Abstract
Gold-bearing beaches at the western end of Foveaux Strait are being fed by the Waiau River, which is dominated by greywacke detritus. At times during the Pleistocene, debris from crystalline rocks in Fiordland has fed the Foveaux Strait beaches, and the greywacke detritus was diverted farther east. In contrast, the Round Hill placer goldfield has been dominated by locally derived fluvial felsic and mafic debris in the late Pleistocene to Recent. Gold particles in the Round Hill goldfield are toroidal in shape, and have been recycled from older beach sediments. Some gold toroids on active beaches may have been recycled from older beach sediments as well, whereas some may have formed in situ. Most gold toroids have encrustations of polymorphic clays and bio-organic matter, locally impregnated with fine-grained authigenic gold, and some toroids hosted in fluvial sediments display delicate authigenic gold overgrowths as a result of the biogeochemical transformation of gold. These biogeochemical processes appear to have little effect on co-existing detrital platinum group minerals such as isoferroplatinum.