Flow structures and mineralisation in thermally altered coal from the Moatize Basin, Mozambique

Abstract

This paper examines changes in structural and petrographic characteristics of organic and inorganic matter developed in a cored 35 m thick Permian coal seam from the Moatize Basin (Mozambique) in proximity to an Early Jurassic intrusive igneous body. Megascopic and microscopic examinations of hand samples and polished blocks, respectively, were carried out to characterise the deformation and petrographic changes of the coal. SEM/EDS analysis was used for mineral identification and modes of occurrence. The upper part of the core is thermally altered, exhibiting intense megascopic folding of the bedding. At microscopic level, vitrinite presents an incipient mosaic coke texture (semi-coke); whereas fusinite and semifusinite layers are deformed, exhibiting folded structures. Coal petrographic characteristics suggests the temperature in the coal at this location reached no higher than 400 °C but was enough for softening and ductile deformation of bands to occur.Apart from epigenetic chamosite occurring exclusively in the upper section, and monazite that appears only in the lower section of the core, microscopic studies showed similar distribution of the mineral species in both sections of the seam, except with a change in their mode of occurrence. Detrital or syngenetic minerals in the lower section of the seam occur also as epigenetic in the thermally affected upper section. Siderite occurs as syngenetic nodules in the lower section but appears as epigenetic fracture filling developed with the cooling of the organic material in the upper section. Quartz occurs as detrital grains in the lower section, whereas in the upper section this mineral occurs also as an epigenetic euhedral mineral filling the fracture system. Fluorapatite occurs as detrital, syngenetic and euhedral epigenetic. The mineralisation in the fractures also gives indications about the fluid circulation associated with the magmatic intrusion. At least three stages of hydrothermal fluids were distinguished (from the first to the last stage): a P/F- and Si-rich fluid, an Fe-, Ca-, Mg-rich fluid and an Al-, Si-rich fluid. These fluids gave origin to the minerals in the fractures and to some of the minerals inside the cell lumina of the inertinite macerals. Given the euhedral shape of some of the epigenetic minerals, there was sufficient time for them to grow into euhedral forms.