The economic significance of deep burial is that the discovered mineralization has to have significant economic value to pay for removal of cover and to justify the increased expense of delineation. Exploration under cover first seeks to identify a Giant Mineral System defined essentially by alteration before exploring that system at a finer scale for economic mineralization. This means that exploration criteria at different scales address different aspects of the system although this variation of criteria with scale has not been widely acknowledged. The control on the location of a mineral system is usually related to crustal architecture and often the architecture is inherited from past events.
However it seems that not all permutations of architecture are equally likely to host a giant mineral system and we do not understand why. Within a mineral system an Eh-pH relationship is commonly observed but not at all understood. It is useful however as a guide to predicting the location of mineralization within a specific architecture. An understanding of this behaviour and its relationship to mineral precipitation will be a significant advance. The project aims to address these issues.
The aim of this project is to develop an integrated framework for the origin of giant hydrothermal deposits with emphasis on defining new exploration criteria at various scales. The project is built upon modern continuum thermodynamics, which is a methodology for crossing length and time scales in a rigorous manner consistent with the laws of thermodynamics.
The project was 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? Mineral Geodynamics: An Atlas of Geodynamic Mineral Systems
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?
The Precious Earth: Understanding Hydrothermal Mineralising Systems 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 instead of 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. 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.
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. 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.