The Vanadiferous Magnetic Iron Ore of the Bushveld Igneous Complex

Abstract The Bushveld Complex contains the largest reserves of vanadiferous iron ore in the world. The ore is found as plug-like bodies and as seams of magnetitite. The plug-like bodies are pegmatoidal in character, and coarse-grained silicates such as diallage, olivine and amphibole commonly render the ore low-grade or useless. These magnetitite pegmatoids are known from the Critical Zone, from the Main Zone and from the Upper Zone of the Layered Sequence of the Complex. Only the Kennedy's Vale magnetitite plug-like mass, which is low down in the Main Zone, has been exploited so far for its vanadium content. The rubble ore of this deposit contains about 2% V2O5. Excepting doubtful, local seams more or less on the horizon of the Kennedy's Vale plug, the real magnetitite seams are encountered only in the uppermost 1,000 feet of the Main Zone and in the lower 4,000 feet of the Upper Zone. In the Eastern Transvaal 26 seams have been distinguished. Among these the uppermost seam, which is about 30 feet thick, is the thickest, but is made up of disseminated ore and of lenses of anorthosite. The Main Seam, 4,000 feet lower down, is 4½− 8½feet thick and consists mostly of massive magnetitite. This latter seam has an exposed strike length of 75 miles in the Eastern and of the order of 125 miles in the Western Transvaal. Rubble ore of this seam averages about 1.6% V2O5 and more than 200,000 tons of this ore are available for exploitation at surface or accessible by shallow open-cast mining in the Eastern Transvaal. Many seams have a sharp basal contact against anorthosite, and a transitional hanging-wall composed of massive ore grading upwards into magnetite-anorthosite and hyperite. A thin, highly puckered veneer of olivine and an intergrowth of pyroxene and plagioclase are developed at the sharp basal contact. The intergrowth could be due to diffusion of iron into the plagioclase. Irregularities at the contact consist of vein-like offshoots of magnetite into the foot-wall and even brecciation of the foot-wall is encountered. The grain-size of the ore in the Main Seam is variable. The closely packed nature of the grains is best explained by enlargement after concentration. The magnetite started to crystallize earlier or simultaneously with the plagioclase, and the seams could have formed in the same manner as any monomineralic rock in a layered sequence. In some of the country rocks magnetite continued to crystallize interstitially to the plagioclase. In these examples hydroxyl minerals such as biotite and amphibole are generally present. Titanium minerals in the ore consist mainly of (1) small ilmenite granules that probably formed earlier or simultaneously with the magnetite, (2) rather rare, broad lamellae of ilmenite which exsolved from magnetite at an early stage, (3) ulvite, which is ubiquitously present and forms the characteristic cloth-pattern, and (4) a wide range of intergrowths of dispersed ilmenite ("proto-ilmenite") with magnetite, maghemite and martite. These intergrowths are considered to have formed from the ulvite cloth-pattern by surface or near-surface oxidation. The cloth-pattern of ulvite seems to have had a profound effect on the formation of maghemite and martite—it promoted maghemitization and retarded martitization. An antipathetic relationship prevails between the V2O5 and the TiO2 content of the ore. The lowermost seam contains about 2% V2O5 and 14% TiO2, the uppermost one about 0.3% V2O5 and 18–20% TiO2. As the titanium is mostly contained in ulvite it is considered that a decrease in oxygen fugacity in the magma determined the increase in the titanium content upward in the layered sequence. The V2O5 content of the magnetitite plugs seems to correspond approximately to that of the seams in their vicinity.