Banded iron formation to iron ore: A record of the evolution of Earth environments? K.A. Evans1, T.C. McCuaig2, D. Leach2, T. Angerer2, and S.G. Hagemann2 1Department of Applied Geology, Curtin University, GPO Box U1987, WA 6845, Australia 2Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia, Crawley, Perth, WA 6009, Australia ABSTRACT of fl uids enter the BIF in a silica-undersaturated Banded iron formations (BIF) are the protolith to most of the world’s largest iron ore state. Here, we combine numerical constraints deposits. Previous hypogene genetic models for Paleoproterozoic “Lake Superior” BIF-hosted with petrological, geochemical and isotopic deposits invoke upwards, down-temperature fl ow of basinal brines via complex silica and characteristics of BIF-hosted iron ore from the carbonate precipitation/dissolution processes. Such models are challenged by the necessary Hamersley province, Western Australia, to con- SiO2 removal. Thermodynamic and mass balance constraints are used to refi ne conceptual strain the conditions associated with silica loss models of the formation of BIF-hosted iron ore. These constraints, plus existing isotope and and carbonate precipitation, and discuss the halogen ratio evidence, are consistent with removal of silica by down- or up-directed infi ltra- fi ndings in the context of the temporal evolution tion of high-pH hypersaline brines, with or without a contribution from basinal brines. The of the world’s atmosphere and oceans. proposed link to surface environments suggest that Paleoproterozoic BIF-ore upgrade may provide a record of a critical time in the evolution of the Earth’s biosphere and hydrosphere. DEPOSIT SCALE PARAGENESIS The mineralogy of BIF, hydrothermally altered INTRODUCTION The challenge for models that involve BIF, and iron ore varies (e.g., Thorne et al., 2004; Banded iron formations (BIFs) record chang- removal of SiO2 from BIF by down-temperature Rosiere et al., 2008; Mukhopadhyay et al., 2008; ing environmental conditions and are a precur- fl ow is that quartz solubility decreases with Angerer and Hagemann, 2010) but some features sor to the world’s largest iron ore deposits. Vast decreasing temperature (e.g., Manning, 1994), are suffi ciently common to allow generalization. volumes of BIFs were deposited on passive so fl uid that moves down-temperature is a poor BIF protolith (Figs. 1A, 1B1, and 1B2) consists margins from ca. 2.6 Ga to the Great Oxygen- agent for quartz removal, unless large volumes of magnetite- and chert-rich bands, iron silicates ation Event (GOE) at ca. 2.4 Ga (e.g., Beukes and Gutzmer, 2008). After 1.85 Ga, formation of iron formations essentially ceased until a Protolith Distal: Proximal. Ore magnetite gradual replacement Hematite, hematite restricted resurgence in the late Neoproterozoic chert of Fe-silicates and carbonate and ore-zone (Young, 1976; Klein and Beukes, 1993) and Fe-silicates chert by Fe- Fe silicates. up to 100s carbonates of meters wide 10s to 100s of more recent minor occurrences. sharp front carbonation front A sharp desilication/ meters wide The temporal distribution of BIFs records a complex interplay between a cooling Earth and changes in mantle plume events, continental carbonate growth and tectonics, evolution of the biosphere mesoband 8 and an increased fl ux of iron to the hydrosphere, magnetite 4 magnetite-1 carbonate which in turn had a fundamental control on the mesoband mesoband oxygen contents of the hydrosphere and redox chert± state of the oceans (Isley and Abbott, 1999; Hol- Fe-silicate Fe-silicate ±magnetite mesoband 6 land, 2005; Bekker et al., 2010). In most giant mplH mesoband mplH Paleoproterozoic BIF-hosted iron ore deposits (~35 wt% Fe) the formation of high-grade (>58 2 wt% Fe) iron ore from Lake Superior–type mar mesoband mar BIF is thought to have occurred post 2.2 Ga, mesoband 5 mm 5 mm after the GOE (Taylor et al., 2001; Rasmussen B 3 5 mm 5 5 mm 7 et al., 2007; Thorne et al., 2009), so the BIF- mplH ore upgrade provides a record of the changing magnetite ankerite chemistry of the hydrosphere and atmosphere in mplH mar chert± Fe-silicate siderite the Paleoproterozoic. ± magnetite mar In the Hamersley province, Western Austra- Fe-silicate magnetite 500 µm 500 µm 50 µm 50 µm lia, the BIF-ore upgrade is proposed to involve 2 4 6 8 basinal brines, meteoric fl uids, and supergene > > CarbonationCarbonation frontfront (CO(CO2 = 0.10.1 molmol lL-1-1–1) ) DesilicificationDesilicationDesilication frontfront enrichment (e.g., Morris et al., 1980; Barley 0.1 0.10.1 0.0170.017 0.017 CarbonationCarbonation frontfront (CO(CO = 0.10.1 mmolol lL-1-1–1) ) OxidationOxidation frontfront et al., 1999; Taylor et al., 2001; Thorne et 2 > C 1 >0.00090.0090.009 al., 2004, 2008). Silica removal is proposed to occur by upward, down-temperature fl ow Figure 1. A: Commonly observed banded iron formation (BIF) alteration stages related to the of basinal brines (e.g., Thorne et al., 2004; formation of iron ore and their characteristics. B(b1–b8): Polished blocks and photomicro- graphs of the typical alteration stages. mplH—microplaty hematite; mar—martite. C: Results Gutzmer et al., 2006; Thorne et al., 2008), and of mass balance calculations to determine the relative distances moved by decarbonation, it is this stage of the BIF-ore upgrade that is desilicifi cation and oxidation fronts. Values indicate distance traveled by front relative to the −1 explored in this paper. carbonation front for a solution with 1 mol L CO2. GEOLOGY, February 2013; v. 41; no. 2; p. 99–102; Data Repository item 2013024 | doi:10.1130/G33244.1 | Published online 13 November 2012 GEOLOGY© 2012 Geological | February Society 2013 of America.| www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 99 ± diagenetic carbonates. Initial hydrothermal lower end of this range. Quartz-hosted fl uid expected. Temperature is the fi rst order control alteration (Figs. 1A, 1B3, and 1B4), which may be inclusions (e.g., Brown et al., 2004) records on quartz solubility, whereas calcite solubility gradational, involves conversion of some of the periods of quartz growth, not quartz removal, depends strongly on pressure (Fig. 2A). Salin- silicates to carbonates. Subsequently, chert and so such inclusions may record either a different ity-driven mineral precipitation/dissolution is quartz are replaced by iron-bearing carbonate event, or a later stage of the silica dissolution minor for geothermal gradients <50 °C km−1 with, in some cases, conversion of magnetite to event that is of interest here. (Fig. 2B). There is no geothermal gradient for hematite, often with no apparent change in vol- Iron oxide oxygen isotopes in unaltered which silica dissolution is accompanied by car- δ18 ume. Reaction fronts may be gradual (millimeter BIF have OVSMOW (Vienna Standard Mean bonate precipitation in the observed quantities, to decimeter scale) or knife-sharp on the scale Ocean Water) between 4‰ and 13‰, whereas so it is necessary to consider infi ltration of out- of individual bands. Quartz may be precipitated hematite and magnetite in altered and miner- of-equilibrium fl uids. δ18 − locally in fault zones within shear veins (e.g., alized rocks have OVSMOW between 9‰ Hagemann et al., 1999; Thorne et al., 2010). and −2.9‰ (Thorne et al., 2009). Carbon iso- Fluid out of Equilibrium with BIF The carbonate-bearing alteration assemblage topes of carbonate minerals lie between −10‰ Infi ltration of high-pH, out-of-equilibrium (Figs. 1A, 1B5, and 1B6) is separated from almost and 0‰, relative to Vienna Peedee belemnite fl uid can drive quartz removal via up- or down- pure (hypogene) hematite iron ore (Figs. 1A, (VPDB). The lower values are typical of unal- temperature fl ow, because quartz is 4 orders of 1B7, and 1B8) by a sharp or gradational reaction tered BIF. Values in altered rocks could record magnitude more soluble at pH 9 than it is at pH front. Carbonate loss is thought to occur without equilbrium with either Paleoproterozoic ocean 6 (Busey and Mesmer, 1977). The desilicifi ca- iron mobility on a scale greater than a few cm water or dolomite in the underlying Wittenoom tion/carbonation and oxidation fronts observed (Taylor et al., 2001). Carbonate dissolution con- Formation. Fluid inclusion Na/Br and Cl/Br in BIFs are suffi ciently sharp that they can be tinues in the weathering environment. ratios record overlapping populations of fl uids: treated as advective chromatographic fronts; Upgrade of BIF in the Hamersley province, (1) seawater that has evaporated to halite satu- broadening by diffusion, dispersion and kinetic Western Australia, is thought to have occurred ration; and (2) meteoric waters that interacted broadening can be neglected. The ratio of the from 2.15 Ga onwards during the waning stages with evaporites (Thorne et al., 2010). distance traveled by a fl uid to that of associated of the Opthalmian orogeny (Rasmussen et al., reaction front is given by 2007). There is evidence for extensional faulting CONSTRAINTS ON SILICA REMOVAL d cc− and orogenic collapse at the proposed time of AND CARBONATE PRECIPITATION fluid = i,rock,final i,rock,initial , (1) d θ−( ) ore upgrade (Müller et al., 2005) and it has been rock cci,fluid,initial i,fluid,final suggested that topographic relief drove circula- Fluids in Equilibrium with BIF tion of surface-derived waters through the fault Quartz solubility is sensitive to pressure, tem- (adapted from Evans et al., 2003). d is the posi- system at this time (e.g., Hagemann et al., 1999; perature (e.g., Manning, 1994), pH (Busey and tion of the front relative to the infi ltration hori- Oliver and Dickens, 1999).