SHRIMP U–Pb zircon ages for the Palaeoproterozoic Kuruman Iron Formation, Northern Cape Province, South Africa: evidence for simultaneous BIF deposition on Kaapvaal and Pilbara Cratons

Abstract

Abstract Stilpnomelane-rich tuffaceous mudrocks are an important component of the Palaeoproterozoic Kuruman and Griquatown Iron Formations in the Northern Cape Province. They document volcanic activity during the deposition of these iron formations, and the zircon populations that they contain record syn-depositional volcanism, resedimentation and post-depositional isotopic disturbance. The trace-element signatures of tuffaceous samples suggest that although zircons are felsic volcanogenic in origin, mixing of felsic ash with basaltic ash and/or muds in the water column was also likely. In addition, these rocks record minor terrigenous input and some samples indicate mixing with iron-rich clays, oxides and smectites. Moreover, they reflect a shift from mafic volcanism (e.g. mafic tuffs in the underlying Gamohaan Formation) towards more felsic volcanism that is a trend also recorded for tuffaceous rocks in the Brockman Iron Formation. The best estimate of the depositional age for the top of the Riries Member, and hence the Kuruman Iron Formation is 2460 Ma, where most of the zircon populations were derived from volcanism coeval with BIF deposition. Ages obtained in this study that are older than the 2465±5 Ma age for the lower Riries Member probably represent resedimented pyroclastic deposits (i.e. give eruptive ages from earlier eruptions) that contain an unimodal zircon population, and as such only provide maximum depositional ages. Although a compacted sedimentation rate (cSR) for the entire Kuruman Iron Formation is 3–4 m per million years, a cSR of about 22 m per million years is deduced for the Riries Member, suggesting that there are cryptic gaps within the Kuruman stratigraphic record. The latter is comparable to the cSR of 33 m per million years calculated for Hamersley Province BIFs. Ages of the tuffaceous horizons in the Kuruman Iron Formation are comparable to those from the Brockman Iron Formation of the Hamersley Province. The 2460±5 Ma age for the top of the Riries Member corresponds to the 2459±3 Ma age for the base of the Joffre Member and the 2463±5 Ma Whaleback Shale Member of the Brockman Iron Formation. Although ca. 2468–2478 Ma ages reflect detrital or syn-depositional zircon populations, they are comparable to the 2470±4 Ma S13 and the 2479±3 Ma S9 macrobands of the Dales Gorge Member, Brockman Iron Formation. Moreover, the zircon-age spectra and Pb-loss histories of both the Kuruman and Brockman Iron Formation tuffaceous sediments are strikingly similar. Without doubt, the Kuruman and Brockman Iron Formations were deposited synchronously. Whether they were a response to global magmatic events or were deposited in the same basin are questions still being debated.