Interpretation of fabrics in ferruginous lag

Lag, a surface veneer of diverse ferruginous fragments, is common on deeply weathered terrain of the Yilgarn Craton, and particularly abundant over mafic and ultramafic rocks. It is a residual accumulation and is dispersed largely by sheetwash. The geochemistry of lag has been used extensively and successfully in the search for gold and base metals in relict and erosional regimes. However, the petrographic potential of lag for rock type identification has not been fully recognised. Ferruginous lag comprises lithorelics or lateritic nodules and pisoliths, which are generally related to the regolith substrate. Their external appearance gives clues to the nature of the regolith; their interior fabrics can contain evidence of the underlying rocks and their weathering history. Iron oxides may form pseudomorphs after pre-existing minerals and fabrics, but identification of these precursors can be difficult. Where ferruginisation occurred during relatively early weathering, a wide variety of primary rock fabrics may be preserved. If it took place during later weathering stages, secondary fabrics, related to authigenic minerals, may also be preserved. If ferruginisation followed pedoplasmation, primary fabric elements are lost, although pockets of earlier fabrics may still be preserved. Preserved primary fabrics are illustrated in lag fragments from sites on the Yilgarn Craton of Western Astralia, namely Beasley Creek (phyllitic schist, mafic and ultramafic schists and deeply weathered Permian tillites), Ora Banda (deeply-weathered peridotites and pyroxenites) and Bottle Creek (phyllic alteration). Pseudomorphs after mineral fabrics may be destroyed by multi-cyclic precipitation of secondary iron oxides. After dissolution of pre-existing minerals, goethite may form colloform structures lining, and even filling, the resultant voids. In some places the fabric is replaced progressively by spongy or massive goethite. In many places, fabric destruction is incomplete, leaving partly erased fabrics. Despite these limitations, careful search for useful primary fabrics in lag is a powerful and non-invasive aid to identification of underlying rocks, particularly in areas of partial profile truncation.

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1996-01-01T00:00:00
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Geoscience Australia Persistent Identifier/https://pid.geoscience.gov.au/dataset/ga/81437

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Australian Geological Survey Organisation

 Canberra
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Robertson, D.M.

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AGSO Journal of Australian Geology and Geophysics

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16:3:263-270

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2018-11-01T00:00:00
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2018-04-20T06:07:23
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2014-06-03T00:00:00

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