Dales Gorge Member Research Articles

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%.

Varve Cycles in the Weeli Wolli Formation of the Precambrian Hamersley Group, Western Australia

Chert and chert-matrix bands, internal striping, comparison with Dales Gorge Member of Brockman Iron Formation

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

Sixteen banded units alternate with seventeen shale units, Precambrian, sequence of mineralization, mineralogy, low-grade metamorphism

Revolution in earth history

Abstract The 2,400‐metre thick Hamersley Group is characterised by its 900 m of banded iron formation, and is the middle of three constituent groups of the Mt Bruce Supergroup, which forms the contents of the 2,300–1,800 m.y. old Hamersley Basin. The Hamersley Basin initially covered about 150,000 km2 of northwestern Australia, and in its present widespread outcrop, the Mt Bruce Supergroup is mostly little disturbed or metamorphosed. Within one iron formation unit 142 m thick, the Dales Gorge Member of the Brockman Iron Formation, within the Hamersley Group, there are three scales of stratification, termed macrobanding, mesobanding (the normal ‘banding’ of banded iron formation) and microbanding. Microbands are thin (0.2–2.0 mm) regular laminae, alternately rich and poor in iron, within chert mesobands. Microbands, mesobands and macrobands may all be correlated over the whole of the present outcrop. Microbands are believed to result from annual seasonal control of the primary precipitation in the basin, w...

Carbon isotopic evidence for the origin of a banded iron-formation in Western Australia

Carbon isotope ratios were determined for carbonates from the banded iron-formation of the Dales Gorge Member of the Brockman Iron Formation and from the Wittenoom and Duck Creek Dolomite Formations of the Hamersley Range area of Western Australia. The iron-formation carbonates had δC 13 values of −9 to −11 %., while the dolomitic limestones had δC 13 values of +2 to −2 %. It is concluded that the iron formation was precipitated in a basin isolated from the ocean, but probably in close proximity to it and that organic activity may have played a significant role in the genesis of the iron-formation. The dolomitic limestones were deposited during periods when the basin was transgressed by the ocean. A consideration of the available data on Precambrian marine carbonates leads to the conclusion that the oceanic reservoir of carbon has remained nearly constant for almost 3 × 10 9 years. The argument previously advanced for Phanerozoic samples, that this constancy implies a constant amount of atmospheric oxygen is reconsidered, and is found to rest on questionable assumptions.

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

Petrographic studies of a bore core section through the Dales Gorge Member of the Brockman Iron Formation, supported by experimental gel runs, indicate a complex paragenesis of associated ferroan silicate minerals in the Wittenoom Gorge area. The primitive silicate phases appear to have been a mixture of montmorillonite and greenalite, while depending on the nature of subsequent diagenesis, these gave way to stilpnomelane and minnesotaite, respectively, with occasionally a little chlorite. Three varieties of stilpnomelane are recognized and can be correlated with different stages of diagenesis. Riebeckite and crocidolite together represent the end products of this sequence, but whereas the former largely represents a metasomatic replacement product after stilpnomelane, crocidolite is essentially a direct crystallization product, which is confined to marked intraformational breccia horizons apparently formed through slumping of the partially consolidated sedimentary mass. Experiments on the formation conditions of crocidolite, show that soda availability and concentration are essential factors, but almost equally critical are an abundance of Fe (super ++) ions and low Eh conditions. Associated magnetite, despite its apparent seeding role, is a serious contender for Fe (super ++) , so to minimize magnetite formation, relatively low pH values are also probably more favorable towards crocidolite formation.