Geochemistry of apatite in Late Permian coals, Bowen Basin, Australia

Abstract

Apatite may be a common accessory mineral in coal seams, but interpretation of its origins can vary from syndepositional mineral detritus, early precipitation during diagenesis, and post coalification precipitation and replacement from hydrothermal or other fluids. It is hypothesised that paragenesis is reflected in the modes of occurrence in different stratigraphic units of the Late Permian in the Bowen Basin. From samples analysed in this study, apatite in the youngest unit, the Rangal Coal Measures, most commonly occurs as pore-apatites (i.e. apatites infilling preserved plant cell cavities of fusinite and semifusinite macerals and commonly associated with kaolinite) and less commonly as fracture-apatites (i.e. apatites infilling micro-fractures) and detrital-apatites intermixed with the organic coal layers (Detrital-B). In the tuffaceous Fort Cooper Coal Measures and equivalents, apatite occurs more commonly as detrital-apatites intermixed with the lithic layers (Detrital-A) (Detrital. Within the basal unit, the Moranbah Coal Measures and its equivalents, the mode is commonly encrusting-apatites (i.e. small apatite crystallites encrusting silicate minerals). Almost all apatites in the coal samples tested by electron microprobe microanalysis (EPMA), regardless of stratigraphic location and mode, are fluorapatite. Although there are some locality-driven enrichments, the minor and trace-element chemistry vary more so by mode of occurrence. For comparison, apatites from dykes intersecting the coal measures and from tuffs were also added to the study. These had lower fluorine (F) but measurable chlorine (Cl) contents and are enriched in light rare earth elements (LREE), similar to the Durango apatite that is of magmatic origins. Most of the detrital-A apatites are also enriched in LREE similar to the tuff-apatites, but with variable enrichment in the rare earth and yttrium elements. Detrital-B apatites have minimal Cl but also show the magmatic LREE trend. The pore- and fracture-apatites exhibited two trends. In the samples from tuffaceous coals (e.g., Fort Cooper Coal Measures), the pore- and fracture apatites tended to follow the magmatic LREE trend. In the Rangal Coal Measures, the pore- and fracture-apatites have elevated F contents, minimal Cl contents and depleted LREE contents (or Middle REE enrichment). Depletion of LREE could occur through leaching in an acidic environment, which is suggested by the abundance of kaolinite. Apatite precipitates under more neutral conditions although pore-apatites are commonly considered “early” and fracture-apatites “late” or post coalification. Their geochemical similarity suggests they have similar fluid origins and/or timing, with geothermal fluids moving through the porosity afforded by the structured inertinite group macerals and fractures. Verification of that geothermal source still requires further work and a technique that can analyse the isotopic composition—e.g., carbon (13C/12C), oxygen (18O/16O) and strontium (87Sr/86Sr)—of micron-sized crystals in situ.