Alison Ord, Bruce Hobbs, Weronika Gorczyk, Mark Munro, Chris Gonzalez
The aim of our research has been to produce an integrated multiscale framework for the origin of giant hydrothermal deposits with an emphasis to date on orogenic gold. The study crosses all the length scales from lithospheric down to thin section. We explore proposals that special lithospheric structural architectures associated with old craton margins are sites for influx of CO2 into the lithosphere so that eventually these architectures control metal sources. A particular interest has been the recognition of structural features that define criticality in the plumbing system. At the mineralising site diagnostic features defined by both the structures developed and the paragenetic sequence result from strong interaction between deformation, fluid flow, thermal transport and chemical reactions. This system is analysed using the principles of non-equilibrium thermodynamics.
The goal is to define measurable parameters that control the size and grade of such systems and that can be used as mineral exploration criteria. In particular the emphasis has been on: (i) Criteria that distinguish a “successful” from a “failed” mineral system and (ii) vectors to mineralisation within a successful system.
The research is specifically designed to answer two issues of importance to mineral explorationists:
(i) What are the crustal-scale architectures that favour the formation of large hydrothermal mineralising systems, and what are the time and length scales involved in the various architectural settings? This is aimed at answering the question: Which region on Earth should I be in? Results to date are assembled in:
Mineral Geodynamics: An Atlas of Geodynamic Mineral Systems which has been developed to provide examples, and interpretations, of tectonic styles at the crustal/lithospheric scale focussed on real examples from around the world.
(ii) Having delineated such a system: What are the small scale (that is, drill-hole scale) characteristics that tell me where I am in the system and how prospective is it? Results to date are assembled in:
The Precious Earth: Understanding Hydrothermal Mineralising Systems which has been developed to encapsulate all learnings from the project, theoretical and applied. It will be published as an eBook by the Geological Survey of Western Australia.
The project has supplied a new model for the introduction of CO2 into crustal settings with the development of a thermodynamic data base for CO2 devolatilisation under mantle conditions. This means that subduction and delamination scenarios for CO2 production can be explored as well as the classical crustal origins for CO2.
At the regional scale we have developed models for criticality in fluid flow systems. Contrary to the established wisdom, the optimal conditions for ore body formation are just before, rather than at, criticality when the crust is in a condition called “bi-fractal” by workers in the Carlin Trend and in Zimbabwe. Such bifractal behaviour is characteristic of many physical and chemical systems just before the onset of criticality. This enables a new view of what constitutes regions of high and low potential for significant mineralisation in that the spatial distribution of alteration should be 'bifractal', indicative of a sub-critical crustal plumbing system.
The theoretical models we have developed indicate that hydrothermal systems should develop as chaotic systems and hence should be multifractal. At the ore body scale we have developed thermodynamic based wavelet technologies that enable the rapid exploration of multifractal geometry in drill-hole data sets for alteration assemblages and gold grades. We have shown that the multifractal characteristics of well-endowed deposits are quite different to those from less endowed deposits and that long range correlations exist in the data sets that enables a new view of ore grade model development.
We have developed a new integrated theory of hydrothermal mineralising systems based on approaches taken by chemical engineers in the design of optimal chemical reactors. This approach enables an understanding of mineralisation distribution in ore bodies and the factors that control the grade and size of these systems; in general, high grades require episodic behaviour of the system whereby temperature and fluid pressure (and hence gold solubility) oscillate. Importantly, bonanza grades of gold require the operation of processes that remobilise gold into localised packages within the system. These processes require the direct coupling between the formation of exothermic pyrite (or arsenopyrite) alteration assemblages and the endothermic deposition of gold. We have developed models that illustrate the critical controls of temperature and fluid pressure fluctuations on the size and grade of orogenic gold deposits. These developments more than achieve our initial goals of placing mineralising systems within a multiscale dynamical framework.
Work in the immediate future involves:
- Extending the existing work, at all scales, to deposit styles other than orogenic gold;
- Developing a probabilistic approach to the targeting and grade-modelling of mineralisation based on nonlinear dynamics rather than classical statistical approaches;
- Extending existing work to two dimensional multifractal analysis especially for structural and potential field data sets with an emphasis on detecting and analysing alteration under cover;
- Consolidating the theoretical framework with an emphasis on specific alteration and mineralising mineral reactions.
This research is funded by grants from the Australian Research Council (Project: LP100200785, Multiscale Dynamics of Ore Body Formation), and MRIWA (Project: M424, Multiscale Dynamics of Hydrothermal Mineral Systems). We thank The Geological Survey of Western Australia, First Quantum Minerals Ltd., AngloGold Ashanti, and Silver Lake Resources for financial support and access to data. We also thank Greg Hall, Julian Vearncombe and Marcus Willson for encouraging us to explore non-linear systems. Sarah Firth, Sandi Occhipinti, Tony Roache, Chris Wijns, Keith Martin, Klaus Gessner, Catherine Spaggiari, Ian Tyler and Cam McCuaig are thanked for discussions and facilitating access to data.