Cleat mineralization of Upper Permian Baralaba/Rangal Coal Measures, Bowen Basin, Australia

Abstract

Coals from the Permo-Triassic Bowen Basin have been investigated using a variety of complementary analytical techniques. Face cleat minerals in the studied samples are dominated by authigenic clays, notably pure illite or illite-chlorite mixtures. Butt cleats and joints are dominated by carbonates, mostly calcite with less abundant ferroan calcite, ankerite and siderite. K/Ar ages of cleat-fill illites and one fracture illite-smectite fill indicate three phases of illite formation during the Triassic: (1) an early phase with ages clustered around 244 Ma ago; (2) a second phase about 232 Ma ago; and (3) the latest phase about 219 Ma ago. Thermal modelling of selected boreholes from the study area and other geological data indicate that the first phase of illitization occurred when the coal seams were about 1000m below the surface and at a temperature of between 70 and 80°C, at a time when the basin was rapidly subsiding. These early illites were stable and retained their 1M polytype even after being exposed to temperatures of 150-190°C during maximum burial in the southeastern Bowen Basin. The second phase of illitization (at 232 ± 3) occurred at about the time of maximum burial. The latest phase took place between 223 ± 3 and 212 ± 3 Ma ago and occurred in a wider temperature range (between 170 and 100°C) during a rapid uplift in the Late Triassic. The Bowen Basin area experienced a second cycle of subsidence that commenced in the Early Jurassic with the 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 carbonate mineralization in butt-cleats and joints took place. Calcite fluid inclusions indicate that calcite was precipitated from meteoric water at about 80°C. The widespread carbonate mineralization 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 Jurassic-Cretaceous times relative to that during the Triassic Hunter-Bowen Orogeny.