Drilling the Bushveld Complex- the world's largest layered mafic intrusion

The fact that surprising new discoveries can be made in layered mafic intrusions (e.g., subtle 100-150 m cyclicity in apparently homogeneous cumulates over 1000s of m) means that we are still in the first-order characterization phase of understanding these objects. Accordingly, we have secured funding from ICDP for a planning workshop to be held in Johannesburg in early 2014, aimed at scientific drilling of the Bushveld Complex, the world's largest layered mafic intrusion. Science objectives include, but are not limited to: 1. Magma chamber processes & melt evolution. How many melts/magmas/mushes were involved, what were their compositions and how did they interact? What, if anything, is missing from the Complex, and where did it go? Did Bushveld magmatism have an effect upon Earth's atmosphere at 2 Ga? 2. Crust-mantle interactions & origin of Bushveld granitoids. Are Bushveld granites & rhyolites crustal melts, differentiates from the mafic magmas or products of immiscibility? How can the evolved isotopic signatures in the mafic rocks (e.g., epsilon Nd to -8) be understood? 3. Origin of ore deposits. What were the relative roles of gravity settling, magma mixing, immiscibility and hydrothermal fluid transport in producing the PGE, Cr and V deposits? We have identified 3 potential drilling targets representing a total of ~12 km of drill core. Exact locations of drill sites are to be discussed at the workshop. Target A- East-Central Bushveld Complex. We propose 3 overlapping 3 km boreholes that will provide the first roof-to-floor continuous coverage of the Rustenburg Layered Suite. These boreholes will represent a curated, internationally available reference collection of Bushveld material for present and future research. Target B- Southeastern Bushveld Complex. We propose a single borehole of ~2 km depth, collared in Rooiberg felsite, and positioned to intersect the Roof Zone, Upper Zone, Main Zone and floor of the Complex. Amongst other things, this site will evaluate the latest differentiates of the Complex: are they represented by felsic Rooiberg lavas (Van Tongeren & Mathez, 2012 J. Petrol. 51, 1891), or a newly-discovered sequence of hornblende quartz monzonites (Cawthorn, 2013, J. Petrol., in press)? Target C- Northern Lobe of Bushveld Complex. We propose 2 shorter ~500 m boreholes sited to (a) fill in missing stratigraphy of 2 existing deep cores (Bellevue & Moordkopje) including the unique 200 m thick troctolitic horizon in the Main Zone, and (b) to study the transgressive Main Zone - Upper Zone boundary several km north of the Bellevue/Moordkopje sites. All boreholes are intended to provide continuous down-hole geophysical measurements including magnetic susceptibility, density, electrical conductivity and radiometric data. There is also potential to obtain deep fluids and gases for biogeochemical and other studies. All interested geoscientists are welcome to attend the workshop. Watch the ICDP website for announcements (www.icdp-online.org).