Coal resources: Southland

Location

The Southland Coal Region includes the extensive lignite deposits underlying the Eastern Southland plains, the Ohai Coalfield in central Southland, and a number of minor coalfields. The main lignite deposits are within 60 km of Invercargill.

Geology

The Eastern Southland lignite seams are within the Late Oligocene to Early Miocene Gore Lignite Measures, fluvio-deltaic deposits at least 500 m thick and overlain by Pleistocene to Holocene gravels. The main lignite-bearing unit is 50-300 m thick and contains multiple-laterally persistent seams up to 18 m thick. Seams are generally only slightly deformed. Ohai Coalfield occupies a fault-controlled depression containing Morley Coal Measures in the Late Cretaceous Nightcaps Group and Beaumont Coal Measures within the Eocene Nightcaps Group. Morley seams are up to 23 m thick and structurally complex.

Coal quality

Southland lignites typically have in-ground moisture contents of 40-65% with 3-20% ash and generally less than 0.6% sulphur contents. Morley coal at Ohai ranges from sub-bituminous A to high volatile bituminous C in rank with low to medium ash and low sulphur contents.

Resources

Coal-in-ground resources of 9392 Mt for the region are dominated by lignites, of which 6200 Mt are estimated to be recoverable. At Ohai, 50 Mt are estimated to be recoverable by underground mining. The region is well explored.

Exploration status

Eastern Southland lignites

Exploration of the Eastern Southland lignite fields was accomplished by drilling along a series of lines across Southland. Under the direction of Lime & Marble Ltd, drilling commenced in 1976 and explored an area of some 2000 sq km, from east of the Oriti River from Invercargill to as far west as Clydevale. Six widely spaced lines of scout holes were initially drilled at a spacing of 2 to 2.5 km, intended to intersect any lignite resource greater than 50 Mt. Where significant lignite intersections were made, short fill-in lines were drilled and the more promising prospects were then drilled down to 750 m spacing. By the time NZCRS drilling was completed in 1978, 21 lines of holes had been drilled. During this twenty month period about 450 holes were drilled.

A total of 4400 Mt tonnes of lignite with opencast mining potential was identified in deposits at Croyden, Waimumu, Mataura, Morton Mains, Waimatua and Ashers-Waituna, part of a discovered resource of some 15 billion tonnes. The NZ Geological Survey completed a separate series of reports on resource estimates and coalfield geology.

Ohai Coalfield

A small NZCRS programme run by Lime & Marble Ltd in association with State Coal Mines explored the eastern parts of Ohai Coalfield in 1987, drilling 12 holes which discovered a significant resource estimated at 88 Mt of coal-in-ground.

Production

Lignites have been mined on a small scale since the mid-19th century, the largest mine currently producing about 140,000 tpa. Output from Ohai has declined since the 1960s to about 140,000 tpa.

Utilisation

Southland coals are mined for local industrial and domestic use.

Key references

Eastern Southland

Carr MJ 1979. Eastern Southland lignite project. Final report. Unpublished report to Mines Division.

Isaac MJ 1980. Lignite deposits of Eastern Southland. NZ Geological Survey report M83.

Isaac M 1990. Lignite resources of Eastern Southland. NZ Geological Survey miscellaneous series map 20.

Isaac MJ, Lindqvist JK 1990. Geology and lignite resources of the East Southland Group, New Zealand. NZ Geological Survey bulletin 101.

Ohai

Bowman RG, Brodie CG, Garlick PR 1987. Ohai Coalfield coal resource assessment, eastern Ohai Coalfield area. Unpublished Landmark Resources report for New Zealand Coal Resources Survey, Mines Division, Ministry of Energy.