Abstract

Central Otago, New Zealand, is drained by a single major river system, the Clutha River, whose active and Recent stream gravels host major placer deposits for most of its length through the region. The valley of the Manuherikia River, a major tributary of the Clutha, hosts eluvial placers in several geologic situations representative of Central Otago as a whole. The Manuherikia catchment is underlain by schist with sparse gold-bearing veins. The schist is unconformably overlain by variable auriferous Miocene fluvial sediments with detrital gold typically about 300 micro m across and a variable silver content of up to 22 wt %. The Miocene sediments have been partially eroded during Quaternary warping and uplift of the schist basement. Gold from the Miocene sediments (and locally, schist veins) has been chemically mobilized on the degraded unconformity to form nuggets up to 2 cm across. Erosion of the unconformity results in alluvial fans containing coarse eluvial gold. Fan sediments have been locally uplifted and recycled into younger fans. Ground-water activity within the fans has chemically mobilized gold, resulting in further addition to nugget rims and an increased grain size. Two distinct chemical gold mobilization processes are identified. During erosion, metal mobility in alkaline soils (pH = ca. 8) under oxidizing conditions results in gold nugget growth with 2 to 6 wt % Ag in the authigenic gold, due to similar solubilities of Ag and Au. Ground water is reduced and acid (pH = 3.5-5), and authigenic gold is almost pure (0.5 wt % Ag), possibly deposited by electroless plating on to preexisting nuggets. These two processes have occurred, and are still occurring, as the Manuherikia catchment evolves tectonically and uplift continues. Ultimately, the valley will be more deeply eroded into schist basement, and eluvial placers will migrate toward the Clutha River. Rugged valleys in the western Clutha catchment have already experienced this deeper erosion and redistribution of gold.

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