Distribution and geochronology of unconformity-bound sequences in paleoproterozoic Elim-Olifantshoek red beds: implications for timing of formation of Sishen-type iron ore and heavy carbonate carbon isotope excursion
- Authors: Da Silva, Richard
- Date: 2012-08-16
- Subjects: Iron ores , Geochronometry , Paleontology, Proterozoic , Carbon isotopes , Geological time , Griqualand West (South Africa)
- Type: Thesis
- Identifier: uj:9517 , http://hdl.handle.net/10210/5946
- Description: M.Sc. , Bracketing the depositional age of the Gamagara/Mapedi to Lucknow and Olifantshoek succession in Griqualand West is important because it not only represents one of the oldest known red bed successions in the world but also hosts some of the first well preserved lateritic soil profiles and carbonates with heavy 13C values traditionally correlated with the so-called Lomagundi carbonate carbon excursion. In addition the ancient supergene very large high-grade hematite iron ore deposits of the Sishen-Postmasburg area on the Maremane dome are associated with the erosional unconformity at the base of the Gamagara Formation (a lateral equivalent of the Mapedi Formation). However, the depositional age of especially the Gamagara/Mapedi to Lucknow succession is under dispute because it has been considered a) correlative to the lower part of the Waterberg Group in the Transvaal area, with the implication that it is younger than the Bushveld Complex with an age of ~2,054 Ga, and b) correlative to the Dwaalheuwel-Magaliesberg succession of the pre-Bushveld Pretoria Group of the Transvaal Supergroup in the Transvaal area. The upper age limit of the Gamagara/Mapedi to Lucknow succession is defined by 1,92 Ga felsic volcanics in the overlying Neylan-Hartley succession of the Olifantshoek Group. The Hartley Lava Formation is overlain by Volop quartzites. This study involves age determinations of detrital zircon populations extracted from the basal Doornfontein conglomerate member of the Gamagara/Mapedi succession, and quartzites of the Gamagara/Mapedi, Lucknow, Neylan, Hartley and Volop Formations at various localities in Griqualand West. Based on field work, three unconformity-bound sequences are defined, namely the Gamagara/Mapedi-Lucknow, Neylan-Hartley and Volop sequences. Most interestingly quartzites of the Gamagara/Mapedi-Lucknow sequence contain abundant zircons with ages similar to that of the Bushveld Complex at ~2,054-2,06 Ga in addition to zircons as young as ~1,98-2,01 Ga. An exception is results on one sample of the Doornfontein Member analyzed so far (it is from the Rooinekke iron ore mine south of Postmasburg) that contains only zircons that are older than the Bushveld Complex with a rather prominent youngest population bracketed between 2,2 Ga and 2,32 Ga. The youngest detrital zircon populations in the Neylan-Hartley sequence are either slightly older than the Hartley lava or contain zircons with similar age to Hartley felsic lavas at 1,92 Ga. This sequence thus appears to have developed immediately prior to and coeval with Hartley volcanism. The overlying Volop sequence contains abundant zircons as young as ~1,89 Ga. The results clearly illustrate that the Gamagara/Mapedi to Lucknow succession is certainly not a lateral correlative of the pre-Bushveld Dwaalheuwel-Magaliesberg succession of the Pretoria Group. Rather it should be considered time-equivalent lower parts of the Waterberg Group in the Transvaal area. This implies that the heavy carbonate carbon excursion known from the Lucknow Formation is at least 100 my. younger than the one known from the upper part of the Silverton Formation along the contact with the overlying Magaliesberg Quartzite. There are thus at least three heavy carbonate carbon excursions, known from Paleoproterozoic cover successions of the Kaapvaal Craton in southern Africa, namely one in the ~2.35 Ga Duitschland Formation, a second in the ~2,1 Ga Silverton Formation of the Pretoria Group of the Transvaal Supergroup and the third in the ~1,98-1,92 Ga Lucknow Formation. It is further known that carbonates with normal open marine 13C values of close to zero occur in stratigraphic intervals between each of the heavy carbonate carbon excursions.
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Provenance and age of Paleoproterozoic red beds of the Elim Group (Griqualand West, South Africa) as determined from detrital zircon age populations
- Authors: Dreyer, Daphne
- Date: 2014
- Subjects: Geology, Stratigraphic , Paleontology, Proterozoic , Geology - South Africa - Griqualand West , Paleontology - South Africa - Griqualand West
- Language: English
- Type: Masters (Thesis)
- Identifier: http://hdl.handle.net/10210/84446 , uj:19221
- Description: Abstract: The Gamagara/Mapedi and Lucknow Formations of the Elim Group, along with the Olifantshoek Group, within the Paleoproterozoic Keis Supergroup, are very important stratigraphic units. Their economic importance is due to the presence of the oldest known red bed successions which hosts well preserved lateritic soil profiles, as well as the ancient supergene high-grade laminated iron ores of the Sishen-Postmasburg area on the Maremane dome. These high-grade iron ore deposits are associated with the pre-ElimlGamagara unconformity, which is located at the base of the conglomerate unit of the GamagaralMapedi Formation known as the Doornfontein Member. The main focus of the study i3 on the lateral correlation of the Elim Group in the Griqualand West area to geology in the Highveld region of South Africa, with specific focus on the depositional age of the Elim Group, i.e. the Gamagara/Mapedi and Lucknow Formations. The depositional age, as well as the lateral correlation of the Elim Group, has however remained a matter of contention, and this succession has previously been correlated with the Dwaalheuwel- Magaliesberg succession of the upper Pretoria Group of the Transvaal Supergroup, and more recently correlated to the Swaershoek- and Alma Formations of the lower Waterberg Group. The correlation of the Elim Group to geology in the Highveld region of South Africa area is however problematic as there is stratigraphic duplication of the red beds in the Elim and Olifantshoek Groups, as well as strata from the Transvaal Supergroup within the Griqualand West area, caused by intense folding and faulting. A higher degree of certainty, regarding the age constraints of the Elim Group, and more specifically the Doornfontein Member of the Gamagara/Mapedi Formation, is thus required to assist with a re-evaluation of the lateral extent and depositional age of the high-grade iron ore deposits in the Highveld area as well as its correlation to geological successions elsewhere in South Africa. The age constraints for the GamagaralMapedi and the lower Lucknow Formations were obtained from U-Pb geochronological studies of detrital zircon populations that were extracted from various samples from the Gamagara/Mapedi and lower Lucknow Formations. The detrital zircon grains were analysed by means of Laser Ablation Inductively Coupled Mass Spectrometry (LAICP- MS) at the analytical facility Spectrum, at the University of Johannesburg... , M.A. (Geology)
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