Breccias form during many geological processes, including faulting (which may be seismic or aseismic), solution and chemical alteration of rocks, volcanic eruption, igneous intrusion, meteorite impact, and tsunami, landslide and submarine slump deposition. Despite over two centuries of research, and their economic importance as ore deposit hosts, the origins of many breccias are highly controversial. Classifications of breccias are problematic: some are purely qualitative and others conflate various processes to describe vastly different geological features.

 

The Chumvale breccia, Mt Isa Inlier. How did this breccia form?

The aim of this project is to understand the fundamental processes of rock fragmentation and transport, using an iterative and dynamic field, fluid chemistry and theoretical analysis.

A major difference between brecciation processes is the rates of fragmentation and transport. Aseismic faulting (creep), solution and chemical alteration of rocks are slow processes compared to seismic faulting, fluid release in some magmatic processes, and meteorite impact. The rate of brecciation has a fundamental economic and genetic significance, which is one focus of the proposal. Combining a field approach to carefully chosen natural examples, with fluid geochemical analysis and theoretical understanding of fracture and transport processes, will allow both inductive and deductive approaches to identifying brecciation processes.

The economic importance of breccias is well appreciated. Worldwide, it has been estimated that 150 Mt of Cu, 1.2 Mt of U, 800t of Au and 80 Mt of Pb and Zn are hosted by breccias (Laznicka, 1988). Their contribution to Australia’s mineral industry is enormous. Several major Australian ore deposits are breccia-hosted, including most notably Olympic Dam, but also, for example, parts of Mount Isa Cu and Pb-Zn, the Ernest Henry Cu-Au mine, and Archean gold deposits in the Yilgarn, including Sunrise Dam. Porphyry deposits both within Australia and the Asian region include many ore bodies within breccias. The importance of breccias to the minerals industry ranges from ground control and grade control (since different types of breccia can have distinctive mechanical properties and contain vastly different and grades of metal), to the fundamentals of ore genesis. Understanding brecciation mechanisms has implications for exploration on the mine and regional scales.



Some of the challenges in working with breccias can now be approached with greater confidence because of advances in methods of analysis. In this project, we will be using:
•    Fractal geometry, which is a natural and powerful tool for physical breccia characteristics.
•    Chemical analysis of breccias, which is reaching new levels of sophistication, with microanalytical approaches to complement more conventional chemical techniques, and isotope geochemistry.
•    Numerical modelling. Both physical and chemical aspects of breccias are increasingly benefiting from numerical modelling approaches.

Looking at breccias in the Mount Isa Inlier, with Basil Tikoff and Kathy Staffier

Literature
Oliver NHS, Rubenach MJ, Fu B, Baker T, Blenkinsop TG, Cleverley JS, Marshall LJ & Ridd PV (2006) Granite-related overpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry District, Australia.  Geofluids, 6, 346 – 358.

Blenkinsop, TG and Duckworth, R. Breccia  EGRU Contribution 36 (Available from EGRU)