Gold placers can be difficult to locate in areas where
tectonic activity and paleogeography have changed significantly since placer
formation. Existing placer exploration models are useful in areas that have a
relatively simple geological history, identifiable relationships with parent
sources, or a history of placer mining. However, exploration becomes
increasingly difficult in areas where tectonic activity and sedimentary
recycling repeatedly redistribute the gold.
Gold placers in the Waitaki River
catchment, North Otago, New
Zealand have no primary source within the
catchment. During the Eocene placer gold was transported by a river
('Buster River') that flowed in a northeast direction from
Central Otago toward a coastline at Maerewhenua, North
Otago. This river and coastal system was inundated by marine
transgression during the middle Tertiary. Since the Miocene its placer deposits
have been uplifted and eroded and any links with parent sources have been
severed. Changes in paleogeography resulted in local reconcentration and local
dilution of placer gold in different areas in the Waitaki catchment. Thin
veneers of old Waitaki River gravels (Kurow Gravels), containing
gold recycled from the older placers, have been deposited unconformably on
strath surfaces cut into Tertiary or older rocks. These gravels along with
remnant Eocene fluvial and shallow marine strata provide potential placer
exploration targets. Uplift and erosion of non-auriferous rocks in the Waitaki
catchment to the west and north of the study area has resulted in considerable
dilution of potential placer deposits downstream, especially in areas of gravel
aggradation.
We present a placer exploration model that is founded on
answering fundamental questions pertaining to the paleogeography and geological
history of the sediments associated with the placers.