Pyrometamorphism of coal measures that overlie underground burnt-coal seams in the southern area of the Rotowaro coalfield, New Zealand, has produced porcellanites that enclose lenses of iron-rich magnetite and hematite-bearing slag-like rocks, which show various degrees of oxidation. Paralavas that are associated with the iron-rich lenses form stalactites around fissures and gas-escape vents and intrude the porcellanites. The slags are unusually rich in iron and contain magnetite, hematite, hercynite, titaniferous magnetite, and minor fayalite and silicate glass. Iron oxides in these rocks exhibit a variety of textures and morphologies, including dendritic, quench, exsolution, and oxidation. The paralavas contain abundant glass, which encloses feathery crystals of fayalite and orthoferrosilite, plagioclase, tridymite, cristobalite, and minor magnetite.

Phase-equilibria data indicate that the paralavas and slags were formed at temperatures in the range of 1000–1600 °C. Composition plots of local coal measure sediments, a siderite nodule, and various porcellanites, slags, and paralavas with respect indicate that the porcellanites are iron-rich when compared to SiO₂, Fe₂O₃, and Al₂O₃ with the unmetamorphosed coal measures. The slags, irrespective of their oxidation state, plot on the iron-enrichment trend shown by the porcellanites. Although the siderite nodule lies within the iron-enrichment trend, most of the slags are more iron-rich than the siderite nodule. The paralavas diverge from the iron-enrichment trend, suggesting that they formed by partial melting of the porcellanites. The Rotowaro samples represent some of the most iron-rich natural slags collected from a combustion-metamorphic environment to date.