This article is an edited excerpt from AusIMM’s upcoming Monograph 33: The Geology and Resources of New Zealand Coalfields
Monograph 33 was commissioned to provide a single introductory reference to the geology, resources and general mining prospects of New Zealand coalfields. It is designed as a high-quality and long-lasting reference book for professionals and companies working in the New Zealand coal sector and geologists with an interest in New Zealand coal deposits.
The volume will be produced in both hard copy and ebook formats. The book will be launched at the New Zealand Minerals Forum (27-29 May 2019) in Dunedin, New Zealand. The book will be available to purchase soon from the AusIMM Library. If you are interested in purchasing the hard copy or digital version of this publication please contact firstname.lastname@example.org.
Building an understanding of New Zealand’s coalfields
That this topic can be summarised at all relies on an extensive body of preceding work. Following the earliest geological surveys, a significant contribution to the foundation work on New Zealand coalfields was through the studies of the Coal Resources Committee from 1938 to the mid-1950s. Some of this work led to the studies of New Zealand coals by RP Suggate and the series of geological coalfield maps and Bulletins published by the New Zealand Geological Survey and its successor organisations between 1952 and 1994. The Greymouth Coalfield monograph of 1952 by M Gage was a pathfinder for the approaches taken to coalfield assessment in New Zealand for the next 40 years.
The stratigraphic and structural settings of New Zealand coalfields and the geological processes that shaped them became better understood through the syntheses published in The Geology of New Zealand in 1978 and the series of monographs on the geology of Cretaceous-Tertiary basins that followed between 1986 and 1999.
A second national Coal Resources Survey was carried out by the New Zealand Government from 1975 to the mid-1980s. Over 1800 exploration holes were drilled. A large body of resource assessment and mining studies were produced, which drew on and added to the earlier work. Since then, more detailed coalfield investigations have been carried out by mining companies.
Purpose and format of Monograph 33
As a result of 150 years of exploration and mining, New Zealand’s coal resources are generally well known. Unlike metallic minerals, coal does not present much exploration opportunity. Most coal exploration information, including some of the earliest work, is in reports and data held by NZ Petroleum & Minerals and available online. That this body of information is so diverse, and that only a small proportion has been published, is one rationale for this monograph; it is to be something of a window into that information.
This book will also be a vehicle for an updated account of New Zealand’s coal resources. New Zealand’s coal resource inventory is dominated by the extraordinary endowment of 12 billion tonnes of lignite. This heavily skews coal resource estimates for New Zealand, disguising the more critical inventory of bituminous and sub-bituminous coals, of which the remaining economically recoverable coal quantities are relatively quite small. One of the objectives of this volume will be to set the geological context for that sensitivity through description of individual coalfields. An attempt is made to deal with the inevitable tension between an economically-based resource classification and one that is geologically-based, but a resource inventory can only be a categorisation at a point in time and the assignment of estimates to those categories can be argued.
Future coal production in New Zealand cannot be forecast with any confidence. A transition from coal to gas began when Taranaki gas came on stream in 1969 and had been largely completed by the 1980s. Gas and coal are now complementary thermal fuels in New Zealand’s energy mix because they contribute in different ways. Coal remains quite intricately embedded in parts of the economy and cannot be quickly displaced. Exports of coking coal remain viable. Alongside that sits a diminishing resource of economically mineable coal, other than lignite, and increasing pressure to reduce emissions from its use.
The monograph will be in two parts. The first is an overview of New Zealand coals and coalfield geology, an outline of the history of coal exploration and mining, discussion of some aspects of the coal industry, and a summary of national coal resources. The second part comprises descriptions of individual coalfields.
The volume is not intended to be a scientific synthesis. The substantial studies that have supported resource assessment, including biostratigraphy, palynology, petrography, geochemistry and geotechnics, are not covered in detail, although the main studies are referenced. Worthwhile additional studies are implied by some areas covered only superficially: the detailed paleoenvironmental studies that have been carried out on most New Zealand coalfields could be synthesised into a coherent account; the results of coal seam gas exploration could usefully be summarised; and the Solid Energy story would make an informative study of public policy and administration.
General geology of New Zealand coalfields
There are coalfields in several regions of New Zealand: Northland, Waikato, northern Taranaki, north-west Nelson, the West Coast of the South Island, eastern and Central Otago, and Southland. There are also extensive coal measures in several deep, mainly offshore, basins where coals have contributed to oil and gas generation.
Broad geological influences on coalfield formation
The Zealandia continental fragment (Mortimer and Campbell, 2014; Mortimer, Campbell et al, 2017) occupied a position on the margin of Gondwana until their separation began about 100 million years ago. There are no Carboniferous coal measures in New Zealand equivalent to those in western Europe and the eastern United States, and no Permian Gondwana coals equivalent to those in Australia, India, South Africa and Antarctica. Thin coal lenses and coalified wood fragments, with rare coal seams, are present in Permian to mid-Cretaceous sedimentary rocks onshore (Suggate, 1990a), and Jurassic coals may have contributed to a petroleum system in Murihiku Supergroup rocks in offshore basins (Uruski and Clare, 2014).
The mid-Jurassic to mid-Cretaceous Rangitata Orogeny coincided with the breakup of Gondwanaland and the opening of the Tasman Sea between Australia and New Zealand. The result was a complete change in the geological environment of New Zealand and a consequent contrast in depositional and deformational patterns between Mesozoic and Cenozoic rocks (see Suggate, 1959a and 1978a; Edbrooke, 2017). Throughout most of New Zealand, this is marked by a contrast in induration between ‘basement’ schists, greywacke and granite, and the overlying Cretaceous–Cenozoic rocks, which include the various coal measure sequences.
New Zealand’s economically significant coals are of Late Cretaceous to Miocene age. This period of 65 million years was characterised by tectonic mobility, preceded and followed by major orogenic episodes. Coal measures accumulated in different places at different times and in different tectonic environments. The nature of depositional basins and patterns of sedimentation varied under these diverse tectonic influences. Being sensitive to geological conditions, the coals themselves have correspondingly complex differences in properties (Suggate, 1959a).
The geology of New Zealand coalfields has been influenced by five main geological episodes, which were not strictly sequential but overlapped in time and place:
1. extensional rifting, forming fault-bounded basins in places from mid to Late Cretaceous
2. widespread peneplanation from Late Cretaceous to Late Eocene, extending at least into the Early Oligocene
3. differential tectonism and the development of sedimentary basins throughout the Tertiary
4. regional marine transgression from Late Cretaceous to Oligocene
5. the tectonic movements of the Kaikoura Orogeny, from Late Miocene to present.
The oldest coal measures of economic importance were deposited during the Late Cretaceous in two general, contrasting depositional settings (Suggate, 1978a; Sherwood et al, 1992). Sedimentation in local, rapidly subsiding troughs and basins resulted in strata characterised by marked variations in sediment type and thickness, eg in the Greymouth, Ohai, and Collingwood coalfields. Rising basin margins supplied large quantities of clastic material, and great thicknesses of sediment accumulated where this tectonic setting persisted. In contrast, marine transgression over already peneplaned rocks began in the Late Cretaceous, eg Green Island and eastern Kaitangata coalfields.
A period of widespread progressive peneplanation (Suggate, 1978a) lasted from Late Cretaceous to Late Eocene in most places, but locally continued at least into the Oligocene or possibly even Early Miocene. Peneplanation took place alongside local differential movement, so that there was no single period of tectonic calm and subaerial erosion, although the Palaeocene was nearly quiescent.
Slow marine transgression over the still-developing peneplain resulted in widespread deposition of coal measures during the Palaeogene (Suggate, 1950 and 1959a), eg Waikato, Reefton, and Canterbury. Differential tectonism continued in Westland, with deposition of coal measures in rapidly subsiding basins, eg Buller and Greymouth. Marine transgression continued until the Oligocene, followed by a post-Oligocene regression that accompanied coal measure deposition in several areas, eg Taranaki, Inangahua and eastern Southland.
‘New Zealand’s coal resource inventory is dominated by the extraordinary endowment of 12 billion tonnes of lignite.’
An increase in the tempo of tectonic events in the Miocene was the precursor to the major uplifts and transcurrent faulting of the Kaikoura Orogeny, which has continued through the Pliocene and Quaternary to the present day and is the main cause of the present deformation of New Zealand’s coalfields (Suggate, 1978b).
Comparison with foreign coalfields
Compared with those in many other countries, New Zealand coalfields are generally characterised by geologically young, discontinuous, structurally disturbed seams. The sedimentary and structural characteristics, and the variations in rank and type of New Zealand coals, have particular implications for exploration, mining and utilisation. Experience developed in Palaeozoic and Permian coals in other countries is not always applicable to New Zealand conditions.
Underground coal mining developed initially in Carboniferous coal measures in western Europe and eastern United States, which have similarities to the Permian Gondwana coaI measures of Australia, South Africa and India. Seams in these Palaeozoic and early Mesozoic coalfields, deposited before the breakup of the Pangaea supercontinent about 200 million years ago, tend to be of great lateral extent, eg the Pittsburgh Seam in the Appalachian Basin of North America underlies an area of 28 000 km2. These seams also tend to have fairly simple geology so that mining conditions can be predicted over distances measured in kilometres. In contrast, few seams of coal on the West Coast of New Zealand extend at mineable thickness for more than a few hundred metres, and generally have marked structural and stratigraphic discontinuities.
The continuity of many Palaeozoic and early Mesozoic coalfields allows high production rates from mines. The biggest United States mines can each produce about 90 Mt (opencast) and 5 to 11 Mt (underground) annually. In contrast, New Zealand’s biggest coalmines have individually produced no more than about 2 Mt (opencast) or about
400 000 t (underground) annually on a consistent basis.
These differences, between the large, ancient coalfields of the old world and the younger, more tectonically influenced coals in New Zealand, are generalisations. Many foreign coalfields have great complexity, including some Palaeozoic to early Mesozoic coalfields such as the South Wales Coalfield, the Ruhr Basin in Germany and the Kunznetsk Basin in Russia. The countries with geological conditions that are generally closer to those in New Zealand are on the Pacific Rim: Japan, Taiwan, Korea, Vietnam, Philippines, Indonesia (Kaliminatan), parts of western Canada and Chile.