The formation and dispersal of pounamu in the Scott Basin/Scott stream area, Wakatipu, New Zealand

Author:

Popham, T. B. (Timothy Bruce)

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Abstract:

Pounamu is a Maori word used to describe nephrite and semi-nephrite. Pounamu occurs in the Whakatipu region of the South Island, New Zealand, in association with the Greenstone Melange. Pounamu formed via metasomatic reaction between two rock types of dissimilar composition, and is comprised of tremolite-actinolite minerals (Ca2Mg4.6Fe0.4Si8O22(OH)2) in a fibrous felted texture. The protolith for pounamu in the Scott Basin area is calcium-rich Caples Terrane-metasediment-metavolcanic and magnesium rich serpentinite, which have been sheared by syn- to post-metamorphic deformation. In situ pounamu in Scott Basin was estimated by scree clast counts and GIS map calculations to comprise between 0.15-1.7 % of the melange source rocks. The in situ surficial resource of pounamu is between 8 - 23 tonnes. The distribution of alluvial source rock material and surrounding country rocks (Caples Terrane) was assessed to quantify the amount and distribution trends of transported material. Rock-type and clastsize were recorded along transect lines laid perpendicular to stream-flow, at seven selected sites down Scott Creek. At the uppermost site overlying in situ Greenstone Melange, alluvial pounamu is 12 % showing considerable concentration above levels estimated for in situ material. Alluvial pounamu in the streambed increases to 15 % at 384 m downstream from the source rocks, then has been diluted to 0 % at 2169 m. No significant resource is expected to be present in the catchment > 2 km from the source rocks. The largest pounamu clast (15 x 31 x 54 cm) was found immediately above the melange source rocks. Metavolcanic rock populations derived from the Greenstone Melange mirror (and are slightly more dominant than) that of pounamu. Therefore, if pounamu is not found in the streambed, it is possible to use the amount of metavolcanic as a proxy for determining pounamu distribution. Pounamu was found to preferentially accumulate in areas of abrupt downstream gradient change, from steep streambed gradients (high energy) to gentle (low energy).

Thesis description:

98 leaves : ill. (some col.), col. maps (2 folded) ; 30 cm.

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OU geology Identifier:

2003Popham

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POLYGON ((168.284710906498645 -44.73638734990837,168.284971242530844 -44.798801191515466,168.25075117989573 -44.800000011118222,168.250451988934458 -44.787364040896016,168.238430688568627 -44.774953348766076,168.24952355239617 -44.770176418070051,168.249197631207494 -44.741581262079379,168.284710906498645 -44.73638734990837))

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http://download.otagogeology.org.nz/temp/Abstracts/2003Popham.pdf

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Citation

Popham, T. B. (Timothy Bruce), “The formation and dispersal of pounamu in the Scott Basin/Scott stream area, Wakatipu, New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/418.

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