Lithostratigraphy of gold-bearing Quaternary gravels, middle Manuherikia Valley, Central Otago, New Zealand

Abstract

The oldest fluvial Quaternary gravels exposed in the middle Manuherikia Valley in the Central Otago goldfield consist predominantly of rounded and angular quartz, with subordinate schist clasts. These sediments occur in a mappable middle Quaternary unit, herein defined as the Tinkers Formation (new name) that is well exposed in historic gold mine excavations. The unit contains placer gold and has been locally mined. The proportion of schist clasts increases up-section in the formation, to make up at least 30% of the gravels. Gravel beds in the formation are interlayered with loess horizons and minor mudstone (1–4 m scale). The Tinkers Formation is unconformably overlain by coarse poorly rounded immature schist gravel that contains dispersed gold but has not been mined. The gravel units have been folded and faulted along the actively deforming Dunstan Range front, from which the gravels have been derived. Ongoing uplift and recycling of sediments and contained gold has yielded younger gold-bearing gravel units. The gravel sequence in the middle Manuherikia Valley is distinctly different from elsewhere in the Manuherikia Valley, where recycled greywacke clasts form a persistent and locally major component of Quaternary sediments. Lithostratigraphic definition and mapping of Quaternary gravels has some advantages over correlation of Quaternary surfaces, which has been the most commonly used approach to Quaternary mapping in the Central Otago goldfield, and the two approaches are complementary.