Greymouth Coalfield, New Zealand: vertical coal rank gradients, depths of burial and palaeotemperatures

Abstract Vertical rank gradients are inferred from lateral variations of Rank(Sr) in successive coal horizons in the Paparoa Group and Brunner Coal Measures in Greymouth Coalfield. The gradients are lowest, c. 3 Rank(Sr)/km, in the centre of the coalfield where the lateral rank pattern is simple. In the west, vertical gradients ranging from 8 Rank(Sr)/km result from contrasting Upper Rewanui and Brunner lateral rank patterns. There, it is inferred that warm fluids migrated from more deeply buried coal measures further east and have enhanced Upper Rewanui ranks. For calculating Brunner burial depths, the high western vertical gradients are rejected and a gradient of 4.0 Rank(Sr)/km is adopted, as it is also east of the Mt Davy Fault Zone where the gradient is unknown. Brunner burial depths increased from 1.8 km in the far west to 4.3 km in the Mt Davy area, and palaeotemperatures increased eastwards from 90–95°C to 150–155 °C.