Abstract Iron enrichment in banded iron formation (BIF)-hosted high-grade iron deposits is the final result of sequential removal or replacement of gangue minerals from the host by hydrothermal and supergene processes. Apart from the presence of the host BIF, structure is the most important control on the location of these deposits. Also, the distinct structural setup of the mineralizing environment results in iron ore of distinct textural features and consequently variable physical properties. In the Hamersley province of Western Australia pre-Upper Wyloo Group extensional faults are most often associated with high-grade hematite deposits in the Paleoproterozoic Brockman Iron Formation. The most important faults provide a fluid pathway between underlying dolomites of the Wittenoom Formation, through a sequence of shales and cherts, and into the overlying BIF. Iron ore in the Kaapvaal province of South Africa is hosted within BIFs of similar age to the Pilbara craton. The BIFs in the Kaapvaal province rest directly on dolomite, and Paleoproterozoic karst structures form the main spatial control on the high-grade iron ore. In contrast, low-angle thrust faults are the principal structural control on large deposits in the Marra Mamba BIF in the Hamersley province. These structures provided a more effective fluid pathway between the BIF and the overlying dolomites. A very similar structural scenario controls the very large Paleoproterozoic iron deposits in the Quadrilátero Ferrífero province in Brazil, although individual deposits are often highly complex due to postmineralization deformation during the Brasiliano orogeny. Structural reconstruction suggests that early structures, particularly thrust faults and tight folds that link a potential fluid source such as the dolomites of the Gandarela Formation with the underlying BIFs, form the most important control on ore formation in this province. Iron deposits hosted by Archean BIFs are less well understood. In the Carajás province of Brazil, fluids derived from granitoid intrusions are interpreted to have caused the initial hypogene alteration of the BIF which later focused the supergene ore fluids that led to high-grade hematite formation. Major structures that linked these granitoids with the BIF were crucial in the formation of the protores. In all these districts, mineralizing structures are those that provided the most effective link between a source of hydrothermal, silica-undersaturated fluids and iron formation, or allowed the influx of surface-derived meteoric waters to control the sites of ore formation in the BIF. Another important effect of structures is that they locally caused a differential pressure gradient during deformation and concentrated fluids into low-strain or dilational sites of iron ore formation. Most high-grade iron deposits formed close to (paleo)-unconformity surfaces and are, therefore, prone to rapid erosion. The structural setting can play a major role in preservation of these deposits. Ore deposits near normal faults in extensional grabens and karst structures are particularly favorable to ore preservation because the faults usually caused downthrow of the mineralized zones and burial by younger sediments. Compressional structures such as thrusts were far less favorable, because they usually caused uplift and erosion of the orebodies within them. Orebodies controlled by these structures require postmineralization preservation events, such as a major postore orogeny, or formed relatively recently, and therefore erosion did not progress far enough to erode them.

Abstract Earliest discoveries of iron ore basins worldwide were commonly the result of regional, government-instigated surveys. After initial discovery, exploration for high-grade iron ores within these basins used ore genesis models from a very early stage. These models included syngenetic, hypogene, and supergene concepts. Systematic prospecting using a supergene genetic model with stratigraphic and structural contributions was hugely successful in identifying major resources in the Hamersley province at an early stage, but the use of this model resulted in significant underestimation of the size of some deposits. With detailed drilling and mining of deposits, many features which seemed at odds with a simple supergene origin were discovered, and modified supergene-metamorphic, syngenetic, or hypogene models were proposed. It was the recognition in these models that high-grade hematite ores could occur concealed beneath unmineralized iron formation that initiated a concealed orebody search by Hamersley Iron from 1980 to 1994. The concealed orebody search was modeled on the CSIRO supergene-metamorphic model. This was followed by a dedicated high-grade hematite search following hypogene concepts by the Hamersley Iron Task Force from 1994 to 2001. These programs were successful in identifying semiconcealed high-grade hematite ores, but none led to a major discovery to replace the Mount Tom Price deposit. Before the concealed orebody search and the Task Force, very limited exploration using a syngenetic model had been carried out at Paraburdoo, which resulted in discovery of the largely concealed Lens 2 deposit. Worldwide, models for the formation of high-grade hematite ores are being refined due to current global exploration activity, which is resulting in greatly improved understanding of the geology of deposits and iron ore provinces. Exploration models are shifting from supergene to hypogene, which has led to deeper drilling and discovery of significant additional resources in some areas. Modern academic and exploration techniques are refining the current ore genesis models and together with the availability of new geophysical methods provide the iron ore exploration geologists with an invaluable set of tools to discover the future, most likely concealed hematite, deposits to satisfy the world's ever-increasing hunger for iron ore.