The Bowen Basin is a complex Permo-Triassic, extensional to foreland basin, extending north-south for about 1000 km. Large commercial coal deposits are known from the Bowen Basin, some of which are currently under investigation as a potential source of Coalbed Methane.
Borehole samples ranging in depth from 300m to 989m from two major intervals- the Upper Permian Rangal Coal Measures and German Creek Coal Measures - show a range of rank from High-Volatile Bituminous Coal (0.81%Ro) to Anthracite (2.54%Ro). The nature and origin of cleat mineralisation in Upper Permian coals from the Permo- Triassic Bowen Basin have been investigated using a variety of complementary analytical techniques.
A total of nineteen authigenic mineral phases have been found in cleats and joints of the Rangal/Baralaba and the German Creek Coal Measures in the study area. These are illite, calcite, kaolinite, ankerite, quartz, chlorite, siderite, barite, pyrite, illite/smectite mixed-layer, strontianite, wollastonite, whewellite, halite, barium phosphate, bjarebyite, analcime, albite, fluorite. However, the overall cleat mineralogy in the studied area is rather simple. The most frequently occurring minerals belong to two groups, (i) clay mineral-dominated by illite found mostly in face cleats and (ii) carbonates-dommoitd by calcite and found mostly in butt cleats. Ankerite dominates the nonsystematic cleat minerals, while kaolinite dominates superimposed cleats.
K/Ar ages and stable isotope geochemistry of face cleat-fill illites (δ18O(VSMOW) = + 6.9 to + 14.4‰, δD = -123 to - 80.1) and one fracture illite/smectite fill indicate three phases of illite formation during the Triassic from deeply circulating meteoric waters as follows: (1) an early phase with ages clustered around 245 Ma B.P., (2) a second phase about 232 Ma B.P. and (3) the latest phase about 219 Ma B. P. Burial history and thermal modelling of selected boreholes from the study area and other geological data indicate that the first phase of illitisation occurred when the coal seams were about 1000m below the surface and at a temperature of between 70°C and 80°C, at a time when the basin was rapidly subsiding. These early illites were stable and retained their IM polytype even after being exposed to temperatures of 150-190°C during maximum burial in the southeastern Bowen Basin. The second phase of illitisation (at 232+3 Ma) occurred at about the time of maximum burial. The latest phase took place between 225±3-212±3 Ma and occurred in a wider temperature range (between 170-100°C) during uplift associated with the Hunter-Bowen Orogeny in the Late Triassic and a subsequent period of post-orogenic extensional subsidence.
Face cleat coals from Banana and Moura areas are dominated by illite, while coprecipitation of illite, chlorite and kaolinite assemblages is noted in face cleat coals from Bluff, Blackdown and Dawson River areas. This change in mineralogy reflects a change in the chemistry of the mineralising fluids during the Triassic. This change was most probably caused by depletion and reduction in activity of K, Ca, Mg and Fe cations within the mineralising fluids as they moved away from their source along the orogenic eastern basin margin towards shallower regions of the basin. Superimposed cleat-fill kaolinite (δ18O = + 8.2 to + 14.1‰, δD = -112 to - 102.5‰) from Emerald coals might also be related to this phase of mineralisation.
Ankerite (δ18O= + 7.7 to + 11.4‰, δ13C (PDB) = - 7.1 to -1.6‰) mineralisation was contemporaneous with non-systematic cleat formation at temperatures between 40 and 100°C during uplift at the latest stage of the Hunter-Bowen Orogeny in Late Triassic times.
The Bowen Basin area experienced a second cycle of subsidence that commenced in Early Jurassic with formation of the Surat Basin, which overlies the Bowen Basin and forms part of the Great Artesian Basin system. Regional uplift and erosion in middle Cretaceous times terminated sediment accumulation in the Surat Basin. During the second cycle of burial widespread carbonates, mainly calcite (δ18O = + 9.8 to + 16.8‰, δ13C = - 7.2 to + 14.7‰) mineralisation in butt cleats and joints took place. Calcite fluid inclusions and stable isotopes indicate that calcite was precipitated from meteoric water at temperatures between 70 and <100°C. The widespread carbonate mineralisation in butt cleats and the near absence of carbonates in the face cleats is here attributed to permeability anisotropy, caused by a change in the direction of lateral compressive stress during the Jurassic-Cretaceous time relative to that during the Triassic Hunter- Bowen Orogeny.