SHRIMP U-Pb zircon ages of tuffaceous mudrocks in the Brockman Iron Formation of the Hamersley Range, Western Australia

Tuffaceous mudrocks are common in the banded iron-formations (BIF) of the Brockman Iron Formation. These tuffaceous mudrocks are either stilipnomelane-rich or siliceous. Their compositions reflect bimodal volcanic activity in the vicinity of the Hamersley BIF depositional site, They also contain complex zircon populations that record resedimentation, syndepositional volcanism and post-depositional isotopic disturbance. The best estimates of depositional age are obtained from siliceous tuffaceous mudrocks in the Joffre Member that contain 2459+/-3 Ma and 2454+/-3 Ma zircon populations most likely derived from felsic volcanism coeval with BIF deposition. These dates constrain the sedimentation rates for the similar to370 m-thick Joffre Member BIF to >15 m per million years. Siliceous tuffaceous mudrocks are not present in the underlying similar to120 m-thick Dales Gorge Member and it is uncertain whether previously reported ages of ca 2479-2470 Ma for this unit reflect detrital/xenocrystic or syndepositional zircon populations in resedimented stilpnomelane-rich tuffaceous mucrocks. The increased abundance of tuffaceous mucrocks in the Joffre Member suggests that a pulse of enhanced igneous and hydrothermal activity accompanied deposition of the bulk of the Brockman Iron Formation BIF after cc 2460 Ma, This preceded and culminated in the emplacement of the 2449+/-3 Ma large igneous province represented by BIF and igneous rocks of the Weell Wolli Formation and Woongarra Rhyolite.