Silicates and their paragenesis in the Brockman Iron Formation of Wittenoom gorge, Western Australia

102

The Hamersley Iron Ore Province of Western Australia is recognized as containing one of the major Precambrian iron formations in the world--the Brockman Iron Formation. The excellent exposure in drill core of the Dales Gorge Member of this formation has provided a unique opportunity for studying iron oxide genesis in this relatively undeformed and only slightly metamorphosed iron formation. Sixteen banded iron formation units alternate with seventeen shale units over a thickness of 400 ft in the sequence studied. The author has established a sequence of mineral development for the iron oxide and associated silicate and carbonate minerals based principally upon textures observed in the banded iron formation macrobands. Chert, hematite, siderite and stilpnomelane are primary minerals and represent the crystallization products of a pre-existing silica-iron oxide-carbonate gel which formed at the bottom of the stable Hamersley Basin. The dominant iron oxide present, magnetite, formed principally during diagenesis by crystallization from hematite and by decomposition of siderite. This type of crystallization was also responsible for the formation of coarse ankerite rhombs. Minnesotaite rosettes, biotite and possibly riebeckite developed during later low-grade metamorphism, which was a response to the deep burial of the rocks.

Section: Mineral Development During Metamorphismmentioning

confidence: 91%

Section: Mineral Development During Metamorphismmentioning

confidence: 91%

Section: Fe•o• = 2 Fe304 + • O_• (2)mentioning

confidence: 91%

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Genesis of Iron-bearing Minerals in Banded Iron Formation Mesobands in The Dales Gorge Member, Hamersley Group, Western Australia

Ayres¹

1972

102

“…On the other hand, riebeckite and stilpnomelane can readily be synthesized in the laboratory using Na-Fe-Si gels as starting materials (Grubb, 1971). Some of the gels may on precipitation contain considerable amounts of alkalis, especially sodium.…”

Section: Precipitation Of Solids In the Central Lakementioning

confidence: 99%

The Depositional Environments of Precambrian Banded Iron-Formations

Eügster¹,

Chou²

1973

120

We propose that Precambrian iron-formations may have formed in the evaporative setting of a playa-lake complex. Hydrologically and sedimentologically such a complex would be similar to the depositional environment suggested for the Green River Formation by Eugster and Surdam (1973). Chemical and mineralogical differences can be accounted for if we assume a Precambrian atmosphere free of oxygen during the depositional interval of banded iron-formations.Following a suggestion of Eugster (1969), magadiite or a sodium silicate gel are considered to be the most likely precursors of the bedded cherts. Magadiite solubility curves have a minimum with respect to silica at a pH of 8.7 and saturation can occur either through evaporative concentration or through flooding of silicarich, high pH brines with fresher waters. Precipitation can take place in a fairly dilute lake. Most Precambrian magadiite must have been converted to chert during early diagenesis, while remaining magadiite interacted with iron minerals to produce riebeckite, a mineral common to iron-formations. Other reasons why magadiite or a sodium silicate gel are attractive as primary precipitates are their plasticity and high water content. These properties may be responsible for some of the diagenetic features observed in iron-formations by Trendall and Blockley (1970), Gross (1972), and Dimroth and Chauvel (1972, 1973).Iron solubiliw and transport have been calculated using the newest set of thermodynamic data available for 25øC. Fe(OH)a is extremely insoluble at pH values considered here (8-9), except at very low oxygen fugacities, such as those of the hematite-magnetite pair. The siderite stability data are conflicting, but siderite is considerably less soluble than calcite at pH below 10. A greenalite stability constant can be estimated using data on other phyllosilicates. Greenalite appears to be very insoluble. Neither siderite nor greenalite are likely to precipitate under equilibrium conditions, but probably require considerable supersaturation.Precipitation of iron minerals occurs in response to changes in pH and fcot at an fo• level between those of the hematite q-magnetite and magnetite q-quartz q-fayalite buffers. Fe(OH)a is precipitated whenever free oxygen is available. The precursor precipitate for magnetite is not clear. Magnetite could form either from Fe(OH)•., a mixture of Fe(OH)• and Fe(OH)a, or by reduction of Fe(OH)a with organic matter, as suggested by Perry and Tan (1973).In the playa-lake model, inflow into the lake is primarily through springs, with precipitation and brine evolution proceeding in the playa flats fringing an alkaline lake. This accounts for the paucity of clastics found in banded iron-formations. Water compositions were of the bicarbonate or of the hydroxide type. The latter are attractive because the lower organic activity of Precambrian soils would have provided less COs for weathering reactions and because the alteration of the abundantly exposed basic and ultrabasic rocks (Petrijohn, 1973) would have produced hig...

“…James (1954), Lepp and Goldich (1964), and many others have favored weathering as the major source of iron. Trendall and Blockley's (1970) study of the BIF of the Hamersley Basin, and the subsequent studies by Grubb (1971), Ayres (1972), and Trendall (1973), allow some new tests of the ability of the three major hypotheses to account for the iron in these deposits and in similar Lake Superior type iron-formations. A few (see for instance Borchert, 1960) have suggested that deep water might have contained a reasonably large concentration of Fe *•, that circulation brought deep water in contact with a somewhat oxygenated atmosphere, and that the oxidation of Fe +• to Fe +8 was followed by the precipitation of ferric hydroxide.…”

Section: Introductionmentioning

confidence: 99%

The Oceans; A Possible Source of Iron in Iron-Formations

Holland¹

1973

356

160

Recent work on one of the major banded iron-formations of the world--that of theHamersley Basin of Australia--has defined the total amount of iron and t•he probable time interval of accumulation of the large iron ore deposits in the basin. The inferred rate of influx of iron into the basin is so large that a derivation of the iron from weathering and transport into the basin by rivers is unlikely. For the same reason derivation of the iron from volcanic sources is also unlikely. Derivation of the iron from the ocean floor is quantitatively possible if deep ocean water containing a few rag/1 of dissolv.ed iron welled up, was oxidized in near surface environments, and deposited its iron in the basin. Iron concentrations of a few mg/1 in deep ocean water are quite reasonable if these water masses were saturated with respect to siderite and calcite.