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Towards a magmatic reaction model for iron-rich ultramafic
pegmatites in the Rustenburg Layered Suite
David L. Reid 1 , I. Veksler 2 , J. Jakobsen 3 , R. Trumbull 4
1. Department of Geological Sciences, University of Cape Town, South Africa, david.reid@uct.ac.za
2. GFZ German Research Centre for Geosciences, Potsdam, Germany, veksler@gfz-potsdam.de
3. GFZ German Research Centre for Geosciences, Potsdam, Germany, jakob@gfz-potsdam.de
4. GFZ German Research Centre for Geosciences, Potsdam, Germany, bobby@gfz-potsdam.de
ABSTRACT
Iron-rich ultramafic pegmatite (IRUP) occurs as irregular bodies replacing the layered sequence associated with the
Merensky Reef and has fascinated workers with their unusual mineralogy, geochemistry and morphology. A
simple magmatic origin is precluded by the extreme iron-rich composition and the highly irregular morphology, the
latter also being inconsistent with injection as a crystal mush. Most workers have proposed some sort of
replacement origin, involving a ferrobasaltic melt reacting with cumulate assemblages to produce the range of
lithologies present in the IRUP bodies (olivine - clinopyroxene ± plagioclase ± magnetite)
Ferrobasaltic magmas are thought to be responsible for the Upper Zone of the Rustenburg Layered Suite and
certain highly differentiated melt fractions are thought to have migrated to the Critical Zone, causing replacement
and disruption of the Merensky Reef, the UG2 chromitite and elsewhere in the layered sequence exposed in the
platinum mines. Liquid immiscibility in the ferrobasaltic system is known to result in separate ultraferrous and
felsic melts, the former being perhaps capable of migrating through and reacting with country rock of variable
compositions to produce IRUP assemblages. The main question has been the composition of this ultraferrous melt
and recognition of its reaction products in the rock record.
While it is possible to characterize the starting materials (pre-exisiting layers of plagioclase - orthopyroxene ±
olivine ± chromite cumulates) and the final products (IRUP suite), the lack of recognisable introduced reactant
preserved in exposures examined requires an inductive approach. The most common lithology in the layered series
is norite (sensu lato) and shows transformation to a wehrlite, often with a transition zone of olivine-gabbro or
troctolite.
We are currently pursuing a mass balance approach to estimate the composition of the introduced ultraferrous melt
(UFM) and the residual melt (RM) that would satisfy the reaction equation below:
NORITE + UFM = OL-GABBRO + WEHRLITE + RM
Other approaches involve trying to derive possible ultraferrous melt compositions from liquid immiscibility
experiments, and studies of melt inclusions in suitable minerals.
KEYWORDS: IRUP, Merensky Reef, magmas, ultraferrous, UFM, RM
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