The Joffre Banded Iron Formation, Hamersley Group, Western Australia: Assessing the Palaeoenvironment through detailed Petrology and Chemostratigraphy
Haugaard et al
The Joffre Member of the Brockman Iron Formation is by volume the largest single known banded iron formation (BIF) in the world. Here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this ∼2.45 billion year old unit. Oxide BIF and silicate-carbonate-oxide BIF dominate the lithology, with minor amounts of interbedded stilpnomelane mudrock, stilpnomelane-rich tuffaceous mudrock and calcareous mudrock. Beside chert and magnetite, the prominent mineralogy is riebeckite, ankerite, hematite, stilpnomelane and crocidolite. The BIF is characterised by an average of 50 wt.% SiO2 and 44.5 wt.% Fe2O3 and an overall low abundance of Al2O3 (less than 1 wt.%), TiO2 (less than 0.04 wt.%), and trace metals such as Cr (less than 10 ppm), Ni (less than 5 ppm) and Mo (less than 0.5 ppm). It has a high ∑REE (rare earth element) content (up to 41 ppm) and a fractionated shale-normalised (SN) seawater REY (rare earth element + yttrium) pattern having an enrichment of HREE (heavy rare earth elements) relative to LREE (light rare earth elements) with an average (Pr/Yb)SN of 0.24. The REY patterns also show a positive LaSN anomaly, no CeSN anomaly and a weakly developed positive YSN anomaly. Iron isotopes (δ56Fe) with positive δ56Fe values of +0.04‰ to +1.21‰ suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides. No epiclastic grains have been found; rather submarine hydrothermal fluids and fine-grained volcanogenic detritus controlled BIF chemistry. The former source is reflected through a constant positive EuSN anomaly throughout the core (average EuSN anomaly of 1.6 with a peak of 2.1 between 100-155 m depth), while the latter source is best reflected through the stilpnomelane-rich tuffaceous mudrock consisting of volcanic ash-fall tuff with relict shards set in a stilpnomelane matrix. The mudrock is overlain by well-preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re-worked volcanic ash formed either on the seafloor or in the water column prior to deposition. An enriched HREE-to-LREE pattern, a high iron content (∼30 wt%), and a δ56Fe value of +0.59‰ collectively imply that the mudrock facies interacted with the Fe-rich seawater prior to deposition. The TiO2-Zr ratio of the BIF and the associated mudrocks suggest a felsic-only-source related to the same style of volcanics as the slightly younger Woongarra rhyolites. Given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics, we speculate that the fine-grained pelagic ash particles may have sourced bio-available nutrients to the surface water. This would have facilitated enhanced biological productivity, including bacterial Fe(II)-oxidation which is now recorded as the positively fractionated 56Fe iron oxide minerals in the Joffre BIF. Alongside submarine hydrothermal input to the basin, the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways, thereby making the Joffre BIF poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses.