Archean cherts in banded iron formation: Insight into Neoarchean ocean chemistry and depositional processes

Interpretations and implications of LA ICP-MS analysis of chert for the origin of geochemical signatures in banded iron formations (BIFs) from the Meadowbank gold deposit, Western Churchill Province, Nunavut

“…Therefore, the Queensland alluvial shale composite (MUQ) was used to normalize the REE + Y values to decrease the influence of terrigenous input. The MUQ represents a bimodal felsic and mafic volcanic provenance , which is similar to the expected average terrigenous input from bimodal greenstone belt volcanism into the Archean ocean (e.g., Bolhar et al, 2005;Thurston et al, 2012). Concentrations of elements were determined using off-line calculations, following the protocol of Longerich et al (1996) using the NIST 612 glass standard as a primary reference material and SiO 2 as an internal standard.…”

“…It is noticed that Gd anomalies reported in previous articles on REE + Y data from BIF precipitated in Archean seawater (e.g., Bolhar et al, 2005;Thurston et al, 2012) were small to nonexistent and is therefore not referred again in this article. Any Gd anomaly present is considered due to a combination of a small positive Gd anomaly associated with seawater (Bau and Dulski, 1996) and slightly− negative Gd anomaly associated with hydrothermal vent fluid (e.g., Allwood et al, 2010).…”

“…5; e.g., Bau and Dulski, 1996;Thurston et al, 2012) which shows: a. Depletion in light rare-earth elements (LREE) relative to heavy rareearth elements (HREE); b. A strongly super-chondritic Y/Ho ratio (i.e., N 27), which produces a positive Y anomaly that is often between 40 and 90; c. A positive La anomaly (La/La* between 1.15 and 1.3); d. Positive Gd anomaly (Gd/Gd* between 1.3 and 1.5); e. A well-developed, negative Ce anomaly resulting from the oxidation of Ce +3 to Ce +4 in the water column.…”

“…The iron-bearing minerals in iron formations precipitated from basin waters and hydrothermal fluids; they include siderite or/and iron oxyhydroxides that were transformed by diagenesis and metamorphism to hematite, magnetite, iron silicates and sulfides. The origin of chert is controversial, but the consensus is that it, like the iron-bearing minerals, originated as either a seawater (e.g., Bolhar et al, 2005;Thurston et al, 2012) or hydrothermal (e.g., Allwood et al, 2010;Thurston et al, 2012) precipitate or by replacement of a precursor volcanic unit due to silicification (e.g., Hanor and Duchac, 1990).…”

Sedimentology of the ∼3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa

“…The negative Ce anomalies that are observed in these samples probably resulted from post-depositional alteration rather than reflecting ambient Archean seawater redox conditions (Hayashi et al, 2004). Previous authors have applied same set of criteria -positive Y, Gd, and La anomalies and LREE depletion -to conclude that Archean and early Paleoproterozoic cherts formed as marine chemical sediments, including, carbonates and chert from the 3.45 Ga Strelley Pool Chert (Pilbara Craton, Western Australia) (Allwood et al, 2010) and BIF-associated chert from the 2.75-2.7 Ga Abitibi greenstone belt (Thurston et al, 2012) and the ~2.7 Ga Woodburn Lake greenstone belt (Gourcerol et al, 2015), both in Canada.…”