Abstract

The Proterozoic rocks of the Cloncurry district preserve the effects of some of the world's largest hydrothermal systems associated with extensive albitisation, brecciation and Na – Ca alteration. These hydrothermal systems are broadly coeval with magmatism, and also host numerous structurally controlled Fe oxide and Cu – Au deposits (ca 1.60 Ga, 1.55 – 1.50 Ga). Fluid-inclusion, stable-isotope, and geochemical data from Cu – Au deposits indicate that the ore-forming fluids were high-T (>300 – 500°C), highly saline (>26 – 70 wt % NaClequiv), typically CO2-bearing, and are mainly considered to be sourced by crystallising intrusions with contributions from other fluid sources and/or host rocks. Fe oxide and Cu – Au mineralisation in the district exhibit a range of interrelationships based upon the metal endowment, relative timing of Fe oxides and sulfides, and Cu:Au ratio. These interrelationships may be divided into four categories: (i) barren magnetite and/or hematite ironstones; (ii) Fe oxide-hosted Cu – Au mineralisation, where relatively Au-rich ore associated with pyrite and hematite overprints older magnetite-rich rocks; (iii) Fe oxide Cu – Au mineralisation, where both Fe oxides and Cu – Au mineralisation are cogenetically deposited; and (iv) Fe oxide-poor Cu – Au mineralisation, where relative Cu-rich mineralisation is associated with pyrrhotite and rare magnetite, and is hosted in relatively reduced rocks such as carbonaceous metasedimentary rocks. These categories reflect variations in fluid redox, f S, aFe, and temperature, as well as host-rock composition. The spectrum from Cu-rich to Au-rich mineralisation is a common phenomenon in Fe oxide – Cu – Au districts and predominantly reflects an increase in the redox of the ore-forming system. The apparent relationship between pH and metal solubility at different redox conditions suggests that Cu – Au mineralisation occurred as a result of decreasing fluid acidity by wall-rock reaction at the site of ore deposition, or potentially by mixing of fluids of different acidity. Fluid mixing provides an effective means to produce high-grade ore deposits via changing pH, cooling, and dilution in hydrothermal systems involving little wall-rock interaction.

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