South Africa - Inkaba.org

The role of footwall reconstitution in the genesis of Merensky
reef types at Northam Platinum Mine
Moses Takalani Mandiwana 1 , D. Reid 2 , C. Van Jaarsveld 3
1. Department of Geological Sciences, University of Cape Town, South Africa, mndmos001@uct.ac.za
2. Department of Geological Sciences, University of Cape Town, South Africa, david.reid@uct.ac.za
3. Northam Platinum Pty Ltd, South Africa, charl.vanjaarsveld@norplats.co.za
ABSTRACT
In spite of their variations in absolute grain size and modal mineralogy, the pegmatitic harzburgites, pyroxenites
and melanorites in the P2 and Normal Merensky reef types at Northam Platinum mine share many similarities. The
stratigraphic horizon equivalent to the P2 reef type is preserved along strike at the Normal reef elevation, where the
original P2 cyclic unit is preserved. Interaction between the P2 cyclic unit norites and the Merensky magma
resulted in replacement by the pegmatitic lithologies listed above and represents an example of footwall
reconstitution. A similar process acting to reconstitute the 4X cyclic unit to produce the Normal reef type is
therefore worthy of consideration.
This project focuses on investigating the P2 cyclic unit at both reef elevations to identify the mineral-magma
interactions that produced the mineralised pegmatitic horizons that form the footwall to the Merensky chromitite
(4A unit). Results will be utilized to gain an insight to the possible origins of the similar lithologies that now
occupy the 4X cyclic unit.
Our approach has been to examine the Normal and P2 reef exposures underground and collect samples for detailed
mineralogical and geochemical analysis. Core obtained from exploration and cover drilling has also been selected
to augment the collection with the unreplaced P2 cyclic unit. Laboratory methods include Optical Microscopy,
XRF and EPMA for comparison of textures, whole rock geochemistry and mineralogy.
KEYWORDS: Normal Merensky Reef; P2 Merensky Reef; P2 Cyclic Unit; 4X Cyclic Unit; Pegmatitic;
Reconstituted Footwall; Merensky (4A) chromitite; 4X chromitite
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