The Joffre banded iron formation, Hamersley Group, Western Australia: Assessing the palaeoenvironment through detailed petrology and chemostratigraphy

Low‐Fe(III) Greenalite Was a Primary Mineral From Neoarchean Oceans

“…Therefore, we suggest that the stilpnomelane particles in Silvergrass 313.6 m are alteration products from earlier greenalite inclusions. Another possibility is that the stilpnomelane reflects detrital grains from volcanic sources (Haugaard et al, 2016;Laberge, 1986;Pickard, 2002). Because all 10 samples that we examined contained wellpreserved greenalite encased in chert, our results indicate that greenalite was a primary mineral forming from Neoarchean seawater.…”

“…The data from this distinct set of particles were consistent with the presence of a 10 Å layered silicate containing K, Fe, Mg, and Al cations, like the complex hydrated phyllosilicate stilpnomelane [(K,Ca,Na)(Fe,Mg,Al) 8 (Si,Al) 12 (O,OH) 36 · nH 2 O] (Figures 1 and S4E). Another possibility is that the stilpnomelane reflects detrital grains from volcanic sources (Haugaard et al, 2016;Laberge, 1986;Pickard, 2002). In BIFs, stilpnomelane is generally observed texturally as forming secondarily, associated with recrystallization and metamorphism (Feininger, 1984;Klein, 1974), and this Silvergrass sample in thin section appeared petrographically to be more recrystallized and altered.…”

“…Iron silicates such as minnesotaite, greenalite, and stilpnomelane have long been identified in BIFs, but the consensus view categorized these as secondary minerals or derived from volcanic input (Beukes & Gutzmer, 2008;Fischer & Knoll, 2009;Haugaard et al, 2016;Klein, 2005). Iron silicates such as minnesotaite, greenalite, and stilpnomelane have long been identified in BIFs, but the consensus view categorized these as secondary minerals or derived from volcanic input (Beukes & Gutzmer, 2008;Fischer & Knoll, 2009;Haugaard et al, 2016;Klein, 2005).…”

“…Additional factors make these pervasive iron silicate inclusions a compelling initial mineral during BIF deposition. Iron silicates such as minnesotaite, greenalite, and stilpnomelane have long been identified in BIFs, but the consensus view categorized these as secondary minerals or derived from volcanic input (Beukes & Gutzmer, 2008;Fischer & Knoll, 2009;Haugaard et al, 2016;Klein, 2005). There had been speculation that primary iron silicates could form in deeper-water and more alkaline environments as ferrous iron and silica reacted, but these minerals were not thought to be the major primary iron phase in BIFs (e.g., Beukes & Gutzmer, 2008;Klein, 1974).…”

Follow the High Subcritical Water

“…The crystallized and amorphous minerals described above are ejected from the conduits, fall on the ocean floor, and form layers, as in Figure 2a in [7], where micromillimeter-size strata of quartz and siderite were observed in a thin sample of the 899 m long BARB3 ICDP drill core. Figures 3 and 4 in Haugaard et al 2016 show the same kind of microbands for the Joffre banded iron formation [46]. Figure 3 shows layers of magnetite, chert (silica), hematite, and riebeckite.…”