The potential for reconstructing primary ocean chemistry from hypogene and supergene altered banded iron formations: An example from Weld Range, Western Australia

Abstract

Pristine banded iron formations (BIF) are established paleo-environmental proxies for reconstructing the elemental and isotopic signatures of the ancient seawater that they precipitated from. Negligible changes in shale-normalised rare earth element patterns in BIF throughout Earth’s history, including features such as low La/Yb ratios, and positive La, Eu, Gd, and Y anomalies, and near- to super-chondritic Y/Ho ratios support the preservation of ancient seawater signatures. Nevertheless, limiting paleo-environmental reconstructions to pristine BIF imparts a significant sampling bias and restricts understanding of the temporal evolution of the oceans. However, altered BIF samples are problematic for paleo-environmental reconstructions due to the risk of disturbance of their primary signatures. Instead, mineral-/fraction-specific analysis potentially provides robust paleo-environmental reconstructions where primary mineral phases are preserved, with the three main mineral fractions in pristine and altered BIF including carbonates (e.g., siderite and ankerite), Fe oxides (e.g., magnetite, hematite, and goethite), and silicates (e.g., quartz and Fe-silicates). This study investigates samples from the ca. 2.7 Ga Weld Range BIF, located in the Youanmi Terrane, Yilgarn Craton, Western Australia. The lower-greenschist facies BIF varies from least-altered to progressively hypogene- and/or supergene-altered. Whole-rock analysis of these rocks revealed the preservation of seawater-like signatures despite significant alteration, such as positive La, Eu and Y anomalies. Additionally, sequential extraction techniques were performed on the least-altered and altered BIF samples to separately analyse the carbonate, Fe oxide, and silicate mineral fractions. In both the least- and hypogene-altered samples all fractions preserved evidence for seawater-like chemistry despite extensive precipitation of secondary hypogene carbonate and Fe oxide minerals in the latter. The seawater-like characteristics preserved in the hypogene carbonate and Fe oxide-fractions are the result of the seawater-magmatic fluid mixture that precipitated hypogene replacement minerals. Therefore, we interpret the silicate-fraction to be the most indicative of the primary seawater that precipitated the Weld Range BIF, where the quartz/chert reflects amorphous silica signatures that are unaffected by low-grade metamorphism and hypogene alteration. The preservation of primary mineral phases (i.e., silicates) and characteristic seawater signatures in the extensively altered Weld Range BIF, suggests that altered BIF should be more widely investigated to improve the breadth and representativeness of global paleo-environmental reconstructions.