Coalgate Bentonite Quarry (Wright's quarry), Coalgate, Selwyn District, Canterbury Region, New Zealandi

Regional Level Types
Coalgate Bentonite Quarry (Wright's quarry)	Quarry
Coalgate	Town
Selwyn District	District
Canterbury Region	Region
New Zealand	Country

New Zealand's largest bentonite quarry. Worked in the summer months, and processed at a plant in Coalgate.

Bentonite in the area in its natural state is non swelling, but is processed with soda ash, to change it to a swelling variety. The clay is sold as a number of products, including as a fine powder for a growth medium and fertilizer, skin care and cosmetics, inhibiting the ingress of water into bore holes, grouting, civil and geotechnical applications such as clay liners in tunnels, land capping, land fill cell construction, and liners to dams, underground slurry walls, and barriers, paper making, and the treatment of waste effluent.

Canterbury Bentonite Ltd began mining here in 1967 at the quarry in the Harper Hills. The operation was later owned by Omya New Zealand Ltd, before being acquired by Transform Minerals in 2006. They went into receivership in 2014, and mid 2015 the company and operations was taken over by Victory Lime 2000 Ltd, a subsidiary of Dunedin based Palmer MR Ltd.

Resources amount to over 10 million tonnes, in the Harper Hills, south-west of Coalgate, and Homebush Ridge, the latter just north-east of Coalgate.

The bentonite is regarded as representing a violent explosive volcanic phase, closing the history of basalt volcanism in the Coalgate area. The Coalgate Bentonite Member is part of the Harper Hills Formation, formed during a marine freshwater environment. The main bed is 62 metres thick, with a lower carbonaceous bed 15 metres thick. It was formed when glassy basalt ash was deposited in a freshwater lake.

The clay minerals in the bentonite have been identified as kaolinite, illite, and montmorillonite, with chlorite, as a calcite-magnesium
ferriferous beidellite deposit. Quartz and feldspar is under 3%.

From the surface is barite concretions to 10 cms in diameter, sand, grit, gravel; then green-fawn coloured bentonite with limonite staining and nodules; a quartz sand layer; black pyritic carbonaceous bentonite; three vesicular tholeiitic labradorite basalt flows; tuff, tuff breccia and brown bentonite. The bentonite tends to change to brown on exposure, but is poorly outcropping, indicated by its late discovery. The beds dip 20-70 degrees south-east.

Bentonite in particular is found as green coloured with limonite nodules; green bentonite without nodules but with wavy limonite bands; fawn bentonite containing abundant calcite, and some manganese nodules; black/grey bentonite, highly carbonaceous and woody to the point of almost brown coal, with partly altered pyrite nodules. Silicified wood is sometimes found near the surface.

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