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Abstract

Gold, bismuth, and copper mineralization at Tennant Creek is hosted by magnetite-hematite replacement bodies in lower Proterozoic sediments of the Warramunga Group. The sediments have been folded about east-west axes, are characterized by a pervasive axial-plane slaty cleavage, and are intruded by pre- and postfolding granites.

Marked structural and stratigraphic control yields lines of lode (ironstones) that can be traced for distances of up to 40 km. Ironstone lodes are restricted to the magnetite-rich Black Eye Member of the Carraman Formation and concentrate adjacent to argillaceous banded iron-formations. They are aligned parallel to the regional axial-plane cleavage, commonly lying in the cores of third-order folds, especially in areas of fold hinge plunge reversal. Faulting and shearing parallel to the cleavage may also play a role in the localization of some lodes.

Gold, bismuth, and copper mineralization and associated alteration form a late-stage overprint on the magnetite lodes, with gold typically concentrated toward the footwall of the ironstone or at its margins in distinct pods associated with chlorite and muscovite. Copper and bismuth mineralization occurs in overlapping zones around these pods, and this zonation is complemented by gangue mineralogy, and trace element and sulfur isotope zonation patterns.

A model for the formation of the ironstone lode involves the movement of hot connate brines into developing fold axes during regional deformation of the Warramunga Group. The fluids in equilibrium with the magnetite in the sedimentary pile reacted with more oxidized horizons (e.g., hematite shales), resulting in the deposition of hematite (or a hydrated precursor) that was subsequently converted to magnetite as equilibrium was restored. Economic mineralization is associated with faulting and fracturing of the ironstone lodes and introduction of hot, saline, relatively reduced and sulfur-bearing solutions. Reaction of these solutions with chlorite in the lodes resulted in its replacement by muscovite with a consequent increase in pH and reduction in fO2 of the fluid. This reaction is likely to have controlled gold, bismuth, and copper deposition. The relative availability of sulfur, metals, and fluid between ironstone lodes is thought to be responsible for the spectrum from unmineralized to copper- and gold-rich ironstone lodes.

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