Growth of alluvial gold particles by chemical accretion and reprecipitation, Waimumu, New Zealand

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Abstract
Coarse gold particles (up to 1 mm) in the Gore Lignite Measures at Waimumu Stream, Waimumu, consist of many small (<0.1 mm) detrital gold flakes diagenetically welded together by reprecipitation of flake margins. During reprecipitation, Ag was leached from the margins of flakes which originally contained up to 8 wt% Ag. Ag‐rich cores are preserved in some flakes, whereas many flakes have been fully reprecipitated, leaving gold with less than 1 wt% Ag. Gold nuggets up to 6 mm across occur in the Waimumu Quartz Gravels at Belle Brook, Waimumu. The interiors of these particles show coarsely crystalline texture, predominantly twinning, with compositions near 5 wt% Ag. This texture continues to the exterior surface of some particles, producing skeletal octahedral crystals. The delicate crystal structure could not survive transport, so formation of the crystals occurred after deposition in the sediment. This crystal structure may have resulted from recrystallisation induced by deformation during transport of the gold particles as they were recycled from the Gore Lignite Measures during local uplift. Further reprecipitation and deposition of new gold (<1 wt% Ag) occurred on and near the margins of large gold particles. The textures in these two examples represent different stages in an overall trend towards coarsening of grain size of alluvial gold derived from fine‐grained (typically <100 μm) gold in Otago Schist quartz veins.