Coal ranks in Permian — Lower Cretaceous rocks of New Zealand

Abstract

Of 36 analyses of coals from Permian to Lower Cretaceous rocks, 32 are used to infer coal ranks, using volatile matter (calculated to the ASTM mineral-matter-free basis) as the rank parameter. Most samples were weathered and a few were of unusually high volatile type, necessitating estimation of volatile contents of the equivalent un weathered coals of normal type. In western rocks of the Kawhia and Southland Regional Synclines (Murihiku Supergroup and Takitimu Group), ranks range from high-volatile Abituminous in Jurassic rocks to semi-anthracite in Permian rocks. The few analysed eastern North Island samples (Torlesse Supergroup) have a small rank range within high-volatile A and medium volatile bituminous. South Island Torlesse samples range from high-volatile A bituminous to meta-anthracite. In Canterbury, the rank apparently diminishes progressively eastwards, with the Middle Triassic Mount Potts Group, unconformable on undifferentiated Torlesse, and the Middle-Upper Jurassic Clent Hills Group, possibly unconformable, conforming to this regional rank pattern; the mid-Jurassic Wakaepa Group, unconformable on Torlesse, is, however, notably lower in rank. An area in the middle Waitaki valley, including the Middle Triassic Corbies Creek Group, is of medium and low volatile bituminous rank. From sparse data on associated rocks, the zeolite and prehnite-pumpellyite mineral facies overlap within the medium volatile bituminous to semi-anthracite range.

Keywords:

• coal rank
• volatile matter
• ASTM coal ranks
• mineral facies
• Torlesse Supergroup
• Murihiku Supergroup
• Takitimu Group
• New Zealand