The origin of discordant Fe-rich wehrlite bodies and the role of metasomatism and liquid immiscibility in the evolution of the Bushveld Igneous Complex and its platinum deposits
Final Report Abstract
The objective of the project was to study enigmatic discordant bodies of very iron-rich and coarse-grained pyroxenite and wehrlite that cut through and replace layered rock series of the Bushveld Complex in South Africa. The bodies, which are called iron-rich ultramafic pegmatite (IRUP), are distinct from other types of discordant bodies of the Bushveld Complex, such as dunite pipes, and have a few, if any, analogues in other layered intrusions. Unlike dunite pipes, IRUPs have low contents of platinum-group elements and destroy economic platinum mineralisation when platiniferous layers of the Bushveld Complex are replaced by IRUP. Field relationships of the discordantbodies with ambient layered rocksof the normal stratigraphy were studied in excellent exposures in the open pit at the Marikana mine in south-western Bushveld. The exposures enabled to trace the 3D shapes of the IRUP bodiesand observe different types of contacts of layered rocks: sharp and more gradational ones. Previous active mining in the open pit also provided possibility to collect fresh IRUP samples, which had not been affected by surface weathering. The samples were used for comprehensive petrographic, mineralogical and geochemical studies. Chemical analyses of rock-forming minerals showed high Ca concentrations (up to An93) in interstitial plagioclase from inner parts of the IRUP bodies and this rules out high alkali content of the IRUP parental magma previously proposed by some researches. Trace element analyses of IRUP minerals helped to put further constraints on geochemical characteristics of the IRUP parental magma and mass balance of reactions between the magma and layered rocks of the Upper Critical Zone (UCZ) of the Bushveld Complex, which host the IRUPs. Major and trace element analyses of the minerals in UCZ layered rocks revealed signs of significant alkali migration, presumably due to mineral-melt reactions and chemical gradients at contacts with chromite-rich layers. Experimental and melt inclusions studies were important parts of the project. Study of crystallised multiphase inclusions in apatite provided new evidence for silicate liquid immiscibility in the Upper Zone of the Bushveld Complex. Experimental studies confirmed stable silicate liquid immiscibility at temperature up to 1200 °C and generation of extremely Fe-rich melts by partial melting of gabbroic rocks. In general, the results of the project appear to support the view that the IRUP parental melt is likely to form by fractional crystallisation of magma saturated in aqueous fluid. Although there is new evidence for Fe-rich melts formed by liquid immiscibility in the Upper Zone of the Bushveld complex, percolation of that melt downwards through many hundreds meters of layered cumulates appears problematic.
Publications
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(2015) Electrochemical processes in a crystal mush: cyclic units in the Upper Critical Zone of the Bushveld Complex, South Africa. Journal of Petrology, 56: 1229-1250
Veksler I.V., Reid D.L., Dulski P., Keiding J.K., Schannor M., Lutz H., Trumbull R.B.
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(2015) Experimental confirmation of high-temperature silicate liquid immiscibility in multicomponent ferrobasaltic systems. American Mineralogist, 100: 1304-1307
Hou T., Veksler I.V.
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(2015) Silicate liquid immiscibility in layered intrusions. In: B. Charlier, O. Namur, R. Latypov, C. Tegner (eds.), Layered Intrusions, Springer Geology, p. 229-258
Veksler I.V., Charlier B.
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(2016) Immiscible iron- and silica-rich liquids in the Upper Zone of theBushveld Complex. Earth and Planetary Science Letters, 443: 108-117
Fischer L.A., Wang M., Charlier B., Namur O., Roberts R.J., Veksler I.V., Cawthorn R.G., Holtz F.
