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Coalgate Bentonite Quarry (Wright's quarry), Coalgate, Selwyn District, Canterbury Region, South Island, New Zealand

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Latitude & Longitude (WGS84): 43° 30' 15'' South , 171° 54' 16'' East
Latitude & Longitude (decimal): -43.50438,171.90450
Age:23.03 to 0.01143 Ma
Geologic Time:Cenozoic : Miocene to Pleistocene
Reference for age:Carlson, J.R., Rodgers, K.A. (1974) The Coalgate Bentonite. A Ferriferrous-Beidellite Deposit from Canterbury, New Zealand. Clay Minerals: 10: 153-172.
Köppen climate type:Cfb : Temperate oceanic climate

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.

Commodity List

This is a list of exploitable or exploited mineral commodities recorded at this locality.

Mineral List

9 valid minerals.

Rock Types Recorded

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Alphabetical List Tree Diagram

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on

Late Miocene
5.333 - 11.62 Ma

ID: 1310128
Late Miocene basalt

Age: Miocene (5.333 - 11.62 Ma)

Stratigraphic Name: Maui Supergroup

Description: Basaltic to trachytic flows, dikes, sills, vent plugs and domes; minor interbedded tuff, breccia, conglomerate, sandstone and carbonaceous mudstone.

Comments: Zealandia Megasequence Extrusive and Intrusive Rocks (Neogene)

Lithology: Basalt, trachyte, tuffite, breccia, conglomerate, sandstone, mudstone

Reference: Edbrooke, S.W., Heron, D.W., Forsyth, P.J., Jongens, R. (compilers). Geology Map of New Zealand 1:1 000 000. GNS Science Geological Map 2. [12]

7.246 - 11.62 Ma

ID: 1368961
Harper Hills Basalt (Burnt Hill Group)

Age: Miocene (7.246 - 11.62 Ma)

Stratigraphic Name: Harper Hills Basalt

Description: Porphyritic and vesicular tholeiitic basalt lava flows. Up to 3 flows, oldest is 15 m thick. Overlain by Coalgate Bentonite.

Comments: Neogene igneous rocks. Age based on K-Ar and underlying fossiliferous stratigraphy

Lithology: Major:: {basalt},Minor:: {clay}

Reference: Heron, D.W. . Geology Map of New Zealand 1:250 000. GNS Science Geological Map 1. [13]

Jurassic - Triassic
145 - 252.17 Ma

ID: 3185705
Mesozoic sedimentary and volcanic rocks

Age: Mesozoic (145 - 252.17 Ma)

Stratigraphic Name: Torlesse Supergroup

Lithology: Mafic volcanic rocks; basalt; chert,greywacke,argillite,limestone

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Data and map coding provided by, used under Creative Commons Attribution 4.0 License

This page contains all mineral locality references listed on This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Carlson, J.R., Rodgers, K.A. (1974) The Coalgate Bentonite. A Ferriferrous-Beidellite Deposit from Canterbury, New Zealand. Clay Minerals, 10:153.
Christie, T., Thompson, B., Brathwaite, B. (2000) Mineral Commodity Report 20-Clays. Institute of Geological and Nuclear Sciences
Carlson, J.R., Grant-Mackie, J.A., Rodgers, K.A. (1980) Stratigraphy and Sedimentology of the Coalgate Area Canterbury, New Zealand Journal of Geology and Geophysics, 23:2, 179-192.
Carlson, J.R., Rodgers, K.A. (1975) The Petrology and Alteration of Tertiary Basalts of the Coalgate Area North west Canterbury, Journal of the Royal Society of New Zealand, 5:2, 195-208.

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