Hekpoort and Ongeluk Formations Chapter 3 Radiometric Ages of the Hekpoort and Ongeluk Formations of the Transvaal Supergroup Revisited 3.1 Introduction The Transvaal Supergroup (Fig 3.1) is a remarkably well-preserved Late Archean to Paleoproterozoic supracrustal succession that is preserved on the Kaapvaal Craton. It crops out in the Transvaal and Griqualand West areas (Fig 3.1). The lower part of the Transvaal Supergroup has been well-dated by single zircon analyses from several tuff beds from the Ghaap Group in Griqualand West and the Chuniespoort Group in the Transvaal area (Fig 3.2)(Altermann and Nelson, 1998; Martin et al., 1998; Sumner and Bowring, 1996; Trendall A.F. in Nelson et al., 1999; Gutzmer and Beukes, 1998). However, for the Pretoria and Postmasburg Groups in the upper portion of the Transvaal Supergroup (Fig 3.2), there are almost no reliable geochronological constraints. Data available on the Pretoria and Postmasburg Groups include whole rock Pb-Pb ages of 2222±12Ma and 2236±38Ma respectively for the Hekpoort lava in the Transvaal and the Ongeluk lava in Griqualand West (Cornell et al., 1996). These ages suggest that the Ongeluk and Hekpoort lavas are lateral correlatives, although the first was deposited in a submarine environment whilst the second was deposited under terrestrial conditions (Cornell et al., 1996). In contrast, Bau et al. (1999) obtained a secondary-lead age of 2394±26Ma for the Mooidraai dolomite that stratigraphically overlies the Ongeluk lava (Fig 3.2)(SACS, 1980; Beukes, 1986). Bau et al (1999), therefore, suggest the radiometric ages previously obtained for the Ongeluk lava, may date the timing of alteration of the lavas, and that the Ongeluk lava may have a primary age that is approximately 170Ma older than previously thought. This suggestion led to a proposed new correlation between the Transvaal Supergroup in the Griqualand West and Transvaal areas in which the Mooidraai Formation of the Postmasburg Group is correlated to the Duitschland Formation at the base of the Pretoria Group (Moore et al., 2001), suggesting that the Hekpoort Formation is younger and does not have an equivalent in Griqualand West. 35 Hekpoort and Ongeluk Formations 36 Hekpoort and Ongeluk Formations On the other hand, detailed lithostratigraphic studies on a regional scale (Beukes et al., 2002A, Dorland, 1999), have reconfirmed the correlation between the Hekpoort and Ongeluk Formations. Both are capped by an oxidized paleosol overlain by red beds, of the Dwaal Heuwel Formation (Transvaal) and the Gamagara Formation of the Transvaal Supergroup in Griqualand West (Holland and Beukes, 1990; Wiggering and Beukes, 1990; Beukes et al., 2002A). The correlation (Fig 3.2) is further supported by the presence of distinct d13C excursions in carbonates that occur in equivalent stratigraphic positions in the Lucknow and Silverton Formations (Fig 3.2)(Swart, 1999; Buick et al., 1998). These observations were used to reaffirm the stratigraphical correlation in which the Gamagara Formation in Griqualand West and the Dwaal Heuvel Formation in the Transvaal area are correlated, implying that the Ongeluk and Hekpoort lavas are time equivalent (Beukes et al., 2002A)(Fig 3.2). A Re-Os age of 2320 Ma obtained for diagenetic pyrite from the Timeball Hill Formation that stratigraphically underlies the Hekpoort lava (Hannah et al., 2002), defines the third radiometric age constraint that is currently available for the upper part of the Transvaal Supergroup i.e. the Pretoria Group (Fig 3.2). In recent years, the Late Archean to Paleoproterozoic Transvaal Supergroup has become the testing ground for controversially discussed concepts, namely a snowball earth event at around 2.22Ga (Kirschvink, 1992, Evans et al., 1997) and the rise of atmospheric oxygen (Holland, 1984; Ohmoto, 1996; Ohmoto, 1997; Beukes et al., 2002A; Evans et al., 2002). These two hypotheses are linked to sedimentary units that are intimately associated with the Ongeluk and Hekpoort lavas. Similarly, the Hotazel Formation that is the host of the giant Kalahari manganese field conformably overlies the Ongeluk Formation. In addition, in many Paleoproterozoic successions of the world, the age of 2.2Ga for flood basalts and dolerite dyke swarms [Pilbara craton, Cheela Springs basalt of the Wyloo Group, U-Pb SHRIMP age of 2209±15Ma (Martin et al., 1999) and the Nipissing dolerite, intruding the Huronian Group, 2217Ma (U-Pb baddeleyite SHRIMP age, Noble and Lightfoot, 1992] are conspicuous and possibly mark the breakup of a hypothetical 37 Hekpoort and Ongeluk Formations 38 Hekpoort and Ongeluk Formations Paleoproterozoic supercontinent (Piper, 1982; Nance et al., 1988; Hoffman, 1989, Evans et al., 2001). An accurate age for the Ongeluk and Hekpoort lavas is, therefore, of both academic and economic importance. During this study, repeated attempts have been made by the author and other members of the RAU Paleoproterozoic Mineralization Research Group to find suitable lithologies from which to isolate zircons and obtain radiometric ages for the Hekpoort and Ongeluk Formations. The results of these attempts are presented in this chapter. 3.2 Regional Geological Setting Lavas of the Hekpoort Formation cover an area of at least 500 000km2 in the Transvaal region, whereas the Ongeluk lava cover an area of at least 300 000km2 in Griqualand West (Fig 3.1). The lavas mark a major volcanic event in the history of the Pretoria and Postmasburg Groups of the Transvaal Supergroup. The Hekpoort Formation is characterized by massive and amygdaloidal lava flows, flow top breccias and some volcanoclastics, typical of a volcanic unit that was terrestrially extruded (Fig 3.3)(Coetzee, 2001). In the Griqualand West area, the Ongeluk Formation contains massive lava flows but also pillow lavas, hyoloclastites and jasper and chert beds that suggest that this unit was extruded subaquaeously (Gutzmer et al., 2001). Regarding the lateral continuity of the Hekpoort and Ongeluk lavas it is important to note that the Hekpoort Formation and other units of the Pretoria Group have been mapped to the westernmost margin of the Transvaal outcrop area of the Transvaal Supergroup at Lobatse in Botswana. Here, the Hekpoort Formation is known as the Ditlhojana Formation (Key, 1983; Fig 3.1). The highly magnetic Asbesheuwels Iron Formation delineates the Griqualand West outcrop area of the Transvaal Supergroup below Kalahari sand in Botswana (Fig 3.4). It extends eastwards from the Griqualand West area to Jwaneng in Botswana. This implies that only about 60km separate the Transvaal and Griqualand West outcrop areas in eastern Botswana (Fig 3.1). In the Griqualand West basin in Botswana, the Hekpoort lava (Ditlhojana Formation) is known to extend as far as Jwaneng, where it is known as the Tsatsu Formation (Fig 3.1)(Tombale, 1986). The 39 Hekpoort and Ongeluk Formations Hekpoort lavas appear on deep seismic information as laterally continuous to those that are known as the Ongeluk lavas in the Northern Cape Province in South Africa (Tinker et al., 2002). There is thus no reason to believe that the two successions are not correlative in contrast to what was suggested by Moore et al. (2001). 3.3 Sampling The Hekpoort lava was sampled in deep drill cores in the Potchefstroom area of the Transvaal outcrop area (Figs 3.3 and 3.5). In this area, the Hekpoort Formation (which is in the order of 750m thick) overlies a unit of reworked volcaniclastic beds, which constitute a transitional zone between the Boshoek and Hekpoort Formations (Fig 3.3). 40 Hekpoort and Ongeluk Formations The lower part of the Hekpoort Formation is comprised of a basal lava flow overlain by volcaniclastics interbedded with thin lava flows. This unit is followed by a lower zone consisting of thin lava flows (individual flows less than 30m thick), a central unit of thick lava flows (each flow 40-60m thick) with flow top breccias, and a top zone consisting of two thick lava flows with a chert and tuff bed at its base (Fig 3.3)(Coetzee, 2001). 41 Hekpoort and Ongeluk Formations The Hekpoort Formation was sampled at a depth of 230m in drill core EBA1, and at a depth of 1078,5m in drill core RHK1 (Fig 3.3). Sample EBA-1 comes from a thin quartzitic volcanoclastic unit within the succession of thick lava flows (Fig 3.3), defined as zone 3 of the Hekpoort Formation by Coetzee (2001). This volcanoclastic unit is 40cm thick. It consists almost entirely of fine- to mediumgrained angular quartz grains. Sample RHK-1 was taken from the base of zone 3 (defined by a succession of thick lava flows) in a thin cherty tuffaceous unit (Fig 3.3). It is overlain by a massive lava flow. The Ongeluk Formation was sampled in the western limb of the Ongeluk-Witwater syncline near Griquatown in the Northern Cape Province of South Africa (Fig 3.6)(Gutzmer and Beukes, 1998). About 50kg of macroscopically fresh pillow lava was collected from outcrop. 3.4 Zircon Analyses Two samples were analysed by SHRIMP at Curtin University of Technology, Perth, namely EBA-1 from the Hekpoort Formation as well as the sample from the Ongeluk Formation. J. Gutzmer performed analyses on the Ongeluk Formation zircons and D.A.D. Evans and H. C. Dorland performed analyses on the zircons of the Hekpoort 42 Hekpoort and Ongeluk Formations Formation. A second sample of the Hekpoort Formation (RHK-1) was analysed by TIMS at the Massachusetts Institute of Technology by S. Bowring. 3.4.1 SHRIMP Analyses of Sample EBA-1 from the Hekpoort Lava A total of 49 zircon grains were analysed for the quartzitic volcanoclastic bed (sample EBA-1) from the Hekpoort lava (Table 3.1, Fig 3.7). Of these, 25 grains were nearly concordant (within 10% discordancy). The average size of the zircons ranges between 100 and 150 µm.