Precise zircon U‐Pb ages from the Marra Mamba Iron Formation and Wittenoom Formation, Hamersley Group, Western Australia

Abstract

The Mt Bruce Supergroup of the Pilbara Craton of Western Australia was laid down in the Hamersley Basin, unconformably over a basement of granite‐greenstone terrane, and consists of the Fortescue, Hamersley and Turee Creek Groups. The base of the ∼230 m‐thick Marra Mamba Iron Formation, the lowest formation of the Hamersley Group, marks the onset of the major banded iron‐formation deposition that characterises the group. Ion microprobe U‐Pb isotope analyses of zircons from a tuff band (NS3) within a high‐grade iron orebody in the uppermost (Mt Newman) member show two distinct, and non‐overlapping, age populations. Fifteen grains from the younger group of 16 have a pooled age of 2597±5Ma (95% confidence), interpreted as the age of syndepositional volcanism; individual grains of the older group of 8, with ages between ca 2950 Ma and ca 2820 Ma, are interpreted as xenocrysts derived from basement rocks transected by the rising magma. The conformably overlying Wittenoom Formation, ∼300–600 m thick, consists largely of dolomitic rocks. Zircons from a submarine fallout tuff within the uppermost (Bee Gorge) member also have two non‐overlapping zircon age populations. Most of the 23 zircons analysed belong to an older population with near‐concordant ages between ca 2750 and ca 2650 Ma, but 12 analysed spots from 7 grains have a pooled age of 2561 ±8 Ma. This is interpreted to be the age of volcanism coeval with deposition. The older zircons were probably derived from tuffs of Fortescue Group age, entrained with the Crystal‐rich Tuff ejectamenta during its explosive, probably phreatoplinian, eruption.