Towards understanding phosphorus distribution in coal: A case study from the Bowen Basin

Abstract

In coal, phosphorus can occur in a variety of minerals but apatite Ca5(PO4)3 (OH, F, Cl) is the most common. This mineral is often observed within the cell-lumens (<20 microns) of inertinite macerals, although fracture-infilling apatite has been reported. Its size and main occurrence within the cell lumens makes it difficult to liberate the apatite by current coal beneficiation strategies. It also reduces porosity and clogs flow paths for gas drainage. Present-day mineral distributions reflect the origins and geological history of the coal and fluids moving through it. Therefore, understanding the geological controls of these distributions within the coal seams can help in prediction and developing mitigation strategies. Interrogation of spatial litho- and bulk chemical-data sets within Permian coals at a single deposit affected by igneous intrusions, faults and seam splits was undertaken. Preliminary results in the study-area show elevated phosphorus contents are common in the roof and floor of the parent coal seam. However, these elevated contents transgress lithotypes, up-dip along the flanks of an anticline that is also proximal to a fault, a dyke and a rider seam split. The findings from this work provided the framework for samples selection for detailed microanalysis. Initial electron probe results show multiple phosphate minerals including: apatite and crandallite [CaAl3(PO4)(PO3OH)OH6]. There are at least two mode of occurrence of the apatite minerals: cell-lumen- and fracture-infilling, with the former most common. The cell-infilling apatite is thought to reflect either syngenetic or epigenetic mineralisation; while the fracture-infilling apatite formed later, post coalification. This is interpreted to have formed via fluids migrating through permeable and porous pathways created during geological deformation. Preliminary infrared spectroscopy results show the fracture-infilling apatite is relatively crystalline and lacking a hydroxyl peak which could reflect fluoridation. Preliminary cathodoluminescence data show this fracture-infilling apatite, as well as those in the pores, appears to be an epigenetic mineral phase that potentially formed from a single-phase fluid. The spatial distribution of bulk chemical data suggests a spatial dependence between elevated phosphorus contents and post- or syn-depositional features. However, a conclusive relationship between mode of occurrence, apatite chemistry and proximity to depositional and post-depositional features has not yet been found. Therefore, alternative analytical methods to discriminate between phosphorus-bearing minerals formed from such processes are proposed.