Mineralogy and geochemistry of platinum group elements in the Aguablanca Ni-Cu deposit (SW Spain)

The Aguablanca Ni-Cu-(PGE) magmatic sulphide deposit is associated with a magmatic breccia located in the northern part of the Aguablanca gabbro (SW, Iberia). Three types of ores are present: semi-massive, disseminated, and chalcopyrite-rich veined ore. The principal ore minerals are pyrrhotite, pentlandite and chalcopyrite. A relatively abundant platinum-group mineral (PGM) assemblage is present and includes merenskyite, melonite, michenerite, moncheite and sperrylite. Moreover, concentrations of base and precious metals and micro-PIXE analyses were obtained for the three ore-types. The mineralogy and the mantlenormalised chalcophile element profiles strongly suggest that semi-massive ore represents mss crystallisation, whereas the disseminated ore represents an unfractionated sulphide liquid and the chalcopyrite-rich veined ore a Cu-rich sulphide liquid. Palladium-bearing minerals occur commonly enclosed within sulphides, indicating a magmatic origin rather than hydrothermal. The occurrences and the composition of these minerals suggest that Pd was initially dissolved in the sulphides and subsequently exsolved at low temperatures to form bismutotellurides. Negative Pt and Au anomalies in the mantle-normalised chalcophile element profiles, a lack of Cu-S correlation and textural observations (such as sperrylite losing its euhedral shape when in contact with altered minerals) suggest partial remobilisation of Pt, Au and Cu by postmagmatic hydrothermal fluids after the sulphide crystallisation

A late Archaean radiating dyke swarm as possible clue to the origin of the Bushveld Complex

The Bushveld Complex in South Africa represents the world's largest intrusion of magnesium-and iron-rich magmas. The Bushveld magmas were emplaced beneath the Transvaal basin(1) similar to 2.06 billion years ago(2,3), but their origin remains elusive. The magmas may have formed in response to an upwelling mantle plume(4), ancient subduction5 or melting triggered by a meteorite impact(6). Here we use U-Pb dating of baddeleyite crystals to date a series of mafic magmatic dykes located east of the Transvaal basin. We find that these dykes formed between 2.70 and 2.66 billion years ago, roughly 600 million years before the Bushveld magmas were emplaced. Collectively, the geometry of the dykes forms a radiating swarm converging towards a focal point in the eastern part of the Bushveld Complex. Such radiating swarms typically record the impact of a mantle plume head that injected large volumes of magma into the crust and at the base of the lithosphere. We propose that subsequent cooling and metamorphism of these mantle-plume-derived rocks caused them to increase in density and sink, triggering subsidence of the Transvaal basin. The dense rocks may later have sunk away into the mantle, with the delamination causing the inflow of hot mantle that initiated production of the voluminous Bushveld magmas about 600 million years after the mantle plume impact