Quaternary warping and uplift of
antiformal schist ranges in Central Otago, New Zealand, has resulted in
recycling of alluvial gold from widespread, generally weakly auriferous, Miocene
fluvial gravels that unconformably overlie the schist, into aprons of fan
sediments along the range margins. Basement erosion during uplift has been
limited and so gold input from sparse gold-bearing quartz veins in the schist
has been relatively minor. In an eluvial cycle on the slopes of the ranges,
chemical remobilization of gold occurs during oxidation and erosion of the
Miocene sediments. The typically fine-grained (300 - 400
m) gold undergoes chemical modification and grain size
enhancement of up to two orders of magnitude, and much of it is redeposited in a
large number of relatively small alluvial fans along the range margins.
Groundwater mobility within the fans results in minor additional gold mobility
and grain size enhancement. During
range uplift, a small number of relatively large axial streams and rivers have
developed in structural lows between growing antiformal folds. Axial streams
'gather' their flow from consequent streams draining the folds, and flow
parallel to a range axis for some distance before cutting across it in an
antecedent gorge. The axial streams rework alluvial fan sediments along the
range margins, and transport mainly fine-grained gold from the fans along the
ranges. This gold is redeposited where the axial streams emerge from a range in
the neighbouring structural basin. Axial streams drain relatively large areas of
range surface, and thus reconcentrate significant quantities of gold in a small
number of relatively large fans and fluvial deposits. The processes of gold
collection and re-deposition by axial streams are essentially the same,
regardless of stream size. Coarse-grained gold produced by chemical
remobilization, is more difficult to transport in fluvial systems, and thus
remains in close proximity to eluvial environment where it was
formed.