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

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 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 mercury and silver contents of gold in quartz vein deposits, Otago Schist, New Zealand

“…Despite the potential usefulness of gold compositions for paragenetic studies of gold deposits in Otago, no data are yet available from hydrothermal veins. This lack of data has been highlighted by Youngson et al (2002) and Falconer (2003), who have identified geochemically distinctive Hg-rich placer gold (up to 10 wt% Hg) in deposits presumed to be derived from hydrothermally emplaced veins in the Otago Schist. Hence, knowledge of the mercury content of gold in Otago Schist hydrothermal veins has potential for linking placer deposits to sources, and determination of the geometry of paleodrainage systems.…”

“…These Hg-bearing alloys are thought to be hydrothermal in origin but no primary source for the gold has been found. The Hg-bearing gold is accompanied by abundant alluvial cinnabar in the Nevis and Nokomai valleys, and some primary genetic relationship between the cinnabar and Hg-bearing gold has been inferred (Youngson et al 2002). Cinnabar-bearing veins occur within basement rocks on the southwestern side of the schist belt ( Fig.…”

“…Au-Ag-Hg alloys occur in late Quaternary placer deposits in the Nevis valley and Nokomai valley on the southwestern side of the Otago Schist belt (Youngson et al 2002) (Fig. 1).…”

Mineralogical and chemical distribution patterns of placers and ferricretes in Quaternary sediments in SE Germany: The impact of nature and man on the unroofing of pegmatites

“…It formed by the reworking of pre-Holocene material. Youngson et al (2002) have recorded such placers by residual concentration on the floor of the slowly incising valleys from New Zealand. The covariance of Mn and Ti in zone A is due to the presence of a significant amounts of pyrophanite (MnTiO 3 ) in the solid solution series with ilmenite (FeTiO 3 ) which is intergrown with rutile in the nigrine aggregates (Dill et al, 2006).…”

Composition and mineralogy of historic gold processing residues, east Otago, New Zealand

“…Quantitative microprobe spot analyses were done using wavelength dispersion and a 5 ^m beam diameter. Mercury content of gold was determined using the data correction method described by Youngson et al (2002).…”