The formation and distribution of gold placers in active
collisional mountain belts are directly related to the tectonic, topographic,
climatic and erosional constraints prevailing on either side of the orogen at
the time of placer formation. In the absence of pre-existing placer deposits,
these factors combine to concentrate placer development on the side of the
orogen facing the prevailing winds, where most of the erosion and uplift occurs.
Drainage network changes and glaciation can significantly alter placer
distribution with respect to source areas within the orogen. After collision
stops, placer formation will proceed on both sides of the orogen as exhumation
of the mountain belt ensues. Incorporation of pre-existing placers formed on
ancient mountain belts into younger placers in subsequent orogens, may be
recognised by anomalous placer distribution compared to the above model. Most of
the relatively young giant placers such as those inNew Zealand, Chile, Bolivia and Columbia, exhibit these general patterns. Their
character and mode of formation have important implications for the formation
and distribution of much older placers, such as those in South Africa's Witwatersrand Basin.