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

Abstract

Dissolution and reprecipitation of gold has occurred in both Miocene and Quaternary fluvial sediments at GaribaldiDiggings. These processes have changed the external appearance of placer gold flakes, obscuring textures imposed by fluvial transport. Our study aimed to investigate the internal structure of placer gold flakes, beneath the surface coating of post-sedimentation gold addition. Gold particles were sectioned and polished, and then the exposed surfaces were abraded with an ion beam to remove the artefact structures imposed by the polishing process and expose the internal structure. The internal structure was examined with high-resolution X-ray micro-analysis to determine relative silver contents, and electron backscatter diffraction to determine the internal structures of individual gold grains making up the gold particles. Analytical data show that the gold particles have ~8 wt% Ag in their cores, and have silver-depleted rims. The boundaries between Ag-bearing and Ag-poor parts of grains are sharp and have irregular shapes. The Ag-bearing gold cores are made up of a small number of large grains which have been internally deformed. The Ag-poor rims are made up of patches of smaller grains that are either largely undeformed or are less deformed than the large grains in the cores. The boundaries between these different grain textures coincide precisely with the sharp compositional boundaries. The Ag-poor rims have apparently recrystallised from original coarser Ag-bearing gold during or after transport in the fluvial system. Recrystallisation occurred by nucleation of silver-poor grains and subsequent strain energy driven grain boundary migration. Strain developed in the grains by hammering during transport was the driving force for the boundary migration. The mobile boundaries provided pathways to remove the silver from the alloy as they migrated through the gold flakes. The combination of internal recrystallisation and external gold dissolution/reprecipitation has resulted in substantial transformation of the gold particle textures during and after transport. These changes inhibit attempts to link placer gold with primary sources during gold exploration programmes.

Topics

• surface
• grain
• silver
• grain boundary
• laser emission spectroscopy
• gold
• texture
• electron backscatter diffraction
• polishing