Developments in Iron Ore Exploration
The most common magnetic mineral is magnetite, which is widespread in banded iron formations (BIFs). It is no surprise that the first application of geophysics in mineral exploration was the search for iron. The method used to detect magnetic minerals, the magnetic method, has had a long and involved gestation period. From its first reported use as a "bump finder," the magnetic method has evolved to become the mainstay of iron exploration. This evolution has mirrored the technological revolution that morphed from the industrial revolution. Magnetometers developed from simple analogue instruments, including compasses and torsion balances, to electronic and atomic-based units in the form of proton precession and optically pumped instruments. A further boost in the magnetometer's usefulness was provided with the development of fast acquisition systems and safe airborne platforms. This was further aided by both the exponential rise in computer power and the advent of a Global Positioning System (GPS). The result has been a geophysical method that can accurately, quickly, and cheaply map the distribution of magnetite in rocks to a level of accuracy that has seen the magnetic method move from bump finder to geologic mapping tool.
High-grade hematite mineralization is generally difficult to directly detect by the magnetic method. The lithologic associations, structural settings, and evidence of geologic processes, however, can be readily mapped out by the method. As the understanding of iron ore genesis evolves, the method can be used to directly test aspects of the genesis model, often on a basin-wide basis. It is anticipated that continuing developments in the magnetic method will further increase its diagnostic capability, while at the same time reduce the unit cost of the information gained.
Another common property of iron is its generally high density. It is no surprise that the gravity method ranks immediately behind the magnetic method as the most efficacious exploration tool for iron. While porosities, and therefore densities of iron ore, have a wide range, the gravity method is nonetheless widely deployed in the search for iron. Gravity methods have seen similar advances in capability to that of the magnetic method, though in a much shorter time frame. This has culminated in the recent development of airborne gravity gradiometry, reducing the time required and the cost of data acquisition while spectacularly increasing survey coverage. As with the magnetic method, further developments in airborne gravity gradiometry will see the acquisition cost continue to decrease, allowing the method to be applied more often.
Other methods, such as radiometric, electrical, multispectral scanning (MSS), and seismic, have niche but important applications. While radiometric and MSS methods are restricted to the surface chemistries of the earth, they have important applications as geologic mapping methods. Though not peculiar to iron exploration, there are iron mineral assemblages and geologic processes that make the methods amenable for that purpose. Similarly, electrical methods, though marginal in iron ore detection, are being used increasingly to aid in geologic mapping and decrease interpretation ambiguity.
The same evolution that has seen vast increases in data collection capabilities has also fed numeric modeling and data transformation advances. Automated modeling schemes that can incorporate a priori information are now the norm. Explosive increases in computing capacity have seen rapid development of multiparameter inversion modeling using multiple geophysical data sets. The aim is no longer fitting a field curve with a single body solution, but developing a "whole of earth" model showing all elements of the detected geology.
It is anticipated that future advances in geophysics will be heavily biased to data collection techniques and further reduction of acquisition costs. This will obviate the need to decide on which geophysical survey to run—multimethod platforms will deliver multiple datasets at high data density and low cost. Although it is unlikely any new techniques will evolve, some methods may see the transition from the lab bench to the field as technology further improves.
Figures & Tables
The spark to put together this volume on banded iron formation (BIF)-related high-grade iron ore was born in 2005 during a steamy night in Carajás where the iron research group from the Universidade Federal Minas Gerais, Vale geologists, Carlos Rosière and Steffen Hagemann, were hotly debating the hypogene alteration genesis for the high-grade, jaspilite-hosted Serra Norte iron ore deposits. A couple of caipirinhas later we decided that the time was opportune to put together a volume that captured the new and innovative research that was being conducted on BIF-related high-grade iron ores throughout the world. We had little problem convincing our South African colleagues Jens Gutzmer and Nic Beukes to join the effort and decided that the 2008 biannual Society of Economic Geologists' (SEG) meeting in South Africa would be the perfect place to present this project through a combined field trip and workshop near Sishen.
The enthusiastic support that we received from the research community, SEG, and industry to put this volume together was generated by the significant increase in exploration activity, and with it the need for more detailed information on what exactly controls the location of high-grade iron orebodies, and renewed research interest around the world in models for the genesis of BIF-related high-grade iron ore, and particularly the relative importance of hypogene and supergene processes in formation of high-grade ore.
This volume concentrates on new research on the characteristics and metallogenesis of BIF-related high-grade iron ores. It contains a state of the art series of papers on established and new iron ore districts and deposits, the different components of the BIF iron mineral system, and how to best explore for this ore type. Although the emphasis of many of the contributions to this volume is on the hypogene aspect of high-grade iron ore formation, it is important to note that most BIF-related iron ore districts have a very pronounced supergene overprint due to deep lateritic weathering. The transformation of many hypogene iron orebodies of reasonable grade and size to the giant deposits exploited today can be related to this geologically recent supergene overprint; most of the past and still much of the present mining of high-grade iron ore relates to soft ore interpreted in most cases to be the direct result of supergene processes. Also mentioned here should be the recent resurgence of a syngenetic model that advocates the formation of chert-free BIF