Neoarchean large igneous provinces on the Kaapvaal Craton in southern Africa re-define the formation of the Ventersdorp Supergroup and its temporal equivalents

Abstract

U-Pb geochronology on baddeleyite is a powerful technique that can be applied effectively to chronostratigraphy. In southern Africa, the Kaapvaal Craton hosts a wellpreserved Mesoarchean to Paleoproterozoic geological record, including the Neoarchean Ventersdorp Supergroup. It overlies the Witwatersrand Supergroup and its world-class gold deposits. The Ventersdorp Supergroup comprises the Klipriviersberg Group, Platberg Group, and Pniel Group. However, the exact timing of formation of the Ventersdorp Supergroup is controversial. Here we present 2789 ± 4 Ma and 2787 ± 2 Ma U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) baddeleyite ages and geochemistry on mafic sills intruding the Witwatersrand Supergroup, and we interpret these sills as feeders to the overlying Klipriviersberg Group flood basalts. This constrains the age of the Witwatersrand Supergroup and gold mineralization to at least ca. 2.79 Ga. We also report 2729 ± 5 Ma and 2724 ± 7 Ma U-Pb ID-TIMS baddeleyite ages and geochemistry from a mafic sill intruding the Pongola Supergroup and on an east-northeast-trending mafic dike, respectively. These new ages distinguish two of the Ventersdorp Supergroup magmatic events: the Klipriviersberg and Platberg. The Ventersdorp Supergroup can now be shown to initiate and terminate with two large ig-neous provinces (LIPs), the Klipriviersberg and Allanridge, which are separated by Platberg volcanism and sedimentation. The age of the Klipriviersberg LIP is 2791-2779 Ma, and Platberg volcanism occurred at 2754-2709 Ma. The Allanridge LIP occurred between 2709-2683 Ma. Klipriviersberg, Platberg, and Allanridge magmatism may be genetically related to mantle plume(s). Higher heat flow and crustal melting resulted as a mantle plume impinged below the Kaapvaal Craton lithosphere, and this was associated with rifting and the formation of LIPs.

Dating detrital zircon from the gold-bearing Ventersdorp Contact Reef in the Ventersdorp Supergroup of South Africa

“…Dates for granitoids in the possible provenance of the VCR are listed in supplementary Table A1. A comprehensive list including intrusive rocks and rocks from outside the Ventersdorp repository can be found in Gumsley et al (2020 define the age of the youngest grains. Eight grains, including six with the youngest 207 Pb/ 206 Pb ages, were dated at Stellenbosch University, analysing up to seven points in each grain.…”

“…The VCR is interbedded with and was covered by Klipriviersberg Group mafic lavas. The 2714 ± 16 Ma ion probe U-Pb date of Armstrong et al (1991, Table 1) for zircon from Klipriviersberg basalt is probably too young, as reviewed by Gumsley et al (2020). Two mafic sills in the Witwatersrand Supergroup, which they dated at 2787 ± 2 Ma and 2789 ± 4 Ma on baddeleyite (Table 1), are plausible feeders to the Klipriviersberg mafic lavas and do correspond geochemically.…”

“…Ion probe data for 20 detrital zircon grains from the VCR were reported by Barton et al (1989). The age of the youngest grain (5.1) was cited by Gumsley et al (2020) as 2780 ± 5 Ma, but that age is actually 2776 ± 102 Ma (2σ) and the point is 65% discordant. The youngest concordant grain (6.1) has an 207 Pb/ 206 Pb age of 2842 ± 80 Ma (2σ) and our re-evalution of the data gives 2785 ± 56 Ma (Table 1).…”

“…In a recent paper we established a precise U-Pb zircon date of 2720 ± 2 Ma for four rhyolite samples from the Makwassie Formation in the Platberg Group (Cornell et al, 2017). Gumsley et al (2020) reported U-Pb baddeleyite ages for three mafic sills and a dyke which were emplaced by intrusion into the Witwatersrand and Pongola supergroups (Table 1). They correlated them with the Klipriviersberg and Platberg groups and postulated the existence of three mafic large igneous provinces (LIPs) and a felsic volcanic province, based on these data and a review of published precise ages of many intrusive and a few extrusive rocks.…”

Lu-Hf analyses of zircon from the Makoppa Dome and Amalia-Kraaipan area: implications for evolution of the Kimberley and Pietersburg blocks of the Kaapvaal Craton

“…Although the Ventersdorp volcanism was long assumed to have started at ca. 2.71 Ga(Armstrong et al, 1991), more recent work has shown activity at 2791-2779 (Klipriviersberg Group), 2754-2709 (Platberg Group) and 2709-2683 Ma (Allanridge Group)(Gumsley et al, 2020). Indeed, a felsic porphyry sample from the Platberg Group of the Ventersdorp Supergroup in the Amalia area was dated at 2729 ± 3 Ma (U-Pb zircon SHRIMP upper intercept age) byPoujol et al (2005).…”