Coals of the West Coast region (South Island, New Zealand) have been investigated by ash constituents analysis and X-ray diffraction of low temperature ash. Combined results from these techniques, together with evidence from sink-float fractions and inter-element relationships, are used to determine the mode of occurrence of major elements, strontium, barium, and nickel. Derivation of formulae for calculating mineral abundance allows the coals to be compared on a statistical basis.

A wide variation in mineralogical character is demonstrated for bituminous coals, particularly the highly diverse marginal marine Brunner coals (Paleocene-Eocene). Coals from the non-marine Paparoa Coal Measures (Late Cretaceous to Paleocene) are relatively uniform.

Mineral assemblages in Paparoa coals are variously dominated by kaolinite, quartz., illlte, or siderite, and accompanied by minor proportions of crandallite, rutile and calcite, Deep circulation of groundwater from overlying marine rocks has locally altered siderite to pyrite. The relative abundance of detrital and syngenetic minerals, and distribution of mlnor ash constituents, is principally controlled by depositional environment and swamp character; differences in source rock composition appear to be of secondary importance. Mineralogical and chemical distinctions can be recognised for samples which have a contrasting coal type. Coals which have been attributed on petrographic evidence to relatively low water table swamps generally contain a highly kaolinitic mineral assemblage, which has resulted from periodic leaching of detrital minerals. Samples from swamps where a high water table is inferred contain abundant quartz, which originated from rafted vegetation,

Distribution of titanium and phosphorus minerals within seams of the Paparoa Coal Measures is complex. Sporadic crandalllte-rich horizons are attributed to interaction of minerotrophic and leaching effects, whereby phosphate in the peat reacted with Al³⁺, Maximum titanium concentrations commonly coincide with abundant crandallite, but the cause of this association is uncertain.

Slderite abundance appears to be controlled by permeability of the coal measure sediments.

Bituminous coals of the Drunner Coal Measures exhibit substantial variation in mineralogical composition as, a result of differing depositional settings. In comparison to the Paparoa seams, Brunner coals lack siderite, but contain locally abundant pyrite, dolomite, and boehmite. Slow and stable peat accumulation and isolation from sediment supply have resulted in some low ash coals, in which the mineral matter often shows mineralogical and chemical evidence of strong leaching of detrital sediment within the peat. In highly acid environments, both silica and alumina are believed to have been mobilised, yielding ultra-low ash coals. Where moderately acid conditions prevailed, decomposition of kaollnite and loss of silica has given rise to abundant boehmite. Boehmite-rich carbargillites at Buller Coalfield are interpreted as areas where aluminium was leached from adjacent high-moor bogs and precipitated in low lying drainage zones. During diagenesis and coalification, the more highly aluminous mineral matter has reacted with other mineral and chemical components to form chlorite, and with organically bound sodium to form dawsonite (NaAlCO₃(OH)₂).

In both Paparoa and Brunner coals, strontium occurs preferentially in crandallite, which ls also a major host to barium. Strontium also occurs in calcium carbonate minerals, and may be organically bound. Sodium occurs to some extent as dawsonite (Brunner coals), or as smectite interlayers in illite (Paparoa coals), but ls generally organically bound. Nickel occurs chiefly in organically bound form, but ls also associated with illite-rich sediment in Brunner coals. In some Paparoa seams, post-depositional enrichment in nickel has occurred via roof and floor.

Mineral matter in West Coast subbituminous coals is dominated by organically bound calcium, and in one area, by sodium. At some localities, horizons of authigenic quartz are present.

The trends in mineral character generally support and extend peat accumulation models based on coal petrology, and thereby have relevance to coal exploration and coalfield development. The mineralogical data are applicable to aspects of coal characterisation, particularly the estimation of mineral matter/ash ratios and correction of volatile matter to mineral matter-free bases.

In general, the geochemical character of the coals presents few problems to conventional methods of coal utilisation. However, if the coals are washed, some potentially adverse elements may concentrate in ash, due to their intimate distribution in the coals.