Changes in geochemistry and mineralogy of thermally altered coal, Upper Hunter Valley, Australia

Abstract

Igneous intrusions thermally and geochemically alter coal, commonly causing economic and safety problems for many coal mines. The effects of two dykes on the inorganic content of the Late Permian bituminous Upper Wynn seam were determined from analyses of 44 samples that were collected along transects approaching the intrusions. Petrographic and XRD data were used to determine sample mineralogy, and INAA and XRF spectrometry were utilised to determine the contents of 57 elements. The mineralogy of the unaltered coal, altered coal and dyke is dominated by carbonates, particularly dawsonite, which formed by epigenetic precipitation at a late stage, after thermal alteration. Ankerite and siderite are the products of thermal alteration and are restricted to the altered coal and dykes. Principal component analysis, correlations and compositional trends approaching the intrusions were used to subdivide the elements into groups and to identify the mineralogical affinity of each group. Geochemical data are more sensitive than mineralogy for defining the size of the alteration halo, and three zones ranging from unaltered coal, through altered coal to highly coked coal are recognised as each intrusion is approached. The alteration halos differ in extent (9.5 and 56 m wide), but their size is not a multiple of the size of the causative intrusion. At the contact between coal and intrusion, concentrations of elements with affinities to some aluminosilicates, oxides, carbonates, sulphides and organic components are enriched, while other aluminosilicate-related elements are depleted. In the altered coal towards the edge of the alteration halo, some aluminosilicate elements are enriched and oxide elements are depleted.