Geochemical and Petrologic Evolution of Chromitites and Other Platinum Group Element-Rich Rocks From Study of the UG2 Layer, Bushvel Complex

American Museum Natural History, New York NY

Investigators

Abstract

Mathez EAR-0106750 The Bushveld Complex and other large layered intrusions have experienced a long and complex evolution following crystal accumulation and while the rocks were still partially molten. During this time the textures, compositions, and in some cases lithologies of the rocks were modified. In the Bushveld Complex this history bears on the fundamental geochemistry of the platinum group elements (PGEs) because the PGE-rich layers, namely the Merensky reef and the UG2 chromitite, exhibit evidence for extensive post-accumulation modification. The objectives of this proposal are to understand the dynamic and chemical evolution of the partially molten rocks and how the PGEs were redistributed during this evolution. The meter-thick UG2 chromitite possesses internal stratigraphic variations in texture and PGE contents. Geochemical profiles for PGEs and other elements will be produced in carefully mapped underground sections. Data on crystal size, shape, and mode will be collected to understand how the textural and geochemical variations are related and when and how they formed. The interstitial silicate minerals will be analyzed for rare earth elements to deduce the role of interstitial melt in textural and geochemical modification. Chromite grains contain inclusions of Os-bearing minerals. Their Os isotopic compositions will be measured and associated with the textural analysis to establish when and how the inclusions formed. Mapping in the mine revealed that the UG2 is associated with several complex rock layers. These will be studied to determine their origins and gain insight into the emplacement of the UG2 and its possible role as a permeability barrier to the flow of interstitial melt and fluid. The Merensky reef is believed to mark the introduction of new magma into an extant magma chamber. The complex stratigraphic profiles of Sr isotopic ratios associated with this layer are inconsistent with simple magma mixing and may have developed later, when the rocks were partially molten and compacting. A Pb isotopic study featuring analysis of individual Pb-bearing minerals will be conducted to gain insight into the geochemical development of the Merensky reef.