. To further our understanding, we have quantified the microstructures and microchemistry of gold flakes from Miocene and Quaternary age alluvial deposits of the Otago goldfield using electron backscatter diffraction and X-Ray microanalysis.0.1 to 0.5mm flakes have rims of pure gold surrounding single or multiple irregular, interior patches of ~8% Ag. The contact of gold rim and silver - rich patches is sharp, precluding a simple diffusional origin. Silver rich patches have coarser grains and coarser Σ3 twins than gold rims. Most grains are strongly deformed, the silver rich patches are more deformed than gold rims. Sharp contacts of Ag - rich patches correspond to general high - angle grain boundaries. We interpret the pure Au rims to have developed by an ambient temperature strain energy driven recrystallization process, with removal of Ag from the alloy occurring by diffusion along migrating grain boundaries.

Topics

• Deposition
• impedance spectroscopy
• grain
• silver
• gold
• annealing
• electron backscatter diffraction
• recrystallization