Economic Geology Vol. 97, 2002, pp. 1037–1049
A New Occurrence of Merensky Reef on the Flanks of the Zaaikloof Dome, Northeastern Bushveld Complex: Relationship between Diapirism and Magma Replenishment ROGER N. SCOON† P.O. Box 2461, Rivonia 2128, South Africa
Abstract Exploration for platinum-group elements has resulted in the discovery of an occurrence of Merensky reef in the eastern limb of the Bushveld Complex, South Africa. The new discovery was made on the Dwarsrand property in the poorly known boundary area between the intensively explored western and central sectors of the eastern limb. This boundary area is largely composed of domes and basins that may, in part, be aligned along a regional lineament, the Wonderkop fault. Transvaal Supergroup metasediments in the core parts of the domes form prominent hills, whereas the Bushveld Complex in the intervening basins is restricted to areas of poor outcrop. The formation of domes is attributed to diapirism triggered by gravitational loading and heating of the Transvaal Supergroup in response to intrusion of the Bushveld Complex. Merensky reef was located beneath cover rocks in an area where the Critical and Main zones are uplifted and attenuated against the southern flank of the Zaaikloof dome. Drill holes, although collared in Upper zone, intersected the Critical and Main zones at relatively shallow depths. As a result of the uplift, the Critical and Main zones, including the Merensky reef, are steeply dipping. The base of the Upper zone forms an intrusive unconformity and the layering in this zone has a much shallower dip. Economically significant grades were obtained on the Dwarsrand property, but exploration was terminated because of poor strike continuity. The new findings suggest that uplift and attenuation of the Bushveld Complex on the flanks of domes in the northeastern part of the Complex are both more extensive and more severe than previously realized. A complex history of intrusion and uplift is implied. Some domes even penetrate into the overlying Bushveld granite. The occurrence of markedly angular unconformities may be contrasted with the layercake sequences typically associated with the Bushveld Complex. Diapirism, in response to crustal loading and heating by ultramafic sequences, occurred in the early stages of magma emplacement. Episodic intrusion and uplift are consistent with multiple replenishment models, in which each of the many dozens of cycles that comprise the Critical and Main zones corresponds to a new magma pulse, with the oldest layers being dragged to higher structural levels in response to the uplift. Periodic reactivation of doming is indicated by the lowermost layers within the Critical and Main zones abutting against the southern flank of the Zaaikloof dome at progressively higher levels. The transgressive nature of the Upper zone and Bushveld granite, relative to the Critical and Main zones, may indicate that the Upper zone formed by new intrusions of magma rather than the generally accepted model of differentiation within the chamber. The discovery of “hidden” Merensky reef at relatively shallow depths beneath transgressive Upper zone and granite could provide the impetus for further exploration possibilities within the Bushveld Complex, particularly on the flanks of regionally extensive domal structures.
Introduction EXPLORATION for platinum-group elements (PGE) has resulted in discovery of an occurrence of Merensky reef in the northeastern part of the Bushveld Complex, South Africa. The exploration programs were carried out by two South African companies, Trojan Exploration and Lydenburg Exploration, in a joint venture agreement directed at locating Merensky reef in an area of the Complex that had not previously been investigated. The new discovery was made on the Dwarsrand property in the boundary area between the Atok and Messina platinum mines (Fig. 1). Whereas the three main limbs of the Bushveld Complex have been intensively explored, structurally complex boundary areas, many of which include major domal features, are poorly known. The first phase of exploration was restricted to the Dwarsrand property (Fig. 2). Merensky reef was discovered here beneath cover rocks in an uplifted sequence on the southern flank of the Zaaikloof dome. A second phase of exploration was carried out on the Zwarthoek and Fortdraai † Corresponding
authoer: e-mail,
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0361-0128/01/3271/1037-13 $6.00
properties, adjacent to Dwarsrand, and on the Goedverwacht and Nooitverwacht properties, on the eastern flank of the Malope dome. The Veeplaats property, which is located between the Zaaikloof and Malope domes, was also investigated. The northeastern part of the Bushveld Complex exhibits an unusual degree of structural complexity and a high metamorphic grade in the country rocks (Hall, 1932). Numerous domes comprising cores of Transvaal Supergroup metasediments occur in this area (Marlow and Van der Merwe, 1977; Sharpe and Chadwick, 1982). Domes are particularly prevalent along the Wonderkop fault, a regionally important lineament. Uken and Watkeys (1997) suggested that the domes in this area formed due to diapirism induced by gravitational loading and intrusion of the Bushveld Complex, with the different shapes of the domes attributed to preservation of oblique sections through variably tilted structures. This new study investigates Bushveld Complex on the flanks of several domes. Layered rocks of the Bushveld Complex may be uplifted by as much as several kilometers on the flanks of the larger domes, a feature that was first recognized by Marlow and Van der Merwe (1977). Some domes even
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FIG. 1. Map showing the three main limbs of the Bushveld Complex, subdivision of the eastern limb, the structural lineaments that bound these sectors (DF = Dwarsrivier fault, SF = Steelpoort fault, WF = Wonderkop fault), and the various localities referred to in the text. The Dwarsrand and associated properties are located in the boundary area between the western and central sectors of the eastern limb.
penetrate into the Bushveld granite, an indication that diapirism was a long-lived process rather than a single event. Bushveld Complex exposed on the flanks of the Zaaikloof dome was previously thought to comprise only Upper zone rocks with the economically significant Critical zone, including the Merensky reef, assumed to occur at depths >3,000 m because of the great thickness of the intervening Main zone (Fig. 3). Evidence presented below demonstrates that uplift forms unconformities within the Bushveld Complex. Merensky reef can, therefore, occur at relatively shallow depths on the flanks of domes that are enveloped by Upper zone at current exposure levels. Layered basaltic intrusions characteristically occur in stable cratonic settings, but field relations demonstrate that intrusion of the Bushveld Complex triggered considerable crustal instability (Daly, 1926). Scoon and Teigler (1994) proposed a relationship between magma replenishment and synBushveld deformation in the western limb of the Complex, and Scoon and Teigler (1995) suggested that attenuated sequences and transgressions in the Steelpoort valley area of the eastern limb are attributable to syn-Bushveld doming. The new findings suggest that uplift and attenuation of the Bushveld Complex on the flanks of domes in the northeastern part of the Complex are both more extensive and more pronounced than previously realized. The successful discovery of Merensky reef on the Dwarsrand property, in conjunction with 0361-0128/98/000/000-00 $6.00
a reappraisal of synintrusion tectonism, could provide the impetus for a new phase of exploration in the Bushveld Complex. Geological Setting Regional geology The three limbs that comprise the Bushveld Complex may be subdivided into sectors, as described by Hatton and von Gruenewaldt (1987). Sectors are subdivided into compartments that are structurally controlled with a unique stratigraphy (Scoon and Teigler, 1994). Complex boundary areas, in which the Lower, Critical, and Main zones are either partially absent or structurally disturbed, separate the sectors and compartments. The Upper zone is generally contiguous between sectors. The eastern limb consists of western, central, and southern sectors with two intervening boundary areas (Fig. 1). The geological setting of the eastern limb, with the exception of the disruptive boundary areas, is well known (Wager and Brown, 1968; Von Gruenewaldt et al., 1985), such that only a brief overview is presented here. Transvaal Supergroup Rocks of the early-Proterozoic Transvaal Supergroup, dominantly metasediments, form the floor to the Complex (Fig. 3). The uppermost formations of the Pretoria Group have been described by Button (1976). From oldest to youngest,
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FIG. 2. Map of the northeastern part of the Bushveld Complex. Based on the published Geological Survey of South Africa 1:250,000 Series, Sheet No. 2428, with some new interpretations from regional mapping, geophysical interpretations, trenching, and drill-hole results. The Wonderkop and Stofpoort faults and a chain of floor-attached domes (Katkloof, Zaaikloof, Adriaanskop, and Malope) demarcate the boundary area between the western and central sectors. The Olifants River cuts through this boundary area. Regional aeromagnetic data enabled the area to the north of the Zaaikloof dome to be reinterpreted despite the fact that magnetism associated with the Upper zone swamps the weakly magnetic signature of the Critical and Main zones. Also shown are properties where the exploration programs were carried out, and the approximate location of drill holes on the Veeplaats and Nooitverwacht properties. The cross section ABC is both schematic and simplified. 0361-0128/98/000/000-00 $6.00
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FIG. 3. Generalized stratigraphic column for the northeastern part of the Bushveld Complex.
these are: Silverton (hornfels and shales with the prominent Machadodorp volcanic member), Magaliesberg (quartzite), Vermont (hornfels with minor quartzite and dolomite), and Lakenvalei (micaceous quartzite with minor hornfels). In the northeastern part of the Complex, formations younger than the Lakenvalei, including the Rooiberg felsites, are absent. In this area, Magaliesberg Formation typically forms the floor to the Complex. Numerous bodies of metasediments occur displaced from the main outcrop of Transvaal Supergroup in the northeastern part of the Bushveld Complex. They may be interpreted as anticlines or domes, uplifted slabs or roof pendants, or wholly enclosed rafts or xenoliths (Fig. 2). Domes are not re0361-0128/98/000/000-00 $6.00
stricted to boundary areas, but they are particularly abundant in such areas. The Steelpoort pericline (Sharpe and Chadwick, 1982) and Eerste Geluk dome (Scoon and Teigler, 1995) are associated with the Dwarsrivier and Steelpoort faults in the boundary area between the central and southern sectors (Fig. 1). Domes associated with the boundary area between the western and central sectors are discussed below. Bushveld Complex The layered rocks of the Bushveld Complex form a 9km–thick sequence in the central sector of the eastern limb (South African Committee for Stratigraphy, 1980). Five principal zonal subdivisions are recognized: Marginal, Lower,
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Critical, Main, and Upper. In addition, the Critical zone may be subdivided into Lower and Upper parts and the Main zone into three subzones (Fig. 3). Marker layers characterize the different zones and may be laterally contiguous for hundreds of kilometers. The Marginal zone, the thickness of which varies considerably, typically consists of relatively fine-grained norite and feldspathic pyroxenite. The ultramafic Lower zone, which was first described in detail by Cameron (1978), is restricted to specific troughlike features (Hatton and von Gruenewaldt, 1987). The Lower Critical zone is entirely ultramafic (Cameron, 1980), whereas the Upper Critical zone includes both ultramafics and norite-anorthosite (Cameron, 1982). Chromitite layers occur in both the Lower and Upper Critical zones (Hatton and von Gruenewaldt, 1987), but are absent from the other zones. The Upper Critical zone hosts the main PGE-rich layers, the Merensky and UG2 reefs (Wagner, 1929). Both reefs occur at the Atok (Mossom, 1986; Lee and Butcher, 1990) and Messina platinum mines (Von Gruenewaldt et al., 1990). The Main and Upper zones have been described by, among others, Von Gruenewaldt (1973) and Molyneux (1974). The Main zone is characterized by relatively monotonous sequences of norite (subzone A) and gabbronorite (subzones B and C). Anorthosite marker layers occur in both subzones A and B. The Upper zone contains numerous thick layers of Ti-magnetite, typically intercalated with magnetite gabbro. The roof rocks in the northeastern part of the Complex consist of younger Bushveld granites and slabs or xenoliths of Transvaal Supergroup metasediments. Physiography The interior of South Africa has been exposed to terrestrial erosion since commencement of the Cenozoic era. This, together with continental uplift and development of the Great Escarpment (Du Toit, 1933; Burke, 1996), has resulted in a remarkable relationship between geology and topography in the northeastern part of the Complex (Hall, 1932). The Transvaal Supergroup metasediments form the rugged Strydpoortberge with elevations of over 2,000 m (Fig. 1). The Lower and Critical zones, together with subzone A of the Main zone, crop out in a well defined valley, part of the Olifants, Motse, and Steelpoort river systems, albeit with prominent ridges. Subzones B and C of the Main zone, together with the Upper zone, comprise the Leolo Mountains, a 150-km–long escarpment. The Bushveld granite caps the flat-lying country on the interior of the escarpment. Geology of the Boundary Area between the Western and Central Sectors of the Eastern Limb The boundary area between the western and central sectors of the eastern limb consists of heavily dissected topography with rugged ridges and hills rising several hundreds of meters above the peneplane. The peneplane has an average elevation of 800 m. The course of the Olifants River approximates to the Wonderkop fault, but forms a deeply incised valley to the east of the Zaaikloof dome. The core parts of the domes in this area are invariably associated with positive topography, with alternating quartzite (with subordinate hornfels) and metashales forming scarp and dip faces. Rocks of the Bushveld Complex underlie near-featureless plains. These plains lie at a similar elevation to the African peneplane (Partridge and 0361-0128/98/000/000-00 $6.00
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Maud, 1987). Karoo Supergroup and Basement rocks underlie the peneplane to the west and north of the Bushveld Complex. The paucity of outcrop of the Bushveld Complex, particularly of the Main zone, is in marked contrast to the mountainous country east of the Olifants River. Main zone rocks within the western sector are also associated with positive topography. Formation of a new erosional surface within the boundary area has been caused by incision of the Olifants River as it cuts into the Bushveld Complex, including Main zone rocks. Four large domes that are, in part, associated with a major structural lineament, the Wonderkop fault, are prominent in this area (Fig. 2). Marlow and Van der Merwe (1977) described the complex relationships between quartzite and Bushveld Complex at the Malope dome. Sharpe and Chadwick (1982) described the Adriaanskop, Malope, and Zaaikloof domes. The southern extension of the Wonderkop fault transects another structurally disturbed area, the most prominent feature of which is the Marble Hall dome (De Waal, 1970). The high grade of metamorphism in the Bushveld aureole in this area, as indicated by cordierite-sillimanite-biotite schist and gneiss, is notable (Kynaston, 1906; Hall, 1932). Prominent outcrops of black, crystalline hornfels within the core of the Zaaikloof dome are metashales of the Silverton Formation that, in the central sector, less than 20 km to the east, have a much lower metamorphic grade. The only published geological map of the area (Geological Survey of South Africa 1:250,000 Series, Sheet No. 2428) depicts the Zaaikloof dome as comprising an anticline of Transvaal Supergroup metasediments. Reinterpretation of the Zaaikloof structure as a dome was crucial to the exploration program. The domal form is readily seen on a satellite image (Fig. 4). The near-circular outcrop of the Machadodorp volcanic member supports the new interpretation (compare Fig. 2 and Fig. 4). Quartzite and hornfels of the Magaliesberg and Vermont Formations dip at approximately 60° away from the core on the southern and eastern flanks, forming prominent ridges some 100 to 200 m above the level of the peneplane. Quartzite of the Lakenvalei Formation forms an additional scarp and dip face in the southeastern part of the Zaaikloof dome. The restricted occurrence of the Lakenvalei Formation is an indication that the dome plunges toward the northwest. A steep ridge composed of subvertical Magaliesberg Formation forms the northern edge of the dome, and a calcrete-covered plain bounds the dome on the northern side. The western edge of the dome is also poorly exposed, as it is cut by the Wonderkop fault. Upper zone rocks on the southern flank of the Zaaikloof dome exhibit the regional southerly dip. Main zone rocks on the eastern flank, however, have a localized north-south strike that has not been previously recorded. The Main zone at this location is unusually thick, possibly due to fold repetition. Bushveld Complex is not exposed on the northern flank of the Zaaikloof dome. The area bounded by the Wonderkop and Stofpoort faults between the Zaaikloof and Katkloof domes has been reinterpreted as a basin with a core of Upper zone enclosed by Main zone (schematic section, Fig. 2). The outlier of Upper zone that crops out to the
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FIG. 4. Satellite Pour l’Observation de la Terre (SPOT) image of the boundary area between the western and central sectors of the eastern Bushveld Complex. Abbreviations: AD = Adriaanskop dome, KD = Katkloof dome, MD = Malope dome, WF = Wonderkop fault, ZD = Zaaikloof dome.
east of the Stofpoort fault supports this hypothesis, as does the presence of two small outcrops of magnetite gabbro within the fault block. Schwellnus et al. (1962) reported the occurrence of chromitite close to the floor contact on the Fortdraai property (Fig. 2), and new mapping has confirmed that the chromitite occurs as layers (not pods or xenoliths) associated with feldspathic pyroxenite. Two layers can be traced along strike for several hundred meters from the Fortdraai property onto the Zwarthoek property (Fig. 5). The association of chromitite and pyroxenite is diagnostic of the Critical zone. Electron microprobe analysis of chromitite grains from the two layers yielded an average Cr/Fe ratio of 1.44, and the whole-rock PGE content for both layers was determined to be
