Marra Mamba Iron Formation Research Articles

Rare earth element and yttrium (REY) geochemistry of 3.46–2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification.

Thermal magnetic analysis on iron ores and banded iron formations (BIFs) in the Hamersley Province: Implications of origins of magnetic minerals and iron ores

<abstract> <p>The genesis models of the iron-ores hosted in banded iron formations (BIFs) in the Hamersley Province of Western Australia have been debated since the iron-ore deposits were discovered in the 1960s. The existing models considered the few physicochemical conditions for the iron-ore enrichment from BIFs. This study incorporates the latest research outcomes in conversions among the major magnetic minerals under different physicochemical conditions with the thermal magnetic analysis for BIFs and iron-ores collected from the Hamersley Province to fill the gap in knowledge highlighted by existing studies of the iron ores and BIFs. The results indicate that the high-grade hematite ores might have been undergone a physicochemical process under hydrothermal conditions between 120 ℃ and 220 ℃ during the major stage of enrichment from the original BIFs in the Brockman Iron Formation. Such physicochemical conditions would require either that the BIF units were buried 4000–5000 m underground with tilted broad channels formed by large-scale deformation in the region that facilitates hydrothermal reactions and leaching by the fluids flowing down deep to 4000–5000 m, somehow similar to the deep-seated supergene model proposed in previous works, or that the BIF units were still buried but the hydrothermal fluids coming up from deeper sources spread widely over the broad channels to ensure the high-grade hematite ores are consistently uniform over the entire deposit. The large-scale martite-goethite deposits in the Marra Mamba Iron Formation might be derived from multiple supergene phases from hematite-martite ores below 100 ℃ in the natural process of oxidization near surface, somewhat similar to the existing model for the channel iron deposits. Magnetite contained within current BIFs and iron ores was least likely derived from primary hematite in BIFs.</p> </abstract>

Petrography of martite–goethite ore and implications for ore genesis, South Flank, Hamersley Province, Western Australia

Martite–goethite (M–G) ores are characterised by the pseudomorphic textural replacement of gangue phases (carbonate, silicate and quartz) in the primary banded iron formation (BIF) by fine-grained goethite. This differentiates them petrographically from rocks enriched in Fe as a result of lateritic weathering and leaching of gangue. A preliminary petrographic study of martite–goethite mineralisation from the Grand Central deposit at BHP’s South Flank in the central Hamersley Province reveals that pseudomorphic textures are abundantly preserved. The mineralisation occurs in the Marra Mamba Iron Formation. Magnetite is converted to martite, and the quartz–silicate–carbonate bands are pseudomorphed by goethite. Of the gangue phases, carbonate and silicate minerals are more susceptible to replacement than is quartz. The replacement of quartz by goethite does not always continue to completion, and relict quartz grains are typically preserved on the lower margin and down-dip extremity of the body of mineralised BIF. The mineralisation appears to take place in three stages. The first stage involves replacement of magnetite by hematite (martitisation) and of the carbonate–silicate–quartz gangue by very fine-grained goethite that pseudomorphs and thus preserves the original texture of the non Fe-oxide portion of the BIF protolith. This ‘Stage 1’ goethite generation appears to be ochreous. Micro-porosity development, most likely developing through dissolution of any remaining quartz that has escaped replacement by goethite, although possibly through the dissolution of goethite after quartz, forms the second stage. The third and final stage is characterised by infill of the micro-porosity to varying degrees by a second generation of coarser-grained goethite that typically rims pores, giving them a comb-textured, drusy appearance. This ‘Stage 3’ goethite generation appears to be brown goethite. This tripartite paragenesis has not been explicitly recognised in previous petrographic studies of M–G mineralisation. Subsequent lateritic weathering affects material in the vadose zone above the pre-mining water-table, introducing additional complexity to the mineralisation. Further dissolution takes place, resulting in meso-scale porosity that is clearly visible to the naked eye. Voids are rimmed by a colloform intergrowth of goethite and hydrohematite. This ‘Stage 4’ goethite is likely to be vitreous goethite. Previous studies have shown that the three different types of goethite (ochreous, brown and vitreous) have distinctly different chemical and physical characteristics, and behave differently during downstream processing of Fe ore, so it is important to understand their genesis and distribution in time and space. KEY POINTS The process of M-G mineralisation involves a tripartite paragenesis: Stage 1—replacement of magnetite by hematite (martitisation) and of the carbonate–silicate–quartz gangue by very fine-grained goethite that pseudomorphs the original texture, Stage 2—dissolution of remaining gangue phases (principally quartz), and Stage 3—infill of the micro-porosity to varying degrees by a second generation of coarser-grained goethite. Overprinting effects of weathering form Stage 4—further dissolution and reprecipitation of Fe as colloform goethite-(hydro)hematite layers lining vugs. Goethites formed during the three different stages are likely to have different chemical and physical properties and this has implications for geometallurgical and materials handling behaviour during processing.

A New Fluid-Flow Model for the Genesis of Banded Iron Formation-Hosted Martite-Goethite Mineralization, with Special Reference to the North and South Flank Deposits of the Hamersley Province, Western Australia

AbstractThe North and South Flank deposits are located on the flanks of the Weeli Wolli anticline at Mining Area C in the central Hamersley Province. Supergene martite-goethite mineralization is hosted within the Marra Mamba Iron Formation and is developed over a strike length of more than 60 km. This multibillion metric ton resource has been drilled out on a 150- × 50- to 50- × 50-m grid, thus providing us with an unprecedented data set for analysis. This study synthesizes the drill hole data and presents a physical process model that can account for the observed distribution of mineralization.A fluid and mass flux model is proposed which envisages a three-stage process: (1) leaching of Fe from banded iron formation (BIF) in the vadose zone by reduced, acidic, meteoric-derived fluids; (2) penetration of an Fe-rich supergene-fluid plume, driven by gravity and focused by bedding-parallel permeability into the body of ambient alkaline groundwater, effecting nonredox, mimetic replacement of magnetite by hematite and of the gangue minerals (carbonate, silicate, and chert) by goethite coupled with the release of silica into the fluid phase; and (3) a change from silica leaching to silica deposition on the downdip margins of the system before the ore-fluid plume is eventually diluted and becomes indistinguishable from the surrounding body of groundwater.Despite the undoubted secondary role played by structurally enhanced permeability, the primary control on ore-fluid hydrology is gravity-driven flow along bedding planes. This central observation explains every observed feature of the three-dimensional distribution of martite-goethite mineralization, and the inherited structural architecture simply provides the context for this process to play out. This type of control is by no means obvious–the ingress of meteoric fluids during later lateritic weathering of the mineralization does not show this control and produces broadly subhorizontal, bedding-discordant zones of overprinting.The fundamental control exerted on the distribution of martite-goethite mineralization by bedding-plane permeability within BIF horizons suggests that the supergene ore-fluid plume created its own porosity via the relevant ore-forming reactions, and that these were in turn controlled by bedding. A corollary of the pseudomorphic replacement process, both the generation of hematite after magnetite and goethite after gangue phases, is that it typically introduces porosity. The mineralizing process thus creates porosity (and potentially permeability) and is likely to be self-propagating as long as there is continuous supply of ore fluid. This putative active porosity-generation process may be an important clue as to the unique conditions of martite-goethite ore formation. Indeed, it may be that the distribution of magnetite is the critical controlling feature of these ore systems, as the nonredox transformation to hematite not only releases Fe2+ to the fluid phase but concurrently introduces porosity. Further research is required to formulate a comprehensive chemical (as opposed to physical) process model for supergene martite-goethite ore formation.Based on the physical process model presented here, the development of a large-scale martite-goethite mineralizing system requires continued delivery of unleached BIF (and, perhaps ultimately, previously mineralized martite-goethite ore) into the vadose zone. The Hamersley Province has been undergoing significant uplift since at least 60 Ma. Preliminary dating of martite-goethite ores from Mining Area C indicates that they formed at about 45 Ma, at a time when the local climate was temperate and wetter than today. The combination of ongoing uplift and a wet, temperate climate is likely to be the key to the widespread formation of martite-goethite deposits in the Hamersley Province.

Learning characteristic natural gamma shale marker signatures in iron ore deposits

Uncertainty in the location of stratigraphic boundaries in stratiform deposits has a direct impact on the uncertainty of resource estimates. The interpretation of stratigraphic boundaries in banded iron formation (BIF)-hosted deposits in the Hamersley province of Western Australia is made by recognizing shale markers which have characteristic signatures from natural gamma wireline logs. This paper presents a novel application of a probabilistic sequential model, named a continuous profile model, which is capable of jointly modelling the uncertainty in the amplitude and alignment of characteristic signatures. We demonstrate the accuracy of this approach by comparing three models that incorporate varying intensities of distortion and alignment in their ability to correctly identify a shale band of the West Angelas member of the Wittenoom Formation which overlies the Marra Mamba Iron Formation in the Hamersley Basin. Our experiments show that the proposed approach recovers 98.72% of interpreted shale band intervals and importantly quantifies the uncertainty in scale and alignment that contribute to probabilistic interpretations of stratigraphic boundaries.

Stratigraphic and structural setting of iron mineralisation at E Deposit (East), Area C, Hamersley Province, Western Australia

E Deposit is located at Area C iron ore operations in the central region of the Hamersley Province, Western Australia. It is hosted by the Marra Mamba Iron Formation, the lowermost unit of the Archaean to Lower Proterozoic Hamersley Group. Mineralisation at E Deposit East is contained within the Mount Newman Member of the Marra Mamba Iron Formation and extends into the lower West Angela Member of the overlying Wittenoom Formation. Use of a combination of geological and geophysical techniques has enabled the geological setting of the deposit to be better understood. Interpretation of detailed mine face mapping, downhole geophysics, geochemistry, geological logging and photography has identified D2-related thrust faulting with substantial south-over-north displacement. Similarly, in-pit mapping at the nearby C Deposit has revealed intense localised D2 deformation including overturning and thrust faulting. The interpreted structure illustrates that iron ore deposits in the central Hamersley Province can be intensely deformed; and they require detailed geological investigation using technical data from mapping and drilling to create robust grade-models for the mining operation.

Journey to the edge of the basin – stratigraphic setting and iron mineralisation at the Davidson Creek and Robertson Range Projects, Hamersley Province, Western Australia

The Robertson Range and Davidson Creek Projects are located in the Hamersley Province about 100 km east of Newman, Western Australia. Significant iron mineralisation within the Marra Mamba Iron Formation is shallowly north dipping at Davidson Creek, and mainly shallow east dipping at Robertson Range, forming part of a regional scale anticline with the granitic and greenstone basement of the Sylvania Inlier as its core. With minor exceptions, the stratigraphy is comparable with that seen in the main Hamersley Basin. Granite and greenstone basement rocks of the Sylvania Inlier are unconformably overlain by shale and volcanic rocks of the Fortescue Group. The overlying Hamersley Group is locally represented by interbedded chert, banded iron-formation (BIF) and shales of the Marra Mamba Iron Formation (Nammuldi, MacLeod and Mount Newman Members), and dolomite, chert, BIF and shale of the Wittenoom Formation. Iron enrichment occurs predominantly in the Mount Newman Member, and to a lesser extent in the West Angela Member of the Wittenoom Formation. Enrichment varies from 20 to 30 m in thickness in elongate bodies up to 3 km in strike length, and generally thins down dip. At Davidson Creek iron mineralisation is a hematite-goethiteochreous goethite assemblage, whereas at Robertson Range this assemblage is consistently overprinted by a goethitic alteration related to hardcap weathering. Near surface hardcap weathering in both project areas comprises more siliceous, aluminous and vitreous goethite.

Iron and carbon isotope evidence for ecosystem and environmental diversity in the ∼2.7 to 2.5Ga Hamersley Province, Western Australia

The largest excursion in kerogen δ13C and bulk/mineral δ56Fe values yet measured in the ancient rock record occurs in rocks of ∼ 2.7 to 2.5 Ga age. New Fe isotope data integrated with previously collected C isotope data on the same samples document the metabolic diversity of microbial communities in the Neoarchean Hamersley Province of the Pilbara Craton in Western Australia. Samples of shales, carbonates, and mixed carbonate/shale lithologies were collected from three drill cores; two cores from the depocenter of the province and one from the margin. Shallow-water clastic/carbonate rocks deposited in the center of the province (Tumbiana Formation) record kerogen δ13C values that indicate C cycling by various anaerobic or aerobic methane pathways, but the restricted range in δ56Fe values indicates little or no Fe redox cycling. Deep-water sediments deposited contemporaneously in both parts of the Hamersley Province (Jeerinah Formation) record slightly positive δ56Fe values in the relatively shallower and suboxic margin, but strongly negative δ56Fe values in the deeper euxinic depocenter of the province, a pattern consistent with Fe cycling via a basin Fe shuttle, driven by bacterial dissimilatory iron reduction (DIR). Kerogen δ13C values from these units indicate coupling of microbial Fe cycling to aerobic methanotrophy or anaerobic oxidation of methane. Younger black shales, intercalated with iron formation (Marra Mamba Iron Formation) in the depocenter, record a shift to near-zero δ56Fe values reflecting an Fe budget dominated by hydrothermal and clastic sources. However, time-equivalent, Fe-rich carbonate/shale lithologies deposited on the margin of the province (Carawine Dolomite) have δ56Fe values that steadily decrease from near zero to strongly negative values. These relatively Fe-rich carbonates may reflect a carbonate trap of a DIR-driven Fe shuttle, similar to the sulfidic trap in the euxinic portion of the Jeerinah Formation. In contrast, younger shallow-water carbonates deposited by turbidity currents in relatively deep water in the center of the province (Wittenoom Formation), have δ56Fe values that correlate with Fe concentrations, a pattern that indicates Fe cycling by Rayleigh fractionation through precipitation of iron oxides from aqueous ferrous iron.

Asteroid mega-impacts and Precambrian banded iron formations: 2.63 Ga and 2.56 Ga impact ejecta/fallout at the base of BIF/argillite units, Hamersley Basin, Pilbara Craton, Western Australia

Abstract The temporal association between late Archaean to earliest Proterozoic asteroid impact ejecta/fallout units and overlying banded iron formations suggests that, in some instances, these impacts were closely followed by significant transformation in the nature of source terrains of the sediments. The Jeerinah Impact Layer (JIL) [B.M. Simonson, D. Davies, S.W. Hassler, Discovery of a layer of probable impact melt spherules in the late Archean Jeerinah Formation, Fortescue Group, Western Australia. Aust. J. Earth Sci. 47 (2000) 315–325; B.M. Simonson, S.W. Hassler, Revised correlations in the early Precambrian Hamersley Basin based on a horizon of resedimented impact spherules. Aust. J. Earth Sci. 44 (1997) 37–48; B.M. Simonson, B.P. Glass, Spherule layers — records of ancient impacts. Ann. Rev. Earth Planet. Sci. 32 (2004) 329–361; A.Y. Glikson, Early Precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47–2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. Astrobiology 4 (2001) 19–50; S.W. Hassler, B.M. Simonson, D.Y. Sumner, D. Murphy, Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups, Western Australia: stratigraphic and depositional implications of re-correlation. Aust. J. Earth Sci. 52 (2005) 759–772; B. Rasmussen, C. Koeberl, Iridium anomalies and shocked quartz in a late Archean spherule layer from the Pilbara Craton: new evidence for a major asteroid impact at 2.63 Ga. Geology 32 (2004) 1029–1032; B. Rasmussen, T.S. Blake, I.R. Fletcher, U–Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah–Carawine impact ejecta layer. Geology, 33 (2005) 725–728.] overlies an argillite-dominated unit (Jeerinah Formation, 2684 ± 6 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and lies directly below a thin volcanic tuff (2629 ± 5 Ma, [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and banded iron formation (BIF) (upper part of Marra Mamba Iron Formation, 2597 ± 5 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]). The Spherule Marker Bed (SMB) [B.M. Simonson, Geological evidence for an early Precambrian microtektite strewn field in the Hamersley Basin of Western Australia. Geol. Soc. Am. Bull. 104 (1992) 829–839; B.M. Simonson, S.W. Hassler, K.A. Schubel, Lithology and proposed revisions in stratigraphic nomenclature of the Wittenoom Formation (Dolomite) and overlying formations, Hamersley Group, Western Australia. Geol. Surv. W. Aust. Rep. 345 (1993) 65–79; S.W. Hassler, B.M. Simonson, D.Y. Sumner, D. Murphy, Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups, Western Australia: stratigraphic and depositional implications of re-correlation. Aust. J. Earth Sci. 52 (2005) 759–772. [5] ], which includes two impact cycles [A.Y. Glikson, Early Precambrian asteroid impact-triggered tsunami: excavated seabed, debris flows, exotic boulders, and turbulence features associated with 3.47–2.47 Ga-old asteroid impact fallout units, Pilbara Craton, Western Australia. Astrobiology 4 (2001) 19–50.], is located at the top of a carbonate/calcareous siltstone-dominated sequence (Bee Gorge Member, Wittenoom Formation, 2565 ± 9 Ma [A.F. Trendall, W. Compston, D.R. Nelson, J.R. deLaeter, V.C. Bennett, SHRIMP zircon ages constraining the depositional chronology of the Hamersley Group, Western Australia. Aust. J. Earth Sci. 51 (2004) 621–644.]) and below a carbonate-poor siltstone–chert–BIF sequence (Mount Sylvia Formation, Bruno's Band [BIF], Mount McRae Shale, 2504 ± 5 Ma [B. Rasmussen, T.S. Blake, I.R. Fletcher, U–Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah–Carawine impact ejecta layer. Geology, 33 (2005) 725–728.]). No ferruginous sediments overlie impact layers hosted by stromatolitic carbonates (

Iron formation-hosted iron ores in the Hamersley Province of Western Australia

Iron formation-hosted iron ore deposits account for the majority of current world iron ore production and consist of three classes: unenriched primary iron formation with typically 25 to 45 wt-%Fe; martite–goethite ore formed by supergene processes, with abundant hydrous iron oxides containing 60 to 63 wt-%Fe; high-grade hematite ores thought to be of hypogene or metamorphic origin overprinted by subsequent supergene enrichment with 60 to 68 wt-%Fe. Individual iron ore deposits range from a few millions of tonnes to over two billion tonnes at >64 wt-%Fe, although most are within the range of 200 to 500 Mt. In the Hamersley province of Western Australia, martite–goethite ores are largely developed in the Marra Mamba iron formation, although high (>0·08 %P) phosphorous mineralisation is also well developed in the stratigraphically higher Brockman iron formation. While the vast majority of high-grade microplaty hematite ore is best developed in the Brockman iron formation, the present paper provides the first textural evidence of locally significant microplaty hematite mineralisation in the Nammuldi member of the Marra Mamba iron formation, in the Chichester Ranges at the Christmas Creek, Cloud Break and Mount Nicholas prospects. Petrographic studies have identified a high-grade Fe texture composed of nanometre scale plates of hematite in mineralised sections of primary microplaty hematite deposits in the Pilbara and elsewhere, well below the normal depth of weathering or dehydration. The population of nanometre to micrometre scale hematite plates is interpreted to represent various stages of nucleation, crystallisation and progressive growth of hematite from the primary ore-forming fluid in areas that were once iron-rich carbonates or silicates in the banded iron formation.

The Genesis of the Hope Downs Iron Ore Deposit, Hamersley Province, Western Australia

The banded iron formation (BIF)-hosted Hope Downs high-grade hematite ore deposits are situated within the Marra Mamba Iron Formation with subsidiary deposits in the Brockman Iron Formation of the Archean to Proterozoic Hamersley Group of Western Australia. The main orebody extends to 260 m below the surface and is unusually rich in martite (pseudomorphous hematite after magnetite) and poor in limonite and goethite compared to other ore deposits of the Marra Mamba Iron Formation. The high-grade hematite ore is mainly within the Newman Member but also occurs in parts of the Nammuldi Member together with low-grade limonitic ore that becomes high grade after calcining. Karst erosion of the overlying Wittenoom Formation has produced steep-sided buried valleys adjacent to the in situ orebodies that contain thick deposits (<160 m) of goethitic and sideritic sediments, including remnants of Robe Pisolite Formation, bedded siderite, hematite gravels, red ochreous detrital material, and enriched scree deposits that are additional sources of ore. The ore consists of low phosphorous martite-limonite-goethite derived from chert-free BIF by supergene weathering. No evidence of the complete carbonate replacement of chert has been found at Hope Downs nor were any traces of preexisting chert bands seen in the ore, despite the abundance of chert bands in BIF elsewhere. A variety of textures and composition shown by cherty BIF adjacent to the orebodies is described from which the origin of the chert-free BIF is inferred, including sedimentary structures consistent with density-current deposition. A model is presented for the origin of the host iron formation and the ore deposits, in which density currents transported reworked iron silicates and hydroxides in colloidal suspension onto an unstable sea floor. The amorphous silica produced during diagenesis of Al-poor iron silicates formed the characteristic chert bands of BIF but some of the hydrous amorphous silica was lost prior to lithification to form chert-free BIF. Weathering of the chert-free BIF produced the high-grade hematite ore that is exposed today.

U-Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah-Carawine impact ejecta layer

Seven spherule beds interpreted to be distal impact ejecta were recently found in late Neoarchean and early Paleoproterozoic successions of northwestern Australia and South Africa. Inadequate age constraints have led to uncertainty about the number of impacts they represent and their correlation. Here we report two new isotopic ages from the Pilbara region of Western Australia. Zircons from a tuff in the Carawine Dolomite, ∼30 m below the Carawine spherule layer, give a 2 0 7 Pb/ 2 0 6 Pb age of 2630 ′ 6 Ma, suggesting that the Carawine and ca. 2.63 Ga Jeerinah spherule layers formed during a single impact, significantly increasing the known area of the strewn field to >17,000 km 2 . This date also requires that the Carawine Dolomite was probably time equivalent to the uppermost Jeerinah Formation and lowermost Marra Mamba Iron Formation, negating previous lithostratigraphic correlations and suggesting that the deposition of banded iron formation was synchronous with carbonate precipitation in a shallow-water platform. A tuff in the Mount McRae Shale yielded a zircon 2 0 7 Pb/ 2 0 6 Pb age of 2504 ′ 5 Ma, providing new age constraints on the overlying Wittenoom and Dales Gorge spherule layers. The new dates show that the Hamersley spherule layers represent three impacts that occurred ca. 2.63 Ga, 2.56-2.50 Ga, and 2.50-2.48 Ga. The three Hamersley spherule beds broadly correlate with three beds in South Africa, highlighting the possible global extent of the impact ejecta fallout. The layers potentially provide three geologically instantaneous markers spanning 150 m.y. across the Archean-Proterozoic boundary, possibly underpinning a chronostratigraphic framework across two continents and constraining late Neoarchean paleogeography.

Marra Mamba Iron Formation stratigraphy in the eastern Chichester Range, Western Australia

The Marra Mamba Iron Formation is the basal member of the Hamersley Group in Western Australia and is host to major iron‐ore deposits in its upper Mt Newman Member. Previous studies have suggested that the Marra Mamba Iron Formation in the eastern Chichester Range was deposited in an isolated basin behind a barrier reef and was greatly reduced in thickness. New drilling covering a large part of the Chichester Range shows that only the lower part of the Nammuldi Member, the lowest unit of the Marra Mamba Iron Formation, remains over most of the range. However, 15 m of the Mt Newman Member, the complete MacLeod Member and the upper part of the Nammuldi Member were drilled in small north‐south synclines preserved near Mulga Downs homestead. This drilling shows that the Marra Mamba Iron Formation is similar in all respects to other occurrences in the Hamersley Province. There is no evidence of facies change or reduced thickness and therefore no evidence for a northern limit to the deposition of the Hamersley Group. Apart from the small synclines near Mulga Downs homestead, the Fortescue River alluvial deposits abutting the Chichester Range are underlain by the Roy Hill Shale Member of the Jeerinah Formation.

Precise zircon U‐Pb ages from the Marra Mamba Iron Formation and Wittenoom Formation, Hamersley Group, Western Australia

The Mt Bruce Supergroup of the Pilbara Craton of Western Australia was laid down in the Hamersley Basin, unconformably over a basement of granite‐greenstone terrane, and consists of the Fortescue, Hamersley and Turee Creek Groups. The base of the ∼230 m‐thick Marra Mamba Iron Formation, the lowest formation of the Hamersley Group, marks the onset of the major banded iron‐formation deposition that characterises the group. Ion microprobe U‐Pb isotope analyses of zircons from a tuff band (NS3) within a high‐grade iron orebody in the uppermost (Mt Newman) member show two distinct, and non‐overlapping, age populations. Fifteen grains from the younger group of 16 have a pooled age of 2597±5Ma (95% confidence), interpreted as the age of syndepositional volcanism; individual grains of the older group of 8, with ages between ca 2950 Ma and ca 2820 Ma, are interpreted as xenocrysts derived from basement rocks transected by the rising magma. The conformably overlying Wittenoom Formation, ∼300–600 m thick, consists largely of dolomitic rocks. Zircons from a submarine fallout tuff within the uppermost (Bee Gorge) member also have two non‐overlapping zircon age populations. Most of the 23 zircons analysed belong to an older population with near‐concordant ages between ca 2750 and ca 2650 Ma, but 12 analysed spots from 7 grains have a pooled age of 2561 ±8 Ma. This is interpreted to be the age of volcanism coeval with deposition. The older zircons were probably derived from tuffs of Fortescue Group age, entrained with the Crystal‐rich Tuff ejectamenta during its explosive, probably phreatoplinian, eruption.

Genetic modelling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia

The banded iron-formation (BIF) of the Hamersley Group, Pilbara Craton, Western Australia, particularly from the well studied Dales Gorge Member, is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km 2, enabling reasonable estimates for the annual input of components to the depository. In the model of this paper, varying supply of materials for the medley of mesoband types, particularly of iron and silica in the oxide BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents and (2) convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied input of pyrochastic material. (1) The surface currents were saturated in silica and carried minimal iron due to photic precipitation, but were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves, whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. (2) Precipitation from convection driven upwelling of high iron solution from MOR or hot-spot activity periodically overwhelmed the delicate seasonal patterns of (1) to produce the iron-dominated mesobands. A wide range of intermediate mesoband types resulted where the deep water supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by the present El Niño). During these periods of oxide-dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photosynthetically produced oxygen. Superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources, changing the meteorological and depositional conditions. Occasional input of extremely fi ash during BIF precipitation produced mesoband (cm) scale variations involving increased carbonate-silicate precipitation. Sustained volcanic periods resulted in S-macroband deposition (chert-carbonate-silicate BIF, with shale), gradually returning to the dominant hematite-magnetite-chert BIF as the volcanic input waned. During volcanic periods, the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron, and by photosynthetic production of oxygen, was modified by turbidity in the atmosphere (aerosols and dust) and in the water (colloids from reactive ash). S Surface-precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone, augmenting the iron content of the zone. Precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub-photic zone exceeded their respective solubilities. During volcanism, the increased availability of nutrients, particularly phosphorus, to surface waters increased the organic contribution despite lower light values, leading to an almost total absence of ferric iron oxides in the S macrobands (i.e. no magnetite or hematite). Cooling of warm, silica-saturated sea-water during these periods of “olcanic winter” increased the ratio of precipitation of silica to iron, which, however, was still controlled by seasonal conditions. Intermediate concentrations of organic matter, insufficient to totally convert the ferric compounds either during precipitation or diagenesis, resulted in overgrowths of magnetite on hematite, and eventually in the substantial conversion of hematite to magnetite, where higher temperatures were achieved during low-grade regional metamorphism. Changes in sea-level to explain facies changes in BIF are not required in this model, but are not excluded. The preferred conditions are for a very low oxygen to anoxic atmosphere, a much higher level of MOR activity than at present, the presence of photosynthetic plankton, the absence of si silica-secreting organisms, and a deep sea-water temperature higher than 20°C. However, none of these conditions is essential to the model. A narrow carbonate bank is postulated for part of the Fortescue River Valley area during Marra Mamba Iron Formation times (basal Hamersley Group), with BIF precipitation on either side. The reef is postulated to have grown northward becoming a major shallow-water carbonate platform on the Pilbara continent during upper Marra Mamba Iron Formation and Wittenoom Dolomite times, but ceased to play an important role in subsequent periods.

Hematite Nanospheres of Possible Colloidal Origin from a Precambrian Banded Iron Formation

Exceptionally small spheres (nanospheres) of hematite (diameters between 120 and 200 nanometers) occur in the Marra Mamba Iron Formation of the Hamersley Basin, Australia. The nanospheres are clustered into small aggregates and may have formed by structural ordering and dehydration of colloidal iron hydroxide particles. Individual spheres consist of numerous thin, curved hematite platelets surrounding a central void that is approximately half the diamter of the sphere; this texture suggests that they formed by a volume reduction of the original colloidal particles by approximately 12.5%. The occurrence of hematite nanospheres supports the hypothesis that some ofthe iron was deposited colloidally during the development ofbanded iron formations, approximately 2.5 billion years ago.

Millimeter-scale variations of stable isotope abundances in carbonates from banded iron-formations in the Hamersley Group of Western Australia.

Several diamond drill cores from formations within the Hamersley Group of Western Australia have been studied for evidence of short-range variations in the isotopic compositions of the carbonates. For a set of 32 adjacent microbands analyzed in a specimen from the Marra Mamba Iron Formation, carbon isotope compositions of individual microbands ranged from -2.8 to -19.8 per mil compared to PDB and oxygen isotope compositions ranged from 10.2 to 20.8 per mil compared to SMOW. A pattern of alternating abundances was present, with the average isotopic contrasts between adjacent microbands being 3.0 per mil for carbon and 3.1 per mil for oxygen. Similar results were obtained for a suite of 34 microbands (in four groups) from the Bruno's Band unit of the Mount Sylvia Formation. Difficulties were experienced in preparing samples of single microbands from the Dales Gorge Member of the Brockman Iron Formation, but overall isotopic compositions were in good agreement with values reported by previous authors. Chemical analyses showed that isotopically light carbon and oxygen were correlated with increased concentrations of iron. The preservation of these millimeter-scale variations in isotopic abundances is interpreted as inconsistent with a metamorphic origin for the isotopically light carbon in the BIF carbonates. A biological origin is favored for the correlated variations in 13C and Fe, and it is suggested that the 13C-depleted carbonates may derive either from fermentative metabolism or from anaerobic respiration. A model is presented in which these processes occur near the sediment-water interface and are coupled with an initial oxidative precipitation of the iron.

A textural and mineralogical study of the relationship of iron ore to banded iron-formation in the Hamersley iron province of Western Australia

Microscope studies of a wide range of ores and fresh, oxidized, or partly iron-enriched, banded iron-formation (BIF) have shown a consistent sequence of alteration of the primary constituents, chert, magnetite with or without hematite, carbonates, and silicates, leading to the formation of martite-(hematite)-goethite ores which form the bulk of the reserves of the Hamersley Iron Province. Primary hematite survives with little change; magnetite oxidizes to hematite (martite) or in part to lacunar phases between magnetite and maghemite (kenomagnetite), which in turn commonly is altered by hydration to goethite. Subsequent leaching of this goethite in preference to martite leads to a variety of skeletal textures which may help identify the original magnetite, despite destruction of martite lamellae by recrystallization. Chert, carbonates, and silicates are either replaced by goethite or leached from the system, resulting in thinning of the orebodies with respect to their parent banded iron-formations. Preservation of macroscopic features, such as mesobanding, and microscopic features, such as pseudomorphs, is compelling evidence of supergene enrichment which, together with residual upgrading by leaching of these iron-formations, produces ores which typically assay 62 percent Fe, ignition loss of 4 to 6 percent, and >0.07 percent P. With increasing maturity, part of the goethite has converted to fine-grained hematite in some of these martite-(hematite)-goethite ores. Though overall the secondary hematite is quantitatively unimportant, some deposits may be significantly upgraded by this process, and it is this variability that is implied by the use of brackets in the classification.A second ore type, characteristic of the Tom Price and Whaleback mines, consists essentially of residual primary components, martite and hematite, with abundant secondary hematite, most of it platy but some anhedral. These martite-hematite ores appear to have formed by a low-temperature process in which some of the goethite or its precursor, in ores of the martite-(hematite)-goethite type, has converted to platy hematite in contrast to the fine-grained and often cryptocrystalline material of the normal ores. This partial transformation has resulted in increased permeability of the deposits which in turn has allowed leaching of the remnant goethite. The Paraburdoo 4E mine represents an arrested stage of development in which a large proportion of this unconverted goethite is still present, whereas the Tom Price and Whaleback deposits represent the most mature ores in the province. In these, leaching has removed the bulk of the goethite, leading to typical assay values of >64 percent Fe, 1 percent loss on ignition, and <0.05 percent P.The supergene ores of the province can be classified in terms of a maturity scale based on the type and amount of secondary hematite replacing goethite. Thus martite-goethite ores without significant secondary hematite are considered to be the least mature. Examples of these have been found in a limited survey of the Marra Mamba Iron Formation ores. The Brockman Iron Formation deposits at Koodaideri, the Brockman syncline, and Metawandy provide examples of more mature ores with increased secondary hematite and are classified as martite-(hematite)-goethite ores. Ancient deposits (early to middle Proterozoic) with abundant secondary platy hematite but with significant residual goethite are classified as the Paraburdoo type. Substantial leaching of this goethite leads to the most mature ores of the province, the martite-hematite deposits of the Tom Price-Whaleback type.

Oxygen isotope study of a Precambrian banded iron-formation, Hamersley Range, Western Australia

Oxygen isotope ratios were determined for quartz, magnetite, ankerite, siderite, riebeckite, hematite and talc in samples of banded iron-formation from the Dales Gorge Member of the Brockman Iron Formation and for quartz, dolomite and calcite in samples of the Wittenoom Dolomite and Duck Creek Dolomite Formations, all from the Hamersley Range area of Western Australia. Additionally, in order to interpret the measured isotope ratios, isotopic fractionations for oxygen between quartz, siderite and magnetite and between these minerals and water as a function of temperature were calculated, using a combination of spectroscopic and thermodynamic data and constraints set by experimental determinations of the fractionations. The Dales Gorge Member was found to have undergone isotopic exchange between minerals at a temperature estimated on the basis of the isotopic fractionations to be above 270°C and probably less than 310°C, during burial metamorphism. At these temperatures quartz and the carbonates were almost completely equilibrated with one another, while hematite apparently underwent negligible exchange. Magnetite may have undergone exchange in some samples but not others, as a result of permeability variations, or it may have been as resistant to exchange as hematite. Riebeckite, and probably talc as well, were also subject to exchange, but to a lesser degree or on a smaller scale than quartz and the carbonates. Hematite formed at temperatures of 140°C or below. Magnetite appears to have formed at temperatures above 140°C, and possibly over a range of temperatures between about 180 and 300°C. The Wittenoom Dolomite and Duck Creek Dolomite samples show apparent lack of equilibrium, due to incomplete exchange or to retrograde effects. A chert from the Wittenoom Dolomite, along with two samples from the Marra Mamba Iron Formation, with δ 18 O values of + 24%. can be considered to set a lower limit of about −11%. on the δ 18 O value of the ocean 2.2 × 10 9 yr ago. Internal fractionations in the Wittenoom Dolomite chert sample may be interpreted as yielding an upper limit on this oceanic δ 18 O value of − 3.5%.