Mount Isa copper orebodies: improving predictive discovery

Abstract

The most likely source for oxidised brines that produced the Mount Isa copper–cobalt deposits is the hematitic meta-sediments of the <1500 Ma South Nicholson Group or its lateral equivalents. Downwards movement of the brines into relatively impermeable basement rocks at about 1370 Ma is suggested to have been a product of reactivation of NNW–SSE faults (e.g. Transmitter fault) during post-Isan inversion. Evidence for the involvement of such structures includes spatial association with major orebodies and mica–dickite–hematite alteration indicative of the passage of hot, acidic and oxidised fluid. Copper depletion and K enrichment in metabasalt of the ca 1830 Ma Eastern Creek Volcanics along such structures has previously been documented. Anhydrite-rich rocks preserved at the northern end of the copper orebodies represent a zone of high fluid input, where the characteristics of the original fluid are best preserved. The most plausible chemical process generating economic Cu is reduction of oxidised brine during reaction with carbonaceous meta-sediments of the Urquhart Shale. Many lithologies in the region would have had the chemical capacity to extract Cu from hydrothermal solutions. The availability of suitable reductant rocks was therefore not a limiting factor on Cu deposition. Rather, the distribution of porosity and permeability would have controlled deposit location. The Urquhart Shale is a particularly favourable host unit on account of pervasive mechanical anisotropy produced by regular interlayering of ductile metapelite and brittle metasiltstone. Strain partitioning during post-Isan inversion resulted in pervasive permeability development in metasiltstone, particularly when steeply dipping. An implication of this research is that rocks of the South Nicholson Group and equivalents could be targetted for sediment-hosted stratiform copper (SSC) deposits.

Key Words:

• Mount Isa
• copper ores
• Proterozoic
• extension
• sedimentary copper
• geochemistry

Acknowledgements

I acknowledge the contribution of many colleagues in the pmd*CRC, notably Peter Jones, Klaus Gessner, Melissa Gregory and Rob Duncan, as well as the numerous geoscientists who have studied the deposits since their discovery in the early part of the twentieth century. Nick Oliver is thanked for a constructive review which greatly improved parts of the manuscript.