A systematic investigation of the exsolution textures in titanomagnetite grains of massive magnetite layers and their associated host rocks from the entire upper zone sequence has revealed noticeable differences that can be related to changes in the oxidation state of the magma during crystallization and cooling. The observed oxidation exsolution intergrowths indicate a higher fo2 during formation of the magnetite layers than during crystallization of the disseminated titanomagnetite, as well as a decrease in the fo2 required to precipitate successive titanomagnetite-rich layers upward in the sequence.Peculiar composite lameliar intergrowths of magnetite and ilmenite in the uppermost magnetite layers and in some magnetite plugs are ascribed to a subsolvus oxidation of ulviispinel in ulviSspinel-rich magnetite. The configuration of the magnetite-ulviSspinel solvus dictates that ulviispinel in Ti-rich magnetite-ulv•ispinel solid solution high in the succession exsolves at considerably higher temperature than in Ti-poor solid solution lower in the sequence. Subsolvus oxidation of ulviispinel to ilmenite at higher temperatures near the top of the intrusion facilitated diffusion of ilmenite to produce the variety of different composite exsolution textures. The textures are not developed where abundant oxidation exsolution of ilmenite has taken place at temperatures above the solvus.

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After a period of relative dormancy, the Lake Victoria Goldfields (LVG) in Tanzania, Kenya and Uganda have emerged as an important site for gold mining and exploration in Africa. The Neoarchaean greenstone belts of the LVG are composed mainly of "Nyanzian" metavolcanic sequences overlain by younger "Kavirondian" clastic metasedimentary strata. Intermediate to felsic plutonic rocks include local, older syn-volcanic bodies and an extensive suite of late, circa 2650-2640 Ma potassic (K)-granitoids, which substantially invade and disrupt the greenstone belts. Gold deposits are widely distributed but particularly concentrated in three belts: Sukumaland, Musoma-Mara and Busia-Kakamega. Many deposits are simply composed of quartz veins in shear zones and fractures in the Nyanzian host rocks (e.g. Bulyanhulu, Buck Reef and Rosterman). Buzwagi and Nyabirama are composed of quartz vein arrays within intrusions. Several are hosted by folded banded iron-formation (Geita, Golden Ridge, Tulawaka, and Nyanzaga). Nyanzaga, however, is a discordant pipe shaped stockwork containing dolomite, silica and sulphides as open-space fill in high level veins. Gokona-Nyabigena at North Mara is closely associated with porphyritic volcanic and sub-volcanic intrusive rocks and is noted for stockwork-disseminated mineralisation and extreme K-feldspar alteration. Field evidence points to a relatively late, syn-orogenic timing for the majority of the deposits irrespective of style. Most deposits were discovered and exploited in colonial times but many have been expanded considerably by modern exploration. Improved geological knowledge has resulted from this private sector exploration using high-resolution aeromagnetic, electromagnetic and radiometric surveys, followed up by regolith and outcrop mapping and systematic drill testing of large areas (rotary air blast (RAB), reverse circulation (RC), aircore and diamond drill core). Lithogeochemistry and geochronological studies have been carried out by several companies in support of their regional exploration programmes. The LVG is now considered mature from an exploration point of view, so future developments will require the investigation of vein deposits to greater depth and/or the discovery of additional examples of the lower grade stockwork-disseminated styles which may have been overlooked during the early rounds of exploration.

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The Upper Critical Zone of the Rustenburg Layered Suite (RLS) in the Swartklip Sector, north-western Bushveld Complex, is considerably attenuated relative to other parts of the Complex. The interval between the UG2 chromitite and the Merensky Reef amounts to as little as 25 m in places. Within this interval, the aggregate thickness of orthopyroxenite-dominated ultramafic layers hosting the UG1 and UG2 chromitites and the Merensky and Bastard reefs does not differ significantly from the area around Rustenburg, to the south. The total thickness of ultramafic lithologies is, in fact, increased by the presence of the 3 to 5 m thick olivine-rich Pseudo Reef Unit, which is developed between the UG2 and Merensky Reef units in the Swartklip Sector, but does not occur in any significant form elsewhere in the Bushveld intrusion. The substantial thinning of the succession is due almost entirely to the fact that plagioclase-rich rocks (norite and anorthosite) between the ultramafic layers are radically thinned in the Swartklip Sector relative to virtually all other parts of the Bushveld Complex. The ultramafic layers, although dominated by orthopyroxenite, are characterized by higher proportions of olivine than in other parts of the Bushveld Complex. In our logging of the substantial number of exploration drill cores that form the basis of this study, we have found it expedient to define stratigraphic units that are either exclusively plagioclase-rich (norite and anorthosite) or plagioclase-poor (consisting of varying proportions of orthopyroxenite, harzburgite and chromitite). This effectively binary system of lithological classification has no overt genetic connotations. Our nomenclature has, in fact, enabled us to rigorously document the nature of contacts between ultramafic and plagioclase-rich units, and thus to identify unconformities between the ultramafic units (orthopyroxenite and harzburgite) and intervening noritic and anorthositic units, which have in the past been ascribed to localized thermo-chemical erosion of pre-existing plagioclase-rich cumulates. Apart from the well-documented evidence of erosional unconformities at the basal contacts of ultramafic units, we also provide evidence for unconformities at the tops of these units.