Abstract The northern limb of the Bushveld Complex of South Africa contains a diverse array of Cr, Ni-Cu-platinum group element (PGE), Fe-V mineralization in mafic-ultramafic rocks and Sn mineralization hosted in granites. The limb has historically been underexplored compared to other parts of the Bushveld Complex and currently represents one of the world’s most interesting exploration frontiers. Successful low-cost open-pit mining of the thick Platreef Ni-Cu-PGE deposit, coupled with rising costs and limited scope for mechanization associated with narrow reef-type deposits in the eastern and western Bushveld, have driven efforts to locate similarly wide magmatic sulfide orebodies at surface or at reasonably shallow depths elsewhere in the northern limb, including recent discoveries of the Flatreef- and Main zone-hosted PGE deposits in the troctolite unit, at Aurora, and in the lower (F) and upper (T) mineralized zones at Waterberg. The Flatreef is hosted within a more consistent series of stratigraphic units than the more varied Platreef located updip, and while it shows similarities in terms of rock types and some geochemical features with the upper Critical zone of the eastern and western Bushveld, strict time equivalence remains to be proven. The various styles of Main zone-hosted PGE mineralization, on the other hand, have no known equivalents in the other limbs of the Bushveld Complex and seem to represent processes and events confined to the northern limb. Potential links based on similar rock types and metal budgets between Aurora and the Waterberg T zone and between the troctolite unit and the Waterberg F zone are attractive but must remain speculative until it becomes clearer whether the northernmost compartment that contains the Waterberg mineralization is linked to the remainder of the northern limb. If both the Flatreef and the Waterberg deposits enter production as planned over the coming decade, they will have dramatic effects on the South African platinum industry and dramatically increase the amount of Pd relative to Pt produced by South Africa due to the Pd-rich nature of all of the northern limb PGE orebodies.

Abstract Ore deposits form by a variety of natural processes that concentrate elements into a small volume that can be economically mined. Their type, character and abundance reflect the environment in which they formed and thus they preserve key evidence for the evolution of magmatic and tectonic processes, the state of the atmosphere and hydrosphere, and the evolution of life over geological time. This volume presents 13 papers on topical subjects in ore deposit research viewed in the context of Earth evolution. These diverse, yet interlinked, papers cover topics including: controls on the temporal and spatial distribution of ore deposits; the sources of fluid, gold and other components in orogenic gold deposits; the degree of oxygenation in the Neoproterozoic ocean; bacterial immobilization of gold in the semi-arid near-surface environment; and mineral resources for the future, including issues of resource estimation, sustainability of supply and the criticality of certain elements to society.

Abstract The Platreef is one of the largest and most valuable Ni-Cu-PGE orebodies on Earth. It is located at the base of the northern limb of the 2.06 Ga Bushveld Complex and stratigraphic relationships with other limbs of the complex and stratiform orebodies such as the Merensky Reef and UG2 chromitite are not clear. The Bushveld Complex intruded along the axis of the >2.9 Ga Thabazimbi-Murchison lineament and this may have acted as a barrier between the northern limb and the rest of the complex for some or all of the intrusion history. Research since the turn of the millenium has demonstrated that the Platreef represents a sill or complex of sills intruded into basement granite-gneiss and sediments of the Transvaal Supergroup. Different sills display variable lithologic units, thicknesses, bulk chemical signatures, and mineralization arising from different inputs of magma and the effects of local wall-rock contamination. Chilling and injection of Main zone gabbronorites took place into already solidified and deformed Platreef, indicating a major break in time between these events. Aspects of mineral chemistry and bulk geochemistry and Nd and Os isotopes in the Platreef overlap completely with the Merensky Reef but not the Upper Critical zone. Conventional and mass independent S isotopes suggest a mantle source of S that was overprinted by addition of local crustal S where the Platreef intruded pyrite-rich shales. Assimilation and introduction of external S is viewed as an ore-modifying process, not as the primary trigger for mineralization. The genesis of the Platreef is more likely to have involved introduction of PGE-rich sulfide droplets with the intruding Platreef magma. These sulfides may have been derived from the same magma(s) that formed the Merensky Reef and which injected up and out along the intrusion walls as the chamber expanded. Alternatively, the sulfides may have formed in pre-Platreef staging chambers where they were upgraded by repeated interactions with batches of Lower zone magma before being expelled as a crystal-liquid-sulfide mush by an early injection of Main zone magma, prior to the formation of the bulk of the Main zone which crystallized above (and partially eroded) the solidified Platreef.

This paper documents an attempt to detect a meteoritic component in both wash-back (resurge) crater-fill breccia (the so-called Exmore breccia) and in suevites from the Eyreville core hole, which was drilled several kilometers from the center of the 85-km-diameter Chesapeake Bay impact structure, Virginia, USA. Determining the presence of an extraterrestrial component and, in particular, the projectile type for this structure, which is the largest impact structure currently known in the United States, is of importance because it marks one of several large impact events in the late Eocene, during which time the presence of extraterrestrial 3 He and multiple impact ejecta layers provide evidence for a comet or asteroid shower. Previous work has indicated an ordinary chondritic projectile for the largest of the late Eocene craters, the Popigai impact structure in Siberia. The exact relation between the Chesapeake Bay impact event and siderophile element anomalies documented in late Eocene ejecta layers from around the world is not clear. The only clear indication for an extraterrestrial component related to this structure has been the discovery of a meteoritic osmium isotopic signature in impact melt rocks recovered from a hydrogeologic test hole located on Cape Charles near the center of the structure, and confirmation of a similar signature in suevitic rocks would have been desirable in order to place constraints on the type of projectile involved in formation of the Chesapeake Bay crater. Unfortunately, the current data show no discernible differences in the contents of the platinum group elements (PGEs) among the suevite, the Exmore breccia, and several crystalline basement rocks, all from the Eyreville core hole. Abundances of PGEs are uniformly low (e.g., <0.1 ppb Ir), and chondrite-normalized abundance patterns are nonchondritic. These data do not allow unambiguous verification of an extraterrestrial signature. Thus, the nature of the Chesapeake Bay projectile remains ambiguous.