RECYCLING AND CHEMICAL MOBILITY OF ALLUVIAL GOLD IN TERTIARY AND QUATERNARY SEDIMENTS, CENTRAL AND EAST OTAGO, NEW-ZEALAND

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-P liocene Wedderburn Formation. Gold has been recycled from older to you nger 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 gol d in source sediments. Gold grains are too small to be hydrodynamicall y equivalent to the maximum cobble size being transported, so streams were capable of transporting all available gold. A small but consisten t increase in gold grain size occurs with decreasing age of the host f ormations, due to minor mobilisation of gold by groundwater, and repre cipitation on pre-existing grain surfaces. Pliocene and younger placer s display evidence of substantial chemical accretion of gold, with gol d grains reaching 2 cm or more in certain Quaternary placers. The incr ease in significance of chemical accretion in Pliocene and younger pla cers is due to the development of alkaline soils in a rainshadow forme d by the rise of the Southern Alps mountain chain, east of the Alpine Fault. Size sorting in streams results in transport of fine-grained go ld, but retention of small nuggets (c. >1 mm) in proximal fan deposits .