Mineralogy and provenance of auriferous Waimumu Quartz Gravels, Southland, New Zealand

Abstract

Waimumu Quartz Gravels, 8 km southwest of Gore, New Zealand, were deposited in Pliocene river channels, and were derived by recycling of older sediments on nearby uplifting mountain ranges. The high proportion of quartz pebbles (>90%) is a result of the decomposition of clay-altered lithic clasts during sedimentary recycling. Most of the quartz was derived from the Otago Schist Belt to the north, including upper greenschist facies metachert clasts with spessartine and piemontite. The quartz gravels at Waimumu Stream mine contain abundant Mn-rich garnet clasts that were derived from Otago Schist. In addition, chromite clasts constitute >10% of the heavy mineral sand fraction, and this may have been derived from the Dun Mountain Ophiolite Belt although the exact source is unknown. Most detrital gold occurs as flakes that have undergone long-distance transport. Rare grains of crystalline gold occur, some of which are in association with prismatic quartz crystals. Gold flakes have minor evidence for authigenic gold overgrowths in association with authigenic marcasite, and gold mobility may have been enhanced by marcasite precipitation and/or oxidation.

Keywords:

• gold
• placer
• chromite
• garnet
• marcasite
• authigenic
• metachert
• Southland
• Otago
• palaeodrainage

Acknowledgements

The study would have been impossible without the enthusiasm and logistical support of Tony Baker and Shane Le Masurier of Nomad Mining. Dave Prior provided expert help with SEM data gathering and manipulation. Able technical support was provided by Lisa Craw, Brent Pooley, Alice Waterman and Damian Walls. Discussions with John Smith, JohnYoungson and Noel Becker were helpful in developing our ideas on Southland placer deposits. Thorough reviews by Mark Rattenbury and an anonymous referee improved the presentation.

Associate Editor: Dr Kari Bassett.

Funding

This research was funded by the Marsden Fund administered by the Royal Society of New Zealand, New Zealand Ministry of Business, Innovation and Employment and the University of Otago.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary data

Table S1. SEM-EDS analyses of detrital minerals from the Waimumu Quartz Gravels at the Waimumu Stream mine, Southland. Typical 1σ uncertainties are indicated. Location: Grid ref NZTM 1278986 4884849. Sample: Placer gold mine processing system, heavy mineral concentrate. Sample archive: University of Otago, OU85059.