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The Merensky Reef at Winnaarshoek, Eastern Bushveld Complex: A Primary Magmatic Hypothesis Based on a Wide Reef Facies

Andrew A. Mitchell^1, and Roger N. Scoon²

¹ School of Geological Sciences, University of KwaZulu-Natal, Private Bag X54001, Durban, 4000, South Africa
² Postnet Suite 291, Private Bag X31, Knysna 6570, South Africa

Corresponding author: e-mail, mitchaa{at}mweb.co.za

The Merensky reef on the Winnaarshoek property, in the central sector of the eastern limb of the Bushveld Complex, belongs to what may be described as a wide-reef facies, in which platinum group element (PGE) mineralization is largely contained within a relatively thick succession of medium-grained, nonpegmatoidal feldspathic orthopyroxenite, bounded top and bottom by thin chromite stringers. The wide-reef Merensky reef facies at Winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb, especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite. At Winnaarshoek, a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer, and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer. In contrast to most occurrences in the western limb, however, both these pegmatoidal orthopyroxenites are only sporadically mineralized, if at all. In addition, rather than being at the base of a feldspathic orthopyroxenite unit, as is the case throughout most of the western limb, the Merensky reef at Winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite.

Despite the gross relative uniformity of the Merensky reef on a property-wide scale at Winnaarshoek, we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution. Our study is based primarily on whole-rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially "normal" (i.e., nonpothole) reef. One of the drill core intersections represents "typical" reef, in which PGE mineralization forms an upper reef, extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite, and a much thinner lower reef centered on the lower chromitite stringer. The other two drill core intersections represent extremes in terms of the vertical distribution of the PGE, one having mineralization confined to the immediate vicinity of the two chromitite stringers, while the other is richly mineralized throughout the reef zone. Peaks in PGE concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene, plagioclase, and chromite, the dominant rock-forming minerals, whereas the cumulate layers below major PGE peaks are the most evolved, as indicated by mineral compositions and elevated whole-rock incompatible element concentrations. Furthermore, the lowest grade Merensky reef intersection has the most chemically evolved footwall, whereas the richest intersection has a relatively primitive footwall.

We consider the origin of the Merensky reef in the context of its position as the culmination of a sequence of several PGE-mineralized, chromite-bearing layers in the Upper Critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next, and the overall cyclicity, are indicative of complex magma replenishment and mixing processes in the chamber. We apply a model of lateral streaming of magma, with chromite crystallization and consequent sulfide and platinum group mineral (PGM) precipitation resulting from magma mixing. Specifically, the Merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the Upper Critical zone. Genetic models also need to account for lateral facies variations in the reef.