Distribution of gold and ore-associated elements within lateritic weathering profiles of the Yilgarn Block, Western Australia

Louisa Lawrance

    Research output: ThesisDoctoral Thesis

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    Abstract

    [Truncated] partly eroded lateritic regolith covers most of the Yilgarn Block of Western Australia. Understanding of this regolith and of the constraints it imposes on the distribution of gold and other ore-associated elements will help optimize exploration procedures for locating concealed and deeply weathered ore deposits. Three previously undisturbed gold deposits in different geomorphological environments were studied. (1) The Hannan South profile, the lowest topographically, occurring beneath a playa, is partly water-saturated and has been eroded to the upper saprolite and overlain by 3-4 m of lacustrine sediments. (2) The Mount Pleasant profile occurs beneath a drainage channel and has been truncated in the lower saprolite and overlain by up to 6 m of alluvial sediments. (3) The Ora Banda profile occurs in a topographically high area where a complete lateritic profile has been preserved.

    These profiles are the product of two major weathering episodes. Evidence for this are the weathered horizons formed and the distribution of iron oxides. Iron oxides are precipitated by ferrolysis under oxidizing conditions in the regolith. Ferrolysis occurs above a redox front,generally associated with the water table, and results in acid conditions. Above the redox front at Hannan South, iron is initially precipitated as hematite, under alkaline to neutral conditions(pH 7-9), where acid produced by ferrolysis is buffered by the dissolution of carbonates. As ferrolysis reactions continue, conditions become more acid (pH 3-5) and hematite is converted to goethite. The downward progression of acid, oxidizing conditions associated with the redox front have greatly enhanced weathering above the redox front. The position of the watertable, and hence the redox front, is controlled by climate and topography.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • The University of Western Australia
    DOIs
    Publication statusUnpublished - 1991

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