Insights into the genesis and diversity of epigenetic Cu – Au mineralisation in the Cloncurry district, Mt Isa Inlier, northwest Queensland

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.