Pounamu : characterisation and resource assessment; Scott Basin, Wakatipu, New Zealand
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Pounamu/greenstone is Aotearoa/New Zealand's icon mineral, a taonga (treasure) of which Ngai Tahu are kaitiaki (guardians). Pounamu in the form of nephrite and semi-nephrite occurs in Scott Basin, Lake Wakatipu, West Otago, New Zealand. It formed within the Greenstone Melange at contacts between tectonic inclusions ("knockers") of metasediment or metavolcanic rock and sheared serpentinite matrix, and was subsequently liberated, concentrated and transported by erosion. Most (90 %) pounamu formed in discontinuous reaction zones typically comprising metasediment - nephrite - semi-nephrite - talcose semi-nephrite - talc magnesite - serpentinite. Approximately 80 % of in situ pounamu occurrences are within 150 m of the Caples - Greenstone Melange contact.
X-ray diffraction and electron microprobe analyses indicate Scott Basin pounamu is predominantly monomineralic, comprised of Mg-rich actinolite. A three-dimensional felted fabric, developed through shearing, gives the rock its hardness and toughness. High quality nephrite is hard (6- 6.5 on the Mohs scale), and is typified by un-oriented, interwoven, short fibres of actinolite. Poor quality semi-nephrite is softer (< 6 on the Mohs scale), and is typified by long, elongate fibres in the plane of the dominant foliation, with interstitial talc. Relict hornblende within pounamu has similar composition to hornblende in the nearby Dun Mountain Ophiolite Belt, and a similar provenance is inferred.
Geochemical analyses (22) of Scott Basin pounamu and associated metasomatic rocks were compared with nephrite and serpentinite samples from Westland, Canada and Australia. Major and minor elements do not discriminate these geographic occurrences and geological environments. Chromium can be used to locate tl1e position of original protolith boundaries within metasomatic zones. Scott Basin nephrite commonly has Cr concentrations < 1000 ppm that indicate nephrite formation has migrated inwards (towards the Cr-poor metasediment protolith) during progressive metasomatism.
Streambed surveys quantified transported alluvial pounamu with progressive distance downstream from Greenstone Melange source rocks. Five techniques were developed and trialled: transect analysis; image area analysis; image point counting; fixed area survey; and fines analysis. Fixed area surveying is recommended for future work. Survey data indicate pounamu: develops Jag deposits above and near melange source rocks at 1 - 11 % of streambed material; decreases to 1 - 7 % immediately downstream; and is further diluted to < 1% at >3 km distance from the source rocks.
Resource assessment calculations show that pounamu occurs at concentrations of 100 - 200 t/km2 in situ; 10,000 - 100,000 t/km2 near situ; and transported alluvial pounamu occurs at 2,000 - 16,000 t/km2 in the upper reaches of Scott Creek, and is diluted to approximately 30 - 300 tjkm2 in the lower reaches. The best estimate of Scott Basin pounamu is 870 t of in situ resource, 4,080 t of near situ resource, and 1,560 t of transported alluvial resource. The total resource of 6,500 t (best estimate) lies within an uncertainty range of 790- 9,600 t. The resource is expected to change with time, however, due to ongoing uplift and erosion. Controlling extraction to < 10 tonnes/year for pounamu, or less for high-quality nephritic material, should enable long-term sustainability.
X-ray diffraction and electron microprobe analyses indicate Scott Basin pounamu is predominantly monomineralic, comprised of Mg-rich actinolite. A three-dimensional felted fabric, developed through shearing, gives the rock its hardness and toughness. High quality nephrite is hard (6- 6.5 on the Mohs scale), and is typified by un-oriented, interwoven, short fibres of actinolite. Poor quality semi-nephrite is softer (< 6 on the Mohs scale), and is typified by long, elongate fibres in the plane of the dominant foliation, with interstitial talc. Relict hornblende within pounamu has similar composition to hornblende in the nearby Dun Mountain Ophiolite Belt, and a similar provenance is inferred.
Geochemical analyses (22) of Scott Basin pounamu and associated metasomatic rocks were compared with nephrite and serpentinite samples from Westland, Canada and Australia. Major and minor elements do not discriminate these geographic occurrences and geological environments. Chromium can be used to locate tl1e position of original protolith boundaries within metasomatic zones. Scott Basin nephrite commonly has Cr concentrations < 1000 ppm that indicate nephrite formation has migrated inwards (towards the Cr-poor metasediment protolith) during progressive metasomatism.
Streambed surveys quantified transported alluvial pounamu with progressive distance downstream from Greenstone Melange source rocks. Five techniques were developed and trialled: transect analysis; image area analysis; image point counting; fixed area survey; and fines analysis. Fixed area surveying is recommended for future work. Survey data indicate pounamu: develops Jag deposits above and near melange source rocks at 1 - 11 % of streambed material; decreases to 1 - 7 % immediately downstream; and is further diluted to < 1% at >3 km distance from the source rocks.
Resource assessment calculations show that pounamu occurs at concentrations of 100 - 200 t/km2 in situ; 10,000 - 100,000 t/km2 near situ; and transported alluvial pounamu occurs at 2,000 - 16,000 t/km2 in the upper reaches of Scott Creek, and is diluted to approximately 30 - 300 tjkm2 in the lower reaches. The best estimate of Scott Basin pounamu is 870 t of in situ resource, 4,080 t of near situ resource, and 1,560 t of transported alluvial resource. The total resource of 6,500 t (best estimate) lies within an uncertainty range of 790- 9,600 t. The resource is expected to change with time, however, due to ongoing uplift and erosion. Controlling extraction to < 10 tonnes/year for pounamu, or less for high-quality nephritic material, should enable long-term sustainability.
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1 v. (various pagings) : ill. ; 30 cm. + 2 maps, folded.
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2006Popham
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POLYGON ((168.286005602953594 -44.730015501152728,168.302611567129986 -44.846456452374163,168.244045625606162 -44.850219675465723,168.223635617541731 -44.829835281733494,168.232816200232577 -44.799732856501393,168.233279745973732 -44.745447329528119,168.286005602953594 -44.730015501152728))
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Popham, T. B. (Timothy Bruce), “Pounamu : characterisation and resource assessment; Scott Basin, Wakatipu, New Zealand,” Otago Geology Theses, accessed June 12, 2025, https://theses.otagogeology.org.nz/items/show/470.