Age and Parental Magma of the Uitkomst Complex, South Africa

De Waal* Sybrand A., Christoph D. K. Gauert*, Richard A. Armstrong^$ and Wolfgang D. Maier*
^{*Department of Geology, University of Pretoria, Pretoria 0002, South Africa; sadw@scientia.up.ac.za. ^$Research School of Earth Sciences, The Australian National University, Canberra, Act 0200, Australia; prise.armstrong@anu.edu.au.

The Uitkomst Complex, a Ni-Cu-PGE-Cr-mineralised layered basic intrusion situated approximately 200 km due east of Pretoria, South Africa, has a concordant zircon 207Pb/206Pb age of 2044(+/-8)Ma. Some non-zero lead loss is indicated and a Monte Carlo simulation yields a discordia intercept at 2055(+45/-17)Ma. These data confirm that the Uitkomst intrusion is coeval with the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Chemical modeling, using simple summation and least squares optimization techniques, shows that the high Mg andesitic Bushveld B1 magma is parental to the conduit as well as the closed system stages of the Uitkomst Complex. The average cumulate of the conduit stage consists of 69(+/-5) percent olivine, 2(+/-1.7) per cent chromite, 29(+/-6) per cent trapped B1 magma and minor (<0.3 percent) sulphide. It is proposed here that the Uitkomst Complex was emplaced along a dilatational opening that formed under brittle shear conditions at a depth of about 8+ km. Early gabbroic magma(Bu)filled the proto-chamber and chilled against the relatively cool wall rocks, but was soon washed away by B1 magma, from which the bulk of the Complex crystallized. The identification of the B1 magma as parental to the Uitkomst Complex has significance in the exploration for similarly mineralized sub-RLS intrusives.}