Contrasting evolution of beach gold on two sides of an active orogen, Southern Alps, New Zealand.

Placer gold on beaches on either side of a major active mountain belt has strongly contrasting compositions and morphologies. Pleistocene-Holocene beach gold placers have formed along the West Coast where silver (Ag)-bearing gold has been eroded from mountains via steep rivers and glaciers, followed by rapid burial in the abundant associated coastal sediments. Deformation along transport pathways has produced flattened gold flakes, but some primary crystalline shapes are preserved. Internal deformation of particles has caused recrystallisation to fine-grained (µm scale) grain structures including mylonitic banding. However, only minor localised (µm scale) Ag leaching has occurred on some particle margins. In contrast, Pleistocene beach placers formed on the southern side of the mountains contain flattened gold particles with a complex transport history involving long-distance (hundreds of km) fluvial transport since Cretaceous. Most primary Ag has been leached from this gold during transport-related recrystallisation to fine (5–20 µm) internal grains. Sand-blasting on the southern beaches with low sediment supply has resulted in further deformation and leaching of Ag to form toroidal shapes with Ag-free delicate extremities. Placer gold which has undergone rapid transport and deposition is useful as a prospecting tool for bedrock deposits in glaciated terranes around the world. [ABSTRACT FROM AUTHOR]