Summary:
The regional dip is generally to the northwest at the northern end of the Rotowaro deposit and to the southwest along the western margin.
There are a series of northwest-southeast trending anticlines and synclines in the central and east of the deposit. Rotowaro is bounded to the southwest by the Mangakotukutuku Monocline, with a net throw of 90m down to southwest, and to the northeast by the extension of the Waipuna Fault scarp. The Awaroa West deposit sits within a faulted graben structure bounded to the east by the Mangakotukutuku Monocline.
For the Waipuna West resource area, there are only minor faults identified, with throws less than 10m in height. These faults are either recorded on historical underground mine plans or interpreted from structure contour plans derived from drillholes. For Awaroa West, large north-south trending faults cut the deposit into two major portions with additional minor faults also identified.
The Rotowaro area is underlain by indurated siltstones, with common sandstones, of the Mesozoic Newcastle Group, which is weathered to a depth of 5-30m. This unit is referred to as “Basement” and has no economic significance.
The Waikato Coal Measures (WCM) lie unconformably on the basement and form the lower part of the Te Kuiti Group. The WCM consist mainly of mudstones and siltstones, often referred to collectively as “fireclay”, with common siderite concretions, referred to as “hardbars”.
There are three major coal seam groups within the WCM: Renown, Kupakupa and Taupiri.
The Taupiri seams are only represented in the Callaghan’s sector of the Rotowaro Coalfield, where they are confined to the structural trough between the Mangakotukutuku Monocline and the Waipuna Fault scarp.
Thickness patterns of seams lying close to basement are influenced by the paleo-relief developed on the basement contact, with thickening and thinning over basement valleys and ridges respectively.
The upper part of the Te Kuiti Group consists of marine to marginal marine claystones, mudstones, sandstones and siltstones which conformably overly the WCM. There is a regional unconformity at the top of the Te Kuiti Group, above which lie the Quaternary deposits of the Tauranga Group, consisting of interlayered alluvial clays, muds and highly weathered volcanic ashes of the Hamilton Formation.
Dimensions
At Rotowaro, the Waipuna West resource model covers approximately 734ha.
Within this area all seams are exposed in the operating mine. Prior to mining the Renown seam roof was as close as 6m below the surface and the floor of the Taupiri Bottom seam is as deep as 290m (-200m RL) below the surface.
The Awaroa West resource model covers 539ha and is adjacent to areas mined out in the old Awaroa and Awaroa NW pits.
Coal seam thickness of the different seams varies considerably throughout the Rotowaro area, from 28m down to <0.5m.
Rotowaro North Extension
The Rotowaro North Extension deposit is located in the Waikato Coalfield.
The Rotowaro North Extension deposit generally dips 3 to 7° to the north. Local dip variations occur adjacent to faults and folds, and seam dip variations are related to differential compaction within the coal seams.
Major faults in the Rotowaro Coalfield were active in the basement rocks before the deposition of the Tertiary units began, several faults continued activity during early coal measure deposition. Faults vary in displacement with the displacement generally increasing northward.
The north-south trending faults tend to be up-thrown to the west and east-west trending faults tend to be up-thrown to the south (Kirk, 1986).
During the late Miocene, the Kaikoura tectonism re-activated many of the late Cretaceous and early Paleogene faults, leading to extensive sub-rectangular block faulting of the Te Kuiti Group.
The Renown Fault is a north-northeast trending fault developed along the western margin of the coalfield, up-thrown to the west with displacement varying from approximately 50m in the south up to approximately 150m in the north.
The Waikokowai Fault is a north trending fault that defines the eastern coalfield boundary. This fault also shows an increase in displacement northward. The fault appears to have been active during the deposition of Tertiary marine formations, as these units appear to be thicker on the downthrown side (Kirk, 1986).
The Hetherington Fault is small scale northeast trending fault that joins the Waikokowai and Renown Faults. The Bain Fault is an east trending fault that increases from approximately 50m displacement near the Waikokowai Fault, to approximately 150m displacement near the Renown Fault.
The area is underlain by indurated siltstones, with common sandstones, of the Mesozoic Newcastle Group, weathered to a depth of 5-30m. This unit is referred to as “Basement”.
The Waikato Coal Measures (WCM) lie unconformably on the basement and form the lower part of the Te Kuiti Group. The WCM consist mainly of mudstones and siltstones, often referred to collectively as “fireclay”, with common siderite concretions, referred to as “hardbars”. There are two major coal seam groups within the WCM in the Project area, Renown and Kupakupa.
The upper part of the Te Kuiti Group consists of marine to marginal marine claystones, mudstones, sandstones, limestones and siltstones which conformably overly the WCM. A regional unconformity at the top of the Te Kuiti Group limits the vertical extent of this unit.
Quaternary deposits of the Tauranga Group unconformably overly the Te Kuiti Group and consists of interlayered alluvial clays, muds and highly weathered volcanic ashes of the Hamilton formation.
Dimensions
The Rotowaro North Extension resource model covers approximately 1,870ha.
Within this area, both the Kupakupa and Renown seams have been mined via underground methods (Renown in the north and Kupakupa in the south) as well as subsequent open-cut mining methods in areas of historically first worked coal. A total area of 420ha has been worked via these methods.
Coal thickness varies across the Rotowaro North Extension area, generally ranging from 9m down to less than 0.5m.