Development of the Platreef in the northern limb of the Bushveld Complex at Sandsloot, Mokopane District, South Africa

• Main Author: Armitage, Paul Edward Blake
• Published: University of Greenwich 2011
• Subjects: 500; QE Geology
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.571426

The Platreef is a Ni-Cu-PGE mineralised tabular body at the base of the Rustenburg Layered Suite in the northern limb of the Bushveld Complex. The reef lies unconformably on a footwall (floor) sequence of Transvaal Supergroup sedimentary rocks and Archaean granite/gneiss basement, and is overlain by a hangingwall (roof) of Main Zone gabbronorites. Structural relationships suggest that the Platreef was emplaced as a broadly horizontal sill-like sheet into the Transvaal Supergroup, but local variations in its thickness and path of intrusion were caused by pre-existing structures in the country rocks. As the Platreef cooled and was nearly crystallised, ductile deformation occurred, possibly as an episode in a longer event. Main Zone magma was emplaced above the deformed, nearly consolidated Platreef and eroded the uppermost portion, locally assimilating mineralised reef. The Main Zone magma also intruded into shear zones as thin dykes down through the Platreef and metasedimentary floor. Structural patterns around a prominent dome in the floor rocks suggest that regional deformation may still have been active when the earliest Main Zone layers were developing, but ceased by Upper Main Zone time. Other studies of the Platreef beyond Sandsloot have shown that its earliest Ni-Cu-PGE mineralisation was orthomagmatic, largely preserved where the floor rocks are unreactive basement granite/gneiss. However, interaction between he Platreef magma and surrounding sedimentary rocks has produced different mineralogical associations and assemblages that were influenced by the local floor and roof rocks along the strike of the reef. At Sandsloot, the floor rocks are represented by reactive siliceous dolomites of the Malmani Subgroup. The Platreef magma caused contact metamorphism and metasomatism of the dolomites, releasing volatiles that entered the reef. These hydrothermal fluids stripped PGE from primary sulphides and redistributed the PGE within the reef and into the metasedimentary country rocks. In places, primary platinum group minerals were overprinted by lower-temperature species.