On the 16th March 2015, the first meeting of the Bushveld Research Group of DST-NRF CIMERA took place in the School of Geosciences at the University of the Witwatersrand (Wits). Sixteen participants attended from Wits and from the University of Johannesburg (UJ) with one member of the team listening in on a Skype link from the UK. 

On the 16th March 2015, the first meeting of the Bushveld Research Group of DST-NRF CIMERA took place in the School of Geosciences at the University of the Witwatersrand (Wits). Sixteen participants attended from Wits and from the University of Johannesburg (UJ) with one member of the team listening in on a Skype link from the UK.

Research rationale and aims 
Within the Bushveld Complex, there is a remarkably persistent variation in modal, textural, geochemical and isotopic composition of the cumulate rocks. Spectacular layering occurs on scales from centimetres to hundreds of metres but in some cases the cyclicity is only apparent in detailed geochemical and geophysical measurements. For many years, layering has been explained in terms of cumulus processes where layers formed by gravitational sorting of crystallizing minerals. However, sorting by density alone does not account for the abundant evidence for reversals in mineral compositions, rock densities and initial isotopic compositions. It is now accepted that the Bushveld Complex has been constructed by the multiple emplacement of magmas that differ in crystal/liquid ratios, bulk compositions, and isotopic signatures. Subsequent cumulate layering appears to be a blend of these varying magmas, but it is unclear where this blending occurred, whether in a deep staging chamber, during remobilization /transfer in a conduit, or during emplacement into the resident magma chamber. Although research has been undertaken for nearly 100 years, much still remains to be understood on the sources of the magmas, the tectonic setting of magma emplacement, what may have contaminated the magmas and where this has occurred, and the variability of the mineralisation on a Complex-wide scale.

In order to address these questions, the multidisciplinary team will collaborate on a Complex-wide approach to address some of these issues and to integrate this new research into a more holistic model for the Bushveld Complex as a whole. The research aims to use innovative petrological, geochemical and geophysical techniques to understand magma dynamics in large chambers, the origin of layering, and the implications for mineralization. Integrated use of geophysical techniques, with structural interpretations, will be linked with isotopic studies and age dating together with innovative approaches to the study of processes of mineralization and the variability of the ore assemblages from different parts of the Complex.

Grant Bybee welcomed everyone to the first meeting of this research team and explained that the purpose was to share ideas and plans in the proposed research through DST-NRF CIMERA and to enhance collaboration between participants. Each member of the team was asked to prepare a short Powerpoint presentation so that everyone would know what was being undertaken by various people. Those attending were:

Judith Kinnaird Mineralisation and contamination of the Bushveld Complex
Whilst it has been known for many years that there were a number of magmas that contributed to the Bushveld Complex, more recently it has become more apparent that mixing between a new and residual magma has been very important in the formation of the Merensky Reef, the chromitite layers and the Platreef. Also, in many cases there has been contamination of these magmas, but more needs to be understood as to what the contaminants were, and where the magma was contaminated. For the Platreef, the addition of sedimentary-derived sulphur from the Duitschland Formation or Malmani dolomite may have been important for PGE mineralisation where magma contamination by footwall or roof rocks occurred. Ongoing and future work will focus on the Waterberg platinum deposit, the Platreef, Nkomati mineralisation and comparisons between the PGM assemblage in normal and potholed Merensky Reef around the Complex.

Marina Yudovskaya New research directions of mineralization and contamination 
Recent published papers have been on a trace-element study and age dating of zircon from chromitites from the Bushveld Complex (Mineralogy and Petrology), petrogenesis of the Lower Zone olivine-rich cumulates beneath the Platreef and their correlation with recognized occurrences in the Bushveld Complex (Economic Geology) and on the role of magmatic and fluid concentrations in the formation of platinum mineralization in the Lower Zone and Platreef (Geology of Ore Deposits). Work completed but not yet published is on combined trace element, U-Pb and Lu-Hf isotope data for zircon from ultramafic and chromititic horizons hosted in the Lower and Critical Zones and the Platreef as well as for acid rocks of the roof of the Complex. The difficulty in this paper is that the εHf is very negative – which is typical of crust, yet everything else is magmatic. The negative Lu-Hf is therefore characterising a very unusual source. Age-dating of zircons in the Bushveld granitic rocks are inherited with ages of 2.8-2.9 Ga. It has been suggested by Matheis that the sub-continental lithosphere (SCLM) with its negative Lu-Hf was the source for the Bushveld and also Palaborwa complex. However, Jan Kramers commented that Os and Sr in these magmas were quite radiogenic so this doesn’t fit with an SCLM source. A potential source for the Os is from black shales. Research on mineralisation in the Platreef and Waterberg platinum project is ongoing.

Ben Hayes Probing magma chamber dynamics in the Bushveld Complex
As a newly arrived postdoctoral researcher, the questions to be addressed with Lew Ashwal and Grant Bybee are: Was the new magma(s) liquids or crystal slurries? What was their role in the formation of cyclic layering? Of particular interest is how cyclic layering developed in large layered mafic intrusions, such as the Bushveld Complex. There is considerable evidence from thin, quickly-cooled, layered sills, for the emplacement of crystal-laden slurries. A potential target to investigate this is within the Main Zone of the Northern Limb of the Bushveld Complex. Here the Bellevue and MO drill cores intersect much of the Upper Zone and Main Zone stratigraphy. Very detailed stratigraphic analyses of the Main Zone (from the Bellevue Core) has revealed cyclic layering, in the order of 50-200 m – with unusual trends such as upward density increases, a corresponding increase in modal clinopyroxene – AND in some cases – a corresponding increase in cpx Mg#. This suggests that magma replenishment played an important role in the development of cyclic layering in the Main Zone of the Bushveld. Further isotopic disequilibrium between coexisting orthopyroxene and plagioclase has been observed in the Main Zone indicating crystallization from discrete magmas. Therefore it is planned to investigate a cyclic package in careful detail through systematic petrography, geochemistry and isotopic profiling. 

Hannah Hughes Magma dynamics and the lithospheric sulphur “reservoir”
During this post-doctoral research together with Prof Judith Kinnaird, Dr Grant Bybee and other members of the DST-NRF CIMERA team, Dr Hannah Hughes intends to investigate populations of upper mantle sulphides, sampled from the shallowest mantle xenoliths beneath the Bushveld. The background to the project stems from work that Hannah and her colleagues are undertaking in the North Atlantic Craton, showing that the precious metal budget of magmas that interacted with the SCLM, depends on S-budget controls related to the age and nature of distinct sulphide populations. The Bushveld Complex is likely to have also had significant interaction with the SCLM and one aspect of Hannah’s work will involve investigating various sulphide populations in the shallowest portions of the Kaapvaal SCLM keel, interrogating the record of the magmatic events held in these sulphide populations and assessing how the SCLM has influenced the Bushveld magmatic event.

Fanus Viljoen Geometallurgy and the Bushveld Complex
A Minerals Liberation Analyser (MLA) housed at UJ is being used to investigate the PGM assemblage in mineralised Bushveld samples, and also this technique can be employed to investigate whether any PGM’s remain in the waste material. 

Rais Latypov Bushveld Complex Research Projects Funded by DST-NRF CIMERA and NRF 
A postdoctoral study by Ria Mukherjee will be on the petrogenesis of the Upper Group chromitites (UG 1) of the Bushveld Complex and work is currently underway at Anglo’s Sipumelele Mine. A postdoctoral study of the petrogenesis of the Pyroxenite Marker around the Bushveld Complex will be undertaken by Nicolas Meriaud. A study of the petrogenesis of the Merensky Reef and UG2 chromitite is planned by Rais Latypov and Sofya Chistyakova (NRF and CIMERA). Parameters such as the Cr:Fe ratios will be evaluated to determine proximity to magma feeder zones. In addition other linked projects funded by the NRF will be on the petrogenesis of the Lower Group chromitites (LG 6) as an MSc project by Ryan McIntosh while a postdoc project by Emma Hunt will make a study of the petrogenesis of potholes associated with the Merensky Reef. 

Sue Webb Seismic investigation of the Bushveld Complex
A lot of 3D seismics have been undertaken on potholed reef. Perhaps an initial step in viewing the whole Complex is to ask industry to share any geophysical data with CIMERA researchers.

Jan Kramers New isotopic studies on the Bushveld Complex
It is suggested that it would be useful to investigate the Ne isotopic profile for the Bushveld Complex, as this has not been done so far. Ancient deep mantle is close to solar ratios, and such studies would contribute to the investigation of the source of the Bushveld magmas.

The team decided that we should have meetings twice a year to present an update of progress. 

Attendees at the first Bushveld Research Group meeting: (from left to right) Grant Bybee, Rais Latypov, Sofya Chistyakova, Matthew McCreesh, Jan Kramers, Judith Kinnaird, Florian Huthmann, Bertus Smith, Marlina Elburg, Ben Hayes, Paul Nex, Sue Webb, Lew Ashwal, Marina Yudovskaya and Fanus Viljoen.