Textures of titaniferous magnetite within the Bushveld Igneous Complex South Africa

Abstract

Magnetite is a major constituent of the Upper Zone of the Bushveld Complex in South Africa. Magnetite occurs first as an intercumulus mineral, then as a cumulus mineral, then as massive monomineralic layers within the succession of gabbroic rocks. This study focuses on the textural intergrowths with the magnetite of the Upper Zone. Intergrowths of ilmenite occur within magnetite and exist between two extremes, namely the cloth microtexture and the trellis microtexture (with a “sandwich” variant). The cloth intergrowth is also characteristic of the ulvöspinel exsolutions in the magnetite. These lamellae usually lie on {100}, with inter-lamellar magnetite-rich blocks. The exsolution microstructure only develops on a small scale, due to the slow rate of the kinetic processes involved in unmixing at the solvus temperature. Within the ulvöspinel cloth microtexture, it is the coarse {100} lamellae pattern which gives the grain the appearance of cloth. Magnetite appears as extremely small blocks between the exsolved lamellae of ulvöspinel. The {100} lamellae of the cloth microtexture generated by exsolution of ulvöspinel occur at a temperature below the magnetite-ulvöspinel solvus. The ulvöspinel completely transforms into ilmenite micro-lamellae through in situ oxidation. Martitization occurs under moderate-temperature, hydrothermal oxidation, resulting in a volume change when hematite replaces magnetite. The calculated volume change increase is around 1.7%; this volume change results in expansion fractures throughout the replaced grains. The magnetite grains are preferentially martitized along grain boundaries and octahedral planes {111} leaving unaltered cores except in few places where it is martitized throughout the grain. Possible processes involved in the formation of the various textures include: (1) Pulses of Fe-Ti rich magmas occur, injecting large amounts of liquid into the Upper Zone. Dense immiscible droplets form and settle out of suspension, accumulating and forming stratified layers. The initial temperature of the liquid is approximately 1150ºC. (2) Upon cooling, titaniferous-magnetite precipitation occurs at around 865ºC, with low oxygen fugacity across the chamber. At this stage, ilmenite is not abundant, due to the presence of Ti as a component of ulvöspinel at high temperatures, rather than ilmenite. (3) At some point during cooling of the solid solution, spinodal decomposition occurs. This starts to produce ulvöspinel lamellae within magnetite grains. At the same time, the stratified layers begin to oxidise, forming ilmenite as a product of ulvöspinel decomposition. (4) External granular diffusion results in ilmenite constituents, diffusing across the grain boundary. When the rates of diffusion decrease, the ilmenite cannot leave the titaniferous-magnetite grain and exsolve. Ilmenite is dependent on the ulvöspinel content, which in turn is dependent on the temperature and oxygen fugacity during cooling. This would indicate why some polished sections showed ilmenite exsolution whilst others do not. (5) The layers cool to a point where spinodal decomposition is no longer favoured and the system shifts from cloth-texture exsolution to trellis-exsolution. The ilmenite component begins to form thin, multi-directional lenses often surrounded by box structures. (6) During oxidation, some magnetite subjected to martitization, is subsequently replaced by martite. The replacement involves an expansion and fracturing of martitized grains. During expansion, syntaxial veins form along grain boundaries, subsequently filled by surrounding Fe-Ti liquid. Depending on the growth rate of the crystals within the vein, either blocky syntaxial veins or blade-like syntaxial veins form. (7) During continued cooling, annealing of euhedral magnetite grains occurs. The annealing process runs concurrently with other processes occurring within the magnetite layers.