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. The shape of the zircons analysed varied between elongate and euhedral (Hek 1.1 and Hek 4.1) to rounded (Hek 6.1; Fig 3.8). Zoning is present in most of the zircons (Hek 1.1). Five significant populations of zircons are defined by 207Pb/206Pb ages at, 2230-2250Ma, 2270-2450Ma, 2500-2520Ma, 2560-2650Ma and 2860-2890Ma (Fig 3.9).

Four nearly concordant zircons (2.1, 11.1, 24.1 and 41.1) are present in the youngest 2230-2250Ma population (Table 3.1). The length of these zircons varies from 80-140 µm, with an average length of 110 µm. The zircons are euhedral to very angular and zoned (Fig 3.8).

43 Hekpoort and Ongeluk Formations

The 2270-2450Ma population (Fig 3.9) is defined by 12 almost concordant zircon grains. The texture of these zircons varies from almost euhedral to subrounded (see appendix II for detailed descriptions; Fig 3.8). The zircons vary in length from 74-113 µm, with an average length of 83 µm.

Three virtually concordant zircon grains define the 2500-2520Ma population (Fig 3.9). The length of these zircons varies from 88-101 µm, with an average length of 95 µm. The zircons are angular to well rounded and zoned (see appendix for detailed descriptions; Fig 3.8).

The 2560-2650Ma population (Fig 3.9) is represented by two virtually concordant zircon grains. Zircon 23.1 is 47 µm in length, subangular to subrounded whereas grain 32.1 is 101 µm in length, subrounded and zoned (Fig 3.8).

44

Table 3.1 Summary of SHRIMP U-Pb zircon data for the Hekpoort Formation.

(1) (1) (1) % Ppm ppm ppm 206Pb/238U 207Pb/206Pb % (1) (1) (1) 206 232 238 206 207 * 206 * 207 * 235 206 * 238 Spot Pbc U Th Th/ U Pb* Age Age Discordant Pb / Pb ±% Pb / U ±% Pb / U ±% Err.corr 1.1 0.14 427 604 1.46 161 2,346 ±19 2,389.5 ± 5.4 2 0.15388 0.32 9.314 1.0 0.4390 0.96 .950 2.1 0.09 112 56 0.52 40.2 2,251 ±21 2,243 ± 11 0 0.14130 0.65 8.14 1.3 0.4178 1.1 .863 3.1 0.05 362 219 0.62 148 2,501 ±20 2,515.5 ± 4.9 1 0.16579 0.29 10.83 1.0 0.4739 0.97 .958 4.1 10.72 1437 1090 0.78 332 1,365 ±41 1,571 ±330 13 0.097 17 3.16 18 0.2358 3.4 .187 5.1 0.16 363 133 0.38 135 2,321 ±25 2,311.0 ± 6.0 0 0.14698 0.35 8.78 1.3 0.4333 1.3 .964 6.1 0.04 162 183 1.17 78.3 2,882 ±34 2,886.1 ± 5.9 0 0.20750 0.36 16.13 1.5 0.5636 1.4 .969 7.1 -- 123 80 0.67 59.1 2,870 ±25 2,877.8 ± 7.3 0 0.20643 0.45 15.96 1.2 0.5607 1.1 .923 8.1 1.17 806 769 0.99 181 1,475 ±13 1,940 ± 68 24 0.1189 3.8 4.21 3.9 0.2570 1.1 .267 9.1 0.06 703 32 0.05 213 1,947 ±16 2,198.4 ± 8.4 11 0.13769 0.48 6.695 1.1 0.3527 0.98 .896 11.1 0.64 236 314 1.37 84.2 2,225 ± 33 2,250 ± 32 1 0.1419 1.9 8.06 2.6 0.4123 1.8 .688 12.1 0.25 719 515 0.74 143 1,339 ± 20 1,956 ± 31 32 0.1200 1.8 3.821 2.4 0.2309 1.7 .694 13.1 0.02 288 212 0.76 118 2,514 ± 36 2,507.3 ± 5.0 0 0.16497 0.30 10.85 1.7 0.4771 1.7 .985 14.1 -- 122 143 1.21 44.0 2,260 ± 36 2,357.2 ± 8.4 4 0.15100 0.49 8.74 2.0 0.4200 1.9 .968 15.1 0.36 1001 368 0.38 233 1,538 ± 23 2,213 ± 44 31 0.1389 2.6 5.16 3.1 0.2694 1.7 .551 16.1 0.15 233 37 0.17 88.1 2,347 ± 34 2,352.1 ± 6.5 0 0.15055 0.38 9.12 1.8 0.4392 1.7 .976 16.1 0.43 458 112 0.25 119 1,690 ± 36 2,172 ± 16 22 0.1356 0.93 5.60 2.6 0.2996 2.4 .932 18.1 16.07 1070 203 0.20 293 1,457 ± 82 2,288 ± 260 36 0.145 14 5.07 16 0.254 6.2 .375 20.1 2.46 664 193 0.30 180 1,720 ± 30 2,028 ± 26 15 0.1249 1.5 5.27 2.5 0.3057 2.0 .803 21.1 0.12 147 134 0.94 73.0 2,940 ± 96 2,884 ± 16 -2 0.2072 1.0 16.52 4.2 0.578 4.1 .971 22.1 18.87 1001 369 0.38 339 1,721 ± 72 1,769 ± 440 3 0.108 23 4.6 25 0.306 4.8 .193 23.1 0.44 174 71 0.42 70.2 2,467 ± 35 2,579.0 ± 7.2 4 0.17219 0.43 11.07 1.8 0.4662 1.7 .970 24.1 0.02 293 120 0.42 99.5 2,145 ± 31 2,248.9 ± 5.9 5 0.14176 0.34 7.72 1.7 0.3948 1.7 .980 25.1 27.80 1279 86 0.07 333 1,225 ±100 1,210 ±1700 -1 0.081 83 2.3 87 0.209 9.4 .108 26.1 0.00 449 207 0.48 174 2,403 ± 34 2,434.2 ± 4.3 1 0.15798 0.25 9.84 1.7 0.4518 1.7 .989 27.1 0.04 271 111 0.42 97.6 2,254 ± 33 2,301.8 ± 6.0 2 0.14618 0.35 8.44 1.7 0.4186 1.7 .980 28.1 2.94 214 128 0.62 78.2 2,211 ± 33 2,314 ± 17 4 0.1472 1.0 8.30 2.0 0.4091 1.8 .865 29.1 0.22 2457 583 0.25 344 972 ± 27 1,466 ± 32 34 0.0919 1.7 2.063 3.4 0.1627 3.0 .872

45

Table 3.1 continued (1) (1) (1) % Ppm ppm ppm 206Pb/238U 207Pb/206Pb % (1) (1) (1) 206 232 238 206 207 * 206 * 207 * 235 206 * 238 Spot Pbc U Th Th/ U Pb* Age Age Discordant Pb / Pb ±% Pb / U ±% Pb / U ±% Err.corr 30.1 0.18 734 163 0.23 212 1,868 ± 27 2,269.9 ± 8.6 18 0.14351 0.50 6.65 1.8 0.3362 1.7 .959 31.1 0.03 503 177 0.36 178 2,222 ± 32 2,609 ± 15 15 0.1753 0.89 9.95 1.9 0.4115 1.7 .886 32.1 0.85 599 312 0.54 233 2,385 ± 34 2,644.1 ± 4.4 10 0.17906 0.26 11.05 1.7 0.4476 1.7 .988 33.1 0.15 918 351 0.39 170 1,258 ± 19 1,777.1 ± 7.5 29 0.10866 0.41 3.229 1.7 0.2155 1.7 .972 34.1 2.12 93 188 2.09 35.6 2,321 ± 35 2,415 ± 48 4 0.1562 2.8 9.33 3.4 0.4334 1.8 .543 36.1 1.34 370 168 0.47 132 2,205 ± 33 2,468 ± 30 11 0.1611 1.8 9.06 2.5 0.4078 1.8 .707 37.1 0.05 465 408 0.91 171 2,295 ± 33 2,332.1 ± 4.5 2 0.14879 0.26 8.77 1.7 0.4277 1.7 .988 38.1 0.06 43 22 0.53 20.8 2,891 ± 44 2,873 ± 11 -1 0.2058 0.66 16.06 2.0 0.566 1.9 .943 41.1 3.69 126 69 0.57 42.6 2,060 ± 31 2,230 ± 35 8 0.1403 2.0 7.28 2.7 0.3765 1.8 .661 42.1 1.00 296 201 0.70 112 2,330 ± 34 2,502.7 ± 6.8 7 0.16454 0.40 9.88 1.8 0.4353 1.7 .974 43.1 0.42 665 125 0.19 181 1,767 ± 26 2,153.4 ± 7.2 18 0.13419 0.41 5.83 1.8 0.3153 1.7 .972 44.1 4.76 2341 884 0.39 494 1,345 ± 26 1,584 ± 260 15 0.098 14 3.13 14 0.2320 2.3 .163 45.1 0.11 941 637 0.70 219 1,547 ± 24 1,960.7 ± 4.9 21 0.12030 0.27 4.500 1.8 0.2713 1.8 .988 46.1 0.31 342 152 0.46 129 2,340 ± 36 2,440 ± 22 4 0.1586 1.3 9.57 2.2 0.4375 1.8 .810 47.1 0.09 491 148 0.31 167 2,150 ± 31 2,286.5 ± 5.5 6 0.14489 0.32 7.91 1.7 0.3959 1.7 .983 48.1 1.54 846 286 0.35 179 1,398 ± 24 2,039 ± 88 31 0.1257 5.0 4.20 5.3 0.2422 1.9 .358 49.1 1.82 265 97 0.38 88.2 2,067 ± 30 2,299 ± 11 10 0.14594 0.64 7.61 1.8 0.3781 1.7 .937 Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in standard calibration was 1.07% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb.

46 Hekpoort and Ongeluk Formations

Four almost concordant zircon grains (6.1, 7.1, 21.1 and 38.1) define the oldest population between 2860-2890Ma (Fig 3.9, Table 3.1). Texturally, these zircons vary from euhedral to rounded. All of these zircons are zoned (see chapter 6 for detailed

47 Hekpoort and Ongeluk Formations descriptions; Fig 3.8) and vary in length from 100-120 µm with an average length of 110 µm.

3.4.2 TIMS Analyses of Zircons from Sample RHK-1 of the Hekpoort Formation Four zircon grains were analysed by TIMS. These zircons yield variably discordant (9- 19%) 207Pb/206Pb ages. Three yield ages between 2495 and 2515Ma, and one yields a 207Pb/206Pb age of 2225±3Ma (Table 3.2, Fig 3.10).

3.4.3 SHRIMP Analyses of Zircons from the Ongeluk Formation A total of 19 zircon grains from the Ongeluk lava were analysed by SHRIMP (Table 3.3, Fig 3.11). The grains were sub angular to well rounded and zoned. They varied in length

48

Table 3.2 Summary of TIMS U-Pb zircon data from the Hekpoort Formation, sample RHK1.

RHK-1 Weigh U Pb 206Pb*/ 208Pb 206Pb/ %er 207Pb/ %er 207Pb/ %er 206Pb/238 207Pb/235 207Pb/206P Corr Pb Blan Fraction t (ppm (ppm 204Pb / 238U r 235U r 206Pb r U U b coef (pg k s (mg) ) ) 206Pb age (Ma) age (Ma) age (Ma) ) (pg) Z5(1) 0.0100 33.28 17.12 8951.3 0.221 0.43149 (.34) 9.7359 (.35) 0.1636 (.07) 2312.5 2410.1 2493.6 0.98 1.0 1.0 6 8 3 4 1 Z3(1) 0.0100 9.0 4.3 1408.2 0.143 0.42380 (.45) 9.6848 (.46) 0.1657 (.08) 2277.7 2405.3 2515.1 0.98 1.8 1.8 2 4 6 Z2(1) 0.0100 15.43 7.24 1348.9 0.163 0.41120 (.29) 9.2895 (.31) 0.1638 (.11) 2220.4 2367.0 2495.7 0.94 3.1 3.1 4 5 9 5 0 Z1(1) 0.0100 15.20 5.90 568.89 0.160 0.33469 (.67) 6.4515 (.70) 0.1398 (.18) 1861.1 2039.3 2224.7 0.96 5.6 3.5 8 1 0 7

Table 3.3 Summary of SHRIMP U-Pb data for zircons from the Ongeluk Formation. Spot U Th Th/U 204Pb/206Pb 4f206 207Pb*/206Pb* 208Pb*/206Pb* 206Pb*/238U 207Pb*/235U 208Pb/232Th 206Pb* (ppm) (ppm) Age C.1-1 210 85 0.4048 0.00007 0.116 0.0643±14 0.1230±31 0.124±2 1.10±3 0.0377±11 752±45 C.2-1 89 7 0.0787 0.00023 0.374 0.0616±43 0.0408±95 0.095±2 0.81±6 0.0502±118 660±149 C.3-1 99 216 2.1818 0.00202 3.236 0.0513±62 0.7135±168 0.086±2 0.61±8 0.0281±8 252±257 C.4-1 1779 810 0.4553 0.00000 0.003 0.1274±2 0.1284±3 0.332±5 5.83±8 0.0937±13 2063±3 C.5-1 212 130 0.6132 0.00003 0.055 0.0567±16 0.2005±40 0.093±1 0.72±2 0.0303±8 482±61 C.6-1 1355 36 0.0266 0.00002 0.032 0.1109±3 0.0071±3 0.226±3 3.46±5 0.0614±27 1814±5 C.7-1 342 97 0.2836 0.00000 0.004 0.0654±9 0.0920±20 0.131±2 1.18±2 0.0425±11 786±29

49

C.8-1 2516 109 0.0433 0.00000 0.007 0.1112±2 0.0130±1 0.334±5 5.12±7 0.1003±17 1819±3 C.9-1 519 14 0.0270 0.00000 0.004 0.1177±4 0.0076±3 0.374±5 6.07±9 0.1055±50 1921±6 C.10-1 459 12 0.0261 -0.00001 0.000 0.1036±5 0.0091±2 0.195±3 2.79±4 0.0694±20 1689±9 Table 3.3 continued Spot U Th Th/U 204Pb/206Pb 4f206 207Pb*/206Pb* 208Pb*/206Pb* 206Pb*/238U 207Pb*/235U 208Pb/232Th 206Pb* (ppm) (ppm) Age C.11-1 719 117 0.1627 0.00021 0.332 0.1236±4 0.0391±8 0.359±5 6.12±9 0.0859±21 2009±6 C.12-1 2237 170 0.0760 0.00001 0.022 0.1181±2 0.0243±2 0.278±4 4.53±6 0.0886±14 1928±3 C.13-1 1455 84 0.0577 0.00004 0.058 0.1094±3 0.0093±3 0.260±4 3.92±6 0.0417±15 1790±4 C.14-1 516 12 0.0233 -0.00001 0.000 0.0586±5 0.0056±2 0.088±1 0.71±1 0.0211±9 552±18 C.15-1 401 108 0.2693 -0.00001 0.000 0.1066±3 0.0737±4 0.425±6 6.25±9 0.1167±18 1743±6 C.16-1 616 79 0.1282 0.00003 0.049 0.0573±7 0.0383±13 0.088±1 0.69±1 0.0262±10 502±25 C.17-1 1917 198 0.1033 0.00008 0.126 0.1102±2 0.0114±3 0.245±3 3.73±5 0.0270±9 1802±4 C.18-1 143 60 0.4196 -0.00000 0.000 0.0576±9 0.1334±23 0.082±1 0.65±2 0.0263±6 514±35 C.19-1 4204 775 0.1843 0.00005 0.086 0.1329±1 0.0516±2 0.344±5 6.31±9 0.0964±14 2137±2

50 Hekpoort and Ongeluk Formations

from 50 to 180 mm. However, most of the zircons are approximately 80mm in length. Two significant populations of 207Pb/206Pb ages are present. The one population are concordant and distributed between 200-800Ma. These very young 207Pb/206Pb ages cannot be primary. However, it is a phenomenon that has been recognized in other studies of zircons from the Transvaal Supergroup, but the reason for their origin remains unknown (ages between 1200-250Ma, Armstrong, written communication, 1997).

The second group of zircon grains yields discordant Paleoproterozoic ages (Fig 3.9). These grains are highly radiogenic, with uranium concentrations as high as several thousand parts-per-million (Table 3.3). They define a crude discordia with an upper intersection of around 2300Ma (Fig 3.11)(Gutzmer and Beukes, 1998).

51 Hekpoort and Ongeluk Formations

3.5. Discussion 3.5.1 Evaluation of Radiometric Ages The attempts to obtain a precise radiometric age of the Hekpoort and Ongeluk lavas by U-Pb-dating of zircon yielded complex results. Although the zircon SHRIMP 207Pb/206Pb ages obtained from sample EBA-1 clearly reflect a detrital zircon population, the

youngest zircons still provide a maximum age limit for the Hekpoort Formation. The youngest, nearly concordant zircons from sample EBA-1 yield 207Pb/206Pb ages around 2240Ma. These zircons display their original euhedral character and are thought to approximate the age of extrusion of the Hekpoort lava. Similarly, the youngest zircon analysed by TIMS form the Hekpoort Formation tuff sample (sample RHK1) yielded a 207Pb/206Pb age of 2225±3Ma, supporting the data acquired by SHRIMP analyses.

52 Hekpoort and Ongeluk Formations

The SHRIMP and TIMS data that was obtained for zircons from the Hekpoort Formation of the Pretoria Group are also in good agreement with the whole rock Pb-Pb age of 2236±38Ma obtained by Cornell et al. (1996). It may therefore be suggested that the Hekpoort lava extruded at approximately 2240Ma.

The radiometric ages of zircons from the Ongeluk lava remain inconclusive. The data for zircons analysed by SHRIMP for the Ongeluk lava is too discordant to calculate any accurate ages (Gutzmer and Beukes, 1998). The older group of 207Pb/206Pb ages for the Ongeluk lava define a crude discordia with an upper intersection of approximately 2200- 2300Ma (Fig 3.11), that may define the age of extrusion of the Ongeluk lava.

The zircon age for the Ongeluk lava is ill-defined and it does not invalidate the secondary-lead age of 2394±26Ma obtained for the Mooidraai dolomite by Bau et al. (1999). However, there are other isotopic age data available that suggests that the Ongeluk lava is in the order of 2250Ma, rather than 2400Ma. For example, a detrital zircon from the Koegas Subgroup stratigraphically well below the Ongeluk Formation yielded a SHRIMP age of 2415±6Ma (Beukes and Gutzmer, 1998)(Fig 3.2). If the secondary-lead age of 2394±26Ma determined for the Mooidraai dolomite (Bau et al., 1999) is the age of deposition for the Mooidraai dolomite, then the Makganyene diamictite, Ongeluk lava, Hotazel banded iron-formation and Mooidraai dolomite were deposited in a very short time interval not exceeding 20Ma. The 2394±26Ma secondary- lead age for the Mooidraai dolomite (Bau et al., 1999) is older than the whole rock Pb-Pb age of 2236±38Ma (Cornell et al., 1996) determined for the Ongeluk Formation. However, the best evidence in support for the assumption that the Ongeluk and Hekpoort Formations are age equivalents is based on the excellent stratigraphic correlation as outlined earlier.

3.5.2 Geological Implications There is strong evidence for the development of an oxidising atmosphere in the Timeball Hill Formation, stratigraphically below the Hekpoort Formation (Dorland, 1999; Dorland et al., 2002; Beukes et al., 2002B; Bekker et al., 2004). The age of the Timeball Hill

53 Hekpoort and Ongeluk Formations

Formation may be bracketed between ca. 2240Ma for the Hekpoort lava and 2320Ma (Re-Os age on secondary pyrite within the lower shales of the Timeball Hill Formation, Hannah et al., 2003). This suggests that there was oxygen in the atmosphere more than 50Ma earlier than the Cloud-Holland Model for the history of atmospheric oxygen has suggested (Holland, 1984; Rye and Holland, 1998, Ohmoto, 1996; Ohmoto, 1997; Beukes et al., 2002A).

The ca. 2240Ma radiometric age of the Hekpoort lava is recognized worldwide as a period of volcanism and dolerite intrusions. Examples of similar age volcanism is found on the Pilbara craton, where zircons from the Cheela Springs basalt of the Wyloo Group yielded an U-Pb SHRIMP age of 2209±15Ma (Martin et al., 1999). The Nispissing dolerite dyke swarm that intrudes the Huronian Group in the Superior Province has an U- Pb baddeleyite SHRIMP age of 2217Ma (Noble and Lightfoot, 1992). This widespread event of volcanism could perhaps be associated with the breakup of an old supercontinent at around 2200-2250Ma (Piper, 1982; Nance et al., 1988. Evans et al., 2001), of which the Kaapvaal craton may have been part.

3.6 Conclusion Zircon SHRIMP and TIMS 207Pb/206Pb ages suggests that the age of extrusion of the Hekpoort lava is approximately 2240Ma, confirming the results of previous Pb-Pb whole rock dating. Poorly constrained zircon SHRIMP results for the Ongeluk lava suggest that it extruded between 2200 and 2300Ma. However, this age is so poorly constrained that the secondary-lead age of 2394±26Ma determined for the Mooidraai dolomite may not be excluded at present to represent a true depositional age. It is suggested that future work be undertaken to remove any remaining uncertainties. An approach to resolve the ambiguity of the radiometric age of the Ongeluk lava (2250 or 2400Ma) may be to measure radiometric ages of detrital zircons within the Makganyene diamictite. If detrital zircons younger ca. 2350Ma would be present in the Makganyene diamictite, it would support the Beukes et al. (2002A) correlation of the Hekpoort and Ongeluk Formations. It may also be worthwhile to process more large samples of the Ongeluk lava for zircon separation and analyses.

54 Hekpoort and Ongeluk Formations

3.7 Reference List Altermann, W. and Nelson, D.R. (1998). Sedimentation rates, basin analyses and regional correlations of three Neoarchean and Palaeoproterozoic sub-basins on the Kaapvaal Craton as implied by precise SHRIMP U-Pb ages from volcanic sediments. Journal of Sedimentary Geology 120, 225-256.

Bau, M., Romer, R.L., Luders, V., Beukes, N.J. (1999). Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): implications for the composition of Paleoproterozoic seawater and ‘dating’ the increase of oxygen in the Precambrian atmosphere. Earth and Planetary Science Letters 174, 43-57.

Bekker, A, Holland, H.D., Wang, P.-L, Rumble III, D., Stein, H.J., Hannah, J.L., Coetzee, L.L. and Beukes, N.J. (2004). Dating the rise of atmospheric oxygen. Nature, 427(8), 117-120.

Beukes, N.J. (1986) The Transvaal sequence in Griqualand West, 819-828. In Annheausser, C.R. and Maske, S. (editors), Mineral deposits of Southern Africa, I. Geological Society of South Africa, Johannesburg, 1020pp.

Beukes, N.J., Dorland, H., Gutzmer, J., Nedachi, M. and Ohmoto, H. (2002A). Tropical laterites, life on land, and the history of atmospheric oxygen in the Paleoproterozoic. Geology, 30(6), 491-494.

Beukes, N.J., Dorland, H.C., Gutzmer, J. (2002B). Pisolitic ironstone and ferricrete in the 2.22-2.4 Ga Timeball Hill Formation, Transvaal Supergroup: Implications for the history of atmospheric oxygen. GSA Annual Meeting, Denver, Abstracts and Program, 125-126.

Buick I.S., Uken, R., Gibson, R.L., and Wallmach T. (1998). High-d13C Paleoproterozoic carbonates from the Transvaal Supergroup, South Africa. Geology, 26, 875-878.

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Buick, I.S., Maas, R. and Gibson, R. (2001). Precise U-Pb titanite age constraints on the emplacement of the Bushveld Complex, South Africa. Journal of the Geological Society, London, 158, 3-6. Carney, J.M., Aldiss, D.T., and Lock, N.P. (1994). The Geology of Botswana. Bulletin 37, Geological Survey of Botswana, 113pp.

Coetzee, L.L. (2001). Genetic stratigraphy of the Paleoproterozoic Pretoria Group in the western Transvaal. MSc thesis (unpublished), Rand Afrikaans University, 212pp.

Cornell, D.H., Schutte, S.S. and Eglington, B.L. (1996). The Ongeluk basaltic andesite formation in Griqualand West, South Africa: submarine alteration in a 2222Ma Proterozoic sea. Precambrian Research 79, 101-123.

De Beers (1998). Aeromagnetic map. Update on the distribution in time and space of southern African Kimberlites. Produced by Ayres, N.P., Hatton, C.J., Quadling, K.E., Smith, C.B.

Dorland, H.C., (1999). Paleoproterozoic laterites, red beds and ironstones of the Pretoria Group with reference to the history of atmospheric oxygen. M.Sc thesis (unpublished), Rand Afrikaans University, Johannesburg, 147pp.

Dorland, H.C., Beukes, N.J., Gutzmer, J. (2002). Oolitic ironstones of the Paleoproterozoic Time Hill Formation. Evidence for a 2.5Ga oxygenated atmosphere. 16th International Sedimentological Congress, Rand Afrikaans University, Abstracts Volume, 81-82.

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Evans, D.A.D., Beukes, N.J., Kirschvink, J.L. (2002). Paleomagnetism of a lateritic paleoweathering horizon and overlying Paleoproterozoic red beds from South Africa: Implications for the Kaapvaal apparent polar wander path and a confirmation of atmospheric oxygen enrichment. Journal of Geophysical Research, 107, no. B12, 1-22.

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Gutzmer, J. and Beukes, N.J. (1998). High grade manganese ores in the Kalahari manganese field: Charaterisation and dating of the ore-forming events. Unpublished Report, Rand Afrikaans University, Johannesburg. 221pp.

Gutzmer, J., Pack, A, Luders, V., Wilkinson, J.J., Beukes, N.J., Van Niekerk, H.S. (2001). Formation of jasper and andradite during low-temperature hydrothermal seafloor metamorphism, Ongeluk Formation, South Africa. Contributions to Mineralogy and Petrology, 142, 27-42.

Hannah, J.L., Stein, H.J., Bekker, A., Markey, R.J., Holland, H.D. (2002). Chondritic initial 187Os/188Os in Paleoproterozoic shale (seawater) and the onset of oxidative weathering. Geochimica and Cosmochimica Acta 67, A-34.

Hoffman, P.F. (1989). Speculations on Laurentia’s first gigayear (2.0-1.0 Ga): Geology, 17, 135-138.

Holland, H.D. (1984). The chemical evolution of the atmosphere and oceans. New York, John Wiley & Sons, 582pp.

Holland, H.D., and Beukes, N.J. (1990). A paleoweathering profile from Griqualand West, South Africa: Evidence for a dramatic rise in atmospheric oxygen between 2.2 and

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Key, R.M. (1983). The geology of the area around Gaborone and Lobatsi, Kweneng, Kgatleng, Southern and eastern districts. Geological Survey of Botswana,. Memoir 5, 117pp. Kirschvink J.L. (1992). Late Proterozoic low-latitude global glaciation: The Snowball Earth. In: Schopf, J.W. and Klein, C (Editors). The Proterozoic Biosphere. Cambridge University Press. Cambridge, 51-52.

Martin, D.McB., Clendenin, C.W., Krapez, B., McNaughton, N.J. (1998). Tectonic and geochronological constraints on late Archean and Palaeoproterozoic stratigraphic correlations within and between the Kaapvaal and Pilbara Cratons. Journal of the Geological Society, London, 155, 311-322.

Martin, D.McB., Li, Z.X., Nemchin, A.A., Powell, C.McA. (1999). A pre-2.2 Ga age for giant hematite ores of the Hamersley Province Australia?. Economic Geology 93, 1084- 1090.

Moore, J.M., Tsikos, H., Polteau, S. (2001). Deconstructing the Transvaal Supergroup, South Africa: implications for Palaeoproterozoic palaeoclimate models. Journal of African Earth Sciences, 33, 437-444.

Nance, R.D., Worsley, T.R., and Moody J. (1988). The supercontinental cycle. Scientific American, 259, 72-90.

Nelson, D.R., Trendall, A.F., Altermann, W. (1999). Chronological correlations between the Pilbara and Kaapvaal cratons. Precambrian Research 97, 165-189.

Noble, S.R. and Lightfoot, P.C. (1992). U-Pb baddeleyite ages of the Kerns and Triangle Mountian intrusions, Nipissing Diabase, Ontario. Canadian Journal of Earth Sciences, 29, 1424-1429.

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Ohmoto, H. (1996). Evidence in pre-2.2 Ga paleosols for the evolution of atmospheric oxygen and terrestrial biota. Geology, 24, 1135-1138.

Ohmoto, H., (1997). When did the Earth’s atmosphere became oxic. The Geochemical News, 93, 12-27.

Piper, J.D.A. (1982). The Precambrian paleomagnetic record: The case for the Proterozoic supercontinent. Earth and Planetary Science Letters, 59, 61-89. Rye, R. and Holland, H.D. (1998). Paleosols and the evolution of atmospheric oxygen: A critical review. American Journal of Science, 298, 621-672.

South African Committee for Stratigraphy (SACS) (1980). Stratigraphy of South Africa. Part 1 (Comp. L.E. Kent). Lithostratigraphy of the Republic of South Africa, South West Africa/Namibia, and the Republics of Bophuthatswana, Transkei and Venda. Handbook Geololgical Survey of South Africa, 8, 690pp.

Sumner, D.Y., Bowring, S.A. (1996). U-Pb geochronologic constraints on deposition of the Campbellrand Subgroup Transvaal Supergroup, South Africa. Precambrian Research 79, 25-35.

Swart, Q.D. (1999). Carbonate rocks of the Paleoproterozoic Pretoria and Postmasburg Groups, Transvaal Supergroup. M.Sc thesis (unpublished), Rand Afrikaans University, Johannesburg, 126pp.

Tinker, J., De Wit, M and Grotzinger, J (2002). Seismic Stratigraphic Constrains on Neoarchean-Paleoproterozoic Evolution of the Western Margin of the Kaapvaal Craton, South Africa. South African Journal of Geology, 105, 107-134.

Tombale, A.R. (1986). Geological map of the Jwaneng area (QDS 2424D), with brief description. Geological Survey of Botswana.

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Trendall, A.F., Compston, W., Williams, I.S., Armstrong, R.A., Arndt, N.T., McNaughton, N.J., Nelson, D.R., Barley, A.M. (1990). Precise zircon U-Pb geochronological comparison of the volcano-sedimentary sequences of the Kaapvaal and Pilbara cratons between about 3.1 and 2.4Ga. Third International Archean Symposium, Perth, Extended Abstracts, 81-83.

Walraven, F. (1997). Geochronology of the Rooiberg Group, Transvaal Supergroup, South Africa. University of the Witwatersrand, Johannesburg, Economic Geology Research Unit Information Circular No. 316, 21pp.

Walraven, F. and Hatting, E. (1993). Geochronology of the Nebo Granite, Bushveld Complex. South African Journal of Geology, 96, 31-41.

Wiggering, H., and Beukes, N.J. (1990). Petrography and geochemistry of a 2000-2200 Ma-old hematitic paleo-alteration profile on Ongeluk basalt of the Transvaal Supergroup, Griqualand West, South Africa. Precambrian Research, 46, 241-258.

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