Au-Ag-Hg and Au-Ag alloys in Nokomai and Nevis valley placers, northern Southland and Central Otago, New Zealand, and their implications for placer-source relationships

John H. Youngson¹

Paul Wopereis²

Luke C. Kerr¹

Dave Craw¹

¹ Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
² L&M Mining
P.O. Box 13442
Christchurch, New Zealand

Abstract 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 a-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 Au-Ag-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 a-phase Au-Ag-Hg and Au-Ag alloy particles. The secondary Au-Ag-Hg 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 pumpellyite-actinolite 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.

Keywords gold alloys; mercury; Au-Ag-Hg alloy; alluvial; placer; Otago; Southland; Otago Schist; Caples Terrane; Nokomai; Nevis; drainage evolution; garnets

Received 20 October 2000; accepted 24 September 2001
New Zealand Journal of Geology & Geophysics, 2002, Vol. 45 : 53–69
0028–8306/02/0053 $7.00/0 © The Royal Society of New Zealand 2002

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