Gold placers in the Nokomai and Nevis valleys are dominantly Quaternary in age. They occur
mainly in western tributaries of the upper Nevis, eastern tributaries of the middle and upper
Nokomai, and in the trunk valleys themselves. Placer gold in both valleys is
dominantly a-phase Au-Ag-Hg alloy. Au-Ag alloy is absent or rare in most of the
deposits, except in the lower Nokomai.
Alpha-Au-Ag-Hg alloy is typically coarse-grained (up to
2 cm), angular, and rarely flattened or folded. Crystalline texture, quartz
intergrowths, and psuedo-hexagonal crystal-pluck cavities are common. Fluvial
transport distance predicted from the maximum Flatness Index ofAu-Ag-Hg alloy
particles is typically less than 10-20 km. Coarse (up to 2 cm) cinnabar is
commonly associated with the Au-Ag-Hg alloy, and both were probably derived from
hydrothermal sources in western tributaries ofthe upper Nevis and eastern
tributaries of the upper Nokomai. Minor secondary Au-Ag-Hg alloy occurs locally
in the lower Nokomai, where it coats or cements detrital a-phase Au-Ag-Hg and
Au-Ag alloy particles.
Au-Ag alloy dominates over Au-Ag-Hg alloy in the lower
half of the lower Nokomai alluvial plain and gravel of a raised channel incised
into semischist basement adjacent to the plain. The Au-Ag alloy is rounded and
commonly flattened and folded. Crystalline texture, quartz intergrowths, and
pluck cavities are rare, and a fluvial transport distance of 25-40 km is
estimated from maximum Flatness Index ofAu-Ag alloy particles. The Au-Ag alloy,
three types of gamet, magnetite, clinozoisite and well foliated schist boulders
in the abandoned channel are not found in the pumpellyite-actinolite facies
bedrock in the Nokomai. These components were derived from greenschist facies
schist and sedimentary sources in Otago, many tens of kilometers north of the
Nokomai catchment. They were transported to the Nokomai either in Wakatipu
Glacier till or fluvio-glacial outwash that entered the valley via Nokomai
Saddle, the confluence with the Mataura, or both.