The Merensky Reef at Winnaarshoek, Eastern Bushveld Complex: Insights from a Wide Reef Facies

Roger Scoon1 and Andrew A. Mitchell2 1P. O. Box 2461, Rivonia 2128, South Africa 2University of Durban-Westville, Private Bag X54001, Durban, 4000, South Africa e-mail: [email protected]

Introduction related to compositional variations in the host The Merensky reef is described from the pyroxenite. This relationship suggests that controls Winnaarshoek property in the eastern limb of the on PGE mineralization are essentially primary Bushveld Complex, South Africa. The Merensky magmatic, with minor postcumulus modification by reef in the western limb of the Complex has been late intercumulus processes. The importance of extensively studied, but information on the reef chromitite layers in the mineralizing process is from the eastern limb comes primarily from the further advanced (Scoon and Teigler, 1994). original studies of Wagner (1929). Winnaarshoek provides an interesting locale for our study as the The Merensky Cycle Merensky constitutes a wide-reef facies with PGE The Merensky cycle is a sequence of mineralization distributed over a height of 1.8 m. layered cumulates that occurs near the top of the The Winnaarshoek facies differs from the widely Upper Critical zone. At Winnaarshoek, this cycle described, and relatively high grade, thin-reef comprises a 2-5 m thick layer of feldspathic Merensky reef facies at Brakspruit in the pyroxenite that is sharply overlain by a sequence of Rustenburg area of the western limb. norite-leuconorite, spotted-mottled anorthosite, and Mineralization at Brakspruit is concentrated into a mottled anorthosite. This is not a simple 0.15-0.25 m thick reef-zone that comprises a layer differentiation cycle (Scoon & Teigler, 1994). A of pegmatoidal feldspathic pyroxenite bounded by notable feature is the paucity of norite-leuconorite; two thin chromitite layers (Viljoen & Hieber, the main components of the cycle are feldspathic 1986). PGE concentrations in the Merensky reef at pyroxenite and anorthosite. Two thin chromitite Winnaarshoek form two distinct peaks separated by layers are generally present within the upper part of a barren middling. The main peak occurs within the the pyroxenite. The upper layer of chromitite is uppermost part of a layer of feldspathic pyroxenite typically located 0.4-0.5 m below the contact with associated with an upper chromitite layer. A the overlying norite-leuconorite. This chromitite is secondary peak is associated with a lower only 8-10 mm thick, but is laterally very persistent. chromitite layer. A layer of pegmatoidal The lower layer of chromitite occurs 1.8 m below feldspathic pyroxenite that occurs below the lower the upper chromitite, is 1-25 mm thick and is chromitite is only sporadically mineralized. The considerably more erratic. A layer of pegmatoidal occurrence of PGE mineralization within a reef- feldspathic pyroxenite occurs below the lower zone defined by two thin layers of chromitite is a chromitite. feature typical of the Merensky reef throughout the Complex, as is the empirical observation that the Merensky Reef wider the reef-zone the lower the mineable grade. Winnaarshoek is one of the few sites in The Merensky reef is a far more complex the Complex where the Merensky reef is readily orebody than may be realized. Cryptic layering of identifiable in outcrop; the reef, in fact, crops out the PGE mineralization presents difficulties with for over 1.5 km on the eastern flank of a low ridge optimizing mining cuts. Mineralization may be and dips gently toward the west. We also mapped developed over several metres, with the highest the Merensky reef in extensive underground grades restricted too a narrow part, or parts, of the workings that were developed during the platinum reef-zone. Cryptic layering is difficult to identify "rush" of 1925-9, and logged several hundred drill in the thin-reef facies, other than the well- core intersections. The following description is a documented association of the highest grades with synthesis of our interpretation of the “typical” chromitite layers, but at Winnaarshoek layering is Winnaarshoek facies. The reef-zone is entirely preserved on a more-or-less cm scale. We enclosed within the basal ultramafics; no demonstrate that the PGE mineralization is closely

mineralization has been recorded in the norite- base-metal sulfides. The main peak of PGE anorthosite component of the Merensky cycle. mineralization is sharply cut-off by the upper The feldspathic pyroxenite consists largely chromitite layer. The pyroxenite above the upper of medium-grained (1-30 mm) orthopyroxene chromitite, which characteristically includes diffuse crystals with intercumulus plagioclase, and exhibits segregations of pegmatoid, is only poorly both poikilitic and porphyritic textures. A mineralized, but does contain abundant base-metal prominent characteristic is the presence of sulfides. The Lower reef is typically restricted to scattered, large (10-20 mm) oikocrysts of light the lower chromitite layer but may include the green clinopyroxene. These oikocrysts are a underlying sporadically mineralized pegmatoid. feature of most of the pyroxenite layers within the The 0.8-1.3 m of pyroxenite between the Lower eastern limb, but are absent from the western limb. reef and the Upper reef is generally barren, as is the Diffuse segregations of pegmatoidal pyroxenite 2-3 m of pyroxenite below the pegmatoid. Locally occur within the pyroxenite, most notably above the entire sequence between the upper and lower the upper chromitite layer. chromitite layers is mineralized, thus forming a Two mineralization peaks are identified, much thicker, composite reef. Locally, the Lower the Upper reef and the Lower reef (Fig. 1). The reef may be richer than the Upper reef. Variations richest mineralization is found in the Upper reef from the norm may be due to proximity to potholes, (colloquially referred to as "top-loading", a particularly in sections where the pyroxenite is characteristic feature of the Merensky reef, considerably thinned. A regional facies change including the thin-reef facies). The Upper reef is also occurs proximal to a large dunite pipe in the typically 0.5-1 m thick and is rich in disseminated northern part of Winnaarshoek.

Figure 1. Whole-rock PGE values and mineral chemistry of plagioclase, chromite and orthopyroxene in the Merensky Reef in borehole WH2. Diagonal hatching on stratigraphic column indicates medium-grained feldspathic pyroxenite, fine cross- hatching pegmatoidal feldspathic pyroxenite, and no ornamentation leuconorite. The lower layer of pegmatoid is laterally contiguous, but the upper layer is patchy and discontinuous. Position of chromitite layers indicated by “Cr”. PGE values are the sum of Pt, Pd, Rh and Au, and Mg# is the Mg-number (Mg/(Mg+Fe2+).

Principal differences between the absence of PGE values from the latter. There is a Merensky reef-zone at Winnaarshoek and that at slight inflection in the Mg-number of the Brakspruit are as follows: (a) the relatively wide orthopyroxene across the boundary between units C separation of the chromitites results in the PGE and D, but wide within-sample compositional mineralization forming two discrete layers. (b) ranges prohibits identification of an equivalent PGE mineralization is situated some several metres inflection in plagioclase and chromite. There is an above the base of the Merensky cycle, whereas at apparent trend of reversed fractionation in all three Brakspruit it occurs at the base. (c) PGEs occur in a minerals in unit D. Unit F has relatively medium-grained pyroxenite and the pegmatoidal fractionated orthopyroxene compositions, pyroxenite is only sporadically mineralized. comparable with those below the lower chromitite rather than with the overlying units. Mineral Chemistry and PGE Distribution All samples discussed here were taken Discussion from borehole WH2. The reef was intersected at a There is no consensus on the origin of the depth of 50 m proximal to the old underground Merensky reef. Data presented here are compatible workings in an area where we have good geological with a magmatic origin for the PGE mineralization. control. Compositional variations in chromite, Scoon & Teigler (1994) interpreted height-related orthopyroxene and plagioclase, determined by trends in the PGE-bearing, sulfide-poor, chromitites electron microprobe analysis, are compared with of the Bushveld Complex (LG1 through UG1) as whole-rock PGE contents (Fig. 1). The most evidence of repeated replenishment of a primitive chromite compositions occur in the upper differentiating magma chamber. The UG2 and and lower chromitite layers, but the highest Cr2O3 Merensky reefs are thought to be a continuum of contents are encountered in disseminated chromite this process, with the higher concentrations of PGE grains within the pyroxenite. Orthopyroxene and attributed to episodic triggering of S-saturation. plagioclase compositions, particularly as regard the Cm-scale layering in the Merensky reef- Mg-number and An/(An+Ab) ratios, respectively, zone at Winnaarshoek suggests recognition of display cyclicity. Based on a combination of PGE, multiple replenishment and magma mixing chromite, orthopyroxene and plagioclase processes. Differentiation trends within the compositional systematics, the pyroxenite between pyroxenite suggest replenishment occurred as the upper and lower chromitites has been divided sequential intrusions of small batches of magma. into a series of units. These divisions are not rigid, PGEs were primarily concentrated into two layers, and should be viewed specifically as an aid to the the lower chromitite, and the upper chromitite description of the Merensky reef succession in together with units A-C of the pyroxenite. Sulfur borehole WH2. The uppermost three units, together saturation and PGE mineralization were triggered with the upper chromitite, form the Upper reef. relatively high in the Merensky reef-zone, as The lowermost three units form the barren lower compared with the thin-reef facies at Brakspruit. part of the pyroxenite between the Upper and Formation of both chromitite layers and/or PGE Lower reefs. mineralization is ascribed to magma mixing due to The uppermost unit (A) carries high PGE repeated replenishment. values and chromite and plagioclase compositions Downward percolation of dense PGE-rich are demonstrably more primitive here than in the sulfide melts within the reef-zone has in part upper part of the underlying unit (B). Units B and obliterated primary trends. Remobilization of C carry significant PGE values, with the latter PGE-rich sulfide melts may have in part been being particularly richly mineralized, and report the caused by additional heat due to repeated most primitive compositions of chromite, replenishment. The distinction between thin-reef orthopyroxene and plagioclase in the succession and wide-reef facies is dependent on separation of between the chromitite layers. An upward the chromitite layers (timing of magma differentiation trend is identified in unit B, but unit replenishment). The width of the Lower and Upper C reports a reversal with the Mg-number of the reefs in a wide-reef facies is in part related to the orthopyroxene increasing upward. These cycles are degree of remobilization. superimposed on otherwise relatively uniform orthopyroxene compositions in the overlying and Summary underlying units. Cm-scale layering within the Merensky reef-zone The most conspicuous difference between in the WH2 borehole succession has a complex units A-C and the underlying units D-F is the

origin and some 11 different layers or units are (d) The lower chromitite layer is typically richly recognized. mineralized. (a) The pyroxenite above the upper chromitite (e) The layer of pegmatoidal feldspathic layer, although poorly mineralized, contains pyroxenite is sporadically mineralized. abundant base-metal sulfides and segregations (f) The footwall pyroxenite is generally barren. of pegmatoidal feldspathic pyroxenite. (b) The upper chromitite layer is laterally References contiguous and contains moderate PGE values. Scoon, R. N. and Teigler, B., 1994, Platinum-group (c) The pyroxenite between the two chromitite element mineralization in the Critical Zone layers is subdivided into six units: of the western Bushveld Complex: I. (i) Unit A, with high PGE values, can be Sulfide-poor chromitites below the UG-2: distinguished from the underlying unit ECONOMIC GEOLOGY, v. 89, p. 1094-1121. B in terms of chromite and Viljoen, M.J. and Hieber, R., 1986, The Rustenburg plagioclase compositions. Section of Rustenburg Platinum Mines (ii) Unit B has moderate PGE values that Limited, with reference to the Merensky are accompanied by an upward Reef. in: Anhaeusser, C. R. and Maske, S. differentiation trend in the Mg- (eds.), Mineral Deposits of Southern number of orthopyroxene. Africa, v. II, Geological Society of South (iii) Unit C contains high PGE values that Africa, p. 1107-1134. are accompanied by a reversed Wagner, P.A., 1929. The Platinum Deposits and differentiation trend in the Mg- Mines of South Africa: Edinburgh, Oliver number of orthopyroxene. & Boyd, 326p. (iv) Units D-F carry no PGEs.