Abstract

Disseminated Fe-Ti oxides and ore-rich concentrations of these minerals in a 1,230-m vertical borehole section through the top of the upper zone of the Bushveld Complex are described. The succession consists of a layered sequence of highly fractionated olivine-rich rocks that range in composition from ferrogabbro to diorite. The Fe content of the olivine increases systematically from Fa 65 at the base to a maximum of Fa 94 at the top of the section. Fe-Ti oxides axe present throughout this interval and are divided into three distinct groups on the basis of their mineralogy and chemistry: (1) an apatite-poor, high Ti magnetite association, (2) an apatite-rich, high Ti magnetite association, and (3) an apatite-poor, low Ti magnetite association. The Ti magnetite in each of these associations exhibits a wide variety of ulvoespinel and/or ilmenite microintergrowths, the development of which can be interpreted in terms of the ambient f (sub O 2 ) at various stages of subsolidus cooling. Certain Ti magnetite crystals are characterized by the development of radiating, eutectoidlike intergrowths of magnetite and ilmenite or ulvoespinel that appear to represent duplex cell decomposition products of magnetite-ulvoespinel solutions.The apatite-poor, high Ti magnetite represents the dominant opaque oxide association and is present both as disseminations and distinct ore-rich layers, particularly toward the base of the study section. The greatest concentration of this association occurs at a depth of between 1,033 to 1,060 m below the top of the intrusion. The lowermost 14 m of this interval consist of a thick composite Ti magnetite layer that contains numerous silicate inclusions and partings. This is believed to represent the equivalent of the uppermost Ti magnetite layer (layer 21) in the eastern Bushveld Complex. Groups of minor Ti magnetite layers are also present at depths of 412 and 225 m below the top of the intrusion. The Ti content of the magnetite increases with increasing stratigraphic height from 55 mole percent ulvoespinel at the base to 65 mole percent near the top of the study section where it gives way to the Ti-poor association. The beginning of Ti-poor magnetite precipitation 225 m below the roof of the intrusion marks the onset of Ti depletion in the residual liquid. Ilmenite is virtually the only Fe-Ti oxide phase present in the uppermost 100 m of the complex. Both these associations formed in response to normal fractional crystallization processes, the formation of discrete ore-rich layers being triggered by episodic increases in f (sub O 2 ) . The apatite-rich, high Ti magnetite association is present both as disseminations and in ore-rich concentrations but is restricted to three relatively narrow horizons within the study section. These are developed at the following depths below the top of the intrusion: 233 to 283 m, 677 to 700 m, and 1,033 to 1,108 m. Discrete ore-rich concentrations consisting almost entirely of Ti magnetite + ilmenite ( approximately 70 vol %) and apatite ( approximately 30 vol %) are present in the lower 2 m of the middle horizon. The upper and lower horizons contain only disseminated opaque oxide-apatite assemblages. The formation of these assemblages is interpreted in terms of the separation of immiscible Fe-Ti-Mn-Ca-P-REE-enriched liquids from the late-stage dioritic residual melt.

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