The nature and origin of titaniferous magnetite-rich layers in the upper zone of the Bushveld Complex; a review and synthesis

Abstract

The occurrence and geologic relationships of Ti magnetite layers indicate that their genesis is intimately related to fractional crystallization processes that were responsible for the formation of their silicate host rocks. The exact mechanism by which these oxide-rich layers form is, however, still debated. Most recent models recognize that precipitation of copious quantities of Ti magnetite is triggered by episodic increases in f (sub O 2 ) , but the process whereby this occurs is not well understood. The nature and occurrence of Ti magnetite-rich layers in the Bushveld Complex is reviewed and the requirements of any satisfactory genetic model are outlined. The formation and subsequent textural evolution of Ti magnetite layers is believed to have taken place as follows:1. Favorable conditions for the precipitation of large quantities of Ti magnetite were created by a lengthy period of fractional crystallization that resulted in the concentration of large amounts of Fe, Ti, and V in the residual magma.2. Large-scale in situ bottom crystallization of plagioclase resulted in a marked increase in the total Fe content and density of the immediately surrounding melt which collected as a dense layer on the bottom of the magma chamber. This dense Fe-Ti-(V)-enriched liquid did not mix with the overlying magma and formed a stagnant layer from which copious amounts of Ti magnetite may have crystallized.3. Crystallization of Ti magnetite is controlled by the Fe 2 O 3 /FeO ratio of the liquid and is a function of f (sub O 2 ) , temperature, and the f (sub H 2 O) /f (sub H 2 ) ratio. The Fe 2 O 3 content of the liquid will also increase during the crystallization of ilmenite and Fe (super +2) -bearing silicates (clinopy-roxene, pigeonitc, and fayalitic olivine). The H 2 O content of the residual liquid will also be increased by the fractionation of anhydrous silicate phases.4. A complex interplay of these factors resulted in the precipitation of relatively large amounts of Ti magnetite required for the development of ore-rich layers. The bulk of the Ti magnetite formed by in situ bottom crystallization. The nucleation and growth of magnetite within the stagnant layer and gravitative settling of these crystals over a short distance (on the order of meters) may have augmented the growth of the layers.5. Variations in the relative proportions of coexisting oxide and silicate phases may be related to differences in concentrations and diffusion rates of ions in the liquid immediately above the crystallizing layer. This resulted in differences in the rates of nucleation and growth of individual phases.6. Precipitation of abundant magnetite lowered the density of the stagnant layer until it reached that of the overlying magma. The two liquids then mixed, thus terminating the cycle of magnetite layer formation and marking a return to silicate-dominated fractionation.7. The ore-rich layers originally consisted of Ti magnetite crystals and variable amounts of silicate crystals which contained a certain amount of interstitial residual liquid. Densification of these layers and expulsion of significant amounts of this liquid were accomplished by annealing at high subsolidus temperatures, possibly augmented by the addition of suitable material from interprecipitate liquids.8. Variations in f (sub O 2 ) during subsolidus cooling resulted in the development of a wide range of ulvoespinel and ilmenite microintergrowths.