Placer Gold in the Eastern Maniototo Basin, Otago, New Zealand: Deposits and Sources

Abundant detrital gold particles in placer deposits of eastern Maniototo basin, southern South Island, New Zealand, have only minor rounding and retain many angular and crystalline features, implying derivation from a nearby source. Likewise, gold particle interior textures have undergone only minor transport‐induced deformation and recrystallisation. This study focused on gold from Holocene schist gravel‐dominated outwash fans formed at the basin margin. Gold in remnants of Eocene quartz pebble conglomerates upslope of the outwash fans has the same morphology as in the fans and was recycled during Pleistocene uplift of the adjacent Otago Schist basement antiform. Pleistocene schist gravel terraces were an intermediate repository for gold during this recycling. Newly defined hydrothermal gold deposits occur at higher levels on the schist antiform amid an extensive (∼10 km wide) soil arsenic anomaly on upper greenschist‐facies basement. These auriferous hydrothermal deposits include quartz veins with pyrite, arsenopyrite and stibnite and a shallow‐dipping zone with arsenopyrite and pyrite in foliation‐parallel shears and associated breccias. There is a mismatch in gold particle size between these hydrothermal deposits (micron‐scale) and the downstream placer gold (millimetre‐scale). Pre‐Eocene existence of a supergene alteration zone is inferred, where surficial gold mobility enhanced gold particle sizes before erosion into the Eocene placers. The detrital gold in the Holocene outwash fans has remained remarkably unaffected by the complex history of erosion, recycling and transport over ∼10 km during the Coenozoic.