The Waikato coal region consists of coalfields developed 30 -35 million years ago during the Eocene-Oligocene period, which formed the Waikato coal measures. The region extends from Drury (near Auckland) in the north to Mangapehi, south of Te Kuiti.
The Waikato coal measures were deposited on an eroded basement of Mesozoic greywackes and argillites. The coal measures are overlain by a succession of marine sediments and an unconformable sequence of much younger essedimentary and volcanic deposits (3 million years ago to recent time).
A lower coal measures unit can be recognised primarily in the Taupiri Seams restricted to the Rotowaro coalfield, whilst the upper measures unit is recognised primarily in the Kupakupa and Renown Seams in the Huntly, Waikare and Maramarua Coalfields further north.
General structural style is that of block faulting with steeply dipping normal faults. One fault set trends north-south and a more frequent set trends northeast-southwest.
Rotowaro Coalfield
The Rotowaro coalfield is located 10 km southwest of Huntly and supports a number of opencast mines including Solid Energy’s Township and Awaroa opencast mines. Both the lower and upper Waikato coal measures are present. The lower measures are up to 200 m thick, while the upper measures reach 100 m. The Taupiri seams (lower measures) are up to 30 m thick, but are often highly split, while the Kupakupa seam (near the base of the upper measures) and the overlying Renown seam, are between 2 m to 11 m and up to 5 m thick, respectively. Historically coal seams at Rotowaro have been widely and intensively underground mined.
Rotowaro Opencast Mine
The topography involves gently undulating hills and low to medium depth valleys. The Taupiri, Kupakupa and Renown seams, are available in various combinations to a maximum economic 12:1 stripping ratio. Weak overburden allows efficient stripping but requires careful design work for stability of backfill dumps and high walls.
Upper Waikato coal measures reach 150 m in thickness in the Callaghan sector with a regional dip of 5 degrees north, although this varies considerably on a local basis. Alluvials, volcanic and swamp deposits mantle the coal measures and marine cover in the lower areas of the northern section of the coalfield. Careful planning is required to dispose of these soils for the long term. Considerable planning effort is expended on water management to ensure mine discharges comply with Rotowaro’s strict environmental consents.
A coal washery and blending plant are used to process and blend coals to optimal product specifications. This enables Solid Energy to maximise the recovery and market value of its coal reserves.
Huntly Coalfield
The Huntly coalfield extends about 10 km north of Huntly and is separated from the Rotowaro coalfield by an area where the coal measures thin out and coal seams are absent. The coal measures here are 50 m to 100 m thick.
Of the three seams present in the upper coal measures – Ngaro, Renown and Kupakupa – the most important is the near-to-basement Kupakupa, which is typically 3m to 15 m thick. In places west of the Waikato River, the Renown and Kupakupa seams merge together and attain a combined thickness of up to 23 m.
Structurally, the area is block-faulted, with interesting north-south trending and northeast-southwest trending normal faults. The regional dip is 10 degrees northwest, with a depth-of-cover approaching 600 m around the northern limit of the coalfield.
Huntly East Mine
The Kupakupa seam mined at Solid Energy’s Huntly East underground mine is undulating and contains large sections of Renown-Kupakupa coal 20m thick identified by exploration. Although surface access is relatively straight-forward, competing land uses (private farmland, transmission lines) and the dense population cause planning constraints.
Faults from 5 m to 25 m throw have been intersected in the existing workings. The general seam thickness allows faults up to 15 m to be worked through, but faults with larger throws required some stone driveage. 3D seismic surveys are now routinely used to provide information on such hazards for reliable mine planning and to minimise the disruption of major faults on production.
Working conditions are stable in the unfaulted areas and, at a mean cover of 200 m, in-situ stress does not impact significantly on roof and rib support. The seam is moderately gassy (methane) and its recognised propensity for spontaneous combustion requires careful ventilation control. This, together with the objective of increasing the recovery rate from the thick seam, are the drivers for current technical mine planning.