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Corresponding author: e-mail, hilke.dalstra@riotinto.com

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.

Introduction

Economic high-grade hematite deposits are almost exclusively hosted by Archean and Proterozoic BIFs and are the world's most important source of iron ore. Other styles, such as magnetite skarns, magnetite sands, or magmatic iron-titanium deposits, are locally important sources of iron ore but generally lack the size and quality of the BIF-hosted deposits. Geologists recognized from a very early stage that the deposits were structurally complex (e.g., van Hise and Leith, 1911), and that structures were the single most important control on the location and geometry of iron orebodies. It was also noted that a large variety of structures could be associated with economic ore concentrations, including plunging synclines, fissures, and fractures (van Hise and Leith, 1911). Early explorers in the Superior province also recognized that the structural controls on orebody locations could vary from district to district. Despite this recognition of the importance of structure, existing ore genesis models pay relatively little attention to the structural setting of deposits and its relationship to the timing of events that formed the deposits.

Syngenetic models generally suffice with the notion that ore concentrations formed in areas where the iron formations were somehow modified as a result of syndepositional structural features (e.g., King, 1989) but often fail to specify these features, probably because they are notoriously hard to detect.

Supergene or supergene-metamorphic models (e.g., Morris, 1985) favor structures which facilitate the downward percolation of surface-derived waters, such as plunging synclines, fractures, or faults. As the mineralization events postdate any structural event, the timing of such structures is not considered important. Morris (1985) for example stated that the general form of the individual ore system is not important. It could be a simple basinal shape, fault, crosscutting intrusion, or a cross fold. What is important is that the BIF acts as an aquifer, that impermeable horizons confine the flow of ground water to the BIF, and that this flow is maintained for long periods. Ribeiro and Carvalho (2002) and Ribeiro et al. (2002) demonstrated that supergene enrichment of BIF through extensive leaching of quartz induced the development of collapse structures causing slump-type folds and the development of kink-bands, probably controlled by preexisting fractures. The development of such structures triggered a progressive disintegration of the BIF facilitating further supergene enrichment.

Initially, hypogene models were proposed as alternatives to the supergene models for ore genesis in the Superior province (Gruner, 1937) to explain features which seemed at odds with simple supergene models, such as the large vertical extent of ore systems, the absence of vertical zonation, the problems of removing silica from BIF with silica-saturated meteoric waters, and evidence which suggested that the orebodies grew from the bottom up. Early hypogene models were not very specific with regards to the structural setting of the ores, although this aspect of the models became more sophisticated over time. Guild (1953, 1957) and Dorr (1965) discussed in detail the structural control on high-grade ores in the Quadrilátero Ferrífero of Brazil, which they suggested were hypogene in origin and controlled by folds and thrusts in the BIF sequence. In the last decade, hypogene models with more detailed structural analysis were proposed for the genesis of the large high-grade hematite deposits of the Hamersley province. Powell et al. (1999) proposed a synorogenic fluid model for the genesis of the Mount Whaleback deposit, whereby heated meteoric waters were expelled into a foreland basin from an uplifted fold and thrust belt at the end of the Ophthalmian orogeny. Brown et al. (2004) refined this model, suggesting that mineralization occurred during post-Ophthalmian extension and associated large-scale hydrothermal activity. Other models rejected the foreland basin setting and highlighted the importance of extensional structures that controlled deposition of the Lower and Upper Wyloo Group rocks for the genesis of high-grade hematite ore in this province (e.g., Taylor et al., 2001). On the basis of detailed structural reconstruction, Dalstra (2006) attributed formation of hypogene hematite-carbonate at Paraburdoo to development of fluid pathways during oblique reactivation of an older rift system (about 2100 Ma) during extension and deposition of the Upper Wyloo Group sediments around 2000

Ma. These protores were subsequently tilted, uplifted, and converted to the high-grade hematite ± goethite ores by supergene processes. In the Quadrilátero Ferrífero, Rosière et al. (2001, 2002) have associated iron ores of granoblastic textures to folds, whereas thrust-controlled ores preserve a pervasive foliation with the development of a lattice-preferred orientation. In the Carajás province high-grade orebodies are also controlled by regional fold hinges (Rosière et al., 2005).

Less attention has been given to the structural setting of the giant hematite-goethite deposits hosted by the Brockman and Marra Mamba BIFs in the Hamersley province. In a recent paper on the Hope Downs deposit in Western Australia, Lascelles (2006) recognized the structural complexity of the orebody but attributes much of this complexity to postmineralization deformation.

This paper briefly reviews the structural frameworks for major Australian, Brazilian, and South African iron ore provinces (Fig. 1) as a prelude to detailed structural settings of individual iron ore deposits within provinces. Finally, it presents structural concepts for formation of high-grade iron ore worldwide and their implications for exploration.

The Hamersley Province, Western Australia

Three compressional (tectonic) events, D2 to D4, with broadly coaxial west to northwest trends affected rocks of the Hamersley province (Figs. 2, 3). Prior to these events, early recumbent isoclinal folds and pods (D1) with northeast-trending axes most likely resulted from vertical loading and deformation in a partly unconsolidated sedimentary sequence.

The pre-Lower Wyloo Group Ophthalmia orogeny D2 (e.g., Tyler and Thorne, 1990) affected most areas, although the intensity of deformation varies widely across the province. In the eastern half of the province, an east-west–trending thin-skinned fold belt developed. Anticlines and synclines typically have wavelengths on the order of 5 to 15 km. Important examples are the Ophthalmia syncline, and the Weeli Wolli, Alligator, and Wonmunna anticlines (Fig. 2). The style of folds suggests the presence of a detachment, possibly along the base of the Fortescue Group, and as a result, the basement granitoid-greenstone terrane is not exposed in the cores of the anticlines. Large thrusts are not common, but smaller D2 thrust faults with displacement on the order of 200 m have been mapped in some prospects and have been exposed by mining and intersected by drill holes in the Marra Mamba deposits in this part of the province. In contrast, large east-west–trending dome and basin folds, formed in the western part of the province, expose granitoid-greenstone terrane in the core of the domes. Important examples are the Milli Milli, Jeerinah, and Rockley anticlines and the Brockman, Hardey, and Turner synclines (Fig. 2). Ophthalmian age folding had only a limited affect on the northern part of the Hamersley province. The presently most accurate estimate for the age of the Ophthalmia orogeny is 2140 Ma (Muller et al., 2005).

Fig. 1.

Locations of high-grade hematite deposits discussed in this paper.

Fig. 1.

Locations of high-grade hematite deposits discussed in this paper.

Fig. 2.

Geologic map of the Hamersley province, showing major structural features and unconformities. Abbreviations: 18EF = 18 east fault, AA = Alligator anticline, BS = Brockman syncline, CHF = Cairn Hill fault, HF = Homestead fault, HS = Hardey syncline, JA = Jeerinah anticline, MA = Milli Milli anticline, MF = Metawandy fault, NF = Nanjilgardie fault, OF = Ophthalmia fault, OS = Ophthalmia syncline, PF = Poonda fault, PH = Paraburdoo hinge zone, PCF = Parry Creek fault, PDF = Prairie Downs fault, PFC = Panhandle fold corridor, TCS = Turee Creek syncline, TS = Turner syncline, RA = Rockley anticline, SBF = Southern Batter fault, SMF = Snowy Mountain fault, VS = Volcano syncline, WA = Wonmunna anticline, WF = Whaleback fault, WHF = Wheelara fault, WWA = Weeli Wolli anticline, WAY = Wyloo anticline.

Fig. 2.

Geologic map of the Hamersley province, showing major structural features and unconformities. Abbreviations: 18EF = 18 east fault, AA = Alligator anticline, BS = Brockman syncline, CHF = Cairn Hill fault, HF = Homestead fault, HS = Hardey syncline, JA = Jeerinah anticline, MA = Milli Milli anticline, MF = Metawandy fault, NF = Nanjilgardie fault, OF = Ophthalmia fault, OS = Ophthalmia syncline, PF = Poonda fault, PH = Paraburdoo hinge zone, PCF = Parry Creek fault, PDF = Prairie Downs fault, PFC = Panhandle fold corridor, TCS = Turee Creek syncline, TS = Turner syncline, RA = Rockley anticline, SBF = Southern Batter fault, SMF = Snowy Mountain fault, VS = Volcano syncline, WA = Wonmunna anticline, WF = Whaleback fault, WHF = Wheelara fault, WWA = Weeli Wolli anticline, WAY = Wyloo anticline.

Fig. 3.

Tectonostratigraphic columns of the Hamersley province (Australia). Arrows indicate the direction of far-field stress direction.

Fig. 3.

Tectonostratigraphic columns of the Hamersley province (Australia). Arrows indicate the direction of far-field stress direction.

The Panhandle folding event, D3, (e.g., Taylor et al., 2001) is particularly well developed in the Turner and Brockman syncline areas and generated the pronounced northwest-trending Bungaroo to Mount Nameless and Panhandle fold corridors (Fig. 2). Folding along this trend predated intrusion of the numerous northwest-trending dolerite dikes. Overprint of Panhandle-age folds on east-west–trending Ophthalmia folds generated an asymmetrical dome and basin pattern and formed the typical mushroom shape of the Turner syncline (Fig. 2). Small-scale thrusts, with maximum displacements on the order of 300 m in the Marandoo Marra Mamba, hosted deposits most likely originated during D3. Recently, the age of this event has been estimated as 2020 Ma (Muller et al., 2005). The Panhandle folding event has in the past been confused with the Capricorn orogeny, D4 (approx 1780 Ma; e.g., Tyler and Thorne, 1990; Krapez, 1999; Tyler, 2005), which is coaxial with the Panhandle event, but postdated the intrusion of the dolerite dikes and also affected younger rocks of the Ashburton basin. So far, only the tilting of Hamersley Group stratigraphy together with the Lower and Upper Wyloo Group unconformities along the southern margin of the province (also called the Paraburdoo Hinge zone) and in the Wyloo dome area can be ascribed to the Capricorn orogeny (Fig. 2).

Several extensional events occurred in the periods between the major compressional events in the Hamersley province. Although these are classified as E1-Ex below, they are generally unrelated to the major deformation events D1-Dx described above.

The first major extensional event (E1) that affected the rocks from the Hamersley province comprised the initial rifting associated with the formation of the Hamersley basin (Fig. 3). Growth faults associated with this event have been mapped in the lower volcanic and sedimentary units of the Fortescue Group.

Detailed prospect-scale mapping and exploration drilling, particularly along the southern margin between the town of Paraburdoo and the Wyloo dome, has identified several younger extensional fault systems, which in timing occupy the periods between compressional events (Figs. 2, 3). The northwest- to north-northwest–striking, predominantly northeast-dipping normal faults that formed during E2 have offsets of up to 1,100 m and predated deposition of the Beasley Quartzite of the Lower Wyloo Group (Table 1). In many areas, later events have tilted these faults to gently east-northeast dips and they are now described as oblique flat faults. The presence of large variation in thickness of BIF conglomerate at the base of the Beasley Quartzite, also called the Three Corner Bore conglomerate, across these faults suggests that locally these faults formed ancient scarps. On the basis of age determinations provided by Muller et al. (2005), their timing is approximately 2100 Ma.

Many of these faults were reactivated during another extension event (E3), just prior to deposition of the Mount McGrath Formation of the Upper Wyloo Group. Additionally, northwest-striking, steep southwest dipping normal faults developed during this event which may have offsets as large as 2,100 m. In some areas, later tilting has brought these structures into steep northeast-dipping orientations with apparent reverse offsets. These two fault sets form a "horst and graben" system along the southern margin of the Hamersley province. Numerous northwest-striking dolerite dikes preferentially intruded in these faults during the later stages of the E3 event at 2008 Ma (Muller et al., 2005). Along the southern margin, from the town Paraburdoo to the Wyloo dome, this horst and graben system was subsequently uplifted, partly eroded, and tilted to the south at 30° to 60° during the Capricorn orogeny. Only those faults that remained in a steep position after tilting were reactivated with strike-slip movement during the later stages of the Capricorn orogeny at about 1740 Ma.

BIF-Hosted Iron Ores of the Hamersley Province

BIF-hosted iron ores are found throughout the Hamersley province. These ores are of two main styles:

1. High-grade (>60% Fe) hematite, often with martite and microplaty hematite (Morris, 1985), but little or no goethite, forms two giant (>900 Mt) orebodies, Mount Whaleback and Mount Tom Price. Moderate size and smaller (<50 Mt) hematite deposits are numerically underrepresented, with only a total of about 20 of such deposits known in the province so far. The major orebodies extend to depths greater than 400 m, are mostly confined to the Brockman Iron Formation, and are most strongly developed in its lowermost unit, the Dales Gorge Member. The importance of structure for the genesis of these deposits was recognized from an early stage (Campana, 1967), but a comprehensive structural model has not yet been published.

TABLE 1.

Fault Systems in the Paraburdoo Ranges and Their Details

Fault systemRepresentativesPresent orientationTimingOriginal orientation
Range parallel flat faults (RPF)66West fault, 44West fault, 32East fault, 42East faultGently N to NNE dippingPre-Mount McGrath (E3)Moderately NNE dipping
Oblique flat faults1 (OF)4West basal fault, 4East basal fault, 23East basal fault, 24East faultGently NE to E dippingPre BRQ, reactivated pre-Mount McGrath (E2-E3)Moderately NE to ENE dipping
Steep reverse faults (SRF)Ratty Springs fault, 18East fault, 23East faultSteeply NE dippingPost-BRQ, pre-Mount McGrath, late dextral reactivation (E3-F4)Moderate-steep SW dipping
Steep normal faults2 (SNF)64East fault, Howies Hole fault, Channar East faultModerately NE dippingPre BRQ?, reactivated pre-Mount McGrath and late dextral (E2?-E3-F4)Moderately NE dipping, rare SW dips
Fault systemRepresentativesPresent orientationTimingOriginal orientation
Range parallel flat faults (RPF)66West fault, 44West fault, 32East fault, 42East faultGently N to NNE dippingPre-Mount McGrath (E3)Moderately NNE dipping
Oblique flat faults1 (OF)4West basal fault, 4East basal fault, 23East basal fault, 24East faultGently NE to E dippingPre BRQ, reactivated pre-Mount McGrath (E2-E3)Moderately NE to ENE dipping
Steep reverse faults (SRF)Ratty Springs fault, 18East fault, 23East faultSteeply NE dippingPost-BRQ, pre-Mount McGrath, late dextral reactivation (E3-F4)Moderate-steep SW dipping
Steep normal faults2 (SNF)64East fault, Howies Hole fault, Channar East faultModerately NE dippingPre BRQ?, reactivated pre-Mount McGrath and late dextral (E2?-E3-F4)Moderately NE dipping, rare SW dips

1Control mineralization at 4West, 4East, and 23/24East

2 Controls mineralization at Channar

2. Traditionally, high-grade (>60% Fe) martite-goethite ores of the Marra Mamba and Brockman Iron Formations have been viewed as supergene deposits, formed from iron enrichment of the precursor BIF by surface derived fluids. The original magnetite crystals are oxidized to hematite (martite), iron silicates and carbonates are oxidized and hydrated to goethite while other carbonates and quartz are leached out and replaced by goethite. The orebodies are extensive, generally flat-lying and, although in places they extend to more than 300-m depth, are generally related to the present land surface.

Until now, the structural controls on these deposits have largely been ignored, but recent evaluation and mining of a number of these (e.g., the Marandoo, Nammuldi, West Angelas, Area C, Hope Downs, and Brockman 2 deposits) demonstrates their structural complexity, and also highlights features, such as the presence of rare magnetite and apatite-bearing ore, which seem at variance with a simple supergene origin. Lascelles (2006) has recently questioned the simple supergene genesis of some of these deposits on the basis of a detailed study of the Hope Downs deposit. He argues for the presence of a precursor, syngenetic "chert-free BIF," which was subsequently weathered to form the present orebodies.

Structural setting of high-grade hematite deposits in the Hamersley province

In order to understand the structural setting of key highgrade hematite deposits in the Hamersley province (Fig. 2), a number of representative cross sections through these deposits and their reconstructed geometry at the time of mineralization are presented in Figure 4. The structural setting of individual deposits is presented below.

Mount Tom Price: The ore at Mount Tom Price is preserved as a folded sheet within and just to the north of the synclinal closure of the Turner syncline (D2, Figs. 2, 3).

Two normal faults, the Southern Batter fault (Figs. 4A, 5A) and the Box Cut fault, are prominent in the mine area but both faults have limited lateral and vertical extents. The main iron orebodies are parallel to the trend of these faults with most enrichment confined to the north of the Southern Batter fault zone. The Southern Batter fault is a northwest-striking, southwest-dipping normal fault or fault splay (Fig. 2) with a maximum throw of up to 300 m. The Box Cut fault juxtaposes high-grade hematite ore of the northeastern Prong deposit against dolomite from the Paraburdoo Member to the north (Taylor et al., 2001). The strike of this fault is east-west, with a steep southerly dip and normal displacement of up to 300 m. The Box Cut fault represents a structural discontinuity that separates the more complexly folded Brockman Iron Formation from the gently synclinal Marra Mamba Iron Formation. There is also a substantial loss of the carbonate units beneath the eastern part of the Mount Tom Price deposit, where the dolomite is dissolved and is represented now by residual, manganiferous shale. The Box Cut fault does not offset the underlying Marra Mamba Iron Formation and is interpreted to compensate for the loss of the dolomite from the Paraburdoo Member below the high-grade hematite ore.

Panhandle-style, west-northwest–northwest–trending folds (D3) are found throughout the mine area. Open-pit mapping at Mount Tom Price has shown that the Southern Batter fault is strongly folded by this event (Taylor et al., 2001). The folds are complex and noncylindrical and have axial planes that fan about a west-northwest axis. Two pronounced D3 synclines, the northeastern and southeastern Prong synclines form the eastern end of the deposit. Importantly, there is no development of an axial planar cleavage associated with these folds in the hematite ore, even though cleavage is locally developed in the surrounding shales. Dolerite dikes extend through a large part of the mine area and are unaffected by, and must therefore be younger than, any of the structures described above (Fig. 4A). Importantly, these dikes display a distinct chloritetalc-hematite alteration in the mine area. This style of hydrothermal alteration is commonly associated with high-grade hematite deposits worldwide (Dalstra and Guedes, 2004). The relative timing of these events constrains the protore formation at Mount Tom Price to post D3 and post emplacement of the dolerite dikes.

Paraburdoo: The ore in the Paraburdoo 4 East and 4 West deposits is hosted in the Dales Gorge and Joffre Members of the Brockman Iron Formation, which dips moderately to steeply to the south (Fig. 4B). The rocks are overlain unconformably by clastic sedimentary rocks and dolomites of the Lower and Upper Wyloo Group (i.e., the Beasley Quartzite unconformity). The rocks at the base of the Lower Wyloo Group display similar southerly dips as rocks from the Hamersley Group consist of conglomerate and sandstone of the Beasley Quartzite. The rocks of the Mount McGrath Formation, including hematite conglomerate, at the base of the Upper Wyloo Group unconformably overlie the units below and dip on average 10° less than the underlying units.

Two major and several subsidiary faults control the location of ore shoots at Paraburdoo (Table 1, see also Dalstra, 2006). Mineralization at 4 West, the westernmost part of the Paraburdoo deposit, is controlled by the 4 West Basal fault, whereas at 4 East, the easternmost part of the deposit, the 4 East Basal fault controls high-grade iron mineralization (Fig. 4B). The faults strike north to northwest, oblique to the general strike of the BIFs, dip gently east or northeast, and have normal displacements. Hematite ore is concentrated in the hanging wall of these faults and is overlain by unmineralized BIF (Fig. 5B). Mineralization is terminated to the east by a northwest-striking, subvertical to steep northeast-dipping reverse fault, the 18 East fault.

Offsets of the Hamersley Group stratigraphy at the 4 West and 4 East Basal faults are up to 1 km but much smaller in the Lower Wyloo Group (<100 m). These faults terminate against the MMG unconformity. The above described relationships suggest that they formed prior to the Beasley Quartzite unconformity (E2) but were subsequently reactivated prior to the MMG unconformity (E3). The tilting of these faults during D4 resulted in their present, much flatter orientation.

Fig. 4.

A. Cross sections 13962E through the Mount Tom Price deposit (modified from Taylor et al., 2001) during protore formation and today. Star shows approximate position of Figure 5A. B. Cross sections 1820E through the Paraburdoo 4East deposit immediately after protore formation and today. Star shows approximate position of Figure 5B. C. Cross sections 2000E through the Giles Mini deposit during protore formation and today. D. Cross sections 7160E through the Mount Whaleback deposit during protore formation and today (modified from Ronszecki, 1992; Dalstra et al., 2001). Star shows approximate position of Figure 5D.

Fig. 4.

A. Cross sections 13962E through the Mount Tom Price deposit (modified from Taylor et al., 2001) during protore formation and today. Star shows approximate position of Figure 5A. B. Cross sections 1820E through the Paraburdoo 4East deposit immediately after protore formation and today. Star shows approximate position of Figure 5B. C. Cross sections 2000E through the Giles Mini deposit during protore formation and today. D. Cross sections 7160E through the Mount Whaleback deposit during protore formation and today (modified from Ronszecki, 1992; Dalstra et al., 2001). Star shows approximate position of Figure 5D.

Fig. 5.

A. The Southern Batter fault in the Mount Tom Price deposit, showing juxtapositioning of high-grade hematite ore in the hanging wall onto oxidized McRae Shale in the footwall of the fault. B. Small flat fault in the Paraburdoo 4East deposit. The fault is characterized by development of a narrow microplaty hematite zone. C. Aerial photograph of the Ophthalmia fault at West Lido. Overturned Brockman BIF with hematite deposits is in fault (dotted line) contact with shales from the Fortescue Group. The fault dips moderately steeply to the south (left side of photo). D. Near-recumbent D2 folds defined by shale bands in hematite ore, Mount Whaleback.

Fig. 5.

A. The Southern Batter fault in the Mount Tom Price deposit, showing juxtapositioning of high-grade hematite ore in the hanging wall onto oxidized McRae Shale in the footwall of the fault. B. Small flat fault in the Paraburdoo 4East deposit. The fault is characterized by development of a narrow microplaty hematite zone. C. Aerial photograph of the Ophthalmia fault at West Lido. Overturned Brockman BIF with hematite deposits is in fault (dotted line) contact with shales from the Fortescue Group. The fault dips moderately steeply to the south (left side of photo). D. Near-recumbent D2 folds defined by shale bands in hematite ore, Mount Whaleback.

The 18 East fault displays a significant offset of the Hamersley Group stratigraphy, but it terminates against the Upper Wyloo MMG unconformity. Relative timing obtained from crosscutting relationships constrains its age to between the depositional ages for the Lower and Upper Wyloo Group (E3).

In order to reconstruct the structural setting during ore formation at Paraburdoo it is necessary to back rotate the hematite conglomerate beds at the base of the Upper Wyloo Group to their near-horizontal depositional setting. Such a reconstruction assumes that these hematite conglomerates formed as a result of uplift and erosion of older high-grade hematite ores.

Rotation of the oblique, flat-lying faults, for example, the 4 East and 4 West Basal faults, over a horizontal west-north-west–striking axis brings them in a northwest to north-northwest strike with moderately steep (48°–72°) northeast to east-northeast dips. Back rotation also indicates that the steeply northeast-dipping 18 East fault originated as a moderately steep southwest-dipping normal fault.

The syn-Upper Wyloo geometry for the 4 East deposit is shown in Figure 4B. This interpretation suggests that hematite ore formed in an extensional "graben," at least 500 to 800 m below the McGrath unconformity. It also shows that the hematite conglomerates at the 4 East deposit were not sourced from the Paraburdoo deposits themselves but more likely from now-eroded ore in the footwall of the 18 East fault. Because the ores in the 4 East and 4 West deposit formed in "grabens they had a higher chance of preservation compared to any ores formed adjacent to the faults on the flanking horst blocks. Only two of such orebodies are preserved in the Paraburdoo ranges (the 11 West and Channar orebodies), compared to 10 orebodies which formed in paleograbens (Dalstra, 2006).

Giles Mini: The Giles Mini deposit is the largest hematite deposit associated with the Ophthalmia fault system along the southern margin of the Ophthalmia range (Fig. 2). In this area, D2 folds trend about east-west and have axial planes with moderate to steep dips toward the south. A steep axial planar cleavage is developed within the less competent rocks, such as shales and dolerites. This cleavage is parallel to the fold axial planes. Overall the axial planes of D2 folds in the southeastern part of the Hamersley province dip about 50° to 80° S but vary due to later refolding (Flynn, 1995).

The Ophthalmia fault system is a major east-west–trending, moderately steeply south dipping series of normal faults, which is recognized from the eastern margin of the Turee Creek syncline to east of the town of Newman, with a total strike length of approximately 150 km (Fig. 2). Several small- and medium-sized hematite deposits are located along its trace. These deposits have a common structural setting, hematite in steeply dipping or overturned Brockman Iron Formation BIF is in faulted contact with shales and basalts of the Fortescue Group (e.g., East Lido, Fig. 5C). The present geometry of the fault system, as demonstrated by detailed mapping at the Giles Mini deposit south of the Ophthalmia range, suggests a normal movement. Over its strike length, the Ophthalmia fault is offset by several smaller northwest- or northeast-trending faults. Along its western segment, the faults are crosscut by northwest-trending dolerite dikes, but no offsets are visible.

The Ophthalmia fault at Giles Mini has an average dip of 55° toward the south (Fig. 4C), but there are large variations, and dips may be significantly flatter or steeper than this. Normal movement along the fault is up to 600 m. In outcrop, the fault is characterized by an up to 100-m-wide zone of intense deformation, with tight recumbent folds and localized small-scale thrusts that display similar vergence as the regional D2 structures. These relationships suggest that the Ophthalmia fault system originated as a thrust during D2 and was later reactivated with normal movement during E2 and E3. At the Giles Mini deposit, the Ophthalmia fault cuts the complex folded Brockman Iron Formation, which is juxtaposed against Marra Mamba BIF and Fortescue Group rocks north of the fault (Fig. 4C). At the deposit the BIF of the Dales Gorge Member is predominantly altered and mineralized to highgrade microplaty hematite, with most of the resource contained in two synclines. These synclines strike east-west and are interpreted to be of Ophthalmian age (D2). Importantly, there is no axial planar cleavage associated with the D2 synclines in the hematite ores.

Recent weathering and oxidation is progressively less deep toward the eastern end of the deposit (120 m below the surface), which results in the preservation of hematite- and magnetite-bearing protores similar to those found at Mount Tom Price (Fig. 4C). Relatively small tonnages of high phosphorous magnetite-carbonate mineralization of intermediate Fe grade occur above the Ophthalmia fault from a depth of about 160 m. High-grade, apatite-bearing magnetite mineralization is located immediately above the magnetite-carbonate mineralization, but below the oxidation surface, followed by high-grade hematite and hematite-goethite mineralization in the modern weathering profile. Locally, barren BIF overlies these ore sequences from about 100-m depth and extends to the surface. A thin layer of lateritic BIF (i.e., the hydrated zone) in places overlies the barren BIF. Mafic rocks of the Fortescue Group underlie the mineralization below the fault and have been altered extensively to chlorite-(Mg-Fe) rich assemblages with a similar geochemical signature as the altered mafic dikes at Mount Tom Price (Taylor et al., 2001). The McRae shales located below the mineralization are also strongly carbonate altered.

Although deformation at the Giles Mini deposit is intense, the structural history is relatively simple, with one dominant folding phase D2, followed by normal faulting and high-grade iron mineralization. Constraining the relative timing of the Ophthalmia fault with respect to the later deformation events is presently not possible at Giles Mini, but the fault is crosscut by dolerite dikes (E3) in the west, suggesting that its timing is most likely syn-E2.

Mount Whaleback: The structure of the Mount Whaleback deposit has been the subject of considerable debate. The structure of the deposit was first described in detail by Ronaszecki (1992) and Powell et al. (1999). The regional structure of the Newman area has been the subject of an unfinished Ph.D. study by Johnsson, data from which was republished by Flynn (1995) in an honors study of the deposit.

Iron mineralization extends to depths of 500 m below the premining ground surface at the western end, where highgrade hematite ore is largely concealed by unmineralized BIF (Fig. 4D). The structural setting of the deposit is complex with a major, late, southeast-dipping normal fault, the Whaleback fault juxtaposing the orebody against the unmineralized Jeerinah Formation to the northwest. This represents a normal displacement of at least a 1,000 m. The host iron formation in the deposit is intensely deformed into small- to medium-scale folds with wavelengths of centimeters to hundreds of meters. The folds are recumbent with west- to west-southwest-plunging fold axes (Fig. 5D). At the western end of the deposit the folds are more upright, with steeply south dipping axial planes. Two important subhorizontal fault zones, the Central fault and the East Footwall fault, crosscut the iron formation and the recumbent folds (Ronaszecki, 1992). At the western end of the orebody, these faults are upright and listric and are terminated by the late Whaleback fault. A cross section shows that unmineralized BIF overlies iron ore, and the contacts between the BIF and iron ore are approximately parallel to the Central and East Footwall fault zones but crosscut through the main D2 folds in the orebody, suggesting that iron mineralization postdated this folding event (Fig. 4D). At least part of the movement on these faults is caused by the loss of volume in the orebody.

Comparing the orientations of regional D2 folds with D2 folds in the mine, Dalstra et al. (2002) argued that the orebody may have been subject to significant postore modification. In the main part of the Mount Whaleback deposit, the D2 folds trend west-southwest and have near-horizontal axial planes. These orientations are clearly anomalous to the more upright, east-west trend of the D2 folds in the Ophthalmia range in general.

If it is assumed that the recumbent style of D2 folding at Mount Whaleback is the result of postmineralization rotation of originally more upright folds, possibly along the Whaleback fault, then by rotating the recumbent folds back to the regional, upright or overturned, east-west orientation of D2 should present the geometry of the orebody pre the Whaleback fault. The resulting geometry is shown in Figure 4D and is very similar to the much smaller Giles Mini deposit, 50 km west of Mount Whaleback, which is characterized by more upright D2 folds and steeper dipping normal faults (Fig. 4C). The Whaleback fault, which postdates iron mineralization (Ronaszecki, 1992) is not shown in this reconstruction.

Postmineralization rotation could also explain the absence of magnetite-carbonate protores at Mount Whaleback. These are generally present in the deepest parts of the mineralized system at the Mount Tom Price or Giles Mini deposits (e.g., Taylor et al., 2001). The reconstructed section of Figure 4D shows that what are now the deepest parts of the present Mount Whaleback orebody were at the time of ore formation the shallowest. The deepest parts of the prerotation orebody are now close to the surface and intensely weathered. No magnetite or carbonate would be expected to survive there. In addition, the postmineralization Whaleback fault has dislodged the orebody from its alteration zone, parts of which may be preserved north of the deposit (Kepert, pers. commun., 2006).

Preservation of the hematite ores was facilitated by the more than 1-km normal movement on the Whaleback fault, which has juxtaposed hematite ore against stratigraphically much lower rocks from the Fortescue Group across the fault.

Structural setting of hematite-goethite deposits of the Hamersley province

Hematite-goethite deposits, particularly those hosted by the Marra Mamba Iron Formation, are of increasing economic importance in the Hamersley province. Several of these deposits are currently being mined or will be mined in the immediate future. These are the Marandoo, Nammuldi, Orebody 29, West Angelas, Area C, Hope Downs, and the Chichester Range deposits. The higher grade Marra Mamba ores typically contain 61 to 64 percent Fe, with a relatively low phosphorous content (0.5–0.7% P). Despite their increasing economic importance, the structural setting and genesis of these deposits remains poorly understood. The following section on the structural setting of the large hematite-goethite deposits in the Marra Mamba Iron Formation demonstrates that these deposits also have a very distinct structural architecture and are different in style to the deposits in the Brockman Iron Formation.

Nammuldi: The Nammuldi deposit is a large hematite-goethite iron ore deposit, typical of those hosted mainly by the Newman Member of the Marra Mamba Iron Formation. The deposit is located along the northern flank of the Brockman syncline, a regional east-west–trending D2 syncline (Fig. 2). At a mine scale, the deposit is located in the northwest-trending Bungaroo-Mount Nameless fold corridor (D3). As a result, the dominant trend of folds in the mine is northwest to west-northwest. Folding is gentle to open with interlimb angles greater than 90°. The deposit comprises several lenses, with the bulk of the iron ore concentrated in Lens C. This lens is characterized by a prominent, gently south dipping thrust, which results in duplication of the Newman Member and the ore zone (Figs. 6A, 7A). In the mine, the thrust was exposed as a thin, deeply weathered zone on which Newman Member hematite-goethite ore was juxtaposed on weathered West Angelas shale (Fig. 7A). The iron ore is thickest close to the thrust, where it exceeds a total depth of 170 m and occupies nearly the entire thickness of the Newman Member. Displacement on the thrust is typically less than 250 m. The thrust is cofolded with the BIF by the west-northwest–trending F3 folds and is therefore interpreted as an older (D2) structure. Total deformation (D2 and D3) in the mine area caused a moderate shortening of 19 percent. The pre-D3 reconstruction through backfolding of the section straightens the thrust, which suggests that thrusting predated folding (Fig. 6A). This reconstruction also shows that ore-BIF contacts (indicated as "future mineralization" in Fig. 6A) after reconstruction are offset. The present day ore-BIF contact crosscuts the thrust without significant offset, suggesting that iron ore formation postdated thrusting and D3 folding.

Fig. 6.

A. Cross section 1000E through the Nammuldi deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7A. B. Cross section 681440 through the West Angelas deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7B. C. Cross section 13100E through the Hope Downs deposit and reconstructed predeformation geometry.

Fig. 6.

A. Cross section 1000E through the Nammuldi deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7A. B. Cross section 681440 through the West Angelas deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7B. C. Cross section 13100E through the Hope Downs deposit and reconstructed predeformation geometry.

Fig. 7.

A. Thrust in the Nammuldi deposit, juxtaposing high-grade hematite-goethite ore in West Angela Shale member. B. Thinly bedded, steeply dipping hematite-goethite ore in the hanging wall of the West Angela thrust.

Fig. 7.

A. Thrust in the Nammuldi deposit, juxtaposing high-grade hematite-goethite ore in West Angela Shale member. B. Thinly bedded, steeply dipping hematite-goethite ore in the hanging wall of the West Angela thrust.

West Angelas: The West Angelas deposit is located in the central Hamersley province on the flanks of a kilometre-scale, gently west plunging anticline (D2) in the Marra Mamba Iron Formation, the Wonmunna anticline (Fig. 2). At West Angelas, the D2 folding is more intense than at Nammuldi, and folds are mostly of closed geometry. Open D3 folds are coaxial with D2 and locally refold the D2 structures (Buerger, 1997). The result is a variation in the dip of the D2 axial planes. An S2 axial planar foliation is locally developed predominantly in the chert layers of unmineralized BIF, but rarely a foliation is preserved in the hematite-goethite ore. The mineralization is distributed over several lenses, with most ore concentrated in deposit A.

Iron ore is thickest in areas of thrust faults (Fig. 6B), exceeding depths of 220 m, and ores occupy the entire Newman Member which is intensely deformed in the hanging wall of the thrusts (Fig. 7B). The thrusts generally crosscut the northern limb of anticlines, with maximum displacements of about 200 m. The thrusts are less folded than the host iron formation. Total shortening across the host sequence is significantly larger than in the Nammuldi deposit and is on the order of 38 percent. Pre-D2 reconstruction through backfolding of the section overcompensates for D2 folding and therefore makes the thrusts highly curved. This geometry is not a ramp-flat structure, because the flat sections of the fault generally do not follow the lithologic contacts with strongest competency contrasts, such as the Newman Member-West Angela Shale boundary. This suggests that the thrusts formed after, or late during D2.

The timing of iron mineralization with respect to D2 is more difficult to determine. Reconstruction also straightens the ore-BIF contacts (indicated as future mineralization in Fig. 6B), which would imply that ore formation predated folding. However, the observation that the ore-BIF contacts crosscut the thrusts without significant displacement implies that iron mineralization postdated thrusting and therefore also the D2 folding event.

Hope Downs: This deposit is situated in the eastern closure of the Weeli Wolli anticline, a regional D2 structure, and together with the Area C deposits located about 20 km to the west, forms an iron resource of well over one billion tons (Fig. 2). The deposit is hosted by intensely deformed Marra Mamba BIF, with a total shortening of over 45 percent (Fig. 6C). The dominant phase of folding, D2 trends east-west and is overprinted by less intense west-northwest–east-southeast–trending folds (D3). Stacked thrusts with displacements of up to 140 m cut the northern limb of the anticline. Iron ore is best developed along these thrusts and exceeds depths of 230 m. Where the orebody is thickest the iron ore occupies the entire Newman Member and parts of the underlying McLeod Member. Again, pre-D2 reconstruction through backfolding makes the thrusts highly curved, suggesting that the thrusts largely postdated D2 folding. Again, the ore-BIF contacts crosscut the thrusts without significant displacement, which implies that iron mineralization postdated thrusting and therefore also the D2 folding event.

The Quadrilátero Ferrífero, Brazil

The Quadrilátero Ferrífero district (Fig. 8A; Dorr, 1969) is located on the southern border of the São Francisco craton (Almeida, 1977), comprising the Archean greenstone terranes of the Nova Lima Supergroup and Paleoproterozoic platformal sediments of the Minas Supergroup that consist of quartzites, metaconglomerates, phyllites, dolomites, and BIFs with enclosed iron orebodies. Both sequences enclose Archean to Paleoproterozoic granite-gneiss domes and are unconformably overlain by Paleo- to Mesoproterozoic rocks of the Espinhaço Supergroup.

The Minas Supergroup comprises four sequences: the Caraça, Itabira, Piracicaba, and Sabará Groups (Fig. 9, see also Dorr, 1969). The Itacolomi Group unconformably overlies the Minas Supergroup. The thickest sequences of BIFs with iron orebodies and subordinate phyllites and metamorphic iron formations (itabirites) belong to the Cauê Formation that, together with carbonate rocks, subordinate phyllites and dolomitic BIFs of the Gandarela Formation, constitute the Itabira Group.

Fig. 8.

A. Geologic map of the Quadrilátero Ferrífero district of Brasil. Abbreviations: Synclines: DBS = Dom Bosco, FC = Fabrica Complex, GS = Gandarela, JMS = João Monlevade, IS = Itabira, MS = Moeda; faults: CF = Curral fault, MF = Mutuca fault; mines: AC = Aguas Claras, AG = Alegria, AN = Andrade, BR = Brucutú, CA = Cauê, CD = Capitão do Mato, CF = Feijão, CM = Corrego do Meio, CO = Conceição CP = Casa de Pedra, CX = Capão Xavier, FB = Fabrica, FZ = Fazendão, MT = Mutuca, PI = Picos, TA = Tamandua, TB = Timbopeda. Inset B shows the distribution of metamorphic zones after Pires (1995), and low- and high-strain domains. AZ = actinolite zone, CZ = cummingtonite zone, GZ = grunerite zone, TAZ = tremolite-anthophyllite zone.

Fig. 8.

A. Geologic map of the Quadrilátero Ferrífero district of Brasil. Abbreviations: Synclines: DBS = Dom Bosco, FC = Fabrica Complex, GS = Gandarela, JMS = João Monlevade, IS = Itabira, MS = Moeda; faults: CF = Curral fault, MF = Mutuca fault; mines: AC = Aguas Claras, AG = Alegria, AN = Andrade, BR = Brucutú, CA = Cauê, CD = Capitão do Mato, CF = Feijão, CM = Corrego do Meio, CO = Conceição CP = Casa de Pedra, CX = Capão Xavier, FB = Fabrica, FZ = Fazendão, MT = Mutuca, PI = Picos, TA = Tamandua, TB = Timbopeda. Inset B shows the distribution of metamorphic zones after Pires (1995), and low- and high-strain domains. AZ = actinolite zone, CZ = cummingtonite zone, GZ = grunerite zone, TAZ = tremolite-anthophyllite zone.

Fig. 9.

Tectonostratigraphic columns of the Quadrilátero Ferrífero (Brazil). Arrows indicate the direction of far-field stress direction.

Fig. 9.

Tectonostratigraphic columns of the Quadrilátero Ferrífero (Brazil). Arrows indicate the direction of far-field stress direction.

The Quadrilátero Ferrífero was affected by at least two orogenic cycles: the Paleoproterozoic Transamazonian and/or Mineiro (2.1–2.0 Ga) and the Neoproterozoic to Early Paleozoic Brasiliano and/or Araçuaí (0.65–0.50 Ga) cycles with metamorphic conditions varying from lower greenschist to amphibolite facies, the latter generally with retrograde metamorphism to greenschist facies (Fig. 8B; Herz, 1978; Pires, 1995).

The regional structure developed from the superposition of two main deformational events (Fig. 9; Chemale et al., 1994). The first compressional event (D1) produced the two main fold and fault directions (F1a/F1b: NE-SW and north-north-west-south-southeast) of the supracrustal sequences and was followed by the uplift of the granite gneiss domes. The uplift phase has been interpreted as the product of an orogenic collapse producing relicts of a metamorphic core complex in the hinterland of the Transamazonian collision zone (approx 2100 Ma, Alkmim and Marshak, 1998) or may also have been influenced by granitic intrusions (Rosière and Rios, 2004), resulting in the typical dome and basin geometry of the Minas Supergroup. The second event D2 (folding event F2) is related to a west-verging fold and thrust belt of Pan-African and/or Brasiliano age (500–650 Ma), causing inversion, amplification, translation, and rotation of the synclines, with a tectonic gradient that divides the area into two main structural domains (Fig. 8B).

The western, low-strain domain displays well-preserved regional synclines and second order folds generated during the first Transamazonian event. These include the Moeda and Dom Bosco synclines and the Bação and Bonfim granite-gneissic domes. In this domain primary features of the iron formation such as microlayering and recrystallized chert pods of the iron formation are well preserved in the Pico do Itabirito, Mutuca, and Tamanduá deposits (Fig. 8A).

The eastern high-strain domain is dominated by thrusts such as the Fundão-Cambotas system and the Fazendão thrust front and by transcurrent shear zones (Engenho fault) that deformed the early-developed synclines (e.g., the Itabira and João Monlevade synclines). This overprint resulted in tight to isoclinal folds and thick mylonitic zones (Chemale et al., 1994) and finally thrusted the Minas metasedimentary rocks onto the Neoproterozoic metasedimentary rocks of the Espinhaço Supergroup (Chemale et al., 1994). In this domain, primary features of the iron formations are partially or totally transposed by a penetrative schistosity, for example, in the Andrade, Conceição, and Morro Agudo deposits.

At least three extensional phases are recognized (Fig. 9). The first event predates deposition of the Minas Supergroup. The second phase is a protracted event that started with the uplift of the gneiss domes immediately after D1 and possibly continued during deposition of the Itacolomi Group in small fault-bounded basins in which these sediments were unconformably deposited over the Minas Supergroup. The third extensional event predated deposition of the Espinhaço Supergroup.

High-grade iron ores (>64% Fe) consist of hard and soft varieties. Hard ore displays a dull to metallic luster and generally forms compact, gray-blue–colored bodies. They may be massive, with or without relics of primary BIF features, such as primary sedimentary bedding or diagenetic structures of the BIF. In the low-strain domain the ores are mainly martite or hematite, although magnetite relics are always present and may constitute nearly 100 percent of some ores. The massive orebodies are strata bound parallel to the structures and range from small mullion-shaped bodies and veins to very large lens-shaped bodies. In the high-strain domains both ore and BIF have a pervasive foliation. The orebodies are shear zone controlled and composed of specularite with variable proportions of (granoblastic) martite as clasts in a mylonitic fabric (Rosière et al., 2001). Soft, gray-blue ores are geochemically very similar to hard ores but are very friable and in many deposits surround the hard orebodies.

In addition to the high-grade hematite ores, there are enormous deposits of rich-BIF ores with iron grades between 50 and 64 percent. In some deposits, these form halos around the higher grade ores. These ores are characterized by the presence of relic quartz crystals in a soft iron oxide matrix. They can be easily mined and the hematite concentrated to a high Fe-grade product.

Structure and Mineralization in the Quadrilátero Ferrífero

The tectonic events related to the iron mineralization are better preserved in the western domain where two main folding directions and related thrust faults are recognized. The first one (F1a) with northeast-southwest axial orientation formed the Serra do Curral and Gandarela synclines (Fig. 8A), while the north-northwest–south-southeast Moeda megasyncline belongs to the F1b structures. Both fold events are probably contemporaneous and are also recognized at meso- and microscale in all units of the Minas Supergroup. North-trending reverse faults and thrusts formed during F1b.

Folds in the BIF are mainly of flexural-slip type, without the development of a pervasive schistosity (Fig. 10A). Mullion-shaped high-grade iron orebodies formed in the hinge zones of the folds or in areas where superposition of two-fold systems produced zones of complex interference (Rosière and Rios, 2004). Examples are the Capitão do Mato and Tamanduá deposits and the eastern limb of the Moeda syncline and the Bocaina and Feijão deposits along the Western Serra do Curral Ridge (Fig. 8A).

Although the orebodies are controlled by folds on the mine scale, a larger order control on the location of mineralization by long-lived faults is suggested by the presence of the Mutuca fault bordering large deposits such as Mutuca, Capão Xavier, Tamanduá, and Capitão do Mato along the eastern margin of the Moeda syncline (Fig. 8A). Another major fault, the Curral fault strikes along the Curral-Rola Moça range and defines the northwest boundary of Quadrilátero Ferrífero. These major structures possibly represent detachment faults related to the opening of the Minas basin and later reactivated as thrusts during its inversion. The spatial association of major massive iron ores (e.g., Águas Claras) with these long-lived faults suggest these structures formed conduits for mineralizing fluids, whereas fold hinges would represent the main loci for fluid-wall-rock reactions to produce large high-grade orebodies.

In the eastern high-strain domain some high-grade orebodies formed under higher temperature conditions with the development of schistose bodies in ductile shear zones related to thrust zones probably related to the Transamazonian D1b event. Examples are the Fazendão, Timbopeba, Alegria, Brucutú, and Cauê deposits (Fig. 8A). In this area, however, there was regional tectonic overprinting of mineralized fold hinges by thrust and transpressive zones formed during the younger Brasiliano orogeny. During the Brasiliano event, Fe remobilization occurred with the development of quartz-hematite veins (e.g., Rosière and Rios, 2004) and local iron mineralization in all stratigraphic sequences including the younger Espinhaço quartzites.

The near-parallel orientation of structures developed in both Transamazonian and Brasiliano orogenies result in some uncertainty regarding the age of some of the mineralized structures and ore deposits. For instance, this is the case of Timbopeba where a large schistose orebody is controlled by a northeast-southwest–striking transpressive zone and in the Casa de Pedra deposit on the southwest extremity of the Quadrilátero Ferrífero, where thrust faults partially control hard hematite bodies in the hinge zone of tight reclined folds, which occur in a much larger mass of soft hematite ore (Figs. 8A, 11)

Fig. 10.

A. Tightly folded BIF in the Serra do Curral. These overturned folds (F1a) have no axial planar cleavage and fold hinges trend northeast. B. Sharp hard hematite-soft hematite contacts just north of the Pico deposit. Hard hematite is developed in fold hinge zone, while soft hematite occurs on the limbs. C. Diamond drillcore CPFSD05 001 depth 163 m from Casa de Pedra, showing sharp transition between hematite-dolomite protore and hematite ore above. D. Tightly folded hematite ore from the Casa de Pedra deposit (core CPFSD05, approx 160 m). Note absence of axial planar cleavage. E. Overview of the Conceição deposit, looking west. The deposit is developed in a thickened refolded D1-D2 hinge zone. The prominent slice of Piracicaba waste rock is bounded by thrusts to the north. F. Hard schistose hematite ore from Conceição.

Fig. 10.

A. Tightly folded BIF in the Serra do Curral. These overturned folds (F1a) have no axial planar cleavage and fold hinges trend northeast. B. Sharp hard hematite-soft hematite contacts just north of the Pico deposit. Hard hematite is developed in fold hinge zone, while soft hematite occurs on the limbs. C. Diamond drillcore CPFSD05 001 depth 163 m from Casa de Pedra, showing sharp transition between hematite-dolomite protore and hematite ore above. D. Tightly folded hematite ore from the Casa de Pedra deposit (core CPFSD05, approx 160 m). Note absence of axial planar cleavage. E. Overview of the Conceição deposit, looking west. The deposit is developed in a thickened refolded D1-D2 hinge zone. The prominent slice of Piracicaba waste rock is bounded by thrusts to the north. F. Hard schistose hematite ore from Conceição.

Deposits in the western low-strain domain

The Feijão deposit is a high-grade hematite orebody in the western branch of the Serra do Curral (Fig. 8A). It comprises hard massive magnetite and/or martite ores that form columnar bodies, mullion structures in the hinge zones of southeast-plunging folds, lenses, or locally developed veins. These ores are commonly massive and devoid of internal structure or are breccias. Metamorphic grade is greenschist facies with the local development of a grunerite-actinolite assemblage in the dolomitic rocks (Pires, 1995). The structure of the Feijão deposit is controlled by the superposition of nearly coaxial folds (F1a,b) with 30° to 40° east-northeast-plunging axis and axial planes dipping steeply south-southeast. The orebodies are sigmoidal or cylindrical in shape and plunge east-northeast parallel to the fold axis, thinning at the fold limbs.

The BIFs are detached from the underlying and upper sequences with the development of disharmonic folds compensated by fold axis rotation during flexural gliding. This deformation mechanism was very effective in creating a pressure gradient from the limbs to the hinge zone during buckling and high-grade ore coincides with zones of low mean stress during the deformation of the BIF.

A similar relationship between folds and mineralization is observed in deposits from Tamanduá to Pico, where highgrade hematite bodies occur in hinge zones of northwest-southeast–trending folds with variable plunges, similar to the structures in the Feijão-Bocaina area (Fig. 10B).

Soft high-grade ore generally envelops such hard bodies. This ore may be seen as the product of supergene enrichment of iron enriched dolomitic BIF or quartz BIF (Spier et al., 2003).

The giant Casa de Pedra deposit (Fig. 11), with reserves of high-grade iron ore of over 1.2 Gt, is situated on the western limb within a group of mines named the Fabrica Complex (Fig. 8A), close to the boundary of the Western and Eastern domains. The Fabrica Complex is also host to the smaller João Pereira and Segredo deposits (in Fig. 8A these are combined as Fabrica). All these deposits display similar structural characteristics and have a strong correlation between thrusts and high-grade hematite ore.

The Fabrica Complex is a structurally intricate area at the confluence of the north-south-trending Moeda syncline and the east-west–trending Dom Bosco syncline (Fig. 8A). The segment of iron formation hosting these deposits is exposed in an open to tight syncline (D1), which plunges to the south-southeast and is bounded to the west and north by thrusts, and to the south by the Engenho fault (Fig. 8A). In the core of the syncline, the Itacolomi Group unconformably overlies the BIF of the Itabira Group.

Fig. 11.

Stacked level plans of the Casa de Pedra deposit, Brasil, showing how the deeper hematite-dolomite ore grades upward into high-grade hematite ore. The hematite ore is surrounded by a halo of soft, enriched BIF. The ore zone sits in a triangular wedge between the northern fault, intruded by dolerite and the southern fault-bounded Nova Lima Group.

Fig. 11.

Stacked level plans of the Casa de Pedra deposit, Brasil, showing how the deeper hematite-dolomite ore grades upward into high-grade hematite ore. The hematite ore is surrounded by a halo of soft, enriched BIF. The ore zone sits in a triangular wedge between the northern fault, intruded by dolerite and the southern fault-bounded Nova Lima Group.

Three stacked, level plans of the Casa de Pedra deposit are presented in Figure 11. In the immediate mine environment, the BIF of the Cauê Formation dips moderately (30°–40°) to the east. The BIF is conformably overlain to the east by thick dolomite of the Gandarela Formation, followed by quartzite and phyllites of the Piracicaba Group. The iron formation is bound to the west by a major thrust, which juxtaposes it over the fyllites and sandstones of the Piracicaba Group in the core of the Moeda syncline, which strikes in a south or south-southwest direction in this area. Chlorite schists of the older Nova Lima Group bound the BIF to the south along a steeply south dipping fault. The third important structure within the mine area is a second order fault which strikes northwest to west-northwest and dips moderately (30°–40°) northeast. This fault, which is intruded by a thick mafic dike, bounds the hematite ore to the north, where it is juxtaposed against dolomite of the Gandarela Formation. Smaller east- to southeast-trending faults offset the lithologic contacts and the older thrusts over meters to hundreds of meters. Both apparent dextral and sinistral displacements are common and this is best explained by block faulting, with vertical movement rather than strike-slip movement.

The lower level plan shows that at depth (>160 m) the orebody is dominated by hematite-dolomite protore. This material is wedged between the Gandarela dolomite and mafic dike to the north and chlorite schists of the basement to the south. An envelope of soft, highgrade hematite surrounds the protore. This hematite is most likely derived from the hematite-dolomite protore by simple supergene leaching of the carbonate (Fig. 10C). The soft hematite ore has sharp contacts with the bordering BIF, which is hard at depth. Upward, the hematite-dolomite protore is progressively leached and converted to soft hematite. The soft hematite in turn is bordered by soft, enriched BIF, which forms a halo around the hematite deposit. This soft BIF forms a narrow margin at depth but dramatically increases in volume in the upper levels of the deposits where it occupies almost the entire thickness of the BIF and represents the main future resource of the deposit.

Since there are not yet detailed structural studies on this deposit, the relative timing of the various structures and the mineralization is not well defined. The thrusts verge westward and have been interpreted by Chemale et al. (1994) as related to the Brasiliano tectonic event: however they are folded around the Fabrica Complex, suggesting they probably originated during the Transamazonian event as early reverse faults and were subsequently reactivated. The fault which terminates the Casa de Pedra deposit to the south is most likely the western continuation of the Engenho fault (Fig. 8A), a major structure with predominantly dextral movement which defines the southern margin of the Quadrilátero Ferrífero. This fault displaces units from the Minas Supergroup and the Itacolomi Group and cuts all major thrusts in the area, suggesting it is late, most likely Brasiliano in age.

Although the hematite ore and the carbonate-bearing protore at Casa de Pedra often preserves tight folds (Fig. 10D), there is generally no cleavage, suggesting that ore formation postdated folding, which is mainly D1 in the deposit area.

Hematite and BIF conglomerates of the St Antonio facies of the Itacolomi Group crop out north of the Casa de Pedra deposit. Assuming that these conglomerates were derived from older BIF with bedded hematite deposits, this suggests that high-grade hematite ore formation at Casa de Pedra postdated the Transamazonian orogeny (~2.1 Ga) but predated deposition of the Itacolomi Group at roughly 2.0 Ga.

Deposits in the eastern high-strain domain

Although the deposits in the western Quadrilátero Ferrífero are mostly controlled by fold hinges and thrusts, in the east several deposits preserve the superposition of a second mineralization event. Specularite ores in this area are also hosted by younger thrust and transpressive shear zones of Brasiliano age.

The high-grade bodies in, for example, the Itabira syncline (Figs. 8A, 12) occupy two distinct settings; first as hard schistose bodies in refolded D1 hinge zones, and secondly as tabular bodies along Brasiliano-age transpressive shear zones. In addition to the high-grade ores, there is a large resource of lower grade soft concentrate ores in schistose BIF. Silicate assemblages of tremolite, anthophyllite in itabirite, and garnet and/or staurolite in the metapelites from the Minas Supergroup in the Itabira syncline indicate intermediate-grade metamorphism between upper-greenschist and lower-amphibolite facies, (Pires, 1995, and references therein).

The Itabira syncline is a northeast-trending regional structure defined by gently east-northeast–plunging, northwest-verging folds (F1a) and thrusts that interfere with second-order east-southeast–plunging reclined folds (F1b). The main schistosity S1a in the area developed during the first folding and thrusting event and is parallel to the axial plane of the F1a folds. A third event is related to a northeast-trending, transpressive zone of Brasiliano age, that also affected the area with thrust sheets crosscutting the 1.67 Ga Borrachudos Granite (Chemale et al., 1994, 1997) . The entire Itabira syncline was near-coaxially refolded during this event (F2) that pushed the entire sequence against the basement with the localized development of a second foliation plane (S2) that locally overprints S1. Younger, gently to moderately steep north or south-plunging, slightly west vergent, mesoscopic folds and crenulations are present throughout the area but do not have any influence on the regional structure (Chemale, 1987).

Over a length of 12 km several high-grade iron deposits are developed along the northern limb of the Itabira syncline from which the largest coincide with structurally thickened (thrusted) parts of the Cauê BIF and east to east-southeast–plunging cross folds (Fig. 12). The large Cauê, Dois Córregos, Periquito, and Conceição deposits are situated in the cross folds (F1b); the Onça and Chacrinha deposits are smaller and located directly along the limbs of the Itabira syncline (Fig. 12).

The flat, east-west–trending, synformal shape of the Cauê deposit follows a refolded thrust plane (Fig. 12). The F1a and F1b fold hinges plunge gently to the northeast and east. The deposit comprises friable, specularitic BIF and subordinate schistose, harder high-grade ores, concentrated in the inflection zone in the western extremity of the open pit.

Fig. 12.

Geologic map of the Itabira district, showing structural features and locations of the major iron ore deposits. Insets show stereoplots (lower hemisphere, equal area) of poles to S1a (contoured, n = 427) and mineral lineations and/or minor fold axes (n = 57) in the Dois Córregos deposit (from Chemale, 1987) and poles to S1a (contoured, n = 134), poles to S2 (triangles, n = 20) and mineral lineations and/or minor fold axes (dots, n =110) in the Conceição deposit.

Fig. 12.

Geologic map of the Itabira district, showing structural features and locations of the major iron ore deposits. Insets show stereoplots (lower hemisphere, equal area) of poles to S1a (contoured, n = 427) and mineral lineations and/or minor fold axes (n = 57) in the Dois Córregos deposit (from Chemale, 1987) and poles to S1a (contoured, n = 134), poles to S2 (triangles, n = 20) and mineral lineations and/or minor fold axes (dots, n =110) in the Conceição deposit.

The Dois Córregos and Periquito deposits comprise refolded orebodies in the central part of the Itabira syncline. In the Dois Córregos deposit (Fig. 12), the ore-controlling structure is compatible with refolding of the northern limb of the east-northeast–trending Itabira syncline (F1a) along east- plunging cross folds (F1b). In a stereonet projection (Fig. 12, see also Chemale, 1987), poles to the S1a foliation follow a great circle distribution, suggesting refolding during the F1b and F2 folding events.

An additional complication is the presence of early thrust imbricates (synchronous with F1a folds), which are refolded by the later F1b event. This is illustrated in cross section E21 of the Dois Córregos deposit (Fig. 13). The overall geometry of the orebody and its country rocks is best explained as a thrust stack that was emplaced during D1a. This stack was subsequently refolded into a tight syncline during F1b. Hard high-grade hematite ore correlates with the thrusts, whereas the softer, enriched BIF most likely formed by supergene processes which have upgraded the specular hematite within BIF in the core of the F1b syncline.

The Conceição deposit is the southernmost deposit with the main orebody located in the refolded hinge zone of the Conceição syncline (Figs. 10E, 12). It preserves an important example of superposition of mineralization events (Rosière and Rios 2004). The deposit contains massive hematite ores which occur as pods surrounded by softer schistose ores. The pods are interpreted to relate to early (D1) thrusts or folds, whereas thinner, sheared orebodies are located along the flanks of the Itabira syncline. In the deposit, F1a and F2 folds are coaxial and plunge moderately to the east-northeast. The macroscopic ore-controlling structure represents a classical type 3 fold interference pattern (Fig. 14; Ramsay, 1962).

In the Conceição syncline two main schistosities developed: S1a which formed during the first-generation F1a folding as a result of the Transamazonian orogeny, and S2, an axial planar foliation which formed during the second-generation F2 folding as a result of the Brasiliano orogeny (Chemale, 1987, Rosière et al., 1997). In a stereonet projection (Chemale, 1987) poles to the S1a foliation follow a great circle distribution, suggesting refolding during the F1b folding event. The ores comprise hard, locally schistose hematite ore (Fig. 10F), soft hematite ore (blue dust), and soft enriched BIF ore. At depth hematite dolomite protore (dolomitic BIF) forms an envelope around the hard hematite ores (Fig. 14).

The main orebody, located within the hinge zone of the Conceição syncline, preserves a relic martite-hematite fabric (S1a) and formed during the early (D1) compressional event. The S1a foliation is overprinted by a specularite fabric (S2) that relates to local transpressive shear zones. Tabular hematite-rich veins trend parallel to these shear zones. This second fabric is characterized by the preferred orientation of the hematite crystals that define a continuous, penetrative, foliation plane with millimeter-large specularite plates displaying straight grain boundaries. The strongly oriented fabric may have facilitated percolation of meteoric waters and extensive leaching of quartz in BIF surrounding the high-grade hematite ores, producing enriched, friable, high-grade schistose soft BIF with iron grades of around 50 percent. These low-grade, soft BIFs can be easily mined and concentrated to a higher Fe grade product, thus contributing significantly to the overall size (tonnage) of the deposit.

Fig. 13.

Cross section E21 through the Dois Córregos deposit. Three sections show the predeformation state, the situation after D1 thrusting, and the present-day geometry.

Fig. 13.

Cross section E21 through the Dois Córregos deposit. Three sections show the predeformation state, the situation after D1 thrusting, and the present-day geometry.

Fig. 14.

Detailed map and section of the Conceição deposit, showing overprint of F2 folds on F1a folds.

Fig. 14.

Detailed map and section of the Conceição deposit, showing overprint of F2 folds on F1a folds.

Iron Ores of the Carajás Province, Brazil

The Carajás province in central Brazil (Fig. 15) preserves several volcano-sedimentary sequences of dominantly very low metamorphic grade. Together, these comprise the poorly defined Itacaiúnas Supergroup. The ca 2.76 Ga Grão Pará Group contains a sequence of mafic-volcanic rocks and BIF units. The latter contain jaspilites that are host to the largest and most important high-grade (Fe ~67%) iron orebodies (e.g., the N4E deposit, Fig. 16A, B) in the Carajás province, with a total ore resource of about 18 Gt. The Grão Pará Group has been mapped in the Serra dos Carajás, which is divided into the northern and southern ranges (Fig. 15). The Grão Pará Group is overlain unconformably by clastic sediments of the Paleoproterozoic Águas Claras Group. Sandstones, siltstones, and minor conglomerates belonging to this unit are preserved in the synclinal structure between the northern and southern ranges (Figs. 15, 16).

In the Carajás province, several large-scale folds partially surround domal structures which comprise granite bodies and granulite cores (e.g., the Piúm Complex, 3.00 Ga; Pidgeon et al., 2000). In the eastern part, the 2.76 Ga old syntectonic calc-alkaline Estrela Complex (Barros, 1997; Barros et al., 2001) produces the discontinuity of the regional structural trend between the Serras do Rabo and Serra Leste (Fig. 15) and thereby modifies large-scale folds, causing localized ductile flattening, development of a schistosity and contact metamorphism. Late, post-tectonic granitoids such as the Carajás granite intruded the sequence at about 1.88 Ga (Gibbs et al., 1986).

Fig. 15.

Geologic map of the Carajás province.

Fig. 15.

Geologic map of the Carajás province.

The structural setting in the Carajás province is dominated by a flattened flexural fold system with axes moderately plunging to the west-northwest. This structure is intersected by several strike-slip faults which trend subparallel to the fold axial planes (Rosière et al., 2005). The Serra dos Carajás represents an S-shaped synform-antiform pair, hereafter named the Carajás Folds, which is partially disrupted by the Carajás shear zone that divides the Carajás Folds in the northern and the southern ranges (Fig. 15).

The west-northwest–east-southeast–trending, sinistral Carajás and Cinzento shear zones represent major structural discontinuities subparallel to the axial plane of the regional folds and were probably formed in order to accommodate deformation by lateral escape during its progressive flattening. Along these faults silicification and hydrothermal alteration is common, with local development of schistosity.

Structure and iron mineralization in the Carajás province

The high-grade hematite ores are generally soft and laminated, characterized by thin hematite laminations and commonly lack any visible relic BIF texture. They lack structural fabrics, suggesting that ore formation postdated the major structural events of the Carajás province. Rarely do they contain rafts of unmineralized BIF, locally associated with anticlines (Fig. 17A). Compact hematite ores and hematite-carbonate protores form less than 10 percent of the resource and are concentrated mainly near the footwall of the deposits (Fig. 17B).

Along the Carajás shear zone iron oxide veins are common (Fig. 17C), suggesting localized iron enrichment, but major hematite deposits are absent. Large-scale hematite-carbonate protore formation was restricted to subsidiary splays associated with the Carajás shear zone. The very large, high-grade northern range iron orebodies developed in zones of greatly enhanced rock permeability at the regional, deca-kilometric hinge zone of the antiformal Carajás fold. The structural control of the large southern range iron deposits is not well understood, mainly because these deposits are as yet undeveloped. They may have developed in the hinges of second-order folds (>10 km) that were likely related to zones of relatively higher permeability for later, postmetamorphic, infiltration of mineralizing fluids (Lobato et al., 2005).

Fig. 16.

A. Geologic map of the N4 mining area (from CVRD data). B. North-south cross section through the N4E deposit. Stereograms (lower hemisphere, equal area) show the distribution of poles to bedding and minor fold axes in the N4W and N4E deposits.

Fig. 16.

A. Geologic map of the N4 mining area (from CVRD data). B. North-south cross section through the N4E deposit. Stereograms (lower hemisphere, equal area) show the distribution of poles to bedding and minor fold axes in the N4W and N4E deposits.

Fig. 17.

A. Raft of undigested BIF in soft laminated hematite ore, N5 deposit, Carajás. Note that this raft preserves an anticlinal structure. Bench height is about 15 m. B. Hard ore near the footwall basalts at N5, Carajás. C. Deeply weathered outcrop of the Carajás shear zone with hematite veins. D. Fault plane on the contact of hematite-chlorite-carbonate–altered footwall basalt and hematite ore of N4E above. Steeply plunging slickenslides indicate normal movement. E. Photograph showing hematite conglomerate over soft laminated ore and shale at Sishen. Channel thickness about 5 m. F. Hematite ore with slickenslided fault plane at Kapstevel North, Sishen South area. G. Face of the Sishen North deposit showing thrusted contact between the Kuruman BIF and hematite ores below and sedimentary rocks of the Postmasburg Group above. Bench height 12.5 m (Source: Kumba data).

Fig. 17.

A. Raft of undigested BIF in soft laminated hematite ore, N5 deposit, Carajás. Note that this raft preserves an anticlinal structure. Bench height is about 15 m. B. Hard ore near the footwall basalts at N5, Carajás. C. Deeply weathered outcrop of the Carajás shear zone with hematite veins. D. Fault plane on the contact of hematite-chlorite-carbonate–altered footwall basalt and hematite ore of N4E above. Steeply plunging slickenslides indicate normal movement. E. Photograph showing hematite conglomerate over soft laminated ore and shale at Sishen. Channel thickness about 5 m. F. Hematite ore with slickenslided fault plane at Kapstevel North, Sishen South area. G. Face of the Sishen North deposit showing thrusted contact between the Kuruman BIF and hematite ores below and sedimentary rocks of the Postmasburg Group above. Bench height 12.5 m (Source: Kumba data).

N4 deposits: In the northern range, iron mineralization in the N4 deposits is preserved as thick, openly folded west- to northwest-dipping sheets enclosed within highly chlorite and hematite altered volcanic rocks (Fig. 16A, B). The Carajás granite or its equivalents crop out less than 8 km southwest and less than 2 km northeast of the deposit (Fig. 16A) and probably underlie the ore system. The folds plunge west to northwest. These are crosscut by two sets of (broadly perpendicular) east-west and north-northwest–trending faults. The east-west-trending faults have been interpreted as normal faults (Fig. 17D), whereas the north-northwest–trending structures were interpreted as strike-slip faults, possibly with an east block down component (Walde, 1986). The mafic wall rocks adjacent to the faults are brecciated and strongly altered to a chlorite-hematite-carbonate assemblage. The timing of these faults is not well constrained, but they postdate deposition of the Paleoproterozoic Águas Claras Group, which unconformably overlies the mineralized sequence at the N4 deposit (Fig. 16A). They also dissect the northern range into several fault-bounded blocks in which the fold hinges plunge variably from southwest, northwest to northeast, suggesting that faulting postdated the main phase of folding at Carajás (Rosière et al., 2005).

The Kaapvaal Province, South Africa

Iron formations of the Transvaal Supergroup crop out locally over a large part of South Africa, with a strike extent of more than 700 km (Fig. 18, inset). Economic high-grade hematite deposits however are confined to two districts, Thabazimbi and Sishen and/or Sishen South (Fig. 18).

Fig. 18.

Geologic map of the Sishen mining area (from Kumba data).

Fig. 18.

Geologic map of the Sishen mining area (from Kumba data).

The early geologic history of the Kaapvaal craton is closely related to that of the Pilbara of Western Australia (summarized by Friese and Alchin, 2007, Fig. 19). The BIFs of the Transvaal Supergroup were deposited in a basin that evolved from 2.7 to 1.9 Ga. The extensive Kuruman/Penge Iron Formation hosts the largest deposits and immediately overlies a thick platformal carbonate unit, the Campbellrand Formation. The BIF is approximately the same age as the Brockman Iron Formation of the Pilbara craton.

At least four major tectonic events have affected the BIF units in the Kaapvaal craton, the Kalahari (ca 2.35 Ga) and Kheiss (ca 1.82 Ga) orogenies and two extensional events which both resulted in north-striking normal faults. The oldest extensional event (E1) predated the Kalahari orogeny and was contemporaneous with deposition of the youngest units of the Transvaal Supergroup (Friese and Alchin, 2007). The same normal faults were subsequently reactivated during renewed rifting- and basin-forming events. Normal faults associated with the second extension (E2) offset the bedded and conglomeratic ores in the Sishen deposit. These faults therefore postdated deposition of the Lower Olifantshoek Group but most likely predated the Kheiss orogeny, although detailed timing relationships are not yet available. Dolerite dikes intrude all the units up to the Dwyka Formation (~310 Ma). These dikes also cut the thrusts of the Kheiss orogeny (Fig. 18). The dikes are unaltered and appear to locally re-crystallize the ore at Sishen, suggesting they postdated ore formation.

Fig. 19.

Tectonostratigraphic columns of the Kaapvaal province (South Africa). Arrows indicate the direction of far-field stress direction.

Fig. 19.

Tectonostratigraphic columns of the Kaapvaal province (South Africa). Arrows indicate the direction of far-field stress direction.

Within the province, two styles of hematite deposits are recognized (e.g., Beukes et al., 2002). The Thabazimbi-style deposits form lenses within, or at the base of, the BIF. Formation of high-grade iron ore was closely associated with hydrothermal alteration of the surrounding iron formation and was not related to present-day or paleoweathering surfaces (Netshiozwi, 2002). The high-grade ore is found along early normal faults or dolerite dikes that crosscut the BIF at high angles. Larger deposits are at or close to the base of the BIF close to the underlying dolomite sequence. In some deposits the ore is hosted in deep troughs in the underlying rock sequence which presumably formed as a result of dissolution of the carbonate. The ores are most likely sourced from below, and high-grade hematite often passes upward into low-grade, primary BIF (Netshiozwi, 2002). The ore types, structural styles, and alteration signatures are very similar to those associated with the high-grade hematite deposits hosted by the Brockman Iron Formation in the Pilbara (e.g., Taylor et al., 2001; Netshiozwi, 2002).

In contrast, Sishen-style deposits (Fig. 18) appear to be related to a major paleounconformity and weathering surface that formed at the time of deposition of the Lower Olifantshoek Group (2.0–2.2Ga). Until now, little evidence of hydrothermal alteration has been found. Although the Sishen-style deposits are located along the western, highly tectonized margin of the Maremane dome, little direct evidence for structural control on ore formation has been found, but there is extensive karsting of the underlying thick dolomite sequence (Beukes et al., 2002).

Ore styles at Sishen and Sishen South comprise massive, laminated, collapse brecciated, and conglomeratic or gritty hematite ores. The latter are part of the Lower Olifantshoek sequence and unconformably overlie the hematite ore at Sishen (Figs. 17E, 20A) and form the major source of iron ore in places. At the unconformity surface, the conglomerates immediately overlie the hematite ore and dip steeply into the karsts. Hematite conglomerates higher in the sequence show little evidence of being affected by the karst and have flatter dips (Fig. 20A). This geometry suggests that karsting occurred during deposition of the conglomerates, and by inference, that karsting was also synchronous with or postdated the ore-forming event. Karst probably formed preferentially along older north-trending normal faults that formed just before the Kalahari orogeny (E1). However, many normal faults offset both the iron ores and the hematite conglomerates and therefore must postdate the mineralizing event. These faults were either synchronous with or postdated deposition of the Olifantshoek Supergroup. Postore faulting is also indicated by surfaces with slickensides on some of the massive hematite ores (Fig. 17F).

Older sedimentary rocks of the Postmasburg Group and the Olifantshoek Supergroup, including lavas of the Ongeluk

Formation and locally diamictites, were thrusted over the mineralized sequence at Sishen during the Kheiss orogeny (Fig. 17G), after formation of the high-grade hematite ore and deposition of the hematite conglomerates.

Preservation of the orebodies at Sishen was facilitated firstly by downwarping of ore along early folds or burial in the karsts (Fig. 20B), secondly by the formation of postore half grabens along the north-trending faults, and thirdly by the presence of tectonic cover of volcanic rocks over the ores (Friese and Alchin, 2007).

Discussion

Structure is the most fundamental control on the location of high-grade iron ore deposits. Structure determines the location of these deposits in two major ways. First, suitable structures form a pathway allowing access for supergene or hypogene fluids from a suitable source area to the site of ore formation, i.e., the BIF. Secondly, structures provide a mechanism for the preservation of mineralized systems by protecting the newly formed orebodies from erosion. Table 2 quantifies the first- and second-order structural controls of significant iron ore deposits in the three major Hamersley, Quadrilátero Ferrífero, and Kaapvaal provinces.

High-grade hematite deposits hosted by the Brockman Iron Formation in Western Australia display a large variation in the overall intensity of deformation. These may vary from relatively weak deformation (e.g., Channar), to more intense deformation, particularly folding (e.g., Mount Whaleback). However, after structural reconstruction of orebodies to remove postore structural events, all deposits have common features, particularly the presence of normal faults or fault arrays which are in many places, but not always, intruded by dolerite dikes. Locally, these faults remain in their original position, but elsewhere, they were modified during later deformation events. At least nine of the presently mined deposits in the Hamersley province, representing the majority of high-grade microplaty hematite deposits, are controlled by normal faults (Table 2).

TABLE 2.

Significant High-Grade Iron Deposits in the Hamersley Province, the Quadrilátero Ferrífero, and the Kaapvaal Province, and Their Main Structural Controls

Hamersley provinceQuadrilátero FerríferoKaapvaal
Main structureSecondary structure(s)No. of deposits
Normal faultDike(s)9(i)2?8(vi)
Reverse fault000
ThrustFolds10(ii)9(iv)0
Fold and/or fold zoneThrust(s)4(iii)19(v)0
KarstNormal fault(s)2(i)0>26(vii)
Hamersley provinceQuadrilátero FerríferoKaapvaal
Main structureSecondary structure(s)No. of deposits
Normal faultDike(s)9(i)2?8(vi)
Reverse fault000
ThrustFolds10(ii)9(iv)0
Fold and/or fold zoneThrust(s)4(iii)19(v)0
KarstNormal fault(s)2(i)0>26(vii)

(i) Brockman-hosted hematite deposits (hypogene)

(ii) Marra Mamba-hosted hematite-goethite deposits

(iii) Brockman-hosted hematite-goethite deposits

(iv) Mainly compact hematite ores (hypogene)

(v) Compact hematite and soft hematite and/or BIF ores

(vi) Thabazimbi-type ores (hypogene)

(vii) Sishen-type ores

Fig. 20.

A. Cross section through the Sishen Middle mine, showing karst development in dolomite below BIF and hematite ore. Note slumping of lower hematite conglomerates into the karst, while conglomerates higher up are less affected (from Kumba data). B. Plan view of the bedded hematite lode at Sishen Middle mine, showing strong downwarping of the ore in a major karst structure (from Kumba data).

Fig. 20.

A. Cross section through the Sishen Middle mine, showing karst development in dolomite below BIF and hematite ore. Note slumping of lower hematite conglomerates into the karst, while conglomerates higher up are less affected (from Kumba data). B. Plan view of the bedded hematite lode at Sishen Middle mine, showing strong downwarping of the ore in a major karst structure (from Kumba data).

Normal faults are believed to form the most effective link between the dolomites of the Wittenoom Formation, a possible source of silica-undersaturated alkaline fluids and the overlying BIFs (Fig. 21A). Particularly, oblique reactivation of older normal fault systems, which formed during an extension event before deposition of the Lower Wyloo Group (E2), during a later rifting event pre-Upper Wyloo Group deposition (E3), appears to be the most effective mechanism to channel these fluids into the iron formation. The coincidence of a regional thermal event, expressed in the form of an extensive mafic dike swarm at approximately 2.0 Ga (Muller et al, 2005), should have assisted in the establishment of large-scale hydrothermal systems which eventually led to the hypogene leaching of silica from the iron formations. Once the hydrothermal system was established, dissolution of silica and other gangue minerals from the faults and the BIF led to increased permeability, allowing increased access of fluid into the BIF. This was a self-reinforcing process which may have resulted in hydrothermal fluid activity eventually being concentrated along relatively few structures. In general this led to few, giant deposits, with smaller deposits being numerically underrepresented.

An extensional tectonic setting with normal faults and grabens also creates a favorable setting for ore preservation. This is shown in Figure 4B and sections in Dalstra (2006) of the Paraburdoo deposits, where most preserved orebodies occur in grabens and survived at least three major phases of erosion. Iron mineralization in horst structures generally was eroded, with remnants of this mineralization being preserved as hematite conglomerates. Preservation of the ores is also favored by localized deep karsting of the dolomite underneath the ores, which formed deep troughs filled with hematite ore. This is best developed in the Mount Tom Price deposit (Taylor et al., 2001), where the northeastern and southeastern Prongs synclines form very deep structures with preserved hematite ore. The Mount Whaleback deposit has a unique preservation history because the orebody is along the downthrown side of a major, postore normal fault, the Whaleback fault.

Presently mined hematite-goethite deposits hosted by the Marra Mamba Iron Formation in Western Australia have a common structural theme, which differs strongly from the hematite-only deposits. These ores were preferentially developed in the steep limbs of folds, where they are intersected by thrusts (Fig. 21B). At least 10 economic hematite-goethite deposits in the Marra Mamba BIF, representing the large majority of presently mined deposits, display this control (Table 2). Although the geometries of most Marra Mamba BIF-hosted deposits are very similar, the detailed structural history in the Hamersley province varies from west to east. Deposits in the western part of the province are controlled by thrusts, which predate mine-scale folding (Panhandle, D3) along west-northwest–trending corridors. In the east, thrusting occurred late in, or after, the main folding event, with folds trending east-west (Ophthalmia, D2). This may suggest that thrusts in all deposits have a common, late-D2 timing. Mineralization is generally thickest in the hanging wall of the thrusts, where the ore is enclosed between thrust slices of the Wittenoom Formation. This suggests that structural interleaving of Wittenoom Formation (dolomite) and BIF was a prerequisite for ore formation in these deposits. It is clear from the structural reconstructions that ore formation postdated the movement along the thrusts. Structural preparation (fracturing) of these sites has increased porosity and permeability of the Newman Member, particularly in the hanging wall of the thrusts. Additionally, interleaving of dolomite with BIF created favorable sites for infiltration of late supergene fluids from the dolomite into the BIF. These supergene fluids oxidized the primary magnetite, leached silica from the rocks, and replaced other gangue minerals with goethite (e.g., Morris, 1985). As more and more silica was removed the permeability increased and fluids penetrated farther into the BIF. Again this was a self-reinforcing process which resulted in the large hematite-goethite deposits presented above. The structural setting of the Marra Mamba deposits is not particularly favorable for their preservation. Active erosion could well be the limiting factor that determines the size of the orebodies. In other words, the orebodies are removed by erosion at the same rate or faster than they can form at depth. A structural setting which is unfavorable for preservation may explain why there are so few known Proterozoic hematite systems within the Marra Mamba BIF.

Folds and thrusts are the most important structural sites for high-grade hematite ore formation in the Quadrilátero Ferrífero (Table 2). In this province, however, the ore hosted by these structures is mostly hard hematite, often underlain by hematite-carbonate protores, suggesting a hypogene origin (see also Spier et al., 2003). Also the intensity of deformation in many deposits within the Quadrilátero Ferrífero is much stronger than in deposits in the Hamersley province. Similar to the Marra Mamba deposits in the Pilbara craton, the most likely source for mineralizing alkaline fluids in the Quadrilátero Ferrífero is the (Gandarela) dolomite which immediately overlies the BIF. Thrusts formed the most effective link between dolomite and BIF.

Fig. 21.

Schematic representations of the relationships of structure, stratigraphy, and iron mineralization. A. BIF and source of alkaline fluids (dolomite) separated by a thick shale sequence. In this situation, steep normal faults form the most effective conduit between the underlying carbonate and the BIF above. Uplift leads to supergene upgrading of carbonate-bearing protores and locally to erosion of the orebodies forming hematite conglomerates. B. Where dolomite sequences immediately overlie BIF, thrust faults form an effective link between the carbonate and BIF. Deep supergene circulation forms thick hematite-goethite deposits. C. Where thick carbonate immediately underlies BIF, large karsts with hematite ore can form. Brecciated BIF within the karst is readily upgraded to high-grade iron ore by deep circulation of fluids through the carbonate and BIF. D. Schematic mineralization model for Carajás from Lobato et al. (2005). Fluids derived from granite intrusion travel upward into BIF, forming hematite or hematite-dolomite protores. These protores are much later upgraded to high-grade hematite ores by supergene processes.

Fig. 21.

Schematic representations of the relationships of structure, stratigraphy, and iron mineralization. A. BIF and source of alkaline fluids (dolomite) separated by a thick shale sequence. In this situation, steep normal faults form the most effective conduit between the underlying carbonate and the BIF above. Uplift leads to supergene upgrading of carbonate-bearing protores and locally to erosion of the orebodies forming hematite conglomerates. B. Where dolomite sequences immediately overlie BIF, thrust faults form an effective link between the carbonate and BIF. Deep supergene circulation forms thick hematite-goethite deposits. C. Where thick carbonate immediately underlies BIF, large karsts with hematite ore can form. Brecciated BIF within the karst is readily upgraded to high-grade iron ore by deep circulation of fluids through the carbonate and BIF. D. Schematic mineralization model for Carajás from Lobato et al. (2005). Fluids derived from granite intrusion travel upward into BIF, forming hematite or hematite-dolomite protores. These protores are much later upgraded to high-grade hematite ores by supergene processes.

Deposits in the Quadrilátero Ferrífero were overprinted by a major late tectonometamorphic event, the Braziliano orogeny, which has complicated the interpretation of the structure and texture of the ores. In some cases this later deformation has led to remobilization of preexisting mineralization and renewed ore formation. This late overprint was also fundamental for preservation, with the largest orebodies in the Quadrilátero Ferrífero located in synclines, formed during the Brasiliano orogeny, which overprinted the early formed thrusts, folds, and hematite ore. As an important secondary effect, strong cleavage development has facilitated later, supergene fluid flow and associated upgrading of the unmineralized BIF to iron grades of >50 percent. Supergene processes have also softened the BIF by partly dissolving quartz grains within it, forming a loose aggregate of quartz and iron oxide grains.

The structural control presented above for the Hamersley deposits may be extended to other hematite deposits elsewhere. Thabazimbi-style deposits of the Kaapvaal province are structurally and geochemically nearly identical to the hematite deposits of the Hamersley province (Table 2). At least eight deposits are associated with normal faults and dikes, and to a lesser degree, karsts. Their genesis and preservation was also probably very similar to the Australian deposits. However, in the Sishen area, and to a lesser degree in the Thabazimbi area, dolomite dissolution below the BIF close to a paleo-unconformity surface was more important for formation of high-grade ore and its preservation than normal faulting. In the Kaapvaal province, dolomites immediately underlie the BIF, and thick intervening shale units, as they occur in the Pilbara, are absent. In the Sishen area, the majority (>25) of deposits are situated in karsts (Table 2). Karst formation through dolomite dissolution could be expected to form a very effective fluid pathway from dolomite to BIF, by forming a large deformed (i.e., brecciated and slumped) depression of BIF within a dolomite karst (Fig. 21C). Mineralizing fluids (hydrothermal or supergene) in such a setting would circulate through dolomite into brecciated BIF within the karsts but would probably leave the adjacent undisturbed BIF relatively unaffected. Factors that control the size of orebodies at Sishen and Sishen South include the size of the karsts and the duration of supergene upgrading. Mineralization ceased when either the BIF within the karsts was entirely converted to high-grade ore or when sufficient younger sediment was deposited on top to stop the supergene fluid flow. Preservation of iron ore at Sishen is favored by slumping of ore into the karsts. Preservation is further enhanced by postmineralization normal faulting, placing the orebodies in downfaulted blocks, and postmineralization thrusting, placing volcanic rocks on top of the ore systems. Karsts play only a minor role in ore formation in other iron ore provinces, for example, in the Hamersley province there are only two known deposits within "karsts," the northeastern and southeastern Prong deposits at Mount Tom Price (Table 2).

The most problematic and less well studied deposits are those where a defined source of silica-undersaturated fluids, such as a carbonate sequence, cannot be easily recognized. Examples are Carajás, but also the Krivoj Rog district in the Ukraine and many smaller deposits hosted by BIF in Archaean greenstone belts, such as the Koolyanobbing and Goldsworthy iron belts, both in Western Australia. Many of these deposits in Archean BIFs have alteration signatures and protores similar to those described for the larger Paleoproterozoic hypogene deposits or their higher temperature equivalents (e.g., Dalstra and Guedes, 2004).

The Carajás hematite ores traditionally have been interpreted to have formed entirely by supergene processes (e.g., Tolbert et al., 1971). Recognition of extensive hematite-dolomite (apatite, talc) alteration of the host BIF below the high-grade hematite ore in the deepest parts of the N4E orebody, and extensive chlorite-hematite alteration of dolerite dikes within the orebodies and footwall basalts, however, led Guedes et al. (2003) and Dalstra and Guedes (2000) to propose that hypogene processes modified and upgraded the BIF to form a carbonate and iron-rich protore. Lobato et al. (2005) suggested a possible link between late granitoid intrusions and the formation of these hematite and carbonate bearing protores. Fault arrays link these intrusions (e.g., the Carajás granitoid) and the sites of protore formation in the BIF (Fig. 21D). Protore formation was further enhanced by the presence of zones of increased permeability in the hinges of major folds. The economic high-grade hematite ores were most likely derived from these protores by supergene leaching of the carbonate in the present tropical climate (Dalstra and Guedes, 2004). Due to the presumed young age of the supergene soft ores, structures that enhance preservation play only a secondary role at Carajás.

Implications for exploration and future developments

Because structures play such a fundamental role in the location of high-grade iron deposits, the identification of prospective structures for iron mineralization has received a major effort in the exploration for these deposits. In the Hamersley province, in the last three decades, more than 80 percent of the concealed targets drilled by Hamersley Iron Pty. Ltd. were defined on the basis of structural criteria, mostly by the identification or proposition of a prospective fault zone or an area of complex folding. However, the majority of these targets failed to define a significant resource, mostly because the detailed structural setting of the prospects was poorly understood or because unprospective structures were targeted. The discussion above has shown that structural controls on BIF-hosted iron ore deposits are multiple and reflect the relationship between the possible source region for the silica-undersaturated fluids and the pathway to the host of the iron ore deposits, the BIF. Understanding this relationship for specific iron ore provinces will give direction to which structures should be targeted for drilling and, therefore, is fundamental for a successful iron ore exploration program.

Structure is also a key factor in preservation of newly formed hypogene or supergene ores because both form relatively close to the present or ancient land surface. In general, orebodies formed in extensional or karstic setting have greater preservation potential than orebodies formed in compressional settings, such as thrusts. The latter are usually relatively young, or need some additional process, like refolding, as is the case in the Itabira district of the Quadrilátero Ferrífero, in order to enhance the preservation potential. Entire provinces may have unfavorable settings for the preservation of high-grade iron ores (e.g., the Labrador province in Canada), even though these may have existed in these provinces abundantly in the past.

Modern analytical techniques are presently providing a wealth of new data on ore types, alteration zonation, fluid sources, and absolute timing of structural events and ore formation. This will undoubtedly lead to greatly improved understanding of basin evolution, ore genesis, and hopefully deliver better "vectors to ore." Promising recent developments include absolute age dating of iron mineralization and alteration, using sensitive high resolution ion microprobes (SHRIMP, McNaughton, pers. commun., 2008), and the definition of oxygen isotope anomalies associated with structures that control mineralization at Mount Tom Price in the Hamersley province of Western Australia (Thorne et al., 2007). SHRIMP age measurements have the potential to link specific basin-forming events to iron ore-forming processes and, therefore, help in the targeting of structures that are associated with these events. Isotope analysis may deliver a powerful tool to discriminate between supergene- and hypogene-formed iron ore and in the near future discriminate between structures that are highly prospective and those that are not prospective for iron mineralization.

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Acknowledgments

Many of the results presented in this paper build on the hard work of many geologists in Pilbara Iron, CVRD, Kumba, MBR, CSN, and BHP. We thank all these company iron ore coworkers and those from universities of Brazil, South Africa, and Australia for stimulating discussions which were fundamental to the publication of this manuscript. R. Bateman, M. Greentree, and S. Hagemann are thanked for their critical and constructive reviews.

Figures & Tables

Fig. 1.

Locations of high-grade hematite deposits discussed in this paper.

Fig. 1.

Locations of high-grade hematite deposits discussed in this paper.

Fig. 2.

Geologic map of the Hamersley province, showing major structural features and unconformities. Abbreviations: 18EF = 18 east fault, AA = Alligator anticline, BS = Brockman syncline, CHF = Cairn Hill fault, HF = Homestead fault, HS = Hardey syncline, JA = Jeerinah anticline, MA = Milli Milli anticline, MF = Metawandy fault, NF = Nanjilgardie fault, OF = Ophthalmia fault, OS = Ophthalmia syncline, PF = Poonda fault, PH = Paraburdoo hinge zone, PCF = Parry Creek fault, PDF = Prairie Downs fault, PFC = Panhandle fold corridor, TCS = Turee Creek syncline, TS = Turner syncline, RA = Rockley anticline, SBF = Southern Batter fault, SMF = Snowy Mountain fault, VS = Volcano syncline, WA = Wonmunna anticline, WF = Whaleback fault, WHF = Wheelara fault, WWA = Weeli Wolli anticline, WAY = Wyloo anticline.

Fig. 2.

Geologic map of the Hamersley province, showing major structural features and unconformities. Abbreviations: 18EF = 18 east fault, AA = Alligator anticline, BS = Brockman syncline, CHF = Cairn Hill fault, HF = Homestead fault, HS = Hardey syncline, JA = Jeerinah anticline, MA = Milli Milli anticline, MF = Metawandy fault, NF = Nanjilgardie fault, OF = Ophthalmia fault, OS = Ophthalmia syncline, PF = Poonda fault, PH = Paraburdoo hinge zone, PCF = Parry Creek fault, PDF = Prairie Downs fault, PFC = Panhandle fold corridor, TCS = Turee Creek syncline, TS = Turner syncline, RA = Rockley anticline, SBF = Southern Batter fault, SMF = Snowy Mountain fault, VS = Volcano syncline, WA = Wonmunna anticline, WF = Whaleback fault, WHF = Wheelara fault, WWA = Weeli Wolli anticline, WAY = Wyloo anticline.

Fig. 3.

Tectonostratigraphic columns of the Hamersley province (Australia). Arrows indicate the direction of far-field stress direction.

Fig. 3.

Tectonostratigraphic columns of the Hamersley province (Australia). Arrows indicate the direction of far-field stress direction.

Fig. 4.

A. Cross sections 13962E through the Mount Tom Price deposit (modified from Taylor et al., 2001) during protore formation and today. Star shows approximate position of Figure 5A. B. Cross sections 1820E through the Paraburdoo 4East deposit immediately after protore formation and today. Star shows approximate position of Figure 5B. C. Cross sections 2000E through the Giles Mini deposit during protore formation and today. D. Cross sections 7160E through the Mount Whaleback deposit during protore formation and today (modified from Ronszecki, 1992; Dalstra et al., 2001). Star shows approximate position of Figure 5D.

Fig. 4.

A. Cross sections 13962E through the Mount Tom Price deposit (modified from Taylor et al., 2001) during protore formation and today. Star shows approximate position of Figure 5A. B. Cross sections 1820E through the Paraburdoo 4East deposit immediately after protore formation and today. Star shows approximate position of Figure 5B. C. Cross sections 2000E through the Giles Mini deposit during protore formation and today. D. Cross sections 7160E through the Mount Whaleback deposit during protore formation and today (modified from Ronszecki, 1992; Dalstra et al., 2001). Star shows approximate position of Figure 5D.

Fig. 5.

A. The Southern Batter fault in the Mount Tom Price deposit, showing juxtapositioning of high-grade hematite ore in the hanging wall onto oxidized McRae Shale in the footwall of the fault. B. Small flat fault in the Paraburdoo 4East deposit. The fault is characterized by development of a narrow microplaty hematite zone. C. Aerial photograph of the Ophthalmia fault at West Lido. Overturned Brockman BIF with hematite deposits is in fault (dotted line) contact with shales from the Fortescue Group. The fault dips moderately steeply to the south (left side of photo). D. Near-recumbent D2 folds defined by shale bands in hematite ore, Mount Whaleback.

Fig. 5.

A. The Southern Batter fault in the Mount Tom Price deposit, showing juxtapositioning of high-grade hematite ore in the hanging wall onto oxidized McRae Shale in the footwall of the fault. B. Small flat fault in the Paraburdoo 4East deposit. The fault is characterized by development of a narrow microplaty hematite zone. C. Aerial photograph of the Ophthalmia fault at West Lido. Overturned Brockman BIF with hematite deposits is in fault (dotted line) contact with shales from the Fortescue Group. The fault dips moderately steeply to the south (left side of photo). D. Near-recumbent D2 folds defined by shale bands in hematite ore, Mount Whaleback.

Fig. 6.

A. Cross section 1000E through the Nammuldi deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7A. B. Cross section 681440 through the West Angelas deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7B. C. Cross section 13100E through the Hope Downs deposit and reconstructed predeformation geometry.

Fig. 6.

A. Cross section 1000E through the Nammuldi deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7A. B. Cross section 681440 through the West Angelas deposit and reconstructed geometry predeformation. Star shows approximate position of Figure 7B. C. Cross section 13100E through the Hope Downs deposit and reconstructed predeformation geometry.

Fig. 7.

A. Thrust in the Nammuldi deposit, juxtaposing high-grade hematite-goethite ore in West Angela Shale member. B. Thinly bedded, steeply dipping hematite-goethite ore in the hanging wall of the West Angela thrust.

Fig. 7.

A. Thrust in the Nammuldi deposit, juxtaposing high-grade hematite-goethite ore in West Angela Shale member. B. Thinly bedded, steeply dipping hematite-goethite ore in the hanging wall of the West Angela thrust.

Fig. 8.

A. Geologic map of the Quadrilátero Ferrífero district of Brasil. Abbreviations: Synclines: DBS = Dom Bosco, FC = Fabrica Complex, GS = Gandarela, JMS = João Monlevade, IS = Itabira, MS = Moeda; faults: CF = Curral fault, MF = Mutuca fault; mines: AC = Aguas Claras, AG = Alegria, AN = Andrade, BR = Brucutú, CA = Cauê, CD = Capitão do Mato, CF = Feijão, CM = Corrego do Meio, CO = Conceição CP = Casa de Pedra, CX = Capão Xavier, FB = Fabrica, FZ = Fazendão, MT = Mutuca, PI = Picos, TA = Tamandua, TB = Timbopeda. Inset B shows the distribution of metamorphic zones after Pires (1995), and low- and high-strain domains. AZ = actinolite zone, CZ = cummingtonite zone, GZ = grunerite zone, TAZ = tremolite-anthophyllite zone.

Fig. 8.

A. Geologic map of the Quadrilátero Ferrífero district of Brasil. Abbreviations: Synclines: DBS = Dom Bosco, FC = Fabrica Complex, GS = Gandarela, JMS = João Monlevade, IS = Itabira, MS = Moeda; faults: CF = Curral fault, MF = Mutuca fault; mines: AC = Aguas Claras, AG = Alegria, AN = Andrade, BR = Brucutú, CA = Cauê, CD = Capitão do Mato, CF = Feijão, CM = Corrego do Meio, CO = Conceição CP = Casa de Pedra, CX = Capão Xavier, FB = Fabrica, FZ = Fazendão, MT = Mutuca, PI = Picos, TA = Tamandua, TB = Timbopeda. Inset B shows the distribution of metamorphic zones after Pires (1995), and low- and high-strain domains. AZ = actinolite zone, CZ = cummingtonite zone, GZ = grunerite zone, TAZ = tremolite-anthophyllite zone.

Fig. 9.

Tectonostratigraphic columns of the Quadrilátero Ferrífero (Brazil). Arrows indicate the direction of far-field stress direction.

Fig. 9.

Tectonostratigraphic columns of the Quadrilátero Ferrífero (Brazil). Arrows indicate the direction of far-field stress direction.

Fig. 10.

A. Tightly folded BIF in the Serra do Curral. These overturned folds (F1a) have no axial planar cleavage and fold hinges trend northeast. B. Sharp hard hematite-soft hematite contacts just north of the Pico deposit. Hard hematite is developed in fold hinge zone, while soft hematite occurs on the limbs. C. Diamond drillcore CPFSD05 001 depth 163 m from Casa de Pedra, showing sharp transition between hematite-dolomite protore and hematite ore above. D. Tightly folded hematite ore from the Casa de Pedra deposit (core CPFSD05, approx 160 m). Note absence of axial planar cleavage. E. Overview of the Conceição deposit, looking west. The deposit is developed in a thickened refolded D1-D2 hinge zone. The prominent slice of Piracicaba waste rock is bounded by thrusts to the north. F. Hard schistose hematite ore from Conceição.

Fig. 10.

A. Tightly folded BIF in the Serra do Curral. These overturned folds (F1a) have no axial planar cleavage and fold hinges trend northeast. B. Sharp hard hematite-soft hematite contacts just north of the Pico deposit. Hard hematite is developed in fold hinge zone, while soft hematite occurs on the limbs. C. Diamond drillcore CPFSD05 001 depth 163 m from Casa de Pedra, showing sharp transition between hematite-dolomite protore and hematite ore above. D. Tightly folded hematite ore from the Casa de Pedra deposit (core CPFSD05, approx 160 m). Note absence of axial planar cleavage. E. Overview of the Conceição deposit, looking west. The deposit is developed in a thickened refolded D1-D2 hinge zone. The prominent slice of Piracicaba waste rock is bounded by thrusts to the north. F. Hard schistose hematite ore from Conceição.

Fig. 11.

Stacked level plans of the Casa de Pedra deposit, Brasil, showing how the deeper hematite-dolomite ore grades upward into high-grade hematite ore. The hematite ore is surrounded by a halo of soft, enriched BIF. The ore zone sits in a triangular wedge between the northern fault, intruded by dolerite and the southern fault-bounded Nova Lima Group.

Fig. 11.

Stacked level plans of the Casa de Pedra deposit, Brasil, showing how the deeper hematite-dolomite ore grades upward into high-grade hematite ore. The hematite ore is surrounded by a halo of soft, enriched BIF. The ore zone sits in a triangular wedge between the northern fault, intruded by dolerite and the southern fault-bounded Nova Lima Group.

Fig. 12.

Geologic map of the Itabira district, showing structural features and locations of the major iron ore deposits. Insets show stereoplots (lower hemisphere, equal area) of poles to S1a (contoured, n = 427) and mineral lineations and/or minor fold axes (n = 57) in the Dois Córregos deposit (from Chemale, 1987) and poles to S1a (contoured, n = 134), poles to S2 (triangles, n = 20) and mineral lineations and/or minor fold axes (dots, n =110) in the Conceição deposit.

Fig. 12.

Geologic map of the Itabira district, showing structural features and locations of the major iron ore deposits. Insets show stereoplots (lower hemisphere, equal area) of poles to S1a (contoured, n = 427) and mineral lineations and/or minor fold axes (n = 57) in the Dois Córregos deposit (from Chemale, 1987) and poles to S1a (contoured, n = 134), poles to S2 (triangles, n = 20) and mineral lineations and/or minor fold axes (dots, n =110) in the Conceição deposit.

Fig. 13.

Cross section E21 through the Dois Córregos deposit. Three sections show the predeformation state, the situation after D1 thrusting, and the present-day geometry.

Fig. 13.

Cross section E21 through the Dois Córregos deposit. Three sections show the predeformation state, the situation after D1 thrusting, and the present-day geometry.

Fig. 14.

Detailed map and section of the Conceição deposit, showing overprint of F2 folds on F1a folds.

Fig. 14.

Detailed map and section of the Conceição deposit, showing overprint of F2 folds on F1a folds.

Fig. 15.

Geologic map of the Carajás province.

Fig. 15.

Geologic map of the Carajás province.

Fig. 16.

A. Geologic map of the N4 mining area (from CVRD data). B. North-south cross section through the N4E deposit. Stereograms (lower hemisphere, equal area) show the distribution of poles to bedding and minor fold axes in the N4W and N4E deposits.

Fig. 16.

A. Geologic map of the N4 mining area (from CVRD data). B. North-south cross section through the N4E deposit. Stereograms (lower hemisphere, equal area) show the distribution of poles to bedding and minor fold axes in the N4W and N4E deposits.

Fig. 17.

A. Raft of undigested BIF in soft laminated hematite ore, N5 deposit, Carajás. Note that this raft preserves an anticlinal structure. Bench height is about 15 m. B. Hard ore near the footwall basalts at N5, Carajás. C. Deeply weathered outcrop of the Carajás shear zone with hematite veins. D. Fault plane on the contact of hematite-chlorite-carbonate–altered footwall basalt and hematite ore of N4E above. Steeply plunging slickenslides indicate normal movement. E. Photograph showing hematite conglomerate over soft laminated ore and shale at Sishen. Channel thickness about 5 m. F. Hematite ore with slickenslided fault plane at Kapstevel North, Sishen South area. G. Face of the Sishen North deposit showing thrusted contact between the Kuruman BIF and hematite ores below and sedimentary rocks of the Postmasburg Group above. Bench height 12.5 m (Source: Kumba data).

Fig. 17.

A. Raft of undigested BIF in soft laminated hematite ore, N5 deposit, Carajás. Note that this raft preserves an anticlinal structure. Bench height is about 15 m. B. Hard ore near the footwall basalts at N5, Carajás. C. Deeply weathered outcrop of the Carajás shear zone with hematite veins. D. Fault plane on the contact of hematite-chlorite-carbonate–altered footwall basalt and hematite ore of N4E above. Steeply plunging slickenslides indicate normal movement. E. Photograph showing hematite conglomerate over soft laminated ore and shale at Sishen. Channel thickness about 5 m. F. Hematite ore with slickenslided fault plane at Kapstevel North, Sishen South area. G. Face of the Sishen North deposit showing thrusted contact between the Kuruman BIF and hematite ores below and sedimentary rocks of the Postmasburg Group above. Bench height 12.5 m (Source: Kumba data).

Fig. 18.

Geologic map of the Sishen mining area (from Kumba data).

Fig. 18.

Geologic map of the Sishen mining area (from Kumba data).

Fig. 19.

Tectonostratigraphic columns of the Kaapvaal province (South Africa). Arrows indicate the direction of far-field stress direction.

Fig. 19.

Tectonostratigraphic columns of the Kaapvaal province (South Africa). Arrows indicate the direction of far-field stress direction.

Fig. 20.

A. Cross section through the Sishen Middle mine, showing karst development in dolomite below BIF and hematite ore. Note slumping of lower hematite conglomerates into the karst, while conglomerates higher up are less affected (from Kumba data). B. Plan view of the bedded hematite lode at Sishen Middle mine, showing strong downwarping of the ore in a major karst structure (from Kumba data).

Fig. 20.

A. Cross section through the Sishen Middle mine, showing karst development in dolomite below BIF and hematite ore. Note slumping of lower hematite conglomerates into the karst, while conglomerates higher up are less affected (from Kumba data). B. Plan view of the bedded hematite lode at Sishen Middle mine, showing strong downwarping of the ore in a major karst structure (from Kumba data).

Fig. 21.

Schematic representations of the relationships of structure, stratigraphy, and iron mineralization. A. BIF and source of alkaline fluids (dolomite) separated by a thick shale sequence. In this situation, steep normal faults form the most effective conduit between the underlying carbonate and the BIF above. Uplift leads to supergene upgrading of carbonate-bearing protores and locally to erosion of the orebodies forming hematite conglomerates. B. Where dolomite sequences immediately overlie BIF, thrust faults form an effective link between the carbonate and BIF. Deep supergene circulation forms thick hematite-goethite deposits. C. Where thick carbonate immediately underlies BIF, large karsts with hematite ore can form. Brecciated BIF within the karst is readily upgraded to high-grade iron ore by deep circulation of fluids through the carbonate and BIF. D. Schematic mineralization model for Carajás from Lobato et al. (2005). Fluids derived from granite intrusion travel upward into BIF, forming hematite or hematite-dolomite protores. These protores are much later upgraded to high-grade hematite ores by supergene processes.

Fig. 21.

Schematic representations of the relationships of structure, stratigraphy, and iron mineralization. A. BIF and source of alkaline fluids (dolomite) separated by a thick shale sequence. In this situation, steep normal faults form the most effective conduit between the underlying carbonate and the BIF above. Uplift leads to supergene upgrading of carbonate-bearing protores and locally to erosion of the orebodies forming hematite conglomerates. B. Where dolomite sequences immediately overlie BIF, thrust faults form an effective link between the carbonate and BIF. Deep supergene circulation forms thick hematite-goethite deposits. C. Where thick carbonate immediately underlies BIF, large karsts with hematite ore can form. Brecciated BIF within the karst is readily upgraded to high-grade iron ore by deep circulation of fluids through the carbonate and BIF. D. Schematic mineralization model for Carajás from Lobato et al. (2005). Fluids derived from granite intrusion travel upward into BIF, forming hematite or hematite-dolomite protores. These protores are much later upgraded to high-grade hematite ores by supergene processes.

TABLE 1.

Fault Systems in the Paraburdoo Ranges and Their Details

Fault systemRepresentativesPresent orientationTimingOriginal orientation
Range parallel flat faults (RPF)66West fault, 44West fault, 32East fault, 42East faultGently N to NNE dippingPre-Mount McGrath (E3)Moderately NNE dipping
Oblique flat faults1 (OF)4West basal fault, 4East basal fault, 23East basal fault, 24East faultGently NE to E dippingPre BRQ, reactivated pre-Mount McGrath (E2-E3)Moderately NE to ENE dipping
Steep reverse faults (SRF)Ratty Springs fault, 18East fault, 23East faultSteeply NE dippingPost-BRQ, pre-Mount McGrath, late dextral reactivation (E3-F4)Moderate-steep SW dipping
Steep normal faults2 (SNF)64East fault, Howies Hole fault, Channar East faultModerately NE dippingPre BRQ?, reactivated pre-Mount McGrath and late dextral (E2?-E3-F4)Moderately NE dipping, rare SW dips
Fault systemRepresentativesPresent orientationTimingOriginal orientation
Range parallel flat faults (RPF)66West fault, 44West fault, 32East fault, 42East faultGently N to NNE dippingPre-Mount McGrath (E3)Moderately NNE dipping
Oblique flat faults1 (OF)4West basal fault, 4East basal fault, 23East basal fault, 24East faultGently NE to E dippingPre BRQ, reactivated pre-Mount McGrath (E2-E3)Moderately NE to ENE dipping
Steep reverse faults (SRF)Ratty Springs fault, 18East fault, 23East faultSteeply NE dippingPost-BRQ, pre-Mount McGrath, late dextral reactivation (E3-F4)Moderate-steep SW dipping
Steep normal faults2 (SNF)64East fault, Howies Hole fault, Channar East faultModerately NE dippingPre BRQ?, reactivated pre-Mount McGrath and late dextral (E2?-E3-F4)Moderately NE dipping, rare SW dips

1Control mineralization at 4West, 4East, and 23/24East

2 Controls mineralization at Channar

TABLE 2.

Significant High-Grade Iron Deposits in the Hamersley Province, the Quadrilátero Ferrífero, and the Kaapvaal Province, and Their Main Structural Controls

Hamersley provinceQuadrilátero FerríferoKaapvaal
Main structureSecondary structure(s)No. of deposits
Normal faultDike(s)9(i)2?8(vi)
Reverse fault000
ThrustFolds10(ii)9(iv)0
Fold and/or fold zoneThrust(s)4(iii)19(v)0
KarstNormal fault(s)2(i)0>26(vii)
Hamersley provinceQuadrilátero FerríferoKaapvaal
Main structureSecondary structure(s)No. of deposits
Normal faultDike(s)9(i)2?8(vi)
Reverse fault000
ThrustFolds10(ii)9(iv)0
Fold and/or fold zoneThrust(s)4(iii)19(v)0
KarstNormal fault(s)2(i)0>26(vii)

(i) Brockman-hosted hematite deposits (hypogene)

(ii) Marra Mamba-hosted hematite-goethite deposits

(iii) Brockman-hosted hematite-goethite deposits

(iv) Mainly compact hematite ores (hypogene)

(v) Compact hematite and soft hematite and/or BIF ores

(vi) Thabazimbi-type ores (hypogene)

(vii) Sishen-type ores

Contents

GeoRef

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