A model concept - earth models for minerals

Materials World magazine
,
8 Jan 2013

The concept of peak oil (and of peak minerals) is based on Earth’s finite resources, and perhaps more importantly, the prices that we are prepared to pay for them. It is unlikely that we will run out of any commodity, it is just that the economic cost of production may outweigh the advantages in use, or consumer demand. Unfortunately there are few developed economic substitutes for many of the metals we use today, again a reflection of price.  

Expenditure on mineral exploration is cyclical, typically following the price of the commodity at a delayed interval. In addition to this driver is the growth in demand in rapidly developing economies, currently focused on China, but with a number of other countries equally keen to improve development and living standards.  

Although the budget for non-ferrous metal exploration reached an all-time high of US$18.2bln in 2011, the number and quality of new finds is declining – particularly of those occurring at the surface. New tools are required that will identify subsurface deposits and offer a predictive model for their discovery. The industry has advanced tools in the form of geophysics and deep drilling technology, but the question is where to apply them – the answer is a predictive model to increase exploration efficiency.  

Perfecting predictions  
For more than 50 years, plate tectonics has underlain concepts that explain the distribution of minerals, and for more than 150 years the geological dictum that the present is the key to the past has held sway. Within these paradigms is a developing earth model that explains why minerals are found where they are, and where they are likely to be found in the future. The oil industry has been the main proponent in developing earth models. They are produced by gathering data from surface and subsurface surveys (for example, maps, satellite images, geological interpretations, geophysics and well logs) and digitally combining them into four dimensional models (3D plus time). A key framework for such models is the concept of sequence stratigraphy, (the predictable response of sedimentary systems to changes in relative sea-level) thereby revealing what geological processes are likely to have occurred at a particular place at a particular time. For oil and gas exploration this framework allows prediction of the accumulation, burial and thermal maturation of organic-rich rocks to produce hydrocarbons. In sedimentary basins, the combined use of biostratigraphy allows a high degree of stratigraphic precision and correlation of disparate geological datasets.  

Resource company Neftex, based in Oxford, UK, is at the forefront of developing an integrated earth model, primarily for the oil industry but more recently applied to mineral exploration. According to Dr Graeme Nicoll, Head of Minerals at Neftex, a predictive earth model should be consistent in its application across regions and terranes while at the same time providing sufficient detail to enable greenfield exploration. Dr Nicoll explains, ‘Our Geodynamic Earth Model is tested with tens of thousands of pieces of data, from drill logs and outcrops, geochronology, geochemistry and palaeomagnetic data. These public domain data are sourced from over 100,000 catalogued industry and academic papers in a searchable database.’ Their model has global coverage and enables detailed plotting of the position of global plate boundaries and geological terranes from the present day to 595 million years ago.  

The model provides the ability to track and delineate major collisional events, subducted margins, major volcanic arc activity, large igneous intrusive events and the Phanerozoic redistribution of mineralrich Archean terranes. Because the model is global, it is possible to relate mineralising activity in one region and directly compare it with another, adding greater insight to the process. Over the past three years Neftex has developed and delineated an array of geodynamic units (GDU) that record unique geological histories. Building upon the former plate tectonic research programme at the Université de Lausanne in Switzerland, the model is undergoing constant refinement, for example with the addition of more than 2,000 oceanic crust segments, definition of intra-cratonic boundaries for crust assembly back to 2.5 billion years ago and identification of zones of attenuated continental crust. The model allows the reconstruction of any Phanerozoic geological data, while PreCambrian terranes and tectonic events are also being identified.  

The validation of a model relies on its ability to identify known mineralisation and to predict targets defined by the established criteria. For example, an exploration model reconstructing the Upper Devonian of Eurasia shows the Siberian and Baltic continental plates separated by the Uralian Ocean (shown above), with the associated arc magmatism shown in yellow (representing a belt 80–250km inboard of the subduction trench). Also highlighted are the predicted former positions of mid-oceanic ridges (shown in red). The intersections (solid black) of these events represent excellent targets for a number of styles of mineralisation such as porphyry copper epithermal gold and volcanogenic massive sulphides. The subduction of ocean plate beneath continental crust provides a mechanism for transferring relatively enriched gold and copper in the oceanic plate to the overriding continental crust for the generation of porphyry copper and gold deposits (as seen at a much later date and distant environment in the copper porphyry mineralisation of Andean South America). The earth model is dynamic, meaning the geology at any one location will change over time with the movement of plates. For example, if all Phanerozoic volcanic arcs are mapped to their present-day locations then some from different geological time periods will overlap in areas where subduction was prolonged, or where subduction settings affected the same area of continental crust repeatedly through geological time. Such areas are likely to have been prone to significant enrichment and fluid flow enhancing the potential for large-scale mineral deposits. The results of this work, mapping out volcanic arcs, show remarkable agreement with around 85% of the known Phanerozoic porphyry copper, epithermal gold and volcanogenic massive sulphide deposits worldwide, thereby providing a predictive framework and global road map for future exploration.  

A variety of mineralisation styles can also be abstracted from the Neftex datasets. So far some 3,500 deposits (porphyry copper/gold, epithermal and orogenic gold and volcanogenic massive sulphides) are incorporated with data confirming geographic  location, age of ore and host rock formation, incorporated commodities, ore hosting minerals and host rock type. Where possible, tonnage/grade and ownership information are also captured.  

The figure below displays the age of a wide range of mineral deposits through time. The peaks represent the number of mineral occurrences recorded against time with specific clusters at approximately 35, 70, 350, 1,800 and 2,700 million years ago. The ability to geodynamically reconstruct these Phanerozoic periods, and in future the Precambrian, will no doubt lead to new models to aid mineral exploration.  

The Neftex Earth Model is the result of significant investment and more than 500 person years of effort, taking into account the multiplicity of component data resources. Within the company there are 75 geoscientists supported by 15 information management and technology specialists who continue to add to the earth model with the specific aim of integrating and interpreting geological data to provide the necessary tools for exploration. While the focus to date has been on providing support to the hydrocarbon industry sector, the company’s earth model is likely to have an equally large impact on mineral exploration.  

For more information, contact Graeme Nicoll, graeme.nicoll@neftex.com