The mineralogy and mineral associations of platinum-group elements and precious metals in the Aurora Cu-Ni-Au-PGE deposit, Northern Limb, Bushveld Complex

Aurora is a platinum-group element (PGE) prospect hosted in the Northern Limb of the Bushveld Complex, South Africa. It is one of only three deposits discovered in the Northern Limb so far to be hosted in the melanocratic-leucocratic gabbroic cumulates of the Main Zone of the Rustenberg Layered Suite (Aurora, Moorddrift and Waterberg T Zone deposits), rather than in predominantly ultramafic rocks (e.g. Platreef). The host cumulates at Aurora have been divided into three principal units and they intrude the dolomites of the lower Transvaal Supergroup. Base metal sulphide (BMS) mineralisation with PGE is present in the leucogabbronorites and gabbronorites of Unit 2, and in coarse grained gabbronorite veins which intrude the peridotites of Unit 1. These veins contain up to 50% interstitial pyrrhotite-pentlandite-chalcopyrite ± pyrite. Unit 2 contains 1–3% pentlandite-pyrrhotite-chalcopyrite assemblages, and 1–5% chalcopyrite ± pyrite/pyrrhotite associated with hydrothermal alteration. The PGE content of Aurora however is predominantly controlled by the presence of platinum-group minerals (PGM), not BMS. LA-ICP-MS analysis of sulphides shows the BMS in Aurora have lower PGE concentrations than other Bushveld magmatic sulphides, with pentlandite carrying much lower concentrations of Pd (average 23 ppm) than the Platreef or the Merensky Reef. SEM-EDS analysis of 26 sections characterised 995 platinum-group minerals (PGM) and precious metal-bearing minerals (PMM), with a total area of 27850 μm2 and an average size of 28.2 μm2. Of the PGM and PMM identified in Aurora 85% (by area) are Pd-Te-Bi minerals, with 6% Pd-Te minerals, 4% electrum and 3% Ag-Te minerals, along with minor Pd-Bi, Pd-As, Pt-Te-Bi, Pt-As and Pt-S minerals that collectively comprise 2% of total area. Only 25% of the PGM and PMM in Aurora are BMS hosted, with the rest hosted in silicates. Of the total PGM and PMM area 22% are hosted in alteration-silicates (quartz, chlorite or actinolite) in an alteration halo around sulphides. Unusually, 52% of the PGM and PMM are spatially removed from BMS, instead hosted in alteration silicates and within cracks in primary silicates away from any BMS. This indicates a multi-stage ore genesis model, with hydrothermal remobilisation of PGE important for ore formation. The style and host rocks for mineralisation in the Aurora deposit are fundamentally different from other deposits in the Northern Limb of the Bushveld hosted in ultramafic rocks, such as the Platreef, GNPA member deposits and the F zone of the Waterberg deposit, all of which contain a greater diversity of PGM and BMS with higher precious metal contents. The mineralisation most similar to Aurora is the T Zone of the Waterberg deposit, located to the north of Aurora, which been suggested to be an along-strike equivalent of the Aurora Main Zone mineralisation. However, despite strong similarities in PGM mineralogy and S isotope signatures there are significant differences in BMS mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically. At present it seems more likely that Aurora and the Waterberg T Zone reflect similar fluid-influenced processes operating in different parts of the Main Zone, perhaps at different times and in different structural basins, rather than a continuous mineralised zone along strike

Spectral simplicity and asymptotic separation of variables

We describe a method for comparing the real analytic eigenbranches of two families of quadratic forms that degenerate as t tends to zero. One of the families is assumed to be amenable to `separation of variables' and the other one not. With certain additional assumptions, we show that if the families are asymptotic at first order as t tends to 0, then the generic spectral simplicity of the separable family implies that the eigenbranches of the second family are also generically one-dimensional. As an application, we prove that for the generic triangle (simplex) in Euclidean space (constant curvature space form) each eigenspace of the Laplacian is one-dimensional. We also show that for all but countably many t, the geodesic triangle in the hyperbolic plane with interior angles 0, t, and t, has simple spectrum.Comment: 53 pages, 2 figure

The Stukpan carbonatite in the Orange Free State Goldfield

The Stukpan Complex is interpreted to be an oval pipe-like body, 3.1 × 1.7 km in diameter, unconformably overlain by Karoo strata and a dolerite sill. One borehole penetrated an intrusive contact between fine-grained amphibole-rich calciocarbonatite and an alkaline amphibolite derived by fenitization from andesitic Ventersdorp lava. A second borehole encountered calciocarbonatite sills as well as red fenite and brecciated quartzite belonging to the Welkom Formation of the Central Rand Group. Geochemical logs show consistently high Sr contents in the carbonatite but low P, Nb, Zr, and REE. Despite erratic Cu and Ni values and sporadic sulphides no indications of economic mineralization have been found. Rb-Sr mineral separate geochronology yielded a well-constrained isochron age of 1354 ± 11 Ma for the carbonatite and confirmed a Ventersdorp (2700 Ma) age for the alkaline amphibolite. -from Author

Mesoproterozoic intraplate magmatism in the Kalahari Craton : a review

The Kalahari Craton was initially stabilized following cessation of Palaeoproterozoic orogenesis in southern Africa at ca. 1.8 Ga. Subsequent Mesoproterozoic intraplate magmatism at ca. 1.4–1.35 Ga formed a series of alkaline and carbonatitic complexes in the southern part of the craton. Original volcanic structures are partly preserved in some of the complexes, and a variety of intrusive rocks (e.g., quartz syenite, nepheline syenite, pyroxenite, ijolite, carbonatite) are present. The Premier kimberlite cluster was emplaced in the same region at ca. 1.2 Ga, but available geochronology indicates that the main alkaline magmatism preceded 1.2–1.0 Ga orogenesis in the Namaqua–Natal–Maud Belt along the southern craton margin. Another, more extensive intraplate magmatic event at ca. 1.1 Ga formed the Umkondo Igneous Province, which is recognized over an area of 2.0 · 106 km2 on the Kalahari Craton, including a detached fragment now located in Antarctica. Much of the province comprises high-level mafic intrusions, but erosional remnants of basalt lava piles and bimodal basalt/rhyolite assemblages are also present. Most of the mafic rocks are continental tholeiites, but trace-element geochemistry reveals distinct subgroups that cannot be related by crustal-level assimilation/fractional crystallization processes or by partial melting of a uniform mantle source. Geochronological and palaeomagnetic data indicate that enormous volumes of tholeiitic magma were emplaced within the province in a narrow time frame at ca. 1112–1106 Ma, which is inferred to record uprise of a mantle plume behind the Namaqua–Natal–Maud Belt