Gold in Quaternary placers in the Nevis and Nokomai valleys is dominantly a-phase Au-Ag-Hg alloy (c.<10 wt% each of Ag and Hg) with subordinate Au-Ag alloy. The α-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 estimates based on maximum flatness index of Au-Ag-Hg alloy particles are typically <10-20 km. Coarse (up to 2 cm) crystalline cinnabar is commonly associated with the AuAg-Hg alloy, and both were probably derived from hydrothermal sources in western tributaries of the upper Nevis River and eastern tributaries of the upper Nokomai River. These sources are possibly related to strands of the Nevis-Cardrona Fault System. Secondary Au-Ag-Hg alloy with up to 38 wt% Hg occurs locally in the lower Nokomai alluvial plain, where it coats or cements detrital α-phase AuAg-Hg and Au-Ag alloy particles. The secondary Au-AgHg alloy has formed by diffusion between detrital gold particles and liquid Hg that is either natural or derived from local breakdown of cinnabar.Au-Ag alloy dominates over Au-Ag-Hg alloy in the lower half of the lower Nokomai alluvial plain and in the gravel of a Pleistocene perched channel incised into pumpellyiteactinolite facies 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 in the Au-Ag alloy, and a fluvial transport distance of 25-40 km is estimated from maximum flatness index. Au-Ag alloy, three types of garnet, magnetite, clinozoisite, and well-foliated schist boulders in the abandoned channel gravel are allochthonous. They were derived from greenschist facies schist and sedimentary sources many tens of kilometres north of the Nokomai catchment, and transported either in Wakatipu Glacier till or fluvioglacial outwash gravel that entered the Nokomai valley via Nokomai Saddle, the confluence with the Mataura River, or both. Mercury minerals in hydrothermal deposits within the Otago Schist appear to be restricted to the Caples Terrane. A gold-mercury association is proposed for the Caples Terrane.
The NNE-striking faults of the Nokomai valley, Southland, New Zealand, are the southern extension of the Nevis-Cardrona Fault System, and have similar deformation style. This fault zone defines the boundary between broad, smooth, central Otago ranges and basins dominated by a deformed regional unconformity cut into the basement, and rugged mountains and deeply incised bedrock valleys to the west. Nokomai valley topography is dominated by bounding ranges: Slate and Hector Ranges to the west, and Garvie Range to the east. These ranges are rising on NNE-striking oblique reverse faults, which dip beneath the ranges in the upper and lower Nokomai valley. The middle reaches of the Nokomai valley are cut by a complex set of NE-ENE strikeslip faults with some reverse component. These faults pass through the headwaters of eastern tributaries of the main Nokomai River, which itself passes through a bedrock gorge. Structural depressions have developed in the upper and lower Nokomai valleys, where the oblique reverse faults strike NNE, and these depressions have accumulated auriferous river gravels. The gold is derived from a variety of sources, including at least one auriferous quartz vein still exposed. Most gold has accumulated in basal lags in gravels in the lower Nokomai, including a major tributary, Victoria Gully. The gravels in the basal lags are older than c. 22 000 yr, based on radiocarbon dating of overlying peats. The whole Nokomai catchment is being uplifted, and early-formed gold accumulations are being eroded and re-concentrated in younger channels. Similar gold recycling processes have occurred along the whole Nevis-Cardrona Fault System, which is at an ideal stage of topographic development for alluvial gold accumulation.
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