Coal rank, coal type, and marine influence in the north Taranaki coalfields, New Zealand

Abstract

A few coal seams in the thin early Miocene Maryville Coal Measures have been mined in the relatively minor north Taranaki coalfields: Mokau, Ohura, Tangarakau, Waitewhena, and Aria. Generally accepted as sub‐bituminous, the coals are discussed in terms of the Rank(Sr) classification to identify more closely the variations of coal type and coal rank. The use of a mineral‐matter‐ and sulphur‐free basis for the analyses is made necessary by the generally moderate to high sulphur contents. The range of volatile matter is 15%, and of calorific value is 1500 Btu/lb (= 3.5 MJ/kg specific energy). In the most closely explored coalfield, Mokau, c. 450 coal analyses from up to 5 seams in 130 drillholes over an area of c. 65 km2 have been used. Analyses from 27 seam sections in Tatu Mine (Tangarakau Coalfield) come from an area of c. 2.5 km2. Analyses from a few mines in Waitewhena and Aria Coalfields, and from widely spaced drillholes in Waitewhena, Ohura, and Tangarakau Coalfields, provide additional data. In Mokau Coalfield, a detailed pattern of lateral rank variation is demonstrated within a Rank(Sr) range from 6.6 to 8.1. The rate of lateral rank variation, up to 1.5 Rank(Sr)/km, is too rapid to result from depth of burial variation. The rank variation is tentatively ascribed to localised differential heating as a result of fluid movement up faults during minor disruption of the downwarping basin at the end of Miocene sedimentation when the coal ranks were imprinted. Such disruption may have produced the older north‐trending faults; the dominant younger east‐northeast‐trending faults are likely to have formed during uplift after the rank pattern was established. The lower ranks over most of the region are considered to indicate the general depth of burial, c. 1.7 km, but a regional decrease in rank along the eastern margin and in the northeast at Aria indicates a probable regional decrease in thickness of the former cover to c. 1.25 km.