Alluvial gold in Central and East Otago occurs in four different ages of quartz conglomerate: Cretaceous-Paleocene Taratu Formation, Eocene Hogburn Formation, early Miocene Dunstan Formation, and late Miocene-Pliocene Wedderburn Formation. Gold has been recycled from older to younger formations. Younger placers also occur in Pliocene and Quaternary strata recycled from the quartz conglomerates, with some dilution by basement clasts. Gold is typically concentrated at the base of units, at or near unconformities. Gold grain size is small (<1 mm; c. 300 mu m mode) in Miocene and older placers, reflecting the grain size of gold in source sediments. Gold grains are too small to be hydrodynamically equivalent to the maximum cobble size being transported, so streams were capable of transporting all available gold. A small but consistent increase in gold grain size occurs with decreasing age of the host formations, due to minor mobilisation of gold by groundwater, and reprecipitation on pre-existing grain surfaces. Pliocene and younger placers display evidence of substantial chemical accretion of gold, with gold grains reaching 2 cm or more in certain Quaternary placers. The increase in significance of chemical accretion in Pliocene and younger placers is due to the development of alkaline soils in a rainshadow formed by the rise of the Southern Alps mountain chain, east of the Alpine Fault. Size sorting in streams results in transport of fine-grained gold, but retention of small nuggets (c. >1 mm) in proximal fan deposits.
