The geology and genesis of high-grade hematite iron ore deposits

Economic Geology, 2004

Petrographic and textural analysis combined with fluid inclusion studies by infrared microscopy of highgrade (>65% Fe) hematite ore samples from the Conceição deposit, in the northeastern part of the Quadrilátero Ferrífero, Brazil, indicate a complex process of oxidation and mineralization during two orogenic events, each developed under different conditions and involving distinct fluids. The earliest mineralization formed massive magnetite-rich orebodies under relatively reducing conditions in the early stages of the Transamazonian orogeny. Magnetite was oxidized (martitized) with the development of porous hematite crystals (hematite I). Possibly during this stage, new hematite crystals were also formed from low-temperature, lowto medium-salinity fluids, as indicated by two-phase fluid inclusions. The origin of these fluids is still uncertain but tentatively interpreted as being modified surface water. The fluids were transported along normal faults and fractures during post-tectonic collapse following the Transamazonian orogeny (2.1-2.0 Ga) and creation of the dome-and-keel structural pattern of the Quadrilátero Ferrífero. These solutions were also likely responsible for the initial oxidation of the iron formations and the development of hematite I. Subsequent uplifted hot basement rocks or post-tectonic plutons were probable heat sources for the regional metamorphism and development of a granoblastic fabric of hematite II grains in the iron formations and high-grade orebodies. However, the ore was only partially recrystallized, as several relics of the early magnetite, martite, and hematite are still preserved in the granular hematite II crystals. During the Brasiliano-Pan-African orogeny (0.8-0.6 Ga), high-salinity fluids, with temperatures varying from ~120°to a maximum of approximately 350°C, penetrated the iron formations along shear zones, crystallizing initially tabular and thereafter platy hematite crystals (hematite III and specularite) forming schistose orebodies. Quartz veins that cut across the ore and envelop specularite plates and ore fragments formed from late-stage, high-temperature, and low-salinity fluids containing CO 2. These later fluids did not alter the ore. Each of these stages of mineralization produced orebodies with distinct features. Recurrent hydrothermal mineralization is thought to have been responsible for the development of giant, high-grade iron ore deposits in structurally favorable sites. Fold hinges with enhanced permeability and deep faults able to conduct the fluids to the surface, repeatedly over time, should be important targets for exploration of new resources.

2009

High-grade hematite ores at the Thabazimbi Mine, Limpopo Province, occur as stratabound bodies in the Early Paleoproterozoic Penge Iron Formation of the Transvaal Supergroup. Iron ores occur at three distinct positions in the Penge Iron Formation (i) basal ore bodies located immediately above a thin oxidised shale unit that marks the base of the Penge Iron Formation in the Thabazimbi area and that may be interpreted as a structural contact towards the underlying dolostones of the Malmani Subgroup; (ii) ore bodies developed immediately above a prominent mafic sill in the Penge Iron Formation; (iii) small, lenticular ore bodies developed in the iron-formation without apparent structural control. Ore bodies in all three stratigraphic positions formed on the expense of the Penge Iron Formation protore, they share very similar mineralogical and textural attributes and can be subdivided into three major ore types with respect to their mineralogy and physical characteristics, namely, (a) carbonate-hematite ore; (b) hard hematite ore; (c) supergene modified ore. Further subdivision into subtypes is possible based on textural attributes. The first stage of iron ore formation at the Thabazimbi deposit is marked by oxidation of ferrous minerals (carbonates and grunerite) and their replacement by hematite. Efficient leaching and replacement of chert in the iron-formation to produce high-grade hematite ores characterizes the second stage of alteration. Stable isotope and fluid inclusion evidence point to a hydrothermal origin of the iron ores. Two hydrothermal fluids were identified, namely a highly saline Ca-Mg-rich brine (S = 27 wt% NaCl equiv , T H = 160ºC) and a Nadominated fluid of intermediate salinity (S = 10 wt% NaCl equiv , T H = 130ºC) that is possibly of meteoric origin. The results obtained in this study are used to propose the following sequence of mineralising events for the Thabazimbi iron ore deposit: (i) Deposition of iron-formation and diagenesis; (ii) contact metamorphic alteration related to the intrusion of the Bushveld igneous complex; (iii) metasomatic oxidation, leaching and residual upgrading that is tentatively linked to structurallycontrolled hydrothermal fluid flow; (iv) supergene modification of existing high-grade ore bodies in post-Gondwana times along the old African land surface.

Hyperfine Interactions, 2010

Iron oxides are dominant minerals in many geo-domains of economical interest, as iron ore mines. Knowing the main mineral transformation pathways is a fundamental step to plan prospecting new mineral deposits. This study aimed at contributing to a better understanding of the chemical and mineralogical processes related to the genesis and transformations of iron oxides involving hematite in an iron-ore mine of the east border of Quadrilátero Ferrífero, Minas Gerais, Brazil. Two representative geo-samples were analyzed with synchrotron radiation X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy, X-ray fluorescence and saturation magnetization (σ) measurements. The iron content varied from 65 to 69 mass% Fe. From XRD data, hematite is indeed the major mineral for all samples but characteristic reflections of goethite and magnetite also appear. For the magnetic sample, σ = 6.9 J T−1 kg−1. 298 K- and 110 K-Mössbauer data allow characterizing hematite in these iron-rich geo-materials.

LACAME 2008, 2009

Iron oxides are dominant minerals in many geo-domains of economical interest, as iron ore mines. Knowing the main mineral transformation pathways is a fundamental step to plan prospecting new mineral deposits. This study aimed at contributing to a better understanding of the chemical and mineralogical processes related to the genesis and transformations of iron oxides involving hematite in an iron-ore mine of the east border of Quadrilátero Ferrífero, Minas Gerais, Brazil. Two representative geo-samples were analyzed with synchrotron radiation X-ray diffraction (XRD), 57 Fe Mössbauer spectroscopy, X-ray fluorescence and saturation magnetization (σ ) measurements. The iron content varied from 65 to 69 mass% Fe. From XRD data, hematite is indeed the major mineral for all samples but characteristic reflections of goethite and magnetite also appear. For the magnetic

Geology, 2022

Hematite and goethite deposits hosted in banded iron formations (BIFs) in the Pilbara craton (Western Australia) represent one of Earth's most significant Fe reserves; however, the timing and tectonic triggers underpinning deposit genesis remain contentious. Uncertainty in ore genesis stems from a lack of direct age measurements, which could aid in correlating periods of BIF mineralization with tectono-thermal events observed elsewhere. Archean-Paleoproterozoic BIFs in the Hamersley Province host extensive martite-microplaty hematite orebodies that formed at 2.2-2.0 Ga, based on indirect constraints. In contrast, combined hematite in situ U-Pb geochronology and (U-Th)/He thermochronology demonstrate that martite-microplaty hematite ores in the Chichester Range crystallized ca. 1.26-1.22 Ga and underwent cratonic denudation between ca. 0.57 and 0.38 Ga. Nanoscale imaging of dated hematite indicates that U-Th-Pb is lattice bound and not hosted in inclusions. New U-Pb hematite ages overlap with other mineral ages reported at the margins of the Pilbara and Yilgarn cratons (1.3-1.1 Ga), where mineral formation was driven by plate reorganization following breakup of the Nuna supercontinent. This age correlation suggests that a combination of increased orogenic (+diagenetic) and heat (+fluid) generative processes resulting from supercontinent reconfiguration was a key trigger for iron ore formation in the Pilbara craton.

2009

21 samples of hematitite and 12 samples of itabirite were collected through different parts of Quadrilátero Ferrífero (QF) area and were analysed for minor elements with the purpose to understand the source of banded iron formation (BIF). Hematite is the main iron phase and magnetite appears as relict. Tourmaline appears sporadically in minor or trace amounts. Trace element concentrations are relatively low with considerable variability and present anomalous values. REE abundance is relatively low. Chondrite normalized pattern shows relatively high degree of fractionation of LREE to HREE and slight positive Eu anomaly similar to some patterns of BIFs around the world. These features suggest a hydrothermal source of iron. Metamorphic overprinting in the eastern domain suggest supergene enrichment model for QF iron formation origin indicated by anomalous values of minor elements. The source for iron might be near the eastern part of QF region indicated by high values of (La/ Yb)N ratio.

Applied Earth Science (Trans. Inst. Min. Metall. B) 2008 VOL 117 NO 3 1

Iron ores from two important Precambrian belts in India are studied in detail. The first of these is the Jilling-Langalota deposit, hosted by banded iron formations along with generations of shales, tuffs belonging to Iron Ore Group of Eastern India and is hosted in the Singhbhum-North Orissa Craton. The second group of ores is from the Chitradurga basin in Eastern Dharwar Craton, Southern India. These form part of the Archaean greenstone belts and show a typical oxidecarbonate-sulphide association. The Jilling-Langalota deposit contains considerable amounts of blue dust that is absent in the Chitradurga deposit. Comparisons are made between the Indian iron ores and those of the Krivoy Rog province of the Central Ukrainian Shield. The Indian iron ores are relatively richer in Fe and contain higher amounts of alumina and phosphorous compared with those of the Krivoy Rog deposit. The Indian iron ore samples contain porous and friable oxides and hydroxides of iron with kaolinite, gibbsite and quartz. In contrast, the ores from Krivoy Rog are massive with negligible clay and a higher quartz content leading to very low alumina and very high silica contents in the ores and slime. The Indian ores and slimes are manganiferous in nature with high alumina, which is deleterious to processing and is due to the presence of intercalated tuffaceous shales and clay. The Eastern Indian iron ore deposits could have been formed due to enrichment of the primary ore by gradual removal of silica. It is believed that the massive ores result from direct precipitation while powdery blue dust is formed owing to circulating fluids, which leach away the silica from the protore. The host rock is exhalatic banded iron formation and the ubiquitous presence of intercalated tuffaceous shales point towards a genesis that could have involved Fe leaching from sea floor volcanogenic rocks. The nature of these ores along with the parting shale is responsible for production of large amounts of alumina rich slime during mining and handling. The detailed mineralogical characterisation studies aided by X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy, physical parameters and chemical characteristics have indicated the presence of various mineral phases and established the nature of iron-bearing and gangue assemblages of the bulk ores and slime samples from the three iron ore deposits. These in turn are useful in understanding the amenability of the ores and slimes for beneficiation and waste utilisation.

Numerous iron ore deposits are hosted within the Meso to Neo-Archean banded iron formations (BIFs) extending across the Singhbhum-Orissa Craton, eastern India. Despite the widespread distribution of BIFs, which forms part of the iron ore group (IOG), heterogeneity in their grade and mineral composition is occasionally observed even within a single ore deposit. Kiriburu-Meghahatuburu iron ore deposit (KMIOD), west Singhbhum district, Jharkhand, eastern India is characterized by a dominant hematite (often martitized) occurrence with a total resource of >150 million tonnes (MT) at 62.85 wt % Fe. Very high-grade blue dust ore (friable and powdery hematitewith~67% Fe), high-grade massive, hard laminated hematitic ores (~66% Fe) and medium to low grade goethitic/lateritic ores (50%–60% Fe) are the common iron-ore lithologies in KMIOD. These ores can be distinguished in the field from their physical appearance, meso-scale texture and spatial occurrences with the host rocks along with the variation in chemical composition. The high-grade ores are characterized by high Fe (>62 wt %), low Al 2 O 3 (1.5–2.5 wt %), low SiO 2 (2.0–4.5 wt %) and low P (<0.06 wt %). Detailed field studies and laboratory investigations on the ore mineral assemblages suggest that the mineralization of high-grade iron ores at KMIOD is controlled by three major parameters, i.e., lithological, paleoclimatic and structural controls. High-grade iron ores such as blue dust seem to be formed during leaching processes through inter-bedded ferruginous shale and banded hematite jasper (BHJ) occurring within BIFs. Structural elements such as folds, joint network, fracture arrays, local faults and steeply dipping bedding planes are surmised as strong controls for the evolution of different iron ore types from the BHJ. Most of the high-grade ores are concentrated at the hinge portions of second generation folds (F 2) owing to the easy access for circulation of meteoric solution along the fractures developed due to release of stresses at the hinge portions aided by supergene ore enrichment processes. The BHJ and interbedded ferruginous shale seem to have been given a significant contribution for the formation of different grades of iron ores over the area. Lithologically, the BIFs are governed by rheological features providing channel ways in the ore enrichment process. The variation in the iron ore mineralogy is caused by the variation in depositional and paleoclimatic environment, structural setting and lithological attributes. Hence, these parameters could be used for future exploration and grade recovery of iron ore resources in the region and in the adjoining areas.

2007

Geochemical process of selective concentration of elements is controlled by the dynamic tectonic evolution of earth in the geological time scale of million of years. Iron crystallizes out from magmatic melt if the evolved basal-tic melt is highly enriched with Fe (high chemical activ-ity of Fe) and suitable pressure temperature (thermo-dynamic) condition to stabilize spinel magnetite and ilmen-ite. Iron is soluble in atmospheric EhpH conditions and is amenable to precipitate as hydroxide, oxyhydroxide, carb-onate and sulphide in localized change in Eh-pH. The sili-cate minerals weather to release Fe which precipitates as goethite, chamosite, siderite, pyrite in sedimentary geochemical environment depending upon the low-temperature thermodynamics prevailing in the depositional site. The phase transition to magnetite andhematite is also possi-ble during metamorphism and martitisation. So, concen-tration of iron in geological set up can occur in widely varied conditions from lacustrine...

2016

Banded iron formations (BIFs) comprise complex textures and mineralogy, which result from fluid-rock interactions related to high and low temperature alteration. The initial iron oxy hydroxide mineralogy and associated phases such as carbonates, quartz, apatite and phyllosilicates were transformed leading to an upgrading of these BIFs into the world’s largest source of iron ore. In low-grade BIFs, a large part of the iron is related to micro- and nano- metric iron-bearing inclusions within micrometric quartz and/or carbonates (mainly dolomite). We studied laminated jaspilitic BIF samples from a drill core containing 26.71 wt. % total iron, 0.2 wt. % SiO2, 0.32 wt.% MnO, 15.46 wt.% MgO, 22.32 wt.% CaO, 0.09 wt. % P2O5, &lt; 0.05 wt.% Al2O3, 0.15 wt. % H2O and 34.08 wt. % CO2 (Aguas Claras Mine, Quadrilatero Ferrifero, Brazil). Bright rose coloured dolomite and quartz bands alternate with massive specular hematite bands. Raman spectroscopy, X-ray diffraction and FIB-TEM analyses revea...